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40 Table 21. Shear friction specimen details and experimental results. P615-3 P615-4 P1035-3 P1035-4 Specimen ID A B A B A B A B Interface Steel 6 #3 A615 6 #4 A615 6 #3 A1035 6 #4 A1035 Material Properties Cast #1: 6020 psi @ 28 days; 7120 psi @ 104 days (age at testing) ' (psi) fc Cast #2: 4220 psi @ 28 days; 5800 psi @ 90 days (age at testing) Avf (in2) 0.66 1.20 0.66 1.20 Acv (in2) 160.4 163.2 165.0 162.5 157.5 160.7 162.5 160.7 = Avf /Acv 0.0041 0.0040 0.0073 0.0074 0.0042 0.0041 0.0074 0.0075 fy (ksi) 67.3 61.5 130.0 126.0 140.0 131.3 fu (ksi) 103.0 102.3 156.0 157.6 174.0 172.3 u 0.153 0.206 - 0.111 - 0.071 Experimental Results at Cracking Shear Load Vcr (kips) 66.2 66.8 50.0 58.2 57.2 72.5 58.4 60.0 cr = Vcr /Acv (ksi) 0.41 0.41 0.30 0.36 0.36 0.45 0.36 0.37 cr (in.) 0.008 0.010 0.006 0.007 0.009 0.011 0.007 0.009 wcr (in.) <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 scr ( ) 23 30 27 28 38 42 24 61 f scr = Es scr (ksi) 0.68 0.87 0.78 0.81 1.11 1.21 0.70 1.78 Experimental Results at Ultimate Shear Load Vu (kips) 112.5 96.5 114.5 129.0 90.0 105.0 135.7 113.5 u= Vu /Acv (ksi) 0.70 0.59 0.69 0.79 0.57 0.65 0.84 0.71 u (in.) 0.025 0.027 0.037 0.038 0.027 0.031 0.032 0.041 wu (in.) 0.008 0.007 0.009 0.008 0.007 0.008 0.008 0.010 su ( ) 238 405 515 410 222 527 529 579 su = Es su (ksi) 6.92 11.74 14.93 11.90 6.44 15.29 15.35 16.79 Comparison with Equation 11 Vni (fy = 60 ksi) 78.1 78.8 111.3 111.0 77.4 78.2 110.8 110.6 Vu /Vni 1.44 1.23 1.03 1.16 1.16 1.35 1.25 1.03 Vni (measured fy) 82.9 83.6 113.1 112.8 122.7 121.7 201.6 196.1 Vu /Vni 1.36 1.15 1.01 1.14 0.73 0.87 0.69 0.58 Note: Shaded entries indicate that if the measured values of fy are used, Equation 11 becomes significantly unconservative when the higher strength A1035 bars are used. at both the top and bottom of the specimens; a ball joint was (displacements at ultimate load were too small to be seen in used at the top to address small alignment discrepancies photographs). (none were observed in any test). A view of the test set-up is Two important shear load values were monitored during shown in Figure 24. The load was applied at a rate of approx- the push-off experiments: the load to cause the initial shear imately 5,000 lbs/min. Once the ultimate shear capacity was crack, referred to as the "cracking shear load" (Vcr); and the reached, loading was continued in displacement control until highest shear capacity obtained, referred to as the "ultimate the specimen failed due to spalling or excessive deformation. shear load" (Vu). After Vcr is attained, shear friction dominates Complete details of the experimental program are provided the behavior of the loaded specimen until Vu is achieved. As in Zeno (2009). described above, the shear friction mechanism arises from the roughness of the concrete interface and the clamping force by 2.6.2 Experimental Results the interface reinforcement. After Vu is achieved, the specimen continues to deform with no further increase in capacity. The A summary of results for applied shear (V), shear displace- crack width increases, reducing the friction component, ment parallel to the interface (), crack width perpendicular although theoretically increasing the clamping force. Addi- to the interface (w), and interface steel reinforcement strain tionally, the roughness of the shear interface is reduced due to (s) for each specimen is given in Table 21. For clarity, the shearing off of the local asperities. gross section shear stress ( = V/Acv) and the apparent stress in the reinforcing steel ( fs = Es s) also are reported. The shear- displacement (V-), shear-crack width (V-w), and shear- Shear Friction Behavior interface steel reinforcement strain (V-s) plots of all specimens The experimental behavior illustrates that the shear fric- are shown in Figure 25. In general, duplicate instruments tion mechanism can be divided into three stages as follows: tracked each other very well; therefore, average values of , w, and s are reported. Figure 26 shows examples of observed Stage 1: Pre-Cracked Behavior. Behavior at loads below test behavior taken well after the ultimate load was achieved the cracking shear load (Vcr) is very similar for all specimens.

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41 face at the time of testing: fc = 5800 psi. As should be expected, this value is largely unaffected by the steel reinforcement. The strain, and therefore stress, in the reinforcing steel at Vcr is negligible--varying only up to 61 in the present study. Hence, shear friction reinforcement does not significantly con- tribute to the shear capacity of the interface up to the instant of cracking. The shear displacement at Vcr was less than 0.01 in. in all specimens. Stage 2: Post-Cracked Behavior. The post-cracked behavior from loads ranging from Vcr to Vu is characterized (a) V- by a softening behavior, larger and visible interface crack widths, and higher interface steel reinforcement strains than in the pre-cracked stage. During the second stage, both and w exhibit a relatively linear relationship with the applied shear load. The shear friction mechanism is engaged in the second stage. The capacity of the now cracked interface to resist shear is primarily attributed to the friction that originates from the roughness of the two concrete surfaces that form the inter- face. The interface surfaces are tied together by the interface steel reinforcement. The ultimate shear capacity of those specimens having #4 bars was 760 psi, which is approximately 20% greater than those having #3 bars (630 psi). These values correspond to 10 fc (psi) and 8 fc (psi), respectively. Sig- (b) V-w nificantly, the ultimate capacity is unaffected by the grade of reinforcing steel, demonstrating the same average value (690 psi) for specimens having A615 and A1035 bars. Significant variability of shear displacement () values was observed ranging from 0.025 to 0.041 in. at Vu. Values of the crack width (w) at Vu show less variability, ranging from 0.007 to 0.010 in. In both cases, this variability appears to be related to the size of the interface steel reinforcement provided: the specimens with #3 bars have somewhat smaller values of and w than those with #4 bars. This observation is expected due to the greater capacity of the specimens having #4 bars and indicates that the apparent stiffness of the cracked speci- mens is unaffected by the bar size; thus, the greater the capac- ity, the greater the displacement. (c) V- s Average interface steel reinforcement strain values (s) at Figure 25. Test results. Vu range between 222 and 579 . Associated with the greater capacity of the specimens having #4 bars, these specimens also exhibited greater bar strains at Vu. In general, the strains It is characterized by a relatively linear relationship between measured in the A1035 bars were marginally greater than the applied load (V ) and shear displacement (), and negli- those measured in the A615 bars. This observation is believed gible interface crack widths (w) and interface steel reinforce- to be associated with the different bond characteristics of the ment strains (s). Prior to cracking, applied load is resisted by bars used and is discussed further below. Because of the still concrete shear associated with the strength of the bond low interface steel reinforcing strains, there is little active between the two surfaces that form the shear interface. The clamping force across the interface in this stage. average value of cracking stress (cr) for the cold-jointed spec- imens tested was found to be 380 psi and have a coefficient of Stage 3: Post-Ultimate Behavior. The behavior follow- variation (COV) of 12%. This stress corresponds to a value of ing achieving Vu is characterized by an increase in , w, and s 5 fc (psi), based on the lower concrete strength at the inter- without any additional increase in applied loading. However,

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42 (a) P615-3B at a slip exceeding 1in. ( can be seen as (b) Distortion of the interface steel reinforcement of displaced horizontal lines representing interface specimen P1035-3A following large slip and cover reinforcement locations) spalling Figure 26. Specimens following testing. as seen in Figure 25, the behavior of the specimens with In fact, as seen in Table 21, the stress in the interface steel rein- A615 interface steel reinforcement in this stage is different forcement is significantly lower than its yield strength when from that of the specimens with A1035 steel reinforcement. the ultimate shear load is achieved. The slight increase in The specimens with A1035 steel reinforcement exhibit con- capacity of the specimens with A1035 interface reinforcement tinued load carrying capacity after the ultimate shear load may be attributed to the enhanced bond characteristics of this is achieved, which can be seen as a plateau in the plots shown steel (Sumpter 2007), because a better bond results in higher in Figure 25. The specimens with A615 steel reinforcement, steel strains and increases the shear friction capacity of the on the other hand, demonstrate a more rapid degradation in interface. Although this study was not intended to investigate post-ultimate load carrying capacity. Although this study is bond characteristics of the bars used and there was no discern- unable to determine the reasons for the different Stage 3 able difference in rib configuration between the A615 and behavior, it is proposed that it may be attributed to the differ- A1035 bars, the data do suggest marginally better bond char- ent bond characteristics of the bars used and is discussed fur- acteristics of the A1035 bars in this instance (Zeno 2009). ther below. Based on the AASHTO design equation (Equation 11), it would be expected that the use of high-strength interface reinforcement would increase the shear friction capacity of Development of Clamping Force the specimens. In fact, if the interface reinforcement had in Interface Steel reached its yield strength during the experiments, it would be In general, it can be seen that, as expected, the shear friction expected that specimens P615-4 and P1035-3, having similar capacity of the specimens increased as the area of interface nominal values of Avf fy, would have achieved similar shear steel reinforcement increased. This increase is because the area friction capacities. However, the interface reinforcement did of interface steel reinforcement is proportional to the clamp- not yield. In fact, the P615-4 specimens had significantly ing force (i.e., fs = fs As, where fs = sEs) and thus, the shear fric- greater capacity than the P1035-3 specimens, even though tion capacity. On the other hand, it can be seen that the use of the latter had high-strength interface reinforcement. These A1035 high-strength steel instead of A615 steel as interface results illustrate that because the ultimate shear capacity is reinforcement did not increase the shear friction capacity of dominated by concrete behavior and is reached well before the specimens significantly. This trend is because, in all of the steel yielding occurs, the clamping force is a function of the specimens, Vu was reached well before steel yielding occurred. steel modulus rather than the yield strength.