Hardened cement paste (HCP) formed from the hydration of mixtures of Portland cement, admixtures and additions has strength and other properties that could make it suitable for use as a construction material in its own right. However, it suffers from two main drawbacks – high dimensional changes, in particular low modulus, high creep and shrinkage, and cost.
Both of these disadvantages are overcome, or at least modified, by adding aggregates to the cement paste, thus producing concrete. The objective is to use as much aggregate as possible, binding the particles together with the HCP. This means that:
- the largest possible aggregate size consistent with the mixing, handling and placing requirements of fresh concrete should be used
- a continuous range of particle sizes from fine sand up to coarse stones is desirable; this minimises the void content of the aggregate mixture and therefore the amount of HCP required, and helps the fresh concrete to flow more easily. Normally aggregates occupy about 65–80% of the total concrete volume.
With one or two notable exceptions, aggregates can be thought of as being inert fillers; for example, they do not hydrate, and they do not swell or shrink. They are distributed throughout the HCP, and it is sometimes useful to regard concrete as a two-phase material of either coarse aggregate dispersed in a mortar matrix, or coarse and fine aggregate dispersed in an HCP matrix.
Aggregates for Concrete
Models based on this two-phase material are of value in describing deformation behaviour, but, when cracking and strength are being considered, a threephase model of aggregate, HCP and the transition or interfacial zone between the two (about 30 – 50 mm wide) is required, since the transition zone can have a significantly different microstructure from the rest of the HCP, and is often the weakest phase and the source of cracks as applied stress increases. There are three general types or groups of aggregate depending on their source:
- primary, which are specifically produced for use in concrete
- secondary, which are by-products of other industrial processes not previously used in construction
- recycled, from previously used construction materials e.g. from demolition.
Primary aggregates form by far the greatest proportion of those used and so we will concentrate on discussing the sources, properties and classification of these.
Types of Primary Aggregate
These can either be obtained from natural sources, such as gravel deposits and crushed rocks, or be specifically manufactured for use in concrete. It is convenient to group them in terms of their relative density.
Normal-density aggregates: Many different natural materials are used for making concrete, including gravels, igneous rocks such as basalt and granite and the stronger sedimentary rocks such as limestone and sandstone. They should be selected to have sufficient integrity to maintain their shape during concrete mixing and to be sufficiently strong to withstand the stresses imposed on the concrete.
Stress concentration effects within the concrete result in local stresses at aggregate edges about three times greater than the average stress on the concrete, and so the aggregates should have an inherent compressive strength about three times greater than the required concrete strength if they are not to crack before the HCP. This becomes a particular consideration with high-strength concrete. Provided that the mechanical properties are acceptable the mineral constituents are not generally of great importance, notable exceptions being those that can participate in alkali–silica reactions and in the thaumasite form of sulphate attack.
All of the above rock types have relative densities within a limited range of approximately 2.55 – 2.75, and therefore all produce concretes with similar densities, normally in the range 2250 – 2450 kg/m3 , depending on the mix proportions.
Gravels from suitable deposits in river valleys or shallow coastal waters have particles that for the most part are of a suitable size for direct use in concrete, and therefore only require washing and grading, i.e. subdividing into various sizes, before use. Bulk rock from quarries, e.g. granites and limestones, require crushing to produce suitably sized material.
The particles are therefore sharp and angular and distinctly different from the naturally more rounded particles in a gravel; we will see in later chapters that particle shape has a significant effect on fresh and hardened concrete properties.
Lightweight Aggregate: Lightweight aggregates are used to produce lowerdensity concretes, which are advantageous in reducing the self-weight of structures and also have better thermal insulation than normal-weight concrete. The reduced relative density is obtained from air voids within the aggregate particles.
Heavyweight aggregates: Where concrete of high density is required, for example in radiation shielding, heavyweight aggregates can be used. These may be made with high-density ores such as barytres and haematite, or manufactured, such as steel shot.
Aggregate classification – shape and size
Within each of the types described above, aggregates are classified principally by shape and particle size. Normal-density aggregates in particular may contain a range of particle shapes, from well rounded to angular, but it is usually considered sufficient to classify the aggregate as uncrushed, i.e. coming from a natural gravel deposit, with most particles rounded or irregular, or crushed, i.e. coming from a bulk source, with all particles sharp and angular (Fig. 1).
The principal size division is that between fine and coarse aggregate at a particle size of 4 mm (although some countries divide at 5, 6 or 8 mm). Coarse aggregate can have a maximum size of 10, 16, 20, 32 or 40 mm (although, again, some countries use different values).
In Europe, the size is described by designation d/D, where d is the smallest nominal particle size and D the nominal largest. We say ‘nominal’ because in practice a few particles may be a smaller than d and a few a little larger than D. Thus:
- 0/4 is a fine aggregate with a maximum particle of 4 mm (with the ‘0’ indicating a near zero lower size limit)
- 4/20 is a coarse aggregate with a minimum particle size of 4 mm and a maximum particle size of 20 mm
- 10/20 is a coarse aggregate with a minimum particle size of 10 mm and a maximum particle size of 20 mm.
The distribution of particle sizes within each of these major divisions is also important both for classification and for determining the optimum combination for a particular mix.
To determine this, a sieve analysis is carried out using a series of standard sieves with, in European practice, apertures ranging from 0.063 to 63 mm, each sieve having approximately twice the aperture size of the previous one, i.e. in the geometric progression 0.063, 0.125, 0.25, 0.5, 1, 2, 4, 8, 16, 32 and 63 mm. Some countries also use supplementary sizes in the coarse aggregate range, e.g. 10, 20 and 37.5 (40) mm in the UK.
The analysis starts with drying and weighing a representative sample of the aggregate, and then passing this through a stack or nest of the sieves, starting with that with the largest aperture.
The weights of aggregate retained on each sieve are then measured. These are converted first to percentage retained and then to cumulative, i.e. total, percent passing, which are then plotted against the sieve size to give a grading curve or particle-size distribution.
Standards for aggregate for use in concrete contain limits inside which the grading curves for coarse and fine aggregate must fall. In the European standard (BS EN 12620:20021 ), these are given in terms of the required percentage passing sieves with various ratios of D and d; Table 1 gives examples of the values for coarse and fine aggregate from this standard.
For fine aggregate the definition of some intermediate values gives a useful addition to these overall limits when considering their use in concrete. The European standard suggests using the percentage passing the 0.5-mm sieve (called the P value), and give ranges of:
- for coarse graded fine aggregate, CP = 5 – 45%
- for medium graded fine aggregate, MP = 30 – 70%
- for fine graded fine aggregate, FP = 55 – 100%.
The overlap of the limits means that it is possible for an aggregate to fall into two classes – which can cause confusion. The grading curves for the mid-points of the ranges for the most commonly used aggregates grades are plotted in Fig. 2. A single number, the fineness modulus, is sometimes calculated from the results of the sieve analysis.
The cumulative percent passing figures are converted to cumulative percent retained, and the fineness modulus is defined as the sum of all of these starting with that for the 125 mm sieve and increasing in size by factors of two, divided by 100. A higher fineness modulus indicates a coarser material; the values for the grading curves in Fig. 2 are given in Table 17.2.
It is important to remember that the calculation is carried out only with those sieves in the geometric progression, not intermediate sizes, and that for coarse aggregate with all particles larger than, say, 4 mm the cumulative percent retained on all sieves smaller than 4 mm should be entered as 100.
During the process of mix design, the individual subdivisions or fractions of aggregates are combined in proportions to give a suitable overall grading for good concrete consistence and stability. This should be continuous and uniform.
Examples for maximum coarse aggregates sizes of 10, 20 and 40 mm that are produced by the mix design process are shown in Fig. 3. These result from using aggregates with ideal gradings; in practice it is normally not possible to achieve these exactly, but they are good targets.
Sieve analysis and grading curves take no account of particle shape, but this does influence the voids content of the aggregate sample – more-rounded particles will pack more efficiently and will therefore have a lower voids content. According to Dewar (1999) it is sufficient to use only three numbers to characterise an aggregate for mix design purposes – specific gravity (or particle relative density); mean particle size; and voids content in the loosely compacted state. We should also mention here the bulk density.
This is the weight of aggregate occupying a unit overall volume of both the particles and the air voids between them. It is measured by weighing a container of known volume filled with aggregate. The value will clearly depend on the grading, which will govern how well the particles fit together, and also on how well the aggregate is compacted. Unlike the relative particle density, which is more useful, it is not therefore a constant for any particular aggregate type.
Other Properties of Aggregates
It is important that aggregates are clean and free from impurities such as clay particles or contaminants that would affect the fresh or hardened properties of the concrete. Other properties that influence their suitability for use in concrete include porosity and absorption, elasticity, strength and surface characteristics.
Porosity and absorption: All aggregates contain pores, which can absorb and hold water. Depending on the storage conditions before concrete mixing, the aggregate can therefore be in one of the four moisture conditions shown in Fig. 4.
In the freshly mixed concrete, aggregate that is in either of conditions (1) or (2) will absorb some of the mix water, and aggregate in condition (4) will contribute water to it. Condition (3), saturated surface dry, is perhaps most desirable, but is difficult to achieve except in the laboratory. It also leads to the definition of the absorption of an aggregate:
Absorption (% by weight) = 100(w2 – w1)/w1
where w1 is weight of a sample of aggregate in the completely (oven) dry state and w2 is the weight in the saturated surface dry state.
Clearly, the absorption is related to the porosity of the aggregate particles. Most normal weight aggregates have low but nevertheless significant absorptions in the range 1 – 3%. Of prime importance to the subsequent concrete properties is the amount of water available for cement hydration, i.e. the amount that is non-absorbed or ‘free’; therefore, to ensure that the required free water:cement ratio is obtained, it is necessary to allow for the aggregate moisture condition when calculating the amount of water to be added during concrete mixing.
If the aggregate is drier than saturated surface dry, extra water must be added; if it is wetter, then less mix water is required. Elastic properties and strength 17.3.2 Since the aggregate occupies most of the concrete volume, its elastic properties have a major influence on the elastic properties of the concrete.
Normal-weight aggregates are generally considerably stronger than the HCP and therefore do not have a major influence on the strength of most concretes. However, in high-strength concrete (with compressive strengths in excess of, say, 80 MPa) careful aggregate selection is important.
There are a number of tests used to characterise the strength and other related properties of aggregates – such as abrasion resistance – that may be important for particular uses of the concrete. A look at any typical aggregate standard will lead you to these.
Surface characteristics: The surface texture of the aggregate depends on the mineral constituents and the degree to which the particles have been polished or abraded. It seems to have a greater influence on the flexural strength than on the compressive strength of the concrete, probably because a rougher texture results in better adhesion to the HCP.
This adhesion is also greatly affected by the cleanliness of the surface – which must therefore not be contaminated by mud, clay or other similar materials. The interface or transition zone between the aggregate surface and the HCP has a major influence on the properties of concrete, particularly its strength.
In principle, any by-product from other processes or waste material that is inert and has properties that conform to the requirements for primary aggregates – strength, particle size etc. – are suitable for use in concrete.
Examples that have been used include power station ash, ferro-silicate slag from zinc production, shredded rubber from vehicle tyres and crushed glass. With materials such as ferrosilicate slag, a problem may be the variability of supply (particularly particle-size distribution) since this was not an issue for the producers.
Clearly with crushed glass and shredded tyres some processing of the waste is first required. Crushed glass is not suitable for high-strength concrete, and there may be some issues with long-term durability owing to alkali–silica reaction between glass and the cement.
Shredded rubber will result in a concrete with a low elastic modulus but this may not be a problem if, for example, shock absorbent properties are required.
Thanks for reading about “aggregates for concrete”.