Experimental Research On Mechanical Properties,Explosive Spalling Behavior And Its Improvements Of Ultra-high-performance Concrete With Coarse Aggregate Exposed To High Temperature

In this paper,a series of ultra-high-performance concretes with coarse aggregate(UHPC(CA))and reactive powder concrete(RPC)with compressive strength of 120?160 MPa at the age of 56 days were prepared using ordinary raw materials.These concretes included plain UHPC(CA),steel fiber reinforced UHPC(CA)or RPC,hybrid fibers(steel fiber and polypropylene(PP)fiber)reinforced UHPC(CA)or RPC,and UHPC(CA)with different combinations of recycled steel fibers,PP fiber and rubber particles.Residual mechanical properties and explosive spalling behaviors of these concretes exposed to high temperature were investigated experimentally.The results will provide important values for design of UHPC structure with requirement of fire resistance,relevant standard specification and scientific evalution on the UHPC structure after exposed to fire.The main research results are as follows:(1)The plain UHPC(CA)has excellent workability,ultra-high strength and ultra-high durability,but low toughness and resistance to shrinkage.The residual mechanical properties and ratios of residual strength to that at 20 ? of the plain UHPC(CA)were higher than those of high-performance concrete(HPC).The steel fiber with high tensile strength exhibited better performance to improve the residual mechanical properties of UHPC(CA),especially residual fracture energy.The maximum residual compressive strength of UHPC(CA)with flat-straight copper-coated steel fiber was slightly higher than 200 MPa at 400 ?.(2)Residual compressive strength and residual splitting tensile strength of all types of fiber reinforced UHPC(CA)and RPC increased firstly and then decreased with the increasing high temperature because of the secondary hydration caused by high temperature.Relationship between the ratio of the residual strength to that at 20 ? and target temperature can be fitted into a quadratic polynomial function or linear function.The residual fracture energy decreased gradually and its relationship with target temperature can be fitted into a linear function.In contrast,the residual fracture energy of the plain UHPC(CA)exhibited a different tendency since it has severe pore-coarsening caused by high temperature.(3)The plain UHPC(CA)encountered severe explosive spalling during heating and its spalling probability and severity was much higher than those of HPC.Incorporating steel fiber can alleviate explosive spalling of UHPC(CA),but cannot prevent UHPC(CA)from explosive spalling.Hybrid fibers can significantly improve the resistance to explosive spalling of UHPC(CA)and RPC,and avoid the explosive spalling of some concrete specimens.Steel fiber with the volume dosage of 0.5%performed better than that with 1.0%volume dosage.The UHPC(CA)incorporating corrugated ordinary high strength steel fiber(d=1.0 mm,1 =35 mm,2200 MPa tensile strength)with the volume dosage of 0.5%and PP fiber exhibited the highest resistance to explosive spalling.(4)Compared with RPC,UHPC(CA)presented a better resistance to explosive spalling,indicating that coarse aggregates can provide a beneficial effect on alleviating the explosive spalling of UHPC(CA),because coarse aggregates not only decreased internal stress,but also increased distribution density of steel fiber in mortar.(5)Moisture content affected explosive spalling of UHPC(CA)and RPC,the greater the moisture content,the more serious the explosive spalling.Many coarse aggregates spalled off from mortar matrix during explosive spalling and remained intact.These findings proved that vapor pressure plays a crucial role in inducing the occurrence of explosive spalling of UHPC(CA),but it is the combination of vapor pressure and thermal stress that finally detennine the occurrence of explosive spalling.(6)The internal vapor pressure in the UHPC(CA)specimens increased with the increasing moisture content,and the maximum value could reach to 5.024 MPa when the moisture content is 100%.With a lower heating rate,the severity of explosive spalling of UHPC(CA)decreased obviously.(7)Recycled steel fiber with rubber(RSFR)significantly improved the splitting tensile strength and the fracture energy of UHPC(CA),especially the fracture energy.Recycled steel fiber without rubber(RSF)alleviated the explosive spalling of UHPC(CA),and hybrid fibers(RSF and PP fiber)remarkably improved the resistance to explosive spalling of UHPC(CA),and the volume dosage of RSF was 0.5%.Thus,RSFR and RSF can substitute for ordinary steel fiber to be applied in UHPC(CA)structures,being of great importance to environment protection.