Study On Mechanical Properties And Flexural Toughness Of Fiber Reinforced Cementitious Composites Under Seawater

The seawater erosion is bringing serious impact on seaport concrete structure in China,however,it can be effectively reduced by fiber reinforced cement composites.In this paper,the mechanical properties and damage law of bending toughness of fiber reinforced cement composites in seawater environment were mainly studied by using steel fiber,steel-polypropylene?PP?hybrid fiber,steel-polyvinyl alcohol?PVA?hybrid fiber and ramie fiber,simulating seawater to make the mechanical properties,dry-wet cycle and freeze-thaw cycle test of fiber reinforced cement composites,comparing standard curing environment.The main results as following:?1?For steel fiber reinforced cement composites,with steel fiber content increased,the ratio of tensile strength to compressive strength,bending strength,and toughness index are increased obviously,and the ability of flexural toughness is improved.When cement amount is 70%of the cementitious materials,fly ash and silica fume amount are 15%,with the increase of steel fiber content,the increase degree of flexural toughness is the greatest.When the content of steel fiber is 2%,synthetic fiber is 0.3%in addition,the compressive strength,splitting tensile strength,bending strength,toughness index,toughness factor and bending toughness ratio of cement-based composite materials can be improved.When the content of 0.3%steel fiber is replaced by synthetic fiber,the toughness factor does not decrease significantly.If the replacement amount is increased,the toughness factor and bending toughness ratio of fiber reinforced cementitious composites will be decreased.?2?With the increase of seawater dry and wet cycle times,the compressive and splitting tensile strengths of fiber reinforced cements composites will increase first and then decrease,and the compressive strength,splitting tensile strength,tensile and bending strength will be decreased gradually as the time of freeze-thaw cycles increased.The attenuation of compressive,splitting,tensile,and bending strength of the steel fiber group are higher than that of the mixed group,and the attenuation rate of steel-PVA group is the lowest.The toughness factor of fiber reinforced cementitious composites are gradually reduced with the increase of dry-wet/freeze-thaw cycle time,and the bending toughness ratio is not obvious.As wet and dry cycles time increases,the toughness indexes I₅ and I₁₀0 of the steel and steel-PP groups will increase first and then decrease,but there is no significant decrease after the steel-PVA group increased.Gradually decrease after the 45 times cycle,but no significant decrease in steel-PVA group.As the number of freeze-thaw cycles increases,the steel fiber groups I₅,I₁₀0 and I₂₀0 gradually increase with cycle times.The steel-PVA group and the steel-PP group I₅ and I₁₀0 did not show a downward trend,but increased slightly,and the steel-PVA group increased more obviously.After more than600 times freeze-thaw cycle,the toughness index I₂₀0 of each group decreased significantly.?3?When steel fiber is added into cement-based materials,the resistance of chloride ion penetration will be reduced,it also can be improved by the combination of steel-PP and steel-PVA fiber.PVA fiber is more benefit to enhance the resistance performance of chloride ion penetration than PP fiber.As the cycles time of freeze-thaw increase,the mass loss rate and chloride ion permeability coefficient will be increased gradually,and the relative dynamic elastic modulus decreased gradually.The 650 times freeze-thaw cycles are the internal damage weak point of steel fiber reinforced cements composites,the mass loss rate and relative dynamic elastic modulus change significantly.Within the first 300 times freeze-thaw cycle,the mass loss rate of steel-PP and steel-PVA groups are basically unchanged,and increased gradually afterwards.?4?The compressive strength of ramie fiber reinforced cementitious composites at 7 days gradually decreased with the increase of ramie fiber content.And when the ramie fiber content are 0.4%and 0.9%respectively,the compressive strength at 28days increased by 2.2%and 8.2%,bending strength was increased by 9.6%and 13.4%.The incorporation of ramie fiber can reduce the early autogenous shrinkage of cement-based materials.The electrical resistivity of ramie fiber reinforced cements composites at 3 d has a linear correlation and polynomial relationship with autogenous shrinkage and compressive strength at 7 d respectively.As the age of dry and wet cycles of seawater and the time increase of freeze-thaw cycles,the relative dynamic elastic modulus,bending strength,toughness index,toughness factor and bending toughness ratio of ramie fiber reinforced cementitious composites are gradually reduced.The research results of this paper enrich the experimental data and theoretical results in the research field of mechanical properties and bending toughness damage of fiber reinforced cement composite under dry wet cycle and freeze-thaw of seawater to a certain extent,and provide an important basis for solving the durability problem of fiber reinforced cement composite under seawater environment.