Internal Curing Of SAP And FLWA In Ultra-high Performance Concrete

Ultra-High Performance Concrete(UHPC)is a new type of composite material with high strength and high toughness,which has broad application prospects in engineering practice.However,due to the low water-to-binder ratio(w/b),high binder material content and no coarse aggregate,the autogeous shrinkage of UHPC is very large,and the cracking problem caused by it is very serious.In addition,UHPC has a dense microstructure,and external moisture is difficult to enter the matrix to compensate for the rapid decrease in internal relative humidity(IRH).As a new type of curing method,internal curing provides an effective source of moisture for the interior of the concrete by introducing water-absorbing materials.This not only reduces the self-dessication phenomenon,but also promotes the hydration of the internal cementitious material and improves the durability.There is less research on the effectiveness of internal curing on UHPC.From the purpose of UHPC shrinkage control,this paper mainly studies the effects of superabsorbent polymer(SAP)and fine lightweight aggregate(FLWA)on the performance of UHPC.The absorption and desorption characteristics of different particle sizes of SAP in tap water and filtrated water were studied by tea bag method,and the effects of different mineral admixtures on absorption and desorption properties of SAP were quantitatively analyzed by ~1H nuclear magnetic resonance(~1H NMR).The results show that the water absorption of SAP in tap water was much greater than the water absorption in the filtered solution.Small particle SAP had a higher water absorption rate and a shorter water absorption time than large particles.SAP had two behaviors in the UHPC paste,water absorption and water release.The addition of silica fume changed the water release characteristics of SAP in the UHPC matrix,and its water release process could be divided into two stages controlled by osmotic pressure and humidity,respectively.The effects of SAP with different particle size and dosage on the strength,shrinkage and durability of UHPC were studied.Combined with microstructural analysis,the mechanism of internal curing on UHPC was investigated.Under the same effective w/b,the addition of SAP and additional water could significantly inhibit the autogenous shrinkage of UHPC,while also reducing the compressive strength and increasing the drying shrinkage of UHPC.As the amount and particle size of SAP increases,the autogeous shrinkage of UHPC was further reduced,and the negative impact on dry shrinkage and strength was also greater.On the other hand,in the case of the same total w/b,the addition of SAP could increase the compressive strength and reduce the vacuum water absorption,carbonization depth and chloride ion permeability coefficient.Although the large pores remaining after the release of the SAP increased the porosity,since they were closed pores,they did not affect the permeability.In addition,SAP and additional water could increase the degree of hydration of the UHPC matrix and reduce capillary porosity.Based on the interaction between fiber and SAP,the effect of SAP particle size and dosage and two different curing methods on UHPC compressive strength,flexural strength,fiber bonding strength and shrinkage performance were studied.Micro-hardness and backscatter scanning electron microscopy(BSEM)was used to explain the effect of SAP on the fiber-interface transition zone(ITZ)in UHPC.The results show that SAP had two effects on the ITZ.Firstly,the internal curing effect of SAP made the ITZ denser,and reduced the micro-cracking of UHPC matrix.Secondly,the residual pores after the SAP releases water would have a negative impact on the ITZ.This dual effect was affected by SAP particle size,dosage,and UHPC curing.Under water-curing conditions,low-dosage(0.3%)of small particle size SAP had the highest flexural strength.In addition,due to the presence of SAP residual macropores,the flexural strength of UHPC decreases with increasing SAP content and particle size.The internal curing provided by SAP could inhibit the shrinkage of UHPC during water and steam curing.In the presence of fibers,the shrinkage inhibition effect of small particle SAP was superior to that of large particles.In addition,in the case of steam curing,UHPC with small particle SAP had the lowest total shrinkage.Based on the internal curing of SAP in UHPC under dry conditions,the internal curing efficiency of SAP under low humidity conditions and the change of microstructure of UHPC were evaluated.Finally,the combination use of shrinkage reducing agent(SRA)and SAP was investigate to solve the drawbacks of drying shrinkage by using of SAP.The results show that SAP could slow down the IRH drop caused by evaporation under drying contion.When the distance from the dry surface exceeds 5 cm,the UHPC matrix at this location was substantially same as the UHPC under sealed conditions.The total shrinkage of UHPC under dry conditions was nonlinearly related to the average moisture loss due to the coupling of self-dessication and evaporation.In terms of size effect,SAP had better shrinkage inhibition effect on larger UHPC specimens than small specimens.In addition,the interaction of the SRA molecule with the functional groups on the SAP hydrogels resulted in the lowest autogeous shrinkage of the combination use of SAP with low dosage and SRA.The use of SAP and SRA resulted in a slight increase in compressive strength at 28d and 90d.Using FLWA as an internal curing material,the effects of pretreatment methods and dosage on UHPC strength,shrinkage and microstructure were systematically studied.The results show that the pretreated FLWA effectively enhanced the flexural strength of UHPC,but the compressive strength was not significantly improved.In addition,the additional water was not conducive to the development of later strength of UHPC.Without the addition of additional water,UHPC increased first and then decreased with the increase of FLWA content.When the content was 15%,the autogeous shrinkage of UHPC was the smallest.The vacuum-saturated FLWA maintains a high relative humidity inside the UHPC for a longer period of time and is more effectively to contro autougenous shrinkage.Both normal soaking and vacuum saturated pretreatment of FLWA reduce drying shrinkage.However,the addition of internal curing water to the matrix has an adverse effect on drying shrinkage.The internal curing water provided by the two pretreatment methods could be used for the later hydration of cement,and enhanced the interface microstructure of FLWA and matrix.