International Concrete Abstracts Portal

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.

An important factor for the success of repair works in concrete structures is to realize sufficient bonding between the repair material and the substrate. Nowadays there are many types of repair material available in the market with improved strength, adhesion, waterproofing and durability properties. There is a need to study the interfacial properties of these repair materials especially in-situ because these properties depend very much on method of placing, curing and environment condition etc. The authors developed a new testing method called the torque test to measure bonding shear strength by twisting concrete cylinder cores. Since concrete is weaker in tension than in shear, slant tension failure often occurred and pure torsional failure could not be achieved. The data from this type of failure are considered the minimum bonding shear strength. To prevent this type of failure, the top concrete cylinder is enclosed in steel pipe. The experimental parameters are the adhesives between two concrete such as epoxy resin, polymer modified cement mortar and cement mortar. The influence of surface roughness of concrete substrate on bonding strength is also investigated. A relationship between bonding shear strength and bonding tensile strength is obtained.

Confinement of concrete by transverse steels (tubes or spirals) is effective in improving ductility of high-strength concrete (HSC). While the circular transverse tubes or spirals can provide much stronger confinement to the compressed concrete than the rectilinear transverse steels (tubes or ties), and circular reinforced concrete column is widely used in building and civil structures, studies and information on the flexural behavior of circularly confined concrete columns are scarce. As a result, there is not yet a reliable ultimate capacity design method directly applicable to circular concrete columns in current design codes. In this paper, a simple ultimate capacity design method is proposed for the circularly confined HSC columns. This simple method utilizes an equivalent rectangular stress block for the compressed concrete in circular column section. Since expressions of the parameters definingu the stress block and the ultimate strain were developed based on an confinement model proposed by the authors, the proposed design method enables engineers to compute directly ultimate capacities of a circular column, and to evaluate the confinement effect of circular transverse steels on the ultimate capacities. The predicted ultimate moment and curvature agreed well with the experimental data of HSC columns confined by circular steel tube and circular concrete-filled steel tubular (CFT) columns available.

This paper presents the results of an experimental study on shear Synopsis: strength enhancement of reinforced concrete (RC) beams with adhesive bonded flexible carbon fiber sheets. A total of eight medium-sized RC beams are tested under four-point bending. Different sheet configurations and fiber alignments are used to clarify their effects on ultimate shear strength of tested beams. It is found that the externally adhesive bonded carbon fiber sheets are effective for enhancing the shear strength of RC beams. Further, it is observed that the sheets with fibers aligned parallel to beam axis are nearly as effective as that of sheets with fibers aligned perpendicular to beam axis. Moreover, the shear strengt increases with the number of sheet layers and the depth of sheets across the beam section. Maximum shear strength is obtained for the beam U-wrapped by sheets over full depth having fibers perpendicular to beam axis. An 119% increase in ultimate shear strength is achieved for this beam with single layer of fiber sheet.

In this study, two types of optical fiber sensors attached on the concrete surface, called a Straight-type sensor and Loop-type sensor were developed. Through bending tests of RC beam specimens, detailed data on beam deflection and optical performance of attached fiber were collected. Overall performance of the optical fiber sensor as a concrete performance monitor is discussed. Test procedures, test results and their discussions are summarized. Based on the experimental result, several conclusions were obtained. 1) The adhesion of epoxy resin to concrete surface is not lost until ultimate concrete failure. 2) The sensitivity of the Straight-type sensor is affected by the angle of concrete cracking and optical fiber intersection. 3) The distance between cracking location and looped circle of optical fiber affects the sensitivity of the Loop-type sensor. 4) Both for the straight type and the loop type, in order to develop a satisfactory level sensor, further research is strongly recommended.