Study On The Mechanical Performance Of High Toughness Early Strength Concrete

The disadvantages of normal concrete,such as slow congealing and hardening reaction rate,low early strength and insufficient tensile strength,limit its application in urgent repair engineering.To improve the mechanical properties,i.e.early strength,tensile properties and toughness of concrete,orthogonal experiment method was used to validate the influence factors on setting time,exothermic rate and the mechanical properties.Then,according to the established design method,a high-toughness rapid-hardening concrete was developed.The main research contents and conclusions are as follows:1.A series of experiments were carried out on the influence of retardant,temperature and fiber content on the setting,hardening reaction and the slump of the cementitious materials.It is indicated the dosage of sodium gluconate and temperature have great influence on the setting of the sulphuraluminate-based cementitious materials.The setting time of cementitious materials increases with the increase in dosage of sodium gluconate and the decrease in the curing temperature,and the time between initial setting and final setting increases as well.Sodium gluconate reduces the exothermic rate of sulphoaluminate cement hydration at the earlier stage,but enhances it at the later stage.Meanwhile,the polycarboxylic-based early strength superplasticizers reduced the exothermic rate at the earlier hydration stage but had little effect on the later exothermic procedure.The increase in PVA fiber volume content results in remarkable slump losses in concrete mixture.2.Orthogonal experiments were performed to investigate the influence of the sodium gluconate,water-binder ratio,the dosage of PVA fiber content on the mechanical behavior of the high-toughness rapid-hardening concrete.The experimental results indicate that sodium gluconate have considerable influence on the early strength of the concrete,and negligible effect on the later strength.It is mentioned that the ratio of sodium gluconate should not exceed 0.2% in order to obatain a suitable setting rate.With the increase in water-binder ratio,the compressive and flexural strengths demonstrate a descending tendency.Meanwhile,both the compressive and flexural strengths of the specimens first increase and then decrease with the increase in PVA fiber content,and the optimal dosage is about 0.4%(by volume)according to the experimental results.3.The surface morphology and microstructure of the high-toughness rapid hardening concrete were studied with the SEM(scanning electron microscopy).The morphology and microstructure shows relvative uniform distribution of fiber in the concrete.The interface between the PVA fiber and concrete shows fine bonding condition in low fiber volume content.However,the interfacial bonding property deteriorates due to fibrous aggregation phenomenon when fiber volume content exceeds 0.6%.In addition,fiber has a negligible effect on compactness of concrete when fiber content less than 0.4%.Once the fiber content exceeds 0.6%,the detrimental influence of fiber on compactness cannot be ignored.The hydration reaction of the sulphuraluminate cement almost completes within 3 days.Large amount of dense ettringites(AFt),a typical hydration products of gelled material,generate during this period of time,and strength of specimens develop remarkably.