Experimental Research On The Durable Properties Of Ultra High Toughness Cementitious Composites(UHTCC)

Ultra High Toughness Cementitious Composites (UHTCC) is featured with its tensile strain-hardening characteristic and outstanding crack controlling capacity. Its tensile strain capacity can be steadily up to 3% and form multiple cracking with tight crack with below 0.1mm. This unique high performance fiber reinforced cementitious composite can be applied into improving the durability of concrete structures, enhancing the seismic resistance of structures, repairing or retrofitting old structures. Design and application of UHTCC have been developed throughout the world among these decades. Many researches on the mechanical properties and structural applications of UHTCC have been carried out, but few investigations on its durability. So this thesis investigates the durable properties of UHTCC based on its design and structural properties excuted by my group. The basic durable properties of UHTCC, including the frost resistance, the carbonation properties and the permeability, are experimental studied firstly. Then the bond characteristic between UHTCC and corroded rebar is investigated. At last the corrosion resistance of RC beams with UHTCC cover has been experimentally studied and theoretically analyzed. The detailed content of this thesis are as follows:(1) Accelerated freezing thawing test on UHTCC has been conducted, and the mass loss, the dynamic elasticity modulus changes and the bend properties of UHTCC are gained. The test results are compared with the ones of ordinary concrete, steel fiber reinforced concrete and air-entrained concrete. The results show that UHTCC behaved good frost resistance without any air-entrained agent. UHTCC can retain high flexural strength and flexural toughness after 300 freezing-thawing cycles. The suggestions on the application of UHTCC in cold area are given.(2) Accelerated carbonation test on UHTCC with and without bending cracks have been carried out. The bending cracks are gained by preloading tests before carbonation test. The test results are compared with the ones of ordinary concrete. The results show that the carbonation rate of UHTCC decreases with age increasing. The carbonation depth of UHTCC at cracks is evidently lower than the one of ordinary concrete at the cracks. Carbonation can slightly enhance the compressive strength and first cracking flexural strength of UHTCC, and have no effect on the its toughness. Suggestions of evaluating the carbonation properties of UHTCC have been given based on the test results and analyses. (3) The permeability of UHTCC and ordinary concrete has been investigated through permeability test, rapid chloride ions penetration test and free chloride ions measurement after ponding test. The results show that UHTCC behaves much better impermeability, lower chloride ions diffusion coefficient than ordinary concrete. Furthermore, the impermeability of UHTCC increased with age goes.(4) The bond behaviors of UHTCC and rebar with different corrosion ratios have been investigated through accelerated corrosion test and pull out test, and compared with the ones of ordinary concrete. The average bond stress to slip curves have been gained with different rebar corrosion ratios. A continuous curve model of bond-slip has been applied for illustrating average bond stress-slip relation. The relationship between average bond strength and rebar corrosion has been analyzed. The results show that UHTCC could delay the occurrence of the bond strength decreasing due to rebar corrosion.(5) 4 kinds of beams, RC beam, reinforced UHTCC beam, RC beam with the concrete cover replaced by 15mm and 50mm UHTCC respectively, are manufactured. The beams are undertaken accelerated corrosion for different period of time. Bending tests are conducted when these beams finished corrosion tests, and the failure mode, flexural bearing capacity, flexural crack pattern have been studied. The results show that UHTCC can effectively restrain cover cracking due to rebar corrosion, retain the bending capacity of members, and control the flexural crack width when subjected to rebar corrosion.(6) Considering the tensile strain capacity of UHTCC, a formula for calculating the critical rebar corrosion ratio when the UHTCC cover cracking due to rebar corrosion through the theory of plasticity and cylindrical cavity expansion theory. The effectiveness of UHTCC for restraining the cover cracking due to rebar corrosion has been proved by a calculated example. The influences on critical rebar corrosion when the UHTCC cover cracking due to rebar corrosion have been analyzed.