Effect of Shrinkage Reducing Agent and Steel Fiber on the Fluidity and Cracking Performance of Ultra-High Performance Concrete

Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects of different types of cementitious materials,chemical shrinkage-reducing agents(SRA)and steel fiber(SF)were assessed.Compared with M2-UHPC and M3-UHPC,M1-UHPC was found to have better fluidity and shrinkage cracking performance.Moreover,different SRA incorporation methods,dosage and different SF types and aspect ratios were implemented.The incorporation of SRA and SF led to a decrease in the fluidity of UHPC.SRA internal content of 1%(NSRA-1%),SRA external content of 1%(WSRA-1%),STS-0.22 and STE-0.7 decreased the fluidity of UHPC by 3.3%,8.3%,9.2%and 25%,respectively.However,SRA and SF improved the UHPC shrinkage cracking performance.NSRA-1%and STE-0.7 reduced the shrinkage value of UHPC by 40%and 60%,respectively,and increased the crack resistance by 338%and 175%,respectively.In addition,the addition of SF was observed to make the microstructure of UHPC more compact,and the compressive strength and flexural strength of 28 d were increased by 26.9%and 19.9%,respectively.

Numerical simulation of central shrinkage crack formation in a 234-t steel ingot

Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.

Hydration Process and Crack Tendency of Concrete Based on Resistivity and Restrained Shrinkage Crack

Hydration process, crack potential and setting time of concrete grade C30, C40 and C50 were monitored by using a non-contact electrical resistivity apparatus, a novel plastic ring mould and penetration resistance methods, respectively. The results show the highest resistivity of C30 at the early stage until a point when C50 accelerated and overtook the others. It has been experimentally confirmed that the crossing point of C30 and C50 corresponds to the final setting time of C50. From resistivity derivative curve, four different stages were observed upon which the hydration process is classified; these are dissolution, induction, acceleration and deceleration periods. Consequently, restrained shrinkage crack and setting time results demonstrated that C50 set and cracked the earliest. The cracking time of all the samples occurred within a reasonable experimental period thus the novel plastic ring is a convenient method for predicting concrete＇s crack potential. The highest inflection time（t_i） obtained from resistivity curve and the final setting time（t_f） were used with crack time（t_c） in coming up with mathematical models for the prediction of concrete＇s cracking age for the range of concrete grade considered. Finally, an ANSYS numerical simulation supports the experimental findings in terms of the earliest crack age of C50 and the crack location.

Influence of Superplasticizers on Strength and Shrinkage Cracking of Cement Mortar under Drying Conditions

The effects of polynaphthalene series superplasticizers（PNS） with a low content of sodium sulfate （H-UNF）,with a high content of sodium sulfate（C-UNF） and polycarboxylate type superplasticizer （PC） on strength and shrinkage cracking of cement mortar under drying conditions were investigated by means of multi-channel ellipse ring shrinkage cracking test, free shrinkage and strength test. The general effect of PNS and PC is to increase the initial cracking time of mortars, and decrease the cracking sensitivity of mortars. As for decreasing the cracking sensitivity of mortars, PC〉H-UNF〉C-UNF. To incorporate superplasticizers is apparently to increase the free shrinkage of mortars when keeping the constant w/b ratio and the content of cement pastes. As for the effect of controlling the volume stability of mortars, PC〉C-UNF〉H-UNF. Maximum crack width of mortars containing PC is lower, but the development rate of maximum crack width of mortars containing H-UNF is faster in comparison with control mortars. The flexural and compressive strengths of mortars at 28-day increase with increasing superplasticizer dosages under drying conditions. PC was superior to PNS in the aspect of increasing strength.

Experimental and Numerical Study of Shear Fracture in Brittle Materials with Interference of Initial Double Cracks

A simultaneous experimental and numerical study of shear fracture of concrete-like materials is carried out using Brazilian disc specimens with initial double edge cracks and fourpoint bending beam specimens with double edge-notches.The interference effects of two cracks/notches are investigated through varied ligament angles and crack lengths.It is shown that shear fracturing paths change remarkably with the initial ligament angles and crack lengths.The cracked specimens are numerically simulated by an indirect boundary element method.A comparison between the numerical results and the experimental ones shows good agreement.