Study on Mechanism of Combined Curing Composed of Precuring in Hot Water and Heating in Dry Air for Optimizing Mechanical Properties and Thermally Explosive Spalling Resistance of Ultra-High Performance Concrete
Author:Niu Xu Zuo
Supervisor:peng gai fei
In recent years,ultra-high performance concrete(UHPC)has been gradually applied to many fields of civil engineering,owing to its ultra-high compressive strength and excellent durability.Heat curing is an effective method to improve mechanical properties of UHPC significantly.Combined curing regime,as a novel approach of heat curing,has attracted much attention from scholars.However,there is not much deeply research on the combined curing and its mechanism.In addition,UHPC with an extremely dense microstructure is prone to explosive spalling under high temperature,leading to an obvious decrease in bearing capacity of structures.Therefore,it is necessary to investigate the resistance of UHPC to explosive spalling at elevated temperature.This thesis mainly investigated influences of hot water-dry air combined curing on mechanical properties and thermally explosive spalling resistance of UHPC,and compared with three types of mono-curing regime,such as 20 ℃ water curing,hot water curing and dry air heating.The optimum combined curing regime was obtained by changing duration and temperature of the dry air heating therein.In order to analyze the mechanism of the combined curing,microstructure of UHPC paste under various curing regimes was determined by means of microscopic testing techniques,such as X-ray diffraction with Rietveld method,thermogravimetric analysis,scanning electron microscope and mercury intrusion porosimetry.Furthermore,the influences of the combined curing on mechanical properties and microstructure of high performance concrete(HPC)were studied and compared with that of UHPC.The main conclusions are as follows:(1)Compared with three types of mono-curing regime,hot water-dry air combined curing can improve the mechanical properties and thermally explosive spalling resistance of UHPC significantly,which is more efficient with prolonged duration and raised temperature of the dry air heating therein.In this paper,250CC-3d(90 °C hot water curing for 2 days + 250 ℃ dry air heating for 3 days)was the optimum curing regime.For plain UHPC,the highest compressive strength(197.1 MPa)and tensile splitting strength(8.0 MPa)were obtained when it was cured under 250CC-3d,exhibiting the increments of 65.1%and 73.9%relative to that under 20 ℃ water curing.Meanwhile,there was no explosive spalling occurrence on the UHPC subjected to 250CC-3d.(2)The combined curing can decrease maximum vapor pressure inside UHPC at elevated temperature remarkably.Meanwhile,the vapor pressure in deep layer of UHPC is higher than that in shallow layer,which is obviously different from that cured under mono-curing regimes.(3)The mechanism for the influences of the combined curing on UHPC is that,a dense structure framework with many amorphous C-S-H gels is formed inside UHPC during the precuring in hot water.When the UHPC is heated under a dry air condition,a high temperature steam environment characterized by relatively high temperature,humidity and vapor pressure can be established in the inner part of framework.Both further cement hydration and pozzolanic reaction can be activated under this high temperature steam condition.It leads to the formation of a large amount of dense hydrates(such as C-S-H gels,tobermorite,xonotlite,katoite and M-S-H),which gradually accumulate in the aforementioned existing aforementioned framework;meanwhile most of free water inside UHPC has been consumed.Therefore,both mechanical properties of UHPC and its thermally explosive spalling resistance are enhanced significantly.(4)The hot water-dry air combined curing also improves the mechanical properties of HPC.However,prolonged the dry air heating duration(from 6 h to 3 d)can lead to a significant decrease in the mechanical properties,which is quite different from UHPC.The compressive strength,tensile splitting strength and fracture energy of HPC subjected to 250CC-3d(90 ℃ hot water curing for 2 days+ 250 ℃ dry air heating for 3 d)decreased by 16.2%,22.8% and 34.9% respectively,compared to that cured under 250CC-6h(90℃ hot water curing for 2 days + 250 ℃ dry air heating for 6 h).(5)Compared to UHPC,a relative looser microstructure is formed inside HPC during the hot water precuring.When the HPC is heated under a dry air condition,a large amount of internal free water can escape to the outside easily,so that it is difficult to build up a high temperature steam environment,which is the case in UHPC.Thus,as to HPC,there is no significant influence of the combined curing on increasing the degree of cement hydration and activity of pozzolans during the dry air heating process.Moreover,dry air heating may cause a pore structure coarsening effect in HPC,and hence a decrease in the mechanical properties significantly.Therefore,the combined curing is a curing regime efficient for UHPC other than for HPC.In general,a novel approach,namely hot water-dry air combined curing,has been provided in this thesis for significantly improving resistance of UHPC to explosive spalling under high temperature and its mechanical properties.Meanwhile,the mechanism of the combined curing was revealed,based on the determination of microstructure of UHPC.It has not only a certain theoretical guiding significance,but also an obvious promoting effect on the further application of UHPC in engineering.