Performance And Application Research Of Expansive Concrete Used For Chemical Self-Stress

Chemical self-stressing technique generates expansive deformation by changing the chemical composition of cement or adding expanders to concrete,producing compressive stress(self-stress)inside the concrete under restraint.Compared with traditional prestressed steel reinforcement,chemical self-stress can be used as an alternative technology in concrete structures.However,the magnitude of self-stress in self-stressing concrete depends on the type and dosage of the expander,while self-shrinkage and drying shrinkage should be controlled to maintain its self-stress level.In this paper,self-stressing concrete was prepared using portlandite-ettringite-based(HCSA),brucite-based(MgO)expansive agent,superabsorbent polymer(SAP),and shrinkagereducing agent(SRA).The effects of the types and dosages of expanders,SAP,SRA,and changes in curing environment on the expansion deformation and mechanical properties of mortar were systematically studied.The mechanisms by which these materials affect the microstructure of the paste were investigated through thermogravimetric analysis(TGA),Xray diffraction(XRD),and scanning electron microscopy(SEM).The dynamic stress development,cracking load,and crack propagation of self-stressing concrete members were analyzed through early-stage stress measurement of steel reinforcement and four-point bending tests under different reinforcement ratios and expander dosages.Moreover,the stress distribution characteristics of the self-stressing bridge deck connection plate were analyzed,and a mathematical model was established.The specific conclusions are as follows:(1)With the addition of a single expansion agent,the compressive strength of the mortar showed a trend of first increasing and then decreasing with the increase of dosage.Among them,at a dosage of 8 wt%,the influence of the expansion agent on the strength of the mortar was the smallest.Compared with ordinary mortar,the addition of MgO and HCS A reduced the strength of the mortar by 3.6%and 2.3%,respectively.Under water curing conditions,the expansion deformation of the mortar under free/limited constraint conditions increased with the increase of the expansion agent dosage.Among them,under the free unconstrained state,the mortar showed "super-expansion" phenomenon,which disappeared under external limited constraint conditions.When two expansion agents were used for co-blending,the larger the proportion of MgO/HCSA under the same total dosage of expansion agent,the smaller the decrease in the strength of the mortar.Among them,when the total dosage of the expansion agent was 18%and the proportion of MgO/HCSA changed from 4:5 to 5:4,the decrease in strength compared with ordinary mortar decreased from 28.9%to 15.7%.Under water curing conditions for co-blending,the early expansion deformation law of the mortar was similar to that of a single HCSA expansion agent.The difference was that when the age approached 56 days,the expansion deformation value of the mortar showed an upward trend.The larger the proportion of MgO/HCSA,the more obvious the upward trend,and the greater the self-stress value generated by the mortar.(2)HCSA in cement slurry generates a large number of flaky Ca(OH)2 crystals during hydration,which are tightly stacked and arranged.A large number of AFt crystals grow in clusters,and both fill the matrix pores and compress each other to promote expansion.MgO hydrates in the cement slurry to form Mg(OH)2,which adheres to the MgO particles.Some Mg2+ diffuses to the cracks and surrounding pores to nucleate and crystallize,presenting a short rod-like shape.When used in combination,the HCSA expansion agent dissolves to produce a large amount of Ca2+.Due to the same ion effect,the high concentration of Ca2+ inhibits the diffusion of Mg2+,allowing more Mg(OH)2 to nucleate and grow in situ.(3)SAP has an adverse effect on the strength of mortar,especially on early strength,because SAP can "release water" in the later stage of hydration when the internal humidity of the slurry decreases,promoting the generation of surrounding hydration products,making the internal structure of the slurry more compact and improving the later strength of the mortar.SRA makes the microstructure of the slurry looser,changes the micro-morphology of hydration products,which results in a decrease in the compressive strength of the mortar.However,both SAP and SRA significantly increase the early expansion deformation of the mortar,and at the same time,significantly reduce the deformation difference caused by wet-dry cycles,making the specimens maintain stable expansion.(4)Chemical self-stress significantly increased the cracking load of concrete components.Among them,when the expansion agent dosage was 0%,16%,and 18%,the self-stress values were 0,2.14,and 4.96 MPa.and the cracking load values of the components were 12,25,and 36 kN.respectively.That is,the generated self-stress increased the cracking load of the component by 2-3 times.When the expansion agent dosage was 18%and the reinforcement ratio was 0.54%,0.91%,and 1.33%,the self-stress values were 3.16,4.96.and 5.84 MPa,and the cracking load values of the components were 36,43,and 49 kN,respectively.That is,as the reinforcement ratio increased,the stress value increased significantly,and the crack resistance of the component improved.(5)The stress distribution state of the cross-section of the self-stressed bridge deck connection plate has been determined,that is,the stress is uniformly distributed under the constraint of the pavement layer at both ends,and the stress gradually changes along the height of the cross-section from the steel bar position under the constraint of the steel bar,approximately showing a quadratic function distribution.Through theoretical analysis and derivation,reasonable assumptions,and appropriate simplification,a cracking load calculation formula applicable to self-stressed concrete connection plates is proposed.The calculation shows that under higher self-stress values and reinforcement ratios,the tensile stress of the connection plate under external loads is smaller.