Effects On Fracture Properties Of Concrete For Using SAP

High-performance concrete has the characteristics of low water-cement ratio and high content of cement and mineral admixture.Although this material has been greatly improved in strength,the water contained in the high-performance concrete is insufficient to meet the requirements of cement hydration and pozzolanic reaction,which result in its self-drying and self-shrinkage.Problems such as early cracking sensitivity have occurred during the practical engineering use.At this time,traditional curing methods have not been able to effectively resolve these contradictions: they may eliminate auto-shrinkage of small-size sections,but have no use of effectively solving the shrinkage of large-size sections.Under such conditions,the different strains produced inside the concrete can exacerbate the adverse effects of autogenous shrinkage.In order to alleviate this problem,researchers have proposed a more effective method by provide internal water supply in recent years.There are two main methods for internal water storage: one is incorporating pre-absorbed lightweight aggregate,and the other is adding super absorbent polymer(SAP).Because the internal curing water is a part of the entire test pieces and evenly distributed in them,it can overcome the low permeability,low water-cement ratio,etc.Internal curing can greatly impact the inherent curing mode,and can also continuously maintain the internal hydration of concrete.High relative humidity can effectively reduce or avoid the self-drying and self-shrinkage.At the present stage,researchers at home and abroad have conducted in-depth studies on the shrinkage,frost resistance,impermeability,mechanical properties,environmental performance,and microstructure of concrete in the internal curing,and have made considerable progress.But the effects on fracture properties of concrete for using SAP is rarely involved.Internal curing is able to greatly reduce the shrinkage and cracks of concrete.Therefore,it is necessary to further explore its impact on the fracture properties of concrete.The effect of SAP on concrete was investigated based on fracture mechanics.With aim at the characteristics of internal curing to improve the early performance of concrete,results of fracture behaviors were compared for concrete with different amounts of SAP and different curing time.The main contents are as follows:(1)The effects of curing time and amount of SAP on compressive strength and tensile strength were studied.The test results show that the compressive strength and splitting strength of the concrete increased,and the increased range first increased and decreased with the increase of the amount.(2)For three-point bending notched beam,the initial cracking load,maximum load and critical crack mouth opening displacement of the concrete were measured,and accordingly the effective critical crack length,initial cracking toughness and unstable fracture toughness were calculated.The effects of curing time and amount of SAP on the initial cracking toughness and unstable fracture toughness were investigated.The results show that the initial cracking load,maximum load,critical crack mouth opening displacement,elastic modulus,effective critical crack length,initial cracking toughness and unstable fracture toughness of the specimen increased after incorporating SAP.The increase in age will increase as usual,but it will not always increase as the amounts of SAP increase.(3)According to the principle and calculation method of the KR resistance curve which has been based on the crack driving criterion,the fracture process zone and the KR resistance curves of the specimens are plotted on the basis of the mode I crack propagation criteria of concrete.And study the effect of SAP on the crack extension resistance,then the fracture energy of the concrete beam is calculated by method of fracture work.The result shows that the introduction of SAP to disperse crack tip stress contributes to the improvement of the crack extension resistance and fracture energy.