Study On The Effect Of Salt-Freeze-thaw Coupling On The Strength Of Recycled Concrete With Waste Fibers

Since the reform and opening up,China’s economy is in and will be in a high-speed development stage for a long time.Old buildings,high-risk buildings and a large number of new buildings will be demolished after their service life.Natural disasters,such as earthquakes,typhoons and so on,will cause buildings to become dangerous buildings,which will produce a large number of waste concrete.The consumption of fibers is enormous.The fibers are characterized by low density,low water absorption,light texture,acid and alkali resistance and strong chemical stability.The mechanical properties of concrete can be improved by adding polypropylene fibers as reinforcing fibers into concrete.From time to time,year after year,a large number of old resources are wasted,can not be folly utilized,and a large number of new resources are depleted.Sooner or later,the concept of recycled concrete emerges as the times require.The coupling effect of salt erosion and freeze-thaw mainly occurs in cold offshore and coastal areas.Freeze-thaw will deteriorate the surface of concrete,and the internal micro-cracks and hole damage will continue to occur,derive and extend.Freeze-thaw causes these physical phenomena of concrete,which makes salt solution enter into concrete more easily and quickly,and accelerates the chemical corrosion of concrete.Salt freezing cycle and salt corrosion interact,promote and aggravate each other,far more obvious than unilateral effects.In this paper,the following tests are earried out to demonstrate the frost resistance of waste fiber recycled concrete:(1)Salt erosion-freeze-thaw cycle was carried out on the specimens by using fast freeze-thaw machine and freeze-thaw medium as solution.(2)Determine and confirm the evaluation index.(3)The strength of the specimens with different freezing cycles was tested.The following conclusions are drawn:(1)The failure process under the coupled action of salt erosion and freeze-thaw is that the surface of concrete is lined and accompanied by shallow mesh filamentary micro-cracks on the surface of concrete.The shedding of paste on the surface leads to the gradual exposure of coarse aggregate under the surface.Then the slurry around the aggregate keeps falling off,and the coarse aggregate breaks away from the surroundings of the mortar and the chemical bonding force,which leads to the peeling phenomenon of the coarse aggregate.Finally,the internal structure of concrete is gradually damaged,more and more aggregates are peeled off,micro-cracks in concrete are erisscrossed,the structure is loose,and the whole concrete is collapsing.Different recycled aggregate replacement rate and different fiber content of waste fiber recycled concrete will be destroyed along with this process,but the destruction process of concrete with high recycled aggregate replacement rate will be shortened,and the addition of waste fiber will inhibit the destruction process.(2)With the change of mass loss rate,the mass does not decrease and rises when the number of freezing cycles is less.Then with the increase of the number of freezing cycles,the;mass loss becomes more and more serious,and the mass loss is faster and faster.(3)Microscopic changes show that the initial micro-cracks in recycled concrete are large and wide,and the capillary holes are large and long,while the fibers in waste fiber recycled concrete will cross around the micro-cracks and capillaries.It is found that salt crystallizes on the surface of concrete crystals and fibers after salt-freezing cycles.With the increase of salt-freezing cycles,micro-cracks become wider and wider,and the pore size of capillary holes becomes larger and wider.(4)The compressive strength,splitting tensile strength and flexural strength of concrete are studied.The loss rate of three kinds of strength is basically consistent with the replacement rate of recycled aggregate,the incorporation amount of waste fibers and the number of salt-freezing cycles.With the increase of recycled aggregate substitution rate,the three kinds of strength show a decreasing trend under the same number of salt-freezing cycles,and the incorporation of waste fibers will inerease the strength to a certain extent.With the increase of the number of salt-freezing cycles,the strength loss rate of recycled concrete will gradually increase,and the strength loss of recycled concrete will be larger than that of ordinary concrete,while the strength loss of recycled concrete mixed with recycled concrete will be much smaller than that of recycled concrete.But overall,the loss of strength is getting faster and faster.