International Concrete Abstracts Portal

Upgrading often becomes a necessity due to changes in usage of buildings due to factors such as deterioration and aging, change in occupancy, or the need for installation of facilities such as air-conditioning, heating, escalators, elevators, additional skylights, or new façade structures. In a number of cases upgrading is related to changes which affect the load bearing components of the structure. Fiber reinforced polymer matrix composites provide an efficient means of both strengthening slabs for enhanced load carrying capacity and for strengthening slabs after installation of cut-outs. This paper reports on a series of tests conducted to assess the comparative efficiencies of a commercially available strip form and a fabric form of material vis-à-vis strengthening ability and ductility. It is shown that material tailoring can result in significant changes in efficiencies. The extension of this to the rehabilitation of cut-outs is also detailed and aspects of an on-going full-scale test program in that area are elucidated.

Much research has been conducted on the short-term behavior of concrete members prestressed with FRP tendons, but relatively little is known about the long-term behavior of these members. This lack of knowledge is one important factor hindering the widespread use of FRP for prestressing. Therefore, a comprehensive study on the long-term response of FRP prestressed concrete members is necessary before FRP can gain acceptance as a viable construction material. This paper describes a part of an on-going test program to investigate the long-term behavior of CFRP prestressed concrete beams. The scope of the program includes eight large-scale T-beams constructed with different levels of prestress. The study considers fully prestressed and partially prestressed sections. For the long-term tests, the specimens are subjected to a sustained loading, higher than the cracking load of the partially prestressed members, maintained constant for several months (about 10,000 hours). The results of the long-term test show similar behavior between beams prestressed with CFRP tendons and that prestressed with steel strands. The ratio of long-term to instantaneous deflection was higher for beams prestressed with CFRP tendons than for the beam prestressed with steel strands. The overall deflection of the CFRP tendons was however smaller than that of the steel prestressed beams.

Thirty-six concrete beams strengthened with glued-on CFRP laminates were tested in bending after being subjected to different numbers of freeze-thaw (F.T.) cycles. In order to evaluate a possible deterioration of the interface bond between the concrete and the CFRP laminate, the development length of the glued-on plate was purposely made smaller than the required value. Parameters investigated were: 1) different strengthening systems; 2) precracking of the beams, prior to strengthening; and 3) number of freeze-thaw cycles (0, 100, 200 and 300). For each number of freeze-thaw cycles three control specimens (RC beams with no externally glued-on CFRP laminates) were also tested. It was generally observed that both the moment capacity and the ultimate deflection decreased with an increase in the number of freeze-thaw cycles. The rate of decrease was larger for precracked than for non-precracked beams. For design purposes the value of the horizontal interfacial shear strength can be taken conservatively as 0.17 ÷f’c.

The effect of FRP material properties on the long-term deflection of concrete beams reinforced with FRP rods was investigated by the experiment of 17 beams reinforced by nine types of FRP rods and a beam reinforced by steel bars. Test results showed that the flexural stiffness of a cracked beam decreased rapidly with a reduction in tensile stiffness of the reinforcing rods. Compared to the short-term deflection of beams, the long-term deflection of the FRP reinforced concrete beams at one week after loading increased on average by 17 percent, and 57 percent at 10 months. The material properties of FRP rods had a great effect on the long-term deflection of beams. The long-term deflection increase of beams with GFRP was the smallest among all of the tested beams, and oppositely, the deflection increase of beams with AFRP was greater than the average. The rate of increase in deflection of the beams reinforced with braided rods was about 10 percent smaller than that of beams with spiral rods. Contrasting, the rate of deflection increase of beams with ribbed rods was about 10 percent greater than that of beams with spiral rods.

A study was undertaken to examine the general behaviour of reinforced concrete beams confined with carbon-fibre-reinforced polymer (CFRP) laminates subjected to accelerated rebar corrosion. Eight small-scale RC beam specimens, 1200 mm long with cross-sectional area of 100 mm by 150 mm, were constructed. Five specimens were strengthened with CFRP laminates using three different strengthening schemes. The tensile reinforcement, 2-10M bars, of six specimens was corroded to 10% mass loss by means of an impressed current. Strain gauges were placed on the CFRP laminates to monitor and quantify tensile strains induced by the corrosion process. The CFRP laminates successfully confined the corrosion cracking, and total expansion of the laminate exhibited a fairly linear and continuous increase throughout the corrosion process.