Early-age Properties Of Polymeric Internally Cured Ultra-high Performance Concrete

Ultra-High Performance Concrete(UHPC)is a new sort of cement-based composite with excellent mechanical properties and durability.In view of the acrylic internal curing agent can not consider strength and autogenous shrinkage at the same time.This study adopted the method of adding polymer curing material to achieve the effect of internal auto-curing and it can effectively improve self-shrinking without sacrificing strength.The effects of sodium carboxymethyl starch-based internal curing agent(CAA-ICA)and alkanolamine internal curing agent(CES-ICA)on the mechanical properties,internal relative humidity(IRH)and autogenous shrinkage of UHPC.The adaptability of polymeric internal curing materials to UHPC was further verified by microstructure testing.The main research achievements are as follows.Aiming at two polymer internal curing materials,the influence of different dosage on mechanical properties of ultra-high performance concrete was investigated.In terms of promoting the development of strength,the carboxymethyl starch sodium internal curing material is better than the alkolamine internal curing material.Under the watercement ratio of UHPC was 0.17,the compressive strength of UHPC with 4% CAA-ICA material can reach nearly 150 MPa at 28 d.The optimum ratio of UHPC by cement,silica fume,fly ash was 72:14:14,steel fiber dosed at 2% by volume and the ratio of ash to sand was 1:1.13.Based on the optimal mechanical properties,the influence of polymer internal curing materials on the volume stability of UHPC was further investigated.The content of curing material less than 5%,effectively inhibit the early autogenous shrinkage.The autogenous shrinking of UHPC increases first and then decreases with the increase of dosage and the autogenous shrinking of UHPC was negatively correlated with IRH.The shrinkage corresponding to the UHPC specimens with CAA-ICA dosed at 4% was247 μm/m,which was 61.3% lower than the reference group;The higher water absorption of CAA-ICA improved the internal relative humidity of UHPC more effectively.The IRH was 81.12% at 7 d and the decrement was 18.88% which was 5.15%higher than the reference group.The effect of internal curing material on the pore structure distribution and porosity of UHPC was studied.The average pore size of UHPC was refined and the pore structure distribution was optimized.The optimal average pore diameter of specimens mixed with CAA-ICA and CES-ICA decreased by 56.3% and 50.9%compared with the reference group at 28 d,and the porosity of specimens with 4%CAA-ICA dosed was 5.83%.The X-ray CT technique was used to visualize the pore structure and distribution of IC-UHPC and the pore volume ratio of UHPC with CAAICA and CES-ICA were reduced by 0.72% and 1.14% compared with the reference group.Based on the various properties of the internal curing materials and the UHPC preparation technology,a hollow thin-walled UHPC pavement stone was prepared.According to the demand of road maintenance,a kind of film which can release water in single channel was prepared.It has the effect of auto-heating and auto-curing which has application value in concrete maintenance work.This study shows that the use of internal curing materials effectively reduces the difficulty of curing ultra-high performance concrete,and provide a good solution to the problem of UHPC cracking due to large early autogenous shrinkage,and have good application prospects in actual road and bridge construction.