This study investigated the punching shear behavior of full-scale, two-way concrete slabs reinforced with glass fiber reinforced polymer (GFRP) bars, which are known as noncorrosive reinforcement. The relatively low modulus of elasticity of GFRP bars affects the large deflection of flexural members, however, applying these to two-way concrete slabs can compensate the weakness of the flexural stiffness due to an arching action with supporting girders. The test results demonstrated that the two-way concrete slabs with GFRP bars satisfied the allowable deflection and crack width under the service load specified by the design specification even in the state of the minimum reinforcement ratio. Previous predicting equations and design equations largely overestimated the measured punching shear strength when the slab was supported by reinforced concrete (RC) girders. The strength difference can be explained by the fact that the flexural behavior of the supporting RC beam girders reduces the punching shear strength because of the additional deflection of RC beam girders. Therefore, for more realistic estimations of the punching shear strength of two-way concrete slabs with GFRP bars, the boundary conditions of the concrete slabs should be carefully considered. This is because the stiffness degradation of supporting RC beam girders may influence the punching shear strength.

Estimating the compressive strength of high strength concrete (HSC) is an essential investigation for the maintenance of nuclear power plant (NPP) structures. This study intends to evaluate the compressive strength of HSC using two approaches: non-destructive tests and concrete core strength. For non-destructive tests, samples of HSC were mixed to a specified design strength of 40, 60 and 100 MPa. Based on a dual regression relation between ultrasonic pulse velocity (UPV) and rebound hammer (RH) measurements, an estimation expression is developed. In comparison to previously published estimation equations, the equation proposed in this study shows the highest accuracy and the lowest root mean square error (RMSE). For the estimation of compressive strength using concrete core specimens, three different concrete core diameters were examined: 30, 50, and 100 mm. Based on 61 measured compressive strengths of core specimens, a simple strength correction factor is investigated. The compressive strength of a concrete core specimen decreases as the core diameter reduces. Such a relation is associated with the internal damage of concrete cores and the degree of coarse aggregate within the core diameter from the extracting process of the cores. The strength estimation expressions was formulated using the non-destructive technique and the core strength estimation can be updated with further test results and utilized for the maintenance of NPP.

The paper describes the mechanical behavior of fine recycled concrete aggregate (FRCA) concrete according to the mineral admixtures. Three types of the mineral admixtures, i.e., fly ash (FA), ground-granulated blast-furnace slag (GGBS), and silica fume (SF), are used and the replacement ratios of FRCA are 50% and 100%. The dosages of the admixtures of FA, GGBS, and SF are determined with the normal dosage (30%, 40%, and 5.0%, respectively) based on the ACI committee reports (No. 232, 233, and 234) and half-normal dosage. The mechanical performance is investigated with the compressive and splitting tensile strength, and elastic modulus. Additionally, the total porosity is measured in natural fine aggregate (NFA) and FRCA 100% replaced specimens by mercury intrusion porosimetry (MIP) for investigating the relationship with the compressive strength. Based on the experimental test results, the mineral admixtures improve the mechanical performance of FRCA concrete. The effective dosages of FA, GGBS, and SF for FRCA concrete are investigated according to the replacement ratio of the FRCA. In particular, FRCA 100% replaced concrete may be possible to be used for the structural concrete members with the specific dosage of the mineral admixtures. The prediction of the splitting tensile strength and the elastic modulus by the codes or previous formulas exhibits underestimated and overestimated results, respectively. The relationship between the total porosity and the compressive strength of the FRCA concrete should be modified with more experimental tests.

This study is to investigate the predictability of the existing punching shear formulas and to evaluate the best predictable punching shear formula including probabilistic evaluation method including the level of 95% probability. The considered punching shear formulas are existing 10 formulas, which are based on the standard codes and newly developed and verified by experimental data. For FRP reinforced concrete slabs, it is more important that the probable uncertainty of predicting formulas for material characteristics, structural dimension, and modeling errors should be considered. The random variables for probabilistic analysis were simulated by Monte Carlo Simulation. The best predictable punching shear formula and its components were analyzed with two analysis methods; the test-to-predicted ratio of punching shear strength and the nominal-to-95% probability ratio which is the ratio of nominal punching shear strength to that at the level of 95% probability. From the test-to-predicted ratio analysis, the important effect of modulus of elasticity of the reinforcement for predicting formulas was found. As using the resulted formula, this study provides a preliminary approach for suggesting a new punching shear formula of concrete slabs reinforced by either FRP or steel bar.

Fiber reinforced polymer (FRP) rebar is a great alternative to the conventional steel reinforcing bar because of its corrosion-free characteristics and high strength. This study introduces a newly developed hybrid FRP rebar which is composed of a conventional steel rebar inside and glass fiber reinforced polymer (GFRP) on the outer surface. The developed hybrid rebar takes advantage of overcoming the low elastic modulus from their composite action and durability as the outer GFRP surface resists corrosion. In addition, this hybrid GFRP rebar is also cost-effective. This study investigated the bonding characteristics between the newly developed hybrid FRP rebar and concrete. A direct pull-out test was conducted following the KS F 2441 testing method. In the test result, the bond strength and slip relationship were investigated for test variables. The rebar outer diameter was 16 mm, and inside steel rebar diameters were 10 and 13 mm with varying thickness of GFRP.