Coupled effects of aggregate size and alkali content on ASR expansion
Introduction
The work presented here is a part of an overall project to develop models to predict the potential expansion of concrete containing alkali-reactive aggregates. These models are intended to take account of several parameters, including the particle size of the reactive aggregates, the total amount of reactive silica and the alkali content of the mixture. In the literature, research on the size effect concerns several types of aggregates. It seems that the aggregate size causing the highest ASR expansion depended on the nature and composition of the aggregate. In spite of numerous investigations, it is difficult to generalize about the effect of the particle size of reactive aggregate, since conflicting results exist concerning the most damaging size which leads to the highest ASR expansion. Some authors obtained significant ASR expansions for very small particles [1], while others obtained insignificant expansion when the sizes of the reactive particles were less than 50 to 150 μm [2], [3], [4], [5]. The use of powder from reactive aggregates like pozzolans, with particle sizes up to about 100 μm, has even been developed to counteract the effect of ASR [6], [7], [8]. Other research works showed a pessimum effect for particles much larger than 100 μm [1], [2], [9], [10], [11]. In most cases, the most damaging effect, leading to the highest ASR expansion, is reached for particle sizes larger than 1 mm. All the results available in the literature were obtained using different experimental conditions. The apparent confusion remaining after this analysis can be related to the nature of reactive aggregates and/or to the different Na/Si ratios of the mixtures, and the combined effect of size and alkali may be as important as the particle size effect alone.
This paper first gives experimental results on the effect on expansion of the particle size of an alkali-reactive siliceous limestone. Special attention is paid to the proportion of alkali (Na2Oeq) in the mixtures and reactive silica in the aggregate. A simple approach is then presented for the assessment of mortar expansion, based solely on correlation between the measured expansion and parameters such as the particle size of aggregates, and the alkali and reactive silica contents. Physicochemical interpretations of mechanisms and microstructural analyses are intentionally omitted, in order to highlight the empirical relationships between the macroscopic properties (expansion), particle size and silica content of reactive aggregate, and the alkali content. This approach is a first step to obtain a predictive model for ASR expansion.
Section snippets
Materials
The cement used was a standard CEM I 52.5R (specific gravity: 3.1, surface area (Blaine): 400 m2/kg). Its chemical composition is given in Table 1. The aggregates used were crushed sands: a non-reactive marble (NR) and a reactive siliceous limestone (R). In order to control the particle size distribution of aggregates in the mortars, the aggregate samples were divided into three particle size fractions: F1 (80–160 μm), F2 (315–630 μm) and F3 (1.25–3.15 mm). F1 was a fraction composed of fine
Soluble silica
The dissolution curves of silica up to 104 days are given in Fig. 1 for size fractions F1 and F3. Both curves reached asymptote values, at dissolved silica (SiO2) values of 9.4% and 12.4% of the total mass of reactive aggregate for fractions F1 and F3 respectively. These results are in accordance with that found by Poyet (11.1%) on a similar aggregate from the same source [13]. It should be noted that the total silica contents were 15.4% for fractions F1 and 20.0% for F3 (Table 1). A similar
Development of an empirical model
A model is proposed to interpret our results (and others found in the literature) and to describe the relationship between the amount of alkali and reactive silica, and the final ASR expansion for mortar cast with reactive siliceous limestone. In a first phenomenological approach, an empirical model has been developed. It is based only on correlations between experimental data. In order to estimate the ASR expansion, the model uses the total amount of alkali in the mortar (AT in Fig. 5), the
Conclusion
The experimental work shows that ASR-expansion was seven times larger for coarse particles (1.25–3.15 mm) than for smaller ones (80–160 μm). The quantification of alkali soluble silica, performed on the different particle size fractions, showed that all the reactive particles contained almost the same amount of reactive silica. Therefore, this factor did not cause the differences in the swelling behavior of the mortars containing the different size fractions. In mortars for which the two size
Notation
- AT
total available alkali content in a mortar
- A0
threshold of alkali per m3 of concrete
- AA
amount of available alkali in a mortar after the threshold A0 is reached
- AAi
amount of available alkali for fraction Fi
- ARi
amount of required alkali for fraction Fi
- ACi
amount of alkali consumed for fraction Fi
- pi
proportion of particles of fraction Fi in the sand composition
- r
amount of required alkali per kg of reactive silica
- si
proportion of reactive silica in particles Fi
- SC
sand content in a mortar
- ɛI
ratio of maximal
Acknowledgments
The authors are grateful to EDF for supporting this work.
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