CONCRETE IiiI ASSOCIATES CTA16. TECHNOLOGY IIi!I. TECHNICAL BULLETIN 74 - Bll NOVEMBER, 1974 DUCTILE P.ULLOUT CONNECTIONS

Transcription

1 CONCRETE IiiI TECHNOLOGY IIi!I ASSOCIATES TECHNICAL BULLETIN 74 - Bll NOVEMBER, PORT OF TACOMA ROAD /TACOMA, WASHINGTON / (206) DUCTILE P.ULLOUT CONNECTIONS 367 CTA16

2 368

3 CCNCRETE IiiiI TECHNOLOGYIIII, ASSOCIATES TECHNICAL BULLETIN 74 - Bll NOVEMBER, PORT OF TACOMA ROAD! TACOMA, WASHINGTON 98421! (206) SYNOPSIS, DUCTILE PULLOUT CONNECTIONS A ductile pullout connection is investigated, including theoretics 1 traatmanl design criteria, and ultimate load tests. The concept is widely used tor connecting columns to footings, walls to footings, columns to columns, walls to walls, and other tensio ll or compression type connections. In this detail, the axial force is transmitted through reinforcing bars projecting from one element and embedded in a grouted conduit in the other. The grout in turn transmits the force to the surrounding concrete and other reinforcement. Bond strength characteristics of three nonshrink grouts J two types of sleeve material, two sizes of reinforcing bars and varying amounts of confinement reinforcement aie invetigat2d. It is suggested that 2-inch cubes be: cast from grout mixes and used in a fashion similar to the use of 6 x 12-inch concrete cylinders for quality assurance. The available bond \ strength can then be considered aqua 1 to 30% of the cube strength in the computation of the required embedment length. The following equations, " are based on res_ults of this study and are recommended for the selection of embedment length and amount of confinement steel: 369

4 Synopsis, cant. Ie Tu Fy where: Ie Tu Eo fb Tu 0 Eo fb b Fy n As fy = embedded length ultimate strength of the grouted bar ACI-capacity reduction factor 0.85 circumference of the bar bond strength == 0.3 times the 2-inch cube strength of the grout b factor determined by tests = 6.5 n As fy number of turns of spiral reinforcement less two turns for anchorage = cross-sectional area of spiral reinforcing bar yield strength of spiral reinforcement The recommended detail of reinforcing steel in the vicinity of the sleeve is shown in Figure CTA-74-Bll

5 INTRODUCTION The SUCCeSS of a precast concrete structure depends largely upon the efficiency of the connections used. Intelligent and economical design of precast concrete connections requires an understanding of precast concrete production, erection, design procedures I tolerances, possible loadings I load factors, and serviceability requirements. Standardization of connections I special attention to reinforcing details in the vicinity of the connection for smooth placing and vibrating of the concrete, coordination of dimensions I bulkheads and blackouts are all factors that influence production economy. Tolerances I field welding, temporary bracing or connections, field concreting or grouting are important erection considerations. Typical loads, volume changes, erection forces, and ductility should be considered in the design. Depending upon their function, connections provide continuity between precast items by transmitting axial loads, shear forces, and/or bending moments. Many types and shapes of hardware have been used successfully. A patented method known as "NMB Splice Sleeve" is shown in Fig. 1. Deform:3d bars are spliced together by inserting them into a sleeve filled with (Master Builder's Embeeo 602) nonshrink mortar. The method is applicable to both preca st and cast-in-place construction. For additional information plea se refer to Reference 3. This report is concerned with a widely used connection for colurr:n to footing, wall to footing, column to column, wall to wall, or for other tension CTA-74-Bll 371

6 or compression type connections (see Fig. 2). In this detail the axial force is transmitted through reinforcing bars projecting from one element and embedded in a conduit filled with grout in the other element. The grout in turn transmits the force to the surrounding concrete and other reinforcement. The objective of this investigation is to develop a grouted dowel connection which is economical, efficient and capable of resisting ultimate loads in a ductile ::nanner. > THEORETICAL TREATMENT In order to achieve the above objective, a clear understanding of the stress pattern around the grouted dowel and of the mechanism of failure under ultimate load is essential. Stress investigations based on elastic theory are complicated and limited to uncracked state. If, however, all modes of failure are carefully studied reinforcing details can be established such that the mode of failure is predictable. When the grouted reinforcing bar shown is subjected to a tensile force T I the followirlg modes of failure are pas sible: 1. Bond failure between the bar and the grout--bar pulls out. 2. Rond failure between the grout and the conduit--bar and grout cylinder pull out. 3 _ Concrete shear cone failure along broken lines. 4 _ Bar yielding then breaking outside the concrete I or pulling out after yielding. 372 CTA-74-Bll

7 (4) I // I // ;-:: \, I /llc (3) / " L.,/ (2) I f "" II I f ( 1) '" " I I!/ : f - ",lfi The first three modes of failure are undesirable because they can occur suddenly and without warning. In the design of such a connection ( it is recommended that the connection be detailed in a manner insuring the ductile behavior of the fourth mode of failure. High-strength-nonshrink grout, adequate embedment of the rebar, corruga tions or interlogking deformations of the sleeve, confinement and boundary reinforcement are all necessary items to avoid a brittle failure. 1. Bond Failure Between the Bar and the Grout--Minimum Ebedment Length: Adequate embedment of the reinforcing bar in the grout is necessary to insure against bond failure. The minimum embedment length can be CTA-74-Bl' 373

8 computed a s follows: (1) where: Ie Tu embedded length ultimate strength of the rebar 0 ACI-capacity reduction factor 0.85 Eo circumference of the bar fb bond strength of the grout The embedment length given in Equation (1) is based on the assuroption that adequate confinement reinforcement around the grouted conduit is provided and contact between the bar and the grout is maintained. When confinement reinforcement is not sufficient, radial cracks may become wide enough due to yielding of the confinement reinforcement so that slippage of the bar or the grout cylinder can take place. 2. Failure Between the Grout and the Conduit: Bond failure is most likely to take place in the case of a smooth conduit. Eq,:,-aUon (I} can also be used, then Eo is the circumference of the conduit and fb is the bond strength between the conduit and the grout or the concrete, whichever is smaller. In the case of corrugated conduits, relative movement between the grout cylinder and the surrounding concrete by the formation of a cylinder of separation between them will r.1obilize the confinement reinforcement. CTA-74-Bll 374

9 computed a s follows: (1) where: Ie Tu embedded length ultimate strength of the rebar 0 ACI-capacity reduction factor 0.85 Eo circumference of the bar fb bond strength of the grout The embedment length given in Equation (1) is based on the assuroption that adequate confinement reinforcement around the grouted conduit is provided and contact between the bar and the grout is maintained. When confinement reinforcement is not sufficient, radial cracks may become wide enough due to yielding of the confinement reinforcement so that slippage of the bar or the grout cylinder can take place. 2. Failure Between the Grout and the Conduit: Bond failure is most likely to take place in the case of a smooth conduit. Eq,:,-aUon (I} can also be used, then Eo is the circumference of the conduit and fb is the bond strength between the conduit and the grout or the concrete, whichever is smaller. In the case of corrugated conduits, relative movement between the grout cylinder and the surrounding concrete by the formation of a cylinder of separation between them will r.1obilize the confinement reinforcement. CTA-74-Bll 374

10 w!1ere: T = applied tensile force N :::: force due to confinement reinforcement T /-' = coefficient of friction to be determined experimenta lly d p = diameter of crack cylinder = confinement pressure, I =-c>j <}= N /"Z /' I I I d At ultimate load, confinement reinforcement should be capable of supplying an ultimate confinement pressure. The corresponding yield strength of circular confinement reinforcement, as in the case of spiral, is computed as follows: d Fy ="2 Ie Pu Tu =7T die/, Pu = 277 Fyji --Radial Cracks Crack Cylinder \I--- Confinement Stee 1 CTA-74-Bl, 376

11 Note that the final result does not depend upon the diameter of the crack cylinder and that the required confinement force is directly proportional to the ultimate tension and inversely proportional to the friction coefficient. 3. COncrete Shear Cone Failure: In order to safequard against this mode of failure, adequate reinforcement is required. Reinforcement is most efficient if placed parallel to the direction of the applied force, as the U -shape reinforcement shown below. However, reinforcement in other directions can be used, and the total area of reinforcement crossing the crack is computed a 5 follows: As = (10) where: n As = L As (cos O<i + e. 5 sin O<i) 1 ASi cross-sectional area of bar No. i crossing the crack = angle between Tu and bar No. i.. Grouted Bar Crack Cone _--''\l 377 CTA

12 4. Design Criteria: Adequate embedment and confinement reinforcement are necessary to develop a grouted rebar. Equation (1) is used to determine the embedment length: but not less than 6". The value of fb (bond strength of the grout) depends mainly upor, the type of the grout used and somewhat on the diameter of the reinforcing bar. The pcr Manual on DeSign of Connections (2) introduces a value of fb = 1200 psi irrespective of the type of grout used. Confinement reinforcement is proportioned as follows: Tu = b F y (Il) b '" 2 TT /', see Equation (9) (12) where: n = number of turns of spiral reinforcement less two turns for anchorage A. s ::::: cross-sectional area of spiral reinforcing bar fy ::::: yield strength of spiral reinforcement The pcr Connection Design Manual introduces a value of 2.8 for the parameter b. However, realistic value of b should be determined experimentally. 378 CTA-74-Bll

13 EXPERIMENTAL STUDY 1. Confined Bond Strength: The confined bond strength of different grout mixes is to be determined. Parameters considered are: 1. Type of grout--(3 types) 2. Age at testing--(l, 3, 7 and 28 days) 3. Size of rebar--(#s and #8 grade 60 rebars) Each bar was grouted for a distance of 5 times its diameter inside a 4" I. D. steel pipe. shown in Figure 3. The steel pipe wa 5 chosen for simplicity of containment of the grout and to provide confinement forces. Three different grout mixes were used and are given in Table I. Pullout test results are given in Tables 2A, 2B and 2C. and are also portrayed in Figure 4. Test results on Master Builder's Embeco 602 grout are given in Reference 3 and are reproduced in Figure 4. In order to establish a relationship between the confined bond strength, fb' and the 2-inch cube strength of the grout, f, Figure 5 was constructed from the data contained in Table 2. The line fb = 0.3 fe represents a lower bound for test values shown in Figure 5 and is, therefore, recommended for use when Equation (1) is applied. 2. Qfltimization ot Confinement Reinforcement: In order to establish a numerical value for the constant b in Equation (1) J to be used in the design of confinement reinforcement, CTA-74-Bll 379

14 sixty reinforcing bars were grouted and tested. The following parameters Were considered: 1. Bar diameter (#5 and #8 grade 60 rebars) Z. Type of grout (Portland Cement and Fondu Ce,ent) 3. Type of sleeve (2-1/2" spiral duct and 3" Flex tube) 4. Amount of confinement reinforcement Confinement reinforcement consisted of spiral placed around the ducts. 'rhe spiral was manufactured from 1/4" diameter cold rolled bars having a yield strength of about 60 ksi. Three levels of confinement were considered for the #5 bars and four levels were considered for the #8 bars. The #5 bars were embedded 10" and the #8 bars were embedded 14" in the grout (see Figure 6). Pullout test results on these bars are given in Tables 3A and 3B and Tables 4A and 48, and plotted in Figure 7. From Figl.,lre 7 it is seen that a value of 6.5 can be used for the constant b. DISCUSSION AND CONCLUSIONS The mosi; significant conclusion of the confined bond strength study is the establishment of a relationship between the 2-inch cube strength of the grout and its confined bond strength. Using four different grouts at different ages and two sizes of rabar the confined bond strength was higher than 30% of the 2-inch cube strength in all cases. It is therefore suggested that 2-inch cubes be ca st from grout mixes used in a fa shion similar to the use of 6 x l2-inch concrete cylinders for quality assurance. The available confined bond strength can then be considered equal to 30% of the cube strength. CTA-74-Bll 380

15 Optimization of confinement reinforcement indicated that a value of 6.5 for the constant b in Equation (11) can be used conservatively. It was also found that flextube ducts possess higher bond strength than spiral ducts. This performance was expected due to the deeper and more closely spaced corrugations in the flextube. A bond strength of 600 psi prevailed for spiral ducts f while no slippage was observed in flextubes. The following design procedure is based on the results of this investigation and is therefore recommended for the design of grouted reinforcing bars: 1. Determin.;:. the required embedment length using Equation (1) and assuming a value of fb equal to 30% of the expected or specified cube strength of the grout. 2. Determine the required amount of confinement reinforcement using Equations (11) and (12), with b taken as Supply sufficient anchoring steel to safeguard against shear cone failure using Equation (10). 4. See Figure 8 for recommended reinforcing detail in the vicinity of grouted dowel connections. CTA-74-Bll 381

16 GROUT DESIGNATION CEMENT SAND ADMIXTURE Wlc RI\TIO Portland Cement Non-Shrink Grout Type III Paving Sand 1 part by volume 1.5 parts by volume Slka Intraplast-N % by weight of cement Fondu Cement Grout Lone Star Paving Sand Fondu Cement 2.5 parts by 1 part by volume volume None 0.35 I Duracal Grout Portland Cement-Gypsum rapid set Lone Star Duraca150'lbs. -_ None None 0.2l (l-1/4 gal. per 50 lbs.) I I o >-l '", '" "' - TABLE GROUT MIXES I

17 AGE GROUT BAR SIZE PULLOUT A V E R A G,E COMPo SYR. FORCE BOND STEEL TENS. STRENGTH STRESS days psi Grade 60 kips psi ksi I 2, , , , # I, # , ,0 # ,210 43,90 36, # ,600 5 I, * , # ,610 71, # , # , TABLE 2A CONFINED 80ND STRENGTH TEST RESULTS - PORTLAND CEMENT GROUT 383 CTA-74-Bll

18 AGE GROUT BAR SIZE PULLOUT A V E R A G E COMPo STR. FORCE BOND STEEL TENS. STRENGTH STRESS days psi Grade 60 kips psi ksi I , #B ,950 7B , 1' , #B 7B.0 4, # , ,5 7B,0 #B ,940 9B ,200, 33.0 I #5 35.0* 5,3BO 106, B 16,550 B2.0 #B 76,0 4,770 94,9 57, # , B *Bar broke. TABLE 2B CONFINED BOND STRENGTH TEST RESULTS - FONDU CEMENT GROUT 384 CTA

19 AGE GROUT BAR SIZE PULLOUT A V E R A G E COMP. STR. FORCE BOND STEEL TENS. STRENGTH STRESS days psi Grade 60 kips psi ksi I 5,100 #8 # , , !. 6,560 2 #8 # B 2, ,160 #8 # , , TABLE 2C CONFINED BOND STRENGTH TEST RESULTS - DURACAL GROUT 385 CTA

20 BAR S P I R A L DUCT P U L L 0 U T SY6E TYPE NO. OF CONF. FORCE BOND STHL prade TURNS FORCE STRESS STRESS 60 n Fy(l), k kips psi ksi C AGE days R 0 U T MODE OF FAILURE(4) CUBE STRENGTH psi, # S (2) , S p(3) 34, S F 34,0 1, ,150 A 4,300 D 4,300 D 5,270 D A g: S 70, S ,770 99, F , #8 S 78, F S F ,300 C (duct bond str. = 640 psi) 4,300 A 4,300 A 5,270 C (duct bond str. 710 psi) 5,270 D 5,270 B (duct bond str. 760 psi) 5,270 D (I) Fy = n As fy = (n) (0.049) (60) = (2.94) (n) kips (2) Galvunized spiral wound duct 2.5 inches diameter (3) Flex tube 3 inches diameter o ;;, "", co - (4) Modes of failure am: A = bar sup B bond failure between grout and duct C = bond failure between duct and concrete D "" bar brok TABLE 3A PULLOUT STRENGTH TEST RESULTS - PORTLAND CEMENT GROUT

21 BAR S P I R A L DUCT P U L L 0 U T SIZE TYPE, NO. OF CONF. FORCE BOND STEEL Grade TURNS F?RCE STRESS STRESS 60 n Fy I), k kips psi ksi C R 0 U T MODE OF FAlLURE(4) AGE CUBE STRENGTH days psi #5 4 II. 8 S (2) III S , (3) , S F ,700 D 3 1l,700 A 3 1l,700 A 8 13,000 D 8 13,000 D w <» 4 1l.9 S S F #8 S F S F (I) Fy n As fy (n) (0.049) (60) = (2.94) (n) kips (2) Galvanized spiral wound duct 2.5 inches diameter 3 II, 700 B (duct bond str. = 550 psi 3 II, 700 C (duct bond str psi) 3 11,700 D 8 13,000 C (duct bond str psi) 9 13,000 D 9 13,000 B (duct bond str psi) D -- (3) Flex tube 3 inches diameter 0 "' '",..,., '" (4) Modes of fallure are: 1\ = bar slip 13 = bond failure between grout and duct C = bond failure between duct and concrete D ;:: bar broke TABLE 38 PULLOUT STRENGTH TEST RESULTS - rondu CEMENT GROUT

22 BAR S P I R A L P U L L 0 U T MODE OF?AILURE(3) SIZE NO. OF CONFIN. FORCE I BOND STEEL Grade TURNS FORCE STRESS STRESS 60 0 F y (2) (kips) (kips) psi ksi,,8 NO ,160! 86.0 A ,180 I 85.7 A ,0 2, A 4 1l , A , A , A , A , A , 2, A ,470 ' 98.1 A #5 NO ,590 I 98.4 A ,380 I 90.3 A ,450 ' 93.6 A 4 1l A , A A , A A (1) Age at test = 7 days Grout compo strength = 6, 000 psi (2) Fy = n As fy = (n) (0.049) (60) = (2.94) (n) kips (3) ModeS of failure are: A =:; bar slip TABLE 4A PULLOUT STRENGTH TEST RESULTS - PORTLAND CEMeNT GROUT 388

23 BAR SPIRAL PULLOUT lode OF FAILURE (3) SE -----r '-----'---- NO, OF CONFIN, FORCE BOND STEEL Grade 60 TURNS n FORCE F y (2) (kips STRESS STRESS (kips) psi ksi #B #5 1_-,N"-0;c-_+--c0C',,,0+4,"8,-,,_,,0-t ,6,"0:-:,-,,8,+-B,--(di!:,l'.ductL_.. :::::::::;:::::::::::::::::::::::::::::=:::-:::= :::t:::,-;";--:!+----;:c-i** - : = '4--_i-_;1-c;1. -o;8_5-;,4-,-, 0;;--t--;;---;-;-;-c,;---r--7;6;;-B'c' 4;;--t_Bo--(d l'r'"ty_d uc tl. 1-_5::_--t--""14",,,7+7c;l,,,_,,0:-t--"2,c' 2:c60"'-t-;B,,9;-:,.9 _J. -6:: +-17"',,,6c-+5::-;3,,,_"0-- t--_-_-_--+-;67;_:."'i+b (dirty duct) -;-B:,-_+-2,"3C',,-,5+7,::B,-,,_,,0'--t ;9,-,8,,,-7 i---j? (!rty duct)--._=-=- ---cci-"o _+--,,2 -,,9 '" 4: ,,6 B=-,,-,O+-_--,--_-_+_ c8 6. I B (d irty due t)! ,S :4 - B_(dFtj,_u2tL._ -'-1;;-2-+-;;-35'c, 3;-+SBn,'-;0; ;-','-;4;-t--;B;- (dirty duct) NO B (dirty duc"'t)'- I 1-_2:;--+_-;:5,;., ii- 9-1(--;;-26C"c;:0.---1f-;;'2-'-" 21;-;0'-+--,-iC83",':.;9:+---';iA,-._ -73 _+"",o:-b:,;, 9::-+=-3 ;..1."0+-;2,",-,,6",,3,,,0 +",1 O,"O",,,O,+A,- 4!l,B , ,0 A 1-_:;,5 _+--+14, 7;-+2 6"":.;5;-,+--;2",-;2 5;;-0 -t--_b:;.:5",c;5'-t--';a' ,6 29,0 2, ,6 A --;-8; t23-'-,75 i28::_;,.0 +2'c,3::-;B0+-;;-90::_;,,,3'+A ,4 2B,0 2, A (I) Age at test = 3 days Grout compo strength == 9,700 psi (2) Fy = n As fy = (n) (0.049) (60) = (2.49) (n) kips (3) Modes of failure are: A = bar slip B == bond failure between grout and duct TABLE 4B PULLOUT STRENGTH TEST RESULTS - FONDU CEMENT GROUT 389 CTA-74-RI!

24 1 _. Embeco 602 Mortar Daformed Bar De:orm2c Bar _ Sleeve t FIGURE 1 NMB SPLICE SLEEVE CTA-74-Bll 390

25 _-1. ". ',>',.. '"... '.. Grout L. J,, FIGURE 2 TYPICAL GROUT DOWEL CONNECTIONS CTA-74-BII 391

26 TEST SPECIMENS -# '- '", f 8 I" :' "'., X'J. ',' 5 W TEST SETUP FIGURE 3 CONFINED BOND STRENGTH TEST CTA-74-Bl1 392

27 " FIGURE 4 CONFINED BOND STRENGTH TEST RESULTS 393 CTA-74-BII

28 ".,._- - --i L..- '_, -, ' f--- L8",-" A A -OJ '" ;C... CJ Z '"... '" OJ Q Z 0 "' Q '" Z - "- Z 0 lj A o c:j 0 I G [] G I FIGURE 4 CONFINED BOND STRENGTH TEST RESULTS 393 CTA-74-Bll

29 '-- I. tr.. :....." I" r #6 n-turn s Spira l WWF 4 x 4-4/4 Spiral Duct CR OSS-SECTION TEST SPECIMENS FIGURE 6 GROUTED DOWEL TEST 395

30 til 0.. -'"... :x:... C) Z... '" til... ::> 0...,..., ::> t Portla nd Cement ( 0. ##5 8 A A.. A A A A A I / YIELD PCI FORMULA *B ' Tu = 2.8 Fy t-----r _ I e I 2 i r;-, 018, ' r ' YIELD CON F IN E M E NT Fy 30 (KIPS) 40 FIGURE 7 PULLOUT TEST RESULTS CTA-74-Bll 396

31 ,,,, Spira 1 per Equation (11) Sleeve with---+t---i>c Corruga tions "'" Auxiliary Rai':lforcing per Equation (10) Sleeve Length per Equation (1) FIG URE 8 RECOMMENDED DETAIL CTA-74-Bll 397

32 REFERENCES 1. Roark, R. J. Formulas for Stress and Strain, third edition. McGraw Hill Book Company, Inc., Page "PCI Manual on Design of Connections for Precast Prestressed Concrete," first edition. Prestressed Concrete Institute, Page "NMP Splice Sleeve." Nissa Master Builders Company I Ltd., New Products Division, 16-26, Roppongi 3-Chome, Minato-Ku Tokyo, Japan (Zone No. 106). 398 CTA-74-Bll