*3.6. Tensile Strength*

As shown in Figure 7, at the age of 28 days, with a W/B ratio of 0.4, a GGBFS content of 20%, and a waste PE content of 0~4%, the tensile strength is 12.0~9.9 MPa; when the W/B ratio is increased to 0.5, the tensile strength is 11.2~8.6 MPa (reduced by 6.7~13.1%); when the W/B ratio is increased to 0.6, the tensile strength is 7.8~5.8 MPa, and the tensile strength is reduced by 35~41.4% in comparison to a W/B ratio of 0.4, meaning that the tensile strength is decreased with increasing W/B ratio.

**Figure 7.** Tensile strength of cement mortar with different W/B ratios and waste PE.

At the age of 56 days, when the W/B ratio is 0.5, the GGBFS content is 20% and the tensile strength is 14.2~12.4 MPa for a waste PE content of 0~4%, indicating that the tensile strength is slightly decreased with the increased waste PE content; at the age of 91 days, the tensile strength is 16~14.3 MPa. As the waste PE is soft and the tensile strength cannot be effectively increased, the tensile strength is decreased with increasing waste PE content. Meanwhile, the tensile strength, flexural strength, and compressive strength show the same trend.

#### *3.7. Ultrasonic Pulse Velocity*

As shown in Figure 8, at the age of 28 days, with a W/B ratio of 0.4, GGBFS of 20%, and different percentages of waste PE, the ultrasonic pulse velocity is 4720~4501 m/s; the pulse velocity is 4386~4173 m/s for a W/B ratio of 0.5, which is reduced by 7.1~7.4% in comparison to that at a W/B ratio of 0.4; when the W/B ratio is 0.6, the pulse velocity is 3825~3611 m/s, which is reduced by 12.8~13.5% in comparison to that at a W/B ratio of 0.5, meaning that the ultrasonic pulse velocity is decreased with increasing W/B ratio.

**Figure 8.** Ultrasonic velocity of cement mortar with different W/B ratios and waste PE.

At the age of 91 days, the control group shows a better ultrasonic pulse velocity of 4903 m/s; the ultrasonic pulse velocity is 4816 m/s for a waste PE content of 1%; and the ultrasonic pulse velocity is decreased to 4607 m/s for a waste PE content of 4%, which represents a 6.1% reduction in comparison to that for the control group. At the age of 56 days, when the substitution amount of GGBFS is 20% and the waste PE fiber material content is 1~2%, the ultrasonic pulse velocity is higher than 4500 m/s, meaning that the concrete quality is good.

When the waste PE content is increased, pores are formed as the specimen absorbs free water so that the ultrasonic pulse velocity is decreased. When the substitution amount of GGBFS is increased, the pulse velocity slowly increases at an earlier age; however, under the effect of pozzolanic reaction, the specimen pores are filled up at a later age, leading to a better density for the specimen, and the pulse velocity grows slowly at the later stage. The internal porosity of the specimen is increased with increasing W/B ratio, resulting in a decrease in the pulse velocity. Furthermore, the ultrasonic pulse velocity is decreased as the content of waste PE fiber material is increased.
