Air Void Structure And Mechanical Properties Of Cement-based Materials Modified By Superabsorbent Polymers

Autogenous shrinkage caused by the self-desscication of concrete has been a major issue in concrete structures.The issue is even heightened by the wide application of high-strength concrete and high-performance concrete with the features of low water-to-binder ratio and high impermeability.As a result,internal curing has been popular in recent years.Its idea is to supply water during the hydration of cementitious materials through internal curing agent such as superabsorbent polymers(SAP).However,there is a lack of efficient and accurate method to characterize the absorption capacity of SAP.Prior to implementing SAP in cement-based materials,the absorption capacity in this environment needs to be known.In addition,it remains unclear how SAP affect the mechanical properties of hardened concrete.The dissertation explains the discrepancy between results from two simplistic methods,and puts forward a new method to estimate the absorption capacity of SAP.The teabag method is modified with different convection conditions during the test,and with various water/cement ratios and hydration times during pre paration of cement slurry filtrate that is used as test liquid.The filtration method is modified with different liquid/SAP ratios.The results reveal that a decrease in absorption capacity observed with the tea-bag method is due to properties of the test liquid and ion exchange between SAP and test liquid.The difference in 24-hour absorption capacities given by the two simple methods is likely due to unequal amounts of interstitial water.It is advised not to use these simple methods as means of estimating SAP absorption capacity in concrete,since the results from these methods do not properly represent the absorption behavior in concrete.Instead,air void analysis of hardened cement paste seems to be a reliable method to register the SAP absorption capacity in cement-based materials.On this basis,this dissertation looks into how the absorption capacity of a superabsorbent polymer(SAP)in cement paste is influenced by a number of variables: cement type,water-to-cement ratio(w/c),additive,dry SAP particle size,mixing time,and temperature.By means of air void analysis of hardened cement paste,the absorption capacity and the specific surface of SAP voids are obtained.Parallel to the direct measurement of specific surface,this parameter is also de rived from dry SAP particle size and absorption capacity.The results show that for the specific type of SAP examined the largest variation in absorption capacity is observed when changing the cement type.Prolonged mixing time can increase the absorption capacity of SAP up to 20%.The influence of a superplasticizer on the absorption capacity of SAP is statistically significant,whereas w/c ratio or dry SAP size has no apparent relation with absorption capacity.A temperature either higher or lower than 20°C can lead to notably higher absorption capacity of SAP.Prediction of specific surface of SAP voids based on measured absorption capacity and calculation seems promising,as it agrees with the specific surface measured directly by air void analysis.In addition,this dissertation investigates the mechanical performance of cement paste modified by SAP.Digital cement pastes consisting of solid phase and gas-filled voids are made and subjected to uniaxial load.The constitutive relation of the solid phase confirms the damage plastic model presented in Abaqus.It is found by numerical simulation that the shape of voids has a small but definite influence on the elastic modulus and strength of cement paste.The size distribution has almost no influence on the compressive strength of traditional air-entrained cement paste,but makes a difference on that of SAP-modified cement paste.The contrast can be well explained by the increased portion of protected zone.This work then presents a model to account for how SAP affect the long-term compressive strength of concrete.SAP can affect the strength of concrete both positively and negatively,but the overall direction and degree remain controversial.By coupling the influence of paste strength and SAP void content,this paper presents a model to calculate the long-term compressive strength of a SAP-modified concrete relative to another one without SAP but with the same water-to-binder ratio(w/b)and the same paste content.Results indicate that in a cement concrete,there exists a critical w/b,above which any amount of internal curing water reduces strength.The critical w/b depends on paste content.At a certain paste content,the largest possible degree of enhancement is greater,when w/b is lower.The model is validated by experimental results,both original and from the literature,and is applicable to concretes whether or not supplementary materials are used.