Shear Strength of Joints in Precast Concrete Segmental Bridges

Shear Strength of Joints in Precast Concrete Segmental Bridges. Paper by Xiangming Zhou, Neil Mickleborough, and Zongjin Li

The authors present a great work, with an impressive number of experimental results, and should be congratulated. Experimental research on dry and epoxied multiplekey joints is very scarce, and to the discussers' knowledge, it is the first time such tests have been performed on highstrength concrete specimens.

Nevertheless, the discussers have some comments and suggestions. It is the intention of this discussion to express some remarks based on previous research, just in case it could be useful to the authors and readers in general.

In the introduction of the article, it is stated that the shear keys serve three functions: aligning the segments during erection, transferring the shear force between segments during service, and ensuring durability of the tendons against corrosion. However, the crucial function of resisting shear during the construction of balanced cantilever bridges, when the epoxy has not hardened and acts like a lubricant, is not mentioned. Stating that keys help prevent corrosion of the internal tendons is not a very accurate assertion; the corrosion protection relies on other techniques such as the use of epoxy in the joints, the sealing of the ducts at the joints with compressed neoprene o-rings, and the duct injection.

Also in the introduction, the AASHTO formula for the design of the keyed joints is claimed to be empirical, whereas it is completely theoretical. From a presumed simple state of stresses within the key, when subjected to axial and shear forces, Roberts and Breen20 deduced the formula later adopted by AASHTO, stating that a key fails when the maximum principal tensile stress equals the concrete tensile strength. Regarding the content of the same paragraph, it is worth noting that a shearing-off failure along the keys only occurs when the shear span-depth ratio (a/d) is extremely low (a/d

It is very difficult to extract definite conclusions from the test program because the object of the research is very dependent on the concrete tensile strength. This magnitude can be derived from the concrete compressive strength with a significant scatter, which of course will be more important for concrete compressive strength in the range of 30 to 80 MPa, as those considered in the test program. In this manner, the comparison of the results is also difficult because compressive stresses for multiple-key specimens (ranging from 0.5 to 2 MPa) are systematically lower than in single-key specimens (ranging from 1 to 4.5 MPa). The compressive stresses observed in the tests seem to also be very low, especially for multiple-key specimens, where the most common service compressive stresses in a concrete box girder bridge is above an average of 0.15[function of]prime;^sub c^ .

Regarding the experimental results and analysis, some points should be discussed. The AASHTO code proposes Eq. (5) to estimate the shear capacity of the joints in PCSB (without safety factor). This formula is the one provided by Roberts and Breen20 and it depends on the tensile strength of the concrete. Actually, this formula was deduced for concrete with a compressive strength up to 55 MPa.20 In such concrete classes, the tensile strength was derived from the compressive strength through the following nondimensional formula (in psi)

[function of]t = 7.5[radical][function of]'^sub c^ (8)

In Reference 5, Eq. (5) does not distinguish between strength levels. This formula, however, was not proposed for high-strength concrete. Hence, the fact that the grade of some test specimens is greater than 55 MPa should have been taken into account when checking the accuracy of Eq. (5). It could be wise to use the tests on the one hand for checking the accuracy of Eq. (5) for conventional strength concrete ([function of]prime;^sub c^ 55 MPa).

The generally accepted statement that the strength of an nkey joint will roughly be n-times the strength of a single-key joint relies on the plastic behavior of the joint. This plastic behavior depends on two main factors: the strength of the concrete and the compressive stresses acting at the joint. The higher the concrete compressive strength, the more brittle the material; thus, the high compressive strength of some concrete used in the test programs will also affect the behavior of the three-key specimens. At the same time, the higher the confinement stresses at the joint, the more plastic its behavior. Due to this fact, Roberts and Breen also limited the validity of their formula to joints where the actuating compressive stress is greater than 0.7 MPa. This stress level is generally lower than the one actuating in a real structure. The low confinement stress would explain the huge differences between the AASHTO predictions and the tests on multiple-key specimens. Actually, when a higher compressive stress is acting at the joint, the AASHTO formula proves to be very accurate for predicting the strength of panel tests with multiple-key joints (up to seven keys).3,22 Average compressive stresses in the mentioned panel tests range between 2.9 and 3.9 MPa (References 3 and 22, respectively).