One of the most damaging actions affecting concrete is the abrupt temperature change (freeze-thaw cycles). The types of deterioration of concrete structures by cyclic freeze-thaw can be largely classified into surface scaling (characterized by the weight loss) and internal crack growth (characterized by the loss of dynamic modulus of elasticity). The present study explored the durability of concrete made with air-entraining agent subjected to 0, 100, 200, 300, and 400 cycles of freeze-thaw. The experimental study of C20, C25, C30, C40, and C50 air-entrained concrete specimens was completed according to “the test method of long-term and durability on ordinary concrete” GB/T 50082-2009. The dynamic modulus of elasticity and weight loss of specimens were measured after different cycles of freeze-thaw. The influence of freeze-thaw cycles on the relative dynamic modulus of elasticity and weight loss was analyzed. The findings showed that the dynamic modulus of elasticity and weight decreased as the freeze-thaw cycles were repeated. They revealed that the C30, C40, and C50 air-entrained concrete was still durable after 300 cycles of freeze-thaw according to the experimental results.
Concrete is considered as one of the most nonhomogeneous and demanding engineering materials used by mankind. The durability [
Concrete has a potential to be damaged if it is subjected to freeze-thaw cycles. The American Concrete Institute (ACI) has established specifications for protection of concrete placed during cold weather. ACI defined cold weather as the period where more than three successive days have a mean daily air temperature less than 40 F (Fahrenheit). The freeze-thaw durability of concrete is of utmost importance in countries having subzero temperature conditions, such as The Arctic Zone, Russia, Northern China, and China. Frost damage, a progressive deterioration which starts from the surface separation or scaling and ends up with complete collapse, is a major concern when concrete is used in colder regions. The deterioration proceeds as freezing and thawing cycles are repeated, and the material gradually loses its stiffness and strength. In addition, the increasing irreversible expansion is induced. So frost damage is a very complex fatigue process. It has been a significant scientific and technical problem to improve the freeze-thaw durability and to prolong the service life of concrete.
Hong-Qiang et al. [
Air-entraining agent [
In this investigation, local materials were utilized. A Chinese standard (GB175-99) [
The mix proportion of air-entrained concrete in per cubic meter.
Cement |
W/C | Cement |
Sand |
Coarse aggregate |
Water |
Air-entraining agent |
Air content | |
---|---|---|---|---|---|---|---|---|
C20 | 32.5 | 0.40 | 339.00 | 642.00 | 1185.20 | 133.80 | 0.85 | 5.5~6.5 |
C25 | 32.5 | 0.40 | 356.00 | 615.20 | 1188.00 | 141.00 | 0.89 | 5.5~6.5 |
C30 | 42.5 | 0.40 | 412.67 | 586.83 | 1186.00 | 164.30 | 1.03 | 5.5~6.5 |
C40 | 42.5 | 0.36 | 467.60 | 568.20 | 1148.00 | 166.00 | 1.17 | 5.5~6.5 |
C50 | 42.5 | 0.32 | 526.00 | 520.00 | 1154.80 | 168.30 | 1.30 | 5.5~6.5 |
Concrete prisms with size of
In this paper, the freeze-thaw test apparatus [
The dynamic modulus of elasticity and weight loss of each specimen were measured before placing the specimens with size of
The surface deterioration of the C30 air-entrained concrete specimens undergoing 0, 200, and 400 cycles of freeze-thaw is shown in Figures
Surface of C30 air-entrained concrete after 0, 200, and 400 cycles of freeze-thaw.
0 cycles of freeze-thaw
200 cycles of freeze-thaw
400 cycles of freeze-thaw
The RDME of C20, C25, C30, C40, and C50 air-entrained concrete after different cycles of freeze-thaw was given in Table
RDME of air-entrained concrete after different cycles of freeze-thaw (%).
Number of freeze-thaw cycles | |||||||||
---|---|---|---|---|---|---|---|---|---|
0 | 50 | 100 | 150 | 200 | 250 | 300 | 350 | 400 | |
C20 | 100 | 99.45 | 99.4 | 98.75 | 96.7 | 83.85 | 64.95 | / | / |
C25 | 100 | 97.60 | 94.35 | 91.55 | 90.75 | 77.35 | 62.8 | / | / |
C30 | 100 | 99.55 | 98.75 | 98.2 | 94.6 | / | 93.9 | 87.3 | 77.05 |
C40 | 100 | / | 98.4 | 98.55 | 99.05 | 98.9 | 97.35 | 96.75 | 95.4 |
C50 | 100 | / | 95.85 | 97.6 | 97.5 | 95.8 | 90.35 | 85.95 | 77.6 |
“/” means: “the measurements were not made.”
The relative dynamic modulus of elasticity is defined as follows:
After 300 cycles of freeze-thaw, the C30, C40, and C50 air-entrained concrete specimens showed a small loss of RDME, while C20 and C25 air-entrained concrete specimens showed considerable loss of RDME, as shown in Figure
RDME of air-entrained concrete after different cycles of freeze-thaw (%).
As seen from Table
Sun et al. [
One type of deterioration of concrete structures by cyclic freeze-thaw is surface scaling. Surface scaling is the loss of paste and mortar from the surface of concrete by the cyclic freeze-thaw or by an internal reaction of aggregate (e.g. alkali-silica reaction in concrete mixed with alkali-reactive aggregate). In extreme cases, the loss of paste can result in loosening of coarse aggregate and gradual reduction in strength of concrete structures. The weight loss will be caused by surface scaling, so the weight loss was measured.
Table
Weight of air-entrained concrete after different cycles of freeze-thaw (Kg).
Number of freeze-thaw cycles | |||||||||
---|---|---|---|---|---|---|---|---|---|
0 | 50 | 100 | 150 | 200 | 250 | 300 | 350 | 400 | |
C20 | 8.930 | 8.920 | 8.860 | 8.770 | 8.720 | 8.660 | 8.540 | / | / |
C25 | 9.417 | 9.380 | 9.270 | 9.150 | 9.080 | 9.050 | 9.005 | / | / |
C30 | 9.960 | 9.930 | 9.940 | 9.940 | 9.890 | / | 9.900 | 9.840 | 9.685 |
C40 | 9.740 | 9.730 | 9.735 | 9.685 | 9.675 | 9.660 | 9.410 | 9.615 | 9.510 |
C50 | 9.960 | / | 9.925 | 9.940 | 9.935 | 9.890 | 9.870 | 9.800 | 9.585 |
“/” means: “the measurements were not made.”
Effect of freeze-thaw cycles on weight loss of air-entrained concrete.
It can be seen from Table
The test results show that the weight loss of C30 air-entrained concrete was 0.20 percent compared with 1.81 percent for plain concrete after 100 cycles of freeze-thaw [
The weight loss of concrete specimens is caused by surface separation or scale off. The weight variation during freeze-thaw cycles is due to movement in and out of water in the specimen and surface separation or scaling (surface scaling is the loss of paste and mortar from the surface of concrete by the cyclic freeze-thaw). As soon as microcracking takes place, the deteriorated zones filled with the surrounding water will cause change in the weight of the specimen. If the mass of surface separation is larger than the water absorbed by the concrete specimens, the weight of the concrete specimens will increase. The weight of the concrete specimens will decrease when the mass of surface separation is less than the water absorbed by the concrete specimens. Compared with plain concrete, the deteriorated zones filled with the surrounding water occurred in the air-entrained concrete needed much more cycles of freeze-thaw.
In actual concrete structures, concrete surface scaled markedly when exposed to deicing salt and freeze-thaw cycles caused by the change of climate. The cycling rate in the laboratory conditions was much higher than that in the natural environment because of the fast change of temperature. Thus, it is reasonable that the scaling observed during the tests was more severe, and the scaling depth of concrete specimens was over 1 mm.
A lot of structures, like bridges, tunnels, dams, buildings, and others, were constructed with concrete material. During the life cycle of these structures, degradations can occur because of mechanical, thermal, or chemical stresses. These often lead to the development of porosity, microcracks, and cracks in the material. Knowing the concrete structure state to prevent or repair damage is needed so the nondestructive characterisation is an important way, and the ultrasonic method is often proposed.
In this work, the ultrasonic velocity of C30 air-entrained concrete was measured with ultrasonic method according to “Testing Code of Concrete for Port and Waterwog Engineering” JTJ 270-98 [
Decreasing percentage of the ultrasonic velocity after freeze-thaw cycles.
Number of freeze-thaw cycles | 0 | 100 | 200 | 300 | 400 |
Loss of the ultrasonic velocity (%) | 100 | 97.7 | 97.6 | 91.0 | 84.7 |
Concrete is a three-phase composite structure at microscopic scale, a cement matrix, aggregate, and the interfacial transition zone between the two. The microcracks will be caused by the action of freezing and thawing cycles; the direction and distribution of microcosmic cracks are stochastic. The microcosmic cracks manifold and become broad as freeze-thaw cycles are repeated. Air-entrained concrete contains billions of microscopic air cells when air-entraining agents were used in concrete. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes. So, comparing the test results in this paper with the conclusion given by other authors [
The effects of freeze-thaw cycles on the RDME and weight loss of C20, C25, C30, C40, and C50 air-entrained concrete were investigated. Based on the experimental work in this study and the discussion about the experimental results, the results of the investigation can be summarized as follows The RDME decreased as the freeze-thaw cycles were repeated. After 100 cycles of freeze-thaw, the RDME decreased to 94.35 and 98.75 percent for C25 and C30 air-entrained concrete, and 64 percent for C30 plain concrete. Therefore, the freeze-thaw durability of air-entrained concrete is much higher than that of plain concrete. After 200 cycles of freeze-thaw, the weight loss was 0.70, 0.67, and 0.25 percent for C30, C40, and C50 air-entrained concrete, and 2.35 and 3.58 percent for C20 and C25 air-entrained concrete. The weight variation during freeze-thaw cycles is due to moveming in and out of water in the specimen and surface separation or scaling. The freeze-thaw durability of plain concrete is poor, but it can be improved greatly when air-entraining agent is mixed into concrete. It demonstrates that ordinary strength concrete can also have a high freeze-thaw durability.
This research work was jointly supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant no. 51121005), the National Natural Science Foundation of China (Grants nos. 51208273, 51222806), a Project of Shandong Province Higher Educational Science and Technology Program (Grant no. J12LG07), and the Program for New Century Excellent Talents in University (Grant no. NCET-10-0287).