Experimental and numerical investigation on thermal properties of alkali-activated concrete at elevated temperatures

Min Yu, Hanjie Lin, Tan Wang, Feiyu Shi, Dawang Li, Yin Chi, Long-yuan Li

a) School of Engineering, Computing and Mathematics, University of Plymouth, Plymouth PL4 8AA, UK

b) School of Civil Engineering, Wuhan University, Wuhan 430072, China

c) Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China

1. Introduction

This paper presents the experimental and numerical investigation on the thermal properties of steel fibre-reinforced alkali-activated concrete (AAC) made by using multiple precursors at elevated temperatures. The temperature-dependent thermal properties such as mass change, thermal conductivity, density, specific heat, and thermal expansion are reported. The effects of temperature heating AAC, coarse aggregate, and steel fibre on the thermal performance of AAC are evaluated quantitatively. Experimental results show that high temperature greatly affects the thermal properties of AAC. Coarse aggregate and steel fibre also have a considerable influence on thermal properties. Based on the test results, a multi-phase mesoscale model is developed to predict the thermal properties considering volume fractions of steel fibre and coarse aggregate, which can be used in the fire safety design of AAC structures.

2. Materials and experiments

Fig. 1 Schematic of the preparation of AAC samples

Fig. 2 Schematic summation of thermal tests

3. Experimental results and discussion

Fig. 3 Test results

4. Mesoscale modelling of AAC at elevated temperatures

Fig. 4 FEM model of the effective thermal parameters

5. Conclusions

The following conclusions can be drawn.

• The steel fibre added in AAC can increase the thermal conductivity but has almost no effect on the specific heat and thermal expansion coefficient of the mixed AAC. The relationship between the effective thermal conductivity of the steel fibre-reinforced AAC and the steel fibre volume fraction can be approximately treated as a linear function.

• The influence of the coarse aggregate on the thermal properties of AAC is dependent on the temperature. For low temperature, the effective thermal conductivity increases with the increased coarse aggregate volume fraction; but for high temperature, it decreases with the increased coarse aggregate volume fraction. The increase of the coarse aggregate volume fraction can slightly decrease the effective specific heat, but can largely increase the thermal expansion of the mixed AAC, particularly when the temperature is high.

• The mass loss is quicker in the AAM than in the AAC. The mass loss in the AAM or AAC with steel fibre is slightly slower than that in the AAM or AAC without steel fibre. The quick mass loss was found in the two temperature ranges; one is between 100oC and 300oC; and the other is between 500oC and 700oC.

• The thermal conductivity of AAC or AAM during the heating process decreases with the increase of temperature. The decrease is much quicker in the AAC than in the AAM and is slightly quicker in the steel fibre-reinforced AAC or AAM than in the non-reinforced AAC or AAM. The thermal conductivity of the AAC or AAM during the cooling process does not change very much with the temperature.

• The variation of the specific heat with temperature follows the up-down-flatten-up-down pattern. The first peak is around 200oC and the second one is at about 800oC. The two “going up” parts correspond to the slow temperature increase in the temperature-history curve of the sample, whereas the two “going down” parts correspond to the fast temperature increase in the temperature-history curve of the sample.

• The linear thermal expansion of AAC is dependent on both the volume fraction of aggregate and temperature. For the AAC studied in the present study the linear thermal expansion increases with the increased temperature until the temperature reaches to 500oC. After then it decreases with further increased temperature. After the temperature exceeds 800oC, it has some recovery.

• The effective thermal properties of steel fibre-reinforced AAC can be evaluated by using the multi-phase model of composites if the thermal properties of individual components used in the mixture are known. Voronoi method and Monte Carlo method can be used to generate the randomly, discretely distributed coarse aggregate and steel fibre in the multi-phase model of the steel fibre reinforced AAC based on their volume fractions.