Damage Characteristics and Micromechanics of Strain-Hardening Cementitious Composites Under High Rate Loading

Victor C. Li1* and En-Hua Yang2

1Department of Civil and Environmental Engineering University of Michigan, Ann Arbor, MI 48109-2125 USA

2Exponent, Inc. 10850 Richmond Ave, Suite 175, Houston, TX 77042

*vcli@umich.edu Keywords: SHCC, ECC, impact-resistant, high-rate loading

Introduction. Strain-Hardening Cementitious Composites (SHCC) is a class of cement based fiber reinforced composite that has been shown to be highly damage tolerant through material as well as structural testing. Most studies, however, have been conducted under quasi-static loading. Recent experimental investigations revealed that the damage tolerant behavior of some SHCC may change as a function of loading rate. In particular, the ductile strain-hardening response under quasi-static loading may switch to a brittle fracture failure as loading rate increases.

This paper discusses some observations of failure modes of SHCC subjected to a range of strain rates under uniaxial tension tests. Strain rates varying by several orders of magnitude indicate that the tensile strain capacity of an SHCC can be lowered from 3% to below 1%. To clarify the source of rate dependency, a new micromechanics theory incorporating rate dependencies of material ingredients and microstructural properties, and accounting for inertial effects under dynamic loads is formulated. The resulting rate dependent strain-hardening criteria pinpoint several micromechanical parameters involving fiber, matrix and interface that could govern the observed changes in the composite response at different loading rates. Experiments conducted at the micromechanical scale further reveal the details of the rate dependencies of each of these parameters. This information is then used to redesign a new class of Engineered Cementitious Composites (ECC) with deliberately tailored micromechanical parameters that minimize the embrittlement effect at high strain-rate. Uniaxial tension tests conducted on these impact resistant ECCs confirmed that high tensile strength and strain capacity could be maintained when the composite microstructure is appropriately tailored.

A variety of experiments, including flexural beams, and circular and rectangular plate specimens, built with the impact resistant ECC, were conducted using drop weight impact loading. The damage characteristics are contrasted with those of control mortar and concrete specimens. In one of these tests, a high speed camera captured the failure modes under repeated drop weight impacts.

Summary: This paper presents new understanding of the micromechanics of damage of SHCC under dynamic loading. It is demonstrated that impact resistance could be designed into this class of materials. Experimental test results suggest that high impact resistant structures could be attained with high load capacities and energy absorption capacities through the use of the impact resistant ECCs.