International Concrete Abstracts Portal

Practicing engineers increasingly favor the use of high-strength concrete and reinforcement in their design. The paper included in this CD present results from recent research studies and examples of practical applications and use of high-strength concrete and steel reinforcement in recent projects. This CD consists of 10 papers that were presented at a technical session sponsored by ACI Committee 441 at the ACI Convention in Toronto. Ontario, Canada in October 2012. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-293

The advancement of material technology has led to higher grades of both concrete and steel strengths. High-strength concrete and steel can decrease the size of structural members and increase the available floor area. In addition, it can decrease the consumption of aggregate and steel, promoting environmental sustainability. This research investigates the shear behavior of high-strength reinforced concrete columns under low axial load. The specified compressive strength of concrete is 70 MPa or 100 MPa. The specified yield strengths of longitudinal and transverse reinforcement are 685 MPa and 785 MPa, respectively. Eight large-scale column specimens were constructed and tested in double bending under lateral cyclic load. Test results showed that all specimens had shear failure without yielding of longitudinal reinforcement as expected in design. Higher concrete compressive strength, higher axial load and smaller spacing of transverse reinforcement resulted in higher shear strength. The peak applied load was reached before yielding of transverse reinforcement. The critical shear crack angle was approximately 30° and 20° for columns with 10% and 20% axial load, respectively. The simplified shear strength equation of the ACI 318 code was conservative for columns tested in this research and for high strength columns collected from literature. However, the detailed shear strength equation exhibited non-conservative results for most of the columns examined.

The present study is an experimental investigation into the performance of high-strength concrete (HSC) square short columns subjected to biaxial bending moments and strengthened by FRP laminates. The main objectives of the study are: to evaluate the strength and deformational enhancement in the structural performance of HSC columns subjected to small biaxial eccentricity when strengthened by externally applied FRP laminates, and to investigate the optimum arrangement and amount of FRP laminates to achieve potential enhancement in structural performance. The study parameters are the number, type and arrangement of FRP layers and the concrete compressive strength. The static axial load small eccentricity (compression-controlled failure) is kept constant corresponding to e/t = 0.125 in two perpendicular directions to the columns principal axes, and the FRP wraps are applied in single or double layers (partial or full column height wrapping). In the present work, test results of eight large-scale concrete columns are presented and discussed. The study has experimentally proven the efficiency of FRP laminates, as a strengthening alternative, in enhancing the strength of biaxially loaded square HSC columns through increasing their axial load carrying capacity (by up to 28%) and flexural capacity (by up to 41%). FRP wraps are also successful in increasing ductility of the strengthened columns. FRP wraps significantly reduced stiffness and strength degradation of HSC columns. Stiffness of strengthened columns is not increased which may be considered an advantage in seismic applications.

National Research Council Canada has recently upgraded its column furnace facility for assessment of columns in fire under not only the applied axial loads but also a potential lateral load. The main goal for this upgrade was to be able to simulate lateral displacement of columns during the fire due to thermal expansion of slabs/floors and to assess a column’s residual seismic/lateral load capacity after fire damage. Furthermore, the new column facility enhancement included a hybrid testing technology in which the column could be tested considering its structural interactions with the remaining of the structure. This paper includes the summary of the new upgrade and testing technology; however, more focus will be on the structural response of a high strength column, with steel fibre, tested using the new upgrade and approach. This includes the fire test of the high strength column specimen as well as lateral load test of the column to determine its residual lateral resistance with fire damage. The results of these tests revealed that fire substantially reduced the residual lateral load/displacement capacity of the high strength concrete column. The new commissioned testing technique/tool could assist researchers to seek and find solutions for more reliable post-fire structural inspection and to develop design tools for the mitigation.

Application of high performance materials in construction combines the advantages of reducing the use of materials, cross-section and reinforcement congestion. Although the application of high strength concrete (HSC) has been gaining popularity in high-rise building construction, parameters affecting the performance of HSC members are still under investigation. The use of high strength steel can result in reduced steel congestion and low cost associated with transportation and installation of rebars. Although many design codes and guidelines apply restrictions on the yield strength of steel reinforcement, high-strength steel (HSS) rebars are still a viable option for longitudinal reinforcement in columns of multi-story moment-frame buildings designed to resist earthquake motions. In this study, a numerical approach has been undertaken for the seismic fragility analysis of RC columns and frames with HSS and HSC. Fragility curves provide the flexibility to deal with the uncertainty in geometric properties, along with the typical uncertainties such as material and ground motion uncertainties. Latin Hypercube Sampling (LHS) technique is employed to quantify the uncertainties associated with different modeling parameters such as concrete compressive strength, yield strength of longitudinal and transverse reinforcement, gross geometries and ground motions. Probabilistic seismic demand model (PSDM) is used to develop fragility curves. The fragility curves thus developed quantify the vulnerability of high strength RC columns and frames and show their effectiveness in reducing the probability of failure compared to regular strength RC columns.