ABSTRACT
Table 2.1 contains a comparison of the constituents of a high quality sprayed concrete and an equivalent strength cast in situ concrete. Considering each component in turn, one may note that:
• the water/cement ratio in sprayed concrete is higher so that the mix can be pumped and sprayed easily;
• ordinary Portland Cement is normally used, in conjunction with cement replacements such as pulverised fly ash (PFA), though special cements are sometimes used;
• the mix is ‘over-sanded’ to improve pumpability (Norris 1999) (see Figure 2.1 for grading curve);
• the maximum aggregate size is usually limited to 10 or 12 mm;
3 4 5 6 7 8 9 1011 1 2 3111 4 5 6 7 8 9 20111 1 2 3 4 5 6 7 8 9 30111 1 2 3 4 35 6 7 8 9 40111 1 2 3
• plasticisers and stabilisers are added to improve workability as in conventional concrete;
• other components may include microsilica, which is added to improve immediate adhesion (which allows the accelerator dosage to be reduced) and to improve long-term density (which improves strength and durability) or steel fibres, which are added for structural reinforcement or crack control.
