International Concrete Abstracts Portal

The use of Fiber-reinforced polymer (FRP) composite materials in new construction and repair of concrete structures has been growing rapidly in recent years. FRP provides options and benefits not available using traditional materials. The promise of FRP materials lies in their high-strength, lightweight, noncorrosive, nonconducting, and nonmagnetic properties. ACI Committee 440 has published several guides providing recommendations for the use of FRP materials based on available test data, technical reports, and field applications. The aim of these document is to help practitioners implement FRP technology while providing testimony that design and construction with FRP materials systems is rapidly moving from emerging to mainstream technology.

Precast, prestressed hollow core (PHC) slabs are among the most common concrete deck system in the world. However, due to the manufacturing constraints and the difficulty in providing internal shear reinforcement, the shear capacity of PHC slabs sometimes dictates the design and reduces the efficiency and economics of PHC slabs. The objective of this research project is to develop an innovative application of externally bonded Fiber-Reinforced Polymers (FRP) sheets by installing the sheets along the internal perimeter of the slab voids to strengthen the webshear capacity of PHC slabs. To explore the feasibility and to optimize the new technique, experimental testing was carried out on eight full-scale single web, I shape, specimens (each of 4575 mm “180 in” long, 300 mm “12 in” thick and a 284 mm “11.2 in” wide) that were cut longitudinally out of the PHC slab. Carbon FRP sheets were bonded along the full perimeter on each side of the web specimens. The test specimens were loaded monotonically until failure under single concentrated load at a shear span/depth ratio of 2.5. The investigated parameters were the width of the FRP strengthened zone (300 “12 in.”, 450 “18 in.”, and 600 mm “24 in.”) and the number of strengthening layers (2 and 4 layers). The test results showed the efficiency of the proposed technique to enhance the shear strength of PHC slabs.

Results are presented from a comprehensive experimental study to assess the occurrence of heat-induced longitudinal splitting cracks in concrete specimens reinforced with CFRP or steel when exposed to severe heating from one side, as would likely occur in a fire in a building. Tests were performed on large- and medium-scale precast CFRP reinforced or prestressed specimens. Large-scale specimens were tested in a standard fire resistance test, while medium-scale specimens were tested using a novel Heat-Transfer Rate Inducing System (H-TRIS) which controls thermal exposure by imposing a time-history of incident heat flux at a specimen’s exposed surface. The formation of thermally-induced longitudinal splitting cracks and failure of the concrete cover to provide sufficient confining action, and thus sufficient bond strength, is shown to be more likely for FRP reinforced or prestressed concrete elements than for those reinforced or prestressed with steel. This appears to be at least partly due to thermo-mechanical incompatibility between CFRP reinforcement and concrete; formation of heat-induced longitudinal splitting cracks is related to rapid thermal expansion of CFRP tendons relative to the surrounding concrete. Many aspects of bond performance at elevated temperature remain poorly understood, and these require additional investigation before FRP reinforced or prestressed elements can be used in fire-rated applications with confidence.

An experimental study was conducted to determine the transfer length of prestressed Glass Fiber Reinforced Polymer bars. This paper is a part of a broad program that studies the long-term behaviour of GFRP prestressed concrete beams. 16 GFRP prestressed concrete beams were cast in this study. The parameters included were; prestressing level; 300 MPa (44 ksi) and 500 MPa (73 ksi), concrete compressive strength; 30 MPa (4440 psi) and 70 MPa (10000 psi), and the GFRP bar diameter;12Φ (No. 4) and 16Φ (No.5). Accurate estimation of the transfer length is necessary for elastic stress calculations at the service limit state and for the shear design of prestressed members. Strain gauges were used to measure strains on the GFRP bars and DEMEC gauges were used to measure the concrete surface strains at the level of the prestressed GFRP bar to determine the transfer length. The transfer length of 16Φ (No.5) GFRP bars in concrete with compressive strength of 30 MPa (4440 psi) was found to be about 17 db, and 14 db for prestressing levels of 500 MPa (73 ksi) and 300 MPa (44 ksi), respectively. The measured transfer length values were used to improve the transfer length estimates provided by the ACI 440.4 R-04 equation by calibrating the material coefficient factor (αt) used in the ACI equation.

Strength and fatigue testing were conducted on concrete specimens strengthened with three different epoxy systems. These specimens were conditioned in elevated water baths, subjected to fatigue loading, then tested for strength. For all three systems, the bond strength of a single conditioned specimen was at least 90 percent of the bond strength of three samples that had been fatigued but not conditioned in elevated water baths. Because of the limited data the results are anecdotal and only preliminary findings can be drawn from this work.