Investigation On The Shrinkage-reducing Effect Of Super-absorbent Polymer In High-strength Concrete And Its Mechanism

Concrete cracking caused by shrinkage is seriously harmful to the safety anddurability of structures. For high-strength concrete (HSC), the early-age moistureshrinkage (including autogenous shrinkage (AS) and drying shrinkage (DS)) is themain cause of cracking. Internal curing using super-absorbent polymer (SAP) is aneffective way to reduce the moisture shrinkage of HSC so as to be in favor of theprevention of cracking. However, the mechanism of shrinkage reduction for SAP hasnot yet been fully disclosed.This article gives a comprehensive study on properties of HSC incorporated withSAP, including the hydration process of cement, the microstructure of hardenedcement paste and the mechanical properties of concrete. The effects of pre-soakedSAP on the shrinkage evolution of concrete at early ages, including AS and DS aregiven special emphasis. The results show that: at a constant effective water-to-cementratio, pre-soaked SAP significantly decreases the AS and DS of HSC. The ASreduction ratio is over90%and the total shrinkage under dry condition decreases to75%of the control group. SAP delays heat release of cement at the early age, butincreases its ultra hydration rate; it changes the distribution of concrete’s porestructure, increasing the quantity of macropores, large and small capillary pores anddecreasing the quantity of medium capillary pores; SAP can maintain a much higherrelative humidity (RH) inside concrete at the same age. Adding SAP has a slightnegative effect on strength development of concrete, especially for concrete at theearly age. Compared with the simple addition of free mixing water, the addition ofpre-soaked SAP makes significant differences in the hydration process, pore structure,mechanical properties and shrinkage deformation of HSC. The differences of thedistribution of the two types of water, the free mixing water and the internal curingwater stored in SAP gel, account for these changes described above.With the help of a capillary stress based model for shrinkage prediction, thestudy on the mechanism of shrinkage reduction for SAP is given on the basis of theobtained experimental results. The calculating results show that the model is wellsuited for fitting and prediction of early-age moisture shrinkage. There are two reasons for SAP to reduce shrinkage on the basis of experimental results and modelanalysis. One is the much smaller ratio of the macro AS and total chemical shrinkagewhen the RH keeps constant at100%, the other is the much higher inner RHcompared with control group of the same age after the RH begins to drop down,which will effectively reduce the driving force of AS and DS. A simple modificationis conducted to make the model better fitted with the experimental results and buildthe continuity between the two separate parts of the model.A self-expanding curve is obtained by model calculation, whose main factors andmechanism are analyzed at the same time. The results show that all the specimensexperience a significant expansion since casting. Both of increasing the water-cementratio and the amount of pre-soaked SAP can increase the amount of early-ageexpansion. The type and composition of cement have a decisive impact on theearly-age expansion. The vast generation of large crystals of Ca(OH)2and ettringite isthe main cause of the early expansion, and the C-K-S-H gel generated in theenvironment of high alkali content also contributes to the occur of early expansion.