HARDENING CONCRETE EARLY THERMAL CRACKING

The reaction of cement with water - hydration - is a chemical reaction that produces heat. If this development of heat exceeds the rate of heat loss, the temperature of the concrete will rise. Subsequently the concrete will cool and contract. Typical temperature histories of some concrete sections are shown in Figure 7.
 
If the contraction were unrestrained there would be no cracking. However, in practice there is always some form of restraininducing tension and hence a risk of cracking. Restraint occurs due to either external or internal influences.

External restraint
Concrete is externally restrained if, for example, it is cast onto a previously hardened base, such as a wall kicker, or if it is cast between two already hardened sections, such as an infill bay in a wall or slab, without the provision of a contraction joint.

Internal restraint
The surfaces of an element of concrete will cool faster than the core, producing a temperature differential, and when this differential is large, such as in thick sections, cracks may develop at the surface. In general, it has been found that by restricting the temperature differential to around 20°C between the core and the surface, little or no cracking will result.

Factors affecting temperature rise
The main factors that affect the rise in temperature are discussed below.

Dimensions Thicker sections retain the heat generated, and will have higher peak temperatures and cool down more slowly. While peak temperatures increase with increasing thickness, above thicknesses of about 1.5 m there is little further increase in temperature.

Cement or combination content The heat generated is directly related to the cement content. For Portland cement concretes in sections of 1 m thickness and more, the temperature rise in thcore is likely to be about 14°C for every 100 kg/m3 of cement. Thinner sections will exhibit lower temperature rises than this.

Cement type Different cement types generate heat at different rates. The peak temperature and the total amount of heat produced by hydration depend upon both the fineness and the chemistry of the cement. As a guide, those cements whosstrength develops most rapidly tend to produce most heat. Sulfate- resisting cement generally gives off less heat than CEM I 42,5and cements that are interground or combined with mineral additions such as pfa or ggbs are often chosen for massive construction because they have the lowest heat of hydration.

Initial temperature of the concrete A higher initial temperature of the concrete results in a greater temperature rise: for example, concrete placed at 10°C in a 500 mm thick section may have a temperature rise of 30°C, whereas the same concrete placed at 20°C may have a temperature rise of 40°C.

Ambient temperature  In cooler weather there is likely to be a greater differential between peak and ambient temperatures, i.e. greater cooling and contraction. During hot weather concrete will develop a high peak temperature but the differential may be lower.

Type of formwork  Steel and CRP formwork will allow the heat generated to be dissipated more quickly than will timber formwork, which acts as an insulating layer. Timber formworand / or additional insulation will reduce the temperature differential between the core and the surfaces.

Admixtures Retarding water-reducers delay the onset of hydration and heat generation but do not reduce the total heat generated. Accelerating water-reducers increase the rate of heat evolution and increase the temperature rise.

The problem of early thermal cracking is usually confined to slabs over about 500 mm thick and to walls of all thicknesses. Walls are particularly susceptible because they are often lightly reinforced in the horizontal direction and the timber formwork tends to act as a thermal insulator, thus encouraging a larger temperature rise. The problem may be reduced by a lower cement content, the use of a cement with a lower heat of hydration or one containing ggbs or pfa. There is a practical and economic limit to these measures,

often dictated by the specification requirements for strength and durability of the concrete itself.

In practice, cracking due to external restraint is best controlled by the provision of crack control reinforcement and the spacing of contraction joints, which should be determined by the designer. It should be noted that reinforcement does not prevent crack formation, although it does control the widths of cracks, and with enough of the right reinforcement, cracks will be fine enough so as not to cause leakage or affect durability. With very thick sections, with very little external restraint, the temperature differential can usually be reduced by insulating, and thereby keeping warm, the surfaces of the concrete for a few days.

Plastic  cracking
There are two types of plastic cracks: plastic settlement cracks,
which may develop in deep sections and often follow the pattern of the reinforcement; and plastic shrinkage cracks, which are more likely to develop on slabs. Both types form while the concrete is still in its plastic state, before it has set or hardened and, depending on the weather conditions, form within about one to six hours aftethe concrete has been placed and compacted. They are often not noticed until the following day. Both types of crack are related to the extent to which the fresh concrete bleeds.

Bleeding of concrete
Fresh concrete is a suspension of solids in water, and after it has been compacted there is a tendency for the solids (both the aggregates and the cement) to settle. This sedimentation displaces the water, which is pushed upwards, and, if the process is excessive, the water appears as a layer on the surface. This bleewater may not always be seen, as it may evaporate on hot or windy days faster than it rises to the surface. The tendency of a concretto bleed is affected by the materials and their proportions. Bleeding can generally be reduced by increasing the cohesiveness of the concrete by one or more of the following means:
n Increasing the cement content
n Increasing the sand content
n Using finer sand
n Using  less water
n Air-entrainment
n Using a rounded natural sand rather than an angulacrushed one.
The rate of bleeding will be influenced by drying conditions, especially wind, and bleeding will take place for longer on cold days. Similarly, due to the slower stiffening rate of the concrete, concrete containing a retarder has a tendency to bleed for a longer period of time and their use will, in general, increase the risk of plastic cracking.

Plastic settlement cracks
Plastic settlement cracks are caused by differential settlement and are directly related to the amount of bleeding. They tend to occur in deep sections, particularly deep beams, but they may also develop in columns and walls. This is because the deeper the section the more sedimentation or settlement that can take place. However, cracks will form only where something prevents the concrete 'solids' from settling freely. The most common cause of this is the reinforcing steel fixed at the top of deep sections; the concrete will be seen to 'break its back' over this steel and the pattern of cracks will directly reflect the layout of the steel below (Figure 8).

Settlement cracks may also occur in trough and waffle slabs (Figure 9) or at any section where there is a significant change in the depth of concrete.

If alterations to the concrete, particularly the use of an air- entraining or water-reducing admixture, cannot be made due to contractual or economic reasons, the most effective way of eliminating plastic settlement cracking is to re-vibrate the concrete after the cracks have formed. Such re-vibration is acceptable provided the concrete is still plastic enough to be capable of bein'fluidized' by a poker, and yet not so stiff that a hole is left when the poker is withdrawn. The timing will depend on the weather.

Plastic shrinkage cracks
These cracks occur in horizontal slabs, such as floors and roads. They usually take the form of one or more diagonal cracks at 0.to 2 m centres  that do not extend to the slab edges, or they form a very large pattern of map cracking. Plastic shrinkage cracks such as those shown in Figure 10 do not usually increase in length or width with the passage of time and seldom have a detrimental effect on the load-bearing capability of suspended slabs or on the carrying capacity of roads. They may occur in both reinforced and non-reinforced slabs.

Plastic shrinkage cracks are most common in concrete placed on hot or windy days because they are caused by the rate of evaporation of moisture from the surface exceeding the rate of bleeding.

It has been found that air-entrainment almost eliminates the risk of plastic shrinkage cracks developing.

Clearly, plastic shrinkage cracks can be reduced by preventing the loss of moisture from the surface of the concrete in the critical first few hours. While sprayed-on resin-based curing compounds are very efficient at curing concrete that has already hardened, they cannot be applied to fresh concrete until the free bleed water has evaporated. This is too late to prevent plastic shrinkage cracking,

Figure 11 :  Polythene sheeting supported clear of a concrete slab by means of blocks and timber. Note that all the edges of the polythene are held down to prevent a wind-tunnel effect.

Remedial measures
The main danger resulting from plastic cracking is the possible ingress of moisture leading to the corrosion of reinforcement. With both plastic settlement and plastic shrinkage cracks, if the affected surface will be protected subsequently either by more concrete or by a screed, no treatment is usually necessary.

Often the best repair is simply to brush dry cement (dampened down later) or wet grout into the cracks the day after they form and while they are still clean; this encourages natural or autogenous healing.


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