Experimental Study On Composition Design And Mechanical Properties Of Ultra-high Performance Seawater Sea-sand Concrete

Ultra-high performance seawater sea-sand concrete(UHPSSC)is a cement-based composite material based on the design method of ultra-high performance concrete(UHPC),which uses seawater and undisturbed sea-sand instead of fresh water and quartz sand.Seawater and sea-sand contain a lot of chloride ions and sulfate ions,which will have a very serious impact on the mechanical properties and durability of concrete.In addition,UHPC is often doped with fiber,which has a significant impact on its mechanical properties.This paper mainly studies the composition design and mechanical properties of UHPSSC,the achievements are as follows:(1)The influence factors of fluidity,flexural strength and compressive strength of UHPSSC were studied by orthogonal design test,it is found that the most influential factors on fluidity,flexural strength and compressive strength of UHPSSC are water binder ratio,water binder ratio and fly ash content in turn;the type and content of fiber have a influence on the flexural and compressive strength of UHPSSC,which shows a positive correlation with the tensile strength and elastic modulus of the fiber.Based on the simple gravity center design method,considering the mobility,flexural strength and compressive strength of UHPSSC,when the water binder ratio is0.2,sand binder ratio is 1.0 and the content of water reducing agent is 2%,the optimal content of cementitious composition is 75% of cement,15% of silica fume and 10% of fly ash.The amount of cement,silica fume,fly ash,seawater and sea-sand per cubic meter of concrete is 554 kg,111kg,74 kg,148kg and 739 kg respectively.(2)The effects of seawater and sea-sand on the flexural strength and compressive strength,hydration process and microstructure of UHPC were studied under different ages.The influence mechanism of seawater and sea-sand on UHPC was explored by analyzing its strength development,hydration rate,hydration products,pore structure and micro morphology.The results are as follows: seawater can enhance the early hydration development of UHPC,and then improve its early flexural strength and compressive strength,and the effect is more obvious when the age is about 3 days;on the other hand,seawater can promote the formation of Friedel salt in UHPC;when sea-sand and seawater are used at the same time,the early hydration of UHPC can be accelerated,which is conducive to the early strength development of UHPC,but it is bad for the later strength development of UHPC,and which will make its pore structure more compact to a certain extent.(3)The effects of volume fraction,fiber length of Ultra high molecular weight polyethylene(UHMWPE)fiber and its mixing with steel fiber on fluidity and mechanical properties of UHPSSC were studied,and compared with steel fiber.The relationship between UHMWPE fiber and UHPSSC was revealed by analyzing the change of its performance index.The results showed that: the fluidity of UHPSSC reduces obviously,the compressive strength and flexural strength、three point bending toughness increases significantly with the add of UHMWPE fiber content;the smaller the aspect ratio of UHMWPE fiber is,the smaller the fluidity of UHPSSC decreases,while the greater the compressive strength、flexural strength、three point bending toughness increases;The mixing of UHMWPE fiber and steel fiber will aggravate the fluidity loss of UHPSSC,and improve the compressive strength、three point bending toughness;when the content of UHMWPE fiber is fixed,which can enhance the flexural strength by increasing the content of steel fiber,while when the content of steel fiber is fixed,which will slightly reduce the flexural strength by raising the content of UHMWPE fiber.