Introduction
On the Loess Plateau in western China, there exists a large number of saline soils, most of which contain salts, typically of sulfate and chloride salts. Since the sulfates in saline soils are sensitive to temperature changes, they are susceptible to salt crystallization expansion and frost heave at low temperatures, which can lead to destruction of concrete structures. Salty soils and freeze–thaw environments cause concrete to be subjected to the coupled effects of simultaneous sulfate attack and freezing and expansion, which further increases the deterioration degree of concrete and seriously threatens the durability of concrete in this area. Adding fibers to concrete can effectively inhibit the plastic shrinkage cracking and improve the durability of concrete such as wear resistance, impermeability and frost resistance. Basalt fiber (BF) has excellent properties such as high strength, light weight, high temperature resistance, high elastic modulus, fracture resistance and corrosion resistance, and it is easy to disperse in concrete and has excellent cooperative work performance with concrete. Therefore, in this paper, the durability of concrete is improved by incorporating an appropriate amount of BF into the concrete to reduce the initial defects and slow down the rate of corrosive ions into the interior of the concrete. Furthermore, the effect of different BF admixtures on the relative dynamic modulus of elasticity, mass loss, compressive strength, and splitting tensile strength of concrete during salt freezing were investigated. The effect of the included BF is also clarified.
Methods
The concrete was prepared by P·O 42.5 ordinary Portland cement, II grade fly ash, 5–20 mm continuous graded gravel and natural river sand fineness modulus of 2.83. The volume content of basalt fiber was 0%, 0.1%, 0.2% and 0.3%. The test was carried out with reference to the quick-freezing method in GB/T 50082—2009 ‘Test method for long-term performance and durability of ordinary concrete’, using HC-HDK9/Y type fast freeze–thaw cycle tester. According to the content of corrosive ions in saline soil in western China, Na2SO4 salt solution with a concentration of 23 g/L was selected for freeze–thaw cycle test. The loss rates of mass, relative dynamic elastic modulus, the compressive strength and splitting tensile strength during the test were tested. At the same time, Numerical simulation tests were carried out using finite element software ABAQUS to analyze the changes of internal stress and strain fields of basalt fiber reinforced concrete (BFRC) during the tests.
Results and discussion
The mass and relative dynamic elastic modulus of BFRC under the action of Na2SO4 solution-freeze–thaw cycle raised first and then decrease with growth of the number of freeze–thaw cycles. The compressive strength and split tensile strength decrease with the increase of the number of freeze–thaw cycles. The addition of BF slows down the formation of corrosion products and expansion products. Under the same number of freeze–thaw cycles, the addition of BF reduces the generation of microcracks in concrete and prevents crack propagation. The addition of BF improves the freeze–thaw resistance of concrete in Na2SO4 solution. And the freeze–thaw resistance performance gradually improves with the increase of the BF volumetric doping within the range of 0.1% to 0.3%. Moreover, the best performance of freeze resistance in Na2SO4 solution is achieved when BF doping is 0.3%.The internal stress and strain fields of BFRC with different BF dosage during Na2SO4 solution-freeze–thaw cycle test decrease with increasing the BF dosage. At the same BF dosage, the stress and strain increase with increasing the number of freeze–thaw cycles, and the dynamic numerical simulation was in good agreement with the experimental results. Using the ‘equivalent temperature load’ method, the salt solution-freeze–thaw cycles test process can be effectively simulated by ABAQUS finite element software. The numerical simulation results also show that the stress field of concrete and BF as well as the strain field of BFRC have changed, and that BF plays the role of transferring and dispersing the stresses during the freeze–thaw cycling process, which leads to the phenomenon of stress redistribution inside the concrete, and improves the salt-freezing resistance of concrete.
Conclusions
In this work, the macroscopic properties and microscopic changes of BFRC with different amounts of BF were studied under the combined effect of Na2SO4 erosion and freeze–thaw. The main conclusions are that the addition of BF significantly reduces the loss rate of concrete mass, relative dynamic elastic modulus, compressive strength and splitting tensile strength during the freeze–thaw cycle of Na2SO4 solution. BF can transfer and disperse the stress, so that the stresses are redistributed inside the concrete during the freeze–thaw process, thus improving the salt-freeze resistance of concrete. In addition, when the BF content is 0.3%, BFRC has the best resistance to Na2SO4 salt erosion-freezing performance. Meanwhile, BF shows an important effect in the evolution of freeze-thaw damage evolution of concrete, which can be visualized effectively by finite element model.