## 1 Introduction
It was found that the shear capacity \(\mathbf{V}\) of the joints increased with the increase of lateral pre-compression stress \(\mathbf{\delta_{n}}\). Consequently, the shear bearing capacity of the keyed joints is proposed as \(\mathbf{V}\) is equal to the product of \(\mathbf{A_{k}}\) (area of the key root) and \(\mathbf{\tau}\) (average shear stress at the key root). In which, \(\mathbf{\tau}\) is linearly related to the lateral prestress \(\mathbf{\delta_{n}}\) and the concrete tensile strength \(\mathbf{f_{t}}\). In 2002, after verifying finite element model through test results, [PERSON] [23; 24] conducted numerical parametric study on the shear bearing capacity of dry joints with various number and shapes of shear keys, concrete qualities etc. On this basis, the shear capacity \(\mathbf{V}\) of the keyed dry joint was considered a combination of a frictional and a shear part [23]. However, different from the recommendation in AASHTO [15], the frictional part included the total area of the joint and not only the smooth parts. The load-bearing capacity of the keys depends on the concrete compressive strength and the area of the failure plane of the keys. Meanwhile, a safety coefficient was considered. Based on the direct shear test results of the keyed dry joints, [PERSON] [17; 18] compared the predictions of ultimate joint strength in terms of the provisions in ATEP [2; 25] and AASHTO [15]. It was shown that results obtained by the provisions in AASHTO were more consistent with test results. Taking concrete compressive strength higher than 50 MPa with comparatively lower tensile strengths into consideration, [PERSON] [17] proposed a new formula be included in the Euro-code. [PERSON] [26] proposed a formula for evaluating the shear bearing capacity of UHPC joints. It was found to have better agreement with the shear test results than that in the AASHTO provisions. Aiming to the existing formulas, further investigations were carried out by researchers. Comparing experimental results with the AASHTO [15] and another design criterion, [PERSON] [27] found that the provisions in AASHTO [15] and Rombach formula [23] underestimated the shear bearing capacity of the single-keyed joint by a value up to 40%, but greatly overestimated the shear capacity of dry multiple-keyed dry joints. Hence, it was suggested to introduce strength reduction factors for multiple-keyed dry joints. Subsequently, to investigate the structural behavior of the keyed dry joints under the direct shear, a numerical analysis model was established by [PERSON] [28], and a parametric study was carried out on predicting the shear capacity of multiple-keyed dry joints. Results showed that the shear capacity predicted by the AASHTO code equation diverges from that predicted by numerical analysis at high confining pressure. This is because the contribution of friction in the total shear capacity decreased with an increase in confining pressure. The same conclusions were demonstrated by [PERSON] [29; 30; 31]. Meanwhile, [PERSON] [32] also investigated the influence of key depth on the shear resistance of the dry joint. Results showed that the predicted results, according to the provisions in AASHTO [15], are more applicable for dry joints with key depth in 35 mm and 40 mm. [PERSON] [2] demonstrated through experiments that the average shear transfer of a single key is higher for specimens with a smaller number of keys. Four flange shear keys are capable of increasing the shear capacity by 14% and the elastic stiffness of the joints by 73%. Compared with simulating results, [PERSON] [33] demonstrated AASHTO standard overestimated the shear capacity of single-keyed dry joints with fixing imperfections at the lower surface of the key by up to 0.602-22.0%, but greatly underestimated that of the rest, and proposed a modified formula with a strength reduction factor. Regarding the shear capacity of the keyed joints in segmental precast bridges, the following points could be summarized according to the above researches.