This paper reports on a series of dynamic splitting tensile tests on 70 mm diameter cored concrete and steel fiber reinforced concrete (SFRC) specimens at moderate strain rates. A modified split Hopkinson pressure bar (SHPB) was specifically developed to test such large diameter heterogeneous specimens. Details of the modified SHPB and a novel striker bar are presented. Dynamic strength magnification of up to 4.5 times the static strength at moderate strain rate was obtained. For the SFRC specimens, hooked-end steel fibers were used with 0.3% fiber volume concentration. A high-speed camera with framing rate up to 106 frames per second was used to record the crack propagation mechanism and the progressive fracture of the specimens in tests. Numerical simulation of the test is briefly presented and discussed. Good approximation of the response is obtained.

High performance fiber reinforced composites are identified by their enhanced elastic behavior, pseudo-strain hardening response and toughened post-peak response. If the composite is adequately reinforced by fibers, the bridging action will transfer the load and multiple cracking will occur. This study investigates the effect of dispersion of fibers on the multiple cracking behavior of fiber reinforced composites. Electronic Speckle Pattern Interferometry technique is used to record the location of crack initiation, sequence of the multiple cracking and corresponding cracking stresses. Microstructural parameters at each crack location are statistically quantified by the theory of point processes. The size of the fiber free areas and fiber clumping are calculated at the crack cross-sections. By using Linear Elastic Fracture Mechanics, fracture toughness of the matrix is calculated. A strong relation between the cracking stress and the fiber free areas in the composite is observed. It is shown that the toughness of the composite depends on the fiber clumping at the first crack cross-section.

The mechanical properties and pore structure of fiber reinforced high strength concrete subjected to different high temperatures and cooling regimes were investigated. The results showed that the residual strength of high strength concrete with or without fibers worsened after exposure to high temperatures. Evident drop in compressive and splitting tensile strengths took place between 400°C and 600°C. However, the decline in flexural strength mainly took place before 400°C. Among the specimens, the concrete with 10% of silica fume, 25% of fly ash and 1% of steel fiber provided the highest residual strength and relative residual strength. Steel fibers played a significant role in preventing the worsening of the mechanical properties, particularly at the temperature from 400°C to 600°C. Thermal stress was not the key factor that caused the spalling or explosive spalling in high strength concrete under high temperatures. The re-curing brought recovery of the residual strength in the concretes damaged at high temperatures due to the remarkable improvement of the pore structure.

Dynamic fracture studies on fiber reinforced cement-based composites were conducted. Contoured double-cantilevered beam specimens were subjected to one rate of quasi-static loading and three rates of impact loading by using a fully instrumented drop weight impact machine. Steel and polypropylene fibers at two dosage rates were investigated, and an analytical scheme was developed to provide the inertial correction to measured loads and obtain crack growth resistance curves (KR Curves) under static and impact loading. KR-Curves were observed to be highly stress-rate sensitive. Comparison between steel and polypropylene fibers indicated a superior performance of the steel fiber under quasi-static loading, but under impact loading, the polypropylene fiber appears to come up to the level of steel fiber.

Fiber-reinforced cement for piezoelectricity and pyroelectricity is introduced, as these phenomena are useful for the sensing of strain and temperature. The use of short steel fibers (8 µm diameter), together with polyvinyl alcohol, as admixtures greatly enhances these effects, thereby attaining longitudinal piezoelectric coupling coefficient 3 x 10-" mN (10kHz), and pyroelectric coefficient 6 x 10$ C/mz.K (10 kHz). The piezoelectric effect is comparable in magnitude to that of PZT. However, due to the high value (2,500) of the relative dielectric constant, the piezoelectric voltage coefficient and pyroelectric voltage are comparable to or even lower than those of plain cement paste or carbon fiber (15 µm diameter) cement paste. Carbon fiber cement paste and plain cement paste are comparable in the piezoelectric coupling coefficient, piezoelectric voltage coefficient and pyroelectric voltage, though the pyroelectric coefficient is higher for carbon fiber cement paste than plain cement paste.