Numerical simulation of early-age thermal stress of high volume fly ash (HVFA) concrete

This paper numerically studied the effect of fly ash (FA) replacement on early-age thermal stress of concrete. Different FA replacement levels (0%, 20%, 50% and 80%) on the thermal stress and cracking risk of concrete have been considered. Results showed that, due to the FA effect on the reduction of hydration heat, the maximum temperature and corresponding compressive stress during the temperature rise stage were both lowered; meanwhile, the strength of concrete was simultaneously weakened, leading to lower cracking stress when the FA replacement increased gradually. Finally, the cracking temperature of concrete decreased when the FA replacement level increased up to 50%, and converted to plateau with higher FA replacement level. Overall, FA replacement level with 50% was recommended for concrete under restraint cracking risk.


Introduction
With the strengthening of environmental protection and the deepening of research on the utilization of solid waste resources, high volume fly ash concrete (HVFA) has also been applied to a certain extent in the field of water conservancy and hydropower [1][2].Roller compacted concrete (RCC) formed by replacing part of cement with fly ash is a common high volume fly ash concrete.However, compared with the normal concrete (NC), which usually takes strength as the evaluation index, the hydraulic environment puts forward new requirements for the thermal crack resistance of high volume fly ash concrete.The fly ash content of high volume fly ash concrete is higher, the pozzolanic reaction is slower, and its early strength is lower than that of NC [3][4][5].Therefore, it is particularly necessary to study the thermal crack resistance of high volume fly ash concrete at early age, which also has a good guiding effect on its application in practical engineering.
In this paper, a three-dimensional finite element software was used to simulate the thermal stress of HVFA concrete based on an established finite element model of concrete temperature stress testing machine (TSTM) and specimen.Effect of fly ash replacement levels on material properties, thermal stress and thermal cracking potential of concrete has been addressed.

Modelling
The finite element model is shown in figure 1. Restraint frame and concrete specimen were simultaneously considered to truly reflect the interaction between these two parts of TSTM.The typical point of model was in the middle of the concrete specimen.By setting a relatively high elastic modulus of restraint frame material, the concrete deformation due to the changing temperature condition was completely restrained by the TSTM restraint system and a fully restraint degree then can be achieved.Finally, thermal stress of concrete was calculated under different temperature histories accordingly.

Input numerical data
In order to investigate concrete properties with different FA replacement levels, concrete specimen with 0%, 20%, 50% and 80% of FA replacement level were selected and corresponding concrete was named based on FA replacement level.Table 1 and table 2 respectively show the tested thermal expansion coefficient and elastic modulus of HVFC concrete [6].Cracking stress was assumed to occur when simulated thermal stress of concrete reached experimental cracking stress of concrete temperature stress test results [6].A stress reduction factor of 0.8 was adopted due to the creep effect of concrete [7].

Numerical results and analyses
Figure 2 shows a typical temperature and stress field of concrete at different temperature stages.It can be seen that the temperature field is quite uniform at the linear part of concrete specimen and the temperature at both ends of concrete specimen is relatively lower due to the interaction between the concrete specimen and the restraint frame.After the concrete mixture was mixed, the hydration of cement and FA started and the hydration heat generated gradually, leading to a considerable temperature rise at an early age stage.Meanwhile, the concrete started to expand due to temperature rise, but these deformations were fully constrained by TSTM and compressive stresses started to develop.The numerical results agree with the experimental test [6].Figure 3 shows that the maximum compressive stresses during the temperature rise stage were -2.1MPa, -0.92MPa, -0.19MPa and -0.08MPa, respectively.The concrete temperature was then gradually reduced due to the hydration heat loss to the environment and the compressive stress correspondingly reduced thanks to the shrink of concrete with the dropping temperature.After second-zero-stress temperature point, the compressive stress converted to tensile stress and continued to increase with lowering temperature.The cracking stress for concrete with the FA replacement level of 0%, 20%, 50% and 80% was 1.75 MPa, 1.58 MPa, 0.98 MPa and 0.35MPa, respectively.The cracking temperature for concrete with the FA replacement level of 0%, 20%, 50% and 80% was 34.5℃ ， 30.1℃, 22.3℃ and 22.5℃, respectively.In a word, disadvantage of FA on lowering cracking temperature of concrete was prominent with the FA replacement level of 60%.

Conclusion
Based on the finite element method, the thermal stress and thermal cracking potential of HVFA concrete was numerically calculated and the effectiveness of FA on lowering the cracking potential of concrete has been addressed based on the index of cracking temperature.Results showed that the cracking temperature was decreased with the increasing level of cement in the concrete mixture substituted by FA.FA replacement level with 50% is desirable for improving the anti-thermal cracking resistance of concrete.This conclusion agrees well with experimental results.

Figure 2 .
Figure2.Typical temperature and stress field of concrete After the concrete mixture was mixed, the hydration of cement and FA started and the hydration heat generated gradually, leading to a considerable temperature rise at an early age stage.Meanwhile, the concrete started to expand due to temperature rise, but these deformations were fully constrained by TSTM and compressive stresses started to develop.The numerical results agree with the

Figure 3 .
Figure 3. Thermal stress evolution of concrete with different FA replacement levels

Table 1 .
Thermal expansion coefficient of concrete.

Table 2 .
Elastic modulus of concrete.