Study On Physical And Mechanical Propertise, Frost Resistance And Microstructure Of Fiber Reinforced High Performance Lightweight Aggregate Concrete

Compared with normal concrete, high performance lightweight aggregate concrete (HPLWAC) which has many unique advantages such as lightweight,high-strength,good durability and so on, will be used widely in high buildings, large span bridges, etc. Former studies show, however, the ratio of tensile strength to compressive strength for HPLWAC is lower than that for normal concrete in the same strength grade; and the higher the strength of HPLWAC is, the more brittle it is. Such brittleness does harm to advantages of high strength materials. Besides, in order to deal with the difficulty of pumping HPLWAC, lightweight aggregates (LWA) usually presoak water under pressure before preparing HPLWAC, but this method will result in bad influences on strength and frost resistance of HPLWAC. The above two problems are worst problems which restrict the application of HPLWAC in engineering.This paper makes analyses and does experiments in order to solving the above two problems. Two types of steel fibers and three kinds of synthetic fibers are added into lightweight aggregate concrete to prepare fiber reinforced high performance lightweight aggregate concrete. Results show the addition of steel fibers can reinforce both strength and toughness of HPLWAC, and its cubic compressive strength is high to 72.7MPa at 28d. All the mechanical properties of HPLWAC can be well improved by adding steel fibers, especially for flexure strengths (increase 81.9% by adding 2.0% steel fibers) and splitting tensile strengths (increase 90.0% by adding 2.0% steel fibers), but its apparent density is increased correspondingly. Compared with adding steel fibers, the toughness of HPLWAC can be improved effectively by adding synthetic fibers, and both its low apparent density and high strength could be maintained. Especially, all the mechanical properties of HPLWAC can be improved obviously by adding PE fibers of both high strength and high elastic modulus. When fiber volume reaches to 0.5%, the increasing effects are as follows: 13.9% for cubic compressive strength, 78.9% for splitting tensile strength, and 30.0% for flexure strength. It is also discovered in experiments that the addition of fibers changes the failure form of HPLWAC, and could reduce its brittleness and improve its toughness effectively, so that toughness index can be increased 14 times at the most.Based on the above experiments, this paper studies the effects of polymer modifying to the physical properties and mechanical properties of HPLWAC. Results show the addition of polymer into HPLWAC will deceases its apparent density, compressive strength and elastic modulus; and increase its tensile strength and flexure strength, especially flexure toughness of fiber reinforced high performance lightweight aggregate concrete. Compared with PE fiber reinforced high performance lightweight aggregate concrete, the polymer-modifying effect to steel fiber reinforced high performance lightweight aggregate concrete is better. When P/C reaches 0.16, toughness index of steel fiber reinforced high performance lightweight aggregate concrete is 32.2 times of that of plain HPLWAC. The reason for this phenomenon is that the modifying effect to interfacial transition zone (ITZ) between steel fibers and hardened cement paste (HCP) is better.This paper researches the difference of water-containing condition of LWA to the strength and frost resistance of HPLWAC. Results show HPLWAC using dry LWA is of good frost resistance, and both strength and frost resistance will decrease when wet LWA are used (decrease 5.5MPa in strength and 9.44% in dynamic elastic modulus). The further study results show that adding steel fibers and PE fibers can both relieve the decrease of strength and frost resistance of HPLWAC using wet LWA. But by comparison, the addition of PE fibers leads to better effect than adding steel fibers, and the best improving effect of frostresistance appears when 0.10% PE fibers are added into HPLWAC.The macroscopic properties of fiber reinforced high performance lightweight aggregate concrete such as mechanical properties and frost resistance are the reflection of its microstructure. Using Scanning Electron Microscope (SEM), the microstructure research of fiber reinforced high performance lightweight aggregate concrete is concentrated in the two types ITZ. One is ITZ between LWA and HCP, and the other is ITZ between fibers and HCP. Many macroscopic properties such as fiber reinforcing effect, polymer modifying effect, and different influence on strength and frost resistance of HPLWAC due to using different water-containing LWA, are analyzed from microstructure point.