Effect of Tensile Strain Capacity of UHPC on the Bond with Steel Reinforcement
Presented By: Amr Soliman
Affiliation: SUNY University of Buffalo
Description: In reinforced concrete structures, the bond of the steel reinforcement is usually governed by the cracking of the surrounding concrete leading to brittle splitting failure. The bridging effect of fibers in Ultra-High-Performance Concrete (UHPC) results in high tensile strength, leading to significantly higher bond strength of steel reinforcement. Furthermore, Strain Hardening Ultra-High-Performance Concrete (SH-UHPC) possesses high tensile strain capacity up to 6% in addition to high tensile strength. Therefore, an SH-UHPC could provide more efficient containment of the steel reinforcement and improve the bond behavior. This study evaluates the effect of strain capacity of concrete on the bond performance by comparing UHPC and SH-UHPC specimens. Double-rebar pullout specimens are made from the two materials, with varying cover thickness and embedment length. It was found that higher tensile strength of concrete led to greater bond strength. On the other hand, higher strain capacity of concrete resulted in a more ductile behavior and greater energy dissipation.
Investigation of Interface Shear Transfer in Engineered Cementitious Composite Members
Presented By: Adham Abu-Abaileh
Affiliation: Bradley University
Description: Interface Shear Transfer (IST) is critical for achieving composite behavior in many concrete structures, such as the connection between bridge girders and cast-in-place decks. This research sought to evaluate the application of Engineered Cementitious Composite (ECC) in IST application compared to Normal Concrete (NC) using Iosipescu 4-point bending shear test (Fig. 1). The ECC has been invented to resolve the need of reinforcement in contrast with the traditional NC members. Test variables included roughness condition of the interface (roughened or smooth), presence of reinforcement (no reinforcement or 2#3 rebar), and the type of concrete (ECC and/or NC). In case of roughened interface members with no interface reinforcement, the results showed that the IST strength of members with ECC at both layers (E-E) was 1.3 times of similar members made with NC layers (N-N). In case of smooth interface with 2#3 interface reinforcement, the IST strength is approximately the same for ECC and NC members. In case of one layer made with ECC and the other layer made with NC, the following observations were made: in case of roughened interface with no reinforcement, the IST strength (15kip) was higher than both cases of E-E (10kip) and N-N (7.5kip); in case of smooth interface with 2#3 reinforcement, the IST strength (25kip) was lower than both E-E and N-N cases (30kip); and in case of roughened interface with 2#3 reinforcement, the IST strength (31kip) was lower than that of the N-N case (36kip).
Fracture Characterization of Interfaces between Ultra-High-Performance Concrete and High-Performance Concrete
Presented By: Ali Cicek
Affiliation: University of Delaware
Description: The adoption of ultra-high-performance concrete (UHPC) to connect precast (high-performance concrete or HPC) bridge deck panels has resulted in improvements in constructability and structural performance of such connections. However, recent field evidence suggests that UHPC connections are susceptible to UHPC/HPC interfacial cracking under service loads. While such cracks may not compromise the structural strength of these bridges in the short-term, concerns were raised about the impacts of interfacial cracks on the serviceability and long-term durability of the connections. This work presents the development of a fracture mechanics test method to accurately characterize the cracking propensity of interfaces between the UHPC and HPC. The test is conducted by loading a bimaterial beam (consisting of UHPC and HPC) notched at the UHPC/HPC interface in a three-point bending setup (Figure 1). Closed-loop feedback was implemented to ensure stable crack growth and control the crack-opening displacement (COD) during the test. HPC substrate hygric state (dry and saturated surface dry or SSD) and surface preparation (as cast and exposed aggregate finish) were varied to evaluate the effect of substrate condition on the fracture energy and tensile strength of the interface. Tension-softening curve parameters for UHPC-HPC interfaces were generated by implementing an inverse analysis approach. The data indicate that UHPC/HPC interfaces are characterized with up to 70% reduction in fracture energy compared to neat HPC. Moreover, there was no significant difference in fracture energy and tension-softening curve parameters between ‘saturated surface dry’ and ‘dry’ interfaces.
Tensile and Shear Capacity of Post-Installed GFRP Bars in Concrete
Presented By: Thomas DeMars
Affiliation: University of Minnesota Duluth
Description: Post-installed reinforcing bars are used to connect an existing and new concrete member. In these applications, uncoated steel reinforcing bars are often used, which are susceptible to corrosion. The use of glass fiber-reinforced polymer (GFRP) reinforcing bars in structural concrete has increased in recent years due to many factors, including their noncorrosive and nonconductive nature, high tensile strength, and nonmagnetic properties. Currently, structural concrete design using GFRP reinforcing bars is covered in ACI 440.1R, which does not include provisions for anchorage to concrete. This research was conducted to begin filling that knowledge gap and determine if the equations for anchorage to concrete with steel reinforcing bars given in ACI 318 are appropriate for use with GFRP reinforcing bars. A series of tests were conducted with tensile and shear loading to characterize behavior of postinstalled GFRP reinforcing bars and allow for comparison of their behavior to post-installed steel reinforcing bars. Specifically, two sizes of both steel and GFRP reinforcing bars (No. 5 and 8) were post-installed in concrete slabs using the same chemical adhesive. The confined and unconfined tensile strength of the bars away from concrete edges and in proximity to concrete edges was established. Additionally, shear tests on No. 5 bars were performed. Using the information gathered from these tests, the applicability of ACI 318 design guidelines for post-installed GFRP reinforcing bars is presented.
Early Detection of Alkali-silica Reactivity in Laboratory Concrete Prisms Using Ultrasonic Coda Wave Monitoring
Presented By: Sina Mehdinia
Affiliation: Portland State University
Description: Current standardized monitoring methods for detecting alkali-silica reaction (ASR) in laboratory concrete prisms rely on manual measurements and are time consuming. Active and passive stress wave-based methods such as ultrasonic and acoustic emission monitoring, respectively, represent promising tools to not only automate the process but also detect the onset of ASR much earlier. In this research, a simple but reliable monitoring method for early detection of ASR in laboratory concrete prisms was developed and evaluated. The approach takes advantage of the highly sensitive coda wave eld of recorded ultrasonic signals. A robust laboratory setup was developed following a year of experimentation. Three concrete mixtures with different target reactivity levels containing various dosages of lithium nitrate were monitored over 84 days. The results were compared to expansion measurements based on ASTM C1293. The proposed approach, based on ultrasonic coda wave elds, correlates well with expansion data, shows to be highly sensitive, and can detect the onset of ASR much earlier than the 1-to-2-year ASTM C1293. A further bene t of this approach is that the measurements are directly relatable to the physical process occurring in the material and can be captured with mathematical prediction models.
Analytical Prediction of Cover Delamination Failure Mode of RC Beams Strengthened with FRP
Presented By: Fahed Salahat
Affiliation: Kansas State University
Description: RC beams strengthened with FRP fail in different mechanisms depending on the geometric and material characteristics of these beams. Among the various failure mechanisms involved, cover delamination and plate debonding are classified as premature failure modes since the strain in the FRP at failure does not reach FRP rupture strain. ACI 440.2R has provisions that considers the plate debonding strain limit but not the corresponding cover delamination limit. However, cover delamination typically occurs at lower strain levels compared to plate debonding. Thus, it is imperative to develop a pertinent design model for cover delamination strain limit. In this study, a new analytical approach is introduced to predict the cover delamination strain limit. The derivation employs nonlinear interfacial shear stress distribution, the trilinear representation of the moment-curvature relationship, and analysis of an experimental database composed of (78) beams failed in cover delamination. Closed form formulas are extracted to calculate the FRP strain limit at cover delamination failure using strain compatibility and force equilibrium. The formulas are easy to evaluate based on basic parameters of the beam problem. The results show good correspondence with experimental data.