Interface Shear Strength at Joints of Ultra-High Performance Concrete Structures

Although various mix proportions of UHPC have been proposed depending on the target properties, the K-UHPC developed in Korea is the main focus in this study (Korea Concrete Institute 2012; Korea Institute of Civil Engineering and Building Technology 2012). Table 1 presents the specific mix proportion of the K-UHPC which is expressed as a ratio of mass. The mixture consists of cement, silica fume, filling powder, fine aggregate, shrinkage reducing agent, expansive agent, superplasticizer, and steel fibers. Coarse aggregates are not included in this mixture. The steel fibers have a diameter of 0.2 mm and a tensile strength of more than 2000 MPa. The length of the fibers can be selected among 13, 16, and 20 mm depending on the required tensile characteristics. The specified compressive strength, cracking strength, and tensile strength of K-UHPC are as high as 180, 9.5, and 13 MPa, respectively. The shape and dimensions of these test specimens and test methods are specified in design recommendations for K-UHPC (Korea Concrete Institute 2012) in detail. The specimen shape for compressive strength is a cylinder with 100 mm diameter and 200 mm height. Cracking strength and tensile strength are measured by the direct tensile test using a specimen with rectangular cross section.

In the case of precast K-UHPC members fabricated at a plant, the standard steam curing process was used to ensure rapid strength development (Park et al. 2015), where 90 °C is maintained for 48 h. More detailed information on each material has been provided in several previous studies and design recommendations for K-UHPC (Korea Concrete Institute 2012; Park et al. 2015).

The joints between UHPC segments were realized by shear keys in this study to increase the shear capacity at the joints. The joint types of this study are largely divided into dry joint with or without epoxy and cast-in-place wet joint. Figure 2 shows the details of each joint type for the specimens with two shear keys. In comparison, the specimens with one shear key have a shear key with the same dimensions as that shown in Fig. 2 at the middle height. The shapes of the shear keys were determined by considering the recommendations in the AASHTO specifications (2003, 2017). According to the AASHTO specifications, the height of the shear key shall be greater than 30 mm and twice the diameter of coarse aggregates. Also, the ratio of height to width of the shear key, where the width is measured at the middle height of the shear key, shall remain less than 0.5 in order to reach a desirable failure mode and provide improved strength of the shear keys. The JSCE specifications (2010) have similar provisions. The shape of the shear key with a height of 30 mm complies with these provisions as shown in the specimens in Fig. 2. However, shear key heights shallower than 30 mm were also attempted in this study to investigate the improved performance of UHPC. The surface of the shear keys in the test specimens remained smooth without additional treatment according to the usual practice.

In the specimens that have dry joints with epoxy applied, a minimum compressive stress of 0.2 MPa and an average stress of 0.28 MPa were applied across the joint using a temporary prestressing system until the epoxy was cured in accordance with the recommendation in the AASHTO specifications (2017). According to the preliminary tests, the compressive strengths of the epoxy were 62.5 and 77 MPa at 1 and 7 days after hardening, respectively, and the shear strength of the epoxy was measured as 24.5 MPa at 7 days.

Figure 3 shows the test set-up of the specimens. A lateral load was first applied on the loading plate (200 mm × 320 mm with the thickness of 30 mm) using a horizontal actuator and was maintained during the loading test. The lateral load simulates the prestress introduced by the prestressing tendons that penetrate the precast UHPC segments in actual construction. In this case, the compressive stresses shown in Table 2 can be introduced by the effective prestressing forces after short-term and long-term losses. The lateral load was introduced to the specimens with wet joint when the filler attained the required strength to resist the compressive stresses in Table 2 after a sufficient curing period. The vertical load was then applied on the loading plate (200 mm × 200 mm with the thickness of 30 mm) up to the ultimate stage at the loading rate of 0.4 mm/min. The loading plates were used for distributing the load uniformly on a specimen. Half of the vertical load is applied at each side.

Concrete strain gauges were attached to the specimen surface and displacement gauges were installed to obtain the load–strain relationship and load–displacement relationship, respectively. They were analyzed in the previous studies (Lee et al. 2011a, b).

The test variables of this study are shown in Table 2. Both the dry joint and wet joint were designed. In the dry joint, the effect of epoxy on the strength improvement was investigated. The effectiveness of cast-in-place UHPC as a wet joint filler between UHPC segments was one of the main concerns. Basically, it can be expected that the strength of a joint needs to be similar to that of the main body to avoid the joint becoming a weak part. The number and height of shear keys were considered as variables to investigate the difference of failure mechanism and ultimate capacity at the joint. Three different curing temperatures of the UHPC filler of a wet joint were considered to account for the inferior condition of the construction site in terms of temperature control, when compared to a precast concrete plant where steam curing is available (Park et al. 2015). Lateral compressive stress was applied at several levels to account for various prestress introduced at the joint by the prestressing tendons that connect a number of segments. As explained earlier, shear keys having a 30 mm-height or shorter were used to investigate the improved performance of UHPC.

The failure modes or cracking patterns of the specimens tested in this study varied depending on the test variables. The shape of the vertical load–displacement relationship and the ultimate strength of a specimen were considerably affected by the cracking pattern.

In particular, the height of the shear key had a significant effect on the cracking pattern. When the shear key height was relatively large, diagonal cracks initiated at the lower face of the shear key which led to shear-off failure of the key as shown in Fig. 4a, b. It was often observed as shown in these figures that the shear-off failure occurred in diagonal direction in the wet joint with the dimensions of Fig. 2b, while the base of the shear keys failed in vertical direction in the dry joint. It is also noted in Fig. 4a, b that the steel fibers of the cast-in-place UHPC wet joint and of the main body of UHPC in the dry joint provided additional resistance to the opening of the diagonal cracks. In the case that the shear key was relatively flat, the failure surface tended to occur along the interface as presented in Fig. 4c or at the lower sloped contact face because of excessively high bearing stress.

Many equations that can predict the shear strength at the concrete joint interface in terms of shear transfer mechanism have been proposed and incorporated into various design codes. However, most of the equations specified in the codes have been derived from the tests of normal strength concrete. Therefore, this study verifies whether the existing equations can also be extended to the range of ultra-high strength of UHPC. The equations largely consist of two types of resistance: friction and cohesion. However, in many equations, the effect of shear keys is not directly accounted for (AASHTO 2017; CEN 2004; fib 2013), and the shear key geometry can only be reflected as the coefficients of friction or cohesion corresponding to a very rough surface or an indented surface in an indirect way. On the other hand, in a few design equations, the shape of the shear keys is reflected as a separate term in a more detailed manner (AASHTO 2003; JSCE 2004, 2010). Because the effect of shear key geometry, including the number of shear keys, on the shear strength is one of the main concerns in this study, the equations presented in the AASHTO guide specifications (2003) and JSCE recommendations (2004, 2010) were analyzed in detail and compared with the test results to verify the applicability of these equations to UHPC.

Equation (1) shows the predictive equation of the AASHTO guide specifications (2003). The form of Eq. (1) is expressed in The International System of Units (SI units) instead of in the US customary units of the original equation. The AASHTO guide specifications state that Eq. (1) can only be applied to the dry joints without epoxy. In comparison, AASHTO LRFD Bridge Design Specifications (2017) only allow either cast-in-place closures or match-cast epoxied joints in the joints of precast segmental bridges. The apparent feature of Eq. (1) is the inclusion of f_pc in both terms. That is, the contribution of shear keys to the shear strength is also affected by compressive stress. The constant 0.6 in the second term represents a friction coefficient.where V_nj is the nominal joint shear capacity (N), A_k is the area of the base of all keys in the failure plane (mm²), f_c′ is the compressive cylinder strength of concrete (MPa), f_pc is the compressive stress in concrete (MPa), and A_sm is the area of contact between smooth surfaces on the failure plane (mm²).

On the other hand, the equation in the JSCE recommendations (2004, 2010) is shown in Eq. (2). The original form of Eq. (2) was first presented in a previous edition of Standard Specifications for Concrete Structures (JSCE 2010) and it was referred to in Recommendations for Design and Construction of Ultra High Strength Fiber Reinforced Concrete Structures (JSCE 2004). Equation (2) features the inclusion of \(f^{\prime}_{cd}\) in both terms. This implies that the concrete strength can contribute not only to the strength of the shear keys but also to the friction characteristics in some circumstances.where V_cw is the shear strength (N), μ is the average friction coefficient of solid contact (0.45), \(f^{\prime}_{cd}\) is the design compressive cylinder strength (MPa), σ_nd is the average compressive stress which acts perpendicular to shear plain (MPa), A_cc is the area of shear plane in compression zone (mm²), b is the coefficient indicating plane configuration (range of 0–1 and 0.5 in the case of the joint of precast members with an adhesive agent), and A_k is the area of compressive side of shear keys (mm²). Note that the definition of A_cc does not coincide with that of A_sm of Eq. (1), while the meaning of A_k is the same as that of A_k of Eq. (1). The friction coefficient of 0.45 is 25% less than 0.6 used in Eq. (1).

The test results were compared in Fig. 8 with the predictions by Eqs. (1) and (2) in various ways depending on the test variables. Note that the joint shear strength is half the failure load in this test set-up. The shear key height is not considered as a variable in either Eq. (1) or (2). Although the AASHTO equation of Eq. (1) was originally intended for a dry joint without epoxy resin, the test data other than this joint type were also compared with Eq. (1) to investigate the applicability of Eq. (1). Even though the coefficient b of Eq. (2) plays an important part in the shear strength, the only value presented in these recommendations is 0.5 for the joint with an adhesive agent such as epoxy. Therefore, referring to a few previous studies (Shin 2016; Watanabe et al. 2007), 0.4 was employed for the wet joint, while zero was assumed for the dry joint without epoxy. In Eq. (2), σ_nd was considered as it is without 50% reduction for additional safety as presented in the JSCE specifications (2010). Also, \(f^{\prime}_{cd}\) originally referred to the design compressive strength that can be obtained by dividing the characteristic compressive strength by a material partial safety factor according to the concept of the limit state design. However, the specified (characteristic) compressive strength of the UHPC of 180 MPa was used instead in these analyses for the purpose of comparing with the test data. Besides, A_k in Eqs. (1) and (2) is 0, 19,000, and 38,000 mm² for zero, one, and two shear keys, respectively. A_sm in Eq. (1) is 64,000, 45,000, and 26,000 mm² for zero, one, and two shear keys, respectively, while A_cc in Eq. (2) has a constant value of 64,000 mm² regardless of the number of shear keys.

As shown in Fig. 8a, the AASHTO equation underestimated the shear strengths in all the joint types, including the dry joint without epoxy. The values resulting from the JSCE equation significantly differed depending on the joint type, and were in good agreement with the test data of the wet joint.

Increasing trend of the shear strength according to the number of shear keys was adequately implemented in Eqs. (1) and (2) as shown in Fig. 8b. The test data as well as the predictive values were almost in linear relation to the number of shear keys. However, the AASHTO equation underestimated the strengths, whereas the JSCE equation overestimated the strengths. If the exponential coefficient b of the JSCE equation was slightly adjusted, the predicted strengths would approach the experimental data. This type of versatility is one of the advantages of the JSCE equation, although some tests need to be performed to select a reasonable coefficient. This issue is covered in the following section.

Figure 8c shows the increasing trend of the shear strength both in the test and prediction as the lateral compressive stress increased. Similar to Fig. 8b, the AASHTO equation tended to underestimate the strengths, while the JSCE equation overestimated the strengths. The JSCE equation was not in linear relation to the lateral compressive stress, as can be expected in the form of Eq. (2).

As mentioned previously, because the height of the shear key is not reflected as a variable in either Eq. (1) or Eq. (2), the trends somewhat differed between the test and prediction, as shown in Fig. 8d. Judging from the context of the relevant specifications, these equations were derived based on the assumption that the key height is more than 30 mm. Nevertheless, the shear strengths for the shear key height of 30 mm also differed considerably.

The analysis results of Sect. 4.1 show that the two representative predictive equations for the joint shear strength do not correctly estimate the experimental results of this study. The AASHTO equation of Eq. (1) originally developed for the dry joint without epoxy largely underestimated the strengths measured in various joint conditions. Figure 8a shows that the AASHTO equation may even result in a lower value than that measured in a dry joint without epoxy. Therefore, it should be careful to apply the AASHTO equation to estimate the joint shear strength of UHPC because this equation shows too much conservatism. Furthermore, Eq. (1) does not include a coefficient that can be adjusted to match the test results of UHPC.

On the other hand, the JSCE equation of Eq. (2) largely overestimated the shear strengths of the dry joints with epoxy. For the wet joints, although Fig. 8a shows good agreement with the test for the compressive stress of 8 MPa (W-1-U70-8-30), further analyses revealed that Eq. (2) overestimated the strengths for other compressive stresses (33.8 and 31.8% for W-1-U70-2-30 and W-1-U70-4-30, respectively). Judging from the form of Eq. (2), the overestimation can be mainly attributed to \(f^{\prime}_{cd}\) included in the first term that refers to the contribution of friction. Specifically, it seems that the first term is highly overestimated when the concrete strength is within the ultra-high range. However, a margin for improvement can be found in Eq. (2). That is, coefficient b can be adjusted to match the test results of UHPC because the previously recommended values of b (0.4 for wet joint and 0.5 for dry joint with epoxy) were derived from the concrete specimens with the range of normal strengths.

Figure 9 presents an example of the possible improvement of Eq. (2) in the case of the dry joint with epoxy when compared to Fig. 8b, c in terms of number of shear keys and lateral compressive stress, respectively. The coefficient b was revised to 0.4 from the original value of 0.5 for the application of epoxy. As a result, the predictive values showed good agreement with the measured values with sufficient accuracy. The improved predictive values even showed slight conservatism, which is desirable from the perspective of safety. The differences from the test data were only 1.1, 4.3, and 11.0% for D-0-E-8-0, D-1-E-8-30, and D-2-E-8-30, respectively, in Fig. 9a. The differences were still as small as 12.5, 3.9, and 4.3% for D-1-E-2-30, D-1-E-4-30, and D-1-E-8-30, respectively, in Fig. 9b.

However, the revised JSCE equation still overestimated the shear strengths for the shear key height lower than 30 mm (38.3 and 16.4% for D-1-E-8-7.5 and D-1-E-8-15, respectively). This indicates that the minimum shear key height of 30 mm specified in AASHTO (2003, 2017) and JSCE (2010) should be ensured if the revised JSCE equation is to be effective.

On the other hand, the analyses of wet joints showed that it was hard for the equation to entirely approach the test data simply by adjusting the value of coefficient b because the trends of overestimation and underestimation coexisted when compared to original JSCE equation with b = 0.4. As was mentioned previously, b = 0.4 is not specified in JSCE specifications (2010) but was proposed in some studies (Shin 2016; Watanabe et al. 2007), in which Watanabe et al. (2007) applied UHPC filler with specified compressive strength of 180 MPa. Therefore, it would be desirable to further investigate the validity of b = 0.4 for the wet joint with more accumulated test data before discussing the revision of the coefficient b for UHPC filler.

In the application of this equation to the actual design of UHPC structures, additional safety factors such as material factor and member factor are employed according to the concept of the limit state design of the JSCE specifications (2010) in order to ensure sufficient safety by overcoming a range of uncertainties.