Shear friction behavior includes the cohesion components resulting from shear transferred through the slip plane and dowel action, and friction components due to clamping force in the reinforcement crossing the interface plane. The mechanism of shear friction is considerably more complex than conventional friction. The mechanism of shear friction is considerably more complex than conventional friction. A “wedging action” developed by the roughness of the shear plane forces the crack to open in direction perpendicular to the interface. This cracking opening induces tension in the reinforcement crossing the plane of interface resulting in a “clamping” force. Furthermore, any compressive force resulting from load conditions crossing the interface also result in a clamping force (Harries et al., 2012). The shear resistance is directly proportional to the normal clamping force through a friction coefficient as shown in Figure 2-23. Moreover, an additional component of shear friction, evident in the experimental data, from cohesion and/or aggregate interlock is proposed by Mattock (1974). Ali et al. claimed that failure of cohesion at the interface to transmit shear force results from loss of contact, which in turn occurs due to crushing of interlocking aggregates and cement paste (Ali and White, 1999). Hence, concrete strength and joint surface condition also affect the shear friction capacity.
Figure 2-23 Shear Friction Mechanisms (Macgregor et al., 1997)
The shear friction behavior can be divided into three stages consisting of precracked behavior, postcracked behavior and post-ultimate behavior (Harries et al., 2012). The behavior during precracked stage is characterized by a relatively linear relationship between the applied load and shear displacement with negligible reinforcement strain. The cohesion component at the shear interface contributes to the shear resistance during this stage. The behavior following the crack appear at the shear interface, referred as postcracked behavior, is characterized by a softening behavior, visible interface crack widths, and low reinforcement strains. The shear displacement continues to have a relatively linear relationship with the applied shear load. Furthermore, the shear friction developed by reinforcement begins to engage in the shear resistance. Following the achievement of the ultimate shear load, the post-ultimate behavior is characterized by an increment in shear displacement and reinforcement strain without any additional increase in applied load. A relatively rapid degradation of shear resistance takes place up to a certain shear displacement (ultimate shear displacement). It is proposed that the failure mechanism
may be attributed to the bond failure of the crossing reinforcement, the aggregate interlock failure, and the concrete failure around reinforcement (Valluvan et al., 1999).
Section 11.6 of the ACI 318-11 and Section 5.8.4 of AASHTO LRFD (2012) specify the nominal shear resistance of shear friction in Table 1. Considering only normal weight concrete and reinforcement oriented perpendicular to the interface, the provisions from AASHTO LRFD (2012) and ACI 318-11 associated with shear-friction are summarized in Table 2-1.
Table 2-1 Shear-friction Provisions in AASHTO LRFD (2012) and ACI 318-11
AASHTO LRFD
(2012), Section 5.8.4 ACI 318-11, Section 11.6
Nominal shear resistance 𝑉𝑛𝑖 = 𝑐𝐴𝑐𝑣+ 𝜇(𝐴𝑣𝑓𝑓𝑦 + 𝑃𝑐) 𝑉𝑛= 𝐴𝑣𝑓𝑓𝑦𝜇 Limitations 𝑓𝑦≤ 60,000 𝑝𝑠𝑖 𝑉𝑛𝑖 ≤ 𝐾1𝑓′𝑐𝐴𝑐𝑣 𝑉𝑛𝑖 ≤ 𝐾2𝐴𝑐𝑣 𝑓𝑦≤ 60,000 𝑝𝑠𝑖
For concrete either cast monolithically or cast on surface intentionally roughened:
𝑉𝑛 ≤ 0.2𝑓′
𝑐𝐴𝑐
𝑉𝑛≤ (480 + 0.08𝑓′
𝑐)𝐴𝑐
𝑉𝑛≤ 1600𝐴𝑐
For all other cases:
𝑉𝑛 ≤ 0.2𝑓′𝑐𝐴𝑐 𝑉𝑛≤ 800𝐴𝑐 Parameter 𝑐, ksi 𝜇 𝐾1, ksi 𝐾2, ksi 𝜆 𝜇 Monolithically cast 0.4 0.4 0.2 5 0.5 1.0 1.4𝜆
Concrete slab on surface intentionally roughened 0.2 8 1.0 0.3 1.8 1.0 1.0𝜆 Other on surface intentionally roughened 0.2 4 1.0 0.2 5 1.5 1.0 1.0𝜆
Cast against surface with no roughening
0.0
75 0.6 0.2 0.8 1.0 1.0𝜆
Notes: 𝐴𝑐𝑣(𝐴𝑐) is area of concrete shear interface; 𝐴𝑣𝑓 is area of reinforcement crossing shear interface; 𝑃𝑐 is
net compressive force; 𝑓𝑦 is yield strength of reinforcement crossing shear interface; 𝜇 is friction factor
(ASSHTO 2012) and coefficient of friction (ACI 318-11); 𝑐 is cohesion factor; 𝐾1fraction of concrete
The ACI 318-11 provisions specify a more conservative shear resistance by neglecting cohesion component and applied compressive forces at the shear interface. However, ACI 318-11 reports in the commentary that the sum of the resistance to shearing of protrusions on the crack faces and the dowel action of the reinforcement can be represented to establish a closer estimate of shear-transfer strength. ASSHTO (2012) introduces a term accounting for the cohesion at shear interface. For simplicity, the term “cohesion factor” is used to capture the effect of cohesion. The values of parameters presented provide a lower bound of numerous experimental data. It should be noted that all parameters resulting from experimental data are subject to the limitations implied by the data set from which they were obtained.
The slip at shear interface, referred as shear displacement, is a critical behavior affecting shear friction resistance. Shear displacement affects the cohesion component of shear friction in an adverse manner, and the clamping force developed in the interface reinforcement (Harries et al., 2012). The value of shear displacement when the shear load first reaches the ultimate value (yield shear displacement) were determined by experimental tests conducted by Harries et al., which ranges from 0.025 to 0.041 in.. Nevertheless, very limited data for the ultimate shear displacement were reported. Available experimental data exhibited the values ranging from 0.25 to 0.45 in. for the ultimate shear displacement (Valluvan et al., 1999).