To make good quality concrete, you need to understand the different constituents and the roles they play.
Cementitious materials for concrete are fine mineral powders. These are called hydraulic materials because, when they are mixed with water, a chemical reaction called hydration takes place. As a result, the cement-water paste gradually changes from a plastic state to form a strong rigid mass that binds aggregate (sand and stone) particles together to make concrete. The ratio between the water and the cement (w:c) in a mix determines the strength of the concrete – the more cement in the mix (for the same water content), the stronger the hardened concrete will be, and vice versa.
Throughout this article we use ‘cement’ to cover all branded cements, bearing the SABS mark, that comply with SABS EN 197-1/SANS 50197-1 strength class 32,5N and are commonly available from hardware stores and builder’s merchants in 50 kg bags. (If considering the use of the cements of other strength classes, refer to the manufacturer’s recommendations on the bag for mix proportions.)
We use the term ‘masonry cement’ for cement which is intended for use in mortars or plasters and complies with SABS ENV 413-1/SANS 50413-1 strength class MC 22,5X.
The rate of strength gain and heat developed by the reaction between water and cementitious materials is governed by the fineness of the cement particles, and their constituents. The letter ‘N’ in a cement strength class designation indicates normal rate of strength gain, and ‘R’ indicates rapid strength gain, particularly at early ages.
Most general purpose cements consist of portland cement and certain specified amounts of extenders (usually ground granulated blastfurnace slag or fly ash) and/or fillers (finely ground limestone). When portland cement is present in a mix and the hydration process has begun, the extenders act as cementitious materials.
Extenders and fillers are blended or interground with portland cement to:
• Save costs because extenders are generally less expensive to produce than portland cement.
• Improve the properties of concrete in the fresh state, or the impermeability and durability of the hardened concrete.
• Benefit the environment by increasing the use of by-products from other industries and reducing CO2 emissions from cement kilns.
The SABS mark on cement bags is a legal requirement, and guarantees that the correct proportions of constituents are present in the cement. The cement must also comply with other physical and chemical requirements as specified in SABS EN 197-1/SANS 50197-1 Cement – Part 1: Comparison, specifications and conformity criteria for common cements.
The strength performance of all mark-bearing cements is regularly monitored and is subject to verification by SABS.
For details of the performance of a particular branded product, consult the manufacturer.
Stone and sand for use in concrete should consist of particles of hard material of rounded or roughly cubical shape, with a fairly smooth surface, and should be free from impurities such as earth, clay, roots, salt, etc.
Providing that stone used for concrete is composed of clean, hard, durable particles, the actual source is less important than the size of the particles. Single-sized 26,5-mm or 19-mm stone is more economical to use, but 13,2-mm stone makes hand mixing easier.
Most quarries, builder’s merchants and hardware stores can provide good crushed stone. The size of the largest particles should not exceed about one-fifth the minimum thickness of the concrete section, or exceed the clearance between reinforcement and formwork, nor should material less than about 7 mm be included.
Weak and porous materials (e.g. furnace bottom ash) yield poor quality concrete of limited strength.
The properties of the sand have a very marked effect on the quality of the concrete, so where possible choose sands carefully.
• The best sands are ‘evenly graded’ – they contain particles of a wide range of sizes, the medium sizes predominating, but with the coarsest (just passing the 4,75-mm sieve) and the finest being present.
• Sand with a ‘poor’ grading (i.e. with mostly single-sized particles, such as desert or beach sand) can be used, but will generally give a mix that is less workable, less economical, more liable to ‘honey-combing’ and less easy to bring to a good finish.
• Sands containing more than about 10% of shale, or of weathered basalt or dolerite (all of which are ark in colour) may cause excessive shrinkage on drying.
• Sand should contain a proportion of very fine material – lack of fines results in a harsh concrete which is hard to compact and to bring to a good finish. It can also lead to excessive ‘bleeding’ where a layer of water appears on the surface of the concrete after placing.
• An excess of fines, on the other hand, tends to cause a reduction in strength and an excess of mortar on the surface. This mortar may craze and will give a surface that will wear rapidly under traffic.
In practice, fine aggregates are usually either natural sands, or sands manufactured by the crushing of rock.
Natural sands sourced from riverbeds or pits should contain at least 5% fines.
River sands are generally reasonably clean and free from clay, and often consist of hard rounded particles which give good workability to the concrete. Deposits are often rather variable, having been sorted into sizes by water flow.
Beach or dune sands tend to be poorly graded. Salts (i.e. chlorides), which may be present in these sands tend to cause efflorescence – a white deposit on the surface of the concrete – and corrosion of reinforcement or steel fixings embedded in the concrete. Nevertheless, such sands, provided that they have been properly washed and processed, are satisfactory for all grades of concrete.
Crusher sands are produced in a controlled manufacturing process which includes crushing, sieving and, where necessary, washing. The products are usually of consistent quality and, if they contain at least 10% fines, are suitable for making concrete.
Admixtures are chemicals in flake, powder or liquid form added to concrete at the mixing stage to modify properties of the mix. They are not usually used for small-scale building projects as their use requires special mix design and dispensing equipment, but we discuss their properties here as they are often used in ready-mixed concrete.
Admixtures are categorised according to their effect. The most commonly used admixtures include:
• Plasticisers to improve workability and provide a more even distribution to the binder (cement) particles through the mix.
• Super-plasticisers to improve workability evenly more markedly, allowing concrete to flow without segregating through heavily congested reinforcement and compact without vibration. They are also used for high-strength concrete. The effect of a super-plasticiser is short-lived, and may disappear as early as 30 minutes after mixing.
• Accelerators speed up the chemical reaction of the cement and water and are useful where rapid setting and high early strengths are required, where rapid turnover of moulds or formwork is required, or where concreting takes place under very cold conditions.
• Retarders slow the chemical reaction of the cement and water leading to longer setting times and slower initial strength gain. They are used when placing concrete in hot weather, to prevent cold joints caused by delays during placing, as well as in concrete which has to be transported for a long time.
• Pigments may be used to colour concrete red, yellow, brown, black, blue, or green. Use only the best quality pigments based on metallic oxides – natural pigments may be cheaper, but the colour they produce is variable and less intense.
Dosage is typically 3 to 5 kg per 100 kg of cement, but the final colour of the dry concrete depends on the combined colours of the cement, sand and pigment. Make up trial mixes using the specific sand and cement and different pigment dosages. Cure the concrete and then allow the surface to dry completely before assessing the colour – the colour of wet concrete is darker and more intense.
When using any type of admixture, pay special attention to correct dosage – overdosing can result in excessive delays in setting.
Drinking water is normally suitable for making concrete mortar.
Murky or dirty looking water, e.g. from a borehole or dam, should be tested by a laboratory before it is used.
Seawater can be used for unreinforced concrete if the appearance of the concrete is unimportant – a white powdery deposit called efflorescence may develop on the surface.