International Concrete Abstracts Portal

This CD-ROM contains 10 papers that were presented at sessions sponsored by ACI Committee 544 at the Spring 2011 ACI Convention in Tampa, FL. The topics of the papers cover durability aspects of fiber-reinforced concrete, ranging from permeability, shrinkage cracking, long-term behavior in chloride environment and resistance to chloride penetration, as well as applications of fiber-reinforced concrete for coupling beams for highrise core-wall structures, beams for bridges, panels and suspended foundation slabs. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-280

Experimental and analytical studies that led to the incorporation of strain-hardening, high-performance fiber reinforced concrete (HPFRC) coupling beams in the design of a high-rise core-wall structure in Seattle, WA, are described. A total of eight HPFRC coupling beams with span-to-depth ratios ranging between 1.75 and 3.3 were tested under large displacement reversals. The tension and compression ductility of HPFRC materials allowed an approximately 70% reduction in diagonal reinforcement, relative to an ACI Building Code (318-08) compliant coupling beam design, in beams with a 1.75 span-to-depth aspect ratio and a total elimination of diagonal bars in beams with a 2.75 and 3.3 aspect ratio. Further, special column-type confinement reinforcement was not required except at the ends of the beams. When subjected to shear stress demands close to the upper limit in the 2008 ACI Building Code (0.83 f’c [MPa] (10 f’c [psi])), the coupling beams with aspect ratios of 1.75, 2.75 and 3.3 exhibited drift capacities of approximately 5%, 6% and 7%, respectively. The large drift and shear capacity exhibited by the HPFRC coupling beams, combined with the substantial reductions in reinforcement and associated improved constructability, led Cary Kopczynski & Co. to consider their use in a 134 m (440 ft) tall reinforced concrete tower. Results from inelastic dynamic analyses indicated adequate structural response with coupling beam drift demands below the observed drift capacities. Also, cost analyses indicated 20-30% savings in material costs, in addition to much easier constructability and reduced construction time.

The paper will review durability data for glass fiber reinforced concrete (GFRC), both accelerated aging and real time data. It will compare real time data with predicted data from accelerated aging tests and describe the design principles that have been established for GFRC based on this durability data. Applications over 20 years old will be reviewed.

Ultra high performance concrete (UHPC) containing steel fibers was used in five beams of the bridge on Route 624 over Cat Point Creek in Virginia. A test beam was also fabricated and tested to failure. The beam had strands but no shear stirrups. Test beam results indicated satisfactory load-carrying capacity. Preparation of the beams involved a longer mixing time and a two-stage steam curing to ensure optimum concrete properties. Testing of specimens at the hardened state showed that UHPC has high strength and high durability attributable to a very low water–cementitious materials ratio, low permeability, a high resistance to cycles of freezing and thawing, and tight cracks.

Durability is nowadays a key-parameter in Reinforced Concrete (RC) structures. Several codes require that structures have a defined service life during which the structural performance must satisfy minimum requirements by scheduling only ordinary maintenance. Durability can be associated to permeability, defined as the movement of fluid through a porous medium under an applied pressure load, which is considered one of the most important property of concrete. Permeability of concrete is strictly related to the material porosity but also to cracking. The former is basically controlled by the water/cement (w/c) ratio while microcracks and cracks are related to internal and external strains or deformations experienced by the RC structures. Shrinkage, thermal gradients and any factor determining volumetric instability, as well as the loads acting on a structure, lead to both microcraking and visible cracking. It is well known that, after cracking, tensile stresses are induced in the concrete between cracks and, hence, stiffen the response of a Reinforced Concrete (RC) member under tension; this stiffening effect is usually referred to as “tension stiffening”. After the formation of the first crack, the average stress in the concrete diminishes and, as further cracks develop, the average stress will be further reduced. When considering Fiber Reinforced Concrete (FRC), an additional significant mechanism influences the transmission of tensile stresses across cracks, arising from the bridging effect provided by the fibers between the crack faces; this phenomenon is referred to as “tension softening”. Fibers also significantly improve bond between concrete and rebars and act to reduce crack widths. The combination of these two mechanisms results in a different crack pattern, concerning both the crack spacing and the crack width. The present paper describes results from a collaborative experimental program currently ongoing at the University of Brescia and at the University of Toronto, aimed at studying crack formation and development in FRC structures. A set of tensile tests (52 experiments) were carried out on tensile members by varying the concrete strength, the reinforcement ratio, the fiber volume fraction and the fiber geometry.