The development and calibration of South Africa’s … Hub Items/Attachments/11697/Industr… · A...

The development and calibration of South Africa’s National

Standards for Water Retaining Structures

Interim Report

by

JA WIUM

Department of Civil Engineering

University of Stellenbosch

December 2007

WRC Project Number K5/1764


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TABLE OF CONTENTS

1. Background ......................................................................................................................... 3

2. Purpose of this document................................................................................................. 3

3. Methodology ....................................................................................................................... 4

4. Current Progress ............................................................................................................... 5

4.1 Introduction ......................................................................................................................... 5

4.2 Industry survey on current design practice : ................................................................. 6

4.2.1 Type of structures .............................................................................................................. 6

4.2.2 Design practice .................................................................................................................. 6

4.2.3 Construction practice ........................................................................................................ 7

4.2.4 Materials .............................................................................................................................. 7

4.2.5 South African conditions ................................................................................................... 8

5. Code evaluation ................................................................................................................. 8

5.1 Introduction ......................................................................................................................... 8

5.2 BS 8110 together with BS 8007 ...................................................................................... 9

5.3 EN 1992-1 together with prEN 1992-3 ........................................................................... 9

5.4 Codes in South-Africa: .................................................................................................... 10

5.5 SABS 0100:1 .................................................................................................................... 11

5.6 Code comparisons........................................................................................................... 11

6. Workshop .......................................................................................................................... 12

6.1 General .............................................................................................................................. 12

6.2 Outcome ............................................................................................................................ 12

6.3 Steering committee ......................................................................................................... 13

7. Assessment and identification of needs ...................................................................... 13

7.1 Current practice : ............................................................................................................. 13

7.2 Design code development and status .......................................................................... 14

7.3 Code comparisons........................................................................................................... 15

7.4 Other needs identified : .................................................................................................. 15

8. Summary and conclusion ............................................................................................... 15

9. References ....................................................................................................................... 16

ANNEXURE A .................................................................................................................................. 17

ANNEXURE B .................................................................................................................................. 21

ANNEXURE C .................................................................................................................................. 29

ANNEXURE D .................................................................................................................................. 43


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Synopsis

The Water Research Commission appointed the Department of Civil Engineering at the University of

Stellenbosch to develop and calibrate a national standard for the design of concrete water retaining

structures in South Africa. This document reports on the progress and on an evaluation of the original project

plan.

An industry survey was undertaken amongst designers, contractors and owners to determine the status of

the current industry. Information was gathered on design methods, standards applied and parameters used

in the design. The survey also provided information on types of structures, materials and methods of

construction.

The current practice in South Africa is to base the design of water retaining structures on the British

Standard. This standard will soon be replaced by the Eurocode. An assessment was made of the

relationship between current standards, being the concrete design code and the code for water retaining

structures. A comparison was also made between the design methods for crack width as performed using

British Standards and the Eurocode.

A one day workshop was held in Stellenbosch where designers, contractors and owners of water facilities

presented the scope and extent of design, construction and maintenance of water retaining structures in

three regions in South Africa. The three regions are Gauteng, KwaZulu-Natal and the Western Cape.

The information gathered to date, and the investigations performed, confirmed the scope and original plan

for the project. Specific needs were also identified which will either be included in this project or which can

form the basis of additional studies, depending on the nature of the specific need.


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1. Background

To assist in design, construction, quality control and maintenance of water retaining infrastructure, it is

important to develop applicable South African National Standards. Extensive research has and is

internationally being focused at deriving rational design rules for Civil Engineering Infrastructure and

buildings and may be exploited for local application. However, careful translation to local conditions,

construction materials and technologies is imperative.

Considering the fact that no South African standard exists for the design of concrete water retraining

structures, a natural step is to develop such a standard for local application. Most frequently, reference is

made to the British Standard BS8007 for this purpose. However, consideration of local conditions, practice

and materials is essential, calling for an in-depth study to ascertain appropriateness of design rules and

guidelines, as well as harmonization of related codes. To achieve such rational and appropriate procedures

and guidelines for the South African Industry, it is necessary that a South African National Standard for

Water Retaining Structures is developed. In addition, the related standards for loading actions (SANS10160)

and relevant construction materials (SANS10100) are to be revised to include appropriate provisions for

water retaining structures.

The Department of Civil Engineering at the University of Stellenbosch was appointed to develop a draft

design code for concrete water retaining structures in South Africa.

The creation of a code of practice for water retaining structures will form part of a basis from which the

quality, durability and maintenance of water retaining infrastructure can be managed in South Africa.

Although not an innovative creation on its own, it will provide local authorities and water authorities a basis

from which systems can be set up in a co-ordinated manner for the management of durable infrastructure.

The establishment of a code of practice for water retaining structures will thus provide one of the building

blocks which is necessary to develop innovative water infrastructure management systems.

2. Purpose of this document

The purpose of this document is to :

- report on the progress of the project with the aim to

- provide an assessment of the original project plan.


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First, the original anticipated methodology of the project is summarized. A more comprehensive description

of the project plan is presented in ANNEXURE A.

The current progress is then presented with a report on actions performed and needs identified. The initial

actions consisted of information gathered through an industry survey and feedback received during a one

day workshop.

Furthermore, evaluations were carried out to access the status and relationship between current codes, and

to make initial comparisons. This document reports on the current status of this evaluation.

Finally, the report presents the way forward and needs which have been identified to date.

The document is finally completed with a summary and a conclusion.

3. Methodology

A comprehensive proposal for an enforceable standard for the design of concrete water retaining structures

is the ultimate goal in this project. The project will focus on making significant steps towards this ultimate

output by developing a design manual for water retaining structures, also referred to as the Design Guide or

Draft Code in this document.

The methodology is described in the original project plan which is presented in ANNEXURE A. The

methodology is briefly summarized in the following paragraphs.

Phase I : Study of existing codes and a comparison thereof :

- Topic 1 : Assessment of Eurocode provisions (EN 1992-3 and EN 1992-1)

- Topic 2 : Assessment of BS 8007 and BS 8110 and comparison of British codes with

Eurocode

Phase II : A Workshop involving participants from industry, academic institutions and owners

- The outcome should be a frame work from which the code can be developed

- Topic 3 : Investigation into local climatic conditions, materials, including aggregates and

concrete practice, reliability standards and general design and construction practice in the

optimization of specifications for a design standard for water retaining structures.

- Topic 4. Identification of activities for the development of a standard will be performed,

including the formulation of a review by committee and ultimately other stakeholders.

Furthermore, identification will be performed of supporting investigations in the process of

calibration and optimization to provide the basis for the standard.


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Phase III : Theoretical investigations as identified under Topic 4 of Phase II. These will be necessary for the

compilation of design guidelines which will form the basis of the proposed design code. The

research project in this phase will consist of the process of calibration and optimization in

support of the design guidelines.

Phase IV : Compilation of design guidelines, based on the results of the research in Phase III. The design

guidelines will be in the format of a Draft Code, which will at the completion of this project, be

coordinated by the Code Committee to become the new Code. The guidelines will also address

the requirements for the reinforced concrete design of low cost underground water storage

facilities using harvested rainwater which is stored in reservoirs for irrigation and livestock.

Phase V : A seminar will be held in Stellenbosch during which designers, local authorities and other stake

holders will be trained in the provisions and application of the design guidelines.

4. Current Progress

4.1 Introduction

In order to assess local conditions, the status of current practice, and to obtain information from

stakeholders, an industry survey was undertaken as part of a 4th year undergraduate research project.

The following actions were undertaken by this study :

- An industry survey on current design practice

- An industry survey on current construction practice and materials

An assessment was also performed of existing codes. The assessment consisted of an evaluation of the

relationship between different codes, and comparisons performed on clauses used for calculating crack

width. Crack width calculation is one of the important parameters in the design of concrete water retaining

structures.

Furthermore, a workshop was held with designers, contractors and clients (owners) to identify needs from

the industry, to obtain buy-in and to get feedback on the proposed project.

Through these three actions (industry survey, code comparison and workshop) the current needs and status

were identified. Furthermore, the proposal for the establishment of a design code for water retaining

structures was brought to the attention of people in practice.

The following three components of the study are therefore presented under separate headings in the

following paragraphs:


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- Industry survey

- Code evaluation

- Workshop

4.2 Industry survey on current design practice :

The information for the industry survey was partly obtained through a final year under graduate (4th year)

research project carried out as partial requirements for the degree of Bachelor of Science in Civil

Engineering (Le Roux, 2007).

Interviews were conducted with designers, contractors and owners of water retaining structures located in

Gauteng, Cape Town and Durban. The names of people who provided information are listed in ANNEXURE

B.

The following paragraphs provide an extract from the information obtained through the industry survey. More

comprehensive information can be found in Le Roux (2007) :

4.2.1 Type of structures

The types of structures considered by persons who participated in the survey include :

- reservoirs (and storage tanks)

o rectangular,

o circular,

o reinforced concrete

o pre-stressed concrete

o flat roofs or dome roofs

- swimming pools

- elevated tanks

- structures for water purification works

4.2.2 Design practice

Analyses of structures are often either performed by hand or by use of available software packages.

Software packages include Prokon, Strap, Strand7 and others. For the design of concrete water retaining

structures, it was confirmed that designers in South Africa to a large extent make use of the British Code for

the design of Water Retaining Structures (BS 8007) to calculate crack widths, determine minimum

reinforcement content, and to obtain details on typical joints.


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The above Code however needs to be used in conjunction with a concrete design code, for which BS 8110

(1997) or SANS 10100 (2000) are often used. It was noted that in most cases the serviceability requirements

(crack widths) determine the design of the structure and not ultimate limit state requirements.

Designers in South Africa are not all aware of the fact that the British Code would soon be replaced by the

Eurocodes (EN 1992-1 and EN 1992-3).

It was noted that most often designers use design parameters for temperatures, restraint factors and

material parameters which they have accumulated from experience, or from more experienced designers in

their companies. There is a general lack of generally available information on South African conditions for

these parameters (such as temperatures and material characteristics). Although the information does exist, it

is not necessarily documented to be accessible to all designers. A very similar situation exists for certain

construction details which designers specify on their drawings. Although joint details may have originated

from the British Code, there may be slight variances for local preferences. This information is accumulated

by designers through experience.

It was therefore identified that a need exists for information on design parameters to be used in South Africa.

This includes information on materials, construction details, parameters used in the design such as

temperatures (hydration temperatures for different concrete mixes, formwork types, and climatic regions) and

construction practice.

4.2.3 Construction practice

The construction of water retaining structures is mostly performed by contractors in South Africa with

previous experience in the field. The importance of good construction quality control was emphasized.

Specific items which were highlighted are concrete cover and curing methods.

An extract from the information obtained through the survey is enclosed in ANNEXURE B of this report. This

includes information on wall and roof construction, joints and workmanship.

4.2.4 Materials

The industry survey included sourcing of information on current construction materials. Information was

obtained on the following materials :

- cement

- aggregate

- extenders

- reinforcement

- jointing material


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The information in the survey is presented in the under graduate research project report of which an extract

is enclosed in ANNEXURE B. the survey served to identify the current practice and draws the attention to the

variety of materials and regions that need to be addressed for the South African context.

4.2.5 South African conditions

In South-Africa and especially the Western Cape, problems can occur due to very pure water (soft water)

found in mountain streams or rain water. This water is “calcium hungry” and leaches calcium hydroxide

[Ca(OH2)] and [C-S-H]-ions from the hardened cement paste. This situation leads to the degradation of the

cement paste and ultimately the exposure of the steel reinforcement.

This problem can be overcome by the implementation of a lime stabilization plant upstream of the reservoir.

These lime stabilization plants are also known as lime stone reactors. It usually consists of a cylindrical

concrete structure filled with big pieces of lime rock. Water is percolated over the rocks inside the reactor and

some of the lime dissolves into the water. Every few months these reactors need to be inspected and the

lime needs to be replenished.

Most consulting engineers in the Western Cape specify that the mountain water needs to be tested to

identify possible problems regarding soft water.

Another problem with pure water is the lack of impurities. Normally these impurities accumulate inside the

fine cracks which develop due to thermal cracking. These impurities will then, after a few days, seal these

initial cracks and create a watertight structure. The lack of these impurities will however lead to the excessive

leakage of the structure.

It is important that this information be shared with all designers and persons who draw up specifications.

5. Code evaluation

5.1 Introduction

The current code used by designers for the design of reinforced and pre-stressed concrete structures in

South Africa is SANS 10100-1 (2000). This code was developed from and based on the British Code BS

8110 (1997). For this reason, quite a number of engineer in South Africa still make use of BS 8110 for their

designs.

For the design of water retaining structures, South African engineers in general use the British Code BS

8007. This code cannot be used on its own and needs to be used in conjunction with BS 8110.


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As a result of developments in Europe, the British Codes will soon be phased out and be replaced by the

Eurocodes. Design of concrete building structures will specifically be performed according to EN 1992-1, and

water retaining structures according to EN 1992-3. The relationship between EN 1992-1 and EN 1992-3 is

similar to the relationship between BS 8110 and BS 8007, in the sense that the design of water retaining

structures needs the general elemental design parts of BS 8110 for the design of structures.

The approach followed by the Eurocodes is that the basic standard will be applicable to all member states,

but individual countries will be able to issue a National Annex with nationally determined parameters for

specific items. In this manner, each country’s design code will consist of a basic Eurocode formulation,

together with a National Annex.

Recent international developments of codes have resulted in the South African structural engineering

fraternity to recognize that an approach needs to be formulated for the next generation of structural design

codes for South Africa. There is a general realization that due to available funding and resources, it would

not be feasible to develop and maintain local codes if these are to be solely South African codes. A seminar

is being planned (February 2008) for stakeholders to discuss the direction which South Africa should take for

its next generation of structural design codes. One of the options which will be debated is the extent to which

new South African codes should adopt or adapt other existing codes such as the Eurocodes.

In this respect, the project leader for this project, Prof J A Wium, has been elected as the chair person for a

committee which will consider the revision of the design code for concrete building structures (currently

SANS 10100- 1 : Part 1 Design).

In the following paragraphs an overview is presented of the relationship between different design codes, their

use in South Africa and in the United Kingdom.

5.2 BS 8110 together with BS 8007

From the survey amongst practicing designers it was established that designers are of the opinion that BS

8007:1987 is still relevant and works for South-African conditions. Most of the engineers feel that the use of

BS EN 1992-3 for the design water retaining structures, which is not currently used locally, will become more

attractive once this code is phased into use in European countries.

None of the engineers that were interviewed acknowledged that they currently use EN 1992-3 for the design

of water retaining structures. This was expected.

5.3 EN 1992-1 together with EN 1992-3

In general EN 1992-3, used in conjunction with the European National Annexes, is not entirely different from

BS 8110 in terms of the design approach. It gives similar answers for those items which have been

evaluated (bending, shear, crack width) and offers scope for more economic structures.


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Overall EN 1992-3 is less prescriptive and its scope is more extensive than BS 8110. For example, it permits

the use of higher concrete strengths. In this sense the new code will permit designs not currently permitted

when using BS 8110, and thus give designers the opportunity to derive benefit from the considerable

advances in concrete technology over recent years. The author believes that, after an initial acclimatization

period, EN 1992-3 will be generally regarded as a suitable code and a step in the right direction. The new

Eurocodes are claimed to be the most technically advanced codes in the world.

In EN 1992-1 concrete strengths are referred to by cylinder strengths, which are typically 10-20% less than

the corresponding cube strengths. The maximum characteristic cylinder strength fck

permitted is 90 MPa,

which corresponds to a characteristic concrete cube strength of 105 MPa. These values are given in Table

3.1 of EN 1992-1:2004.

The design of flexural elements to EN 1992-1 is in practice very similar to that of BS 8110. However EN

1992-1 does not generally give element specific design guidance, but more the general principles to be

applied. This approach should be welcomed, as it is less restrictive and may encourage innovative design

methods. EN 1992-1 also allows for reinforcement strengths of up to 600 MPa. The new set of Eurocodes

are logically set out and arranged to avoid repetition.

The EN 1992-3 design code is more specific with regards to the design crack width specifications and the

classification of the water tightness of the structure (refer to Section 7 of EN 1992-3). The calculation of crack

width is thus more laborious with a large number of parameters to attend to.

5.4 Codes in South-Africa:

In South-Africa the use of Eurocode 2 (EN 1992-1) is currently very limited due to the lack of available

information regarding values which are contained in the European National Annexes. A South African

equivalent of the National Annex has not been prepared. There appears to be a current lack of interest

amongst designers due to the fact that these codes have not yet been fully implemented in Europe. There is

however significant interest from the side of code writers and developers in adopting or adapting Eurocode

formulations for a new generation of South African Codes (see information about the Code Summit in

Section 5.1 above).

The design processes in these codes are complicated by the continuous reference to national annexes and

other design codes which are not freely available in South-Africa. No definite design formulae are provided

for the design of concrete members, the basic principles are given and the designer basically has to derive

his/her own formulae.

The EN 1992-3 code makes no provision for structures at very high or very low temperatures. There is also

no reference to any water-stop or jointing material specifications.


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5.5 SABS 0100:1

This design code is the primary concrete design code for South-African and its development was based on

the British concrete design code BS 8110.

The South-African design code for the design of water retaining concrete structures will be used in

conjunction with this code. It would therefore be advantageous to prepare the proposed South-African design

code for water retaining structures according to the format of SABS 0100:1 to enhance readability and

compatibility (also to be dependent on the future format of SANS 10100:1).

5.6 Code comparisons

The code currently used by designers in South Africa for the design of concrete water retaining structures is

BS 8007. This code provides a calculation method and guidance for crack width calculations and is used in

conjunction with BS 8110, the design code for concrete structures.

The Eurocode equivalent for the calculation of crack width and design of concrete structures is EN 1992-3

(liquid retaining structures) and EN 1992-1 (concrete structures).

An initial comparison was made between the crack width calculation methods of BS 8007/BS 8110 and EN

1992-3/EN 1992-1. Only flexural cracking was considered and no tensile cracking due to thermal effects was

taken into account. The purpose of the comparison was to determine the extent to which the two codes

(British vs. European) might differ or provide similar results.

The parameters used for the comparisons are presented in ANNEXURE C. The same ANNEXURE also

provides graphs showing the comparison between the two codes.

The design crack width depends on the specific conditions, the choice of the designer, and specifications in

the codes. The value which is very often used when BS 8007 code is applied, is a maximum crack width of

0.2mm. It can be observed from the comparisons between the two codes, that the two codes compare very

well when a crack width of 0.2mm is targeted. For other crack widths there appears to be slight difference

between the codes. This difference increases as the crack width with moves away from the 0.2mm target.

Further comparisons are required for the crack width calculations by the two codes. Such a comparison

should include thermal cracking due to restrained imposed deformations.

A study was also identified in which the reliability of the crack width calculations should be determined. The

reliability level for each of the two codes will be determined. This investigation will form the subject of a MSc

Eng study by Mrs Christian Mcleod from the University of KwaZulu-Natal.


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6. Workshop

6.1 General

Phase II of the project included a workshop which was held in Stellenbosch on 17 October 2007. The

following paragraphs provide an overview of the workshop.

The purpose of the workshop was three-fold :

- to expose designers, contractors and client organizations to the project for the development of a

design code for water retaining structures.

- to obtain input and feedback from the participants in order to involve a wider audience.

- to identify practitioners who would serve as members on working group for the development for the

code (steering committee).

The workshop was arranged to include participants of a variety of organizations and regions. To achieve this

aim, a strategy was followed to invite participants from the following regions and affiliations :

- three regions were decided upon : Western Cape, KwaZulu-Natal and Gauteng

- it was decided to invite representatives of each of the following disciplines from each region :

o designer

o contractor

o client body (owner).

Apart from the above participants, staff and students from the University of Stellenbosch and the University

of KwaZulu-Natal also participated in the workshop. The names, affiliations and topics of presentations of the

participants to the workshop can be seen on the enclosed workshop program which is presented in

ANNEXURE D.

ANNEXURE D also includes a summary of the most important information presented by participants at the

workshop.

6.2 Outcome

Following the presentations at the workshop, a discussion was held on the topic of a design code for water

retaining structures in South Africa. The following are some salient points which were raised during the

discussion :


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- Information on local conditions such as heat of hydration, material characteristics for different types

of cements and mixes, water quality and purity are not generally available. Specialist design firms

have been able to compile some information through experience. This type of information should be

made available to a broader spectrum of designers

- Although not considered to be part of this project, serious consideration should be given to the

development of a design handbook or manual which can be used by designers.

- The use of pre-cast concrete in the construction of especially rural water retaining structures should

be given more consideration. Guidelines should be developed to allow designers and contractors to

use this type of construction. This will require less site work, less reliance on inexperienced site

laborers, and should ensure a better quality product.

- A new code should not aim to be a local developed code, but should be based on international

norms and developments.

6.3 Steering committee

Participants in the workshop are available to become part of a steering committee for the development of a

code for water retaining structures. The participants are keen that a new code should be developed from an

existing code such as EN 1992-1 and EN 1992-3.

7. Assessment and identification of needs

The actions performed to date provided extensive information on current practice, scope of existing codes

and the needs in South Africa for design of water retaining structures.

Although not described in the initial project plan, by commencing the project with an industry survey proved

to be very useful and beneficial. The survey, the code evaluation and the interaction with participants at the

workshop identified the specific needs for the project, but also identified further needs which are beyond the

scope of this project.

With reference to the original project plan, it can be concluded that Phase I and II of the project have been

carried out. These actions confirmed the original project plan and set the stage for the subsequent phases.

The following paragraphs provide a summary of the needs which have been identified through this first two

Phases of the project.

7.1 Current practice :

From the industry survey it was noted that most often designers use design parameters, such as hydration

temperatures, restraint factors and material parameters which they have accumulated from own experience,


14

or from more experienced designers in their companies. There is a general lack of available information on

South African conditions for these parameters. Although the information does exist and has been

accumulated over the years, it is not necessarily contained in the Codes used by the designers. A very

similar situation exists for certain construction details which designers specify on their drawings. Although

joint details may have originated from the British Code, there may be slight variances for local preferences.

These details have been accumulated by designers through own experience.

It was therefore identified that a need exists for information on design parameters to be used in South Africa.

This includes information on materials, construction details, parameters used in the design such as

temperatures (hydration temperatures for different concrete mixes, formwork types, and climatic regions) and

construction practice. The material information includes quality of water used in construction and the criteria

required to verify water for impurities, hardness, and other relevant parameters.

The gathering of information on local material characteristics will form the subject of a MSc Eng thesis. A

student is current being sought for this project to start in 2008.

7.2 Design code development and status

Recent international developments of codes have resulted in the South African structural engineering

fraternity recognizing that an approach needs to be formulated for the next generation of structural design

codes for South Africa. There is a general realization that due to available funding and resources, it would

not be feasible to develop and maintain local codes. A seminar is being planned (February 2008) for

stakeholders to discuss the direction which South Africa should take for its next generation of structural

design codes.

Through the use of existing codes (BS 8110 and BS 8007) in the code comparisons that have been made for

this project, as well as through the application of new codes (EN 1992-1 and EN 1992-3) it has become clear

that a development of code for water retaining structures is a development which goes hand in hand with the

development of a concrete design code. The process to formulate a strategy for the development of a

concrete design code has already commenced, with a code seminar planned for February 2008.

Furthermore, the project leader for this project been elected as the chair person for a committee which will

consider the revision of the design code for concrete building structures (SANS 10100 Part 1). It has already

been decided that this revision should either be based an adoption or an adaptation of the Eurocode (EN

1992-1).

The development of the code for water retaining structures can not and will therefore not proceed in isolation


15

7.3 Code comparisons

It was found through initial comparisons between the British Code (BS 8007) and the Eurocode (EN 1991-3)

that the two codes compare very well when the standard design crack width is targeted. For other crack

widths there appears to be slight difference between the codes.

Further comparisons are required for the crack width calculations by the two codes. Such a comparison

should include thermal cracking due to restrained imposed deformations.

A study was also identified in which the reliability of the crack width calculations will be determined. This

investigation will form the subject of a MSc Eng study by Mrs Christian Mcleod.

7.4 Other needs identified :

Although not considered to be part of this project, other items have been identified which needs specific

attention. These needs can be addressed as a continuation of the current project, or can also run in parallel.

These items are briefly listed below :

- consideration should be given to the development of a design handbook or manual which can be

used by designers.

- the use of pre-cast concrete in the construction of especially rural water retaining structures should

be given more consideration. Guidelines should be developed to allow designers and contractors to

use this type of construction.

8. Summary and conclusion

The purpose of this project is the compilation of a draft code for the design of concrete water retaining

structures in South Africa. An initial project plan included Phase I and II of the project which consists of :

- an evaluation of relevant codes

- a workshop to be held with role players in the industry.

These two phases have now been completed and it has shown that the original project plan is still relevant.

Through an industry survey of design and construction practice, valuable information has been compiled

about conditions and practice in South Africa. This information will now be used as background for the

continuation of the study.

The industry survey assisted to confirm some aspects of the study, such as :

- a need for a compilation of local knowledge on materials and conditions

- the relevance of the current design codes

- the need for guidance on the design of concrete water retaining structures.


16

The survey and workshop also served to identify specific needs which are considered to be beyond the

scope of this project :

- the compilation of a design manual (or handbook) which can be used in conjunction with the Code

- the need to develop the use of pre-cast concrete as a construction method for rural water storage

reservoirs

9. References

1. Le Roux, W. South African Practice for the design of water retaining concrete structures. Project

report in partial fulfillment for the degree of bachelor in Civil Engineering at the University of

Stellenbosch, 2007.

2. SANS 10160 General Procedures for Loading in Buildings and Industrial Structures. DRAFT. South

African Institute for Civil Engineers, 2006.

3. SANS 10100-1 : South African standard. The structural use of concrete. Part 1 : Design. The South

African Bureau of Standards. Pretoria. 2000.

4. BS EN 1990 : Eurocode 0. Basis of structural design. BSI. 2002

5. BS EN 1991-4 : Eurocode 1. Actions on structures. Silos and tanks. BSI 2006.

6. BS EN 1992-1 : Eurocode 2. Design of concrete structures. General requirements. BSI. 2004

7. BS EN 1992-3 : Eurocode 2. Design of concrete structures. Liquid retaining and containment

structures. BSI. 2006

8. BS 5337 : The structural use of concrete for retaining aqueous liquids. BSI. 1976

9. BS 8007 : British Standard code of practice for design of concrete structures for retaining aqueous

liquids. BSI. 1987

10. BS 8110 : British Standard. Structural use of concrete. Part 1. Code of practice for design and

construction. BSI. 1997.


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ANNEXURE A

ORIGINAL PROJECT PLAN

A Methodology

A comprehensive proposal for an enforceable standard for the design of concrete water retaining structures

is the ultimate goal in this project. The project will focus on making significant steps towards this ultimate

output by developing a design manual for water retaining structures, also referred to as the Design Guide or

Draft Code in this document.

The methodology described below was originally identified for the project. This methodology is briefly

presented in the following paragraphs. This presentation is then followed by a description of the work

performed to date, followed by a proposal for the way forward.

A.1 Phase I :

Phase I of the project was defined as follows :

The relationship between BS8007, BS5337, BS8110 and SANS 10100:2003 needs to be studied in the light

of the future replacement of British structural design standards by the Eurocodes. Applicable Eurocodes

include BS-EN 1992 Structural concrete design, in particular BS-EN 1992-1 Design of Concrete Structures.

General requirements and BS-EN 1992-3 Liquid retaining and containment structures.

The development of a code of practice for water retaining structures is thus a process by which existing

knowledge and experience will be considered, assessed, and applied to the conditions in South Africa and to

the benefit of its people. Whereas the usual work method involves human resource training in the form of

full-time post graduate students who are guided by the academic team members to perform the research,

the first phase will involve the research specialists directly to a large extent.

The research for this phase of the project consists of desk top studies and literature reviews.

The following subjects are being addressed :

- Topic 1. Assessment of Eurocode provisions for the design of water retaining structures from

BS-EN 1992-3 Liquid retaining and containment structures, its related general concrete

standard BS-EN 1992-1 General rules and rules for buildings, the related standards on

actions BS-EN 1991-4 Actions in silos and tanks and the related aspects from BS-EN 1990

Basis of the design of structures.

- Topic 2. The present use of BS 8007 for water retaining structures in South Africa is

facilitated by the fact that the local standard for concrete structures SANS 10100 is


18

referenced to BS 8110. The implications of changes in British practice are to be assessed in

order to provide guidance on implications for South African practice.

A.2 Phase II :

Phase II of the project was envisaged as follows :

To establish the support base, but also ensure the relevance of the research activity and to obtain buy-in by

all stake holders, a workshop will be held in Stellenbosch as part of Phase II of the project. Keynote

contributions will be solicited from national participants and potentially one international specialist.

Representatives from local authorities, consulting engineering firms, academic institutions, and construction

companies will be invited. It is envisaged that the workshop will entail one day, of which the morning session

will be devoted to formal presentations and the afternoon for round table discussions.

The outcome of the workshop will be a comprehensive framework for the development of the Code, as well

as the selection of a steering committee to which the research project can report. The steering committee will

also act as the SAICE committee responsible for the development of the revised Code for Water Retaining

Structures.

Specific research subjects that will follow upon the workshop are :

- Topic 3. Investigate the implications of local climatic conditions, materials, including

aggregates and concrete practice, reliability standards and general design and construction

practice in the optimization of specifications for a design standard for water retaining

structures.

- Topic 4. Identification of activities for the development of a standard will be performed,

including the formulation of a review by committee and ultimately other stakeholders.

Furthermore, identification will be performed of supporting investigations in the process of

calibration and optimization to provide the basis for the standard.

Where necessary supporting research will be identified, particularly regarding local conditions, materials and

practice. For this purpose, specific attention will be paid to identify the conditions on which existing codes

are based, as well as the conditions for which the new provisions will be developed. Together with

information gained from Topic 3 above, this information will be used in the motivation for the development of

new code provisions.

The efficiency of this process will be largely improved by reference to BS-EN 1992-3 and associated

standards.


19

The research on Topics 3 and 4 will consist of desk top studies and will be performed by post graduate

students in Stellenbosch and at the University of KwaZulu-Natal.

A.3 Phase III :

Phase III was defined as follows :

This Phase will consist of the theoretical investigations as identified under Topic 4 of Phase II. These will be

necessary for the compilation of design guidelines which will form the basis of the proposed design code.

The research project in this phase will consist of the process of calibration and optimization in support of the

design guidelines.

A wealth of existing information is available on the effect of different materials which determine concrete

characteristics such as creep, shrinkage and moduli of elasticity . The effect of aggregate is of particular

importance. This information will be evaluated for its incorporation into the code. It is not envisaged that

further material testing will be performed to add to this database of information. Where information is lacking

for the design guidelines, current information will be used as a fall back option.

However, sample tests will be performed on structural elements (beams or slabs) to confirm the calculation

methods adopted. Material tests will be performed to determine the characteristics of materials for these

tests.

For the execution of these projects postgraduate students will be drafted and the research will be performed

by post graduate students in Stellenbosch and at the University of KwaZulu-Natal.

A.4 Phase IV

The actions for Phase IV of the project were envisaged as follows :

This phase will consist of the compilation of design guidelines, based on the results of the research in Phase

III. The design guidelines will be in the format of a Draft Code, which will at the completion of this project, be

coordinated by the Code Committee to become the new Code. The final completion of a Design Code is not

included in this proposal. The reason being that the administrative process of formatting, printing and the

public commentary process can be rather lengthy, a process normally managed by the code committee. The

project does however include the preparation of the Draft Code which includes all the necessary technical

information as required for the Code.

The guidelines will also address the requirements for the reinforced concrete design of low cost underground

water storage facilities using harvested rainwater which is stored in reservoirs for irrigation and livestock.


20

The work will be performed partly by a postgraduate student but also by an undergraduate student or young

engineer. The work will be performed in Stellenbosch.

A.5 Phase V

Phase V of the project was planned as described below :

Upon completion of the design guidelines (Draft code), a seminar will be held in Stellenbosch during which

designers, local authorities and other stake holders will be trained in the provisions and application of the

design guidelines. During this seminar, the dissemination of the compiled information will be transferred to

the practitioners by presentation of the guidelines as well as by suitable examples which will serve the

purpose of a design aid.. Local authority representatives will be schooled in the application of the document

which will enable them to oversee the provision of durable, economic infrastructure to the benefit of the

community.


21

ANNEXURE B

INDUSTRY SURVEY

Sources of information for industry survey:

The following people were interviewed and presented information to be used in this project (Le Roux, 2007):

Mr. H van Dalsen. Africon - Pretoria. July 2007. Designer.

Mr. K Bokelman. Africon - Cape Town. September 2007. Designer.

Mr. G Mosterd. Bergstan - Cape Town. September 2007. Designer.

Mr. H Niehaus. Ninham Shand - Cape Town. September 2007. Designer.

Mr. W Jerling. Stefanutti & Bressan Civils - Johannesburg. Workshop, 17 October 2007.

Contractor.

Mr. S Scruton - City of Durban. Workshop, 17 October 2007. Client.

Mr. A Singels - City of Cape Town. Workshop, 17 October 2007. Client.

B.1 CONSTRUCTION PRACTICE AND MATERIALS

The information presented in this ANNEXURE is an extract from project report of the 4th year undergraduate

research project by W le Roux (2007). More comprehensive information can be found in the project report.

B.1.1 Construction

Various different construction methods exist and this chapter provides a brief overview of some of the

methods used to construct water retaining structures.

The following aspects will be discussed in this chapter:

Walls

Roofs

Foundations and floor slabs

Joints

Workmanship

B.1.1.1 Wall construction

Some designers specify that no vertical construction joints may be used when walls are cast for circular

reservoirs (Figure 4.1). This means that walls shall be cast in lifts of a height that permits each lift to be

poured without interruption in one continuous operation. A single circular section is cast monolithically,

then the shuttering is raised and another ring is cast. This process is repeated until the reservoir is

completed. Normally about four or five lifts is specified depending on the size of the reservoir and the

according amount of concrete in a single pour.


22

Other designers favor a different method of wall casting which allows vertical construction joints. The

most popular method will be to cast quarter panels with two lifts, thus eight individual casts will be made.

B.1.1.2 Roof construction

Two main types of roofs are normally constructed:

a. Flat slab

A flat slab roof without beams or drops is the most convenient form of roof to construct and is the

easiest to maintain and clean.

b. Dome

These types of roofs can be found on some of the older municipal reservoirs. These roofs are built

by specialist contractors with the necessary experience and equipment.

Modern reservoirs are rarely built with these types of roofs due to:

The difficulty and complexity of the construction process.

These roofs are not aesthetically pleasing.

It is difficult to seal the interface between the roof and the wall.

These types of roof are approximately 10 percent more expensive than flat slab roofs.

B.1.1.3 Foundations and floor slabs

Floor slabs are usually constructed as reinforced single panel slabs. These single slabs are normally

cast in a rectangular shape for rectangular reservoirs or in a radial pattern for circular reservoirs. The

construction joints are normally specified on the layout drawing together with the arrangement in which

the individual panels should be cast.

Often engineers specify that floor slabs may be cast without construction joints. This method however

requires a substantial increase in the amount of steel to prevent cracking due to plastic and drying

shrinkage and thermal movement. The method of casting alternate strips is not encouraged by

designers.

B.1.1.4 Joints

Horizontal construction joints are permitted in the reservoir wall in positions indicated on the drawings or

as approved by the Engineer. No vertical joints are however permitted unless specified by the Engineer

under special circumstances, such as water conditions that is conducive to autogenous healing.

a. Causes of joint failure

The main causes of joint failures include :

Poor workmanship and detailing which includes:


23

Incorrect positioning of joints.

Incorrect positioning of water stops.

Incorrect positioning of the joint fillers.

Poor preparation of concrete between pours.

Joints opening beyond the normal operating range of the sealants.

Failure of jointing materials including:

Deterioration of joint fillers or sealers – through age embrittlement or shrinkage.

Splitting of water stops – due to embrittlement or over extension.

b. Effects of joint failures

When joint failures occur the following effects can be seen on the structure :

Leakage inwards or outwards from the reservoir.

Cracking of the construction materials.

B.1.2 Workmanship

Poor workmanship at the time of construction can result in a number of defects. In concrete construction

the two most common deficiencies, which result in porous concrete, are air pockets and honeycombing.

Air pockets or entrapped air are usually the result of insufficient compaction (vibration). Honeycombing

occurs when the cement paste does not completely fill the spaces between the coarse aggregate

particles. It may be caused by either incomplete compaction or by grout loss at joints in the formwork.

Other defects that may occur as a result of poor working practices include:

Poor preparation of kickers.

Cold joints forming leakage paths.

Misplaced or moved water stops.

Loss of section due to failure of previous repairs.

Poor grouting of shutter tie holes (walls), pipes and ducts.

One of the most important factors which affects the quality and performance of the final concrete is

curing. It is essential that curing starts relatively soon after the initial set of the concrete to achieve a

durable and impermeable concrete. Some of the curing methods favored by engineers include

:

A constant fog spray applied to the concrete surface.

Members covered in wet hessian and wrapped in plastic sheeting.

Membranes painted onto the concrete surface to retain internal concrete moisture.


24

B.1.3 Types of materials

This chapter reports on the various types of materials which are used in the construction process as well as

the properties of these materials. This information is based on the interviews that were held with the

designers and also information obtained from literature.

The following materials will be discussed in this section:

Cement

Aggregate

Extenders

Reinforcement

Jointing materials

B.1.3.1 Cement properties and content

The minimum cement content of a concrete mix should be 325 kg/m3. This content should however not

exceed 400 kg/m3 for ordinary Portland cement (opc) or cement containing ground granulated

blastfurnace slag (GGBS) or 450 kg/m3 where cements containing pulverized fuel ash/fly ash (PFA/FA)

is used. A maximum water/cement ratio of 0.55 is used for normal Portland cement. When PFA or GGBS

is included into the mix this ratio should be reduced to 0.50.

Typical cement choices for water retaining structures include:

50/50 portland cement/GGBS

70/30 portland cement/FA

B.1.3.2 Aggregate properties

Normally coarse aggregates with a low coefficient of thermal expansion are preferred. The proportions of

the various aggregates shall be such as to produce a well designed concrete mix with a density of at

least 2400 kg/m3 and minimum 28 day strength of 35 MPa. The following types of aggregate are

frequently used in South-Africa:

Dolomitic aggregates

This type of aggregate is normally specified in the construction of sewage treatment works.

Dolomitic aggregates are however not widely available throughout South-Africa. In the Western

Cape it is difficult to procure this type of aggregate.

Hornfels

This type of aggregate is mostly used in the Western Cape where Dolomite aggregates are

scarce. The problem with Hornfels is the reaction between the aggregate and the cement,

known as the alkaline-silicate reaction (ASR). This reaction occurs when the silica in the

aggregates become susceptible to attack by alkalis (Na2O and K2O) originating from the cement

or other sources. During this reaction an alkali-silica gel is formed which can absorb water and

swell, generating internal stresses resulting in cracking and disruption of the aggregate and


25

cement paste. In some cases pop-outs are formed where an isolated reactive particle occurs

close to the surface of the concrete.

The following precautions may be taken to prevent ASR:

o Limit the alkali content of the concrete mix.

o Use replacement materials like GGBS or PFA

B.1.3.3 Extenders

It is of cardinal importance to specify the amount and type of extender which should be included in the

mix. If the percentage of extender is not specified then most of the contractors assume that the mix

should contain a 50:50 ratio of opc:extender.

B.1.3.4 Fly Ash or Pulverized Fuel Ash

Fly Ash provides various benefits when used in conjunction with ordinary Portland cement. These

advantages include:

Increased abrasion resistance

Enhanced durability

A reduction in the reaction between silica aggregates and alkalis in the cement (ASR).

Fly Ash is however not widely available. Fly Ash is not available in the Western Cape or KwaZulu-Natal.

In these regions designers opt for the use of ground granulated blastfurnace slag as an extender to the

concrete mix. The maximum allowable proportion of Fly Ash in the concrete mix should not exceed 35

percent.

B.1.3.5 Ground Granulated Blastfurnace Slag

GGBS increases the density of the concrete and results in a more durable concrete. In the Western

Cape designers specify Corex slag which originates from the Corex steel works at Saldanha.

The necessary care needs to be taken when implementing slag into the concrete mix. If the correct

curing is applied then the slag will result in an excellent concrete mix. If insufficient curing is however

allowed, a great amount of plastic shrinkage will occur which will lead to the development of plastic

cracks.

The amount of slag included in the mix can be increased to as much as 50 percent of the total mix.

Increased amounts of slag lead to the reduction of panel sizes which can be cast at once. This is due to

the increased plastic shrinkage. In normal water retaining structures the amount is limited to a maximum

of 20 percent.


26

B.1.3.6 Reinforcement properties

Normally high-strength steel with a ribbed or a deformed surface and a yield or proof stress of 460 MPa

will be prescribed as reinforcing steel. BS 8007 prescribes a material partial safety factor of γm=1.15 for

reinforcing steel.

Fixing of reinforcing bars on site by welding and heating is not permitted since the tensile strength of the

reinforcing bars is influenced when they are heated above a certain temperature.

When prestressing steel is used all cutting thereof shall be performed with a high speed abrasive cutting

wheel or by a method approved by the engineer. Flame cutting will not be permitted under any

circumstances.

Under certain circumstances where highly corrosive materials are contained special measures can be

taken to improve the durability and corrosion resistance of the reinforcement. Normal reinforcing bars

may be hot-dip zinc galvanized, epoxy-coated or even replaced by stainless steel reinforcement. These

measures are however not widely used in South-Africa due to the increased cost thereof. The availability

of stainless steel reinforcing bars also varies considerably throughout South-Africa. These measures

would typically be applied at splash walls exposed to seawater, sewage treatment works or in places

where highly corrosive water is a problem like in the Middle East. In South-Africa designers favor the

method of increased concrete cover rather than altering the reinforcement type.

The following paragraph will discuss the method of protection and the effectiveness of galvanized

reinforcement.

The zinc coating provided by galvanizing is more anodic than the steel reinforcing bars and so will

corrode preferentially, thus protecting the steel. Zinc is however susceptible to rapid attack where high

levels of residual chlorine are present. The additional short period of protection provided by the zinc

coating may not justify the increased cost of galvanizing.

a. Causes of steel reinforcement corrosion

The corrosion of reinforcement is normally caused due to:

Chemical attack, resulting in loss of alkalinity of the concrete from:

Carbonation

Acid attack

Carbonate imbalances

Chloride attack

Cracking

Exposure of prestressing bands to water leaking outward or groundwater.


27

b. Effects of reinforcement corrosion

The corrosion of steel reinforcement leads to:

The loss of structural integrity due to the loss of a steel section.

The loss of bond and the reduction in concrete cross-section as a result of spalling.

Cracking of the concrete due to the expansive corrosion products.

B.1.3.7 Jointing materials

Jointing materials may be classified as:

a. Joint fillers

They consist of compressible sheets or strips of material fixed to the face of the first-placed concrete

and against which the second-placed concrete is cast.

They provide the initial separation between the faces of the concrete and compress under the

predetermined expansion from each face of the concrete.

Only non-rotting and non-absorbent materials, polyethylene, should be used as joint fillers.

b. Waterstops

Waterstops are preformed strips of durable, impermeable material that are wholly or partially

embedded in the concrete during construction.

They are located across joints in the structure to provide a permanent liquid-tight seal during the

whole range of joint movements.

Waterstops are normally made from durable Poly-Vinyl Chloride (PVC), stainless steel, copper or

rubber compounds.

The problem with PVC waterstops is that they are difficult to bend and hold in place inside the

formwork. Stainless steel is not widely available throughout South-Africa and copper is relatively

expensive.

c. Joint sealing compounds

These materials (or sealants) are impermeable, ductile materials that are required to provide a liquid-

tight seal by adhesion to the concrete throughout the range of joint movements.

The sealing performance is obtained by permanent adhesion of the sealing compound to the

concrete each side of the joint only and most sealants should be applied in conditions of complete

dryness and cleanliness.


28

The long-term performance of a joint sealing compound depends on its formulation, the

workmanship with which it is prepared and applied as well as the circumstances of the structure.

Joint sealing compounds are normally made up of thermoplastics like bitumen rubber or else

chemically curing elastomeric compounds like elastomeric polyurethane is used.


29

ANNEXURE C

COMPARISONS BETWEEN CODES

C.1 DESIGN PARAMETERS AND ASSUMPTIONS

This chapter provides the information on design parameters and assumption which were used in the

calculations for the comparison of BS 8007 and EN 1992-3.

The following design parameters have been used in the crack calculations which follow in the follow section :

C.1.1 Material density

Density of earth = 18 kN/m3

Density of concrete = 24 kN/m3

Density of water = 9.81 kN/m3

C.1.2 Material strengths

Compressive cube strength of concrete (fcu) = 30 - 35 MPa when using BS 8007:1987.

Compressive cylinder strength of concrete (fck) = 28.57 MPa when using prEN 1992-3.

Corresponds to cube strengths of 35 MPa. (refer to Table 3.1, EN 1992-1-1).

Yield strength of reinforcement (fy) = 450 - 460 MPa.

Modulus of elasticity of steel (Es) = 200 GPa.

C.1.3 Thermal parameters

Coefficient of thermal expansion (α) = 10 x 10-6

K-1

. (refer to prEN 1992-3, page 8)

T1 = 15˚C (refer to BS 8007:1987, Table A.2).

T2 = 15 - 20˚C (Designers in Cape Town and Pretoria normally specify these values, depending

on the season).

C.1.4 Design crack widths

Design crack width = 0.2 mm when using BS 8007:1987.

Design crack width = 0.2 - 0.05 mm when using prEN 1992-3 (refer to prEN 1992-3, page 11).

Concrete cover = 40 mm for normal non-aggressive potable water (see BS 8007:1987, page 6).

C.1.5 Load factors and conversion factors (see Tables 9.1 and 9.2)

Partial load factor for water γL= 1.4 when using BS 8007:1987.

Partial load factor for water γL= 1.2 when using prEN 1992-3 (refer to EN 1992-1).

Conversion factors: 1 N/mm2 = 1 MPa / 1 kN/mm

2 = 1 GPa.


30

Design assumptions

The following assumptions were made for the crack width calculations :

Only consider cracking due to bending.

Consider a wall section of unit width (b = 1 m).

Consider a wall of thickness h ≤ 1 m.

Effective depth of concrete (d) = 330 mm (or else d = h - d’).

No pre-or-post tensioned tendons will be used, only reinforcing bars.

Cracking will be determined due to long-term loading.

Calculation of creep factor according to SABS 0100:1, Figure C.1, page 191.

Calculation of effective modulus of elasticity of concrete:

When using BS 8007:1987 refer to SABS 0100-1, page 177.

When using prEN 1992-3 refer to EN 1992-1-1, page 130.

Use only high bond ribbed bars.

No moment redistribution is allowed.

Reinforcement only consists of one diameter bar and not a mixture of bars.

TableC.1 Partial safety factors

SABS 0100:1 ULS 1.2 Dn + 1.6 Ln

SLS 1.1 Dn + 1.0 Ln

BS 8110 ULS 1.4 Dn + 1.6 Ln

SLS 1.0 Dn + 1.0 Ln

EC 2 ULS 1.35 Dn + 1.5 Ln

SLS 1.0 Dn + 1.0 Ln

Table C.2 Material partial safety factors

BS 8110 Concrete γm=1.5

Reinforcing γm=1.15

EC 2 Concrete γm=1.5 (subject to national annex)

Reinforcing γm=1.15 (subject to national annex)


31

C.2 COMPARISON BETWEEN BS 8007 AND prEN 1992-3 THROUGH CRACK CALCULATIONS

This section presents the comparison of the two design codes through the calculation of crack widths in a

mature concrete section.

Only flexural cracking is considered in this section. No tensile cracking or cracking caused by thermal effects

is taken into consideration.

Both BS 8007:1987 and prEN 1992-3 have their own set of design formulae for the calculation of crack

widths in concrete due to bending.

When using BS 8007:1987 the following simple procedure can be used to determine the surface crack

widths of a concrete section:

1. Calculate the service bending moment (Ms).

2. Calculate the depth of the neutral axis, lever arm and the steel stress by elastic theory.

3. Calculate the surface strain allowing for the stiffening effect of the concrete.

4. Calculate the crack width.

When using prEN 1992-3, a similar procedure is applied for the calculation of crack widths:

1. Calculate service bending moment.

2. Calculate the neutral axis depth of the cracked section.

3. Calculate the stress in the tension steel (σs).

4. Calculate the maximum crack spacing.

5. Calculate the crack width.

Comprehensive Excel spreadsheets were prepared containing the design formulae of both BS 8007:1987

and prEN 1992-3. The data for the graphs that are presented here was compiled from these Excel sheets.

The two design codes are compared to each other by means of graphs. Certain specific parameters were

incremented and the effect on the crack width was tabulated. These tabulated values were then plotted to

visualize the trend of the design formulae. In the top right hand corner of each graph the parameters are

listed which were used during the analysis

As a verification (of the calculations and the software!) a commercial software package (PROKON) was used

to calculate crack widths. The Concrete Crack module in this software has the functionality of changing

between design codes. The user can specify which design code he/she wants to use for the crack

calculation. Both sets of design codes in the software (PROKON) were compared to each other as well as to

the spreadsheet calculations described above.

The following four data sets are presented in each of the seven figures listed below:

BS8007 – Data from the spreadsheets when using BS 8007:1987.


32

EC 2 – Data from the spreadsheets when using prEN 1992-3.

Prokon_BS – Data from Prokon when using BS 8007:1987.

Prokon_EC – Data from Prokon when using prEN 1992-3 (Eurocode 2).

For details regarding the source data refer to Section B1 above.

The following seven graphs were compiled to compare the design formulae:

Figure C.1 Crack width versus service moment

Figure C.2 Crack width versus reinforcement area

Figure C.3 Crack width versus section thickness

Figure C.4 Crack width versus steel ratio

Figure C.5 Crack width versus moment ratio

Figure C.6 Crack width versus bar spacing

Figure C.7 Crack width versus bar diameter

The figure below will be used to guide the reader through the set of graphs. It will be repeated at the start of

each page and the appropriate block, which corresponds to the specific graph on that page, will be

highlighted. This method will be used to illustrate progression and thus guide the reader through the set of

graphs. A brief discussion of each graph also follows directly after each graph.

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio


33

C.2.1 Crack width versus service moment

Figure C.1 Crack width versus service moment

Figure C.1 presents an interesting comparison between the design codes and the manner in which the

crack width increases with an increased service moment.

It is clearly shown that the service moment is directly proportional to the predicted crack width which

develops within the concrete.

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio


34

There is a slight difference between the BS 8007:1987 and the Eurocode 2 spreadsheet-values.

The values obtained when using Prokon with BS 8007:1987 almost exactly match the values when using

the BS 8007-spreadsheet.

When using Eurocode 2 in Prokon, the values differ quite a lot from the spreadsheet values as can be

seen with the bottom line which lies below the other three sets of values.


35

C.2.2 Crack width versus reinforcement area

Figure C.2 Crack width versus reinforcement area

Figure C.2 shows that the crack width increases dramatically when the reinforcement area dips below

the value of 2000 mm2/m. There is however little change in the crack widths when increasing the

reinforcement area above this value. This also shows that a minimum area of reinforcement of 2000

mm2 is required to restrict the crack widths to 0.2 mm.

The values obtained from Prokon correspond almost exactly to the values from the spreadsheet

calculations.

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable Steel

Ratio

Variable

Moment Ratio


36

C.2.3 Crack width versus section thickness

Figure C.3 Crack width versus section thickness

Figure C.3 shows a great variance in the crack widths calculated by the different design formulae.

These graphs differ a lot due to the different assumptions made regarding the surface zones of

reinforcement.

A. BS 8007 specifies the following surface zone depths :

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio


37

For h < 500 mm, assume each reinforcement face controls h/2 depth of concrete. Where h is the

total depth of the slab.

For h > 500 mm, assume each reinforcement face controls 250 mm depth of concrete, ignoring

any central core beyond this surface depth.

B. prEN 1992-3 specifies :

The effective tension area should be taken as having a depth equal to 2.5 times the distance from the

tension face of the concrete to the centroid of the reinforcement, although for slabs the depth of this

effective area should be limited to (h-x)/3 where x is the depth to the neutral axis. An overall upper limit

of h/2 also applies.

This means that the surface zone depth is the minimum value of:

2.5 x (h-d)

(h-x)/3

h/2


38

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio

C.2.4 Crack width versus steel ratio

Figure C.4 Crack width versus steel ratio

Figure C.4 shows that the values obtained from the two different codes vary considerably at higher steel

ratios where the area of steel required exceeds the area of steel provided.

The Prokon values from the two codes however correspond to their respective values from the

spreadsheet.

This graph shows that a steel ratio of 0.5 is required to restrict the crack widths to a value of 0.2 mm.


39

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio

C.2.5 Crack width versus moment ratio

Figure C.1 Crack width versus moment ratio

Figure C.5 shows that in the region where the crack width is in the range of 0 - 0.2 mm, the value of all

four data ranges corresponds.


40

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio

Above the maximum allowable crack width of 0.2 mm the data values become scattered and the

Eurocode 2 values moves away from the other three data sets. Following from Figure 11.5 we can see

that for this specific section an ultimate moment (Mu) of almost half the moment of resistance (Mr) is

required to restrict the crack width to 0.2 mm.

C.2.6 Crack width versus bar spacing

Figure C.22 Crack width versus bar spacing


41

Graphs

Variable

Service

Moment

Variable

Reinforcement

Area

Variable

Section

Thickness

Variable Bar

Diameter

Variable Bar

Spacing

Variable

Moment Ratio

Variable Steel

Ratio

From Figure C.6 it is apparent that all four data sets exhibit an almost exponential trend at which the

crack width changes relative to the spacing of the reinforcement bars.

Again, the Eurocode 2 values obtained from Prokon does not match the values of the other three data

sets. At a crack width of 0.2 mm the other three data sets lie on the same line. This figure also shows

that for this section a maximum bar spacing of 250 mm is allowed to restrict the crack widths to 0.2 mm.

C.2.7 Crack width versus bar diameter

Figure C.3 Crack width versus bar diameter


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For smaller bar diameters Eurocode 2 predicts larger crack widths than calculated with BS 8007.

The Prokon values almost exactly match the values from the spreadsheets.

Following from Figure C.7 it seems that the maximum allowable bar diameter, to restrict crack widths to

0.2 mm, is a 25 mm bar for this specific concrete section.


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ANNEXURE D

WORKSHOP HELD IN STELLENBOSCH ON 17 OCTOBER 2007


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A CODE FOR THE DESIGN OF WATER RETAINING STRUCTURES IN SOUTH AFRICA

WORKSHOP 17 OCTOBER 2007

PROGRAM 17 October 2007 : 08h15 : Coffee available 08h30 : Welcome and introductions : 08h40 : Introduction and overview of research project Jan Wium, University of Stellenbosch 08h55 : Practice in KwaZulu-Natal : A client’s perspective Simon Scruton, City of Durban 09h10 : Practice in KwaZulu-Natal : A designer’s perspective Selvan Naidu, Naidu Consulting 09h25 : Practice in Gauteng : A client’s perspective Jan Kruger, Tshwane City Council 09h40 : Practice in Gauteng : A contractor’s perspective Werner Jerling, Stefanutti & Bressan Civils (Pty) Ltd 10h00 : Tea 10h30 : Practice in Gauteng : A designers perspective Jan Wium for Erhard Kruger, HGK Consulting 10h40 : Practice in the Western Cape : A client’s perspective Arne Singels, City of Cape Town 10h55 : Practice in the Western Cape : A contractor’s perspective Alex Pospech, CSV Construction 11h10 : Practice in the Western Cape : A designer’s perspective Jan Wium for Hennie Niehaus, Ninham Shand 11h40 : Crack width calculations to BS8007 and EN 1992-3 Christina Mcleod, University of KwaZulu-Natal 11h55 : Reliability of design for water retaining structures Johan Retief, University of Stellenbosch 12h15 : Discussion All 13h00 : Lunch 13h45 – 14h30 : Summary and closure


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D.3 Summary of workshop held at the University of Stellenbosch

A brief summary of the views and concerns which was raised at the Workshop on 17 October 2007 at the

University of Stellenbosch is given below.

D.3.1 Mr. S Scruton. City of Durban : Client.

No thin walled or floating roof used on reservoirs.

Precast concrete only used for chamber roofs.

Bostik ditsit used to seal roof slabs at joints.

Access hatches 1 big and 1 small.

Vela VKE provides asset management program to eThekwini that stores attribute of all reservoir.

Vandalism is a big problem with fences and hatches.

Two types of reservoirs: Urban and Rural.

Need to implement sieves at inlet to reduce debris in reservoir.

eThekwini monitors all reservoirs on a 24 hour basis (fiber optics also used) with GSM loggers that react

on alerts.

D.3.2 Mr. S Naidu. Naidu Consulting : Designer

Temperatures used T1 = 40˚c and T2 = 30˚c.

Roof covering with stone for long term creep effects (in rural areas people steal the rock).

Curing methods: Fog sprays and hessian wrapped in plastic sheeting.

Gateway traffic circle is a big underground 3.2 Ml reservoir.

D.3.3 Mr. W Jerling. Stefanutti & Bressan Civils – Johannesburg : Contractor.

Dome roof is 10 % more expensive than flat roof.

No specification for pre-cast reservoirs.

No maintenance record for pre-cast reservoirs.

Testing of structures should be better specified.

Not all contractors have good quality control systems in place.

Good, skilled laborers are extremely had to find.

Mines are a potential for construction of water retaining structures where contractors and designers are

often used with insufficient experience.

Be careful of sealing materials like xypex and hypalon bandages.

Subsoil drainages have two functions: i) pressure relief, ii) leak detection.

Leakage detection with different subsoil drains for different panel and manholes connected for each

drain to detect leaks.

Cost of curing with water is expensive in certain inland areas.

Waterbars : Rubber – difficult to place.

PVC – bend and difficult to hold in place.


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Stainless steel – not available everywhere.

Copper – expensive.

D.3.4 Mr. A Singels. City of Cape Town : Client.

Curing by painting on membrane to retain concrete internal moisture.

Reservoir at Plattekloof pre-cast beams and slabs used.

Last tender pre-cast slabs came out cheaper when compared to cast in-situ.

Slabs which are sloped should be cast with lower part first and upper part later, because concrete will

not get under waterstop of lower slab if done other way around.

D.3.5 Ms. Christina Mcleod. University of KwaZulu-Natal : Lecturer.

Eurocode does not cover: - Structures at very high or low temperatures.

- Does not touch subject of waterstops or jointing materials.

BS 8007:1987 does not necessarily cover aggressive water.

Eurocode is more specific to crack widths and classification of water tightness EN 1992-3.

D.3.6 Mr. H G Kruger : Consultant, Pretoria

BS 8007 does not provide a calculation method for crack width under bending and axial tension. A

method was published by Kruger and Atkinson in 2005.

Roof slab are not post tensioned for his designs

Pre-cast is not used.

Concrete specification is durability driven and not strength.

Materials used :

o marine structures : slagment and OPC

o Sewerage pump stations : OPC + PFA + silica fume blend

o All others : OPC + PFA

o (minimize the usage of slagment)

Design codes : BS 8007, BS 8100, New Zealand code

Temperatures used : own experience

Very little use of expansion joints to create maintenance free structures.

Sliding joints used between wall and base for post tensioned structures, also for roof slab and walls.

No curing compound allowed, only water curing.

A South African Code requires information on :

- heat of hydration

- relative humidities

- Temperatures

- Code commentary

- More sketches and less words

- Good contact with Robin Atkinson of the UK, specialist.


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D.3.7 Mr. H Niehaus : Consultant, Cape Town

Reinforced concrete, pre-stressed and pre-cast pre tensioned.

Generally use 35 MPa concrete, cement type CEM I (class 42.5) with a specification of 25% GGCS

(slagment) by mass. Minimum cement content as per SBS 1200G table 5.

Design codes used : BS 8007 with BS8110, using load factors from BS 8110. Crack widths using BS

8007 (either by hand or using software such as Prokon)

Temperature used in design is according to BS 8007. Some special tests were done for particular

projects.

Details that are used are in-house developed but based on BS 8007 information.

Construction problems relate mainly to :

o honeycombed concrete

o shrinkage cracking due to inadequate curing or insufficient expansion joints.

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