Study On Early Shrinkage And Mechanism Of Ultra-high Performance Concrete Under Environmental Factors

Ultra-high-performance concrete(UHPC),possessing outstanding mechanical properties and durability,exhibits immense potential for application in the field of engineering construction.However,the issue of early shrinkage of UHPC is of significant concern,as it may give rise to cracks and damage,ultimately compromising the service life of structures.As a result,the phenomenon of UHPC shrinkage has garnered widespread attention.Nonetheless,the existing literature has predominantly focused on UHPC’s autogenous shrinkage,while early-age drying shrinkage’s importance has been neglected.The capacity to comprehend the theoretical underpinnings of UHPC’s shrinkage cracking is hampered by this approach,and the application of UHPC is impeded,rendering it incapable of meeting diverse engineering requirements.Thus,in this study,the early-age drying shrinkage and formation mechanism of UHPC under dry conditions were investigated systematically to provide a comprehensive perspective on this issue.In this study,early shrinkage tests were conducted on UHPC with different powder gradations,and the main areas of investigation included: 1)the impact of environmental factors on early UHPC shrinkage;2)the formation mechanism of UHPC shrinkage;and 3)effective UHPC shrinkage reduction mechanisms.As a result,the causes of early UHPC shrinkage in dry environments were explained,and suitable measures for shrinkage reduction were proposed,providing theoretical and technical support for UHPC’s engineering application.(1)The optimal UHPC mix proportion was obtained through powder gradation experiments,and the impact of evaporation on early UHPC shrinkage was analyzed based on this.The study showed that evaporation not only promoted concrete drying shrinkage but also inhibited the hydration reaction of cementitious materials,delaying the autogenous shrinkage of concrete in dry environments.Therefore,it was concluded that the dry shrinkage of UHPC at the age of 24 hours was about 51%～53% of the total shrinkage,indicating that if early curing is improper,UHPC’s autogenous shrinkage may not necessarily be the dominant factor in early shrinkage.Combining evaporation and pore structure distribution,it was found that the critical pore size of UHPC at 24 hours was close to the mesopore size(～50 nm),which generated large shrinkage stress.Therefore,early drying shrinkage became one of the main factors affecting the volume stability of UHPC.(2)To further explore the early-age shrinkage issue of UHPC as stated in(1),an analysis was conducted on the shrinkage pattern and formation mechanism of UHPC under the influence of environmental factors.The study revealed that under a wind speed of 0.25 m/s or humidity of 40%,the critical pore diameter of UHPC approached the most probable pore diameter(～32 nm),causing a substantial increase in the shrinkage rate of UHPC.The most probable pore diameter reflects the pore volume characteristics of concrete.Therefore,it was concluded that the growth of the critical pore volume under environmental factors was the main cause of UHPC shrinkage development.Furthermore,the critical pore diameter is an essential parameter related to shrinkage.However,when evaporation becomes intense and the critical pore falls within the mesopore range,the effect of pore diameter parameters on shrinkage is less significant than that of pore volume parameters.To quantitatively evaluate the evolution pattern of early-age shrinkage in UHPC,a shrinkage prediction model based on critical pore parameters was established and validated by measured values.(3)To address the issue of UHPC shrinkage mentioned,an external curing mechanism was established to improve the curing efficiency while controlling the curing time.The study revealed that curing UHPC for 6 hours at 90% humidity after the initial setting led to a significant increase in the critical pore diameter of the concrete,thereby relieving the growth of pore stress.This resulted in a 68.9% decrease in drying shrinkage and a 26.9% decrease in total shrinkage of UHPC at 24 hours,indicating that the external curing measure had a high curing efficiency.After early-stage curing,the critical pore diameter of UHPC is typically higher than the mesopore diameter(～50nm),and the change in pore volume within the corresponding pore diameter range is small,limiting its effect on shrinkage.Therefore,the critical pore diameter is considered a key indicator to explain the shrinkage mechanism under external curing conditions.(4)The advantages of different curing mechanisms were complemented by utilizing a combination of internal and external curing techniques,which resulted in a further reduction of early shrinkage of UHPC.It has been shown by research that drying shrinkage can be effectively suppressed by external curing at 90% humidity,but autogenous shrinkage cannot be reduced.Better effects are observed for internal curing with superabsorbent polymers(SAP),which can slow down the self-deformation of the matrix.However,after adding SAP with a cementitious material dosage of 0.1%,the compressive strength of the UHPC decreased by 13.0%,which is not conducive to ensuring the mechanical properties of the concrete.Composite curing is a method by which the shortcomings of external curing are compensated for,the amount of SAP is reduced,and the negative impact of internal curing is mitigated.With the incorporation of 0.05% SAP,the autogenous shrinkage of UHPC was reduced by 11.2%.The synergistic effect of 0.05% SAP and 90% humidity significantly reduced concrete evaporation,and the critical pore size of the matrix increased to the range of 150-350 nm,effectively delaying the growth of pore stress.Therefore,the total shrinkage of UHPC in 24 hours decreased by 40.4%,indicating that composite curing can effectively suppress the early shrinkage of UHPC.