Effects of Steel Fiber and Specimen Geometric Dimensions on the Mechanical Properties of Ultra-High-Performance Concrete

Ultra-high-performance concrete (UHPC) is an advanced concrete with superior mechanical strength, ductility and durability properties. However, the influence of steel fiber on its constitutive laws and the specimen geometric dimension effect on its strength had not been paid enough attention. To investigate the effect of steel fibers on the properties of UHPC, specimens with different fiber volume contents and fiber types were tested. Meanwhile, the mechanical properties of UHPC at different ages from 3 days to 28 days were conducted. Moreover, specimens with various geometric dimensions were also prepared to study the effect of specimen geometric dimensions (dog-bone-shaped, prism and cylinder specimens) on the properties of UHPC. The results indicated that elastic modulus, tensile peak stress and the corresponding strain increased as the fiber volume content and curing age increased. Specimens with hooked-end fibers exhibited better tensile performance than those with straight fibers. Furthermore, different geometric dimensions of specimens significantly influenced the tensile properties of UHPC. Based on the experimental results, conversion factors were suggested for the transformation of strength obtained from specimens with different geometric dimensions to reference specimens. In addition, both compressive and tensile constitutive laws were proposed to generate the stress–strain relationship of UHPC

Shear performance of high-strength friction-grip bolted shear connector in prefabricated steel–UHPC composite beams: Finite element modelling and parametric study

As a competitive alternative for accelerated bridge construction (ABC), prefabricated steel–ultra-high-performance concrete (UHPC) composite beams containing high-strength friction-grip bolt (HSFGB) shear connectors offer numerous advantages, including reduced on-site construction time and ease of replacing/removing deteriorated components. However, the failure mechanism of HSFGBs in UHPC remains unclear due to the lack of internal stress analysis, which hinders the design of these innovative composite beams. To clarify the shear performance of HSFGBs in prefabricated steel–UHPC composite beams, an effective finite element model (FEM) considering the non-linearities of materials and geometry was developed through ABAQUS. Based on the experimentally verified model, the internal stress transfer mechanisms of HSFGBs and the failure mechanism of precast UHPC were revealed. According to the extension parametric analysis results, a stronger HSFGB presented better shear performances in terms of ultimate shear strength, initial shear stiffness and slip capacity. Adopting oversized holes with appropriate bolt-to-hole clearance can improve the constructional efficiency without considerable strength and stiffness reduction. HSFGBs with low bolt pretension exhibited unfavorable initial shear stiffness, while smaller slip capacity in high bolt pretension conditions. A smaller ductility was observed as the steel beam tensile strength and slab concrete strength increased. Additionally, the ACI 318–19, Eurocode 3, and AASHTO LRFD specifications underestimated the shear strength of HSFGBs, whereas the Eurocode 4 presented acceptable predictions in determining the ultimate shear capacity of HSFGBs in prefabricated steel–UHPC composite beams

Numerical Analysis on Shear Behavior of Joints under Low Confining and Eccentric Loads

The joints of precast concrete segmental beams (PCSBs), which are in complex stress status and susceptible to failure, are very important parts of the structure. In this paper, a finite element model was established to study the shear performance of single-keyed joints. The plastic damage model was used to simulate the cracking of specimens. Three types of single-keyed joints were investigated, including the dry joint with normal concrete (NC), dry joint with steel fiber-reinforced concrete (SFRC), and epoxied joint with NC. The cracking pattern, ultimate shear strength, and load-displacement curve for these specimens were obtained. Based on these numerical simulation models, extended analyses in terms of low confining pressures and eccentric loads were performed. It has been found that the influence of fiber-reinforced concrete should be considered. The ultimate shear strength of specimens reduced with the reduction of confining pressure. When an eccentric load was applied, a lower shear capacity would be obtained. Under the low confining stress, the AASHTO LRFD 2014 provision underestimated the shear strength of single-keyed dry joints with both NC and SFRC, while the shear capacity of single-keyed dry joints with both NC and SFRC has been overestimated under the eccentric loads

Horizontal Shear Behaviors of Normal Weight and Lightweight Concrete Composite T-Beams

Abstract This paper presents the results of recent research on the interface shear behavior of normal weight and lightweight concrete composite T-beams. In the experimental program 12 beams and necessary control cylinders were tested to provide experimental cases with the variables of interface preparation, clamping stress and lightweight slab concrete strength. Compared with 7 equations developed previously, it has been found that those formulas, especially the ones from current AASHTO and ACI design codes, give a conservative theoretical prediction of horizontal shear capacity of composite T-beams. Based on the experimental results, a more accurate equation was developed to predict the interface shear transfer strength of composite concrete T-beam. By comparing the experimental results of previous beam tests and shear-friction push-off tests for different types of concrete with both rough and smooth interface published in literature, it has been found that the proposed formula is reliable in predicting the horizontal shear strength of concrete composite T-beams

Shear behavior of short studs in steel-thin ultrahigh-performance concrete composite structures

Steel-concrete composite structures gradually tend to be thinner and lighter in modern bridge engineering. Ultrahigh-performance concrete (UHPC) as an innovative solution has been used to upgrade the behavior of composite structures and accelerate construction, and short stud connectors are the key elements to guarantee the effective connection of steel and concrete components. This study conducted push-out test to explore the failure modes and load-slip relationships of short studs in steel-thin UHPC composite structures (STUCs). The experimental findings revealed that the fracture of the stud shank and local concrete crushing dominated the failure modes of all specimens. Increasing stud diameter could enhance shear strength, while arranging short studs densely and decreasing stud height could result in the reduction of shear capacity of a single stud. The experiment data were employed for the construction and verification of the finite element models. The impact of several parameters on the shear strength was investigated via the validated numerical models. The shear strength increased approximately linearly with stud diameter for short studs in thin UHPC layers, but the cover thickness and UHPC strength had a slight impact. The shear capacity of short studs couldn't be negatively impacted by reducing the aspect ratio to 3.16. In addition, the ultimate shear capacity significantly decreased due to the grouped stud effect for grouped short studs when the spacing between the studs was less than 70 mm. The shear capacity was conservatively predicted by these specifications for short studs in thin UHPC layer, according to the comparison of the simulated data with the existing construction specifications. Finally, a new equation considering multiple parameters was proposed and validated by the test data, which could precisely predict the shear strength of short studs in STUCs