# Shear Tests of Deep Hollow Core Slabs Strengthened by Core-Filling
## 1 Introduction  
Prestressed hollow core slabs (PHCSs) can reduce self-weight due to the presence of voids in the section, and because they are prefabricated in a factory as precast members, ensure excellent quality and reduced construction duration [1; 2; 3; 4]. In addition, PHCSs exhibit high flexural strength and excellent deflection control performance because prestressing strands are placed in the bottom flange of the member, and consequently they are widely applied to long-span structures [5; 6; 7; 8]. However, due to the characteristics of the PHCS, which has a large void ratio, the web is very thin, and the prestress is not fully effective in the transfer length region which results in a lack of web-shear strength at the member ends subjected to high shear forces [9; 10; 11; 12; 13; 14]. [PERSON] and [PERSON] (2006) [13] reported that the thicker the depth of the PHCS, the lower the web-shear strength, and thus an additional strength reduction factor should be applied to ensure adequate safety for PHCS with a thickness greater than 315 mm. The ACI 318-08 [15] code, reflecting their findings, stipulates that the web-shear strength of PHCS with a thickness of more than 315 mm without minimum shear reinforcement should be reduced to half of the design shear strength (\(\phi V_{cw}\)) calculated using a code equation, and this also applies to the ACI 318-14 [16] code. In particular, since shear reinforcement cannot be placed in PHCS manufactured by using the extrusion method, the case of deep PHCS with a thickness greater than 315 mm can lead to very uneconomic design results. Many previous researchers [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14] have made various attempts to increase web-shear strength without using shear reinforcement, and examined the rationality of the additional strength reductionfactor for deep PHCS members specified in the design code. [PERSON] et al. (2018) and [PERSON] et al. (2019) collected many web-shear test results on PHCS, based on which they pointed out that the additional strength reduction factor (0.5) specified in ACI 318-14 ([PERSON] et al., 2019) can provide excessively conservative design results. In addition, [PERSON] et al. (2018) performed a regression analysis and suggested a simplified equation to accurately estimate the web-shear strength of PHCS, regardless of the thickness of the member, even if the additional strength reduction factor is not taken into account. [PERSON] and [PERSON] (2018) conducted finite element (FE) analysis to identify the shear stress distributions and crack patterns of PHCSs according to member heights and section details including void shapes. From their analysis results, it is found that the shear stress distribution is sensitive with respect to the section shapes of PHCSs, and it is adequate to apply the fracture mechanics approach for the analysis of shear crack propagation. [PERSON] and [PERSON] (2018) carried out pull-off and shear tests of PHCS composites with topping concrete, based on which they analyzed the shear strengthening effect of topping concrete considering the bond strength between the PHCS unit and the topping concrete. In their analysis, the shear strength of the PHCS with a thickness of 200 mm increased up to 35% when topping concrete was cast with a thickness of 80 mm, and it matched well with the test results. [PERSON] et al. (2019) reported shear test results of four deep PHCSs with heights ranging from 320 to 500 mm and established FE model to simulate their shear behavior. However, the results of FE analysis were very sensitive, depending on the dilation angle used in their concrete damage plasticity model. In overall, there are many empirical and numerical approaches to accurately estimate the shear behavior and strength of PHCSs, and they contributed to the better understanding of the shear resistance mechanism of PHCSs. However, a few studies have been conducted on the shear behavior of PHCSs strengthened by core-filling concrete.