Confinement of High-Strength Concrete

SP 176-6

Confinement of High-Strength Concrete by M. Saatcioglu, P. Paultre and S.K. Ghosh

Synopsis: Recent research on confinement of high-strength concrete (HSC) is reviewed. The emphasis is placed on the effects of confinement parameters and related experimental research. A review of analytical models proposed for HSC is also presented. The results indicate that for similar strength and deformability, HSC requires higher confinement pressure than normal-strength concrete. The level of lateral pressure required can be provided by increasing the volumetric ratio and grade of confinement reinforcement. The efficiency of pressure can be improved by reducing the spacing of lateral reinforcement in both the longitudinal and crosssectional planes. When properly confined, HSC exhibits ductile stress-strain characteristics. The analytical models developed for normal-strength concrete cannot be used to describe stress-strain characteristics of HSC. A number of models have been proposed for HSC that produce good correlations with experimental data.

Keywords: Columns (supports); confinement; ductility; earthquake-resistant structures; high-strength concrete; reinforced concrete; structural design

105

106 Saatcioglu, Paultre and Ghosh Murat Saatcioglu, F ACI, is a professor of structural engmeenng at the Uruvers1ty of Ottawa, Canada. He is a member of the ACI Committees 368, Earthquake Resisting Elements and Systems; 340 Design Aids; 441, Reinforced Concrete Columns; and 442, Response of Buildings to Lateral Loads. Dr. Saatcioglu is also a member of the Canadian National Committee on Earthquake Engineering. ACI member Patrick Paultre is a professor of structural engineering at the University of Sherbrooke, Sherbrooke, Canada. He is a member of the ACI Committees 352, Joints and Connections; 368, Earthquake Resisting Elements and Systems; and 441, Reinforced Concrete Columns. Dr. Paultre is also the technical secretary ofCSA Committee A23.3, Design of Concrete Structures for Buildings. S. K. Ghosh, FACI, is Director ofEngineering Services, Codes and Standards for the Portland Cement Association, Skokie, Illinois. He is a member of the ACI 318 Standard Building Code Committee and the chair of the ACI Committee 435 on Deflections. Dr. Ghosh is also a member of the Provisions Update Committee of the Building Seismic Safety Council and the ASCE Committee 7.

INTRODUCTION

Strength of concrete used in the construction industry has been increasing gradually over the past two decades. HSC with strengths of up to 130 MPa are currently being used in major metropolitan centers throughout North America. HSC exhibits superior performance while offering savings in material and construction costs. In spite of the advantages offered by HSC, however, the increase in brittleness of concrete with strength is often questioned for structural applications. This is especially true for seismic applications where inelastic deformability of members is relied on for dissipation of seismic energy. Deformability of concrete can be improved through confinement. Hoops, overlapping hoops, cross ties and spirals are used to confine the core concrete in reinforced concrete members. The confinement steel requirements for normalstrength concrete are reasonably well established in current building codes (1,2). However, research findings for HSC are still scarce in the literature. A number of projects have been carried out in the U.S., Canada, Japan, New Zealand and Norway to investigate confinement of HSC and associated improvements in strength and deformability. This paper provides a summary of recent findings resulting from these r~search programs. Although the emphasis is placed on research carried out in the U.S. and Canada, research data obtained from other parts of the world is also used in presenting some ofthe conclusions. Characteristics of confined HSC have been researched experimentally by

HSC in Seismic Regions 107 testing columns under concentric loading, and analytically by developing models for concrete stress-strain relationship. A major difficulty in testing HSC columns is the high level of loads required for such tests, which may be beyond the capacity of most laboratories, especially if full or near-full-size columns are to be tested. This becomes a major drawback for most researchers who may have to limit the scope of their research to small-size specimens. Both small-scale and large-scale columns are addressed in this paper, with appropriate references made to the specimen size. Improvements in strength and deformability due to confinement are discussed separately.

STRENGTH OF CONFINED HSC

Strength of unconfined concrete in a structural member may be different than that obtained through standard cylinder tests. This difference is usually attributed to differences in size, shape and concrete casting practice between a column member and a standard cylinder. Therefore, it is important to establish the in-place strength of concrete by testing columns of realistic size. The in-place strength of normalstrength concrete in columns is typically taken as 85% of the cylinder strength. Tests of two 250 mm square columns with 124 MPa and 81 MPa cylinder strengths, conducted by Saatcioglu and Razvi (3) indicated 0.89f, and 0.92f, as in-place strength of concrete, respectively. The average value reported by Cusson and Paultre (4) from tests of 235 mm square columns with 100 MPa concrete was 0.88f,. Tests conducted by Yong et al. (5) on small scale specimens with 152 mm square cross-section indicated 0.87f, to 0.97f, for concretes with 84 MPa to 94 MPa cylinder strength. The current Canadian Standard CSA A23.3-1994 (2) recommends values of up to 0.90f, to be used as in-place strength of unconfined concrete in columns. Strength of concrete improves through confinement. Razvi and Saatcioglu (6,7) showed by experimental and analytical research that the increase in strength due to confinement could be expressed independently of unconfined concrete strength in the member (f, This implies that the gain in strength due to confinement is essentially a function of confinement parameters that produce lateral confinement pressure. Hence, the same confinement reinforcement producing the same lateral pressure would produce approximately the same incremental strength enhancement in normal-strength and high-strength concretes. The increase in strength ofHSC becomes lower in percentage terms, while the actual incremental strength gain remains approximately constant. Therefore, higher confinement pressure is needed for HSC if the same percentage strength enhancement is required as for normal-strength concrete. Table 1 includes sample test data illustrating lower percentages of strength enhancement attained in higher strength concretes with the same confinement reinforcement. Similar conclusions were also derived from experimental data by Cusson and Paultre (8). Figure 1 illustrates that higher strength concretes require higher lateral confinement pressure for the same percentage strength enhancement. 0 ).

108 Saatcioglu, Paultre and Ghosh The lateral confinement pressure required for strength enhancement can be increased by increasing either the amount (volumetric ratio) or the grade of the confinement steel. Table 2 provides a summary of sample experimental data illustrating the effects of volumetric ratio on strength enhancement. In all cases the confined concrete strength increases with increased volumetric ratio p,. However, the improvement in strength is more evident in columns with efficient tie arrangement and/or close tie spacing. The strength gain with increase in volumetric ratio can be negligibly small within the low range of lateral pressure. The confinement pressure can also be increased through the use of highgrade transverse reinforcement. Muguruma et al. (9) showed experimentally that high-strength steel was fully effective as confinement reinforcement. Lateral reinforcement with a yield strength of 1363 MPa was reported to have yielded at or near the peak load, under concentric compression. The researchers also reported that high-strength steel was more effective controlling bar buckling than an equivalent amount of normal-strength steel. The beneficial effects of high-strength steel in confining columns under concentric loading were also reported by Polat (10), Sugano et al. (11), Nagashima et al. (12), Sakai et al. (13), Li (14), Cusson and Paultre (8) and Saatcioglu and Razvi (3,6, 7). Although the beneficial effects on deformability were more obvious, strength enhancement was also attained in HSC columns confined with high-grade reinforcement. Table 3 illustrates sample data obtained from different research programs, illustrating the effect of transverse steel grade on strength enhancement. Cusson and Paultre (8) showed that there was a limit until which high-strength steel improved the strength ofHSC. The researchers showed that the high-strength steel improved concrete strength only in well confined columns. The spacing and arrangement of confinement reinforcement, observed to play important roles on normal-strength concrete, also play important roles on HSC. Close spacing of ties in the longitudinal direction, and tie legs in the transverse direction (as dictated by reinforcement arrangement) improve the efficiency of lateral pressure and produce near-uniform confinement pressure. While it is important to increase the confinement pressure through sufficiently high volumetric ratio and strength of transverse steel, it is also important to maximize the efficiency of this pressure by using closely spaced reinforcement both in the longitudinal and transverse directions. Table 4 and Fig. 2 contain test data obtained by Martinez et al. ( 15) and Cusson and Paultre (4), respectively, indicating higher strength enhancement in columns with closely spaced ties. Similarly, Table 5 and Fig. 3 contain test data obtained by Razvi and Saatcioglu (6) on columns with different arrangements of tie reinforcement, producing different spacing of tie legs in the cross-sectional plane. The results indicate that columns with a 12-bar arrangement show higher strength gain due to confinement than companion columns with 8-bar and 4-bar arrangements. Similar effects were also observed by Yong et al. (5) and Sakai et al. (13), although Nagashima et al. (12) reported no significant difference in behavior between 6-bar, 8-bar and 12-bar arrangements. Similarly, no significant difference was reported by Cusson and Paultre ( 4) between 8-bar and 12-bar arrangements as illustrated in Fig. 4, although the researchers stressed the poor

HSC in Seismic Regions 109 performance of 4-bar arrangement. Cusson and Paultre (8) expressed tfie efficiency of confinement in terms of the confinement index. They defined the confinement index for HSC as I e = f 1jfco = K e p w f he/ f co' and showed the variation of strength enhancement with confinement index as illustrated in Fig. 1.

DEFORMABILITY OF CONFINED USC

HSC exhibits brittle behavior under concentric compression, developing explosive failure immediately after the attainment of its strength. It is, however, possible to improve inelastic deformability of HSC through confinement. The confinement pressure required in HSC columns is higher than that required for normal-strength concrete to attain similar deformabilities. This implies that the confinement requirements of current building codes (1,2), developed for normalstrength concrete columns, could lead to brittle response in HSC columns. Table 1 and Fig. 5 illustrate the variation of column deformability with concrete strength. The test data indicate that the axial strain ductility decreases as concrete strength increases. Inelastic deformability ofHSC can be improved, up to 1.5% axial strain and higher, if sufficiently high confinement pressure is provided. One possibility is to increase the amount of confinement reinforcement in proportion to the increase in concrete strength. Table 2 indicates that an increase in the volumetric ratio of reinforcement results in improvements in strain ductility. If the required increase in pressure is provided by increasing the volumetric ratio of transverse reinforcement alone, however, the volumetric ratio may reach unrealistically high values, making construction impossible. While some increase in the volumetric ratio may be necessary to improve concrete stress-strain characteristics beyond the peak load, especially in terms of stabilizing the longitudinal compression reinforcement, additional increase in lateral pressure may be provided by using higher grade confinement reinforcement. Figure 6 shows a sample comparison, illustrating the improvement in the stress-strain relationship of confined concrete attained by using high-strength confinement reinforcement. Figure 7 illustrates that, in a sample comparison, a reduction in the volumetric ratio from 3.05% to 1.32% can be compensated for by a proportional increase in the grade of confinement steel from 400 MPa to 1000 MPa, without altering the stress-strain characteristics significantly. These two figures clearly show the beneficial effects of high-strength confinement reinforcement on the deformability ofHSC. Razvi and Saatcioglu (16) concluded after an extensive evaluation of test data that the required lateral pressure can be provided by increasing the volumetric ratio and grade of lateral reinforcement to a certain level such that the product p,fyt is increased in proportion to the increase in concrete cylinder strength. Similar observations were made by Sugano et al. (11) who recommended that the ratio p,fy/f, should be at least 0.2 to obtain ductile behavior. Nagashima et al. (12) reported that 120 MPa concrete columns failed suddenly at approximately 1% strain when pJ;./f, was less than 0.15, and developed approximately 2% strain when this ratio was in excess of0.15. Nishiyama et al.(17)

110 Saatcioglu, Paultre and Ghosh reported that 4% volumetric ratio of800 MPa steel was required to obtain ductile behavior in columns with 110 MPa concrete, producing p,:t;,lf, ratio of0.29. While recommended values ofthe p.f;/f, ratio in the literature vary between 0.15 and 0.30, lower values can be used when the spacing of ties in the longitudinal direction and the spacing of tie legs in the cross-sectional plane are close enough to produce efficient lateral pressure. Tables 4 and 5 indicate that HSC columns with the same p,:t;,lfc ratio develop higher ductilities when the spacing parameters are favorable. Figure 3(a) includes a sample comparison of experimentally recorded stress-strain relationships for I24 MPa concrete columns confined with a p,fJfc ratio of 0.10. The results indicate that the column with a 12-bar arrangement developed significantly higher ductility than the companion column with a 4-bar arrangement. A similar comparison is presented in Fig. 3(b) for 92 MPa columns with 8-bar and I2-bar arrangements, having a p,~/f, ratio of 0. 13. These comparisons clearly show that while the p,fy/f, ratio may be an important parameter indicating the level of confinement, it is not sufficient to show differences in concrete confinement resulting from differences in tie arrangement and tie spacing. The spacing parameters can be adjusted in design to achieve the desired efficiency in confinement pressure. Tests conducted by Razvi and Saatcioglu (6) have shown that the use of a wider tie spacing can be compensated for by the selection of a superior tie arrangement. Figure 8 illustrates a sample comparison of two companion columns with different spacings and tie arrangements. In this specific example, the increase in tie spacing by a factor of 1.4 was compensated by changing the tie arrangement from 8 bars to 12 bars without altering the stressstrain relationship significantly. Furthermore, the efficiency, spacing and the volumetric ratio of transverse reinforcement play significant roles on the level of tensile stress developed in transverse reinforcement. The high-strength transverse reinforcement may not develop its yield strength at peak concrete strength when the ties are placed with wide spacing, especially when the volumetric ratio of steel is excessively high and/or the axial compression is very low. In such cases the applicability of the ratio p,fy/f, becomes questionable. Cusson and Paultre (8) proposed an iterative method to compute the actual stress in transverse reinforcement at peak strength of confined concrete. They expressed the gain in deformability in terms of confinement index, I, = ~jf,o = KePwfhc/( as illustrated in Figs. 9 and I 0. Based on a comparative study of the behavior of several confined HSC columns, the researchers suggested that a confinement index greater than 5% was necessary if moderate confinement was needed and greater than 20% if high confinement was desired. As an example, a column with r, = 100 MPa and fyh = 770 MPa with a confinement index of I6% showed a strength gain of 51% and a strain gain at peak of223%. 0,

Closely spaced circular hoops and spirals exhibit superior confinement characteristics when compared with square ties of similar properties, in normalstrength concrete. Similar observations were made in HSC columns. Razvi and Saatcioglu (6) tested companion HSC columns with circular and square sections. Figure II shows the comparison of250 mm circular and 250 mm square columns, both with I24 MPa concrete. The results indicate that a square column with a 4-bar

HSC i~ Seis.mic ~egions. 111

arrangement showed a rapid strength decay at 0.3% axtal stram wh!le the Circular column was able to develop 0.7% strain without any strength loss, followed by a gradual strength decay. However, in the same research program it was shown that the efficiency of square ties could be improved to produce similar behavior as that of circular HSC columns. Figure 12 shows that the behavior of a square column can be similar to that of a circular column, developing axial strains in excess of 1%, when confined with a 12-bar tie arrangement and 1000 MPa transverse reinforcement. It may be concluded from the forgoing discussion that the deformability requirements for HSC can be met if concrete is confined with an efficient arrangement of transverse reinforcement, and a p,f)f'c ratio of approximately 0.2.

ANALYTICAL MODELS FOR STRESS-STRAIN RELATIONSHIP OF CONFINED HSC

A number of models have been proposed to describe the stress-strain relationship of HSC. A detailed review of existing models is presented elsewhere (18). The following sections provide an overview of these models with proper references to original publications for further details. Ahmed and Shah (1982)-- Ahmed and Shah (19) were among the first group of researchers who proposed a model for confined HSC. The model was developed based on data obtained by testing concretes of up to 69 MPa strength. A triaxial stress-strain relationship of concrete and a stress-strain relationship of confinement reinforcement were used to construct the model through an iterative procedure. The model was verified using tests of spirally reinforced HSC cylinders. Martinez. Nilson and Slate (1982) -- Martinez et al. (15) developed a model for 21 MPa to 83 MPa normal-density and 21 MPa to 62 MPa low-density concretes. It was based on tests conducted on concrete cylinders confined with spiral reinforcement of Grade 414 MPa and lower. The specimens did not contain any longitudinal reinforcement. Strength enhancement was expressed in terms of lateral confinement pressure and the spacing of confinement steel. Ductility improvement was expressed in terms of concrete strength, lateral pressure and spacing of confinement reinforcement. A second degree mathematical expression was specified for the ascending branch, followed by a linear descending segment which was defined by an expression specified for the strain value at 15% strength decay. Muguruma. Watanabe. Iwashimizu and Mitsueda (1983) -- Muguruma et al. (9) modified the model that had been developed by Watanabe et al. (20) in 1980 for concretes in the range of 26 MPa to 61 MPa strength, confined with Grades 161 MPa to 1353 MPa reinforcement. The modification was based on small scale square columns confined with 1390 MPa lateral steel, without any longitudinal reinforcement. The concrete strength considered in the test program varied between

112 Saatcioglu, Paultre and Ghosh 34 MPa and 88 MPa. A confinement coefficient was defined to express the strength of confined concrete. Strain at peak stress was defined in terms of concrete mix proportions. Two second order parabolas were suggested for the ascending portion of the curve, one describing the initial part up to the peak stress of unconfined concrete, and the other describing the portion between the peak stresses of unconfined and confined concretes. The descending branch was specified to be a linear segment. Fafitis and Shah (1985) -- Fafitis and Shah (21) had developed a model for concretes with strengths of up to 62 MPa on the basis of small cylindrical specimens. The model was modified in 1985 to include higher strength concretes (22), based on the tests conducted by Muguruma et al. (9) and Martinez et al. (15). The model was developed for circular columns and can be used for square columns with well distributed laterally supported longitudinal reinforcement. The confining pressure for square columns was calculated approximately by assuming a circular column with an equivalent diameter. The ascending branch was a parabolic curve, followed by the descending branch, which consisted of an exponential curve reaching zero stress at an infinite strain. Yong. Nour and Nawy (1988) --Yang et al. (5) proposed a model based on experimental data obtained from tests of small-scale square columns. The range of concrete strength considered was 84 MPa to 94 MPa. Three sets of parameters were used to establish the model. These included the peak stress and corresponding strain, the stress defined as inflection stress and corresponding strain, and the stress and strain at an arbitrarily selected point on the descending branch. The proposed peak stress was similar to that suggested by Sargin (23). The strain corresponding to peak stress was also similar to that originally proposed by Sargin (23). The stressstrain relationship consisted of three parts. The first part formed the ascending branch. The second part consisted of a polynomial similar to that proposed by Sargin (23). The curves forming the first and second parts were joined at the peak with slope continuity to provide a smooth continuous curve. The third part consisted of a linear constant stress segment at 30% of peak stress. Bjerkeli. Tomaszewicz and Jansen (1990) -- Bjerkeli eta!. (24) proposed a generalized model applicable to circular, square and rectangular sections. The behavior of confined HSC was assumed to be controlled by three parameters; concrete strength, confinement pressure and section geometry. The model was developed for both normal-density and low-density concretes of up to 90 MPa and 70 MPa, respectively. The stress-strain relationship was a modified version of that proposed by Martinez et al. (15). It consisted of three parts. The ascending part was a polynomial, followed by a linear descending branch which continued as a linear constant stress segment beyond a certain minimum stress value. Muguruma Watanabe and Komuro (1990) -- Muguruma and Watanabe (25) modified two of the previously proposed models using test data on eight largescale HSC columns with well distributed longitudinal reinforcement. The columns were tested under constant axial load and lateral load reversals. The first model

HS~

if! Seismic

Region~

113

modified was that proposed by Muguruma et al (9), ongmally developed usmg test data on small-scale columns. It was found that the model did not reproduce the curvature ductility of columns tested under reversed cyclic loading. Hence, it was modified by reducing the slope of the descending branch to obtain better agreement with the test data. The second model modified was that originally proposed by Kent and Park (26) for normal-strength concrete. It was found that the model overestimated the curvature ductility of the columns tested in their program. Therefore, the model was modified by increasing the slope of the descending branch.

Muguruma, Nishiyama. Watanabe and Tanaka (1991) -- Muguruma et al. (27) modified one of the models already modified for HSC, based on their tests conducted on four large scale specimens under constant axial compression and lateral load reversals. The original model had been developed by Muguruma et al. (9), and was later modified by Muguruma and Watanabe (25) in 1990. The new series of tests included columns with 130 MPa concrete. The results indicated that the constant stress segment at the end of the descending branch had to be increased from 30% to 50% of peak stress to better correlate with test data. The polynomial ascending branch and the linear descending branch remained the same as those suggested in the earlier model.

Nagashima. Sugano. Kimura and Ichikawa (1992) -- Nagashima et al. (12) proposed a model based on the effectively confined core area concept originally proposed by Sheikh and Uzumeri (28) for normal-strength concrete columns. The model reflected the effects of distribution oflongitudinal reinforcement and spacing of ties on confinement. The stress-strain relationship consisted ofthree parts. The expression proposed by Popovics (29), also used by Mander et al. (30) was adopted as the ascending part. The descending part was a linear segment, changing slope at 30% of the peak, forming the third segment with a constant stress. Muguruma. Nishiyama. Watanabe (1993) -- Muguruma et al. (31) modified the model previously proposed in 1983 (9) and modified twice in 1990 (25) and 1991 (2 7), using their earlier tests. The recent modifications included a new expression for the strain at peak stress in terms of concrete strength instead of the mix-design parameters used previously, a reduction of about I 0% in the modulus of elasticity, and new expressions for the descending branches. The model was reported to be applicable to both normal-strength and high-strength concretes within the 20 MPa to 130 MPa strength range. Li (1994) -- Li (14) proposed a generalized stress-strain relationship for confined and unconfined HSC. The model was developed on the basis of a large volume of tests data, covering a wide range of concrete strengths. It was reported to be applicable to circular and square columns confined with normal and highstrength reinforcement. A continuous curve was proposed to represent the stressstrain relationship of unconfined concrete. The proposed curve was a modified version of that proposed by Popovics (29). Expressions were proposed for strength and ductility of confined concrete on the basis of concrete strength and the type of

114 Saatcioglu, Paultre and Ghosh confinement reinforcement. Accordingly, different sets of expressions were suggested for HSC confined by circular normal-strength steel, HSC confined by rectilinear normal-strength steel, concrete confined by circular high-strength steel and concrete confined by rectilinear high-strength steel. Azizinamini. Koska. Brungardt and Hatfield (1994) -- Azizinamini et al. (32) modified the model developed by Yong et al (5) based on their test data, as well as those obtained by Yonge et al. (5). The model consisted of a linear ascending branch, followed by a linear descending branch with a constant residual stress at 30% of the peak stress. Cusson and Paultre (1995) --Cusson and Paultre (33) developed a model for confined HSC in 1993, based on their tests conducted on large-scale HSC columns. The model is based on the effectively confined core area concept originally developed by Sheikh and Uzumeri (28) and later adopted by Mander et al. (30). It is defined by three parameters, consisting ofthe peak stress of confined concrete and corresponding strain, and the strain at 50% strength decay. All of these three parameters are related to the confinement index 1,. The stress-strain relationship consists of two parts. The expression for the ascending part was adopted from Popovics (29) and the expression for the descending branch was adopted from Fafitis and Shah (22). In 1995, Cusson and Paultre (8) modified their original model on the basis of additional column tests conducted by the authors and by N agashima et al. (12). Because strength and ductility gain due to confinement depend on the stress in transverse bars at peak concrete strength, the researchers developed an iterative procedure to predict the stress in transverse reinforcement. The procedure resulted in good agreement with test data. The modified stress-strain relationship was constructed using the same expressions as before. However, some of the empirically derived coefficients were modified based on the new data. Razvi and Saatcioglu (1996) -- Razvi and Saatcioglu (7) developed a model for confined HSC based on their tests conducted on a large number of nearfull-size circular and square columns, and the analytical model developed earlier by Saatcioglu and Razvi (34) for normal-strength concrete. The concrete strength considered in the experimental program ranged between 60 MPa and 124 MPa and the yield strength of confinement steel ranged between 400 MPa and I 000 MPa. The model was based on the same equivalent uniform confinement pressure concept presented earlier for normal strength concrete (34), incorporating variations in pressure resulting from different arrangements of lateral reinforcement. The effect of higher grade steel was included in computing the lateral pressure. The stressstrain relationship consisted of a curved ascending branch followed by a linear descending segment with a constant residual stress at 20% of peak stress. The expression used for the ascending part was adopted from Popovics (29). Both the strength and the slope of the descending branch depended on the equivalent uniform lateral confinement pressure. The model was verified against a large number of circular and square column tests.

HSC in Seismic Regions 115 SUMMARY AND CONCLUSIONS

The results of recent experimental research on confinement of HSC are reviewed. Strength and deformability of confined concrete are discussed. A summary of available analytical models for HSC are presented. The following conclusions can be drawn based on the research data highlighted in the paper: - The limited test data reported on concentrically tested plain HSC columns indicate that the unconfined in-place strength of concrete in columns vary between 0.87 f, and 0.97 fc for concretes in the range of 80 to 125 MPa. - Strength of HSC can be increased by confinement. The strength enhancement obtained is generally limited to 100% of unconfined inplace strength of concrete, although for concretes of 100 MPa and higher the strength enhancement is usually limited to 50%. Most HSC columns with realistic volumetric ratio and spacing of confinement reinforcement show less than 50% strength enhancement due to confinement. - The confinement parameters known to play important roles on normalstrength concrete, also play important roles on HSC. - For the same percent increase in strength, higher confinement pressure is required to confine HSC relative to that needed for normal-strength concrete. The confinement pressure can be increased by increasing the volumetric ratio and the strength of transverse reinforcement. The research data indicate that for the same arrangement and spacing of confinement reinforcement, the pJJf, ratio can be used as an indication of confinement steel requirements for HSC. The recommended values for this ratio range between 0.15 and 0.30, depending on the arrangement and the spacing of confinement reinforcement. Alternatively, the confinement index, defined as I, = ~j( 0 = ~Pwfhc/(o may be used to express confinement pressure requirements for HSC. Confinement index of greater than 5% and 20% appear to be necessary for moderate and high confinement, respectively. - High-strength transverse reinforcement was reported to be effective in confining HSC and may be used as an alternative to increasing the volumetric ratio of transverse reinforcement to unrealistically high proportions. However, caution must be exercised in using high grade reinforcement, since the maximum steel stress developed may be less than the yield strength, depending on the efficiency of confinement. - Analytical models developed for the stress-strain relationship of confined normal-strength concrete cannot be used for HSC. Analytical models have recently been developed for HSC, showing good correlations with experimental data.

116 Saatcioglu, Paultre and Ghosh NOTATIONS

I,

K, PC poe pocc

s

P,Pw Ec

Eo! Ec50c

concrete compressive strength based on a standard cylinder test; strength of concrete in column core; strength of unconfined concrete in column core; stress in transverse steel at peak concrete strength; equivalent lateral pressure; yield strength of transverse reinforcement; cross-sectional dimension of a square column or diameter of a circular column; confinement index; confinement effectiveness coefficient; axial load carried by concrete; axial load capacity of total concrete section; axial load capacity of core concrete; center-to-center spacing of transverse reinforcement along column height; volume of transverse reinforcement divided by volume of core concrete; axial concrete strain; strain corresponding to the peak stress of unconfined concrete in column; strain corresponding to 50% of confined concrete strength on the descending branch; strain corresponding to 50% of unconfined concrete strength on the descending branch; strain corresponding to 85% of confined concrete strength on the descending branch;

REFERENCES 1.

ACI Committee 318, "Building Code Requirements for Reinforced Concrete and Commentary (ACI 318-95)," American Concrete Institute, Detroit, 1995, 369 pp.

2.

CSA, "Design of Concrete Structures for Buildings (CAN3-A23.2-94)," Canadian Standards Association, Ontario, 1994, 199 pp.

3.

Saatcioglu, M., and Razvi, S., "Behaviour of Confined High-Strength Concrete Columns," Proceedings of CPCA/CSCE Structural Concrete Conference, May 19-21, 1993, Toronto, Canada, pp.37-50

4.

Cusson, D., and Paultre, P., " High-Strength Concrete Columns Confined by Rectangular Ties," Journal of Structural Engineering, ASCE, Vol. 120, No.3, March 1994, pp.783-804.

5.

Yang, Y.K., Nour, M.G., and Nawy, E.G., "Behaviour of Laterally Confined

HSC in Seismic Regions 117 High-Strength Concrete Under Axial Loads," ASCE Journal oT Structural Engineering, Vol.l14, No.2, 1988, pp. 332-351. 6.

Razvi, S. R. and Saatcioglu, M., "Tests of High-Strength Concrete Columns under Concentric Compression." Research Report 96-03, Ottawa-Carleton Earthquake Engineering Research Centre, Department of Civil Engineering, The University of Ottawa, Ottawa, Ontario, Canada, 1996, p.147.

7.

Razvi, S. R. and Saatcioglu, M., "Concrete Confinement Model for NormalStrength and High-Strength Concrete Columns." Research Report 96-04, Ottawa-Carleton Earthquake Engineering Research Centre, Department of Civil Engineering, The University of Ottawa, Ottawa, Ontario, Canada, 1996.

8.

Cusson, D. and Paultre, P., "Stress-Strain Model for Confined High-Strength Concrete." ASCE Journal of Structural Engineering, Vol. 121, No.3, 1995, pp. 468-477.

9.

Muguruma, H., Watanabe, F., Iwashimizu, T., and Mitsueda, R., "Ductility Improvement of High Strength Concrete by Lateral Confinement," Transactions of the Japan Concrete Institute, Vol. 5, 1983, pp. 403-410.

10. Polat, M.B., "Behaviour of Normal and High Strength Concrete under Axial Compression,"M.A.Sc thesis, Department of Civil Engineering., University of Toronto, Ontario, Canada, 1992, 175pp. 11. Sugano, S., Nagashima, T., Kimura, H., Tamura, A., and Ichikawa, A., "Experimental Studies on Seismic Behaviour of Reinforced Concrete Members of High Strength Concrete," High Strength Concrete, Second International Symposium, ACI SP-121-5, Detroit, 1990, pp.61-87. 12. Nagashima, T., Sugano, S., Kimura, H., Ichikawa, A., "Monotonic Axial Compression Test on Ultra-High-Strength Concrete Tied Columns," Proceedings of 1Oth World Conference on Earthquake Engineering., Madrid, 1992, pp. 2983-2988. 13. Sakai, Y., Hibi, J., Otani, S., and Aoyama, H., "Experimental Study ofFlexural Behaviour of Reinforced Concrete Columns using High Strength Concrete," Transaction ofThe Japan Concrete Inst., Vol.12, 1990, pp. 323-330. 14. Li, Bing, "Strength and Ductility of Reinforced Concrete Members and Frames Constructed Using High Strength Concrete," Research Report No. 94-5, University of Canterbury, Christchurch, New Zealand, May 1994, p. 389. 15. Martinez, S. Nilson A.H., and Slate, F.O., "Spirally Reinforced High-Strength Concrete Columns," ACI Journal, Sept.-Oct. 1984, pp. 431-442. 16. Razvi, S., and Saatcioglu, M., "Strength and Deformability of Confined High-

118 Saatcioglu, Paultre and Ghosh Strength Concrete Columns," ACI Structural Journal, Vol.95, No.6, 1994, pp.678-687. 17. Nishiyama, M., Fukushima, 1., Watanabe, F., Muguruma, H., "Axial Loading Tests on High-Strength Concrete Prisms Confined by Ordinary and HighStrength Steel," Proceedings of the Symposium on High-Strength Concrete, June 93, Norway, pp. 322-329. 18. Razvi, S.R., "Confinement ofNormal and High-Strength Concrete Columns." Ph.D. Dissertation, Department of Civil Engineering, the University of Ottawa, Ottawa, Ontario, Canada, 1995, p.415. 19. Ahmed, S.H., and Shah, S.P., "Stress-Strain Curves of Concrete Confined by Spiral Reinforcement," ACI Journal, V.79, No.6, Nov.-Dec. 1982, pp.484-490. 20. Watanabe, F., Muguruma, H., Matsutani, T., and Sanda, D., "Utilization of High Strength Concrete for Reinforced Concrete High-Rise Buildings in Seismic Area," Utilization of High Strength Concrete Proceedings, Stavanger, Norway, Tapir Publisher, June 1987, pp. 655-666. 21. Fafitis, A and Shah, S. P., "Prediction of Ultimate Behaviour of Confined Columns Subjected to Large Deformation," ACI Structural Journal, Vol. 82, July-August 1985, pp.423-433. 22. Fafitis, A and Shah, S. P., "Lateral Reinforcement for High-Strength Concrete Columns," ACI Special Publication, SP-87-12, American Concrete Institute, Detroit, 1985, pp.213-232. 23. Sargin, M., "Stress-Strain Relationship for Concrete and Analysis of Structural Concrete Sections," Study No.4, Solid Mechanics Division, University of Waterloo, 1971, pp.167. 24. Bjerkeli, L., Tomaszewicz, A, and Jensen, J.J., "Deformation Properties and Ductility of High Strength Concrete," High Strength Concrete, Second International Symposium, ACI SP-121-12, Detroit, 1990, pp.215-238. 25. Muguruma, H., Watanabe, F., and Komuro, T, "Ductility Improvement ofHigh Strength Concrete Columns with Lateral Confinement," High Strength Concrete, Second International Symposium, ACI Sp-121-4, Detroit, 1990, pp.47-60. 26. Kent, D.C., and Park, R., "Flexural Members with Confined Concrete," Journal of Structural Division, ASCE, Vol.97, 1971, pp. 1969-1990. 27. Muguruma, H., Nishiyama, M., Watanabe, F., and Tanaka, H., "Ductile Behaviour of High Strength Concrete Columns Confined by High Strength Transverse Reinforcement," ACI International Conference on "Evaluation and

HSC in Seismic Regions 119 Rehabilitation of Concrete Structures and Innovations in Design, " SP-128-54, Vol.2, Hong Kong, December 1991, pp. 877-891. 28. Sheikh, S.A., and Uzumeri, S.M., "Analytical Model for Concrete Confinement in Tied Columns," Journal of Structural Engineering, ASCE, Vol.l08, No.5, December 1982, pp. 2703-2723. 29. Popovics, S., "Analytical Approach to Complete Stress-Strain Curves," Cement and Concrete Research, Vol.3, No.5, Sept. 1973, pp.583-599. 30. Mander, J.B., Priestley, M.J.N., and Park R., "Theoretical Stress-Strain Model for Confined Concrete," ASCE Structural Journal, Vol. 114, No.8, Aug. 1988(b ), pp. 1804-1826. 31. Muguruma, H., Nishiyama, M., and Watanabe, F., "Stress-Strain Curve Model for Concrete with a Wide-Range of Compressive Strength," Proceedings of the Symposium on High-Strength Concrete, June 93, Norway, pp. 314-321. 32. Azizinamini, A, Kuska, S.S.B., Brungardt, P., and Hatfield, E., " Seismic Behaviour of Square High-Strength Concrete Columns," ACI Structural Journal, Vol. 91, No.3, May-June 1994, pp. 336-345. 33. Cusson, D., and Paultre, P., "Experimental Study ofHigh-Strength Concrete Columns Confined by Rectangular Ties," Proceedings of the Symposium on High-Strength Concrete, June 1993, Norway, pp. 136-145. 34. Saatcioglu, M., and Razvi, S., "Strength and Ductility of Confined Concrete," Journal of Structural Engineering, ASCE, Vol. 118, No.6, 1992, pp. 15901607.

120 Saatcioglu, Paultre and Ghosh TABLE 1-EFFECTS OF CONCRETE STRENGTH ON STRENGTH ENHANCEMENT AND STRAIN DUCTILITY RATIO h s f,, p, roo (Mpa) (mm) (mm) (MPa) (%)

Column

Researcher

ND65-3 ND95-3 NDIIS-3

Bjerkeli et al.(24)

63 87 100

ISO ISO ISO

25 25 25

613 613 613

III-4 V-3

Ahmed and Shah( 19)

38 66

76 76

13 13

414

PSS-SC* PSS-7A* PSS-9A*

Muguruma et al.(9)

41 56 75

147 147 147

so so

PHS-58* PH5-9A*

Muguruma et al.(9)

43 63

NCI66-I NCI68-I

Martinez eta!.( IS)

50 69

CS26* CS3*

f,Jfw

E,/Eot

3.2 3.2 3.2

1.82 1.55 1.25

6.5+ 5.2 3.5

414

3.2 3.2

1.26 1.12

7.2 4.5

50

1364 1364 1364

2.1 2.1 2.1

1.43 1.18 1.06

5.1 3.5 1.8

147 147

50 50

1364 1364

2.1 2.1

1.39 1.25

4.4 2.5

102 102

6 6

414 414

7.5 7.7

1.92 1.66

12.6 4.1

Razvi and 51 Saatcioglu(6) 105

250 250

55 55

570 570

2.16 2.16

1.50 1.22

6.4 2.8

CS-24* CS-19* CS-8*

51 Razvi and Saatcioglu(6) 78 105

250 250 250

85 85 85

400 400 400

3.24 3.24 3.24

1.42 1.33 1.12

7.2 3.6 3.2

CC-2 CC-22

Razvi and 51 Saatcioglu(6) 78

250 250

135 400 135 400

1.35 135

1.22 1.14

3.2 1.9

CC-3 CC-21

Razvi and 51 Saatciog1u(6) 78

250 250

70 70

0.80 0.80

1.34 1.19

3.9 2.6

Note:

* indicates (h x h) square cross-section.

660 660

All others are circular sections with

h diameter. While the same reinforcement arrangement is used within a group, the arrangement may vary between the groups. + indicates a higher capacity that could not be reached due to the termination of test.

HSC in Seismic Regions 121 TABLE 2-EFFECTS OF VOLUMETRIC RATIO ENHANCEMENT AND STRAIN DUCTILITY RATIO Column

Researcher

IPsi ON STRENGTH

h s fyt fco (MPa) (mm) (mm) (MPa)

p,

fjf,

0

EssfEoJ

(%)

NC164-3 NC166-1

Martinez et al.(lS)

so so

102 102

5 6

379 414

2.2 7.5

1.27 1.92

3.2 12.6

NC83-2 NC166-3

Martinez et al.(lS)

58

so

102 102

6 6

414 414

3.4 7.5

1.64 1.88

2.7 11.9

CC-11 CC-10

Razvi and Saatcioglu(6)

105 105

250 250

60 60

660 400

0.9 3.1

1.18 1.28

1.8 3.4

CC-21 CC-15

Razvi and Saatcioglu( 6)

78 78

250 250

70 60

660 400

0.8 3.1

1.19 1.35

2.3 2.9

PSS-9A * PDS-9A*

Muguruma et al.(9)

75 76

147 147

so so

1364 1364

2.1 4.2

1.06 1.25

1.8 7.6

CS-6* CS-8*

Razvi and Saatcioglu( 6)

lOS lOS

250 250

85 85

400 400

1.1 3.2

1.10 1.12

1.6 3.2

CS-7* CS-9*

Razvi and Saatcioglu( 6)

lOS lOS

250 250

120 400 120 400

1.0 3.1

1.09 1.27

1.7 2.7

CS-17* CS-19*

Razvi and Saatcioglu( 6)

81 92

250 250

85 85

1.1 3.2

1.09 1.33

3.4 4.6

400 400

Note: * indicates (h x h) square cross-section. All others are circular sections with h diameter. While the same reinforcement arrangement is used within a group, the arrangement may vary between the groups.

122 Saatcioglu, Paultre and Ghosh

TABLE 3-EFFECT OF STEEL YIELD STRENGTH (f vt ) Column

Researcher

Tie s b, fyt fco (MPa) (mm) (mm) Arrng. (MPa)

Ps

fjf, 0

(%)

CS-13

Razvi and

78

224

55

8-bar

570

2.16

1.10

CS-15

Saatcioglu(6)

69

224

55

8-bar

1000

2.16

1.39

9A 6HB

Li (14)

63 52

204 204

50 50

Circ. Circ.

445 1318

0.55 0.49

1.18 1.31

3B 2HC1

Li (14)

72

204 204

20 20

Circ. Circ.

445 1318

1.39 1.20

1.50 1.78

5B 3HC3

Li (14)

209 209

35 35

8-bar 8-bar

445 1318

1.32 1.14

1.13 1.27

2B lHCI

Li (14)

83

209 209

20 20

8-bar 8-bar

445 1318

2.31 2.00

1.49 1.82

12 7

Nishiyama (17)

96 93

214 214

60 60

12-bar 462 12-bar 813

0.93 0.93

1.20 1.30

11 3

Nishiyama (17)

96 93

214 214

45 45

12-bar 462 12-bar 813

1.24 1.24

1.22 1.32

HL08LA HH20LA

Nagashima (12) 100 100

200 199

35 35

12-bar 807 12-bar 1368

1.12 1.25

1.33 1.48

HL06LA HH15LA

Nagashima (12) 100 99

200 199

45 45

12-bar 807 12-bar 1368

0.87 0.97

1.18 1.29

83

72

83

72

Note: All columns have a square cross-section except when indicated as having a circular reinforcement arrangement in which case they have a circular cross-section.

HSC in Seismic Regions 123 TABLE 4-EFFECT OF TIE SPACING (s) ON STRENGTH ENHANCEMENT AND STRAIN DUCTILITY RATIO h s fyt Ps (mm) (mm) (MPa) (%)

Column

Researcher

NC83-2 NC243-2

Martinez et al.(15)

58 58

102 127

6 22

414 414

NC83-1 LC243-1

Martinez et al.(l5)

54 57

102 127

7 28

414 414

fco (MPa)

fc/fco

EgsfEoi

3.4 3.1

1.64 1.32

2.7 1.8

2.8 2.4

1.23 1.01

2.1 1.3

Note: All columns have a circular cross-section with "h" diameter.

TABLE 5-EFFECT OF TIE ARRANGEMENT ON STRENGTH ENHANCEMENT AND STRAIN DUCTILITY RA TID h s Tie fco Ps fcJfco EssiEoi ~ (MPa) (mm) (mm) Arrng. (MPa) (%)

Column

Researcher

CS-13 CS-14

Razvi and Saatcioglu( 6)

78 78

250 250

55 55

CS-2 CS-3

Razviand Saatcioglu(6)

105 105

250 250

55 8-bar 570 55 12-bar 570

8-bar 570 12-bar 570

Note: All columns have (h x h) square cross-section.

2.16 1.10 2.16 1.21

3.5 6.9

2.16 2.16

2.7 2.8

1.15 1.22

124 Saatcioglu, Paultre and Ghosh

2.25

.....

0 ~

-

.....

• :Experimental results from Cusson and Paultre (1994) o : Experimental results from Nagashima et al. ( 1992)

2.00

~

~

...

1.75

c

f cc

f=1.0+2.1 f co

0

le

)0.7

co

0

0

;;

ns 1.50

~

... c ... tn

0

.c

C)

I

1.25

~

1.00 0.75 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Effective Confinement Index, f1e I fco Fig. 1-Effect of confinement on peak concrete strength (8)

0.35

HSC in Seismic Regions 125

u

o..g o..u

2.0

~

u

0..0

-

• -

First hoop yielding s = 50 mm (2A-2D)

-

s = 100 mm (3A-3D)

1.5

o..u ..

2C

"t:J

cu

20

0

-e ....1 Q)

1.0

CJ

r:: 0 0 0.5

.. Q)

>

cu

Q)

~

0.0

Concrete Axial Strain Ec Fig. 2-Effect of tie spacing (4)

126 Saatcioglu, Paultre and Ghosh

140

CS-3

___ ,

,

120

\

f'c

''

_100 cu

a.

::

''

80

''

en en 60 Cl)

...

en

''

''

lD

40 20

''

''

''

''

''

cs- 1

= 124 MPa

''

0 0

1

0.5

1.5

2

2.5

3

Strain (%) (a) 140

cs -14

m1

120 _100 cu

a.

::

'

80

en en 60 Cl)

-

''

= 92 MPa

\ \

]',,, _________

' ...

...

en

f'c

40

20 0 0

0.5

1

1.5

2

2.5

Strain(%) (b) Fig. 3-Effect of reinforcement arrangement on confined core (6)

3

HSC in Seismic Regions 127

a..o8

-

2.0

a_u

"'C

cu

0 1.0 .J

-g Q)

~

0

0.5

0 Q)

>

+i

cu

0.0 0:: 0.00 Q)

0.01

0.02

0.03

0.04

Concrete Axial Strain £c Fig. 4-Effect of tie configuration (4)

0.05

...... N

= Cf.l

-= I» I»

n

c;·

::

1A

-a c

I»

...

12

;::::;:'

CD I»

1~

-

::II

=-

u

...--

C')

Q8

0 0

:::r Cl en :::r

f'.=60MPa

m Q6

~

w Q4

02 QO QO

0.5

1.0

1.5

2.0

2.5

Strain(%)

(a) Fig. 5A-Effect of concrete strength (6)

3.0

3.5

4.0

CS-24 f'c=60 MPa

1.4 1.2

•• =3.24%

1~

m

.u ~

~ ~ ~

D

QB f 'c= 124 MPa

Q6

00

Q4 Q2

::c en n

QO

::I

QO

0.5

1.0

1.5

2.0

2.5

Strain(%)

3.0

3.5

4.0

en 2!. en

3

n

= ell

(b)

=

= en

::I

Fig. 58-Effect of concrete strength (6)

N CD

130 Saatcioglu, Paultre and Ghosh

140 f'c

120

= 124 MPa

_100

ca

0..

-:: t /) t/)

Q)

80 60

1..

CJ)

40 20 0 0

0.5

1

1.5

2

2.5

Strain ( 0/o) Fig. 6-Effect of transverse steel strength on confined core (6)

3

HSC in Seismic Regions 131

140 f~

120 _100

Ps=

ta

D.

==

I

en en 60 G)

-

I

'

...

en

0

1.32%

fyt =1000 MPa

80

= 92 MPa

40 20 0 0

0.5

1

1.5

2

2.5

3

Strain (o/o) Fig. 7-Trade·off between volumetric ratio and strength of transverse reinforcement (6)

132 Saatcioglu, Paultre and Ghosh

100

.,

s =120 mm

-

80

CD

40

P.=

D.

...."" tn

~

ll_

~~

ca

:E en en

3.06%

' ..

= 60 MPa

fyt= 400 MPa

•

/ .... ----

60

f~

, ,_

,, ,,

,, ,,

s =85 mm

---,, __ _

20

o;---.----.---.---.--~--~--~--~---

0

0.5

1

1.5

2

2.5

3

3.5

4

Strain (o/o) Fig. 8-Trade off between spacing and arrangement of transverse reinforcement (6)

HSC in Seismic Regions 133

0.030

• : Experimental results from Cusson and Paultre (1994) o : Experimental results from Nagashima et al. ( 1992)

0.025 0

u

w

u u

0.020

w

ft

c:::

ns 0.015

(!)

tee- teo=

c:::

0.21

(fr f7 co

ns 0.010 ....

....

tJ)

0.005 0.000 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Effective Confinement Index, f 1e I Fig. 9-Effect of confinement on peak strain of concrete

teo

0.35

134 Saatcioglu, Paultre and Ghosh

0.05

• :Experimental results from Cusson and Paultre (1994) o : Experimental results from Nagashima et al. ( 1992)

:I 0

II)

CJJU 0.04 0

u

0

II)

wu

-

0

0.03

c cu

(!)

0

0

•

0.02

0

~

-u

Ec50c- tc50u

:.;:;

:I

c

0

=0.15 (

r )1.1 f

co

0

0.01

0.00 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Effective Confinement Index, f1e I fco Fig. 10-Effect of confinement on ductility of concrete

0.35

HSC in Seismic Regions 135

140 f~

= 124 MPa

fyt= 400 MPa

120 _100

ca

a.

-

:2 80 tn tn

Q)

I

I I

60

ll..

en

40

ps =

3.33%

s =55 mm

20 0 0

0.5

D 1

1.5

2

2.5

Strain (%) Fig. 11-Effect of cross-sectional shape on confined core concrete (6)

3

136 Saatciogl u, paultre and Ghosh

f~

...

'

\

= 124 MPa

fyt= 1000 MPa

..

'-- --,

0 I

--

...

i ~[§!] P8 = 2.17%

s =55 mm s =55 mm

20 0

.

~-------;-:------.-----r-

;:-0

u

0.5

1

1.5

2

Strain(%)

fig. . of a · . 12-Companson With efficient reinfo rcement Circular column and arrangement (6) a square column

2.5

3