Successful placing of concrete is not enough. It is necessary to ensure that it comes through the first few days of its life without mishap, so that it goes on to have the required mature properties. Immediately after placing, before the cement’s initial set the concrete is still in a plastic and at least semi-fluid state, and the component materials are relatively free to move.
Between the initial and final set, it changes into a material which is stiff and unable to flow, but which has no strength. Clearly it must not be disturbed during this period. After the final set hardening starts and the concrete develops strength, initially quite rapidly.
Early Age Properties of Concrete
In this article we will discuss the behaviour of concrete during each of these stages and how they affect construction practice. The hydration processes and the timescales involved and their modification by admixtures and cement replacement materials have been described in past articles. In particular, we discussed the exothermic nature of the hydration reactions, and we will see that this has some important consequences.
Behaviour After Placing
The constituent materials of the concrete are of differing relative particle density (cement 3.15, normal aggregates approx. 2.6 etc.) and therefore while the concrete is in its semi-fluid, plastic state the aggregate and cement particles tend to settle and the mix water has a tendency to migrate upwards.
This may continue for several hours, until the time of final set and the onset of strength gain. Inter-particle interference reduces the movement, but the effects can be significant. There are four interrelated phenomena – bleeding, segregation, plastic settlement and plastic shrinkage.
Segregation and bleeding: Segregation involves the larger aggregate particles falling towards the lower parts of the pour, and bleeding is the process of the upward migration or upward displacement of water. They often occur simultaneously (Fig. 1).
The most obvious manifestation of bleeding is the appearance of a layer of water on the top surface of concrete shortly after it has been placed; in extreme cases this can amount to 2% or more of the total depth of the concrete. In time this water either evaporates or is re-absorbed into the concrete with continuing hydration, thus resulting in a net reduction of the concrete’s original volume. This in itself may not be of concern, but there are two other effects of bleeding that can give greater problems, illustrated in Fig. 1.
Firstly, the cement paste at or just below the top surface of the concrete becomes water rich and therefore hydrates to a weak structure, a phenomenon known as surface laitance. This is a problem in, for example, floor slabs, which are required to have a hard-wearing surface. Secondly, the upward migrating water can be trapped under aggregate particles, causing a local enhanced weakening of the transition or interface zone between the paste and the aggregate, which may already be a relatively weak part of the concrete, and hence an overall loss of concrete strength.
However, in most concrete some bleed may be unavoidable, and may not be harmful. The combined effects of bleed and particle settlement are that after hardening the concrete in the lower part of a pour of any significant depth is stronger than that in the upper part, possibly by 10% or more, even with a cohesive and well produced concrete.
Plastic settlement: Overall settlement of the concrete will result in greater movement in the fresh concrete near the top surface of a deep pour. If there is any local restraint to this movement from, say, horizontal reinforcing bars, then plastic settlement cracking can occur, in which vertical cracks form along the line of the bars, penetrating from the surface to the bars (Fig. 2).
Plastic shrinkage: Bleed water arriving at an unprotected concrete surface will be subject to evaporation; if the rate of evaporation is greater than the rate of arrival of water at the surface, then there will be a net reduction in water content of the surface concrete, and plastic shrinkage, i.e. drying shrinkage while the concrete is still plastic, will occur.
The restraint of the mass of concrete will cause tensile strains to be set up in the near-surface region, and as the concrete has near-zero tensile strength, plastic shrinkage cracking may result (Fig. 19.3). The cracking pattern is a fairly regular ‘crazing’ and is therefore distinctly different from the oriented cracks resulting from plastic settlement.
Any tendency to plastic shrinkage cracking will be encouraged by greater evaporation rates of the surface water, which occurs, for example, with higher concrete temperature or ambient temperature, or if the concrete is exposed to wind.
Methods of reducing segregation and bleed and their effects: A major cause of excessive bleed is the use of a poorly graded aggregate, a lack of fine material below a particle size of 300 mm being most critical. This can be remedied by increasing the sand content, but if this is not feasible for some reason, or if a particularly coarse sand has to be used, then air entrainment can be an effective substitute for the fine particles.
Higher bleeds may also occur with higher consistence mixes, and if very high consistence is required it is preferable to use superplasticisers rather than high water contents. Microsilica, with its very high surface area, is also an effective bleed-control agent.
Bleed, however, cannot be entirely eliminated, and so measures must be taken in practice to reduce its effects if these are critical. Plastic settlement and plastic shrinkage cracks that occur soon after placing the concrete can be overcome by re-vibrating the surface region, particularly in large flat slabs.
All concretes, no matter how great or small their tendency to bleed, must be protected from moisture loss from as soon after placing as possible, and for the first few days of hardening. This will not only reduce or eliminate plastic shrinkage cracking, but also ensure that there is an adequate supply of water for continued hydration and strength gain.
This protection is called curing, and is an essential part of any successful concreting operation, although often overlooked. Curing methods include:
- spraying or ponding the surface of the concrete with water
- protecting exposed surfaces from wind and sun by windbreaks and sunshades
- covering surfaces with wet hessian and/or polythene sheets
- applying a curing membrane, usually a spray applied resin seal, to the exposed surface; this prevents moisture loss, and weathers away in a few weeks. Extended periods of curing are required for mixes that gain strength slowly, such as those containing additions, particularly fly ash and ggbs, and in conditions of low ambient temperature.
Strength Gain and Temperature Effects
Effect of temperature: The hydration reactions between cement and water are temperature dependent and their rate increases with curing or storage temperature. The magnitude of the effect on the development of strength for concrete continuously stored at various temperatures at ages of up 28 days is apparent from Fig. 4.
There is, however, evidence that at later ages higher strengths are obtained from concrete cured at lower temperatures, perhaps by as much as 20% for concrete stored at 5°C compared to that at 20°C (Klieger, 1958). Explanations for this behaviour have been conflicting, but it would seem that, as similar behaviour is obtained with cement paste, the C-S-H gel more rapidly produced at higher temperatures is less uniform and hence weaker than that produced at lower temperatures.
There also appears to be an optimum temperature for maximum long-term strength of between 10 and 15°C, although this varies with the type of concrete. The hydration reactions do still proceed at temperatures below the freezing point of water, 0°C. In fact they only cease completely at about -10°C.
However, the concrete must only be exposed to such temperatures after a significant amount of the mix water has been incorporated in the hydration reactions, since the expansion of free water on freezing will disrupt the immature, weak concrete. A degree of hydration equivalent to a strength of 3.5 MPa is considered sufficient to give protection against this effect.
Maturity: Temperature effects such as those shown in Fig. 4 have led to the concept of the maturity of concrete, defined as the product of time and curing temperature, and its relationship to strength. For the reasons given above, –10°C is taken as the datum point for temperature, and hence:
maturity = ∑t(T + 10) …….(1)
where t and T are the time (normally in hours or days) and curing temperature (in °C), respectively.
Figure 5 shows the relationship between strength and maturity for concrete with three water:cement ratios. These results were obtained with each mix being cured at 4, 13 and 21°C for periods of up to 1 year; the results for each mix fall on or very near to the single lines shown, thus demonstrating the usefulness of the maturity approach.
If the temperature history of a concrete is known, then its strength can be estimated from the strength– age relationship at a standard curing temperature (e.g. 20°C).
Figure 5 shows that over much of the maturity range:
strength = a + b log10(maturity) ……(2)
which is a convenient relationship for estimating strength. The constants a and b will be different for different mixes and will generally need to be established experimentally.
A slightly different approach is to express the maturity as being equivalent to a certain number of days at the standard curing temperature of control cubes (normally 20°C). On this basis, for example, a maturity of 1440°C hrs has an equivalent age of 3 days at 20°C. Equation 1 then becomes
equivalent age at 20°C = ∑kt …….(3)
where k is the maturity function. Various forms for this function have been proposed, as summarised by Harrison (2003).
Heat of hydration effects: As well as being temperature dependent, the hydration of cement is exothermic. The opposite extreme to the isothermal condition is adiabatic (i.e. perfect insulation or no heat loss), and in this condition the exothermic reactions result in heating of a cement paste, mortar or concrete. This leads to progressively faster hydration, rate of heat output and temperature rise, the result being substantial temperature rises in relatively short times (Fig. 6).
The temperature rise in concrete is less than that in cement paste as the aggregate acts as a heat sink and there is less cement to react. An average rise of 13°C per 100 kg of cement per m3 of concrete has been suggested for typical structural concretes.
When placed in a structure, concrete will lose heat to its surrounding environment either directly or through formwork, and it will therefore not be under truly adiabatic or isothermal conditions, but in some intermediate state. This results in some rise in temperature within the pour followed by cooling to ambient.
Typical temperature–time profiles for the centre of pours of varying depths are shown in Fig. 7; it can be seen that the central regions of a pour with an overall thickness in excess of about 1.5 – 2 m will behave adiabatically for the first few days after casting.
Such behaviour has two important effects. First, the peak temperature occurs after the concrete has hardened and gained some strength and so the cool down will result in thermal contraction of the concrete, which if restrained will result in tensile stresses that may be sufficiently large to crack the concrete.
Restraint can result from the structure surrounding the concrete, e.g. the soil underneath a foundation, or from the outer regions of the concrete pour itself, which will have been subject to greater heat losses, and therefore will not have reached the same peak temperatures, or from reinforcement within the concrete. The amount of restraint will obviously vary in different structural situations.
As an example, a typical coefficient of thermal expansion for concrete is 10 × 10-6 per C degree, and therefore a thermal shrinkage strain of 300 × 10-6 would result from a cool down of 30°C.
Taking a typical elastic modulus for the concrete of 30 GPa, and assuming complete restraint with no relaxation of the stresses due to creep, the resulting tensile stress would be 9 MPa, well in excess of the tensile strength of the concrete, which would therefore have cracked.
Rigorous analysis of the thermal strains and the consequent stresses is complex but in structural concrete, control of the likelihood and consequences of any cracking can be obtained by design of the reinforcement system and in pours of any substantial size to limit the temperature differentials. Insulation by way of increased formwork thickness or thermal blankets will have some beneficial effect but, more commonly, or in addition, low heat mixes are used.
If strength or durability criteria mean that a sufficiently low cement content cannot be used, then either a low-heat Portland cement can be used or, more conveniently, the use of additions; fly ash or ground granulated blast furnace slag (ggbs) are effective solutions, as shown in Fig. 8.
As alternative or additional measures, the temperature of the fresh concrete can be reduced by pre-cooling the mix water or the aggregates, or by injecting liquid nitrogen.
Second, much of the concrete will have hydrated for at least a few days after casting at temperatures higher than ambient, and the long-term strength may therefore be reduced, owing to the effects described above. Typical effects of this on the development of strength are shown in Fig. 19.9. By comparing Fig. 19.9a and Fig. 19.9b it can be seen that fly ash and ggbs mixes do not suffer the same strength losses as 100% Portland cement mixes.
Measurement of the concrete’s properties after being subjected to such ‘temperature-matched curing’ is therefore extremely important if a full picture of the in-situ behaviour is to be achieved.
Thanks for reading about “early age properties of concrete.”