Civil engineers are responsible for the production, transport, placing, compacting and curing of fresh concrete. Without adequate attention to all of these the potential hardened properties of the concrete, such as strength and durability, will not be achieved in the finished structural element.
It is important to recognise that it is not sufficient simply to ensure that the concrete is mixed and placed correctly; the behaviour and treatment of the concrete during the period before setting, typically some six to ten hours after casting, and during the first few days of hardening have a significant effect on its long-term performance.
Properties of Fresh Concrete
The concrete from the point of delivery by pump, skip, conveyor belt or wheelbarrow and compaction by internal poker vibrators or external vibratrors clamped on to the formwork. The aim of all of these practices is to produce a homogeneous structure with minimum air voids as efficiently as possible; it is also necessary to ensure that the concrete is then stable and achieves its full, mature properties.
We therefore need to consider the properties when freshly mixed, between placing and setting, and during the early stages of hydration. We will discuss the former in this article, and the latter two in the next article.
Experience in mixing, handling and placing fresh concrete quickly gives concrete workers (and students) a subjective understanding of its behaviour and an ability to recognise ‘good’ and ‘bad’ concrete.
A major problem is that a wide variety of subjective terms are used to describe the concrete, e.g. harsh, cohesive, lean, stiff, rich, which can mean different things to different people and do not quantify the behaviour in any way. However, the main properties of interest can be grouped as follows:
- Fluidity. The concrete must be capable of being handled and of flowing into the formwork and around any reinforcement, with the assistance of whatever equipment is available. For example, concrete for a lightly reinforced shallow floor slab need not be as fluid as that for a tall narrow column with congested reinforcement.
- Compactability. All, or nearly all, of the air entrapped during mixing and handling should be capable of being removed by the compacting system being used, such as poker vibrators.
- Stability or cohesiveness. The concrete should remain as a homogeneous uniform mass throughout. For example, the mortar should not be so fluid that it flows out of or segregates from the coarse aggregate.
The first two of these properties, fluidity and compactability, have traditionally been combined into the general property called workability, but this has now been replaced by the term consistence in some current standards, including those in Europe. We will use the latter term in this book, although the two can be considered synonymous.
Although consistence (or workability) might seem a fairly obvious property, engineers and concrete technologists have struggled since concrete construction became popular early in the last century to produce an adequate definition. Two examples illustrate the difficulty:
- ‘that property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished’ (ACI, 1990)
- ‘that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity’ (ASTM, 1993).
These both relate to the requirements in very general terms only, but the biggest problem is that neither makes any reference to a quantitative measurable property, which engineers need and have for most other properties, e.g. elastic modulus, fracture toughness, etc., etc.
As we will see, the measurement of consistence is by no means straightforward. In general, higher-consistence concretes (however defined or measured) are easier to place and handle, but if higher consistence is obtained, for example, by an increased water content, then a lower strength and/or durability will result if no other changes to the mix are made. The more widespread use of plasticisers and superplasticisers has therefore been a key factor in the trend towards the use of higher-consistence concrete in recent years in many countries.
It is clear that a proper understanding of the fresh properties and the factors that affect them is important. Achieving a balance between consistence and strength is part of the mix design process.
For most concrete about 65 – 80% of the volume consists of fine and coarse aggregate. The remainder is cement paste, which in turn consists of 30 – 50% by volume of cement, the rest being water. Cement paste, mortar and concrete are all therefore concentrated suspensions of particles of varying sizes, but all considerably denser than the mix water.
Surface attractive forces are significant in relation to gravitational forces for the cement particles, but less so for the aggregate particles, where the main resistance to flow comes from interference and friction between them. The behaviour is therefore far from simple.
Measurement of Consistence
Fundamental properties: Rigorous measurement of the flow behaviour of any fluid is normally carried out in a rheometer or viscometer of some sort. We do not have space to describe these, but they apply a shear stress to the fluid and measure its consequent rate of shear, for example in a concentric cylinder viscometer an inner cylinder or bob is rotated in an outer cylinder or cup of the fluid.
Any respectable undergraduate fluid mechanics textbook will describe such instruments, and a test will result in a flow curve of shear stress vs. shear rate. Several such tests have been developed for concrete, involving either a mixing or a shearing action, and for which the apparatus is of sufficient size to cope with coarse aggregate particles of up to 20 mm (RILEM, 2000).
There is general agreement that the behaviour of fresh paste, mortar and concrete all approximate reasonably closely to the Bingham model illustrated in Fig. 1.
Flow only starts when the applied shear stress reaches a yield stress (ty) sufficient to overcome the interparticle interference effects, and at higher stresses the shear rate varies approximately linearly with shear stress, the slope defining the plastic viscosity (µ). Thus two constants, ty and µ, are required to define the behaviour, unlike the simpler and very common case of a Newtonian fluid that does not have a yield stress, and which therefore requires only a single constant, viscosity.
Because at least two data points are required to define the flow curve, the first satisfactory test that was devised to measure this on concrete was called the twopoint workability test (Tattersall and Banfill, 1983; Tattersall, 1991; Domone at al., 1999).
Single-point tests: A large number of simple but arbitrary tests for consistence or workability have been devised over many years, some only being used by their inventors. These all measure only one value, and can therefore be called single-point tests. Four are included in European standards, and have also been adopted elsewhere, and are therefore worth considering in some detail. The simplest, and crudest, is the slump test (BS EN12350-2, Fig. 2).
The concrete is placed in the frustum of a steel cone and hand compacted in three successive layers. The cone is lifted off, and slump is defined as the downward movement of the concrete.
A true slump, in which the concrete retains the overall shape of the cone and does not collapse, is preferred, which gives a limit to the slump measurement of about 180 mm. A shear slump invalidates the test, and may indicate a mix prone to segregation owing to lack of cement or paste.
A collapsed slump is not ideal, but the trend mentioned above of the increasing use of high-consistence mixes, which produce collapsed slumps with little or no segregation, means that slump values up to, and even above, 250 mm are considered valid in many standards. For such very high consistence mixes an alternative is to measure the final diameter or ‘flow’ of the concrete, which is more sensitive to changes in the mix than the change in height.
Indeed, for selfcompacting mixes the slump-flow test is carried out without any initial compaction when filling the cone. As a general guide, mixes with slumps ranging from about 10 mm upwards can be handled with conventional site equipment, with higher slumps (100 mm and above) being more generally preferred and essential to ensure full compaction of the concrete in areas with limited access or congested reinforcement.
However, some zero-slump mixes have sufficient consistence for some applications. The degree of compactability test (BS EN 12350-4, Fig. 3), which has replaced the compacting factor test in many standards, is able to distinguish between low-slump mixes.
A rectangular steel container is filled with concrete by allowing it to drop from a trowel under its own weight from the top of the container. It is therefore only partially compacted. The concrete is then compacted, e.g. by vibration, and its final height measured. The difference between the initial and final heights is a measure of the amount of compaction the concrete undergoes when loaded into the container, and will be lower with high consistence concrete.
The degree of compactability is defined as the ratio of the initial height to the final height. Values over 1.4 indicate a very low consistence, and as the consistence increases the value gets closer and closer to 1.
In the Vebe test (BS EN 12350-3, Fig. 4), the response of the concrete to vibration is determined. The Vebe time is defined as the time taken to completely remould a sample of concrete from a slump test carried out in a cylindrical container.
Standard vibration is applied, and remoulding times from 1 to about 25 seconds are obtained, with higher values indicating lower consistence. It is often difficult to define the end-point of complete remoulding with a sufficient degree of accuracy. The flow table test (BS EN 12350-5, Fig. 5) was devised to differentiate between high consistence mixes. It is essentially a slump test with a lower volume of concrete in which, after lifting the cone, some extra work is done on the concrete by lifting and dropping one edge of the board (or table) on which the test is carried out.
A flow or spread of 400 mm indicates medium consistence, and 500 mm or more high consistence. Apart from only giving a single test value, these four tests (or five if we consider the slump-flow test to be distinct from the slump test) all measure the response of the concrete to specific, but arbitrary and different, test conditions.
The slump, slump flow and flow table tests provide a measure of the fluidity or mobility of the concrete; the slump test after a standard amount of compaction work has been done on the concrete, the slump-flow test after the minimal amount of work of pouring into the cone, and the flow table test with a combination of compaction work and energy input.
The degree of compactability test assesses the response of the concrete to applied work, but the amount of work done in falling from the top of the container is much less than the energy input from practical compaction equipment such as a poker vibrator.
The Vebe test comes closest to assessing the response to realistic energy levels, but it is the most difficult test to carry out and it is not able to distinguish between the types of high-consistence mixes that are becoming increasingly popular. There is some degree of correlation between the results of these tests, as illustrated in Fig. 6, but as each of the tests measures the response to different conditions the correlation is quite broad.
It is even possible for the results to be conflicting – e.g. for say the slump test to show that Mix A has a higher consistence than Mix B, and for the degree of compactability test to give the opposite ranking. The result therefore depends on the choice of test, which is far from satisfactory.
The slump and slump-flow tests clearly involve very low shear rates, and therefore, not surprisingly, reasonable correlations are obtained with yield stress (e.g. Fig. 7). No correlation is obtained with plastic viscosity. The test therefore indicates the ease with which the concrete starts to flow, but not its behaviour thereafter.
Despite their limitations, single-point tests, particularly the slump test and, to a somewhat lesser extent, the flow table and slump-flow tests, are popular and in regular use, both for specification and for compliance testing of the concrete after production.
Perhaps the main reason for this is their simplicity and ease of use both in the laboratory and on site, but specifiers and users must be aware of the potential pitfalls of over-reliance on the results. 18.3 Factors affecting consistence Lower values of yield stress (ty) and plastic viscosity (µ) indicate a more fluid mix; in particular, reducing ty lowers the resistance to flow at low shear stresses, e.g. under self-weight when being poured, and reducing m results in less cohesive or ‘sticky’ mixes and increased response during compaction by vibration, when the localised shear rates can be high.
Some of the more important effects of variation of mix proportions and constituents on ty and µ, shown schematically in Fig. 8, are as follows:
- Increasing the water content while keeping the proportions of the other constituents constant decreases ty and m in approximately similar proportions.
- Adding a plasticiser or superplasticiser decreases ty but leaves m relatively constant. In essence, the admixtures allow the particles to flow more easily but in the same volume of water. The effect is more marked with superplasticisers, which can even increase m, and can therefore be used to give greatly increased flow properties under self-weight, while maintaining the cohesion of the mix. This is the basis for a whole range of high-consistence or flowing concretes.
- Increasing the paste content will normally increase µ and decrease ty i.e. the mix may start to flow more easily but will be more cohesive or ‘stickier’, and vice versa.
Replacing some of the cement with fly ash or ggbs will generally decrease ty, but may either increase or decrease m, depending on the nature of the addition and its interaction with the cement.
- The small bubbles of air produced by air-entraining agents provide lubrication to reduce the plastic viscosity, but at relatively constant yield stress. An important consequence of these considerations is that yield stress and plastic viscosity are independent properties, and different combinations can be obtained by varying the mix constituents and their relative proportions. There is a great deal of information available on the effect of mix constituents and proportions onconsistence measurements using single-point tests, particularly slump. Many mix design methods take as their first assumption that, for a given aggregate type and size, slump is a direct function of the water content. This is very useful and reasonably accurate – other factors such as cement content and aggregate grading are of secondary importance for slump, but are of greater importance for cohesiveness and stability. The effectiveness of admixtures, particularly plasticisers and superplasticisers, is also often given in terms of slump.
Loss of Consistence
Although concrete remains sufficiently workable for handling and placing for some time after it has been mixed, its consistence continually decreases. This is due to:
- mix water being absorbed by the aggregate if this is not in a saturated state before mixing
- evaporation of the mix water
- early hydration reactions (but this should not be confused with cement setting)
- interactions between admixtures (particularly plasticisers and superplasticisers) and the cementitious constituents of the mix.
Absorption of water by the aggregate can be avoided by ensuring that saturated aggregate is used, for example by spraying aggregate stockpiles with water and keeping them covered in hot/dry weather, although this may be difficult in some regions. Evaporation of mix water can be reduced by keeping the concrete covered during transport and handling as far as possible.
Most available data relate to loss of slump, which increases with higher temperatures, higher initial slump, higher cement content and higher alkali and lower sulphate content of the cement.
At an ambient temperature of 20°C, slump may reduce to about half its initial value in two hours, but the loss is more acute, and can have a significant effect onconcrete operations, at ambient temperatures in excess of 30°C. The rate of loss of consistence can be reduced by continued agitation of the concrete, e.g. in a ready-mix truck, or modified by admixtures, particularly retarders.
In hot weather, the initial concrete temperature can be reduced by cooling the constituents before mixing (adding ice to the mix water is a common practice) and the concrete can be transported in cooled or insulated trucks. In principle, re-tempering, i.e. adding water to compensate for slump loss, should not have a significant effect on strength if only that water that has been lost by evaporation is replaced.
Also, studies have shown that water can be added during retempering to increase the initial water:cement ratio by up to 5% without any loss in 28-day strength (Cheong and Lee 1993). However, except in very controlled circumstances, retempering can lead to an unacceptably increased water:cement ratio and hence lower strength, and is therefore best avoided.
Early Age Properties of Concrete
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.
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.