Early Age Shrinkage And Long-term Stability Performance Of Ultra High Performance Concrete

Ultra High Performance Concrete?UHPC?is a kind of ultra-high strength cement-based composite material with compressive strength over 120 MPa,high toughness,and excellent durability.It usually uses cementitious materials?cement,silica fume,fly ash,mineral powder,etc.?with different partic le sizes,fine aggregate,and steel fibers to achieve close packing at ultra-low water-binder ratio?W/B?around0.2,thus the production of a new cement-based material with excellent performance.Such components and characteristics can render UHPC structures with significantly reduced size and dead weight,improved seismic resistance,and seawater corrosion resistance.The self-desiccation and chemical reaction process of cement during hydration can lead to capillary pressure,and thus autogenous shrinkage of concrete.Such autogenous shrinkage usually occurs in concrete with W/B lower than 0.42 and it increases with the decrease of W/B.High autogeusos shrinkage values can result in potential cracking of concrete.Due to the extremely low W/B of UHPC,its autogensous shrinkage is several times or even an order of magnitude greater than that of conventional concrete.The potential cracking can eventually lead to reduced mechanical strength and durability of UHPC.In view of the potential problems existing in UHPC at early and late ages,this study mainly focues on the change of autogeneous shrinkage,dry shrinkage,and long-term stability of UHPC,as well as their influcencs on microstructure.It mainly includes the following three aspects:1)the effect of steel fiber content on autogeneous shrinkage of UHPC using corrugated tubes and rings testing methods,and comparision in effect of slag and fly ash on autogeneous shrinkage.2)infucence of steel fiber content on autogeneous and drying shrinkage values of UHPC and proposed a equation to better describe the change in drying shrinkage of UHPC with time and steel fiber content.3)changes in strength,mass,and length of UHPC specimens made with 25%slag or 25%fly ash when exposed in out-door,water,and seawater enviroments.In addition,TGA,MIP,XRD,SEM,and phenolphthalide analyses were used to investigate the microstructure of matrix and fiber-matrix interface.The relationship between microstructure and long-term performance was then clarified.In terms of the results,the following results can be drawn:?1?The influence of steel fiber content on the autogeneous shrinkage of UHPC was studied using corrugated tubes and ring testing methods.It was found that the autogensous shrinkage decreased with the increase of steel fiber content,which was attributed to the strengthening effect of fibers through crossing.Meanwhile,the closed ring method was more suitable in measuring the autogenous shrinkage of UHPC.The autogenous shrinkage of UHPC mainly occurred in the first 24h,which accounts for 98%of the total shrinkage value.At 72 h after casting,it remained almost stable.The incorporation of 2%-3%steel fibers in UHPC significantly improved strength and restrained autogenous shrinkage.The ring testing method can provide relatively complete and uniform constraint for UHPC,which can better reflect the combined effect of shrinkage and stress relaxation of UHPC under constraint conditions.?2?Effect of steel fiber content on autogenous and drying shrinkage of UHPC were evaluated and a equation to better describe the change in drying shrinkage of UHPC with time and steel fiber content was proposed.It was shown that the dry shrinkage value of UHPC first increased quickly and then slowed down after 7 days.Steel fiber content showed a significant effect on drying shrinkage of UHPC.With the increase of fiber content,the drying shrinkage was obviously decreased.However,high steel fiber content decreased the restraining degree for drying shrinkage.For example,the drying shrinkage value was decreased by 1.5%with fiber content increased from 2%to 3%.This was mainly attributed to the detrimental effect associated with fiber-matrix interface.High steel fiber content can increase the amount of weak interfaces,and thus decreasing the restaining effect of fibers and then dyring shrinkage.Besides,the restraining effect of using 25%fly ash on drying shrinkage was greater than that of 25%slag.?3?Investigation of the variations of strength,mass,and length of UHPC with age when exposed in outdoor,water,and seawater environments.The miscrostructure of specimens after certain exposion was analyzed and its relationshiop with long-term performance was clarified.It was found that the compressive strength of UHPC exposed in tap water for 2 years increased continuously.The strength of UHOC specimens exposed to outdoor conditions was lower than that of the samples soaked in water,while the strength for specimens exposed in seawater was hindered.The UHPC specimens exposed to outdoor conditions and tap water exhibited shrinkage,but the UHPC specimens exposed to seawater displayed expansion.The shrinkage of specimens exposed to outdoor conditions was 10 times greater than that of specimens submerged in water.For mass,the UHPC specimen exposed to outdoor conditions showed a decrease trend,due to water loss,while the UHPC specimen exposed to tap water and seawater slightly increased.The Ca?OH?₂?CH?content in the inner layer near the surface of the samples that immersed in seawater was the lowest,and CH leaching is the main reason for the derease in strength.SEM and TGA/DTG results showed that the reduction of CH conent was accompanied by the formation of calcium carbonate?calcite?,which was due to carbonization under outdoor conditions.However,in tap water and seawater environments,CH was transformed into other products,including Mg?OH?₂,ettringite,Friedel's salt,and sulfoaluminate.As ettringite expands with time,more small and large pores form in the specimens that merged in seawater.More mesopores form inside the specimens exposed to the outdoor environment perhaps due to the formation of calcite.