Enhancing mechanical properties of engineering cementitious composite by defoamer

•The effect of defoamer on the mechanical properties of ECC was investigated.•Adding defoamer could improve compressive strength, tensile strength, and tensile capacity of ECC.•With increasing defoamer content, mechanical properties of ECC increased and then decreased.•The energy criterion and strength criterion were satisfied for the specimens with defoamer. The pores introduced by the inclusion of fibers affect the mechanical properties of engineering cementitious composite (ECC). In this paper, the effect of defoamer on the mechanical properties of ECC was investigated. Different contents (0.05 wt.%, 0.10 wt.%, 0.15 wt.%, and 0.20 wt.%) of defoamer were used for this purpose. The results showed that the inclusion of defoamer could significantly increase the compressive strength, first-crack strength, ultimate tensile strength, and tensile strain of ECC. A dosage of 0.15% was the optimal defoamer content for enhancing the mechanical properties of ECC, as further increasing defoamer content beyond 0.15% compromised the efficiency of the improvement. The inclusion of defoamer reduced the total number, size, and volume of pores in ECC, resulting in the increase in the fracture toughness of matrix and fiber/matrix interfacial bond. Based on the micromechanics theory, the inclusion of defoamer increased the pseudo strain hardening(PSH) indexes that govern the multi-cracking behavior. The results from micromechanics analysis well explained the trend of tensile properties of ECC with different defoamer contents.