Quasi-static tension experiments on long unnotched matrix and SHCC specimens

To understand the effect of increasing strain rates on the mechanical properties of SHCC, it is important to assess the individual strain rate sensitivities of the matrix, of the fiber and of their common interface. The quasi-static compressive strength of the matrices is given as a reference and also for justifying the term “high-strength” attributed to M2, since it is difficult to assign this feature based on its tensile strength alone. Table 4.7 presents the average values of the quasi-static tensile strength of both cementitious matrices under investigation as measured on long unnotched cylindrical specimens as well as their average compressive strength values as measured on cubes with 100 mm side length. The average compressive strength is 44.8 MPa for M1 and 141.8 MPa for M2, while the average tensile strength of M1 is 2.8 MPa and that of the high-strength matrix M2 is just 3.4 MPa. Most probably, the relatively low tensile strength of M2 is caused by air voids resulting from its high viscosity in the fresh state as well as by distributed, tiny clumps of unhydrated cement and silica fume which form due to the insufficient shear stresses during mixing due to the absence of large aggregates and fibers.

Table 4.7. Average compressive and tensile strength values of the normal-strength M1 and high-strength M2 matrices; standard deviations are given in parentheses.

Matrix type and specimen geometry M1-cube M2-cube

Compressive strength [MPa] 44.8 (0.3) 141.8 (8.5)

Matrix type and specimen geometry M1-L M2-L

Tensile strength [MPa] 2.8 (0.6) 3.4 (0.3)

Figure 4.32 presents the stress-strain curves obtained from the long SHCC specimens in quasi-static tension tests. The curves are quantitatively very similar to those presented in Section 4.2 and by Mechtcherine et al. (2011a) on considerably larger cylindrical specimens 75 mm in diameter, 250 mm effective length and 70 mm gauge length. The first-crack stress for M1-PVA-L is the highest among the three tested types of SHCC. It is also higher than the tensile strength of the non-reinforced matrix M1 as given in Table 4.7. This demonstrates the effect of the strong chemical bond between the PVA fibers and the cementitious matrix,

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which involves the fibers’ hindering the propagation of micro-cracks before the formation of a larger crack recognizable by the first stress drop in the stress-strain curve.

The first-crack stress of PE-L is, with 2.4 MPa, considerably lower in comparison to M1-PVA-L; see Table 4.8, and lower than the tensile strength of the normal-strength cementitious matrix M1 (2.8 MPa) as well; see Table 4.7. The hydrophobicity of the HDPE fibers leads to a weak frictional bond with the normal-strength cementitious matrix, meaning that the fibers are activated only after crack formation. Considering the high volume fraction of fibers, their random orientation and not perfectly uniform distribution (due to poor fresh state properties) as well as hydrophobic nature of the HDPE fibers (thus, lack of chemical bond to the matrix), fiber-matrix interfaces are likely to act as micro-defects. The author assumes that this is the main mechanism leading to the reduction in the first-crack stress of the composite in comparison to the tensile strength of the constitutive cementitious matrix as observed in the investigation at hand. The assumption regarding the weak composite action between HDPE fibers and the cementitious matrix before crack formation is also supported by the results obtained for M2-PE-L, which yielded first-crack stress lower than the tensile strength of the non-reinforced high-strength matrix M2, i.e., 3.0 MPa vs. 3.4 MPa. Curosu et al. (2017) showed that polymer micro-fibers which form a strong interfacial bond with the surrounding cementitious matrix may increase significantly the first-crack stress of SHCC, provided that the Young’s modulus of the fibers is higher than that of the matrix.

Despite the relatively low first-crack stress, M2-PE-L yielded the highest tensile strength with an average value of 6.5 MPa. The other composites reached average tensile stresses of only 3.8 MPa (M1-PVA-L) and 3.6 MPa (M1-PE-L); see Table 4.8. The fracture surfaces of M2-PE showed partial fiber rupture, indicating an adequate utilization of the fibers’ mechanical properties. At the same time, the fracture surfaces of M1-PVA-L exhibited a highly pronounced fiber rupture, the maximum protruding length of the fibers being less than 2 mm, significantly less than half of fiber length. This was caused not only by the strong interfacial bond but also by the slip-hardening pullout behavior of the PVA fibers from the cementitious matrix under low strain rates (Kanda and Li 1998b; Boshoff et al. 2009). In this case the tensile strength of the composite was chiefly limited by the tensile strength of the PVA fibers.

In contrast to M1-PVA-L, the fracture surfaces of M1-PE-L showed complete fiber pullout since neither a chemical bond (like in M1-PVA) nor a dense and uniform surrounding matrix (like in M2-PE) enabled a proper fiber anchorage; this explains the lowest tensile strength.

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Figure 4.32. Stress-strain curves of the SHCC under investigation as obtained in quasi-static tension tests on long unnotched specimens.

M2-PE-L reached the highest ultimate strain before softening, of 3.6% on average, as a result of highly pronounced multiple cracking. The micro-defects, which have a negative influence on first-crack stress of M2-PE, exhibit a beneficial effect on the quasi-ductility of the high-strength SHCC by playing the role of crack initiators. Furthermore, the high strain-hardening ratio of M2-PE, i.e., the ratio of tensile strength to first-crack stress, is advantageous for more pronounced multiple cracking. The strain capacity of M1-PVA-L was the lowest compared to the other two SHCCs. An important reason for such a behavior was poor multiple cracking due to the excessively strong fiber-matrix interfacial bond causing rupture of a considerable number of crack bridging fibers during crack formation already. M1-PE-L yielded an average ultimate strain of 1.7%, i.e., marginally higher than that of M1-PVA-L.

Table 4.8 presents the work-to-fracture and the specific fracture energies of the SHCC investigated. Work-to-fracture was calculated as the area under the stress-strain curves up to failure localization (softening), while specific fracture energy corresponds to the area under the stress-crack opening displacement (COD) as measured in the softening phase of the experiment. The average work-to-fracture of M2-PE-L is, with a value of 206.5 kJ/m³, approximately four times higher than the fracture energies of the other two SHCC. M1-PVA-L and M1-PE-M1-PVA-L yielded comparable results of 52.7 kJ/m³ and 53.5 kJ/m³, respectively. The lowest specific fracture energy (1.6 kJ/m²) was measured for M1-PVA-L, which can be traced back to the extensive fiber rupture, while M2-PE-L yielded the highest values of 5.7 kJ/m² on average as a result of the highest tensile strength and moderate fiber pullout. M1-PE-L also

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showed a high average specific fracture energy of 4.9 kJ/m² ensured by the extensive fiber pullout.

Table 4.8. Average values of the mechanical properties of SHCC obtained from quasi-static tension tests on long unnotched specimens; standard deviations are given in parentheses.

SHCC type and specimen geometry M1-PVA-L M1-PE-L M2-PE-L

First crack stress [MPa] 3.1 (0.5) 2.4 (0.3) 3.0 (0.6)

Tensile strength [MPa] 3.8 (0.9) 3.6 (0.4) 6.5 (0.8)

Ultimate strain [%] 1.5 (0.8) 1.7 (0.8) 3.6 (1.3)

Average crack spacing [mm] 15.3 (6.6) 9.3 (1.4) 2.6 (0.4)

Work-to-fracture (up to softening) [kJ/m³] 52.7 (35.3) 53.5 (24.8) 206.5 (81.7) Specific fracture energy (softening regime) [kJ/m²] 1.6 (0.8) 4.9 (0.8) 5.7 (1.5)