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CRACKING IN CONCRETE AND

METHODS OF MINIMISING CRACKING POTENTIALS 1. INTRODUCTION

Engineers are particularly concerned with cracks in structural concrete. These cracks may be produced before the concrete has set or after it has hardened. Broadly, they are called plastic cracks when they are formed before the concrete has set. The term refers to the concrete in its fresh state and not to be confused with "plasticity" which is a term used to describe the property of a material, i.e. flow under constant stress. The two types of plastic cracks are plastic settlement cracks and plastic shrinkage cracks.

The cracks induced in concrete in its hardened state may be the result of

applied forces or environmental conditions. Thus the cracks produced by environmental conditions may occur even before a structure is in service and may continue to be produced concurrently with the applied forces.

The term "non-structural" cracking indicates only that the cracks are not

the result of structural loading. It does not imply that they may not influence the performance of the structural element, although generally the effect is minor.

Concrete is a highly complex composite. It is made up of numerous

components of various properties. Two concrete mixes may have the same mix proportions but not the same property, e.g. compressive strength. Another pair of mixes may have the same property, e.g. compressive strength, but not the same mix proportions. A third pair may have the same value for a particular property, e.g. compressive strength, but not the same for another property, e.g. modulus of elasticity. Even for a given mix, its properties may vary with time and with the history of the environment in which it is placed.

Within a concrete mix, the various components have different properties.

For example, the strength and modulus of elasticity of its weakest component, air, are negligible compared to those of its strongest component, aggregate (of normal weight)? In the case of such a complex and heterogeneous composite, it can be expected that the internal stress distribution under either physical loading or environmental conditions is both complex and varying from point to point within the mass.

The scope of the presentation covers the causes of cracking, particularly

when, where and why cracks are produced and how such cracking potential may be minimised.

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2. PLASTIC CONCRETE CRACKS Freshly mixed concrete is considered to be in its plastic stage, i.e. before

setting and hardening. This may be taken as the time to reach 27.6 MPa (4,000 psi) penetration resistance by the ASTM C 403 method or the corresponding method given in BS 5075 for testing of retarding admixtures.

2.1 Fresh Concrete

When concrete has been placed and compacted into a form or mould, the

initial spatial distribution of the various components is established. At this stage, the components are:

(a) a well distributed system of coarse aggregate particles, (b) a system of fine aggregate particles filling the inter-particle space

between the coarse particles, (c) cement particles (including mineral admixtures such as pfa, ggbs or

cfs, if added) filling the voids between all the aggregate particles, (d) water voids (including chemical admixtures, if used) forming

interconnected capillaries, and (e) air bubbles (entrained and/or entrapped). This complex system of solids, liquids and gas changes from their initial

spatial distribution as the denser solid particles, particularly the coarse aggregates drift downwards whilst the lighter components, water and air, rise upwards to the exposed top surface. This behaviour has been described as bleeding, settlement and segregation. As hydration takes place, the rate of the movements is rapidly reduced and ending when hardening takes place. The duration of time, during which gradual stiffening results in hardening, may be prolonged by the addition of retarders. The initial degree of fluidity of the mix may also be modified by varying the dosage of plasticiser besides adjustments to the mix proportions.

2.2 Plastic Settlement Cracking

After concrete has been placed, stiffening begins. However, if the

concrete contains a set-retarding admixture, the process is very much slower. The duration of this plastic stage may extend from an hour or two for a plain mix but up to several hours depending on the dosage of retarding admixture added. During this period, the few hours after placing, the concrete continues to settle. It the downward movement of the concrete is obstructed e.g. by the top reinforcement bars or formwork tie-bolts, the concrete tends to flow around them and hence resulting in a line of crack along the direction and directly over these obstructions. In addition to the cracks above the bars, the settlement of concrete may also cause to void to form immediately below the bars.

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When this occurs, it is difficult to detect and to fill this void after the

concrete has hardened. It is important that revibration of the upper layer of the

concrete is carried out to close both the top cracks and to fill the voids before the

concrete has passed the time for formation of cold joint, i.e. the time to reach a

penetration resistance of 3.5 MPa. Remedial measures after the concrete has

hardened may seal the cracks to protect the reinforcement but the loss of bond

not recovered. If this is significant structurally, the top layer may have to be

removed to below the bar level and recast.

Similar problems may arise from differential rate of settlement results in

plastic settlement cracks. In the case of waffle slabs, the greater depth of the

ribs may cause a crack to form at the location of change in depth. For columns

with a conical column head, the change in section may lead to arching action of

the concrete flow resulting in a near horizontal crack near the bottom of the

transition. For very large diameter columns with heavy reinforcement near the

surface, the friction near the formwork may cause the concrete to settle at the

different rate from the interior volume. If the cover is small, a horizontal crack

may appear just above the region where the splicing bars from the lower floor are

terminated.

In such cases, changes in the mix design to provide better cohesion by

using higher percentage of fine aggregate or adding condensed silica fume. It is

best to avoid over retardation for the top layer to minimise the potential for plastic

settlement cracking. The method of placing thick sections, e.g. a thick pile cap

or a deep raft foundation, is often by building up in lifts at one or more corners of

the plan area to its full height and then move progressively over the area. With

this approach, the top layer has the same retardation time as the rest of the

concrete, which is needed to prevent formation of cold joints. However, the top

layer does not have similar need and the high retardation provides more time for

the process of settlement before the concrete reaches setting. The alternate

approach of placing the entire area in horizontal layers enables the concrete for

the top layer to be designed with a nominal retardation suitable for the time

transportation from plant to site. The lower layers will have also reached partial

set by the time the top layer is placed. This will also reduce the total plastic

settlement for the whole depth. In the case of very large areas in plan, the total

plan area may be divided into separate cells with partitions making up of special

mesh for end stops, e.g. Hyrib. This is to reduce the retardation time needed if

the whole area is filled to the same height at a time. The planning and

organisation of the logistic for ensuring the supply of mixes with different set-

retardation times is directed to the appropriate locations are vital to the success

of such an operation. The alternate to this is to introduce revibration of the

finished surface just prior to the time for cold joint (3.5 MPa penetration

resistance), to close any settlement cracks that may have developed.

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2.3 Plastic Shrinkage Cracking

Shrinkage of concrete is the result of drying out of the moisture within its mass. This can occur before the concrete has hardened and continue for months and years. Cracks may appear in an exposed concrete surface very soon after it has been finished or even, in some cases, before finishing has been completed. Such cracks are called plastic shrinkage cracks. They are produced by the rapid drying of the concrete surface whilst the concrete is still in its fresh or plastic state. They are generally discontinuous and very seldom extend to a free edge when there is very little restraint to movement. In areas without top reinforcement, they are typically in a diagonal direction of relatively short length (up to 300 mm). The pattern may be modified if top reinforcement bars are present. The slightly smaller thickness of the cover over a bar may align the crack along part of its length.

The loss of moisture is most commonly due to evaporation of water from

the surface, but a dry sub-base or formwork materials may suck water from the concrete and directly lead to cracking or aggravate the effects of surface evaporation. Thus plastic shrinkage cracking is most common with large horizontal surface in the case of slabs and pavement. Drying out of the concrete surface begins when the rate of evaporation exceeds that of bleeding. Thus plastic shrinkage increases with the cement content of the mix and with lowering of the water/cement ratio. Hence, there is a complex relationship between bleeding and plastic shrinkage. A higher bleeding capacity leads to more shrinkage, but on the other hand it slows down the rate of drying out for a given rate of evaporation and so reduces potential plastic shrinkage cracking. The rate of evaporation from the concrete surface depends on the combined influence of wind velocity, relative humidity of the air, the temperature of the air and the temperature of the concrete. If the rate of evaporation exceeds 0.5kg/m2/h (0.1Ib/ft2/h), loss of moisture may exceed the rate of bleeding. Potential plastic shrinkage cracking is high when the rate exceeds 1kg/m2/h (0.2lb/ft2/h). A chart is often used to estimate the rate of evaporation under given environmental conditions and the temperature of the concrete.

Potential plastic shrinkage cracking may be minimised by reducing the

wind velocity with windbreakers, by reducing the temperature of the concrete and by avoiding excessive delay in setting of the concrete. The best approach is to ensure that concrete surface is kept wet until the surface has been finished and curing to begin as soon as finishing is complete. Site control to achieve this is most important particularly when specification calls for a dry shake hardener finish is to be applied. The introduction of the hardener with added cement when bleeding is ending together with the mechanical trowel finishing leads to a very try finished surface. Any delay in curing is likely to result in plastic shrinkage cracking. Experience has shown that it is vital that the curing process follows immediately after the completion of this type of surface finish to avoid potential plastic shrinkage cracking.

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Methods of curing include the use of sprayed or painted curing compounds, wet hessian or wet sand (both of which should be overlaid with airtight polythene cover sheets to prevent loss of moisture), or other curing systems.

Plastic shrinkage cracks may be very fine but can penetrate the full depth

of a slab or a deep beam. Since they are usually developed within the first few hours after finishing of the concrete surface when the bond strength with steel reinforcement is still very low, the provision of shrinkage reinforcement (against drying shrinkage in hardened concrete) does not prevent such cracks. However, the addition of fibres, e.g. polypropylene fibres, has been reported to be effective in improving the ultimate tensile strain of fresh concrete against plastic shrinkage cracking. Often these cracks are not noticed at early ages until their width is increased by subsequent drying shrinkage. They are difficult to be fully grouted even by low viscosity polymers. Even when they may not influence structural behaviour significantly they should be sealed (as far as possible) to reduce the rate of ingress of moisture and oxygen. These are essential for corrosion of steel to take place.

2.4 Formwork Settlement Cracks

Formwork should be designed to have sufficient rigidity and the supporting

props strong enough not to settle or deflect significantly under the weight of the fresh concrete and equipment plus their operators. If the support is inadequate, the downward or sideways movement of the formwork may lead to the formation of tension cracks on the surface of the partially set concrete. When such cracks are found in hardened concrete, the remedial measures should include a structural check in addition to grouting to restore structural action.

2.5 External Sources of Vibration

Before fresh concrete has developed adequate strength, it can be severely disturbed by external sources of vibration, e.g. pile driving nearby, heavy vehicle movement or other construction activities. Depending on the severity of such vibrations, surface cracks and/or internal microcracks of the interfacial bond between cement paste and aggregates or steel reinforcement bars. It is advisable that fresh concrete be kept from such disturbances for at least the first 24 hours after placing.

3. HARDENED CONCRETE CRACKS

Cracks may exist in hardened concrete even before it has been subject to

any physical loading. Such cracks are usually the result of environmental loading combined with the heterogeneity of concrete.

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3.1 Composition of Concrete

The volumetric composition of normal weight concrete indicates that in general, aggregates make up to 65 to 75% by volume, and the balance consisting of cement paste. For a typical concrete mix with a cement content ranging from 300 to 500 kg/m3, the volumetric fraction of cement in concrete is about 10 to 15% by volume of concrete. Water content in the range of 160 to 200 kg/m3 contributes 16 to 20% by volume of concrete. There is always a small volume of air, up to 3% when admixtures (other than air-entraining admixtures) are used. Chemical admixtures consist of about 30 to 40% by mass of solids only and are seldom more than a few kilograms per cubic metre of concrete and contribute a negligible volume in the concrete.

The mechanical properties of the components according to Newman are

shown below:

Property

Anhydrous cement

Hardened Cement paste

Natural aggregates

Elastic modulus (psi x 106)* 7 to 8 1 to 4 5 to 10

Poisson's ratio 0.25(?) 0.25 0.10 to 0.25

Ultimate compressive strength (psi)* 50,000(?)

2,000 to 20,000

10,000 to 50,000

Ultimate tensile strength (psi)* 2,000(?)

200 to 1,000

200 to 2,000

Specific gravity 3.1 to 3.2 1.7 to 2.2 2.5 to 2.7

Drying shrinkage (microstrain) Negligible (?)

2,000 to

3,000

Negligible (with a few exceptions)

Specific creep (microstrain/psi)*

Negligible (?) 1 to 3 Negligible (with a few exceptions)

Thermal coefficient (microstrain/°C) 6 to 12 10 to 20 6 to 12

* 1 psi = 0.00698 MPa The stress-strain relationship for both hydrated cement paste and

aggregates are approximately linear over its entire range. However, the composite material, concrete, does not have a linear stress-strain relationship. The departure from linearity is due to the behaviour of the interfacial bond between cement paste and aggregates and in the development of microcracks in this interfacial bond.

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3.2 Drying Shrinkage Induced Cracking

Drying shrinkage is the term used to describe the reduction in volume of concrete arising from the loss of water due to chemical reaction or physical movement out of the concrete. Curing may maintain near saturation of the concrete for normal size members. Subsequent exposure to unsaturated environment leads to moisture moving out of the concrete resulting in a reduction in volume. The magnitude of shrinkage is in the order a few hundred microstrains for normal weight aggregate concrete. It develops at a rapid rate at early ages and continues for many years (shrinkage has been monitored up to 30 years). The amount of shrinkage at time, t is often related to the ultimate shrinkage, εsh by the modeled:

εsh(t) = εsh [t/(b + t)], where a and b are constants

The value of the constant b is often in the range of 20 to 50 days, implying

that this is the period of time at which half of the ultimate shrinkage has occurred. In the construction of a long structure, it may be advantageous by leaving a pour strip, to allow most of the long-term shrinkage to take its course before the whole structure is made continuous.

When concrete surface is sealed or in the interior of a large mass,

continuing hydration uses up water which is not replaced from external source such as curing. This is known as "self-desiccation" and the reduction in volume is called autogenous shrinkage. The order of shrinkage is only about 100 microstrain after 5 years.

Shrinkage that is free from restraint leads to only a negligible reduction in

volume. However, when it is restrained in some way, cracking may develop. Drying shrinkage occurs at a very slow rate in full size members and relaxation due to creep is significant. It is only when the tensile stress created by restraint and after its reduction by creep, exceeds the ultimate tensile strength that cracking takes place. In the case of autogenous shrinkage, minor bond cracks may develop at the interface between cement paste and aggregates which act as restraints.

3.3 Differential Thermal Cracking

When concrete is subject to heating and cooling arising from daily or seasonal temperature changes, the different components within the concrete respond differently due to their different coefficient of thermal expansion. The differential movement of these components may lead to microcracks in the interfacial bond between cement paste and aggregates or embedded reinforcement steel. Such cracking is minor and seldom propagates to any significant extent unless the thermal changes are very high, e.g. in the case of a fire.

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The range of both the daily and seasonal temperatures is small within the tropics, generally within 10°C. A variation of ±5°C produces a strain of the order

of 50 microstrain. Since concrete is not a good conductor of heat, the interior of the mass is subject to a much less change in temperature and thermal strain induced cracking is unlikely. Thus this factor is not significant in the design of structural elements. On the other hand, in the tropics an exposed concrete roof slab is heated to a high temperature in the afternoon. Temperatures up to 50°C have been recorded. A heavy downpour of tropical rain occurs frequently and the sudden thermal sock of a fully restraint slab, particularly when no top reinforcement is provided at mid-span, may lead to differential thermal cracking. Another similar situation is during fire fighting when water is sprayed onto the hot concrete. In general, it is not only the space rate of temperature difference (physical gradient) but also the time rate of temperature change (creep relief) combined together that determines the potential thermal cracking.

The case of casting of thick sections with a high cementitious content is

covered in a later section under potential early thermal cracking.

4. CRACKING DUE TO CHEMICAL ATTACK Only the commonly encountered chemical attacks are considered. The

details on the mechanism of attack and prevention methods are not included. Cracking due to chemical attack is divided into two groups:

(a) directly induced cracking in concrete (b) indirectly induced cracking in concrete

4.1 Directly induced Cracking The chemical reaction between component of concrete and the chemical

may result in cracking directly. The two most common types are:

4.1.1 Alkali-silica reaction (ASR) This involves the reaction between the soluble alkalis (Na2O and K2O) in

cement and the reactive silica in certain types of aggregates. The resultant gel formed by the reaction has a high capacity of imbibing water and swell. The expansion of the gel gives rise to forces sufficient to cause spalling of the concrete.

The best means to prevent such occurrence is to either avoid the use of

aggregates that are known to be reactive, or by selecting cements that have an Na2O equivalent (%Na2O + 0.658%K2O) content below 0.6% or 3.0 kg/m3. In general, if the service environment is dry, (below 75% RH), there is insufficient water to enable the gel to produce enough swelling to cause spalling.

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4.1.2 Sulphate attack

Sulphate attack is a well-known phenomenon and it is not intended to present the mechanisms of attack in details. The main chemical reaction is between the sulphate from an external source, e.g. from the soil, ground water or seawater and the tricalcium aluminate in cement. The reaction product is ettringite (sulphoaluminate corrosion) which forms with a large increase in volume (over twice its initial volume). The expansion disrupts the concrete, which spalls off exposing new concrete for further reaction. The other reaction is between the sulphate and the calcium hydroxide present in the pore liquids of concrete, forming calcium sulphate (gypsum corrosion). Magnesium sulphate, which is present in seawater, may also react with the silicates thereby disrupting the concrete matrix. Depending on the type of sulphates and their concentration, the attack varies in severity.

The method of minimising sulphate attack is to reduce the amount of

tricalcium aluminate in cement. Typically, a sulphate resisting Portland cement limits its tricalcium aluminate content to less than 3.5%. The recent revised edition of BS 5328 : Part 1 : 1997 provides details for the selection of appropriate types of cement and mix proportions for various concentration of sulphates. 4.2 Indirectly Induced Cracking

The ingress of chloride or carbon dioxide into concrete results in chemical reactions which do not directly produce cracking in the concrete. The reaction of carbon dioxide with calcium hydroxide in the pore fluids of concrete reduces the alkalinity of the concrete. This results in the depassivation of the steel reinforcement bars which may then corrode if water and oxygen are available. The presence of chloride leads to depassivation even when the alkalinity is still high. The products of corrosion (rust) can be several times larger in volume than the materials entering into the reaction. The expanding forces lead to overstressing of the concrete cover. Initial cracking may be transverse to the steel bars but later splitting along the length of the bar results in spalling of the cover concrete.

The best way to minimise the ingress of chloride or carbon dioxide is to

reduce the permeability of the concrete to these chemicals combined with an adequate cover depth so that depassivation does not reach the level of the steel reinforcement bars during the design service life of the structure. Remedial measures include the replacement of cover concrete, application of coatings, addition of corrosion inhibitors, calcium nitrite and cathodic protection for the steel. Alternatively, epoxy coated reinforcement may be used. In recent years, non-metallic reinforcements, e.g. carbon fibres, are being developed to avoid corrosion. However, most of these are still in the development stage and costly. Their common use in practice will take some time yet.

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5. PHYSICAL LOAD INDUCED CRACKING

In reinforced concrete design, the tensile stress in bending is assumed to be contributed by steel reinforcement bars only. The contribution by concrete in tension is ignored. In general, the tensile stress in the steel bars at serviceability limit state already exceeds that of the ultimate tensile strength of concrete. Hence tensile cracks are accepted but their crack width is limited to 0.1 to 0.3 mm depending on its applications.

From the discussion on bleeding and segregation of fresh concrete and

the difference in the response of the various components in concrete to environmental changes. minor internal cracking exists even before any load is applied. For concrete in compression, as the stress level increases there are four nominal stages of cracking intensity in relation to its stress-strain relationship:

(a) Below about 30% of the short-term strength, the degree of pre-

existing bond cracking is small and the cracks are stable with little tendency to propagate. In addition, there is some new crack initiation at localised regions of stress concentration accounting for the slight deviation from linearity at this low stress level.

(b) Between about 30 and 50% of the short-term strength, the cracks

may propagate but only very slowly. They are mainly interfacial bond cracks growing in a stable manner with very little cracking in the cement matrix. The stress-strain relationship showy. more deviation from linearity.

(c) When 50% of the short-term strength is exceeded, cracks begin to spread into the matrix and a more extensive and continuous network of cracks begins to be developed. More and more of the originally isolated bond cracks are connected to the matrix cracks.

(d) Beyond 75% of the short-term strength, more rapid crack growth within the cement matrix takes place and eventually linking up with other cracks to form an unstable system leading to fracture

5.1 Creep Fracture

When concrete is subject to a sustained loading, the strain increases

under constant stress. This is called creep strain. For a sustained stress level of about 70-80%, the propagation of cracks may lead to eventual fracture, known as creep fracture. At lower sustained stress levels e.g. under service loads, creep continues for many years (30 year creep has been reported) but fracture is not reached although deformation may be excessive.

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6. EARLY THERMAL CRACKING

The casting of thick sections is a topic of special interest. This is an area of concrete construction where transfer of technology developed for temperate region calls for critical review and modifications to suit the hot and humid tropical region.

The hydration of the cementitious fraction of a concrete mix generates a substantial amount of heat, as this is an exothermic reaction. The rate of heat evolution is high during the first few days after placing. At the exterior region of the concrete mass, heat can dissipate into the environment. For the interior region, the poor thermal conductivity of concrete prevents significant heat dissipation. Thus a temperature differential is developed with the centre remaining at high temperatures for a long time, particularly when the exterior surface is insulated to reduce the temperature differential. In the surface region, similar to the case of a thin section, the temperature rise is dependent on the rate of heat evolution and the rate of heat loss. For the interior of a thick section, the temperature rise is higher and hence the temperature differential increases. The peak temperature depends mainly on the total heat evolved more than the rate of evolution. The contraction of the cool external zone is restrained by the warmer interior. This is the internal restraint. The resultant surface tensile strain may lead to surface cracking if the restrained component exceeds the tensile strain capacity of the concrete at the time of the occurrence.

The actual thermal strain in concrete is the product of the coefficient of thermal expansion, the difference in temperature and a restraint factor (based on the external restraints). In order to avoid differential thermal cracking, the tensile strain capacity. TSC should exceed the thermal strain developed:

TSC > α(T1 − Te)R

where TSC = tensile strain capacity

α = coefficient of thermal expansion of concrete

T1 = temperature at the hottest part of a section Te = temperature at the surface of a section R = restraint factor

6.1 Tensile Strain Capacity

The TSC of concrete depends mainly on its aggregate. Typical values in BS 8110 : Part 2 : 1995 are shown below:

Aggregate Gravel Granite Limestone Sintered pfa

TSC x 10-6 70 80 90 110

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6.2 Coefficient of Thermal Expansion

The coefficient of thermal expansion for the aggregate type in the above table are shown below: 6.3 Restraint Factor

Typical values of external restraint recorded in various structures are provided in BS 8110 : Part 2 : 1995 are shown below:

Pour configuration Restraint factor [R]

Thin wall cast on to massive concrete base 0.6 to 0.8 at base 0.1 to 0.2 at top

Massive pour cast into binding 0.1 to 0.2

Massive pour cast on to existing mass concrete 0.3 to 0.4 at base 0.1 to 0.2 at top

Suspended slabs 0.2 to 0.4

Infill bays, i.e. rigid restraint 0.8 to 1.0

6.4 Estimated Temperature Differential For Cracking

The following estimated temperature differentials above which cracking may occur, are computed using the TSC and restraint factors in tables above:

Aggregate Type

Limiting temperature differential(°C) for restraint factor of

1.0 0.8 0.6 0.4 0.2

Gravel 7.3 9.1 12.2 18.3 36.5

Granite 10.0 12.5 16.7 25.0 50.0

Limestone 16.0 20.0 26.7 40.0 80.0

Sintered pfa 19.6 24.5 32.7 49.0 98.0

The tabulated values show that in the case of a raft foundation, the

probability of differential thermal cracking is very low, even at the base where the restraint is higher. On the other hand, for the case of a thin wall cast on to massive concrete base, the probability of cracking is very high due to the high restraint factor of 0.8 at the base. 7. EFFECT OF HIGH TEMPERATURE ON CONCRETE STRENGTH

The major effects of temperature on the strength development of hardened concrete are due to high temperature during setting and for curing.

Aggregate Gravel Granite Limestone Sintered pfa

Coefficient of thermal expansion 10-6/°C 12.0 10.0 8.0 7.0

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7.1 Setting Temperature

Several researchers have reported the influence of high temperature during setting on strength. In particular, Neville explains that a rapid initial rate of hydration may lead to the formation of products of a poorer physical structure (probably more porous), so that a large part of these pores will remain unfilled even with continuing hydration. This concept has been further extended by Verbeck and Helmuth that the rapid rate of initial hydration at higher temperatures retards the subsequent hydration and leads to non-uniform distribution of products of hydration within the paste. Such products have a poorer physical structure, probably more porous. These pores may remain unfilled and the resultant higher gel/space ratio will result in a lower strength. In terms of the gel/space ratio concept of Powers and Brownyard, the lower gel/space ratio areas result in a lower strength of the paste as a whole. Notably Price and Klieger have studied the influence of high temperature on the rate of strength development as well as on the long-term strength that may be achieved. In most cases, the reported effect of up to 40°C compared to 20°C' is around 10 to 15% for 28-day strengths of 30 to 40 MPa. However, for early temperatures at 50°C, around 30% had been indicated by the work of Klieger as well as by Verbeck and Helmuth. However, their studies were based on a high setting temperature followed by a lower curing temperature. This is the situation for concrete cast on a hot summer day, which is followed by cooler days. However, the situation in the hot wet tropical environment is different. Both the setting and curing temperatures remain high throughout. Under these conditions, studies by those in the tropics, such as the reported effects observed by Ackroyd and Rodes in Nigeria, Quao in Ghana and locally by Tam showed that 28-day strength of comparable mix proportions do not show any reduction but instead there is an increase at curing temperatures of around 30°C. In fact, the results show that similar strengths can be obtained with an increase in water/cement ratio of about 0.08. Thus on an equal 28-day strength basis, concrete in the tropics will be of a lower quality and hence poorer durability than corresponding mixes in temperate countries. Recent temperature-matched curing studies for cement containing partial replacement with mineral admixtures such as pulverised fuel ash by Mani et al or ground granulated blast furnace slag Bamforth show even much later ages under temperature-matched curing tests before the strength is observed to be less than those under the normal (lower) standard curing temperature.

7.2 Curing Temperature

A higher curing temperature is the basis for accelerated curing of products in the precast concrete industry. However, the combined effect of higher setting and, curing temperatures experienced in the tropics compared to the temperate climate produces a cross over of the strength development with time curves. Although, this generally occurs beyond the age of 28 days, the rate of early age strength gain is much higher due to the higher temperature. This is reflected in the higher earlier age strength relative to that at the age of 28 days.

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The effect high curing temperature on early age (1 to 3 days) strength is often beneficial in construction. It enables earlier removal of formwork for which the recommended 10 MPa strength (BS 8110) can be achieved often within 1 to 2 days. For precast and prestressed concrete, the savings in time for reuse of the moulds can be significant.

In site practice, as well as in production of concrete, it is common to estimate 28-day strength based on earlier age strength. Because of the rapid development of early strength under higher curing temperature, the relative strength between early age and 28-day strength is much higher in the tropics. Tam showed that the ratio between the strengths at 7 and 28 days should be modified so as not to overestimate the 28-day strength - an unsafe practice. Tam and Sri Ravindrarajah further reviewed this "age factor" for different curing temperatures in the case of water or air curing for data up to the age of 5 years. For better estimates, more elaborate methods such the hyperbolic function proposed by Chin or Tam with or without the modification by Carino or the recent approach of maturity function -in ASTM C 1074 may be adopted. This last method takes into account the effect of temperature by obtaining the activation energy for cement hydration experimentally.

The reduction is long term strength that is commonly reported for Ordinary Portland cement is modified when supplementary cementitious materials, such as pulverized fuel ash (pfa) or ground granulated blast furnace slag (ggbs) are used. Recent studies by Bamforth and Mani et al have shown that with the replacement of cement by such materials, there is no reduction in strength even under mass concrete conditions as simulated by a temperature-matched-curing process. In general, the peak temperature reached is also lowered and the time to reach this peak temperature delayed with resultant benefit in reducing potential early age thermal cracking discussed in the next section. 8. CONCRETING OF THICK SECTIONS

The effect of high temperature is of special interest when casting thick sections such as transfer girders and thick pilecaps and when pouring very large volume of concrete in a single continuous operation such as thick raft foundations. With the increasing use of higher strength grades, the cement content of concrete mixes is much higher than those associated with mass concrete in dam construction. The combination of these factors has given rise to specifications which are based on the experience in temperate climate. When such specifications are applied to the tropical climate, suitable adjustments should be made to account for the difference in climatic pattern and ambient environment. Some of these factors are reviewed and guidelines for satisfactory concreting of such construction are proposed.

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8.1 Temperature Control

The are two stages in temperature control. The first is the initial concrete temperature and the second is the peak temperature that the hardened concrete may reach. 8.1.1 Initial temperature

The factors influencing the initial temperature are the temperature of the constituents and the mix proportions. The temperature of the aggregates, which occupy the biggest proportion of the mix, will have the largest influence. This is the reason why shielding of the mix constituents from direct solar radiation in the tropics is beneficial in preventing mix temperature becoming much higher than ambient temperature. However, the easiest approach to lower mix temperature is to use mixing water which is chilled or replaced by ice. ACI Committee 305 has provided an estimate of the various effects.

The temperature of a freshly mixed concrete can be estimated based on the

proportions and the specific heat for the various constituents of a mix (ACI Committee 305) and may include the use of ice to partially or completely replace the mixing water. This controls the initial temperature of fresh concrete. Allowance must also be made for the rise in temperature due to the heat capacity of the mixer drum (and transport truck drum) as well as the gain in heat from the environment and the agitation during transport, and pumping. In general, the mix temperature can be lowered by 6° to 11°C either by chilled water or ice. Further lowering can only be achieved by liquid nitrogen injection into the freshly mixed concrete. It has been estimated that this method may reduce initial concrete temperature to about 10°C when the concrete near the injection nozzle becomes frozen. The use liquid nitrogen to cool the fine aggregate before its batching has also been reported by Japanese researchers Kurita et al. This has a better efficiency than injection into fresh concrete from the discharge port of a mixer truck.

As an indication, the resultant temperature of a mix is given by:

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iicacacafafafawwwccc

HMHMHMHMHM

FMHTMHTMHTMHTMT

++++

−+++=

1

where M = mass in kg T = temperature in °C H = specific heat of ingredient, in kJ/kg F1 = latent heat of fusion for ice = 335 kJ/kg and the subscripts c, w, fa, ca represent cement, water, fine aggregate, and coarse aggregate respectively.

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In the case of ready-mixed concrete where the cement is kept in silos, the cement temperature is generally much above the other ingredients. In the tropics, the cement temperature is likely to be above 40°C and at times above 60°C. It can be seen that from the equation given above, the change in cement temperature alone may be estimated for a typical mix of the following proportions:

Cement = 350 kg/m3 Water = 175 kg/m3 Total aggregates = 1,875 kg/m3

Taking the approximate specific heat for the solids, Hs (cement and

aggregates) as 0.92 kJ/kg/°C and for water, Hw as 4.2 kJ/kg/°C, the change in the initial concrete temperature of 1°C may be caused by a change in the temperature of either the cement, water or aggregates approximately as shown below:

Cement temperature = 8.6°C, or Water temperature = 3.8°C, or Aggregate temperature = 1.6°C

The above indicates that an increase in concrete temperature by 1°C is

produced by every 10°C rise in cement temperature. This change in temperature of the mix may be compensated for by a decrease in the water temperature of about 3.8°C. However, the same increase in concrete temperature may be produced by an increase of the aggregate temperature of only about 1.6°C. 8.1.2 Peak Temperature

In order to minimise the rise in temperature, the total heat of hydration of the mix may be reduced by suitable choice of the chemical composition of the cement. Special low heat cement and partial replacement of Portland cement with supplementary cementitious materials such as pfa and ggbs are available for such purposes. The use of plasticizers to reduce cement content for a given water/cement ratio offers another means to reduce the total heat of hydration of a mix. Currently available high range water-reducing admixtures (super-plasticizers) can provide a mix equivalent in workability and strength with a reduction of up to 20-30% in both the water and cement contents.

In actual construction of thick sections, the situation is not one of truly adiabatic condition but that there will always be some loss of heat from the concrete to the environment. Under this partially adiabatic condition, the delay in hydration by the introduction of pfa or ggbfs will result in not only a lower peak temperature but also for it to occur at a later age of the concrete. Thus the cracking resistance to the mix is higher by the time when the peak temperature is reached. The differential temperature is often reduced in the process as observed by Mani et al.

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The peak temperature as well as the differential temperature can be controlled by circulating cooling water through pipes embedded within the large concrete mass. This is a technique taken from the casting of mass concrete dams. The cost to provide such a cooling system and later to grout up the pipes may not justify its use except for special cases. Some recent examples of casting of thick sections in the local climatic conditions are described by Lee et al and Tam et al. 9. SIGNIFICANCE OF CRACKING

Cracking of concrete in the tension zone is an accepted criterion in reinforced concrete design. Hence the presence of cracking in a member does not necessarily imply failure. The significance of cracking may be divided into 4 levels of increasing severity:

Level 1: cracks which spoil the aesthetic appearance of a structure. Level 2: cracks which affect serviceability, e.g. water leakage, damage

to finishes. Level 3: cracks which impair durability and may lead to reduction in

load carrying capacity. Level 4: cracks which affect structural integrity.

9.1 Level 1 Cracks

All cracks are unsightly and invoke human reactions to the safety of the structure. The prestige of the structure also determines the tolerance of cracks which do not impair durability, serviceability or structural integrity. Campbell-Allen proposed a set of prestige numbers and the acceptable viewing distance from which cracks of a given width may be noticed:

Prestige Number

Description

Viewing distance - m for crack width (mm) of

0.1 0.2 0.3

1 Little used and scarcely seen storage areas 1 1 1

2 Parking stations and garages 1 1 1

3 Factory and commercial buildings 1 2 2

4

5 Domestic buildings 1 2 4

6

7

8 Prestige public buildings, public works and offices 3 6 8

9 Monumental buildings 3 8 >8

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9.2 Level 2 Cracks

Early thermal cracks and plastic shrinkage cracks may be continuous through the full thickness of a section. Often they are less than 0.2 mm and may seal themselves if autogenous healing (i.e. sufficiently wet) takes place. However, cracks greater than 0.2 mm are unlikely to seal themselves and leakage of water and sound are generally not acceptable. Hence they should be grouted to provide acceptable degree of serviceability. 9.3 Level 3 Cracks

There is no agreed opinion on the importance of crack width (up to 0.5 mm) and its effect on durability. Generally, crack width is limited to a range of 0.1 to 0.4 depending on the service environment. Some studies (Beeby) show that there is no unique correlation between crack width and corrosion of steel bars. In particular, cracks perpendicular to the bars do not lead to significant corrosion beat those parallel to bars are critical. However, there is a danger that deep pitting corrosion may occur when the anode area is small. If funds permit, It is desirable to seal all cracks even if the seal is only partially penetrating the full depth of a crack. This is to increase its durability life even though it may be marginal in effectiveness. 9.4 Level 4 Cracks

These cracks are usually wide and are caused by structural action. They indicate possible overloading in flexure or shear (depending on the nature of the cracks) or settlement of supports. Before they are grouted or repaired by replacement of the spalled concrete, their cause(s) should be established and prevented from re-occurring. 9.5 Repair Techniques and Repair Materials

Repair techniques and repair materials form a specialised aspect of construction. Not much of these have reached codified stage and specialist literature and advice are necessary at this stage of development. However, there are moves to draft standards on requirements for repair materials and codes of practice for methods of repair. For general guidance, rigid type of repair materials should only be used with dormant cracks, i.e. cracks which are unlikely to change in width. For live cracks which may be subject to further movement, flexible repair materials are needed and they should be able to accommodate the expected movement. (cracking/980618)

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REFERENCES ACI Committee 305 (1991), Hot weather concreting, Manual of Concrete Practice, Part 3, 1991, American Concrete Institute, Farmington Hill, Michigan, 1991. Ackroyd, LW and Rodes, FG, An investigation of the crushing strengths of concrete made with three different cements in Nigeria, Journal, ICE, London, February 1964 Bamforth, PB, In-situ measurements of the effect of partial Portland cement replacement using either flyash or ground granulated blastfurnace slag on the performance of mass concrete, Proc. lnstn. Civ. Engrs. (Part 2), September 1980, Vol. 69, pp 777-800. British Standards Institution, Methods of specifying concrete, BS 5328:Part 1: 1997, London, BSI, 1997. British Standards Institution, Structural use of concrete, Part 2, Code of Practice for special circumstances, British Standard BS 8110:Part 2:1985, London, BSI, 1985 Fitzgibbon, ME, Large pours for reinforced concrete structures, Concrete, Vol. 10, No. 3, March 1976, pp 41, Concrete Society, London. Fitzgibbon, ME, Large pours - 2: Heat generation and control, Concrete, Vol. 10, No. 12, December 1976, pp 33-35, Concrete Society, London. Fitzgibbon, ME, Large pours - 3: Continuous casting, Concrete, Vol. 11, No. 2, February 1977, pp 35-36, Concrete Society, London. Harrison, TA, Early age thermal crack control in concrete, Construction Industry Research and Information Association Report 91, London, CIRIA, 1981. Klieger, K, Effect of mixing and curing temperature on concrete strength, ACI Journal, Vol. 54, No. 6, June 1958, pp 1063-1081, American Concrete Institute, Farmington Hill, Michigan, 1958. Kurita, M, Goto, S, Minegishi, K, Negami, Y and Kuwahara, T, Precooling concrete using frozen sand, Concrete International, Vol. 12, No. 6, June 1990, pp 60-65, American Concrete Institute, Farmington Hill, Michigan, 1990. Lee, SL, Tam, CT, Swaddiwudhipong, S and Mani, AC, Temperature distribution and thermal stresses in thick concrete pours, Proc. 3rd Intn. Conf. On Struct. Fail., Singapore, April 1991, pp 17-33, Singapore Concrete Institute, Singapore, 1991

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Neville, AM, Properties of concrete, 4th Ed., Pitman, London, 1997. Powers, TC and Brownyard, Studies of the physical properties of hardened Portland cement paste (9 parts), ACI Journal, Vol. 43, Oct. 1946 to April 1947, American Concrete Institute, Farmington Hill, Michigan, 1946. Price, WH, Factors influencing concrete strength, ACI Journal, Vol. 47, No. 2, February 1951, pp 417-432, American Concrete Institute, Farmington Hill, Michigan, 1951. Quao, HNO, The age-strength relationship of concrete under tropical conditions, Bulletin RILEM, No. 24, September, 1964. Tam, CT, The relationship between strength and maturity of concrete in tropical conditions, Journal Dept. of Civil Engineering, University of Malay, Vol. 7, pp 60-74, Kuala Lumpur, Malaysia, 1968. Tam, CT, Swaddiwudhipong, S, Mani, AC and Lee, SL, Concreting of thick sections in the tropics, Proc. Durable Concrete in Hot Climates, San Juan, October 1992, SP-139, pp 143-155, American Concrete Institute, Farmington Hill, Michigan, 1993. Verbeck, GJ and Helmuth, RH, Structure and physical properties of cement paste, Proc. 5th Intn. Symp. On Chemistry of Cement, Tokyo, Vol. III, pp 1-32, 1968.