1、This report reviews the state of the knowledge of the behavior ofhigh-strength concrete (HSC) columns. High-strength concrete, as used inthis report, is defined as concrete with compressive strength exceeding 70MPa (10,000 psi). The report provides highlights of research available onthe performance
2、of HSC columns under monotonically increasing concen-tric or eccentric compression, and with incrementally increasing lateraldeformation reversals and constant axial compression.Research results are used to discuss the effect of cover concrete and param-eters related to transverse reinforcement on s
3、trength and ductility of HSCcolumns subjected to concentric load.The behavior of HSC columns subjected to combined axial load and bend-ing moment is discussed in terms of variables related to concrete and trans-verse reinforcement. In addition to discussion on flexural and axialcapacity, this report
4、 also focuses on seismic performance of HSC columns.Keywords : axial load; bending moment; columns; cover concrete; ductil-ity; flexural strength; high-strength concrete; longitudinal reinforcement;seismic design; transverse reinforcement.CONTENTSChapter 1Introduction, pp. 441R-1Chapter 2Performance
5、 of HSC columns underconcentric loads, pp. 441R-22.1Effect of cover concrete2.2Effect ofvolumetric ratio of transverse reinforcement2.3Effect oflongitudinal and transverse reinforcementstrength2.4Effect oflongitudinal and transverse reinforcementarrangementChapter 3Performance of HSC columns underco
6、mbined axial load and bending moment, pp.441R-53.1Flexural strength3.2Ductility of HSCcolumns under combined axialload and bending momentChapter 4Recommended research, pp. 441R-11Chapter 5References, pp. 441R-12Chapter 6Notation, pp. 441R-13CHAPTER 1INTRODUCTIONOne application of high-strength concr
7、ete (HSC) has beenin the columns of buildings. In 1968 the lower columns of theLake Point Tower building in Chicago, Illinois, were con-ACI 441R-96High-Strength Concrete Columns:State of the ArtReported by joint ACI-ASCECommittee 441S. Ali Mirza* Atorod Azizinamini* Perry E. AdebarChairman Subcommit
8、tee Chair SecretaryAlaa E. Elwi Douglas D. Lee B. Vijaya RanganRichard W. Furlong James G. MacGregor* M. Ala SaadeghvaziriRoger Green Sheng- Taur Mau Murat Saatcioglu*H. Richard Horn, Jr. Robert Park Arturo E. SchultzCheng-Tzu Thomas Hsu Patrick Paultre* Lawrence G. SelnaRichard A. Lawrie Bashkim Pr
9、ishtina Shamim A. SheikhFranz N. Rad*Subcommittee members who prepared this report.ACI committee reports, guides, standard practices, designhandbooks, and commentaries are intended for guidance inplanning, designing, executing, and inspecting construction.This document is intended for the use of ind
10、ividuals who arecompetent to evaluate the significance and limitations of itscontent and recommendations and who will accept responsibil-ity for the application of the material it contains. The AmericanConcrete Institute disclaims any and all responsibility for theapplication of the stated principle
11、s. The Institute shall not be li-able for any loss or damage arising therefrom.Reference to this document shall not be made in contract docu-ments. If items found in this document are desired by the Archi-tect/Engineer to be a part of the contract documents, they shallbe restated in mandatory langua
12、ge for incorporation by the Ar-chitect/Engineer.ACI 441R-96 became effective November 25, 1996.Copyright 1997, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by anymeans, including the making of copies by any photo process, or by electronic or
13、mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission inwriting is obtained from the copyright proprietors.441R-1structed using 52 MPa concrete.1More recently, several highrise buildin
14、gs1-4have utilized concrete with compressivestrengths in excess of 100 MPa in construction of columns.Many studies4-9have demonstrated the economy of us-ing HSC in columns of high-rise buildings, as well as lowto mid-rise buildings.10In addition to reducing columnsizes and producing a more durable m
15、aterial, the use ofHSC has been shown to be advantageous with regard tolateral stiffness and axial shortening.11Another advan-tage cited in the use of HSC columns is reduction in costof forms. This is achieved by using HSC in the lower storycolumns and reducing concrete strength over the height ofth
16、e building while keeping the same column size over theentire height.The increasing use of HSC caused concern over the ap-plicability of current building code requirements for designand detailing of HSC columns. As a result, a number of re-search studies have been conducted in several countriesduring
17、 the last few years. The purpose of this paper is tosummarize major aspects of some of the reported data.The major objectives of reported studies have been toinvestigate the validity of applying the current buildingcode requirements to the case of HSC, to evaluate similar-ities or differences betwee
18、n HSC and normal-strengthconcrete (NSC) columns, and to identify important pa-rameters affecting performance of HSC columns designedfor seismic as well as non-seismic areas. These concernsarise from the fact that requirements for design and detail-ing of reinforced concrete columns in different mode
19、lcodes are primarily empirical and are developed based onexperimental data obtained from testing column speci-mens having compressive strengths below 40 MPa.The reported information can be divided into two gen-eral categories: performance of HSC columns under con-centric axial load; and performance
20、of HSC columnsunder combined axial load and bending moment. This re-port gives the highlights of the reported data in each ofthese categories. In this report, HSC is defined as concretewith compressive strength greater than 70 MPa.CHAPTER 2PERFORMANCE OF HSCCOLUMNS UNDER CONCENTRIC LOADSThe majority
21、 of reported studies12-27in the field of HSCcolumns concern the behavior of columns subjected to con-centric loads. Understanding the behavior of columns underconcentric loads assists in quantifying the parameters affect-ing column performance. However, conclusions from thistype of loading should no
22、t necessarily be extended to thecase of combined loading, a situation most frequently en-countered in columns used in buildings.Reported data indicate that stress-strain characteristics ofhigh-strength concrete, cover concrete, and parameters relat-ed to confining steel have the most influence on re
23、sponse ofHSC columns subjected to concentric loads. The effect of thefirst parameter is discussed in Sec. 3.1. The remaining twoparameters are discussed in the following sections.2.1Effect of cover concreteFigure 1 shows a schematic load-axial deformation re-sponse under concentric loads of HSC colu
24、mns with trans-verse reinforcement. As concrete strength increases, theascending portion of the curve approaches a straight line. Ingeneral, spalling of the cover concrete is reported12-27to oc-cur prior to achieving the axial load capacity of HSC col-umns, as calculated by the following equation:(1
25、)where:PO= Pure axial load capacity of columns calculated ac-cording to the nominal strength equations of ACI318-89fc=Concrete compressive strengthAg=Gross cross-sectional area of columnAst=Area of longitudinal steelfy=Yield strength of longitudinal steelThe 1994 edition of the Canadian Code for Des
26、ign of Con-crete Structures also uses this equation for computing Po, ex-cept that the factor 0.85 is replaced byin which fcis in MPa. Hence, Pocalculated by the Canadiancode will be somewhat less than that calculated by ACI318-89.Point A in Fig. 1 indicates the loading stage at which coverconcrete
27、spalls off. The behavior of HSC columns beyondthis point depends on the relative areas of the column and thecore and on the amount of transverse reinforcement provid-ed. Following spalling of the cover concrete, the load-carry-ing capacity of columns generally drops to point B in Fig. 1.Beyond this
28、point, Bjerkeli et al.,19Cusson et al.,25andNishiyama et al.28report that it is possible to increase themaximum axial strength of columns up to 150 percent of thatcalculated by the ACI 318-89 provisions and obtain a ductilebehavior by providing sufficient transverse reinforcement.The effect of the a
29、mount of transverse reinforcement isPo0.85 f cAgAst()Astfy+=10.85 0.0015 f c()0.67=441R-2 ACI COMMITTEE REPORTFig. 1Schematic behavior of HSC columns subjected toconcentric axial loads, incorporating low, medium, andhigh amounts of transverse reinforcementshown schematically in Fig. 1 and will be di
30、scussed furtherin later sections.The loss of cover concrete in HSC columns before reach-ing the axial capacity calculated by ACI 318-89 is contraryto the observed behavior of concrete columns made of NSC.Collins et al.29provide the following explanation for the fac-tors resulting in early spalling o
31、f cover concrete in HSC col-umns. According to those authors, the low permeability ofHSC leads to drying shrinkage strain in cover concrete,while the core remains relatively moist. As a result, tensilestresses are developed in the cover concrete as shown in Fig.2a. Moreover, longitudinal steel, as d
32、epicted in Fig. 2b, pro-motes additional cracking. The combination of these twomechanisms (see Fig. 2c) then results in the formation of acracking pattern that, according to those authors, is responsi-ble for early loss of cover concrete, thereby preventing HSCcolumns from reaching their axial load
33、capacity predicted byEq. (1) prior to spalling of cover concrete.Early spalling of concrete cover may also be initiated bythe presence of a closely spaced reinforcement cage that sep-arates core and cover concrete. Cusson et al.25attributed thespalling of the cover to planes of weakness created by t
34、hedense steel cages. They state that spalling becomes moreprevalent as the concrete strength increases.Saatcioglu and Razvi27,30also observed early spalling ofcover concrete in their tests. Those researchers indicated thatthe presence of closely spaced reinforcement cage betweenthe core and the cove
35、r concrete provided a natural plane ofseparation, which resulted in an instability failure of the cov-er concrete under high compressive stresses. The spalling intheir tests occurred at a stress level below that correspondingto the crushing of plain concrete.2.2Effect of volumetric ratio of transver
36、sereinforcementIn the case of NSC, an increase in the amount of transversereinforcement has been shown to increase strength and duc-tility.31The same observation has been reported19,25,27forthe case of HSC, though to a lesser degree. Some researchershave attributed this phenomenon to the relatively
37、smaller in-crease in volume during microcracking of HSC, resulting inless lateral expansion of the core. The lower lateral expan-sion of core concrete delays the utilization of transverse re-inforcement.Reported data12-27,30indicate that in the case of HSC, lit-tle improvement in strength and ductil
38、ity is obtained whenthe volumetric ratio of transverse reinforcement is small. Forinstance, Bjerkeli et al.19report that a volumetric ratio of 1.1percent was not sufficient to generate any improvement incolumn behavior, while the use of 3.1 percent resulted in col-umns performing in a ductile manner
39、.Sugano et al.,32Hatanaka et al.,23and Saatcioglu etal.27,30report a correlation between the non-dimensional pa-rameter, Sfyt/fc, and axial ductility of HSC columns subject-ed to concentric loads. Figure 3 shows the relationshipbetween this parameter and axial ductility of columns withdifferent comp
40、ressive strengths. In this figure, the axial duc-tility of columns is represented by the ratio 85/01, where85is the axial strain in core concrete when column load onthe descending branch is reduced to 85 percent of the peakvalue and 01is the axial strain corresponding to peak stressof plain concrete
41、. For each pair of columns compared, simi-lar reinforcement arrangements and tie spacings were main-tained. As indicated in this figure, columns of differentcompressive strength having the same Sfyt/fcvalue result inalmost the same axial ductility, provided that certain mini-mum limitations are met
42、for the volumetric ratio and spacingof transverse reinforcement.30441R-3HIGH-STRENGTH CONCRETE COLUMNSFig. 2Factors promoting cover spalling in high-strengthconcrete columns (adapted from Ref. 29)Fig. 3Columns with different concrete strengths showingsimilar axial ductility ratios (fc= concrete comp
43、ressivestrength based on standard cylinder test) (adapted from Ref.30)Fig. 4Comparison of experimental and calculated con-centric strengths of columns (adapted from Ref. 30)Figure 4 shows the relationship between the parameterSfyt/fcand the ratio of experimentally obtained axial loadcapacity for 111
44、 HSC columns to that predicted by Eq. 1.From this plot it could be observed that columns with a lowvolumetric ratio of transverse reinforcement may notachieve their strength as calculated by ACI 318-89; howev-er, well-confined columns can result in strength in excessof that calculated by ACI 318-89.
45、 Excess strength of col-umns with relatively higher amounts of transverse rein-forcement is generally obtained after spalling of coverconcrete. This strength enhancement comes as a result of anincrease in strength of the confined core concrete.2.3Effect of longitudinal and transversereinforcement st
46、rengthThe yield strength of the confinement steel determinesthe upper limit of the confining pressure. A higher confin-ing pressure applied to the core concrete, in turn, resultsin higher strength and ductility. Figure 5 shows normal-ized axial load-axial strain response of core concrete forfour pai
47、rs of HSC columns.25For each pair of columns,all parameters were kept constant except the yieldstrength of the transverse reinforcement. The yieldstrength of transverse reinforcement for columns 4A, 4B,4C, and 4D and columns 5A, 5B, 5C, and 5D was approx-imately 400 MPa and 700 MPa, respectively. As
48、 indicatedin this figure, for well confined columns (C and D), in-creasing the yield strength of transverse reinforcement re-sults in an increase in strength and ductility. However, fortype A columns, where only peripheral ties are provided,the gain in strength and ductility is negligible. Reportedd
49、ata of HSC columns17,25,27indicate that whenhigh-strength concrete is used in well-confined columns,the full yield strength of transverse reinforcement is uti-lized. On the other hand, in a poorly confined HSC col-umn, tensile stresses that develop in the transversereinforcement remain below yield strength even at thetime of column failure.2.4Effect of longitudinal and transversereinforcement arrangementWell-distributed longitudinal and transverse reinforce-ment results in a larger effectively confined concrete areaand more uniform distribution of th
