الفهرس 
LITERATURE REVIEWOnly 14 pages are availabe for public view
Abstract
The main objective of this research was to evaluate the behavior of punching shear of UHSC flat slabs (with and without shear reinforcement) subjected to centric and eccentric loading. Also to study the efficiency application of using UHSC as a DROP panel for strengthening NSC flat slabs to punching shear under centric and eccentric loading.
The experimental program was carried out by dividing the specimens in two groups. In the first group (group A), six slabs were tested under centric loading. In the second group (group B), four slabs were tested under eccentric loading. The variables included in the research work for two groups were the effect of using UHSC with and without shear reinforcement (shear studs) on punching shear behavior and for strengthened specimens; the variable parameter was using a different thickness of UHSC DROP panel for strengthening NSC slabs to punching shear.
The predicted values of the punching shear capacity for the experimental program slabs studying the behavior of using the UHSC slabs were derived using different codes (ACI, CEB-FIB model 90, ECP and CSA). A comparison between the values of punching shear capacity obtained by the experimental results and the values predicted were carried out. It concluded that CEB-FIB model 90 is the best model to calculate the prediction of punching shear strength of UHSC slab.
In addition to the experimental work, numerical analysis using nonlinear software finite element program (ANSYS v.12) was constructed for all specimens on both groups to study the compatibility in behavior between experimental and numerical analysis. ANSYS models were constructed to simulate the tested specimens for centric and eccentric loading.
A parametric study with total 96 models was constructed to get more insight study of all parameters as illustrated previously.
7.2 Conclusion
Based on the results obtained from experimental, numerical investigations and parametric study the following main conclusions can be drawn:
1 – Using UHSC has significant effect in increasing the punching shear strength capacity. For the UHSC specimens subjected to punching shear under centric and eccentric loading, the ultimate loads were increased by 87% and 72%, respectively, compared to the NSC specimens.
2 – Using UHSC has significant effect in increasing the initial cracking load. For the UHSC specimens subjected to punching shear under centric and eccentric loading, the cracking load was increased by 155% and 111%, respectively, compared to the NSC specimens. This behavior is related to higher tensile strength and modulus of elasticity of UHSC compared to the NSC specimens.
3 – Using UHSC has inverse effect in the ductility. The ductility factor of the UHSC specimens subjected to punching shear under centric and eccentric loading was decreased by 18% and 21%, respectively, compared to the NSC specimens. This behavior led to the rupture being more brittle when compared with the NSC specimens.
4– The UHSC specimens with shear reinforcement (shear studs) subjected to centric and eccentric loading gave punching shear capacity by a ratio of 11% and 9%, respectively which is higher than the UHSC specimens without shear reinforcement. Also the UHSC specimens with shear reinforcement improved the ductility factor by a ratio of 16.5% and 19, respectively, which is higher than the UHSC specimens without shear reinforcement. from pervious results, it is concluded that providing shear reinforcement to the UHSC specimens had a slight effect in improving the punching shear capacity, however, it improved the ductility of UHSC slabs.
5– The final shape of the punching cone is completed after the column stub starts to penetrate through the slab.
6– Crack pattern of all UHSC specimens with shear reinforcement did not change the shape of punching failure on the tension face but shifted the failure surface away from the column face.
7– NSC specimens which were strengthened by adding different thicknesses UHSC DROP panel, resulted in a significant increase in the punching shear strength capacity compared to the original NSC specimens. This behavior is due to the increase of perimeter of loading and the high stiffness of the whole section.
8 – Crack pattern of all strengthened NSC specimens did not change the shape of punching failure surface on the tension face but shifted the failure surface away from the column face beyond the perimeter of DROP panel.
9 – Based on observations from the experimental work, we noticed that the UHSC specimens subjected to centric loading, the critical shear perimeter was at 2.5d distance from the face of the column, and thus the critical shear perimeter can be defined by: bo = 4c + 5 π d. For UHSC specimens subjected to eccentric loading, the critical shear perimeter was at 2.5d distance from the face of the column, and thus can be defined by: bo = 4c + 2.5 π d.
10 – A comparison was carried out between test results and the predicted values was conducted based on a reference provisions from ACI, CEB-FIB model 90, ECP and CSA. It concluded that CEB-FIB model 90 is the best model to calculate and predicted punching shear strength of UHSC slab.
11 – A numerical model using FEM (ANSYS ®12) is presented. The simulated models showed a good agreement prediction with the test results. For both of UHSC slabs (with and without shear reinforcement) and the strengthened NSC slabs , showed different results for punching shear capacity between experimental results and the predicted values which were obtained from the numerical models ranged from - 6% to +16%. Other variables which were observed at the experimental program -maximum deflection under the column, maximum strain in the flexure steel and maximum strain in the shear studs- showed a good agreement with prediction values of the simulated models.
12 – Some variables which were not covered in the experimental program that effects on the punching shear capacity were demonstrated by a theoretical study (parametric study).
13 – Based on the model results of the parametric study, it can be concluded that the increase of concrete compressive strength caused an increase in concrete punching strength. It is also noticed that the rate of increase in punching strength is decreased with increasing the compressive strength especially for slabs with a compressive strength over 130 MPa.
14 – Based on the parametric study, it can be concluded that using UHSC DROP panel for strengthening NSC slabs had a significant effect in increasing the punching shear capacity to the whole section, for DROP panel thickness ranging from 0.50 to 0.75 d. If we went above this value of thickness (0.75d), increasing concrete strength would not be effective because the punching failure would occur outside the limits of the DROP panel. Therefore, we conclude that we must perform an accurate calculation to choose the ideal thickness of UHSC DROP panel to ensure we get the best result without extra cost.
15 – Based on the parametric study, the proposed equation used to predict the nominal punching strength without and with shear reinforcement of normal and ultra-high strength concrete, showed consistent results with ANSYS results and experimental results.
16 – For the UHSC DROP panel- strengthened NSC slab, The proposed equation used to predict the required thickness of UHSC DROP panel, showed consistent results with ANSYS results.
7.3 Recommendations
For future works, it is recommended to:
1. Additional testing on UHSC slab should be performed with extra variations in the following parameters, such as slab thickness, slab aspect ratio, loading area, loading plate aspect ratio, dynamic effects, restraint conditions fiber volume and contribution of tensile reinforcement.
2. Study the effect of FRP bars and FRP mesh laminate on punching shear behavior of UHPC slabs.
3. Application of UHSC as a composite concrete section for repairing and strengthening of the NSC ordinary flexural elements.
4. Study the behavior of UHSC slabs exposed to fire.
5. Study the seismic behavior of UHSC slab-column connections.