Group 14 Assignment 3

ASSIGNMENT 3ADDITIONS AND ADMIXTURES

Concrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast

Concrete

GROUP 14

Makintane MofokengSivuyile Ngobozana

Reatile PitsoMfundo TaliweCharlie Visser

Kevin Volmink (Group Leader)

Word Count7145

GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete

Group Assignment 3: Additions and AdmixturesQ1. (a) Specify a suitable concrete for the application given in Group Assignment 2, Question 2, using a blend of cement A from this assignment and ground granulated blast furnace slag. The sea wall will be located on the North-East coast of Scotland. (b) State the limiting values for the proportions of the concrete that you have specified and give the sources of the limits. Discuss the reasons for these limits in terms of the specific aspects of the concrete technology involved. You should make appropriate assumptions about the concrete properties required. Give the reasoning and analysis that have led to your specified concrete. Q2. A precast concrete manufacturer has been producing structural elements with compressive strength classes of up C35/45, using CEMII/A-V 42,5N cement and a lignosulphonate-based plasticiser. He wishes to extend his range of elements to strength classes up to C90/105 to include 4m x 500mm diameter reinforced concrete columns with a high-quality as-cast surface finish. He wishes to demonstrate to clients that sustainability issues have been fully considered over the full range of his products. (a) Prepare a report for the manufacturer, discussing the potential constituent materials and production procedures that should be considered, including the advantages and disadvantages in each case. Recommend your preferred options. (b) Propose a concrete mix design that would be suitable for the columns, giving reasons for your proposal. Requirements: Overall maximum length 7,000 words (excluding report title page, contents, reference list and appendices) with each diagram, figure etc. within the main text to count as 150 words. Key diagrams, figures etc. should not be relegated to appendices. The number of words or word equivalents should be declared on the title page.


EXECUTIVE SUMMARYThe marine environment provides a severe test of the durability of reinforced concrete structures due to chloride-induced corrosion of reinforcement. The durability of concrete in this environment is based on complex interactions between the environment, materials and structure that affect the long-term performance of marine structures. It is therefore of imperative to consider all the important factors when designing concrete for durability.

Since precast is manufactured in a controlled casting environment it is easier to control the mix, placement, and curing. The economy achieved in pre-cast construction is minmizing the amount to be spent in transport and handling of pre-cast members.

The mix design of HSC is controlled by super-plasticizers (Owens, 2009); the following factors should be considered: (Newman, 2003)

The free water to cement ratio in ranges of 0.25 to 0.3 should be selected based on previous data

The cement should be chosen to achieve maximum compressive strength

The saturation super-plasticizer dosage has to be chosen based on flow cone

The optimum aggregate proportions should be chosen

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TABLE OF CONTENTSPage No

1. INTRODUCTION 11.1 Objectives 11.2 Report Structure 11.3 Scope 1

2. PART 1: CONCRETE WITH GGBS BLENDED CEMENT IN A SEA WALL2

2.1 Specification of a concrete to be used in the construction of a sea wall 22.1.1 Introduction22.1.2 Environment 22.1.3 Binder Type and Content 32.1.4 Concrete Cover 42.1.5 Proposed concrete mix 4

2.2 Admixtures 42.2.1 Plasticisers 42.2.2 Air-entraining Admixtures 5

2.3 Aggregates 52.4 GGBS Properties 5

2.4.1 Plastic state 52.4.2 Hardened state 6

2.5 Durability 62.5.1 Curing 62.5.2 Permeability 62.5.3 Alkali Silica Reaction (ASR) 62.5.4 Sulphate Attack 62.5.5 Chloride ingress 6

2.6 Limiting Values 62.6.1 Cement and GGBS62.6.2 Aggregates 9

2.7 Conclusion 13

3. PART 2: CONCRETE ADMIXTURES IN PRECAST CONCRETE 143.1 Introduction 143.2 Constituent materials 14

3.2.1 Cement 143.2.2 Aggregates 153.2.3 Admixtures 16

3.3 Production Procedure 163.3.1 Specification of concrete 173.3.2 Tolerances 173.3.3 Structural drawings173.3.4 Shop drawings 183.3.5 Casting and erection sequences 183.3.6 Formwork 183.3.7 Reinforcement 19

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3.3.8 Concrete placement 193.3.9 Curing and release 203.3.10 Release agents 203.3.11 Stripping and repair 203.3.12 Concrete strength for handling 203.3.13 Storage 20

3.4 Advantages of Pre-cast Concrete 203.5 Disadvantages of Pre-cast Concrete 213.6 Mix design proposal 21

3.6.1 Introduction213.6.2 Water cement ratio 223.6.3 Cement 223.6.4 Superplasticizer 223.6.5 Proposed mix design 22

4. References 24

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LIST OF FIGURES

Figure 2-1 Primary transport mechanisms in the various exposure zones of a sea wall (Domone, 2001).................................................................................................................3

Figure 2-2 Compressive strength development in pastes of pure cement compounds (Taylor, 1997).................................................................................................................................8

Figure 3-1 Typical ranges of cement content for different concrete strengths classes (strength classes according to EN206:1) (Owens, 2009)................................................15

Figure 3-2 Optimisation of aggregate distribution..................................................................15Figure 3-3 Effect of super-plasticizer on HSC (Nagataki, 1989)............................................16

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LIST OF TABLES

Table 2-1 Limiting values of cement and GGBS compounds..................................................7Table 2-2 Limiting values for Aggrgates...................................................................................9Table 3-1 Commercial HSC mix designs from North America (Burg and Ost, 1992)............21

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1. INTRODUCTION

1.1 Objectives

The objective of this report is to address the questions as required in assignment 3 as partial fulfilment of the requirements for the Advanced Concrete Technology (ACT) Diploma for 2012/2013.

1.2 Report Structure

Assignment 3 addresses concrete additions and admixtures in question 1 and 2 respectively. This report is therefore divided into two parts:

1. Concrete with GGBS blended cement in a sea wall

2. Concrete Admixtures in Precast Concrete

1.3 Scope

Question 1 calls for the specification of a concrete mix to be used in the construction of a sea wall on the North-East coast of Scotland using a CEM I 52,5R cement, from assignment 2, extended with ground granulated blast furnace slag (GGBS). The limits of the proportions of this concrete must be stated and the reasons for these limits discussed.

Question 2 requires a report for a precast concrete manufacturer wishing to extend his product range to manufacturing precast concrete columns using High Performance Concrete (HPC). The report is to discuss the potential constituent materials and production procedures that should be considered as well as propose a mix design suitable for constructing the columns.

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2. PART 1: CONCRETE WITH GGBS BLENDED CEMENT IN A SEA WALL

2.1 Specification of a concrete to be used in the construction of a sea wall

2.1.1 Introduction

The marine environment provides a severe test of the durability of reinforced concrete structures due to chloride-induced corrosion of reinforcement (Alexander and Mackechnie, 2003).

The durability of concrete in this environment is based on complex interactions between the environment, materials and structure that affect the long-term performance of marine structures.

It is therefore of imperative to consider all the important factors when designing concrete for durability, these include:

1. Environment

2. Selection of suitable binders and w/c ratio

3. Minimum cover depths

4. Allowance for construction factors such as placing, curing and appropriate concrete grade for structural purposes.

2.1.2 Environment

It is important to identify the relevant condition of exposure from the beginning. In this case the structure is a sea wall which will be cast in-situ and will be located in North-East coast of Scotland.

According to Domone (2001) the concrete in a sea wall structure is subject to various transport mechanisms resulting in penetration of aggressive agents. These mechanisms are as a result of the different exposure zones on the sea wall as shown in Figure 2-1.

The concrete in sea water is exposed to a number of possible degradation processes simultaneously, including the chemical action of the sea salts, wetting and drying in the tidal zones and just above, abrasion from sea waves and water-borne sediment and, in some climates, freezing and thawing. (Domone, 2010).

Therefore in accordance with BS5600-2:2006 the sea wall exposure designation is class XS2 for the parts of the structure that are submerged and XS3 for the parts of the structure in the tidal zones.

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2.1.3 Binder Type and Content

(a) Binder Type

The type of binder used in the concrete protecting the reinforcement has a major influence on durability since the material controls the rate at which the aggressive agents move through the cover concrete.

Chloride ingress into concrete is not only determined by the permeability of the pore system but also by interactions between the material and the diffusent that depletes the concentration and constricts the pore structure.

Concrete containing fly ash or slag have been shown to have exceptional chloride binding characteristics and produce material of high chloride resistance (Mackechnie, 2001)

In this case CEM I extended with GGBS has been specified.

(b) Binder Content

With the condition of exposure selected, the recommended strength class and cover can be selected from table A.4 of BS8500-2:2006.

Therefore the recommended concrete strength based on XS2 exposure:

C35/45, w/c ratio of 0.40 and a minimum binder content of 380 kg/m3

(BS8500, 2006).

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Figure 2-1 Primary transport mechanisms in the various exposure zones of a sea wall (Domone, 2001)

GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete2.1.4 Concrete Cover

The potential durability of reinforced concrete is greatly enhanced if adequate cover to reinforcement is specified and more importantly achieved on site (Mackechnie, 2001).

A minimum cover of 30+c is recommended, where c = 20 mm according to BS8500:2-2006

Therefore the target cover = 50 mm

2.1.5 Proposed concrete mix

• C35/45

• w/c ratio 0.40

• Minimum binder content 380 kg/m3

• CEM IIIA

• Binder proportion = 40/60 (CEM I/GGBS)

• Intended working life 50 years

• Cover to reinforcement = 30+c, which is equal to 50mm (BS8500, 2006)

Target mean cube strength = 45+ (5X1.64)

= 53.2

= 55 MPa

55 MPa = 400 kg/m3 cement

Based on w/c of 0.4

Cement for free water cement = 450 kg/m3

Therefore:

Total Cement = 450kg/m3

Portland cement = 180 kg/m3

GGBS = 270kg/m3

Fine Aggregate = 500 kg/m3

Coarse Aggregate = 1150 kg/m3

Water = 180 kg/m3

Slump = 90 mm

Expected mean cube strength = 60Mpa

2.2 Admixtures

2.2.1 Plasticisers

The recommendation on admixtures is the plasticizers, plasticizer will be used purely water reduction and improved workability.

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The admixture will be used moderately to avoid retardation especially in cold weather and high slag additions as recommended. In light of the above, the recommended admixture dosage is 0.4%.

2.2.2 Air-entraining Admixtures

Since the sea wall will be constructed in Scotland where temperature sometimes gets below freezing point an air-entraining admixture is recommended for freeze and thaw resistance which can result into concrete cracking if not managed.

A dosage of 0.4% of air-entraining admixture is recommended as this concrete will be exposed to extreme weather.

The air-entraining admixture will also help reduce the bleeding potential as the mix has high slag content (Newman and Choo, 2003).

2.3 Aggregates

The sea wall will be located in North-East Scotland; it will contain siliceous gravel aggregate as stated. A maximum aggregate size of 20mm is recommended BS8500:2006.

The benefits of a using 20mm aggregate will be:

• Economic factor, not just for aggregate but the whole mix design

• The risk of shrinkage is minimised as there is reduced surface area which helps with water reduction and less paste.

• 20mm can be handled and placed with ease and still get a good cohesive concrete.

2.4 GGBS Properties

It is very important to take note of the properties of GGBS that will affect the concrete in the plastic and hardened state:

2.4.1 Plastic state

(a) Water demand/Workability

Concrete made with GGBS cements will lower the water content of the mix due to smoother surface texture of the GGBS particles and delay the chemical reaction. So laboratory trial mixes will determine the final mix water.

(b) Stiffening times

The concrete will take longer to stiffen because GGBS is slower to react with water compared to Portland cement.

(c) Bleeding and settlement

GGBS might increase bleeding and settlement especially in proportions in the excess of 40%.

(d) Heat of hydration and early age thermal cracking

GGBS is very good at reducing heat of hydration as the proportion of slag is increased. This is beneficial in large pours enabling greater heat dissipation and reduced temperature rise which will reduce the likelihood of thermal cracking.

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GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete2.4.2 Hardened state

(a) Compressive strength and development

The early age rate of strength development is slower than Portland cement due to the fact that GGBS hydrates slower than Portland cement. Slag cement concrete gains higher strength at a later age due to the prolonged hydration reaction. This has to be taken into consideration.

(b) Formwork pressures

The use of slag will lead to increase in formwork pressures. The increase in pressure will be pronounced in high lifts greater than 4m cast at low temperatures less than 5°C and low placing rates of less than 0.5m/h. So the contractor needs to take this into account.

(c) Form work striking times

The slower rate of gain of early strength of concrete with high levels of GGBS may require the extension of form work striking times. (Newman and Choo, 2003)

2.5 Durability

2.5.1 Curing

It may be necessary to cure concrete containing slag for longer periods especially in cold weather conditions

2.5.2 Permeability

Concrete containing slag if well cured it is beneficial in terms of long term permeability

2.5.3 Alkali Silica Reaction (ASR)

The use of slag is a very effective way of reducing alkali silica reaction.

2.5.4 Sulphate Attack

Concrete containing GGBS have higher resistance to attack from sulphates than those with only Portland cements. This improved resistance is related to the overall reduction in C3A content of the blended cement as GGBS contains no C3A, and to the inherent reduction in permeability.

2.5.5 Chloride ingress

Slag cement have more resistance to the ingress of chloride ions than Portland cements. This is due to the reduced permeability of the slag cements and also the chlorides chemically combine with slag hydrates which has the effect of reducing the mobility of chlorides .This improved resistance has the potential for minimising the risk of corrosion of the reinforcement in concrete. (Newman and Choo, 2003)

2.6 Limiting Values

2.6.1 Cement and GGBS

Table 2-1 details the limiting values of the cement EN 197-1 and GGBS BS EN 15167-1 compounds.

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The following equations from ASTM C150-77 were used to calculate the cement clinker composition in Table 2-1.

Equation 1

Equation 2

Equation 3

Equation 4

Cement A Limits Typical range

Chemical composition Cement GGBSSiO2 % 19.5 21-22 36CaO % 61.0 65-66 40Fe2O3 % 4.1 4-4.5 0.5AL2O3 % 3.5 5-5.5 10SO3 % 2.8 1 0.2NA2O % 0.6 0.7 0.4K2O % 0.4 0.8 0.7MgO % 3.0 6 8Free lime % 0.8 2LOI % 1.2Insoluble residue

% 2.8

Physical propertiesSSA m2/kg 420 250Glass content % ≤95Soundness mm ≤10Compressive strength (BS EN 196-1)2 days MPa 30.5 ≥1228 days MPa 55.5 ≥32Calculated mineral compositionC3S % 67.5 45-65C2S % 5.0 10-30C3A % 2.3 5-12C4AF % 12.5 6-12LSF % 97.9 95-97

Tricalcium silicate of 67.5 % and low dicalcium silicate of 5 % for cement A will generate high heat and high early strength which will have an effect on shrinkage of the concrete, therefore durability will also be affected but the combination with GGBS will results to slower early strength, low heat of hydration and therefore the

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Table 2-1 Limiting values of cement and GGBS compounds


pore structure within the concrete will tends to be more refined, decreasing permeability and providing a greater protective pore ration.

Rasheeduzzafar et al (1992) had stated that there is a considerable evidence that concrete with Portland cement with high levels of C3A are more resistant to corrosion than those with sulphate resisting cements. Mehta (1981) concluded from the laboratory tests and theoretical calculations that concrete in marine environment require Portland cement containing at least 8 % C3A in order to bind and remove chloride ion from solution. According to Verbeck (1968) the rate of cracking associated with corrosion of beams in seawater decreased as the C3A content increased.

With the theories in mind it is clear that sulphate resistance will be improved when slag is introduced, due to the deduced C3A contents and thus the reduction in permeability. The research that was done has shown that for every 1% P2O5

presence, the alite is lowered by 9.9% and the belite solid solution is increased by 10.9%.

Tetracalcium aluminoferrite for cement A is 12.5 % which is higher than the norm of between 6 and 12 %. Many studies as mentioned by Taylor (1997) have shown that the hydration of C4AF is analogous to that of C3A but proceeds more slowly. According to Newman and Choo (2003) the C4AF makes the cement more resistant to seawater and results in a somewhat slower reaction which evolves less heat.

Equation 5

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Figure 2-2 Compressive strength development in pastes of pure cement compounds (Taylor, 1997)


Equation 6

The silica reaction ratios (SR) were calculated from equation 5 and were found to be 2.6 for cement A. According to Newman and Choo (2003) the silica ratio range should be between 2 and 4.

Alumino ratios were found to be 0.9 using equation 6 for cement A. High aluminate content cement of not between 1 and 4 as per Newman and Choo (2003) could results in an undesirable temperature rises in concrete.

2.6.2 Aggregates

Table 2-2 provides a summary of the limiting values and sources of these limits for the aggregates. The aggregate limits of particular importance to the concrete used in the construction of a sea wall are listed below and should be taken special note of in testing the suitability the aggregates.

Chloride content

Low density material content

Shell content

Aggregate alkali reactivity

Aggregate PropertyFine

Aggregate Limits

Coarse Aggregate Limits

Source Reasons for Limits

Chloride content (mass %, as Cl-)Concrete Type1. Concrete for

prestressing2. Reinforced

concrete3. Non-

reinforced concrete

0.010.030.03

---

SANS 1083:200

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Chloride salts may increase the setting rate of the concrete, cause efflorescence in hardened concrete and accelerate the rate of corrosion of steel embedded in the concrete. Newman and Choo (2003)

Fineness Modulus 1.20 - 3.50 - SANS 1083:200

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The limits guide the usage of fine aggregate but can be extend beyond these limits provided the concrete mix is proportioned accordingly. (Commentary on SABS 1083:1994)

Size Distribution

Standard Sieve Size(mm)

37,526,519,013,2

Natural source

Percentage mass

passing

(19 mm Aggregate)Percentage

mass passing

10085 - 1000 - 50

SANS 1083:200

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The finer fractions of the sand, namely the minus-150-μm and minus-300-µm, play a major role in determining the cohesiveness and workability of the mix and that sand

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Table 2-2 Limiting values for Aggrgates


9,56,74,752,361,18

0,6000,3000,1500,075

90 – 100

5 - 250 – 5 (10)

0 - 250 - 5

≤ 2

deficient of these fractions tend result in a harsher mix more prone to segregation. Also excessive dust (minus-75-μm) material can contribute to a higher water demand and possibly higher shrinkage in the hardened concrete. The limit for the dust portion on aggregates derived from a crushed source may be increased as indicated in parenthesis provided the fine aggregate complies with clay content requirements. (Commentary on SABS 1083:1994)

Organic impurities content

Not darker than

reference solution

- SANS 1083:200

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Organic materials may retard setting or hardening of cement paste. (Commentary on SABS 1083:1994)

Soluble deleterious impurities content (% strength of cubes made with washed sand)

≥ 85 ≥ 85 SANS 1083:200

6

Impurities such as sugars, organic impurities and sulphate salts, which are water soluble, interfere with the strength development of Portland cement products. (Commentary on SABS 1083:1994)

Dry-shrinkage (% shrinkage of reference aggregate)Concrete Type1. Concrete for

prestressing2. Reinforced

concrete3. Non-

reinforced concrete

≤ 150≤ 200≤ 235

≤ 150≤ 175≤ 235

SANS 5836:200

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Concrete shrinks due to loss of moisture in drying. This shrinkage is reduced by the restraining influence of the aggregate. Illston and Domone, (2001) However aggregates which themselves are prone to shrinkage will contribute to the dry-shrinkage of the concrete. (Commentary on SABS 1083:1994)

Low density materials content

Presence Presence SANS 5837:200

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Low density materials such as coal/lignite and chert are prone to separation by floatation or freeze-thaw damage respectively and must be identified if present in aggregates. (Commentary on

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SABS 1083:1994)Sand equivalent value (%)

≥ 65 - SANS 5838:200

6

The Sand Equivalent Value is a measure of the silt and clay content relative to the sand fraction in the fine aggregate. (Commentary on SABS 1083:1994)

Soundness (%) ≤ 15 - SANS 5839:200

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Unsound aggregate may cause local scaling, surface cracking and disintegration of the concrete it is used in. (Commentary on SABS 1083:1994)

Shell content (%)Aggregate fraction1. ≤ 5 mm2. > 5 mm ≤ 10

mm 3. > 10 mm

No limit

≤ 20≤ 8

BS 882:1992

The shell content of the aggregate influences the workability of the concrete due to the plate shape or irregularity of coarser shell particles found mainly in dredged marine gravels. These shell particles adversely affect the workability of the concrete and hence the water requirement and can cause voids increasing the permeability of the concrete. (Commentary on SABS 1083:1994)

Aggregate crushing value (%)

- ≤ 29 SANS 1083:200

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Where the compressive strength of the aggregate is not a significant property of the material where it is considerably higher than that of the cement paste. Therefore the ACV and 10% FACT values are useful in the assessment of the general quality of the aggregate. (Commentary on SABS 1083:1994)

FACT value (10% fines aggregate crushing value) (kN)1. Concrete

subject to surface abrasion

2. Concrete not subject to surface abrasion

- ≥110

≥ 70

SANS 1083:200

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Water absorption (%)

≤ 1.0 ≤ 1.0 SABS 1083:197

6

Water absorption of aggregates influences the workability of fresh concrete and subsequently the long-term durability. An aggregate with high water absorption withdraws mixing water into the aggregate resulting in a rapid loss in

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workability. This can be greatly increased by the use of certain water reducing admixtures. (Commentary on SABS 1083:1994)

Particle and relative densities

- - SANS 5844:200

6

There are no limits on these values as they are only used in the mix design process to convert data from mass to volume.

Bulk densities and voids content

- - SANS 5845:200

6

There are no limits for these values however to obtain concrete mixtures having low voids ratios requires low voids ratios of the constituent materials. Newman and Choo (2003)

Flakiness index (%) - ≤ 35 SANS 1083:200

6

The Flakiness Index describes the shape and the interlocking properties of the aggregate. This property is of particular concern in concrete paving where very flaky aggregates can lead to excessive ravelling and spalling on joints. (Commentary on SABS 1083:1994)

Polished-stone value

- ≥ 50 SANS 5848:200

8

The polished stone value is a measure of the aggregates resistance to becoming smooth or slippery under the action of traffic which is of particular importance in industrial pavements.

Potential reactivity of aggregates with alkalis (%)

10 days - 0.08% and 12 days -

0.10%

10 days - 0.08% and 12 days -

0.10%

SANS 6245:200

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Alkali aggregate reactivity occurs when the constituents of the aggregate such as silica, silicates and carbonates in certain mineral forms react with the alkaline hydroxide in the pore water derived from the cement. The most common is Alkali-Silica Reaction (ASR). The product of ASR is a gel which can destroy the bond between the aggregate and the hardened cement paste. This gel also absorbs water and swells causing

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cracking and disruption of the concrete. Illston and Domone, (2001)

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2.7 Conclusion

Slag cement combination is the best suitable concrete to use in such an aggressive environment but proper care needs to be taken when working with slag cement especially at such high dosage.

The durability of reinforced concrete structures in the marine environment is not only depended on the mix design, good site practise particularly to placing, compacting and curing of concrete will improve the concrete durability. Some of these activities are almost impossible to specify hence proper site supervision is necessary during placing.

As a result of slower early age strength development in GGBS concretes, the pore structure within the concrete tends to be more refined, decreasing permeability and providing a greater protective pore ratio. Greater GGBS proportions of 60 % may lower resistance to freeze/thaw cycles, it can be concluded from the above results that cement A can be suitable for use in concrete sea wall with 60 % cement replacement of GGBS.

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3. PART 2: CONCRETE ADMIXTURES IN PRECAST CONCRETE

3.1 Introduction

High Strength Concrete (HSC) is a product of research over a period of three decades to improve the performance of ordinary concrete. The HSC achieves strengths which cannot be achieved by ordinary concrete. (Owens, 2009)

High and very high strength concrete up to 700Mpa is already being produced; elements with strength of 350 MPa have been constructed. The precast industry has offered and achieved strengths up to 180 MPa and this gives the edge over concrete poured on site which is generally limited to 50Mpa. (BIBM, 2005)

3.2 Constituent materials

3.2.1 Cement

The high strength concrete can be produced with most available Portland cements and the coarsely grounded cements are usually not suitable. In Norway special cements for high strength concrete have been developed with lower tricalcium aluminates (C3A) and elsewhere normal commercial cements are used. (Newman, 2003)

CEM II A-V has 6 to 20 percent by mass of fly ash and this will reduce shrinkage cracks if the volume change is restrained and will also result in positive influence of fresh and hardened concrete properties. (Owens, 2009)

Silica fume is normally blended with the binder because it is three times efficient than Portland cement, it is blended at 5-15 percent by weight of the total binder. (Newman, 2003)

The binder content of HSC is usually between 380 and 500kg/m3 and Figure 3-1 below indicates typical ranges of cement content. (Owens, 2009)

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3.2.2 Aggregates

The aggregates should have low free water demand and rounded aggregates should be chosen over crushed rock fines if possible. The clay and silt of aggregates should be reduced to be as low as possible. The cement content in excess of 500kg/m3 for HSC allows for coarse aggregates to be used compared to conventional concrete. (Newman, 2003) The optimised grading of different aggregates suitable for HSC is indicated in Figure 3-2 below.

The fine aggregates should have the fineness modulus ranging from 2.7 to 3.0 and coarse aggregates should be limited to 16mm to achieve high packing density,

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Figure 3-3 Typical ranges of cement content for different concrete strengths classes (strength classes according to EN206:1) (Owens, 2009)Figure 3-4 Optimisation of aggregate distribution


concrete with coarse aggregates sizes of 19.0 to 26.5mm have achieved good results in South Africa. (Owens, 2009)

3.2.3 Admixtures

The admixtures are important in HSC to produce workability at low free water to cement ratios which are normally below 3.0, melamine-based, naphthalene-based and polycarboxylate ether super-plasticizers at dosages up to 3 percent by cement weight have yielded good results. (Newman, 2003)

The effect of super-plasticizers is ascribed to the following mechanisms: (Owens, 2009)

Creation of electrostatic forces which result in repulsion of particles

The reduction of surface tension of water which results in improved lubrication of particles

The retardation of hydration process between cement and water particles resulting in increased free water.

The effect of different dosages of super-plasticizers in HSC is illustrated in Figure 3-3 below. (Nagataki, 1989)

There is a saturation point with super-plasticizers beyond which further increase in dosage will have no effect on the workability; this can be determined by flow cone. (Newman, 2003)

3.3 Production Procedure

Precast concrete construction is a method of prefabricating concrete in discrete elements and erecting and incorporating them by crane into their final position in the building structure. In precast concrete construction, there are two separate phases of design. The first, the structural design, is for the in-service condition and is usually carried out by the project design engineer as part of the design of the complete structure. The second, the design for erection, is for the handling, transportation and erection of the individual elements and structure during the erection process. It may be carried out independently of the structural design by the project design engineer or by the erection design engineer.

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Figure 3-5 Effect of super-plasticizer on HSC (Nagataki, 1989)

GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete3.3.1 Specification of concrete

The concrete specification should be clearly shown on the drawings and include any special requirements, for example, colour, cement content and water-cement ratio. The specification of the strength of concrete should take into account the strength required at lifting as well as the requirements for in-service loading, durability and ease of construction.

Precast concrete elements should be designed for the loads and conditions likely to be experienced during the manufacturing, lifting, transportation, erection, braced and in-service phases. In addition to the normal design considerations, special consideration should be given to:

construction loads

handling and transport loads

erection loads

wind load on the braced elements prior to incorporation into the structure

Erection-load design should consider variations to the precast element load distribution during lifting, rotation and impact during placement

3.3.2 Tolerances

Recommended tolerances will be as per design standard. Because precast concrete elements cannot be manufactured to exact dimensions, provision should be made in the design for dimensional variation. Where required tolerances are less than the recommended values, the specific requirements should be clearly stated on the drawings.

3.3.3 Structural drawings

Drawings and details must provide sufficient information for the shop detailer to prepare detailed shop drawings. The information provided on structural drawings should include:

date and issue number of the drawing

plans and elevations clearly indicating the structural framing and precast element layout

structurally critical dimensions

reinforcement required for in-service loads and conditions

framing connection locations and required type (e.g. cast-in) and the capacity of the fixing inserts

levelling pad details

structural design criteria affecting construction

the concrete specification including all special requirements to meet in-service loadings and conditions and a note that all concrete must meet the strength requirements at the time of lifting nominated on the shop detail drawings

base connection details, for example, grouting sequence of dowel connections

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GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete3.3.4 Shop drawings

The project design engineer shall ensure that the shop drawings comply with the structural design. The builder shall check the shop drawings for compliance with dimensions.

3.3.5 Casting and erection sequences

The casting and erection sequences of the precast elements should be agreed. Prior to manufacturing the precast elements, the parties involved in the design, manufacture transport and erection process should liaise and have planned the complete construction and erection sequences.

The casting and erection sequences of the precast concrete elements should be agreed between the builder, precaster, erector and, where necessary, the erection design engineer and project design engineer. The precaster, in association with the builder and erector, should prepare plans showing the erection sequence and bracing layout. The casting and erection sequences should take into account the required crane capacity and configuration

3.3.6 Formwork

Forms must be square, and features like reveals, chamfers and blockouts must be set correctly. Typically, there is less tolerance due to alignment of patterns, connection details and/or abutting pieces, etc. The form joints must be correctly aligned and sealed to prevent leakage. Silicone is commonly used to seal joints and should be applied prior to the form release agent. All fasteners, such as screws or nail heads, must be properly covered so that their image is not transferred to the finished surface; one method is to use a sandable epoxy resin. Forms must be properly seasoned so that concrete does not bond to them. Reactive release agents should be used in accordance with the manufacturer’s recommendations. Petroleum-based products may change the colour of the concrete finish and should be avoided. Moisture absorption by forms will also discolour the concrete surface and can be prevented with proper form preparation

Precast concrete construction usually requires multiple uses and early stripping of formwork and these requirements should be taken into account in the design of the formwork. Formwork or mould design for precast concrete elements can have a direct bearing on how elements are cast and handled and on the loads imposed during manufacture. In particular, the following should be noted:

Surface finish requirements can influence the preferred orientation of a precast element in the mould. The quality of the finish of vertical mould faces may differ from that cast against a horizontal surface. Two-stage casting can be used to avoid this problem.

Moulds for elements such as beams and columns may require special provisions to accommodate prestressing.

Generally, the side forms should be released or removed prior to releasing stressing strands. Stop ends should be detailed to accommodate sliding of the component during release

• Both suction and friction can be reduced by the use of high quality mould release compounds

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• Suction on flat mould surfaces is increased by the presence of water. Suction pressure can be relieved by lifting gently at one end or edge of the element

• Friction forces are increased by vertical or near vertical sides on a mould. To reduce friction, mould sides should be detailed with adequate draw, or should be released to allow them to spring back. To avoid overloading lifting inserts, the mould can be vibrated while gently lifting one end of the precast element

3.3.7 Reinforcement

The reinforcing steel should not be set on chairs or supports that touch an exposed surface. These supports may become exposed over time, impairing the finish. Secure reinforcing cages by other means. Cages can be suspended from the top of the form. However, this needs to be taken into account when designing the form. Reinforcing steel should be installed after the form release agent is applied so that no release agent contaminates the steel, which would prevent it from bonding to the concrete

Reinforcement for precast concrete is usually preassembled into a rigid cage using a template before the steel is placed in the form. Cage assemblies shall be constructed to close tolerances, and various pieces shall be rigidly connected by tying or wielding.

3.3.8 Concrete placement

Prior to placing concrete, the arrangement must be inspected for compliance with the shop drawings. In particular, this must include checks on:

formwork dimensions

formwork stability

edge details and penetrations

connection details

insert locations, types and fixing to reinforcement

reinforcement sizes, locations and fixing

bond-breaker effectiveness

The inspection shall be carried out by a trained and competent person who was not involved in the original set-up. For stack casting, an inspection shall be done prior to the casting of each panel. Bond-breaker effectiveness can be checked by sprinkling water over the casting bed. It should form into beads if the bond is effective.

The concrete supplier shall be advised of:

the specified characteristic concrete compressive strength

the concrete compressive strength required at time of lifting

the maximum aggregate size

the slump

special design requirements, if any, e.g. colour, cement content and water to cement ratio

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GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete the site access, required rate of supply and the method of placement, e.g. type of pump

Vibrators shall be used to compact the concrete. Particular attention and care shall be paid to vibrating the concrete around the inserts and adjacent to the corners and edges. Concrete must be placed in a uniform manner and properly spread over the area before commencing vibration.

3.3.9 Curing and release

The strength, water tightness and durability of concrete depend on the concrete being adequately cured. Release agents used in the manufacture of precast concrete elements should be checked for compatibility with the curing compound and other applied finishes and joint sealants.

3.3.10 Release agents

Before a release agent is chosen for use in the manufacture of the precast element, it shall be checked for compatibility with the curing compound and other applied finishes and joint sealants. A proven proprietary combination curing compound or release agent should be used.

3.3.11 Stripping and repair

Formwork shall be carefully stripped to prevent damage. If the precast element is damaged, the proposed repair system shall be submitted to, and approved by, the project design engineer before being attempted.

3.3.12 Concrete strength for handling

Precast elements shall not be removed from the moulds and placed in storage until the concrete strength has attained the minimum value required for lifting as specified on the shop drawings.

3.3.13 Storage

The storage area shall be large enough for elements to be stored properly with adequate room for lifting equipment and for manoeuvring trucks and cranes. The area shall be reasonably level and hard surfaced with adequate drainage to ensure that a safe workplace can be maintained. Elements should not be stored directly on the ground. Generally, two discrete support points should be provided unless specifically noted otherwise by the project design engineer. Timber supports raised above the ground or dedicated racking systems should be used in all cases. Elements should be stored in such a manner that each element supports only its own weight without any load being imposed by other elements.

3.4 Advantages of Pre-cast Concrete

Since precast is manufactured in a controlled casting environment it is easier to control the mix, placement, and curing

Quality can be controlled and monitored much more easily

Weather is eliminated as a factor-you can cast in any weather and get the same results, which allows you to perfect mixes and methods

Less labour is required and that labour can be less skilled

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GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete On site, precast can be installed immediately, there is no waiting for it to gain strength

and the modularity of precast products makes installation go quickly

Repeatability-it's easy to make many copies of the same precast product; by maximizing repetition, you can get plenty of value from a mould and a set-up

Accelerated curing, by heating the precast parts, greatly increases strength gain, reducing the time between casting the part and putting it into service

With the ability to so tightly control the process, from materials to consolidation to curing, you can get extremely durable concrete

3.5 Disadvantages of Pre-cast Concrete

If not properly handled, the pre-cast units may be damaged during transport.

It becomes difficult to produce satisfactory connections between the pre-cast members.

It is necessary to arrange for specific equipment for lifting and moving of pre-cast units.

The economy achieved in pre-cast construction is partially balance by the amount to be spent in transport and handling of pre-cast members. It becomes, therefore, necessary to locate the pre-cast factory at such a place that transport and handling charges are brought down to the minimum possible extent.

3.6 Mix design proposal

3.6.1 Introduction

The mix design of HSC is controlled by super-plasticizers (Owens, 2009); the following factors should be considered: (Newman, 2003)

The free water to cement ratio in ranges of 0.25 to 0.3 should be selected based on previous data

The cement should be chosen to achieve maximum compressive strength

The saturation super-plasticizer dosage has to be chosen based on flow cone

The optimum aggregate proportions should be chosen

Table 1 below indicates commercial mix designs from North America (Burg and Ost, 1992).

Material 1 2 3 4 5Cement (kg/m3) 564 475 487 564 475Fly ash (kg/m3) - 59 - - 104Micro silica (kg/m3) 1068 1068 1068 1068 1068Coarse agg (kg/m3) 647 659 676 593 593Water (L/m3) 158 160 155 144 151Super-plasticizer (L/m3) 11.61 11.61 11.22 20.12 16.45Retarder (L/m3) 1.12 1.04 0.97 1.47 16.45Free water/cement ratio 0.281 0.287 0.291 0,220 0.231

90 day cylinder strength 86.5 100.4 96.0 131.8 119.3

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Table 3-3 Commercial HSC mix designs from North America (Burg and Ost, 1992)


(MPa)3.6.2 Water cement ratio

The main requirements for successful and practical HSC are a low water/cement ratio combined with high workability and good workability retention characteristics.

When doing a mix design for a HSC it is of upmost importance to ensure the correct constituents are being used.

It should be recognized that there is no single or unique composition for high strength concrete. HSC can be made with a range of materials and mix designs which will produce slightly differing properties. (Price 2003)

3.6.3 Cement

The first change would have to be the cement. CEM I 42.5 R cement is especially suitable for manufacture of High performance concrete, as it has desirable characteristics in terms of workability and strength development. The maximum achievable long term strength for this type of cement is however limited and strength classes above 105 MPa, CEM I 52.5 R has proven to give more satisfactory results. Here it must be considered that, because of its higher fineness, the use of CEM I 52.5 R results in a higher water demand and higher heat of hydration and may require a retarding agent to prevent rapid stiffening.(Fultons 2009) The cement composition should be selected to maximize strength and other performance requirements. At its simplest this will be Portland cement blended with 5–10 per cent silica fume.

3.6.4 Superplasticizer

A new type of Superplastecizer will have to be used and not just the lignosulphonate-based plasticizer. Suitable superplasticizer for HSP would be sulphonated melamine formaldehyde (SMF) and sulphonated naphthalene formaldehyde (SNF) or the newer polycarboxylate acid (AP) and polycarboxylate (PCE). (Fulton’s 2009)

HSC produced by conventional mixing technologies are usually prepared with water-cement ratios in the range of 0.22 to 0.40, and their 28 days compressive strength is about 60 to 130 MPa when normal density aggregates are used.

3.6.5 Proposed mix design

The proposed mix design for strength classes up to C90/105 for 4m x 500mm diameter reinforced concrete columns is as follows.

(a) Mix design 1

Cement (kg/m3) 475

Fly ash (kg/m3) 59

Microsilica (kg/m3) 24

Coarse agg. (kg/m3) 1068

Fine agg. (kg/m3) 659

Water (l/m3) 160

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Superplasticizer (l/m3) 11.61

Retarder (l/m3) 1.04

Free water/cement ratio 0.287

90-day cylinder strength (MPa) 100.4

(b) Mix Design 2

Cement (kg/m3) 564

Microsilica (kg/m3) 89

Coarse agg. (kg/m3) 1068

Fine agg. (kg/m3) 593

Water (l/m3) 144

Superplasticizer (l/m3) 20.12

Retarder (l/m3) 1.47

Free water/cement ratio 0.220

90-day cylinder strength (MPa) 131.8

(Commercial HSC mix designs used in North America)

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4. REFERENCES

1. ACI Manual of Concrete Practice Part 5. (1995). Mansory Precast and Special. Farming Hills. USA

2. Addis B and Goldstein G, editors, Commentary on SABS 1083:1994, Cement and Concrete Institute, Midrand, 1995, p22

3. BIBM, p90

4. British Standards Institution BS 882:1992 Specification for aggregates from natural sources for concrete.

5. BS EN 151167-1:2006 Ground granulated blast furnace slag for use in concrete mortar and grout.

6. BS EN 15167-1 :Portland cement extenders Part 1:GGBS

7. BS8500. (2006). Method of specifying and guidance for the specifier. British Standard Institute. pp 19, 28

8. Domone P. (2001).Part 3: Concrete. In Construction Materials: their Nature and Behaviour. 3rd edition, (eds Domone P and Illston J), Spon, London, pp. 89-223

9. Domone P. (2010).Part 3: Concrete. In Construction Materials: their Nature and Behaviour. 4th edition, (eds Domone P and Illston J), Spon, London, pp. 83-208

10. EN 197-1:2000 Composition, specification and conformity criteria for common cements.

11. Fulton. Fulton's concrete technology. Midrand: Cement & Concrete Institute(Midrand 2009).

12. Illston J M and Domone P L J Construction Materials Their Nature and Behaviour, 3rd edition. Spon Press, London, 2001, p148

13. J Mackechnie (2001). Predictions of reinforced concrete durability in the marine environment. Department of Civil Engineering: University of Cape Town

14. J Newman and B Seng Choo (2003), Advanced Concrete Technology. Processes. Concrete mix design Butterworth-Heinemann, Oxford pp. 1/16

15. John Newman and Ban Choo (2003), Advanced concrete technology, Constituent Materials:Elsevier, Butterworth-Heinemann pp 1/1 -1/44.

16. John Newman and Ban Choo (2003), Advanced concrete technology, Constituent Materials:Elsevier, Butterworth-Heinemann pp 3/18 -3/33.

17. John Newman, Ban Seng Choo (2003), Advanced Concrete Technology. Constituent Materials. Cement additions, Butterworth-Heinemann, Oxford, pp.3/22

18. John Newman, Ban Seng Choo, Advance Concrete Technology, Processes, Bryan K. March (2003), Precast concrete structural elements, Butterworth-Heinemann, Oxford, p21/11 – p21/14.

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GROUP 14ASSIGNMENT 3 ADDITIONS AND ADMIXTURESConcrete with GGBS Blended Cement in a Sea Wall and Concrete Admixtures in Precast Concrete19. M Alexander, J Mackechnie (2003). Concrete mixes for durable marine. Journal of the

South African Institution of Civil Engineering, pp20-25Annual book of ASTM standards: 1977 Standard specification for Portland cement C150-77

20. Mehta P.K (1981). Effect of cement composition on corrosion of reinforcing steel in concrete.

21. Nagataki, Effective utilization of industrial by-products,1989, p84

22. National Precast Concrete Association. Neuwald, A (2010).Quality in, Quality out: a look at steel fabrication, forming and consolidation equipment: Available from : www.precast.org

23. Newman J and Choo B S Advanced Concrete Technology Constituent Materials. Elsevier Butterworth and Heinemann, Oxford, 2003, p 8/12

24. Newman N and Choo , Advanced concrete technology, 2003, p3/4- 6

25. Newman, J. Choo, BS. Advanced Concrete Technology – In B . Price. Section 3/1

26. Owens G, Fulton`s concrete technology, 2009, p298-350

27. Precast and Tilt-up Concrete for Building, Melbourne,<htt://www.worksafe.vic.gov.au/../CP200802PrecastTiltupAndConcreteElements.pdf [Accessed 17 April 2012]

28. Rasheedussafar, AL-Saadoun SS, and AL-Gahtani AS (1992). Reinforcement corrosion resisting characteristics of silica fume blended cement concrete.ACI material Journal, pp 334-337

29. Standards South Africa SANS 1083:2006 Aggregates from natural sources – Aggregates for concrete.

30. Taylor H.F.W (1997). Cement chemistry, Thomas Telford, London

31. Verberck GJ (1968). Field and laboratory studies of the sulphate resistance of concrete. University of Toronto, Toronto

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B - 1

Group 14 Assignment 3

Documents

Transcript of Group 14 Assignment 3