3.2 Flexural strength
Figure 4 displays the flexural strength of carbon fiber foam concrete at different ages.
The flexural strength of group C2 with 0.6wt.% carbon fiber increases significantly and reaches 3.6 MPa at 28d, 188.62% higher than that of group O at the same age. The strength values at different ages are sorted from large to small as group C2 > group C3 > group C1 > group O. It indicates that there is an optimal dosage of carbon fiber in improving the flexural strength. Han et al.[35] found that the enhancement mechanism of carbon fiber on cementitious materials can be divided into three aspects. First, the removal or fracture of carbon fiber can absorb energy to improve mechanical properties. Second, hydration products can be tightly bonded to carbon fiber, indicating good adhesion between carbon fiber and cement slurry. Third, carbon fiber can inhibit crack growth.
Figure 5 shows the flexural strength of foam concrete with composite admixture.
As depicted in Figure 5, adding carbon fiber and graphite significantly enhances the flexural strength. The flexural strengths of group C2S1 at 3d and 28d are 4.1 MPa and 4.3 MPa respectively, 262.83% and 249.59% higher than those of Group O at the same age. When the graphite content exceeds 2wt.%, the flexural strength of this group declines, but it is still in the range of 3.4 MPa to 3.7 MPa and higher than the flexural strength of group O. Compared with group C2, the flexural strength values of group C2S1 and group C2S2 mixed with graphite still improve. It shows that the synergistic effect of carbon fiber and graphite can strengthen the flexural strength of specimens, and the specimens with composite admixture has better flexural strength than that mixed with carbon fiber alone when the graphite content is less than 5wt.%. As for the enhancement mechanism, firstly, carbon fiber can absorb energy, adhere to hydration products, and suppress crack growth. Secondly, graphite has the stabilizing effect on foam cement slurry.