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38 Avffs Avffs Avf V V = Avffstan w V Avffs N Avffs Avffstan Figure 23. Shear friction analogy proposed by Birkeland and Birkeland (1966) (redrawn). the clamping force is passive in nature. The crack must open fy = yield strength of interface shear reinforcement; sufficiently to develop the "design" clamping force, Avf fs. fy 60 ksi; Loov and Patnaik (1994) conclude that a slip of = 0.02 in. fc = concrete compressive strength; is required to result in yield of conventional reinforcing steel = "friction factor" (see below); having fy = 60 ksi. They additionally point out the inconsis- c = "cohesion factor" (see below); tency of limiting slip (a previous proposal by Hanson [1960], K1 = fraction of concrete strength available to resist inter- for instance, suggested limited slip to = 0.005 in.) to a lower face shear (see below); and value since this may be insufficient to generate fy in the inter- K2 = limiting interface shear resistance (see below). face reinforcement. Most critical to this discussion is the fact The factors , c, K1 and K2 are given based on the interface that only limited data are available for steel interface rein- forcement having a nominal yield capacity greater than condition as follows: 60 ksi. Kahn and Mitchell (2002) report a study where the Interface condition c (ksi) K1 K2 (ksi) actual yield stress of the interface reinforcing steel was either 70 or 83 ksi. In this study, they report significantly increased Monolithically cast 0.400 1.4 0.25 1.50 scatter in shear friction prediction reliability when using the Slabs on 1/4 in. amplitude 0.280 1.0 0.30 1.80 roughened surface measured values of fy and conclude that fy should not be taken Other on 1/4 in. amplitude 0.240 1.0 0.25 1.50 to exceed 60 ksi for design. Additionally, when normalized by roughened surface concrete strength, the experimental results show no effect Cast against surface with 0.075 0.6 0.20 0.80 no roughening resulting from the different values of fy. The understanding of the shear friction resisting mechanism has evolved to recog- The inclusion of the cAcv term in Equation 11 (which is nize the complex nature of the crack interface behavior and reported to account for the effects of cohesion and aggregate to include the effects of aggregate and cement matrix proper- interlock) requires that minimum interface reinforcement ties, dowel action of the interface reinforcement, and the also be provided (Avf 0.05Acv/fy) since the design shear, Vu, localized effects of interface reinforcement within the inter- facial area (Walraven and Reinhardt 1981). Nonetheless, code could be less than cAcv, technically requiring no reinforce- approaches remain based on simple formulations derived ment across the interface. The parameters of Equation 11 are from the work of Birkeland and Birkeland (1966). highly empirical and have been calibrated over a relatively Considering only normal weight concrete and interface narrow band of parameters; most significantly, limited data reinforcement oriented perpendicular to the interface, the pro- exist for fy > 60 ksi. visions from AASHTO (2007) 5.8.4 to calculate the nominal shear friction capacity, Vni, are as follows: 2.6.1 Experimental Program An experimental study, intended as a series of proof tests Vni = cAcv + ( Avf f y + Pc ) of Equation 11 for shear interfaces having high-strength Vni K 1 fcAcv A1035 reinforcement was carried out. This test program is the Vni K 2 Acv (Eq q. 11) only known study of shear friction behavior to include high- strength steel. Where: Typical push-off specimens, having dimensions and details Acv = area of concrete shear interface; shown in Figure 24, were used in this study. This specimen Avf = area of interface shear reinforcement; geometry is commonly used for such tests. The applied load Pc = permanent net compressive force across interface; is concentric with the test interface, which is therefore effec-

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39 (a) Test Specimen (b) Instrumentation (c) Specimen Prior to Testing Figure 24. Test specimen details and instrumentation. tively subjected to only shear stress. The shear is resisted by provided in Appendix A. The concrete used was a conventional the concrete along the test interface and the steel ties crossing 4000 psi mix having a w/c ratio of 0.44 and 1-in. maximum the interface. For these tests, the interface was placed as a aggregate size. As noted in Table 21, the concrete strength on "cold joint" with the concrete on one side placed and allowed either side of the interface was 4220 psi and 6020 psi. For sub- to cure for 14 days prior to the placement of the other side of sequent shear capacity calculations, the lower value is used. the interface. Prior to placing the second side of the interface, Two types of interface steel reinforcement were tested: ASTM it was cleaned of laitance and roughened to create a surface A615 and A1035 with nominal yield strengths of 60 and condition with at least 1/4-in. amplitude. The interface was 100 ksi, respectively. Two bar sizes of each steel type were horizontal during concrete placement; thus, the interface may tested: #3 and #4. All specimens had three double-legged ties be thought of as representing the interface between a precast crossing the interface; thus, the interface reinforcing ratios concrete girder and cast-in-place concrete deck. The interface were 0.0041 and 0.0075 for the specimens having #3 and #4 steel reinforcement, therefore, represents the stirrup exten- ties, respectively. sions or interface shear reinforcement along such a cold joint. The instrumentation used in the experiments consisted of The parameters measured during the experiments were mag- three strain gauges, one located on each of the interface ties nitude of the shear load, displacement parallel to the shear approximately 3 in. from the interface, and eight linear variable interface, crack width perpendicular to the shear interface, displacement transducers (LVDTs) as shown in Figure 24. The and strain in the steel reinforcement across the test interface. strain gages were located away from the interface in order to The specimen designations and measured material pro- improve their reliability and ensure that they were not dam- perties of the eight push-off specimens tested are shown in aged. Thus, the actual bar strain at the interface is expected to Table 21. Four types of duplicated specimens were tested. Spec- be greater than that measured by the gages since some of the imen labels are preceded with "P" (push-off); the numbers bar stress is transmitted back into the concrete over this short "615" or "1035" indicate the type of steel reinforcement used development length. As the concrete is damaged during test- (ASTM A615 and A1035, respectively); the numbers "3" or "4" ing, the difference in strain between the interface and mea- indicate the size of the interface steel reinforcement (#3 and #4, surement location becomes less significant. respectively); and the letters "A" and "B" are used to identify the Testing consisted of the application of a monotonically duplicated specimens. increasing load to the top and bottom surfaces of the speci- All measured concrete and steel reinforcing bar material mens until the ultimate shear capacity of the test interface was properties are reported in Table 21. Detailed material data are reached. The load was applied through a 10-in. diameter plate