Behavior of High-Strength Concrete Rectangular Columns

Transcription

1 Seventh International Congress on Advances in Civil Engineering, October11-13, 26 Yildiz TechnicalUniversity, Istanbul, Turkey Behavior of High-Strength Concrete Rectangular Columns S. Kim, H. C. Mertol, S. Rizkalla, P. Zia, North Carolina State University, Department of Civil Construction and Environmental Engineering, Raleigh, NC, USA A. Mirmiran Florida International University, Department of Civil and Environmental Engineering Miami, FL, USA Abstract This paper summarizes the test results of an extensive research program sponsored by the US Transportation Research Board of the National Research Council to examine the behavior of high-strength concrete rectangular columns subjected to concentric and eccentric loading conditions. The variables considered in this investigation were concrete strength ranging from 7.9 ksi (55 MPa) to 16.5 ksi (114 MPa), longitudinal and transverse reinforcement ratios. Test results were combined with reported data in the literature to examine the validity of the current AASHTO LRFD Bridge Design Specification for high-strength concrete up to 18 ksi (124 MPa). Research findings indicate that the current specification overestimate the load carrying capacity of columns with high-strength concrete under both concentric and eccentric loading conditions. This paper recommends several provisions to the current AASHTO LRFD Bridge Design Specifications to extend the use of high-strength concrete up to 18 ksi (124 MPa) for axially and eccentrically loaded short columns. Keywords: High-strength concrete, Rectangular, Column, Concentric, Eccentric Introduction The use of high-strength concrete (HSC) for bridges and high-rise buildings as become very popular due to development in concrete technology and availability of various types of mineral and chemical admixtures such as silica fume, fly ash, retarders, and superplasticizers. HSC could lead to smaller member sizes for compression members and therefore provide considerable savings associated with material costs and reduction of dead loads. Furthermore, due to the superior durability of HSC, significant reduction of the maintenance requirements and an increase in the service life of the structure can be achieved. 1

2 Most of the current design specifications, such as AASHTO LRFD Bridge Design Specifications, are based on tests conducted using normal-strength concrete (NSC). The research reported in this paper was performed to evaluate the behavior of high-strength concrete columns and to provide recommended provisions to extend the current AASHTO LRFD Bridge Design Specifications to include concrete strengths up to 18 ksi (124 MPa) for short columns. Test Specimens Experimental Program A total of thirty rectangular columns with concrete strengths ranging from 7.9 ksi (55 MPa) to 16.5 ksi (114 MPa) were tested under monotonically increasing concentric and eccentric loading. The test parameters for concentric loading included concrete strength, specimen size, longitudinal and transverse reinforcement ratios. For eccentric loading, the parameters were concrete strength, specimen size and eccentricity of the applied load. The concrete cover used was ½ in (13 mm) to the face of the tie for all the test specimens. All columns were reinforced with six longitudinal steel bars and confined with #4 (φ13) bars as transverse reinforcements. The two ends of the test specimens were reinforced with closely spaced ties and confined with external steel tubes, as shown in Figure 1, to avoid premature failure at the two ends of the test specimens. All columns were cast vertically to simulate typical column construction practice as shown in Figure 2. Details of concentric and eccentric columns are given in Table 1 and Table 2. Geometric overview of the columns and instrumentations are shown in Figure 3. s15r7-ρ3 s15r7-ρ3 Figure 1 Typical Test Specimen Figure 2 Casting Specimens 2

3 Table 1 Details of Concentrically Loaded Columns Column ID Longitudinal Reinforcement Transverse Reinf. Concrete b h L (mm) ρ No. & Size l f y Spacing ρ h f yh f c (%) (MPa) (mm) (%) (MPa) (MPa) 8R9-ρ1 6 # R4½-ρ1 6 # R9-ρ2.5 6 # R4½-ρ2.5 6 # R9-ρ4 2 #7 + 4 # , R4½-ρ4 2 #7 + 4 # , R9-ρ # R9-ρ2.5 6 # R9-ρ4 2 #7 + 4 # , R9-ρ1 6 # R4½-ρ1 6 # R9-ρ2.5 6 # R4½-ρ2.5 6 # s14r7-ρ2 6 # s14r7-ρ3 6 # s14r7-ρ4 6 # s15r7-ρ2 6 # s15r3½-ρ2 6 # s15r7-ρ3 6 # s15r3½-ρ3 6 # s15r7-ρ4 6 # s15r3½-ρ4 6 # Table 2 Details of Eccentrically Loaded Columns Longitudinal Reinforcement Trans. Reinf. Concrete Column b h L e (mm) ID (mm) ρ No. & Size l f y Spacing f c (%) (MPa) (mm) (MPa) 8RE ,414 8RE RE #7 + 4 # RE , RE s14re s16re # s16re Concentric b P L Test Region Eccentric Strain Gauges on Figure 3 Geometric Overview and Instrumentations h h Long. Steel Trans. Steel b LVDT s e P Figure 4 Eccentric Loading 3

4 Material Properties The three target concrete strengths considered in this investigation were developed after laboratory and plant trial batches (Logan 25). The corresponding water to cementitious material ratios for target strengths of 1 ksi (69 MPa), 14 ksi (97 MPa) and 18 ksi (124 MPa) were.3,.26 and.25, respectively. Retarders and superplasticizers were used in all mixes to achieve reasonable workability for the HSC. Three 4 8 in. (1 2 mm) cylinders were cast for each test specimen to obtain concrete strength at the time of testing. Both the longitudinal and transverse reinforcement used for test specimens were Grade 6 steel. A 22 kip (979 kn) capacity MTS testing machine was used to determine the fundamental properties of the longitudinal and transverse reinforcement. The yield stress of longitudinal reinforcement ranged from 58 ksi (4 MPa) to 67 ksi (462 MPa). The transverse reinforcement exhibited non-linear behavior within the yielding range, without a well defined yield point; therefore, the.2 percent offset method was used to determine the yield strength. Instrumentation and Test Set-Up The axial shortening of the columns was measured using four 1 mm pi gages, located at the mid region of the test specimens. Two of the pi gages were attached to the threaded rods embedded in the core concrete while the other two gages were mounted on the concrete surface. Two additional 1 mm pi gauges were used to measure the transverse deformations at the mid region. The strains in the longitudinal and transverse reinforcement were measured using electric resistance strain gages, which were attached to two longitudinal and two transverse reinforcements for concentric loading case and four longitudinal and four transverse reinforcements for the eccentrically loaded specimens. Three linear variable displacement transducers (LVDT) were used to measure the lateral deflections of the eccentrically loaded specimens. Readings from the pi gages, strain gages, applied load and stroke of the testing machine were recorded using a Vishay Data Acquisition System during testing. A 2, kip (8896 kn) capacity compression testing machine was used to apply the compression load monotonically at a rate of.36 mm/min. The tests continued until a significant drop in load-resistance of the columns. Thin layers of hydrostone were used at the top and the bottom ends of each column for leveling and to ensure uniform distribution of the applied load across the cross section. For eccentric tests, the load was applied with specific eccentricities through a specially designed curved plates and roller bearing assembly as shown in Figure 4. Concentrically Loaded Columns Test Results and Analysis Typical measured axial load-axial shortening behaviors of concentrically loaded columns are shown in Figure 5. No cracks were observed up to the measured peak load in most of the columns except in some specimens which were subjected to small unintentional eccentricities during testing. At the peak load, the concrete cover suddenly 4

5 spalled off explosively at the mid region of the column for all the test specimens with larger tie spacing as shown in Figure 6(a). Spalling of the concrete cover for the columns with closer tie spacing occurred more slowly, as shown in Figure 6(b). Spalling of the concrete cover was also accompanied by some loss of core concrete and resulted in a sudden drop in load carrying capacity of the columns. This was more pronounced for columns with higher concrete strength as shown in Figure 5. Relatively higher residual resistance remained after the peak load was measured for columns with closer tie spacing. This behavior suggests that the remaining resistance of the column is highly dependent on the local bucking resistance of the individual longitudinal reinforcement f c =11~15 Mpa (14.6 ~ 15.2 ksi) b h = mm (7 9 in.) s15c3½ -ρ2 5 4 Load (kips) 6 s15c3½ -ρ4 s15c7-ρ4 s15c7-ρ2 3 2 Load (kn) Axial Shortening (in.) Figure 5 Load-Axial Shortening Graphs for Concentrically Loaded Columns (a) Column with Larger Tie Spacing (b) Column with Closer Tie Spacing Figure 6 Typical Failure Shapes of Concentrically Loaded Columns In general, at failure the measured longitudinal reinforcement strains exceeded the yield strain of the reinforcement. At this stage, the measured transverse reinforcement strains 5

6 were considerably lower than the yield strain of the transverse reinforcement. Test results indicated that the test specimens with wide transverse reinforcement spacing, designed according to the provisions specified by AASTHO LRFD Bridge Design Specifications, did not provide sufficient confinement to the concrete core. For some of the columns with closer spacing of ties, the transverse reinforcement yielded at a later stage of loading. The average measured axial concrete strains corresponding to the peak load ranged from.22 to.29. The nominal axial load carrying capacity of a column (P n ) can be determined using the equilibrium equation as follows: P = kf ( A A ) + f A n c g s y s where the parameter k is the ratio of the in-place concrete strength to the control cylinder compressive strength, f c ; A g is the gross area of the column; f y is the yield strength of longitudinal reinforcement and A s is the area of longitudinal reinforcement. The current k value specified by the AASHTO LRFD provision for concentrically loaded column is.85 for NSC. The values of k parameter from the tested concentrically loaded rectangular columns in this experimental program as well as other reported tests in the literature, are shown in Figure fc (MPa) Experimental ( mm) Experimental ( mm) Cusson et al. Yong et al. Martinez et al. k Sheik et al..8.6 AASHTO LRFD k = fc (ksi) Saatcioglu et al. Sharma et al. Nagashima et al. AASHTO Proposed Figure 7 Comparison of k Parameters of Concentrically Loaded Columns Figure 7 suggests that the k parameter decreases with increasing concrete strength and is less than.85 for HSC. Test results for this research program which include concrete strengths ranging from 7.9 ksi (55 MPa) to 15.4 ksi (16 MPa) confirm the findings of other researchers. Using a value of.85 for the k parameter may not be appropriate for the concentrically loaded column with HSC. Based on a regression analysis of the collected data, the following equation provides the proposed k values which are the lower bound for the 9 percentile of the test data. The proposed equation maintains the current value of k specified by the AASHTO LRFD Bridge Design Specifications and 6

7 reduces the value for compressive strengths of concrete up to 18 ksi (124 MPa). 69 k =.85.3( fc 69).75 for fc > 69 f c in MPa Eccentrically Loaded Columns Typical measured axial load-axial shortening behaviors of eccentrically loaded columns with eccentricities of e/h values of 1 and 2 percent are shown in Figure 8. The figure also shows concentrically loaded column to reflect the effect of load eccentricity on the behavior of the column. In most of the columns, no cracks were observed on the compressive side of the column up to the maximum load. In some cases, the cracking sound could be heard at loads slightly lower than the maximum load. At the peak load, spalling of the concrete cover and buckling of the longitudinal reinforcement were observed simultaneously at the extreme compression face, as shown in Figure 9. As the peak load was reached, inclined flexural cracks propagated quickly through the tension side. The load carrying capacity of eccentrically loaded columns was reduced due to the presence of the moment resulting from the eccentricity. The maximum measured concrete compressive strain at the compression side ranged from.25 to.46 at the peak load and consequently the strain in the longitudinal reinforcement exceeded the yield strain. Load (kips) Axial Shortening (mm) R9-ρ4 (e %) 8RE1 (e=1%) 8RE2 (e=2%) fc = 54 Mpa (7.9 ksi) Load (kn) Axial Shortening (in.) Figure 8 Load-Axial Shortening Graphs for Eccentrically Loaded Columns 7

8 (a) Column with 1 % Eccentricity (b) Column with 2 % Eccentricity Figure 9 Typical Failure Shapes of Eccentrically Loaded Columns Prediction of the load carrying capacity is based on a proposed equivalent rectangular stress block (RSB) representing the stress distribution of concrete in the compression zone for flexural members at ultimate strength. The RSB, with an intensity of α 1 f c, is assumed to be applied over a zone bounded by β 1 c from the extreme compression fiber with an ultimate concrete strain, ε cu, where α 1 and β 1 are the RSB parameters, f c is the cylinder concrete strength and c is the depth of the compression zone from the extreme compression fiber. The values for RSB parameters were determined based on specially designed bracket specimens tested also in this program and reported in a separate paper (Mertol et al. 26). The proposed RSB parameters were based on an extensive research program to extend the applicability of the AASHTO LRFD Bridge Design Specifications for concrete strengths up to 124 MPa (Mertol et al. 26). These RSB parameters are given in Table 3. It should be noted that both of the relationships assume the ultimate concrete compressive stain of.3 at the extreme compression fiber. Table 3 Rectangular Stress Block Parameters RSB α 1 β 1 AASHTO LRFD* Mertol et ( fc ) for fc > al. (26)*.85.58( fc 69).65 for fc > 69 ( fc ) * f c in MPa for fc > 27.6 The test results of eccentrically loaded columns were compared to axial load-moment interaction diagrams developed by using these RSB parameters. The diagrams for the two different concrete strengths are shown in Figure 8. The solid and the dashed line represent the interaction diagram using RSB parameters specified by the current AASHTO LRFD Bridge Design Specifications and the proposed value by Mertol et al (26), respectively. Figure 8(a) indicates that RSB parameters specified by the AASHTO LRFD Bridge Design Specifications produce conservative prediction for concrete strengths up to 69 MPa. For concrete strengths beyond 69 MPa, Figure 8(b), RSB parameters specified by the AASHTO LRFD Bridge Design Specifications overestimate the capacity of eccentrically loaded HSC columns whereas the predictions based on RSB parameters proposed by Mertol et al. (26) produce a more conservative estimation. 8

9 8 Exp. AASHTO 8 Exp. AASHTO Proposed 6 6 P (kn) 4 P (kn) 4 2 fc =54 MPa ρ = 4 % M (kn-m) 2 fc =114 MPa ρ = 4 % M (kn-m) a) 8RE1 and 8RE2 a) 17RE1 and 17RE2 Figure 8 Interaction Diagrams based on Different RSB Parameters Conclusions A total of thirty rectangular columns with concrete strengths ranging from 7.9 ksi (55 MPa) to 16.5 ksi (114 MPa) were tested under monotonically increasing concentric and eccentric loading. The test parameters for concentric loading included concrete strength, specimen size, longitudinal reinforcement ratio, and amount of transverse reinforcement. For eccentric loading, concrete strength, specimen size and two different load eccentricities (1% and 2% of the depth of the section) were considered as test parameters. The test results were used to evaluate the provisions specified by the AASHTO LRFD Bridge Design Specifications (24) for concentrically and eccentrically loaded HSC members. The test results indicate that: 1. Using a value of.85 for the k parameter for concrete strengths beyond 69 MPa could overestimate the load carrying capacity for concentrically loaded columns. Based on the regression analysis of the collected data combined with the experimental results of this research, the following equation is proposed to extend the current AASHTO LRFD Bridge Design Specifications for HSC. 69 k =.85.3( fc 69).75 for fc > 69 fc in MPa 2. For concrete strengths beyond 69 MPa, RSB specified by the current AASHTO LRFD Bridge Design Specifications overestimates the capacity of eccentrically loaded HSC columns. The proposed RSB parameters to estimate the ultimate strength for NSC and HSC are: 69 α1 =.85.58( fc 69).65 for fc > β1 = ( fc 27.6).65 for fc > 27.6 fc in MPa fc in MPa 9

10 Acknowledgements The authors would like to acknowledge the support of the NCHRP project and the Senior Program Officer, David Beal. The authors also thank the contributions of Henry Russell of Henry Russell, Inc. and Robert Mast of Berger/ABAM Engineers, Inc. who served as consultants for the project. The contribution of Ready Mixed Concrete Company and the personnel of the Constructed Facilities Laboratory are greatly appreciated. The authors would also like to acknowledge the helpful efforts provided by the graduate research assistants, Andrew Logan, Wonchang Choi and Zhenhua Wu. References American Association of State Highway and Transportation Officials (24) AASHTO LRFD Bridge Design Specifications. Third Edition including Interims for 25 and 26, Washington, DC, USA. Cusson, D. and Shah, S. P. (1995) Stress-strain model for confined high-strength concrete. Journal of Structural Engineering, ASCE, Vol. 121, No.3, pp Logan, A. T. (25) Short-term material properties of high-strength concrete. M.S. Thesis, Department of Civil, Construction and Environmental Engineering, North Carolina State University, Raleigh, USA. Martinez, S. et al. (1982) Short-term mechanical properties of high-strength concrete. Department Report No.82-9, Structural Engineering Department, Cornell University, Ithaca, USA, 98 p. Mertol et al. (26) High-strength concrete for flexural design of bridge girders. Proceedings of Seventh International Conference on Short and Medium Span Bridges, Montreal, Canada, in press. Nagashima, T. et al. (1992) Monotonic axial compression test on ultra-high-strength concrete tied columns. Proceedings of Tenth World Conference on Earthquake Engineering, Madrid, Spain, pp Saatcioglu, M., and Razvi, S.R. (1998) High-strength concrete columns with square sections. Journal of Structural Engineering, ASCE, Vol. 124, No. 12, pp Sharma, U.K. et al. (25) Behavior of confined high strength concrete columns under axial compression. Journal of Advanced Concrete Technology, Vol. 3, No. 2, pp Sheikh, S.A. and Uzumeri, S.M. (198) Strength and ductility of tied concrete columns. Journal of Structural Engineering, ASCE, Vol. 16, No.5, pp Yong, Y. K. et al. (1988) Behavior of laterally confined high-strength concrete under axial loads. Journal of Structural Engineering, ASCE, Vol. 114, No.2, pp