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Structural Analysis of Strengthened RC Slabs

Davvari, Mohammadtaher

[Thesis]. Manchester, UK: The University of Manchester; 2018.

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Abstract

In this thesis, the experimental programmes and numerical investigations are described that have been conducted to partially cover the knowledge gap in the field of strengthening two-way reinforced concrete (RC) flat slabs. The conducted studies demonstrate that the most common method to strengthen two-way RC slabs is by applying fibre reinforced polymers (FRP) on the tension surface of the slabs. Applying prestressed FRP to strengthen two-way flat slabs combines the advantages of both FRP strengthening and prestressing to enhance the efficiency of the strengthening methods. Hence, two previous studies on strengthening two-way flat slabs with non-prestressed and prestressed FRP are analysed to clarify the effect of different strengthening methods on the behaviour of slabs. Both studies demonstrate the benefits of applying non-prestressed FRP to enhance the structures’ capacities. However, for the case of strengthening RC slabs with prestressed FRP, the results seem to be controversial and more studies are necessary to arrive at a conclusion on whether it is feasible to strengthen RC slabs with prestressed FRP. Further analysis indicates that there is an optimum percentage of prestressing for the FRPs applied to RC slabs. Increasing the prestressing ratio of FRP to the optimal percentage increases the ultimate load capacity of the RC slabs. However, increasing the prestressing ratio of FRPs beyond the optimum value can cause de-bonding and loss of composite action, which prevents the RC slab from reaching its expected ultimate load capacity. The optimum prestressing ratio of FRP depends on the prestress load as well as the concrete tensile strength and slab depth. Eventually, a formula is proposed to estimate the optimum FRP-prestress ratio considering the effective parameters in both concrete and FRP. Moreover, this thesis elaborates on an investigation that was conducted to make a comparison between the effects of different strengthening methods such as FRP strengthening, applying vertical (shear) reinforcement, and their combination, on the behaviour of flat slabs with different conditions (tensile reinforcement ratios). To conduct the investigation, eight slab specimens were cast, which were classified into two categories: low and high tensile reinforcement ratios. The strengthening methods included applying FRP sheets to the tension surface of the RC slabs externally, applying vertical (shear) reinforcement, and a combination of both methods. The experimental and validated numerical results demonstrate that the most efficient strengthening strategy is a combination of strengthening methods in both categories. Strengthening with FRP sheets improves the slabs load capacity in both categories. However, applying vertical (shear) reinforcement does not significantly affect the behaviour of RC slabs with a low tensile reinforcement ratio. From the resulting analyses, it was concluded that the strut and tie model of the FRP strengthened structure changes compared with the control specimen. This enables researchers and designers to justify how FRP strengthening enhances the punching strength of the slab, an aspect that has not been explained in previous studies. The results also show that applying vertical (shear) reinforcement in the critical punching area strengthens the critical compressive strut of the RC slab. This shifts the critical punching area from the column vicinity to the outside of the shear reinforced zone and enhances the RC slabs load capacity. A comprehensive parametric study using calibrated finite element models has also been conducted to analyse the effect of varying different parameters such as tensile reinforcement ratio and compressive reinforcement as well as the pattern, number and thickness of FRP strips (applied to strengthen RC slabs) on the behaviour of flat slabs. The results demonstrated that enhancing the tensile reinforcement ratios (including both steel reinforcements and FRP strips) can enhance the ultimate load capacity of the strengthened slabs, but reduces the ductility of the structure. Based on the results, flat slabs with compressive reinforcements could reach more load capacity and deflection (which resulted in having more ductility) as compared with the samples that do not include compressive reinforcements.

Bibliographic metadata

Type of resource:
Content type:
Administered thesis
Form of thesis:
Traditional
Type of submission:
Doctoral level ETD - final
Thesis title:
Structural Analysis of Strengthened RC Slabs
Degree type:
Doctor of Philosophy
Degree programme:
PhD Civil Engineering
Publication date:
2018-07-18T11:39:49
Institution:
The University of Manchester
Location:
Manchester, UK
Total pages:
219
Abstract:
In this thesis, the experimental programmes and numerical investigations are described that have been conducted to partially cover the knowledge gap in the field of strengthening two-way reinforced concrete (RC) flat slabs. The conducted studies demonstrate that the most common method to strengthen two-way RC slabs is by applying fibre reinforced polymers (FRP) on the tension surface of the slabs. Applying prestressed FRP to strengthen two-way flat slabs combines the advantages of both FRP strengthening and prestressing to enhance the efficiency of the strengthening methods. Hence, two previous studies on strengthening two-way flat slabs with non-prestressed and prestressed FRP are analysed to clarify the effect of different strengthening methods on the behaviour of slabs. Both studies demonstrate the benefits of applying non-prestressed FRP to enhance the structures’ capacities. However, for the case of strengthening RC slabs with prestressed FRP, the results seem to be controversial and more studies are necessary to arrive at a conclusion on whether it is feasible to strengthen RC slabs with prestressed FRP. Further analysis indicates that there is an optimum percentage of prestressing for the FRPs applied to RC slabs. Increasing the prestressing ratio of FRP to the optimal percentage increases the ultimate load capacity of the RC slabs. However, increasing the prestressing ratio of FRPs beyond the optimum value can cause de-bonding and loss of composite action, which prevents the RC slab from reaching its expected ultimate load capacity. The optimum prestressing ratio of FRP depends on the prestress load as well as the concrete tensile strength and slab depth. Eventually, a formula is proposed to estimate the optimum FRP-prestress ratio considering the effective parameters in both concrete and FRP. Moreover, this thesis elaborates on an investigation that was conducted to make a comparison between the effects of different strengthening methods such as FRP strengthening, applying vertical (shear) reinforcement, and their combination, on the behaviour of flat slabs with different conditions (tensile reinforcement ratios). To conduct the investigation, eight slab specimens were cast, which were classified into two categories: low and high tensile reinforcement ratios. The strengthening methods included applying FRP sheets to the tension surface of the RC slabs externally, applying vertical (shear) reinforcement, and a combination of both methods. The experimental and validated numerical results demonstrate that the most efficient strengthening strategy is a combination of strengthening methods in both categories. Strengthening with FRP sheets improves the slabs load capacity in both categories. However, applying vertical (shear) reinforcement does not significantly affect the behaviour of RC slabs with a low tensile reinforcement ratio. From the resulting analyses, it was concluded that the strut and tie model of the FRP strengthened structure changes compared with the control specimen. This enables researchers and designers to justify how FRP strengthening enhances the punching strength of the slab, an aspect that has not been explained in previous studies. The results also show that applying vertical (shear) reinforcement in the critical punching area strengthens the critical compressive strut of the RC slab. This shifts the critical punching area from the column vicinity to the outside of the shear reinforced zone and enhances the RC slabs load capacity. A comprehensive parametric study using calibrated finite element models has also been conducted to analyse the effect of varying different parameters such as tensile reinforcement ratio and compressive reinforcement as well as the pattern, number and thickness of FRP strips (applied to strengthen RC slabs) on the behaviour of flat slabs. The results demonstrated that enhancing the tensile reinforcement ratios (including both steel reinforcements and FRP strips) can enhance the ultimate load capacity of the strengthened slabs, but reduces the ductility of the structure. Based on the results, flat slabs with compressive reinforcements could reach more load capacity and deflection (which resulted in having more ductility) as compared with the samples that do not include compressive reinforcements.
Keyword(s):
Thesis main supervisor(s):
Thesis co-supervisor(s):
Degree grantor:
Language:
en

Record metadata

Manchester eScholar ID:
uk-ac-man-scw:315276
Created by:
Davvari, Mohammadtaher
Created:
18th July, 2018, 10:39:49
Last modified by:
Davvari, Mohammadtaher
Last modified:
14th August, 2019, 10:44:13

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