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]. Hawkins and Ghosh (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 (
) 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 reduction factor for deep PHCS members specified in the design code. Lee et al. [
5] and Park et al. [
6] 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 [
16] can provide excessively conservative design results. In addition, Lee et al. [
5] 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. Brunesi and Nascimbene [
17] 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. Girhammar and Pajari [
18] 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. Nguyen et al. [
19] 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. Investigating the shear strengthening effect of the core-filling method is very important because it has been widely applied as a shear strengthening method of PHCSs in construction fields due to its simple working process [
12,
20,
21].
Table 1 shows the shear strength equations for reinforced concrete (RC) and prestressed concrete (PSC) members specified in the current design codes [
16,
22,
23]. In practice, the shear contribution of concrete (
) of the PHCS with core-filling concrete is often simply calculated as
, but in that case, the
could be overestimated.
Palmer and Schultz [
12] conducted web-shear tests on 400 mm thick PHCS strengthened by the core-filling method, and analyzed the web-shear strength equation presented in ACI 318-05 [
24]. Note that there is no provision for the additional strength reduction factor (0.5) in ACI 318-05. Based on their test results, Palmer and Schultz reported that when calculating the web-shear strength of PHCS with filled cores, the shear contribution of the filled cores should be reduced by as much as 50% of the shear strength calculated using a code equation, in order to obtain accurate analysis results. That is, it turned out that the filled cores made a partial contribution to the shear strength of the member but failed to achieve 100% of their shear capacity in the test. On the other hand, Hegger et al. [
25] performed shear tests of PHCSs applied in a slim floor system, in which two specimens were reinforced with core-filling concrete. In their tests, however, the shear strengths of the PHCSs did not increase by the core-filling concrete.
As mentioned above, previous researchers reported different test results on the shear strength of PHCS with filled cores, which is considered to be because of a very different bond condition, often poor, between the PHCS unit and filled cores. Thus, it is very hard to estimate the shear strengthening effect of the core-filling method. In structural design codes, such as ACI 318-14 [
16], CSA-A23.3-14 [
22], and Eurocode2 [
26], estimating the shear strength of PHCSs with core-filling concrete is not provided.
In this study, experimental research was carried out to investigate the shear resistance mechanism of PHCS strengthened by the core-filling method. A total of five PHCS specimens with a thickness of 400 mm was fabricated by using the extrusion method, in which a machine extrudes low-slump concrete and compacts it to form a hollowed section along the long-line prestressing bed. The main test variables were set to the number of filled cores and the shear reinforcement ratio. During the test, the member behavior and crack patterns of PHCS specimens were measured in detail.
After the test, the members were cut with a wire saw to observe the crack patterns that occurred in the filled cores, and to identify whether the filled cores effectively contributed to the shear resistance mechanism. Based on the test results, this study also examined whether the current design codes adequately evaluate the shear strengths of PHCS with filled cores, and proposed a modified equation to accurately estimate the shear strengths of PHCS members strengthened by the core-filling method.
The significance of this research is summarized, as follows:
Investigating the shear reinforcing effect of core-filling concrete.
Identifying the effects of shear reinforcement placed in core-filling concrete.
Investigating the composite action between the PHCS unit and core-filling concrete by observing their shear crack patterns.
Developing a simple equation to accurately estimate the shear strengths of PHCS reinforced by the core-filling method.
2. Test Program
In this study, five PHCS specimens with a thickness of 400 mm were fabricated as shown in
Table 2 and
Figure 1. The width of the PHCS unit (
) was 1200 mm, the width of one web (
) was 55.2 mm, the total width of the web (
) was 276 mm, and the void ratio was 56%. Eleven prestressing strands with a diameter of 12.7 mm were placed in the lower part of the section, while three 9.5 mm diameter strands were laid in the upper part of the section, to control the tensile stress generated during the introduction of prestress. As shown in
Table 2, the first character in the names of the specimen indicates a shear reinforcement ratio (
). NR is a specimen without shear reinforcement. LR means a specimen with a relatively low
(i.e.,
), while HR means a specimen with a relatively high
(i.e.,
). The number that follows is the ratio of the number of filled cores to the number of entire cores. For example, LR-2/4 represents a specimen in which two of the four voids are filled with normal strength concrete.
The tensile strength (
) of the prestressing strands used in the specimens was 1860 MPa, and the effective prestress introduced into the PHCS unit was approximately 1200 MPa, which was about 65% of
.
Figure 1b–d shows that the top flanges were opened, and in-filled concrete was casted in the specimens, except for the reference specimen (NR-0/4). In addition, as shown in
Figure 2, the core-filling concrete was placed for the position of the section, which is up to 1500 mm away from both ends of the member.
Table 3 shows the mix proportions of the PHCS unit and the core-filling concrete. The concrete compressive strengths of the PHCS unit (
) and filled cores (
) were 66.0 and 25.1 MPa, respectively, which were measured from cylinder tests.
Figure 3 shows the details of helix shear reinforcement in LR-2/4 and HR-2/4 specimens. The amount of shear reinforcement was determined to be more than the minimum shear reinforcement ratio (
) specified in ACI 318-14 [
16], where
was calculated as below:
where,
is the yield strength of the shear reinforcement, which was 400 MPa. The minimum shear reinforcement ratio (
) calculated using Equation (1) was 0.127%, and the diameters of shear reinforcements placed in LR-2/4 and HR-2/4 specimens were 8 and 12 mm, which correspond to the reinforcement ratios (
) 0.175% and 0.395%, respectively.
Figure 4 shows the specimen loading details. In this study, the length (
) of the PHCS specimen was 7000 mm, and shear tests were conducted on both ends of the member. In other words, two shear test results were obtained for each specimen. In the shear test, the lengths of the left and right shear spans were set differently to induce shear failure in the transfer length region. The shear span-to-depth ratio (
) was 2.78, and a concentrated load was applied on the top of the specimens at 1000 mm away from the inner end of the supporting plate with a loading rate of 1.0 mm/min. In order to measure the deflection of the specimens, a linear variable differential transformer (LVDT) was installed at the bottom of the section located at the loading point. In addition, the shear contributions of the helix reinforcements in LR-2/4 and HR-2/4 specimens were measured from the strain gauges. As shown in
Figure 5, the gauge attachment locations were determined by considering three crack patterns that can occur in the shear span [
4,
22,
26,
27]: (1) the straight line that makes an angle of 35° to the member axis from the support; (2) the straight line that connects from the support to the loading point; and (3) the straight line from the bottom of the critical section, which is distance
from the support, to the loading point [
22].