International Concrete Abstracts Portal

The paper investigates the shear strength and the failure modes of high strength concrete beams with and without steel fibers ( fC = 110 MPa). Sixteen reinforced high strength concrete beams (3600x350~200 mm) were tested under different combinations of shear force and bending moment. The beams were singly reinforced and without shear (web) reinforcement. The test results indicated that the addition of steel fibers to high strength concrete increases the ultimate shear strength, improves the brittle characteristic and transforms the failure mode into a more ductile one. The average gain of the ultimate shear strength due to the addition of steel fibers varied from about 14% to 14 1% depending on the shear span to depth ratio and the longitudinal steel ratio. Four modes of failure of reinforced high strength concrete are clearly distinguished as; diagonal tension, shear compression, shear flexure, and flexural failure. In general, cracks in fiber reinforced concrete beams are closer, narrower, and more than those in beams without fibers. This reflects the effect of steel fibers in redistributing the stresses beyond cracking.

In the late 1990’s stainless steel has gained increased acceptance as an economically viable material for concrete reinforcement in bridge decks. Its ease of fabrication and outstanding resistance to corrosion caused by chlorides and other corrosive effluents have allowed it to be used in new bridge construction as well as rehabilitation projects. Corrosion studies have indicated that the material may have easily a life cycle of between 50 and 100 years even with heavily salted highways. Additions of nitrogen and special processing have also allowed the project designers to save weight and reduce the use of special membrane layers and other concrete additives to the bridge design. This therefore in part defrays the higher initial cost of using stainless steel reinforcement as compared to either epoxy coated or plain black steel. Before 1995, several projects in the USA, the UK and in Europe had employed 304 grade stainless as reinforcement. In North America in the last several years, higher-grade alloys of stainless such as 3 16LN and Duplex 2205 have been used more prevalently. This paper will examine the benefits of the use of these alloys. We will address why they have become increasingly accepted as well as the reasons for their selection in specific bridge projects in North America. We will also examine the specific role of moly and nitrogen as alloying elements in stainless steel rebar. A general review of corrosion mechanisms, potential problems and the physical and mechanical properties of these alloys as compared to other potential alloy selections will also be presented.

Experimental results of the bond behavior of 10, 13, 19, and 25-mm diameter bars corroded to various levels are presented. A total of forty specimens were tested to obtain the bond - slip behavior. The corrosion level ranged from 0 to 4.79 percent diameter loss. Externally applied current was used to induce controlled amount of corrosion. The results were analyzed to study the influence of bar diameter and corrosion level on bond strength, slip at peak load, and overall bond - slip behavior. The results indicate that exponential reduction in bond strength occurs once the diameter loss exceeds a fraction of a millimeter. At a diameter loss of O.l-mm, the strength reductions are substantial. The reduction in slip at maximum load is even more significant with corrosion. The actual magnitude of corrosion can be predicted using the applied current input with reasonable accuracy.

Fiber reinforced polymer composite (FRPC) wraps are increasingly being used for rehabilitation and strengthening of concrete structures This paper presents the results of an experimental study on the tensile performance of cement-based specimens wrapped with FRPC sheets subjected to wet-dry and freeze-thaw cycles. The tensile strength values were evaluated using the ASCERA hydraulic tensile tester. This simple testing technique provides a uniform stress distribution throughout the specimen, thus minimizing eccentricity and gripping effects, which can be of a significant source of error. Cement-based specimens were wrapped with three different types of FRP tow sheets: two carbon and one glass. Test variables included the type of fiber (Cl, C5, and GE) and the environmental exposure conditions. The specimens were conditioned in three different environments, as follows: a) room temperature (23C), b) 300 wet-dry cycles using salt water and c) 300 freeze/thaw cycles. At the end of each exposure, ultimate strength and load-extension behavior were obtained and then compared to the performance of unconditioned samples. Results show that specimens wrapped with carbon fiber reinforced polymer (CFRP) experienced no reduction in strength due to exposure, whereas specimens with glass fiber reinforced polymer (GFRP) experienced a significant reduction in strength. Fractography was used to identify the failure initiating flaw and failure mode for the fractured tensile specimens.

In this paper, shear strengthening of RC beams by confining shear critical region is experimentally and theoretically investigated. Confinement is implemented using vertical post-compression force in the shear critical region with steel plate, bolts and angles. The loading tests include (1) loading beams to first major shear crack, then shear strengthening, and reloading to complete failure, (2) strengthening beams from beginning and loading to complete failure, and (3) loading unrepaired beam to complete failure. Influences of concrete strength, shear span length, longitudinal tensile reinforcement, level of post-tensioning, presence of shear reinforcement, use of continuous plates, and presence of shear crack are studied by testing twenty-four RC beams. Test results show that in the presence of post-compression stress, as low as 0.04f'c for strengthening, shear strength increases significantly and the mode of failure of the beams changes from brittle shear to ductile bending. Also, for this level of post-compression, influence of all other parameters is limited, while a ductile failure is dominant. Calculations based on Mohr’s theory also indicate that this method of strengthening is very effective, and provides ductile flexural behavior in beams with inadequate shear strength.