A study has been carried out to understand the effect of polypropylene fibres on the shrinkage cracking behaviour of lightweight concrete using a restrained ring test. Polypropylene fibres of 0 to 1·0% by volume in lightweight aggregate concrete and 0 to 2% by volume in foamed concrete were used. Steel and alkali-resistant glass fibres of 0·5% by volume were also used in the lightweight aggregate concrete for comparison. The effect of fibres on compressive and tensile strength, modulus of elasticity, flexural toughness and free shrinkage properties are also studied. The results indicate that the polypropylene fibres have less influence on the compressive strength, modulus of elasticity and free shrinkage of lightweight aggregate concrete and foamed concrete. However, the addition of fibres resulted in an increase in the tensile strength in both the concretes. Polypropylene fibres have shown comparable performance in improving toughness in both lightweight aggregate concrete and foamed concrete, while a significant effect in enhancing the flexural toughness and controlling shrinkage cracking behaviour was observed with an increase in the fibre content. At comparable polypropylene fibre percent, flexural toughness and shrinkage cracking resistance of lightweight concrete were found to be lower than that of normal weight concrete. Among the different fibres used in lightweight aggregate concrete, steel and glass fibres have exhibited better and poorer performance than polypropylene fibre, respectively in terms of flexural toughness and shrinkage cracking behaviour.

A study was undertaken to investigate the effect of water to cementitious material (w/c) ratio (ranging from 0 . 22 to 0 . 60) on the air-void system (air content ranging from about 10 to 70%) of foamed concrete, and their effects on the mechanical properties of foamed concrete without sand. The cementitious material used in this study was blended cement with 50% ordinary Portland cement (OPC) and 50% ground granulated blastfurnace slag (ggbs) by weight.The linear traverse method was used to characterise the air-void system, referring to air content, average air-void size, air-void frequency and spacing factor. The results show that although the same quality of foam was introduced, the air-void size and frequency, and hence spacing factor, of the foamed concrete varied with different w/c ratios and air contents, which in turn affected the mechanical properties of the concrete. For the different w/c ratios and air contents adopted, it was found that an air-void system with a spacing factor of about 0 . 05 mm, air-void size of 0 . 15 mm and air content of 40% was optimal in terms of achieving a high strength to weight ratio. The compressive strength of foamed concrete seems to be influenced by the spacing factor, w/c ratio and air content in relation to density. The results also indicated that the inclusion of air-voids in foamed concrete had a greater effect on compressive strength than the modulus of elasticity and it increases with increase of w/c ratio. The results of a numerical analysis concurred with the experimental observations.

An experimental study has been carried out to understand the behaviour of high-strength lightweight aggregate concrete reinforced with mono and hybrid fibres. Control, mono (single), double hybrid and triple hybrid fibre reinforced concretes were cast using macro-fibres of steel and micro-fibres of steel and polypropylene. This paper presents the segregation resistance, compressive and tensile strength, flexural toughness, stress-strain behaviour, shrinkage and water absorption of lightweight aggregate concrete C with and without hybrid fibres. The results indicate that the stability of lightweight aggregate (segregation resistance) increases when LWAC is reinforced with fibres in general and hybrid fibres in particular. Lightweight aggregate concrete with mono and hybrid fibres has shown a significant increase in tensile strength when compared with plain lightweight aggregate concrete; however, it has shown a marginal decrease in compressive strength. Lightweight aggregate concrete with mono and hybrid fibres displayed a pseudo hardening response in the post-crack region, while lightweight aggregate concrete with hybrid fibres has shown superior performance to lightweight aggregate concrete with mono fibres in terms of flexural toughness, post-crack strength and durability. The effect of water to cement ratio on the properties of lightweight aggregate concrete with hybrid fibres was also studied.

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