William D. Cook

Minimum Shear Reinforcement in Normal, Medium,and High-Strength Concrete Beams

This paper presents the evaluation of minimum shear reinforcement requirements in normal, medium, and high-strength reinforced concrete beams. Twelve shear tests were conducted on full-scale beam specimens having concrete compressive strengths of 36, 67, and 87 MPa. Different amounts of minimum shear reinforcement were investigated including the traditional amounts required by older codes and the amounts required by the 1989 ACI Code (revised 1992) and the 1994 CSA Standard. The performance of the different amounts of shear reinforcement are discussed in terms of shear capacity, ductility, and crack control at service load levels. An assessment of the 1989 ACI and 1994 CSA provisions for minimum shear reinforcement is also presented.

Response of Steel Fiber-Reinforced Concrete Beams with and without Stirrups

This paper will discuss how a series of nine full-scale reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC) beams were tested to study the effects of steel fibers on shear capacity, failure mechanism, and crack control. Six of the specimens were constructed without shear reinforcement. In addition, three specimens were detailed in accordance with the minimum shear reinforcement requirements of CSA A23.3-04 to examine the influence of fibers on ductility. The results demonstrate that the addition of fibers leads to improved shear resistance in shear-deficient beams. The addition of fibers in beams that contain minimum shear reinforcement results in improved ductility and crack control. A procedure for predicting the shear resistance of SFRC beams is also presented in this paper.

Benefits of concentrated slab reinforcement and steel fibers on performance of slab-column connections

Six two-way slab-column specimens, designed to fail in punching shear, were tested. The parameters investigated were the placement of steel fiber-reinforced concrete (SFRC) in the slab and the concentration of slab reinforcement around the column. The effects of these parameters on the punching shear capacity, negative moment cracking, and stiffness of the two-way slab specimens were investigated. Currently, there is no beneficial effect in the 1995 American Concrete Institute Code for using FRC or concentrated slab reinforcement near the column in the calculation for the punching shear resistance. The beneficial effects of concentrating the slab reinforcement near the column and of using FRC are demonstrated.

Behavior of Columns Constructed with Fibers and Self-Consolidating Concrete

Steel fiber reinforced concrete was first developed in the 1960s, but the use of this material in load-carrying structural elements has not yet gained wide acceptance. This paper describes a series of 13 full-scale axial compression tests that was conducted on reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC) columns. The objectives were to gain a better understanding of the performance enhancements that can be gained from the use of SFRC in columns and to examine if the provision of fibers would permit a reduction of confinement reinforcement. The specimens, which were detailed with varying amounts of transverse reinforcement, were cast using self-consolidating concrete (SCC) that contained various quantities of fibers. The results demonstrate that the addition of fibers leads to improved load-carrying capacity and post-peak response, and greatly delays cover spalling. The findings also show that the addition of steel fibers can partially substitute for the confinement reinforcement in columns, thereby improving constructability while achieving significant confinement. Although an addition of moderate amount of fibers to SCC can result in an adequately workable concrete mixture, there is a limiting fiber content above which the SCC mixture can lose much of its workability.

Early Age Compressive Stress-Strain Properties of Low-,Medium, and High-Strength Concretes

The paper presents a detailed study of early age compressive stress-strain responses of low- (30 MPa), medium (70 MPa) and high strength (100 MPa) concretes. In particular, the stress-strain responses during the first 72 hr after casting are closely followed. Tests were carried out on 100 x 200-mm (4x8-in.) concrete cylinders. The influence of concrete strength on temperature rise due to temperature-matched curing is presented. The effect of three different curing conditions ont he development of concrete compressive strength and elastic moduli is described : 1) temperature-matched, 2) sealed, and 3) air-dried curing.

Improving Punching Shear Behavior of Glass Fiber-Reinforced Polymer Reinforced Slabs

This study compares the behavior of slab-column connections reinforced with steel or glass fiber-reinforced polymers (GFRP). The effects of concentrating the reinforcement in the immediate column region and the influence of using steel fiber-reinforced concrete (SFRC) in the slab were also studied. The punching shear capacity, stiffness, ductility, strain distribution, and crack control were investigated. Concentrating the slab reinforcement and the use of SFRC in the slab enhanced the punching behavior of the slabs reinforced with GFRP bars. Predictions using different design guidelines are compared to the experimental results obtained from the FRP reinforced slabs tested in this study and those tested by other researchers. Results show that the GFRP-reinforced slabs have significantly lower punching shear capacities, lower post-cracking stiffness and greater deflections than slabs reinforced with steel reinforcing bars. Concentrating the top mat of flexural reinforcement results in beneficial effects on punching shear strength, postcracking stiffness, and crack control. The presence of 0.5% by volume steel fibers in the concrete significantly improves punching shear capacity and crack control.

Creep, shrinkage, and thermal strains in normal, medium, and high-strength concretes during hydration

This paper presents an experimental study on early-age shrinkage and thermal and creep strains of normal (30 MPa), medium (70 MPa), and high-strength (100 MPa) concretes subjected to sealed and air-dried curing. It was found that demolding at very early ages resulted in greater shrinkage and thermal strains in high-strength concrete than in medium strength concrete, which in turn showed greater strains than the normal strength concrete, which in turn showed greater strains than the normal strength concrete. It was also observed that creep of the high-strength concrete containing naphtalene-based superplasticizer is much more sensitive to the age of loading than the normal and medium strength concretes, with very early-age loading resulting in significantly higher creep. Predictions using the CEB-FIP creep expressions agree reasonably well with the measured creep strains, except for the case of high-strength concrete loaded at very early ages (i.e., less than 24 hr).

Punching Shear Behavior of Two-Way Slabs Reinforced with High-Strength Steel

The punching shear behavior of slabs reinforced with high-strength steel reinforcement was studied and compared with that of slabs reinforced with conventional steel reinforcement. The high-strength steel selected for this research conforms to ASTM A1035-07. The influences of the flexural reinforcement ratio, concentrating the reinforcement in the immediate column region, and using steel fiber-reinforced concrete (SFRC) in the slab on the punching shear resistance, post-cracking stiffness, strain distribution, and crack control were investigated. In addition, the test results were compared with the predictions using various design codes. The use of high-strength steel reinforcement and SFRC increased the punching shear strength of slabs, and concentrating the top mat of flexural reinforcement showed beneficial effects on post-cracking stiffness, strain distribution, and crack control.

Behavior of Rectangular Columns Constructed with SCC and Steel Fibers

AbstractExtensive research has shown that properly detailed and closely spaced transverse reinforcement in reinforced concrete columns can ensure ductile behavior during earthquakes. However, in regions of high seismicity, detailing requirements can result in heavily congested sections; the use of self-consolidating concrete (SCC) can facilitate construction in these situations. Although extensive research exists on the axial load behavior of traditional concrete columns, only limited research exists on the behavior of columns constructed with SCC. Research over the past two decades has also shown that use of steel fiber-reinforced concrete (SFRC) can improve the strength and ductility of columns by delaying cover spalling and improving core confinement. Recent research has also shown that the combined use of SCC and steel fibers can ease problems associated with the workability of traditional fiber-reinforced concrete. This paper presents the results from an experimental program that was conducted to stu...

Seismic Response and CFRP Retrofit of Poorly Detailed Shear Walls

AbstractThe reversed cyclic loading behavior of full-scale shear wall specimens was investigated before and after retrofit. The wall specimens were designed and detailed to simulate nonductile reinforced concrete construction of the 1960s, having lap splices of the longitudinal reinforcement in the potential plastic hinge region, and having inadequate confinement of the boundary regions. The lap splices in these walls failed in a brittle manner before yielding occurred in the main flexural reinforcement. The use and effectiveness of carbon fiber-reinforced polymer (CFRP) wrap for improving the lap splice behavior and the shear strength of the walls were investigated. The walls were tested under reversed cyclic loading with loading applied near the tip of the walls. The retrofit technique improved the displacement ductility, energy dissipation, and prevented premature failure of the lap splices. The target ductility level of 2.0 was achieved with this CFRP retrofit.