How to Produce Concrete Paving Block (2)
- Selection of materials
3.1 Cement
Cement should comply with SABS EN 197-1. The strength class should be 42,5 or higher because blocks are required to develop good strength relatively quickly to permit early delivery without excessive breakages. (A 32,5 strength class cement may be used if the blocks are to be subjected to elevated temperatures and/or good curing, eg steam curing.) Note that it is illegal to sell cement not bearing the SABS mark.
3.2 Aggregates
General
Natural aggregates used in the manufacture of concrete paving blocks should meet the requirements for aggregates for concrete given in SABS 1083:2002 Aggregates from natural sources – Aggregates for concrete. Slag aggregates may also be used if they can be shown to be physically and chemically sound. Waste materials, or materials not in demand, are often sought after as these are generally relatively cheap. But the use of such materials could be at the expense of quality or result in increased costs due to the need to use higher cement contents to maintain quality. These materials might also create compaction difficulties which could adversely affect productivity and durability.
The performance of aggregates at the moulding stage and in the hardened block depends on the combined effects of particle size, grading, particle shape, and hardness. Each of these properties is discussed below.
Size
The recommended maximum nominal size of aggregate is 13,2-mm. However, the maximum size generally used is 9,5-mm. Smaller sizes (-4,75 mm) may be used to suit circumstances or may be specifically selected to obtain a particular surface texture. Generally, the use of coarse particles results in savings in binder provided the mix is properly proportioned. If coarse aggregate particles are too big, or if too much coarse aggregate is used in the mix, it may be difficult to achieve good compaction and acceptable surface texture.
Grading
Continuous grading will facilitate compaction. Guidelines for grading are given in Table 1. If a material is unsatisfactorily graded for use on its own, good grading may be achieved by blending two or more materials. This is particularly true of crusher sands.
Table 1: Recommended aggregate grading for making paving blocks
Particle shape
Fineness modulus 3,2–4,2
Because paving blocks are manufactured from semi-dry mixes, chunky particle shape will facilitate compaction. This property is more likely to be found with natural sands. On the other hand, good green strength is required as the units are extruded and handled straight after compaction in the mould. Here crusher sand is suitable because of its elongated particle shape. It may therefore be beneficial to use a blend of natural sand (for easy compaction) and crusher sand (for green strength).
Hardness
For concrete subject to abrasion, SABS 1083 specifies that coarse aggregate should have a minimum 10% fines aggregate crushing test (FACT) value of 110-kN. Fine aggregate, ie crusher sand, derived from such rock will be satisfactory. However, there is no specification requirement in SABS 1083 for fine aggregate. Sands containing large amounts of unsound weathered material should be avoided. Natural sands with a high silica content are suitable.
Sand and stone chip particles derived from rock types available commercially can differ considerably in hard-ness, eg 10% FACT of limestone is about 100-kN; that of andesite lava could exceed 450-kN. Abrasion resistance depends on many factors but the single most important factor is the degree of cementing of particles at the sur-face. Factors like surface texture, shape, cement content, compaction and curing are therefore important.
All aggregates complying with SABS 1083 should give satisfactory wear performance. But, assuming that aggre-gate particles are well cemented at the surface of the block, for most modes of wear the service life of block paving can be extended by using the harder aggregate types. To enhance wear resistance, selected aggregates may be used in a richer topping layer about 15-mm thick moulded simultaneously with the base concrete.
3.3 Pigments
Quality pigments are commercially available to add colour to paving blocks. Dosage, which will depend on the colour selected and the natural colour of the aggregate and cement being used, is generally 5%, but not more than 10%, by mass of the cement. Experimentation may be required to determine the correct dosage because the colour of the finished product in a dry state is influenced by density, curing and surface texture.
3.4 Chemical admixtures
Concrete paving blocks are manufactured from semi-dry mixtures which possess poor flow properties even under vibration. Using a water-reducing or plasticizing admixture to improve compactibility may be cost-effective.
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