To determine the durability of concrete in the actual temperature and humidity of the tunnel environment, this study investigates the mechanical properties, permeability of chloride ion, relative dynamic elastic modulus, and mass loss ratio of concrete specimens cured in the temperature which varied from normal, 40, 60, 75, and 90°C, and the humidity was kept at 90% continuously. Experimental results reveal that the hot temperature curing environment may benefit early stage strength development but reduce the long-term strength. It is proved that 60°C is a critical point. At above 60°C, the strength of the concrete material and its resistance to chloride ion permeability showed a decreasing trend; however, in the appropriate temperature range, the frost resistance properties of the concrete are improved with increasing temperature.
During the development of the Western world, mining techniques, and the understanding of underground engineering, gradually improved. The creation of deep strata tunnels in many countries has highlighted the damage that can be caused by highly geothermal environments; these issues are of increasing significance in underground engineering [
Lee et al. [
Nowadays, studies into concrete durability are no longer limited to a single environment but can be performed taking multiple factors into consideration. Jin et al. [
For the study of chloride ion erosion resistance, Zhang et al. [
Based on the above analyses, research into the effects of freeze-thaw cycling and corrosion on accelerated shotcrete has been limited and unsystematic [
In view of this, and to prevent the corrosion of concrete structures due to temperature, chloride ions, freeze-thaw conditions, or a combination of these factors, this study was performed at normal temperature, 40, 60, 75, and 90°C, at a relative humidity of 90%. The mechanical properties, permeabilities of chloride ion, relative dynamic elastic moduli, and mass loss ratios were determined in order to better understand concrete durability.
The Qirehataer Hydropower Station is located at the middle and lower reaches of the Tashkur River. The diversion tunnel passes through a highly geothermal environment, between Y7 + 010 and Y10 + 355 (Figure
Diversion tunnel layout chart.
Water vapour gushing into the entrance [
These experimental conditions in this work are based on the actual temperature and humidity of the tunnel environment [
Concrete test conditions.
Test condition | Temperature (°C) | Age (d) | ||||
---|---|---|---|---|---|---|
90% RH | 40 | 60 | 75 | 90 | Normal (20) | 3, 7, 28 |
The concrete curing room for different temperatures.
As shown in Table
Raw materials for the concrete used in this study.
Material | Cement | Gravel | Sand | Coal ash | Water-reducer | Accelerator | Fibre |
---|---|---|---|---|---|---|---|
Type | P.042.5 | 5–15 mm continuous grading | Machine-made sand, medium sand with 2.70 fineness modulus | II with 18.5 fineness | UNF-2A naphthalene type | HZC-1 Type | Roycele RS2000 |
Experimental concrete mix composition.
Material utilization amount (kg/m3) | ||||||||
---|---|---|---|---|---|---|---|---|
Water-cement ratio | Water | Cement | Coal ash |
Sand | Gravel | Water reducer |
Accelerator |
Fibre |
0.42 | 196 | 373 | 93 | 895 | 860 | 3.27 | 18.67 | 1 |
Uniaxial compression tests were carried out on cubic concrete specimens of 100 mm side length, for 3, 7, and 28 days, following test methodology for the mechanical properties of ordinary concrete (GB50081-2002, China). A TAW-2000 computer-controlled, electrohydraulic servo rock triaxial testing machine was used for these tests. When the final concrete compressive strength was obtained, it was multiplied by a size conversion factor of 0.95.
In this experiment, the resistance to chloride ion penetration test method (RCM) was conducted based on the “Standard Test Method for Long Term Performance and Durability of Ordinary Concrete” (STPDC) of China standard (GB-T50082-2009). The size of the mould that was used in this test was
Chloride ion penetration test.
Test device
Chloride ion transport coefficient measurement
The non-steady-state chloride transport coefficient of concrete is calculated as follows:
For the concrete freeze-thaw test, this work used the rapid freeze-thaw method, which was carried out on samples cured for 28 d. The specimen size was 100 mm × 100 mm × 400 mm. A comparison group was set up in accordance with GB-T50082-2009 “Standard Test Method for Long-term Performance and Durability of Ordinary Concrete.” Freeze-thaw analyses were performed every 25 cycles. The maximum number of freeze-thaw cycles was 100. The mass and relative elastic modulus of each concrete sample were measured after 25, 50, 75, and 100 freeze-thaw cycles. According to the test method specifications, when the relative dynamic elastic modulus of the specimen dropped to 60%, testing was halted.
The compressive strength values of the concrete at 3, 7, and 28 days are shown in Table
Compressive strength values for cubic concrete test blocks (MPa).
Temperature | RH% | 3 d | 7 d | 28 d |
---|---|---|---|---|
Normal | 90 | 23.81 | 29.64 | 32.25 |
40°C | 28.71 | 33.06 | 34.73 | |
60°C | 34.06 | 35.68 | 37.26 | |
75°C | 26.76 | 27.48 | 28.99 | |
90°C | 25.11 | 25.19 | 26.91 |
The compressive strength test results can be divided into three sets of working conditions, namely, 3-day, 7-day, and 28-day curing times. When the curing temperature of the specimen is in the range of around 60°C, the compressive strength of the concrete gradually increases with increasing temperature. Specifically, when the temperature reached 60 degrees, the concrete test cubes reached the maximum strengths of 34.06 MPa, 35.68 MPa, and 37.26 MPa, for samples cured for 3, 7, and 28 d, respectively. Compared to the data from concrete cured at normal temperature, the strengths measured at 60°C showed increases of 43.05%, 20.38%, and 15.53%, respectively. However, when the temperature exceeded 60°C, the strength values began to decrease, indicating that concrete strength is improved only over a certain temperature range, and up to a threshold value. In addition, the compressive strength of the concrete specimens, cured at 60°C, increases linearly with time, up to 28 d. With increasing temperature, the compressive strength of the concrete decreases, but the overall strength is still higher after 28 d than at 3 or 7 d curing ages. These results indicate that temperature does indeed have a significant influence on the compressive strength of concrete, but it is not the only factor to consider.
At temperatures between 40 and 90°C, the UCS after 28 days of curing increased by 7.2–35.45%, when compared to the equivalent 3-day sample, and by 4.42–8.80% when compared to the equivalent 7-day sample. This indicates that, with increased curing time, the UCS gradually increases under conditions of high humidity. However, the rate of increase relaxes once the curing time exceeds 7 days. This is because, in the high humidity environment, higher temperatures are conducive to improving the rate of the hydration reaction of cement, promoting this reaction and resulting in a rapid rise in strength over the initial curing time. With increasing age, the influence of temperature becomes more and more obvious. The free water in the capillary inside the concrete gradually evaporates and can no longer participate in hydration reactions at later stages. As a consequence, porosity increases and the rate of strength improvement decreases.
The effect of temperature on chloride ion permeability is shown in Figure
Effect of temperature on chloride ion permeability.
Concrete morphology after chloride ion penetration.
75°C
90°C
Based on the above analyses, compared with the rest temperatures, we found that concrete cured at 60°C has the best resistance to chloride ion penetration, because this temperature environment accelerates the rate of concrete hydration, and the formation of the hydrated gel is unable to spread to the concrete surface. This colloid blocks the entry of water and makes the internal hydration reaction incomplete. At the same time, because of the high temperature, the concrete moisture evaporation rate is accelerated, resulting in more porosity cracks in the structure that cause a decrease in the compactness of the concrete.
Figure
Relationship between the number of freeze-thaw cycles and the relative dynamic elastic modulus under different temperatures.
Specimen after 75 freeze-thaw cycles.
Specimen after 75 freeze-thaw cycles at 75°C
Specimen after 75 freeze-thaw cycles at 90°C
It can be seen from Figure
Compared to the relative dynamic elastic moduli of concrete at 75°C and 90°C, the relative dynamic elastic moduli of concrete at normal temperature, 40°C and 60°C, were 84.85%, 86.80%, and 95.37%, respectively, after 25 freeze-thaw cycles. The relative elastic moduli of the concrete specimens were 79.40%, 78.02%, and 86.10% after 50 freeze-thaw cycles. The results are better than those at 75°C and 90°C. After 100 freeze-thaw cycles, the relative elastic modulus of the cured specimen under the normal temperature decreases to 57.77%. The relative elastic modulus of concrete specimens at 40°C and 60°C was 63.08% and 68.11% after 100 freeze-thaw cycles. Accordingly, within a certain temperature range, there is a positive correlation between temperature and the ability of the concrete to resist freezing and thawing. The higher the temperature, the better the performance of concrete to resist freezing and thawing. Beyond the certain temperature range, with the increase in temperature, the antifreezing performance of concrete will weaken. A proper curing temperature is advantageous for the concrete hydration reaction, as the reaction can proceed fully. Gel obtained through the hydration reaction can fill the entire internal pore and reduce the number of large pores that is advantageous for improving the antifreezing performance of concrete.
The quality loss of concrete specimens under different temperatures changes with freeze-thaw cycles, as shown in Figure
Relationship between the number of freeze-thaw cycles and the mass loss ratios under different temperatures.
As shown in Figure
The above results indicate that the quality of the sample showed a downward trend with an increase in the number of freeze-thaw cycles. The mass loss of the concrete specimens at 40°C and 60°C is the least for 100 freeze-thaw cycles, and their frost resistance is the best. The order of frost resistance of concrete under five temperature conditions is 60°C > 40°C > normal temperature > 75°C > 90°C. This indicates that a higher curing temperature increases the porosity of concrete and decrease the binding force of concrete. Its frost resistance is worse in comparison. Within an appropriate temperature range, the frost resistance of concrete improves with an increase of temperature.
In this study, the performance of concrete, including its mechanical properties, permeability of chloride ion, relative dynamic elastic modulus, and mass loss ratio, was investigated to clarify concrete durability under the different temperatures. The following conclusions can be drawn from the study: Under the condition of high humidity, the early strength of concrete increases obviously with the increase of curing temperature, and the later strength increases little. When the curing temperature is higher than the critical temperature, the concrete strength decreases with the increase of temperature. The increasing of curing temperature can promote the hydration reaction, which is beneficial to the hardening and integrity of concrete. However, when the temperature is too high, under the action of temperature and hydration heat, the water in the concrete has evaporated in the form of gas, which forms pores or voids, and affects the occurrence of hydration reactions. This results in the decrease of chloride ion resistance and freeze-thaw resistance of concrete. Moreover, concrete durability decreased significantly. An excessively high curing temperature increases the porosity of concrete and decreases the binding force of concrete. In addition, the distribution of hydration products is not uniform. The mass loss of concrete is severe under freeze-thaw cycles. Within an appropriate temperature range, the frost resistance of concrete improves with an increase in temperature. In the construction process of the concrete supporting system in the high temperature water diversion tunnel, the rock wall temperature and the ambient humidity should be measured in time. When the rock wall temperature is too high, spray cooling and other methods should be taken to reduce the temperature of the rock wall, which can reduce the impact of high temperature on the durability of concrete.
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge the support received from Funding research projects in Hebei Education Department (Grant no. QN2015157).