International Concrete Abstracts Portal

The purpose of the present investigation was the study of the effect of specimen size on the compressive strength of concrete reinforced with steel spirals and with fibers. Concrete cylinders of different sizes were loaded under uniaxial compression. The cylinders, ranging in size from 60 x 120 mm to 100 x 400 mm, were reinforced with short fibers (steel and polyolefin) at volume percentages of 1% and 2%. Some specimens also contained transverse steel reinforcement (0 5 mm) at pitches of 25 and 50 mm. There was a reduction in compressive strength as the specimen size increased in all cases (plain concrete, fiber reinforced concrete or spirally reinforced concrete). The effect of the fibers and of the steel spirals was a reduction in the brittleness of composite with increasing size of the specimens. The experimental results confirm that the size effect laws proposed in the literature for compressive strength provide a satisfactory prediction of the reduction in compressive strength with increasing specimen size.

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.

Damage due to recent earthquakes and the high costs of repairing old and building new structures emphasizes the need for novel ways of designing cost-effective and more seismically resistant buildings. To address this problem, the structural system considered in this paper is constructed with composite materials that display excellent earthquake-resistant properties such as high strength, high ductility and increased energy dissipation capacity. The particular composite used in this project is High StrengthLightweight Aggregate Fiber Reinforced Concrete (HS-LWA FRC). Its use in concretefilled steel tubular (CFT) columns allows for both: (a) improved seismic resistance and (b) possibly a faster and more cost-effective method of construction, than is the case with conventional buildings. The main goal of this research was to manufacture and study the seismic performance of such a composite CFT column. The ductility, stiffness degradation, energy dissipation and hysteretic response of the CFT member are reported. Testing of the CFT revealed that adequate strength was developed in the critical section, with respect to the predetermined yielding of the system. The use of fibers prevented an otherwise extremely brittle failure of HS-LWA concrete and increased its capacity for lateral expansion, leading to a ductile response of the critical column zones. As a result, large levels of ductility and energy dissipation capacity were measured during testing.

This narrative discusses design and construction aspects related to a steel fiber reinforced concrete bonded overlay repair project on the existing Houston Beltway 8 Freeway. Several previous steel fiber reinforced paving projects are reviewed. An experimental program involving steel fibers complete in 1983 (1-610 Overlay Project in Houston) is comprehensively reviewed with special attention paid to reflective cracking reduction. Design considerations such as mix design, modulus of rupture, residual strength factors, overlay bond performance, and evaporation rate are discussed. Fatigue endurance limit laboratory test data are presented and a correlation to design applications using residual strength factors is suggested.