Experimental Study On Freeze-thaw Cycle Of Basalt-polypropylene Hybrid Fiber Reinforced Concrete

The surface of concrete is easily spalled after freeze-thaw cycles,which will lead to steel corrosion and structure failure,so freeze-thaw damage is one of the important factors that lead to the durability problems of concrete structure.Fiber reinforced concrete has many advantages such as crack resistance,wear resistance,erosion resistance,frost resistance and is concerned by scholars and engineering.Moreover,it is difficult to comprehensively improve the performance of concrete by using a single kind of fiber in theory,and only adding different fibers rationally can improve the overall performance of concrete.So it is significant to solve the practical engineering problems by researching the hybrid fiber reinforced concrete.At present,domestic and foreign researches on hybrid fiber concrete are very few and not enough to guide the project,so further studies are essential.The thesis mainly studies the basic mechanical properties and frost resistance of basalt-polypropylene hybrid fiber concrete,and analysizesthe mechanism of fiber reinforcement to provide the basis for further theoretical research and engineering application in the future.The research content is as follows:Basalt fiber and polypropylene fiber with single and mixed were added to C60 concrete respectively,and 12 mix proportions with different fiber volume content were designed.Basic mechanics of fiber reinforced concrete including compressive,splitting tensile and flexural properties were tested.The results showed thatthe three trials of strength with the increase of the total fiber contentincreased at first and then decreased.Polypropylene fibre has a significant influence on compressive and splitting strength,concretewith 0.15% BF content has the highest flexural strength and the increasing range was 16.6%,mixed fiber groups showed the "negative hybrid effect".Overall concrete had best basic mechanics performance with 0.2%total fiber dosage.The freeze-thaw cycle test was used to study 12 groups of concrete frost resistance performance.The results showed that the quality losses of concrete were not obvious,but the addition of basalt fiber and hybrid fiber had significantly inhibited on the degradation of the relative dynamic elastic modulus and flexural strength of concrete.The trend of the performance of polypropylene fiber reinforced concrete was similar to concrete without fiber,and even had the accelerated deterioration trend.The experimental study showed that basalt fiber and mixed with two kinds of fiber can significantly enhance the frost resistance of concrete and inhibit strength degradation.SEM and mercury injection test were carried out to study micro structure of the concrete.SEM test result showed that the fiber can effectively restrain the growth of cracking,transmit stress and relieve stress concentration.When meeting the fiber,the crack will be stopped or kept expanding through avoiding fibers which extend the transmission path and consume more energy.The basalt fiber will begin to disperse unevenly when the volume content was more than 0.30%,which will form weak points in the interior of concrete.Mercury intrusion porosimetry results showed that the addition of fiber can make the volume of harmless pore and more harmful pore increase,while the volume of less harmful pore and harmful pore decrease.That means the existence of fiber will increase the number of pores in concrete,which may reduce the macro performance,but meanwhile,the fiber restrains the development and cut through of pore,so the effect of the fiber was various.