Effect of limestone fines content in manufactured sand on durability of low- and high-strength concretes
Introduction
The use of manufactured sand (MS) has been increasing in China because good quality natural sand is not economically available in many areas. Because of the crushing process, MS differs from natural sands in its grading, particle shape, and texture; and typically has between 10% and 20% fines (the particles passing the 75 μm sieve). In MS these fines usually are most likely smaller size fractions of crushed aggregate, while in natural sands the fines can be clays or other deleterious particles. The current Chinese National Standard GB/T 14684-2001 specifies that the limit of the rock fines content in MS is, respectively, 3%, 5% and 7% [1], based on the used concrete strength grade (defined as the 28d compressive strength standard value) such as higher than 60 MPa, between 60 and 30 MPa and lower than 30 MPa. Such specifications severely limit the amount of fines that can be used, even though higher fines contents can improve the properties and the performance of the resulting concrete and reduce the cost of the mixture in which they are used as research in China and abroad has shown. In other countries, limits of up to 25%, and commonly around 15%, have been established [2], [3], [4].
The effect of inclusion of limestone fines in MS on fresh and hardened concrete has been a major research topic for many years. It has been established that 10–15% of limestone fines could be allowed in MS without harmful effects on the physical and mechanical properties of concrete [5], [6], [7], [8], [9], [10], [11], [12]. However, little work has been done so far on the effect of fines in MS on the durability of concrete. Therefore, this paper reports the results of an experimental investigation on the influence of limestone fines as replacement of fine aggregate on concrete compressive strength and durability.
Section snippets
Materials
Two ordinary Portland cements of 32.5 and 52.5 grades were, respectively, used for preparing low- and high-strength concretes. The properties of the cements are shown in Table 1. Two coarse aggregates, crushed limestone from same mineralogical sources were selected with different maximum size (31.5 and 25 mm for low-strength and high-strength concretes, respectively). The manufactured sand was made from pre-washed crushed limestone, however, not all the fines were removed by washing and the
Workability and compressive strength
The test results for workability of MS concretes are shown in Table 4. It can be observed that with the increment of limestone fines content in MS, the slump for series L is increased and the cohesiveness and segregation of fresh concretes are improved, and the slump for series H is little influenced, but the slump flow value falls down gradually. It is suggested that when the fines content is, respectively, 10% to15% for series L and 7% to 10% for series H, the mixes achieved better
Conclusions
The following conclusions can be draw from the results of this study:
- (1)
The incorporation of up to 15% limestone fines as a partial replacement for fine aggregate in low-strength MS concrete or 10% limestone fines for in high-strength MS concrete improved compressive strength.
- (2)
For low-strength MS concretes, the resistance to chlorine ion permeability increased and the freeze–thaw resistance decreased linearly with increasing percentage of limestone fines replacement. For high-strength MS concrete,
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