The Constructor Notes

8/12/2019 The Constructor Notes

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CONCRETE FORMWORK DESIGN CONSIDERATIONS

Designing and building formwork effectively requires a basic understanding of

howconcretebehaves as it exerts pressure on formwork.Concrete exerts lateral pressure on

the formwork. The formwork is designed based on these lateral forces.Lateral concrete pressure on formwork is affected by:

1) Height of concrete pour 2) Concrete pour rate 3) Weight of concrete 4) Temperature 5)

Type of cement 6) Vibration 7) Concrete slump (watercement ratio) 8) Chemical additives

1) Height of concrete pour:Before concrete hardens, it acts like a liquid and pushes

against the forms the way water presses against the walls of a storage tank. The amount of

pressure at any point on the form is directly determined by the height and weight of

concrete above it. Pressure is not affected by the thickness of the wall.

Fig: Lateral concrete pressure on formwork

2) Concrete pour rate:Concrete pressure at any point on the form is directly proportional

to the height of liquid concrete above it. If concrete begins to harden before the pour is

complete, the full liquid head will not develop and the pressure against the forms will be

less than if the pour were completed before any of concrete hardened.

Once concrete hardens it cannot exert more pressure on the forms even though liquid

concrete continues to be placed above it. The following diagrams illustrates how form

pressure varies when the pour rate is increased from one level to another level. For ease of

explanation, it is assumed that concrete hardens in one hour (typically) at 21C.
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Fig: Concrete pressure on formwork during hardening

When the pour rate is increased the pressure also increases as shown below:

Fig: Concrete pressure on formwork due to higher pour rate

3) Weight of Concrete:Pressure exerted against the forms is directly proportional to the

unit weight of concrete.Light weight concrete will exert less pressure than normal weight

concrete as shown below:

Fig: Pressure on formwork due to normal and lightweight concretes

4) Temperature:The time it takes concrete to harden is influenced greatly by its

temperature. The higher the temperature of the concrete, the quicker it will harden. Most

formwork designs are based on an assumed average air and concrete temperature of 21C.
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At low air temperatures, the hardening of concrete is delayed and you need to decrease

your pour rate or heat your concrete to keep the pressure against the formwork from

increasing. Ideally, concrete should be poured at temperatures between 16C and 38C.

Outside this temperature range there is often insufficient moisture available for curing. If

adequate water for curing is not available or freezes, the strength of the concrete will suffer.5) Type of Cement:The cement type will influence the rate at which concrete hardens. A

high early strength concrete will harden faster than normal concrete and will allow a faster

pour rate. When using a cement which alters the normal set and hardening time, be sure to

adjust the pour rate accordingly.

6) Vibration:Internal vibration consolidates concrete and causes it to behave like the pure

liquid. If concrete is not vibrated, it will exert less pressure on the forms. ACI recommended

formulas for form pressures may be reduced 10% if the concrete is spaded rather than

internally vibrated. Re-vibration and external vibration result in higher form loads than

internal vibration. These types of vibration require specially designed forms.7) Concrete Slump:When concrete has very low slump, it acts less like a liquid and will

transmit less pressure. When using concrete with a slump greater than 100 mm, the

formwork should be designed to resist full liquid head.

8) Chemical additives: When using chemical additives i.e. retarders, plasticizers, etc.

make sure to refer to the vendors application data.


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METHODS OF INCREASING STRUCTURAL STIFFNESS

The methods of increasing structural stiffness oftall buildings are by providing central

core,shear walls,tubes, braced frame and double tube. These are discussed in detail below.

1. Central Core:

By constructing a central core, the stiffness of the building is

greatly increased. A central core is used to house stairs and

lifts and building services. This method allows the building to

keep an open facade. Following figure shows the central core

of a building with a 55 column.

2. Shear Walls:

Shear walls are constructed at opposite ends of a building to

provide stiffness in a particular direction. Shear walls are

particularly useful in non-squarebuildings,where the wind

forces predominantly come from one direction. The interior of

the building is kept clear. Shear walls only provide minimal

torsional stiffness.

3. Tube system:

A tube system is essentially two sets of shear walls. The tube

system allows the building to be stiff in all directions of

loading. The building will also have a high-torsional

resistance. The interior of the building is kept clear.

The tube system must be pierced for windows. These

openings must be kept to minimum.

4. Braced Frame:

Braced frame is the simple structure with bracing that help to increasethe structural stiffness. A braced frame is similar to floor bracing, but it

does not depend on the stiffness provided by the floor system, rather on

the addition of another diagonal cross members.
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This type of bracing is lighter than its counterpart with floor bracing. The building has good

stiffness.

Compared to the system with floor bracing, this system is less easy to construct. The faade

detailing can be interesting but also expensive.

5. Double Tube System:

A double tube system is a combination of the central core

and tube system as shown in figure below. This

combination of both systems allows the building to be

extremely stiff.

The building has high torsional resistance. For the tube

system, apertures for windows must be kept to minimum.

The central core take much valuable space. This type of

system is used for very tall building.


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COOLING TOWERS AND ITS SYSTEMS

Cooling towers are commonly used to remove excess heat that is generated in places such

as power stations, chemical plants and even domestically in air conditioning units. This

equipment has recently developed into an important part of many chemical plants. They

represent a relatively inexpensive and dependable means of removing low-grade heat from

cooling water.

Cooling towers might be classified into several types based on the air draft and based on

the flow pattern. Each type of cooling tower has its own advantages and disadvantages;

thus the proper selection is needed based on the system operation. Besides, the material

selection of cooling tower is also important. Cooling towers tends to be corrosive since it

always has direct contact with the water. Proper material selection or additional water

treatment is then needed to keep the cooling tower safe.

Cooling towers are heat removal devices used to transfer process waste heat to the

atmosphere. Cooling towers make use of evaporation whereby some of the water is

evaporated into a moving air stream and subsequently discharged into the atmosphere. As

a result, the remainder of the water is cooled down significantly.

Fig: Schematic Diagram of a Cooling Tower System

There are several important factors that govern the operation of cooling tower:

The dry-bulb and wet-bulb temperatures of the air.


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The temperature of warm water. The efficiency of contact between air and water in terms of the volumetric mass transfer

coefficient and the contact time between the air and the water.

The uniformity of distribution of the phases within the tower. The air pressure drop. The desired temperature of the cooled water.


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CALCULATE EARTHQUAKE FORCES ON BUILDINGS AND STRUCTURES

Earthquake resistant design of structures has become integral part ofstructural design of

any structure. The guide to earthquake resistant design of building and structures are given

by IS1893 2002 in India. Here we will talk about how to calculate theearthquake

forces for buildings and structures as per above mentioned code. First step to calculate earthquake loads onstructure is to identify the

earthquake zone for which structure needs to be designed. This earthquake zones

are displayed in a map on page 6 of the code.

After earthquake zone has been identified, the following steps are followed:

1. Calculate design horizontal seismic coefficient, Ah, which is given by (cl. 6.4.2 of

IS1893 2002:

Where, Z is the zone factor, given in table 2 of IS1893 2002.

I is the importance factor of the structure depending on the function or use. This factor can

be obtained from table 6 of the code.

R is response reduction factor. This value is obtained from table 7 of the code. The value of

1/R shall not be more than one.

Sa/g is average response acceleration coefficient. This value depends on time period of

structure and on soil type. This can be obtained from clause 6.4.5 of the code.

2. Calculate design seismic base shear for the structure (VB). This is the total design

lateral force along any principal direction. This is calculated as:

VB= Ahx W

Where Ah= horizontal seismic coefficient as calculated above in step 1.

W = Total weight of the structure.

3. Now calculate the distribution of design forces on the structure. The seismicdesign base shear calculated in step above is distributed on the structure as design seismic

forces. This is calculated as below:
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Where Qi = Design lateral force at floor i

Wi = seismic weight of the floor i

hi = height of the floor i from the base

n = number of storeys of the building at which masses are located.

4. Distribution of horizontal seismic forces on structure: These forces are

distributed on the vertical elements of the building resisting lateral forces.

Example:

Consider a two-bay two-storied building for which earthquake forces need to be calculated.

Consider total weight of the building as 1000 kN. Top roof has the weight of 200 kN and

both floors have weight of 400 kN as shown in figure below. The calculation of earthquake

seismic forces will be as shown below:


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Total Weight of the Structure< ?xml:namespace prefix =

"o" /> Wi= 1000 kN

Zone Factor (Zone IV) Z = 0.24

Importance factor (Table6) I = 1

Response Reduction Factor R = 3

Average response acceleration coefficient

Let time

period T 0.373358 (Cl. 7.6)

Sa/g 2.5

Design Horizontal Seismic coefficient Ah 0.10


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Base Shear 100 kN

Storey Base Shear Height Wi Wixhi2 Qi

Design

Force

3 100 8.5 200 14450 57.00197 9.50

2 100 5 400 10000 39.44773 6.57

1 100 1.5 400 900 3.550296 0.59

Total 25350 (in kN)


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ROLE OF CONSTRUCTION SITE ENGINEER

On any one contract there may be a number of engineers with varying amounts and types

of experience. The duties and responsibilities of an engineer are typically as follows, many

of these will be delegated to other engineers on the site according to their experience and

ability:

Setting out the works in accordance with the drawings and specification Liaising with the project planning engineer regardingconstruction programmes Checking materials and work in progress for compliance with the specified requirements Observance of safety requirements Resolving technical issues with employers representatives, suppliers, subcontractors

and statutory authorities

Quality control in accordance with CSIs/procedures method statements, quality plansand inspection and test plans, all prepared by the project management team and bysubcontractors

Liaising with company or project purchasing department to ensure that purchase ordersadequately define the specified requirements

Supervising and counselling junior or trainee engineers Measurement and valuation (in collaboration with the project quantity surveyor where

appropriate)

Providing data in respect of variation orders and site instructions Preparing record drawings, technical reports, site diary Job review of subordinate staff
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SELF COMPACTING CONCRETE WITH TESTS

For several years beginning in 1983, the problem of thedurability of concrete structureswas a major topic of interest in Japan. The creation of durable concrete structures requires

adequate compaction by skilled workers. However, the gradual reduction in the number of

skilled workers in Japans construction industry has led to a similar reduction in the qualityof construction work. One solution for the achievement of durable concrete structures

independent of the quality of construction work is the employment of self-compacting

concrete, which can be compacted into every corner of a formwork, purely by means of its

own weight and without the need for vibrating compaction(Fig. 1). The necessity of this

type of concrete was proposed by Okamura in 1986. Studies to develop self-compacting

concrete, including a fundamental study on the workability of concrete, have been carried

out by Ozawa and Maekawa at the University of Tokyo. The prototype of self-compacting

concrete was first completed in 1988 using materials already on the market (Fig. 2). The

prototype performed satisfactorily with regard to drying and hardening shrinkage, heat ofhydration, denseness after hardening, and other properties. This concrete was named High

Performance Concreteand was defined as follows at the three stages of concrete:

(1) Fresh: self-compactable

(2) Early age: avoidance of initial defects

(3) After hardening: protection against external factors

At almost the same time, High Performance Concrete was defined as a concrete with high

durability due to a low water-cement ratio by Professor Atcin et al. Since then, the term

high performance concrete has been used around the world to refer to high durability
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concrete. Therefore, the authors have changed the term for the proposed concrete to Self-

Compacting High Performance Concrete.

Self-compactability of fresh concrete

Mechanism for achieving self-compactabilityThe method for achieving self-compactability involves not only high deformability of pasteor mortar, but also resistance to segregation between coarse aggregate and mortar when

the concrete flows through the confined zone of reinforcing bars. Okamura and Ozawa have

employed the following methods to achieve self- compactability (Fig. 3) (1995):

(1) Limited aggregate content

(2) Low water-powder ratio

(3) Use of super-plasticizer

The frequency of collision and contact between aggregate particles can increase as the

relative distance between the particles decreases and then internal stress can increase when

concrete is deformed, particularly near obstacles. Research has found that the energy

required for flowing is consumed by the increased internal stress, resulting in blockage of

aggregate particles. Limiting the coarse aggregate content, whose energy consumption is

particularly intense, to a level lower than normal is effective in avoiding this kind of

blockage.


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Highly viscous paste is also required to avoid the blockage of coarse aggregate when

concrete flows through obstacles (Fig. 4). When concrete is deformed, paste with a high

viscosity also prevents localized increases in internal stress due to the approach of coarse

aggregate particles. High deformability can be achieved only by the employment of a super-

plasticizer, keeping the water-powder ratio to a very low value.The mix proportioning of self-compacting concrete is shown and compared with those of

normal concrete and RCD (Roller Compacted concrete for Dams) concrete (Fig. 5). The

aggregate content is smaller than conventional concrete that requires vibrating compaction.

The ratios of the coarse aggregate volume to its solid volume (G/Glim) of each type of

concrete are shown in Fig. 6. The degree of packing of coarse aggregate in SCC is

approximately 50% to reduce the interaction between coarse aggregate particles when the

concrete deforms. In addition, the ratios of fine aggregate volume to solid volume (S/Slim)

in the mortar are shown in the same figure. The degree of packing of fine aggregate in SCC

mortar is approximately 60% so that shear deformability when the concrete deforms maybe limited. On the other hand, the viscosity of the paste in SCC is the highest among the

various types of concrete due to its lowest water-powder ratio (Fig. 7). This characteristic is

effective in inhibiting segregation.

Test for Self CompactibilityThere are three purposes for self-compactability tests relating to practical purposes.

Test (1): To check whether or not the concrete is self-compactable for the structure.

Test (2): To adjust the mix proportion when selfcompactability is not sufficient.

Test (3): To characterize materials.

As Test (1), the so-called U-flow test or Box test is recommended (Figs. 8, 9 and 10). The

U-flow test was developed by the Taisei Group (Hayakawa 1993). In this test, the degree of

compactability can be indicated by the height that the concrete reaches after flowing

through an obstacle. Concrete with a filling height of over 300 mm can be judged as self-

compacting. The Box-test is more suitable for detecting concrete with higher possibility of

segregation between coarse aggregate and mortar.
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If the concrete is judged to be having insufficient selfcompactability through test (1), the

cause has to be detected quantitatively so that the mix proportion can be adjusted. Slump-

flow and funnel tests (Fig. 11) have been proposed for testing deformability and viscosity,

respectively.

Flow and funnel tests for mortar or paste have been proposed to characterize materials

used in selfcompacting concrete, e.g. powder material, sand, and super-plasticizer. Testing

methods for the mortar properties were al so proposed and the indices for deformability and

viscosity were also defined as in (Figs. 12 and 13).

Factors of self-compactability in terms of testing results.The factors making up self-compactability were described in terms of the test results for

fresh concrete and mortar below.(1) Influence of coarse aggregate depending on spacing size.


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It is not always possible to predict the degree of compaction into a structure by using the

test result on the degree of compaction of the concrete into another structure, since the

maximum size of coarse aggregate is close to the minimum spacing between the reinforcing

bars of the structure. For example, the relationship between coarse aggregate content in

concrete and the filling height of the Box-type test, which the standard index for self-

compactability of fresh concrete, is shown in Figs. 14 and 15. The relationship between the

filling height through obstacle R1 and that through R2 varied depending on the coarse

aggregate content. That test result shows that the influence of coarse aggregate on the

flowability of fresh concrete largely depends on the size of the spacing of the obstacle. It

can be said that the self-compactability of fresh concrete has to be discussed in terms of

solid particles as well as in terms of liquid.

2) Role of mortar as fluid in flowability of fresh concrete

Sufficient deformability of the mortar phase in concrete is required so that concrete can be

compacted into structures by its self-weight without need for vibrating compaction. In


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addition, moderate viscosity as well as deformability of the mortar phase is required so that

the relative displacement between coarse aggregate particles in front of obstacles when

concrete is to flow around such obstacles can be reduced and then segregation between

coarse aggregate and mortar can be inhibited. The necessity for viscosity was confirmed by

Hashimotos visualization test. The indices for mortar deformability and viscosity wereproposed by using mortar flow and funnel test results. The relationship between mortar

deformability and viscosity and the self-compactability of fresh concrete is shown assuming

a fixed coarse aggregate content (Fig. 15). The existence of an optimum combination of

deformability and viscosity of mortar for achieving self-compactability of fresh concrete was

demonstrated.

(3) Role of mortar as solid particles

In addition to its role as a liquid mentioned above, mortar also plays a role as solid

particles. This property is so-called pressure transferability, which can be apparent when

the coarse aggregate particles approach each other and mortar in between coarse

aggregate particles is subjected to normal stress (Fig. 16). The degree of the decrease in

the shear deformability of the mortar largely depends on the physical characteristics of the

solid particles in the mortar (Fig. 17) (Nagamoto 1997).

For example, the difference in the relationships between the funnel speeds of mortar and

concrete due to differences in the fine aggregate content in mortar are shown in Fig. 18. It

was found that the relationship between the flowability of mortar and concrete cannot

always be unique due to differences in the characteristics of the solid particles in the

mortar, even if the characteristics of the coarse aggregate and its content in concrete are

constant.

(4) Influence of coarse aggregate -Content, shape and grading-

The influence of coarse aggregate on the sel fcompactability of fresh concrete, especially

flowability through obstacles, can be equal despite the shape of the coarse aggregate

particles shape as long as the ratio of coarse aggregate content to its solid volume in

concrete is the same (Fig. 22). However, the influence of the grading of coarse aggregate


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has also to be considered if the spacing of the obstacles is very close to the maximum size

coarse aggregate. For example, the relationships between the size of the concrete funnels

outlet and the flow speed through it depends on the fineness modulus of coarse aggregate

FM even if the property of the mortar phase is the same (Figs. 23 and 24). It was found out

that the flow speed of concrete through a funnel with an outlet width of 55 mm was largelyinfluenced by the grading of the coarse aggregate.

State of the art on Self-Compacting Concrete

Current status of Self-Compacting ConcreteSelf-compacting concrete has been used as a special concrete only in large general

construction companies in Japan. In order for self-compacting concrete to be used as a

standard concrete rather than a special one, new systems for its design, manufacturing and

construction of self-compacting concrete need to be established. Various committee

activities on self-compacting concrete have been carried out as a result. Among them, a

system by which the ready-mixed concrete industry can produce self-compacting concrete

as a normal concrete would seem the most effective since, in Japan, as much as 70% of all

concrete is produced by the ready-mixed concrete industry. Assuming a general supply from

ready-mixed concrete plants, investigations to establish the following items have been

carried out mainly at the University of Tokyo since the development of the prototype.

(1) Self-compactability testing method

(2) Mix-design method

(3) Acceptance testing method at job site

(4) New type of powder or admixture suitable for self-compacting concrete

Of those items, (1) has already been mentioned in this paper. (2), (3) and (4) are described

below.

Mix-design method(1) Rational mix-design method

Self-compactability can be largely affected by the characteristics of materials and the mix

proportion. A rational mix-design method for self-compacting concrete using a variety of

materials is necessary. Okamura and Ozawa (1995) have proposed a simple mix

proportioning system assuming general supply from ready-mixed concrete plants. The

coarse and fine aggregate contents are fixed so that self-compactability can be achieved

easily by adjusting the water-powder ratio and superplasticizer dosage only.

(1) The coarse aggregate content in concrete is fixed at 50% of the solid volume.

(2) The fine aggregate content is fixed at 40% of the mortar volume.

(3) The water-powder ratio in volume is assumed as 0.9 to 1.0, depending on the properties

of the powder.

(4) The superplasticizer dosage and the final water-powder ratio are determined so as to

ensure selfcompactability. In the mix proportioning of conventional concrete, the water-


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cement ratio is fixed at first from the viewpoint of obtaining the required strength. With self-

compacting concrete, however, the water-powder ratio has to be decided taking into

account self-compactability because self-compactability is very sensitive to this ratio. In

most cases, the required strength does not govern the water-cement ratio because the

water-powder ratio is small enough for obtaining the required strength for ordinarystructures unless most of the powder materials in use is not reactive.

The mortar or paste in self-compacting concrete requires high viscosity as well as high

deformability. This can be achieved by the employment of a superplasticizer, which results

in a low water-powder ratio for high deformability.

(2) Adjustment of water-powder ratio and superplasticizer dosage

The characteristics of powder and superplasticizer largely affect the mortar property and so

the proper water powder ratio and superplasticizer dosage cannot be fixed without trial

mixing at this stage. Therefore, once the mix proportion is decided, self-compactability has

to be tested by U-flow, slump-flow and funnel tests. Methods for judging whether the water-powder ratio or superplasticizer dosage are larger or smaller than the proper value by using

the test results, and methods for estimating the proper values are necessary. The

relationships between the properties of the mortar in selfcompacting concrete and the mix

proportion have been investigated and then formulated. These formulae can be used to

establish a rational method for adjusting the water-powder ratio and superplasticizer dosage

to achieve appropriate deformability and viscosity.

4. Properties of Hardened SCC

Structural PropertiesThe basic ingredients used in SCC mixes are practically the same as those used in the

conventional HPC vibrated concrete, except they are mixed in different proportions and the

addition of special admixtures to meet the project specifications for SCC. The hardened

properties are expected to be similar to those obtainable with HPC concrete. Laboratory and

field tests have demonstrated that the SCC hardened properties are indeed similar to those

of HPC. Table 3 shows some of the structural properties of SCC.

Table 3 Structural Properties of SCC2

Items SCC

Water-binder ratio (%) 25 to 40

Air content (%) 4.5-6.0

Compressive strength (age: 28 days) (MPa) 40 to 80

Compressive strength (age: 91 days) (MPa) 55 to 100


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Splitting tensile strength (age:28 days) (MPa) 2.4 to 4.8

Elastic modulus (GPa) 30 to 36

Shrinkage strain (x 10-6) 600 to 800

Compressive StrengthSCC compressive strengths are comparable to those of conventional vibrated concrete made

with similar mix proportions and water/cement ratio. There is no difficulty in producing SCC

with compressive strengths up to 60MPa.

Tensile StrengthTensile strengths are based on the indirect splitting test on cylinders. For SCC, the tensile

strengths and the ratios of tensile and compressive strengths are in the same order of

magnitude as the conventional vibrated concrete.

Bond StrengthPull-out tests have been performed to determine the strength of the bond between concrete

and reinforcement of different diameters. In general, the SCC bond strengths expressed in

terms of the compressive strengths are higher than those of conventional concrete.

Modulus of ElasticitySCC and conventional concrete bear a similar relationship between modulus of elasticity and

compressive strength expressed in the form E/(fc)0.5, where E = modulus of elasticity, fc =

compressive strength. This is similar to the one recommended by ACI for conventional

normal weight concrete.

Acceptance test at job site

Since the degree of compaction in a structure mainly depends on the self-compactability of

concrete, and poor self-compactability cannot be compensated by the construction work,

self-compactability must be checked for the whole amount of concrete just before casting at

the job site. However, conventional testing methods for self-compactability require sampling


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and this can be extremely laborious if the self-compactability acceptance test is to be

carried out for the whole amount of concrete. A suitable acceptance test method for

selfcompactability has been developed by Ouchi et al (1999).

(1) The testing apparatus is installed between the agitator truck and the pump at the job

site. The whole amount of concrete is poured into the apparatus.(2) If the concrete flows through the apparatus, the concrete is considered as self-

compactable for the structure. If the concrete is stopped by the apparatus, the concrete is

considered as having insufficient selfcompactability and the mix proportion has to be

adjusted.

Superplasticizer suitable for Self-Compacting ConcreteThere is more room for improvement for admixtures such as superplasticizer suitable for

self-compacting concrete. In order to achieve this purpose, characterization of materials is

indispensable. The requirements for superplasticizer in self-compacting concrete are

summarized below.

(1) High dispersing effect for low water/powder (cement) ratio: less than approx. 100% by

volume

(2) Maintenance of the dispersing effect for at least two hours after mixing

(3) Less sensitivity to temperature changes.

There have been many examples of the development of new type of superplasticizer for

self-compacting concrete. Characterization of the dispersing effect of superplasticizer

independent of the effect of water flow is indispensable.

Segregation-inhibiting agentIt has been found that it is possible to manufacture selfcompacting concrete with constant

quality, especially self-compactability. However, any variation in material characteristics can

affect self-compactability. The most influential variant is the water content of fine

aggregate, which results in variations in the water content of the concrete itself. To solve

this problem, some general construction companies employ a segregation-inhibiting agent.

This type of agent is effective in making selfcompactability less sensitive to the variation of

the water content in the concrete. Various agents are available for this purpose in Japan

(Hibino 1998).

Applications of Self-Compacting Concrete in Japan

Current condition on application of selfcompacting concrete in Japan


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After the development of the prototype of selfcompacting concrete at the University of

Tokyo, intensive research was begun in many places, especially in the research institutes of

large construction companies. As a result, self-compacting concrete has been used in many

practical structures. The first application of selfcompacting concrete was in a building in

June 1990. Self-compacting concrete was then used in the towers of a prestressed concrete

cable-stayed bridge in 1991 (Fig. 29). Lightweight self-compacting concrete was used in the

main girder of a cable-stayed bridge in 1992. Since then, the use of self-compacting

concrete in actual structures has gradually increased. Currently, the main reasons for the

employment of self-compacting concrete can be summarized as follows.

(1) To shorten construction period.

(2) To assure compaction in the structure: especially in confined zones where vibrating

compaction is difficult.

(3) To eliminate noise due to vibration: effective especially at concrete products plants.

The volume of self-compacting concrete in Japan is shown in Fig. 30. The production of self-

compacting concrete as a percentage of Japanese ready-mixed concrete, which accounts for

70% of total concrete production in Japan, is only 0.1%. The current status of self-

compacting concrete is special concrete rather than standard concrete.

Other applications of self-compacting concrete are summarized below.

Bridge (anchorage, arch, beam, girder, tower, pier, joint between beam & girder)

Box culvert

Building

Concrete filled steel column

Tunnel (lining, immersed tunnel, fill of survey tunnel)

Dam (concrete around structure)

Concrete products (block, culvert, wall, water tank, slab, and segment)

Diaphragm wall


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Tank (side wall, joint between side wall and slab)

Pipe roof

Large scale constructionSelf-compacting concrete is currently being employed in various practical structures in order

to shorten the construction period of large-scale constructions.

The anchorages of Akashi-Kaikyo (Akashi Straits) Bridge opened in April 1998, a suspension

bridge with the longest span in the world (1,991 meters), is a typical example (Fig. 31)

(Kashima 1999). Self-compacting concrete was used in the construction of the two

anchorages of the bridge. A new construction system that makes full use of the

performance of self-compacting concrete was introduced for this purpose. The concrete was

mixed at the batcher plant next to the site, and was then pumped out of the plant. It wastransported 200 meters through pipes to the casting site, where the pipes were arranged in

rows 3 to 5 meters apart. The concrete was cast from gate valves located at 5-meter

intervals along the pipes. These valves were automatically controlled so that the surface

level of the cast concrete could be maintained. The maximum size of the coarse aggregate

in the self-compacting concrete used at this site was 40 mm. The concrete fell as much as 3

meters, but segregation did not occur, despite the large size of coarse aggregate. In the

final analysis, the use of selfcompacting concrete shortened the anchorage construction

period by 20%, from 2.5 to 2 years.

Self-compacting concrete was used for the wall of a large LNG tank belonging to the OsakaGas Company. The adoption of self-compacting concrete in this particular project had the

following merits.

(1) The number of lots decreased from 14 to 10 as the height of one lot of concrete was

increased.

(2) The number of concrete workers was reduced from 150 to 50.

(3) The construction period of the structure decreased from 22 months to18 months.


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In addition, a rational acceptance test for self-compactability at the job site was newly

introduced. The concrete casting was completed in June 1998.

Concrete productsSelf-compacting concrete is often employed in concrete products to eliminate vibration noise

(Fig. 32). This improves the working environment at plants and makes the location ofconcrete products plants in urban areas possible. In addition, the use of self-compacting

concrete extends the lifetime of mould for concrete products (Uno 1999). The production of

concrete products using self-compacting concrete has been gradually increasing.

Necessity for new structural design and construction systems.Using self-compacting concrete saves the cost of vibrating compaction and ensures the

compaction of the concrete in the structure. However, total construction cost cannot always

be reduced, except in large-scale constructions.This is because conventional construction

systems are essentially designed based on the assumption that vibrating compaction of

concrete is necessary. Self-compacting concrete can greatly improve construction systems

previously based on conventional concrete that required vibrating compaction. This sort of

compaction, which can easily cause segregation, has been an obstacle to the rationalization

of construction work. Once this obstacle is eliminated, concrete construction can be

rationalized and a new construction system, including formwork, reinforcement, support and

structural design, can be developed. One example of this is the so-called sandwich

structure, where concrete is filled into a steel shell. Such a structure has already been

completed in Kobe, and could not have been achieved without the development of self-

compacting concrete (Shishido et al. 1999).

Summary

Since a rational mix-design method and an appropriate acceptance testing method at the

job site have both largely been established for self-compacting concrete, the main obstacles

for the wide use of self-compacting concrete can be considered to have been solved. The

next task is to promote the rapid diffusion of the techniques for the production of self-

compacting concrete and its use in construction. Rational training and qualification systems

for engineers should also be established. In addition, new structural design and construction

systems making full use of self-compacting concrete should be introduced.

When self-compacting concrete becomes so widely used that it is seen as thestandard

concrete rather than a special concrete, we will have succeeded in creating durable and

reliable concrete structures that require very little maintenance work.


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AERATED CONCRETE & ITS PROPERTIES

Aeratedconcrete is made by introducing air or gas into a slurry composed of

Portlandcement or lime and finely crushed siliceous filler so that when the mix sets and

hardens, a uniformly cellular structure is formed. Though it is called aeratedconcrete it is

really not a concrete in the correct sense of the word. As described above, it is a mixture ofwater, cement and finely crushed sand. Aerated concrete is also referred to as gas concrete,

foam concrete, cellular concrete. In India we have at present a few factories manufacturing

aerated concrete.

A common product of aerated concrete in India is Siporex.

Manufacturing of Aerated ConcreteThere are several ways in which aerated concrete can be manufactured.

(a) By the formation of gas by chemical reaction within the mass during liquid or plastic

state.(b) By mixing preformed stable foam with the slurry.(c) By using finely powdered

metal (usually aluminium powder) with the slurry and made to react with the calcium

hydroxide liberated during the hydration process, to give out large quantity of hydrogen

gas. This hydrogen gas when contained in the slurry mix, gives the cellular structure.

Powdered zinc may also be added in place of aluminum powder. Hydrogen peroxide and

bleaching powder have also been used instead of metal powder. But this practice is not

widely followed at present.

In the second method preformed, stable foam is mixed with cement and crushed sand slurry

thus causing the cellular structure when this gets set and hardened. As a minor modification

some foam-giving agents are also mixed and thoroughly churned or beaten (in the same
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manner as that of preparing foam with the white of egg) to obtain foam effect in the

concrete. In a similar way, air entrained agent in large quantity can also be used and mixed

thoroughly to introduce cellular aerated structure in the concrete. However, this method

cannot be employed for decreasing the density of the concrete beyond a certain point and

as such, the use of air entrainment is not often practised for making aerated concrete.

Gasification method is of the most widely adopted methods using aluminium powder or such

other similar material. This method is adopted in the large scale manufacture of aerated

concrete in the factory wherein the whole process is mechanised and the product is

subjected to high pressure steam curing i.e., in other words, the products are autoclaved.

Such products will suffer neither retrogression of strength nor dimensional instability.

The practice of using preformed foam with slurry is limited to small scale production and in

situ work where small change in the dimensional stability can be tolerated. But the

advantage is that any density desired at site can be made in this method.

Properties of Aerated ConcreteUse of foam concrete has gained popularity not only because of the low density but also

because of other properties mainly the thermal insulation property. Aerated concrete is

made in the density range from 300 kg/m3 to about 800 kg/m3. Lower density grades are

used for insulation purposes, while medium density grades are used for the manufacture of

building blocks or load bearing walls and comparatively higher density grades are used in

the manufacture of prefabricated structural members in conjunction with steel

reinforcement.

COMPRESSIVE STRENGTH OF CONCRETE CUBES

Compressive strength ofconcrete:Out of many test applied to theconcrete,this is the

utmost important which gives an idea about all the characteristics of concrete. By this single

test one judge that whether Concreting has been done properly or not. For cube test two

types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm

depending upon the size of aggregate are used. For most of the works cubical moulds of

size 15 cm x 15cm x 15 cm are commonly used.This concrete is poured in the mould and tempered properly so as not to have any voids.

After 24 hours these moulds are removed and test specimens are put in water for curing.

The top surface of these specimen should be made even and smooth. This is done by

puttingcement paste and spreading smoothly on whole area of specimen.

These specimens are tested by compression testing machine after 7 days curing or 28 days

curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the
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Specimens fails. Load at the failure divided by area of specimen gives the compressive

strength of concrete.

Following are the procedure for Compressive strength test of Concrete Cubes

APPARATUSCompression testing machine

PREPARATION OF CUBE SPECIMENS

The proportion and material for making these test specimens are from the same concrete

used in the field.

SPECIMEN

6 cubes of 15 cm size Mix. M15 or above

MIXING

Mix the concrete either by hand or in a laboratory batch mixer

HAND MIXING

(i)Mix the cement and fine aggregate on a water tight none-absorbent platform until the

mixture is thoroughly blended and is of uniform color

(ii)Add the coarse aggregate and mix with cement and fine aggregate until the coarse

aggregate is uniformly distributed throughout the batch

(iii)Add water and mix it until the concrete appears to be homogeneous and of the desired

consistency

SAMPLING

(i) Clean the mounds and apply oil

(ii) Fill the concrete in the molds in layers approximately 5cm thick

(iii) Compact each layer with not less than 35strokes per layer using a tamping rod (steel

bar 16mm diameter and 60cm long, bullet pointed at lower end)

(iv) Level the top surface and smoothen it with a trowel

CURING


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The test specimens are stored in moist air for 24hours and after this period the specimens

are marked and removed from the molds and kept submerged in clear fresh water until

taken out prior to test.

PRECAUTIONSThe water for curing should be tested every 7days and the temperature of water must be at

27+-2oC.

PROCEDURE

(I) Remove the specimen from water after specified curing time and wipe out excess water

from the surface.

(II) Take the dimension of the specimen to the nearest 0.2m

(III) Clean the bearing surface of the testing machine

(IV) Place the specimen in the machine in such a manner that the load shall be applied to

the opposite sides of the cube cast.

(V) Align the specimen centrally on the base plate of the machine.

(VI) Rotate the movable portion gently by hand so that it touches the top surface of the

specimen.

(VII) Apply the load gradually without shock and continuously at the rate of

140kg/cm2/minute till the specimen fails

(VIII) Record the maximum load and note any unusual features in the type of failure.

NOTE : Minimum three specimens should be tested at each selected age. If

strength of any specimen varies by more than 15 per cent of average strength,

results of such specimen should be rejected. Average of there specimens gives the

crushing strength of concrete. The strength requirements of concrete.CALCULATIONS

Size of the cube =15cm x15cm x15cm

Area of the specimen (calculated from the mean size of the specimen )=225cm2

Characteristic compressive strength(f ck)at 7 days =


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Expected maximum load =fck x area x f.s

Range to be selected is ..

Similar calculation should be done for 28 day compressive strength

Maximum load applied =.tones = .N

Compressive strength = (Load in N/ Area in mm2)=N/mm2

=.N/mm2

REPORT

a) Identification mark

b) Date of test

c) Age of specimen

d) Curing conditions, including date of manufacture of specimen

f) Appearance of fractured faces of concrete and the type of fracture if they are unusual

RESULT

Average compressive strength of the concrete cube = .N/ mm2(at 7 days)

Average compressive strength of the concrete cube =. N/mm2(at 28 days)

Percentage strength of concrete at various ages:The strength of concrete increases with age. Table shows the strength of concrete at

different ages in comparison with the strength at 28 days after casting.

Age Strength per cent

1 day 16%

3 days 40%

7 days 65%

14 days 90%

28 days 99%


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Compressive strength of different grades of concrete at 7 and 28 days

Grade of

Concrete

Minimum compressive

strength N/mm2at 7 days

Specified characteristic compressive

strength (N/mm2) at 28 days

M15 10 15

M20 13.5 20

M25 17 25

M30 20 30

M35 23.5 35

M40 27 40

M45 30 45

PILES AND PILE CAPS

When the bearing capacity ofsoil immediately below the structure is insufficient for a

spread footing, then piles are used to transfer the load to deeper, firmer strata. Piles may

also be used where the soil is particularly affected by seasonal changes, to transfer the load

below the level of such influence.

The load carrying (bearing) capacity of a pile is the sum of the end bearing capacity and the

skin friction capacity between the peripheral area of the pile and the surrounding soil. The

contribution of each differs widely depending on the ground conditions. For example, the

skin friction resistance in sandy soils is small compared to clayey soils.

Usually, the load to be supported exceeds the bearing capacity of a single pile, and so a

group of similar piles is used.

The group is capped by a spread footing or a cap to distribute the load to all piles in the

group. Where there are a large number of closely spaced piles, rather than provide

individual caps, it may be more economical to provide just one large cap, thus forming apiled raft.
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RETAINING WALL DESIGN PRINCIPLES

Retaining structures

Generally speaking, any wall that sustains significant lateral soil pressure is a retaining wall.

However, the term is usually used with reference to acantilever retaining wall,which is a

freestanding wall without lateral support at its top. For such a wall, the major design

consideration is for the actual dimensions of the ground-level difference that the wall serves

to facilitate. The range of its dimensions establishes some different categories for the

retaining structure as follow

(a) Curbs

Curbs are the shortest freestanding retaining structures. The two most common forms are

as shown in Figure 1(a), the selection being made on the basis of whether or not it is

necessary to have a gutter on the how side of the curb. Use of these structure is typically

limited to grade level changes of about 0.6m or less.

(b) Short retaining walls

Vertical walls up to about 3m in height are usually built as shown in Figure 1(b). these

consists of a concrete or masonry wall of uniform thickness, vertical wall reinforcing, and

transverse footing reinforcing are all designed for the lateral shear and cantilever bending

movement plus the vertical weights of the wall, footing, and earth fills.

When the bottom of the footing is a show distance below grade on the low side of the wall

and/or the lateral passive resistance of the soil is low, it may be necessary to use anextension below the footing called a shear key to increase the resistance to sliding. The

form of such a key is shown in Figure 1(c)
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Figure 1:- Retaining Structure

(c)Tall retaining walls

As the wall height increase it become less feasible to use the simple construction shown in

Figure 1(b) or (c). The overturning moment increases sharply with the increase in height of

the wall. For very tall walls one modification used is to taper the wall thickness. This permits

the development of a reasonable cross section for the high bending stress at the base

without an excessive amount of concrete. However, as the wall becomes really tall, it is

often necessary to consider the use of various bracing technique, as shown in Figure 2.


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Figure 2:- Tall Retaining Walls

Design Considerations.

In the design of free-standing retaining walls, the following aspects need to be investigated:

(a) the stability of soil around the wall;

(b) the stability of retaining wall itself;

(c) the structural strength of the wall;

(d) damage to adjacent structures due to wall construction.

The magnitude of the earth pressure which will be exerted on a wall is dependent on the

amount of movement that the wall undergoes.

It is usual to assume for free-standing retaining walls that sufficient outward movement

occurs to allow active (minimum) earth pressures to develop. The designer must ensure

that sufficient movement can take place without affecting the serviceability or appearance of

the wall.


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Where it is not possible for the required outward movement to occur, for instance due to

wall or foundation rigidity, higher pressures will develop and the wall must be designed for

these.

Basic LoadingThe basic pressure loading to be considered for the design is:

Normal Loading = static earth pressure + water pressure + pressure due to live loads or

surcharge.

In general, the resulting design pressure for earth retaining structures should not be less

than the pressure due to a fluid of unit weight 5kN/m3.

Other considerations

The possible occurrence of other design cases, or variation of the one above, caused by

construction sequence or future development of surrounding areas should also be

considered. For instances, additional surcharges may need to be considered and allowance

made for any possible future removal of ground in front of the wall in connection with

services, particularly if the passive resistance of this material is included in the stability

calculations. The effect of excavation on the wall bearing capacity may also need to be

considered.

For the determination of earth pressure, it is usual to consider a unit length of the cross-

section of the wall and retained soil. A unit length is also used in the structural design of

cantilever walls and other walls with a uniform cross-section.

Support of existing fill slopes

Fill slopes constructed in Hong Kong prior to 1977 are likely to have been end tipped or

inadequately compacted. Such slopes may be subject to liquefaction under conditions of

heavy rainfall, vibration or leakage from services, and resulting mud flows may have serious

consequences. Undercutting of the slope toe, for retaining wall construction, will increase

the risk of failure.

Soil PropertiesGeneralFor all walls higher than 5 meters, especially those with sloping backfill, the soil properties

of natural ground and backfill should be estimated in advance of design from tests on

samples of the material involved. In addition, special attention should be paid to the

determination of ground water levels, particularly with respect to maximum probable

values.


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For less important walls, an estimation of the soil properties may be made from previous

tests on similar materials. A careful visual examination of the materials, particularly that at

the proposed foundation level, should be made and index tests carried out to ensure that

the assumed material type is correct.

Selection and use of backfill

The ideal backfill for a minimum section wall is a free draining granular material of high

shearing strength. However, the final choice of material should be based on the costs and

availability of such materials balanced against the cost of more expensive walls.

In general, the use of fine-grained clayey backfills is not recommended. Clays are subjected

to seasonal variation in moisture content and consequent swelling and shrinkage. This effect

may lead to an increase in pressure against a wall when these soils are used as backfill. Due

to consolidation, long term settlement problems are considerably greater than with cohesion

less materials.

For cohesive backfills, special attention must be paid to the provision of drainage to prevent

the built-up water pressure. Free draining cohesionless materials may not require the same

amount of attention in this respect. They may still require protection by properly designed

filter layers.

In Hong Kong, backfill for retaining walls usually comprises selected decomposed grainite or

decomposed volcanic rock. This material is in general suitable for backfill provided that it is

properly compacted and drainage measures are carefully designed and properly installed to

prevent build-up of water pressure.

In fact, rock fill is a very suitable material for use as a backfill to retaining walls and

consideration should be given to its use when available. In general, the rockfill should be

well graded and have a nominal maximum size of 200mm. A well-graded densely

compacted rockfill should not have more than about 2% finer than if it is to remain

free-draining.

Earth Pressure

The earth pressure which acts on an earth retaining structure is strongly dependent on the

lateral deformations which occur in the soil. Hence, unless the deformation condition can be

estimated with reasonable accuracy, rational prediction of the magnitude and distribution of

each pressure in the structure is not possible.


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The minimum active pressure which can be exerted against a wall occurs when the wall moves sufficiently for

outwards for the soil behind the wall to expand laterally and reach a state of plastic

equilibrium.Similarly, the maximum passive pressure occurs when the wall movement is towards the

soil. The amount of movement necessary to reach these failure conditions is dependent

primarily on the type of backfill material. Some guidance on the movements is given inTable 1.

Table 1:- Wall displacements required to develop active and passive earth

pressure

Soil State of Stress Type of

Movement

Necessary

Displacement

Sand Active Parallel to wall 0.001H

Active Rotation about base 0.001H

Passive Parallel to wall 0.05H

Passive Rotation about base >0.1H

Clay Active Parallel to wall 0.004H

Active Rotation about base 0.004H

Passive -

Effects of surcharges

Load imposed on the soil behind the wall should be allowed for in design.

Uniform surcharge loads may be converted to an equivalent height of fill and the earth

pressures calculated for the correspondingly greater height. For example, the buildings with

shallow foundation may be taken as a uniform surcharge of 10kPa per storey. (Figure 1.3)

For example, the standard loading for highway structures in Hong Kong are expressed in

terms of HA and HB loading as defined in BS5400: part 2: 1978. In the absence of more

exact calculations, the nominal load due to live load surcharge may be taken from Table 2.

Table 2:- Suggested surcharge loads to be used in the Design of Retaining

Structures


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Road Class Types of Live loading Equivalent Surcharge

Urban Trunk

Rural trunk

(road likely to be regularly

used by heavy industrial

traffic)

HA +45 units of HB 20kPa

Primary Distributor

Rural main road

HA + 37.5 units of HB 15kPa

Footpath, isolated from

roads, play areas

5kPa

.

Figure 3: Surcharge load cases


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Effect of water

The presence of water behind a wall has a marked effect on the pressures applied to the

wall. When the water intersect the walls, a hydrostatic pressure will exert against the wall,

together with uplift pressures along the base of the wall. Even when there is no water in

direct contact with the wall, such as when adequate drainage is provided, there is anincreased pressure on the wall due to the increase earth pressure. The effect of water

behind the wall is significant; the total force may be more than double that applied for dry

backfill. Many recorded wall failures can be attributed to the presence of water.

The height to which water can rise in the backfill, and the volume of flow, are both of prime

concern. To determine these the ground water conditions must be established. These may

be best derived form the observation of groundwater conditions prior construction using

piezometers. Notwithstanding the results of groundwater monitoring, the groundwater level

assumed for design should be not lower than one-third of the retained height.

The effect of leakage from services can be significant. There is evidence from field

measurements and failures in Hong Kong that this leakage contributes substantially to both

perched and main groundwater tables. Where inadequate drainage is provided behind a

retaining structure, there may be a damming effect which would result in raising

groundwater levels locally and in the general areas. Such a rise may adversely affect

thestability of slopes and retaining walls. Effective drainage measures should always be

provided in such cases.

Stability of Retaining walls

The stability of a free standing retaining structure and the wall contained by it is determined

by computing factors of safety (or stability factors), which may be defined in general terms

as:

Fs= Moments or forces aiding stability / moments or forces causing instability

Factors of safety should be calculated for the following separate modes of failure and should

apply to the 1 in 10 year groundwater condition:

(a) sliding of the wall outwards from the retaining soil,

(b) overturning of the retaining wall about its toe,

(c) foundation bearing failure, and

(d) larger scale slope or other failure in the surrounding soil.
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The forces that produce overturning and sliding also produce the foundation bearing

pressures and, therefore, (a) and (b) above are inter-related with (c) in most soils.

In cases where the foundation material is soil, overturning stability is usually satisfied if

bearing criteria are satisfied. However, overturning stability may be critical for strongfoundation materials such as rock and so on.

In general, to limit settlement and tilting of walls on soil materials, the resultant of the

loading on the base should be within the middle third. For rock foundation material, the

resultant should be within the middle half of the base.

When calculating overall stability of a wall, the lateral such pressure is calculated to the

bottom of the blinding layer, or in the case of a base with a key, to the bottom of the key

where the actual failure mechanism extends to that point.

Concrete framing system

There are many different reinforced concrete floor system, both cast in place and precast.

The cast-in-place systems are generally of one of the following types:

(a) One-way solid slab and beam

(b) Two-way solid slab and beam

(c) One-way concrete joist construction

(d) Two-way flat slab or flat plate without beams

(e) Two-way joist construction, called waffle construction.

Each system has its distinct advantages and limitations, depending on the spacing of

supports, magnitude of loads, required fire rating, and cost of the construction. The floor

plan of the building and the propose for which the building is to be used determine loading

conditions and the layout of supports. Whenever possible, columns should be aligned in

rows and spaced at regular intervals in order to simplify and lower the cost of the building

construction.

One-way joist construction

Figure 2.1shows a partial framing plan and some details for a type of construction that

utilize a series of very closely spaced beams and a relatively thin solid slab. This system is
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generally the lightest (in dead weight) of any type of flat-spanning, poured-in-place

concrete construction and is structurally well suited to the light loads and medium spans of

office buildings and commercial retail buildings.

Figure 2.1:- Typical Concrete One-way joist construction

Waffle construction

Waffle construction consists of two-way spanning joists that are formed in a manner similar

to that for one-way spanning joists. The most widely used type of waffle construction is the

waffle flat slab, in which solid portions around column supports are produced by omitting

the void-making forms. An example of a portion of such a system is shown in Figure 2.2.

However, at points of discontinuity in the plan, such as at large openings or at edges of the

building- it is usually necessary to form beams. These beam may be produced at projections

below the waffle, as shown in Figure 2.2.


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Figure 2.2:- Typical concrete waffle construction

On the other hand, if beam are provided on all of the column lines, as shown in Figure 2.3,

the construction is analogous to the two-way solid slab with edge supports. With this

system, the solid portions around the column are not required, since the waffle itself does

not achieve the transfer of high shear or development of the high negative moment at the

column.

As with the one-way joist construction, fire ratings are low for ordinary waffle construction.

The system is best suited for situations involving relatively light loads, medium to long

spans, approximately square column bays and a reasonable number of multiple bays in each

direction.


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Figure 2.3:- Waffle construction with column-line beams within the waffle depth

Two-way solid slabs

If reinforced in both directions, the solid concrete slab may span two ways as well as one.

The widest use of such a slab is in flat slab or flat plate construction. In flat slab

constriction, beams are used only at points of discontinuity, with the typical system

consisting only of the slab and the strengthening elements used at column supports. Typical

details for a flat slab system are shown in Figure 2.4. Drop panels consisting of thickened

portions square in plan are used to give additional resistance to high shear and negative

moment that develops at the column supports. Enlarged portions are also sometimes

provided at the tops of the columns to further reduce the stresses in the slab.


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Figure 2.4:- Concrete flat slab construction with drop panels and column caps

Two-way slab construction consists of multiple bays of solid two-way-spanning slabs with

edges supports consisting of bearing wall of concrete. Typical details for such a system are

shown in Figure 2.5.


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Figure 2.5:- Two-way spanning concrete slab construction with edge supports

Two way-solid slab constructions is generally favored over waffle construction where higher

fire rating is required for the unprotected structure or where spans are short and loadings

high. As with all types of two-way spanning system, they function most efficiently where the

spans in each direction are approximately the same.

Composite construction: concrete plus structural steel

Figure 2.6shows a section detail of a type of construction generally referred to as

composite construction. This consists of a poured concrete spanning supported by structural

steel beams, the two being made to interact by the use of shear developers welded to the

top of the beams and embedded in the cast slab. The concrete slab may be formed by use

of plywood sheets, resulting in the details as shown in Figure 2.6. however, a more popular

form of construction is that in which a formed steel deck is used in the usual manner,

welded to the top of the beams. The shear developers are than site-welded through the

deck to the top of the beam. The steel deck may function essentially only to form the

concrete, or may itself develop a composite action with the poured slab.


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Figure 2.6:- Steel frame with poured-in-place concrete slab

THERMAL CRACKING OF CONCRETE AND PREVENTION

Thermal Cracking of Concrete and Prevention

Temperature difference within a concrete structure may be caused by portions of the

structure losing heat of hydration at different rates or by the weather conditions cooling or

heating one portion of the structure to a different degree or at a different rate than another

portion of the structure.

These temperature differences result in differential volume change, leading to cracks. This is

normally associated with mass concrete including large and thicker sections ( 500mm) of

column, piers, beams, footings and slabs. Temperature differential due to changes in the

ambient temperature can affect any structure.


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The temperature gradient may be caused by either the centre of the concrete heating up

more than the outside due to the liberation of heat during cement hydration or more rapid

cooling of the exterior relative to the interior. Both cases result in tensile stresses on the

exterior and, if the tensile strength is exceeded, cracking will occur. The tensile stresses are

proportional to the temperature differential, the coefficient of thermal expansion, theeffective modulus of elasticity (which is reduced by creep), and the degree of restraint. The

more massive is the structure, the greater is the potential for temperature differential and

restraint.Hardened concrete has a coefficient of thermal expansion that may range from 4

to 910-6 per deg. F. When one portion of a structure is subjected to a temperature

induced volume change, the potential for thermally induced cracking exists. Special

consideration should be given to the design ofstructures in which some portions are

exposed to temperature changes, while other portions of the structure are either partially or

completely protected. A drop in temperature may result in cracking in the exposed element,

while increases in temperature may cause cracking in the protected portion of the structure.Preventive Measures:

Reducing maximum internal temperature. Delaying the onset of cooling. Controlling the rate at which the concrete cools by insulating the exposed concrete

surface during first 5 days. This could be done by 50mm thick thermocol sheets encased

with polythene sheet laid over concrete surfaces already covered with hessian cloth and

water sprinkler keeping the hessian wet. The temperature gradient between core of

concrete and the surfaces should not be allowed to be more than 150C.

Increasing the tensile strength of concrete. Reducing the concrete temperature at placement up to say 32 0 C. Using low heat of hydration cement or using fly ash replacement of part of cement. Keeping steel formwork warm by air heating during winter. Use of thermally insulating material as formwork. Keeping insulating formwork for longer duration. Low grade of cement, OPC 33 grade is the best. Cement with high C2S content.Repairs: Sealing and grouting ofconcrete cracks.

Concrete Crazing:Crazing is the development of a network of fine random cracks or fissures on the surface

ofconcrete caused by shrinkage of the surface layer.
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These cracks are rarely more than 3mm deep, and are more noticeable on over floated or

steel-troweled surfaces. The irregular hexagonal areas enclosed by the cracks are typically

no more than 40mm wide and may be as small as 10mm in unusual instances (Fig. 1(a) &

(b)).

Generally, craze cracks develop at an early age and are apparent the day after placement or

at least by the end of the first weak. Often they are not readily visible until the surface has

been wetted and it is beginning to dry.

They do not affect the structural integrity ofconcrete and rarely do they affect durability.

However crazed surfaces can be unsightly. Crazing in concrete usually occurs because of

wrongconstruction practices like:

Poor or inadequate curing. Intermittent wet curing and drying. Excessive floating Excessive laitance on surface. Finishing with float when bleed water is on the surface. Sprinkling cement on the surface to dry up the bleed water. Over vibration loading extra bleed & laitance on surface.Preventive Measure for Concrete Crazing: Proper and early start of curing. Use of curing compound on the surface. Never sprinkle dry cement or a mixture of cement and fine sand on the surface of the

plastic concrete.
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SOIL TESTS REQUIRED FOR DEEP

FOUNDATIONS

Deep foundations are those where the depth of foundation is generally greater than two

times of width of footing (D = 2B). Deep foundations are required due to variousreasons.Read hereabout why deep foundations are required and types of deep

foundations.

The soil tests required for deep foundations are:1. While the composition and depth of the bearing layer for shallow foundations may vary

from one site to another, most pile foundations in a locally encounter similar deposits. Since

pile capacity based onsoil parameters is not as reliable from load tests, as a first step it is

essential to obtain full information on the type, size, length and capacity of piles (including

details of load settlement graph) generally adopted in the locality. Correlation of soil

characteristics (from soil investigation reports) and corresponding load tests (from actualprojects constructed) is essential to decide the type of soil tests to be preformed and to

make a reasonable recommendation for the type, size, length and capacity of piles since

most formulae are empirical.

2. If information about piles in the localityare not available or reliable, it may be

necessary to drive a test pile and correlate with soil data.

3. Standard penetration test (SPT)to determine the cohesion (and consequently the

adhesion) to determine the angle of friction (and consequently the angle of friction between

soil and the pile and also the point of resistance) for each soil stratum of cohesion less soil

of soil.

4.Static cone penetration test (CPT)to determine the cohesion (and subsequently the

adhesion) for soft cohesive soils and to check with SPT result for fine to medium sands.
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Hence for strata encountering both cohesive and cohesion less soils, both SPT and CPT tests

are required.

5. Vane shear testfor impervious clayey soils.

6. Un-drained triaxial shear strengthof undisturbed soil samples (obtained with thin

walled tube samplers) to determine cohesion (c) and angle of internal friction ( ) for clayeysoils (since graphs for correlations were developed based on un-drained shear parameter).

In case of driven piles proposed for stiff clays, it is necessary to check with the c and

from remoulded samples also. Drained shear strength parameters are also determined to

represent in-situ condition of soil at end ofconstruction phase.

7. Self boring pressure meter testto determine modulus of sub-grade reaction for

horizontal deflection for granular soils, very stiff cohesive soils, soft rock and weathered or

jointed rock.

8. Ground water condition and permeability of soilinfluence the choice of pile type to

be recommended. Hence the level at which water in the bore hole remains are noted in thebore logs. Since permeability of clay is very low, it takes several days for water in the drill

hole to rise upto ground water table.

Ground water samples need to be testedto consider the possible chemical effects

onconcrete and the reinforcement. Result of the cone penetration test for the same soil

show substantial scatter. Hence, they need to be checked with supplementary information

from other exploration methods.

Pressure metersare used to estimate the in-situ modulus of elasticity for soil in lateral

direction. Unless the soil is isotropic, the same value cannot be adopted for the vertical

direction.

Composition and Types of SoilsSoil grains consists of inert rock materials (cobble, gravel, sand and silt) often combined

with significant amounts of clay (say more than 5%). While inert silt grains may be angular

or rounded (thus contributing to greater or less angle of internal friction), particles of clay

are small platelets with negative charges on both faces which attract the positively charged

ends of water molecules. Thus bond is responsible for the cohesion ends of water molecules.

This bond is responsible for the cohesion C of clay.

Silt or sand with appreciable amounts of clay (say more than 15%) behaves like clayey soil

since the permeability of clay is of the order of 10-7cm/s compared to 10cm/s for sand. This

capacity of the clay to hold the water molecules for long even when the pressure is applied

on the soil, greatly influences its behaviour, i.e. shear strength, compressibility and

permeability.
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Simple and Quick Methods of Field Identification of Soils:1. Fine sand is differentiated from siltby placing a spoonful of soil in a glass of jar or

test tube, mixing with water and shaking it to a suspension. Sand settles first, followed by

silt which may take about five minutes. This test may also be sued for clay which takes

more than 10 minutes to start settling. The percentages of clay, silt and sand are assessed

by observing the depths of the sediments.

2. Silt is differentiated from clay as follows:

a) Clay lumps are more difficult to crush with fingers than silt. When moistened, the soil

lump surface texture is felt with the finger. If it is smooth, it is clay, if rough it is silt.

b) A ball of the soil is formed and shaken horizontally on the palm of the hand. If the

material becomes shiny from water coming to the surface, it is silt.

c) If soil containing appreciable percent of clay is cut with a knife, the cut surface appears

lustrous. In case of silt, the surface appears dull.

3) Field indication for the consistency of cohesive soils are as follows:

Stiff consistency: Cannot be moulded with the finger. Medium consistency: Can be moulded by the fingers on strong pressure. Readily

indented with thumb nail.

Soft consistency: Easily moulded with the fingers.4) Colour of the soil indicatesits origin and the condition under which it was deposited.

Sand and gravel deposits may contain lenses of silt, clay or even organic deposits. If so, the

presumptive bearing capacity is reduced.


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Based on the field identification of the soil, the presumptive bearing capacity of the soil can

be guessed by referring to Table-2 of IS:1904 1986. The objectives of preliminary soil

investigations are to drawn up an appropriate program for detailed soil investigation and to

examine the sketch plans and preliminary drawings prepared by the Architect from the point

of suitability of the proposed structure.

TO DETERMINE THE PARTICLE SIZE DISTRIBUTION OF ASOIL BY SEIVING

Theory:

The soil is sieved through a set of sieves. The material retained on different sieves is

determined. The percentage of material retained on any sieve is given by

Where = mass of soil retained on sieve n

M= total mass of the sample.

The cumulative percentage of the material retained

Where , etc are the percentages retained on sieve 1, 2 etc which are coarser than

sieve n. The percentage finer than the sieve n

Equipment:

1. Set of fine sieves, 2mm, 1mm, 600micron, 425, 212, 150, and 75 micron.

2. Set of coarse sieves, 100mm, 80mm, 40mm, 10mm, and 4.75mm.

3. Weighing balance with accuracy of 0.1% of the mass of the sample.

4. Oven

5. Mechanical shaker

6. Trays
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7. Mortar with a rubber covered pestle.

8. Brushes

9. Riffler

Part-I: Coarse Sieve Analysis

Procedure:

1. Take the required quantity of the sample. Sieve it through a 4.75mm Is sieve. Take the

soil fraction retained on 4.75mm IS sieve for the coarse sieve analysis (Part-I) and that

passing through the sieve for the fine sieve analysis (Part-II).

2. Sieve the sample through the set of coarse sieves by hand. While sieving through each

sieve, the sieve should be agitated such that the sample rolls in irregular motion over the

sieve, the material retained on the sieve may be rubbed with the rubber pestle in the

mortar, if necessary. Care shall be taken so as not to break the individual particles. The

quantity of the material taken for sieving on each sieve shall be such that the maximum

mass of material retained on each sieve does

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