Study On Preparation And Volume Stability Of Lightweight Low Shrinkage Ultra High Performance Concrete

With the continuous advance of concrete technology,ultra-high performance concrete（UHPC）materials have been developed on the basis of reactive powder concrete（RPC）,with its ultra-high mechanical properties and durability,show unique advantages in bridge construction.UHPC is used to prefabricate bridge components,which can effectively improve the bearing capacity of the bridge,reduce the section size of the components,and reduce the reinforcement ratio and the self weight of the components.However,due to the high incorporation amount of cementitious materials,the low water binder ratio and the principle of dense design,UHPC is faced with the problems of high density（2600-2800kg/m³）and large shrinkage(up to 8.0×10^-4).It needs steam curing or autoclaving to improve the volume stability and crack resistance of the prefabricated components.The steam curing and autoclaving processes result in high energy consumption and cost,which limits the popularization and application of UHPC prefabricated components.In order to solve the abovementioned problems,financially supported by the National Natural Science Foundation Project"preparation of lightweight and low-shrinkage ultra-high performance concrete and its microstructure formation mechanism"（51878003）,this paper have designed a new type of low shrinkage lightweight ultra-high performance concrete（LUHPC）material which can be cured in standard environment.The low shrinkage LUHPC is prepared by the most compact packing theory,and the mechanical properties,volume stability and durability of lightweight aggregate are systematically studied.The main work and achievements of this paper are as follows:The conbined design method of low density and ultra-high strength and low shrinkage for LUHPC was proposed.By changing the composition of cementitious materials and using Andreasen&Andersen calculation method to mix the particle size distribution of light aggregate,and adjusting the steel fiber content,sand binder ratio and the adaptability of additives,the collaborative design and optimization of low shrinkage LUHPC are completed.The slump of the as-prepared UHPC can reach higher than 200 mm,its apparent density is less than 2000 kg/m³,the 28 d compressive strength is higher than 100MPa,and the 56 d drying shrinkage is less than 6.5×10^-4.The influence of lightweight aggregate（particle type,water absorption,substitution）on the workability,mechanical properties and volume stability of low shrinkage LUHPC was studied systematically.The mechanism of improving the mechanical properties and volume stability of UHPC by"internal curing"effect and"arched shell"effect of lightweight aggregate was analyzed.With the increase of water absorption rate of lightweight aggregate,the 28 d compressive strength and splitting tensile strength of lightweight and low shrinkage UHPC are increased first and then decreased,while workability becomes better and volume stability decreases;With the increase of substitution amount of lightweight aggregate,the workability and mechanical properties of UHPC are increased first and then decreased,but the volume stability of UHPC is significantly improved;the spherical lightweight aggregate can act as roller in UHPC mixture and improve the workability of the mixture;The internal curing effect of pre wet spherical lightweight aggregate can promote the hydration of the around cement paste,which forms a spherical interfacial transition zone（ITZ）with mechanical properties higher than cement matrix,namely,"arched shell".On the other hand,the internal curing effect of pre wetting lightweight aggregate can regulate the early internal relative humidity of UHPC,reduce the self shrinkage of UHPC and reduce the interface micro cracks,thus effectively improving the volume stability of UHPC.The influence mechanism of lightweight aggregate（particle type,water absorption and substitution）on the durability of low shrinkage LUHPC was revealed.The impermeability,frost resistance and sulfate attack resistance of LUHPC are slightly better than those of UHPC prepared with crushed stone.As compared with the irregular interface formed by crushed stone,the arched shell interface formed by UHPC is more resistant to the migration of water molecules and SO₄^2-in the pores.With the increase of water absorption of lightweight aggregate,the impermeability,frost resistance and sulfate attack resistance of UHPC are improved.The erosive properties all increased first and then decreased.When the water absorption rate of lightweight aggregate is too high,the excessive water released by lightweight aggregate becomes free water,which makes the compactness of interfacial area reduce,and the blocking effect of resistance to harmful ion penetration and transmission is obviously reduced,resulting in poor durability of UHPC;With the increase of lightweight aggregate substitution,the impermeability,frost resistance and sulfate attack resistance of lightweight and low shrinkage UHPC also show a trend of first strengthening and then weakening,but the durability of LUHPC with even a 100%substitution of lightweight aggregate is better than that of UHPC.The lightweight aggregate with high water absorption can play a strong role in internal curing,improve the volume stability and compactness of ITZ in UHPC,so as to improve the durability of UHPC.However,when the replacement amount of super absorbent lightweight aggre gate is too high,the pre wet water released by lightweight aggregate exceeds the optimal release amount,resulting in the decrease of durability of UHPC.Therefore,there is an optimal correlation between water absorption and substitution amount of lightweight aggregate.The research of this paper can provide technical support for the design and development of lightweight prefabricated bridge components.