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European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Shear Strengthening of Continuous RC T-Beams Using Deep Embedment Technique

Vesna Raicic

The University of Bath

 

Overview

The historical, economic and social importance of strengthening and rehabilitation of reinforced concrete (RC) structures has become an important part of civil engineering. It is not only the need to protect our built heritage but also the demand to strengthen significant numbers of ‘new’ concrete structures. Gold and Martin (1999) emphasised the significant redundant building space in the UK, much of which was constructed in the 1960s and 1970s. Together with these buildings, those from 1980s, 1990s and pre-and-post World War II need to be adapted to meet the requirements of the 21st century. A great number of public and residential buildings, along with infrastructure, need structural strengthening in order to meet changes in society and as a response to upgrading of design standards.

A lack of structural ductility can lead to a brittle and catastrophic failure of a structure. Dissimilar to old codes that didn’t have strict rules for increasing ductility (irregular stirrups, spacing and lack of concrete cover), current codes require a high amount of shear reinforcement. Fibre-reinforced polymer (FRP) materials are well established for such structural strengthening. Shear failure of reinforced concrete (RC) is very dangerous and occurs suddenly with no advance warning. Significant numbers of research findings have confirmed that shear capacity of RC beams can be significantly improved by using the Externally Bonded technique (EB) (Triantafillou 1998; Khalifa et al. 1998; Pellegrino and Modena 2002; Chaallal et al. 2011) or the Near Surface Mounted (NSM) technique (De Lorenzis and Nanni 2001; Barros and Dias 2005; Chaallal et al. 2011). International and national guidelines and codes (Concrete Society Technical Report 55, ACI, etc.) give design tools for FRP reinforcement on the basis of experimental research. However, despite numerous studies, shear behaviour before or after strengthening is still not fully understood, particularly in continuous concrete structures, which are the norm, as there are very few simply supported concrete structures, in reality.

While Externally Bonded Reinforcement (EBR) or Near Surface Mounted (NSM) techniques are used widely for flexural strengthening, upgrading shear resistance is altogether more difficult. In practice, beams are frequently cast monolithically with the top slab in which case full FRP wrapping to help resist shear is not a feasible option. So, U-wrapping or side-wrapping is sometimes used. However, these methods usually do not allow the FRP material to be anchored into the compression zone, with the consequence that truss action cannot be mobilised.

 Fig.1 - Representative Image

 

 

Aims and Objectives

The present work aims to contribute to a deeper understanding of the complex shear mechanisms in continuous RC beams strengthened using the Deep Embedment technique. The eventual aspiration of this work is to produce valuable data for formulation of design guidelines on shear strengthening of these beams. In order to meet this demand, a profound understanding of mechanical behaviour of strengthened systems is essential as well as the ability to identify the most appropriate strategy for structural analysis of this complex problem.

This gives the following key research objectives:

  • To determine effective FRP/steel bars contribution to overall shear capacity in situations of continuity where the maximum shear force is accompanied by the maximum bending moment,  observing the influence of the distance between the bars and the angle of their insertion; and
  • To deliver robust, mechanics-based design code formulations for FRP shear strengthened RC structures.

Concerning the above motivation, this work will also strive to evaluate previous research work done on strengthening of continuous RC beams in shear. It will thoroughly study the influences that affect the behaviour of such strengthened systems.

 

Methodology and Results

In order to fulfil the research objectives a targeted experimental compaign has been developed. The work is divided into three linked stages focused on different aspects of the study. In the first stage of the research programme shear capacity and failure mechanisms of reinforced concrete push-off specimens strengthened with deep embedded FRP/Steel bars across a shear plane are investigated. An emphasis is put on the minimum bar anchorage lengths required for effective shear transfer. The second stage will be concerned with global continuous T-beam performance, investigating the influence of the type, angle of insertion and distance between FRP/steel bars. In the final stage, mechanics-based design models will be developed based on the experimental work carried out in the first two stages.

Push-off specimens

Attempts have been made by many previous researchers to estimate the shear capacity of a beam using various analytical, experimental and numerical methods. One of the most widely used and accepted theories used to predict shear capacity of concrete elements is the shear-friction theory and its modifications (Nielsen, 1984).

Since DE is a fairly new technique, more research needs to be conducted on the behaviour of deep embedded bars and the extent to which these bars are capable of transferring shear forces across a known shear plane. In order to do that, it is necessary to isolate parameters that can affect this shear-strengthening system. Birkeland and Birkeland (1966) carried out tests on initially uncracked steel reinforced push-off specimens to develop a shear friction hypothesis. This approach was further developed by Hofbeck et al. (1969), Mattock et al. (2001) and Walraven (1981) on both cracked and uncracked specimens. Investigation of shear capacity of concrete reinforced with FRP materials has also been done in order to determine its efficiency, (Ibell and Burgoyne, 1999). More recently, Grusova et al. (2013) have studied the effectiveness of FRP sheets on the resistance to shear in steel reinforced concrete by using push-off specimens. Therefore, it is of great importance to deepen our understanding of failure mechanisms in DE strengthened steel reinforced concrete as well as the contribution of FRP/steel bars to the total shear friction capacity.

For this purpose, ten initially uncracked push-off specimens were cast in the Laboratory of the University of Bath, see Fig. 2. They were classified into four categories depending on the type, anchorage length and angle of insertion of the bars. An emphasis was placed on the minimum bar anchorage lengths required for effective shear stress transfer, see Fig.3. Assuming that the shear discontinuity develops at an angle of approximately 450 with respect to the horizontal beam axis, the 450 and 900 push-off geometries were chosen to investigate vertical and inclined Deep Embedded bars, respectively, see Fig.4.

 Fig.2 - Reinforcement of the specimens and their appearance after casting

 

 Fig.3 - Strengthened specimens

 

 Fig.4 - Vertical and inclined DE bars

 

Ultimate capacities Pu obtained from the tests are presented in the Table 1. Further analysis of the data is in progress and will be presented in the next report.

 Table 1 - Ultimate capacities of push-off specimens

 

March 2016

Continuous reinforced concrete T-beams

The behaviour of FRP shear reinforcement in the negative moment region in continuous structures is not clear yet. Continuous RC beams behave differently from simply supported beams, as large shear forces co-exist with large negative bending moments at the same location. It is demonstrated in Fig. 5 where on the side right of the point load maximum moment and maximum shear force act simultaneously.

 Fig.5 - Bending moments (M) and shear forces (V) in continuous beam

 

An alternative strategy for shear strengthening of reinforced concrete beams with FRP/steel bars, Deep Embedment (DE) or Embedded Through Section (ETS) has been developed (Valerio and Ibell, 2003; Valerio et al. 2009; Chaallal et al. 2011; Mofidi et al. 2012). In this technique, DE bars are epoxy bonded into previously drilled holes (vertical or inclined) through the cross section of the RC beams. In this way, tension and compression chords are directly connected and bond between FRP bars and concrete is much better, which makes this technique superior in comparison with Externally Bonded (EB) and Near Surface Mounted (NSM) methods (Valerio et al. 2009), (Fig. 6).

 Fig.6 - Deep Embedment technique

 

Nine full-scale two-span continuous beams strengthened with DE bars and one unstrengthened control beam will be tested to failure. Dimensions of the specimens were adopted through consideration of average geometry of typical continuous reinforced concrete beams in buildings, cast monolithically with the top slab. All test specimens have the same dimensions and internal reinforcement as shown in Fig. 7 (a) and (b). The specimens are 3840 mm long (L) with spans of 2400 mm (L1) and 1290 mm (L2). The flange width (bf) and total height of the vertical cross section (h) are 350 mm. The thickness of the web (bw) and the flange (hf) are 150 mm and 100 mm, respectively. The effective depth (d) is 320 mm. The shear span-to-effective depth ratio (a/d) is 3.0.

 Fig.7 - Longitudinal cross section (a) and vertical cross section (b)

 

The beams are divided into three groups depending on the type of the strengthening bars used, which could be CFRP, GFRP or STEEL (Table 2). The control specimen, not strengthened with Deep Embedded bars, is labelled as CON. Key parameters are type of DE bars, their spacing and the angle of their insertion. Each group consists of three beams strengthened in the shear zone using three different configurations: a) vertical bars spaced at 150 mm (one bar between two shear links), b) vertical bars spaced at 75 mm (two bars between two shear links) and c) inclined bars (450) at 150 mm (each bar crossing two shear links). DE bars used in this study are all of diameter 6 mm. In addition, strengthening configurations are illustrated in Table 3.

 Table 2 - Beam groups and strengthening schemes

 

 Table 3 - Strengthening configurations of groups I, II and III

 

Based on the currently accepted approach, predictions of the shear capacity of strengthened beams start from the assumption that the ultimate shear capacity (Vu) is equal to the sum of the shear capacity contributions of the concrete (Vc), steel (Vs) and DE bars (VDE) (Eq. 1).

Although significant questions exist over the validity of this equation (Grusova et al. 2013), it is adopted as the starting point in this research, for further investigation to be carried out later. A simplified methodology developed in the work of Valerio and Ibell (2003) and Valerio et al. (2009) was adopted in order to calculate the contributions of Deep Embedded bars to the shear resistance of the beams. This methodology was based on a conservative 450 truss analogy and was used to predict the increase in shear capacity due to the strengthening bars (Equations 2, 3 and 4).

Where: s is the bar spacing, σ_f is the stress limit for the bars, z is the effective lever arm, Af is the cross sectional area of each bar, h is the height of the beam, l_(b,max) is the required anchorage length at each end of the embedded bars, Ef is the Elastic modulus of the FRP bar, ε_f=0.004 (strain limit in FRP bars), and τ_b is the bond stress. The stress limit σ_f is equal to the yield strength of the steel bars or to the product εf Ef for the FRP bars, and the value of the bond stress is taken equal to 20MPa (as determined in the work of Valerio et al. 2009). Table 4 summarises the predicted shear capacities of the beams. Shear capacity of the control specimen (CON) is predicted to be 84 kN. Cp represents increased shear capacity of control specimen by contribution of DE bars in percentages.

 Table 4 - Predicted shear capacities of the beams

 

Tests results will be presented in the next report.

 

June 2016

Push-off specimens

Table 5 shows that strengthened specimens reached significant increase in ultimate capacity in comparison with the control specimen. The average increase in ultimate capacity reached around 50%. Deep Embedded bars significantly improved the capacity of specimens even in no presence of shear links across the shear plane. Increasing the DE bar anchorage length resulted in a higher contribution to shear resistance.

Characteristic crack patterns are shown in the Figure 8.

 Table 5 - Ultimate capacities of push-off specimens

 

 Fig.7 - Longitudinal cross section (a) and vertical cross section (b)

 

Figure 9 shows the curves for applied load versus maximum displacement at the point load for the control and strengthened specimens. They represent typical behaviour during a push-off test.

 

 Fig.9 - Load vs displacement for Group I (A), Group II (B), Group III (C) and Group IV (D)

 

Continuous reinforced concrete T-beams

Table 6 summarizes the experimental results obtained from the tests for all the test groups. Shear-strengthened beams experienced significant increase in capacity with respect to the control beam. Deep Embedment strengthening system significantly enhanced the shear capacity of RC beams even in presence of a minimum amount of transverse steel reinforcement. 

 Table 6 - Description of test specimens

 

Figure 10 shows the curves for applied load versus maximum displacement at the point load for the control and strengthened beams.

 

 Fig.9 - Load vs displacement for Group I (A), Group II (B) and Group III (C)

 

Further analysis of the data is in progress and will be presented in the next report.

 

Dissemination Activities

  • Abstract published at the proceedings of 17th Young Researchers Conference (14/04/2015 Institution of Structural Engineers)
  • Poster presented at the COST Action TU1207 meeting (22-24 October 2014, TU Kaiserslautern, Germany)
  • Poster presented at endure Mid Term Review Meeting (29th June-3rd July 2015, Empa – Swiss Federal Laboratories for Materials and Technology, Dübendorf, Switzerland)
  • SMAR2015 - The Third Conference on Smart Monitoring, Assessment and Rehabilitation of Structures, Antalya, Turkey. Participation in the special session: Presentation competition for Early Stage Researchers.
  • Seminar presentation of the research project at the Department of Architecture and Civil Engineering, University of Bath
  • Paper excepted at the 11th fib International PhD Symposium in Civil Engineering Aug 29 to 31, 2016, The University of Tokyo, Tokyo, Japan
  • Paper excepted The 8th International Conference on Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2016)
  • Oral presentation at Young Researcher's Forum III (12 Apr 2016 ) - Innovation in Construction Materials, Department of Civil and Environmental Engineering, Imperial College London
  • Poster presented at the COST Action TU1207 meeting (04-06 April 2016, University of Technology (TUL), Lodz, Poland)

Attendance at meetings

  • COST Action TU1207 meeting (Working Groups Progress Meeting, Management Committee Meeting: 22-24 October 2014, TU Kaiserslautern)
  • endure meeting 4 and training school (26-30 January 2015, University of Ghent, Belgium)
  • CIV6170/4170 - FRP Composites in Construction – training course 27 April - 14 May 2015, University of Sheffield
  • COST Action TU1207 Meeting (General meeting - Next Generation Design Guidelines for Composites in Construction: 19-21 May, Università degli Studi del Salento, Lecce, Italy
  • endure Mid Term Review Meeting (29th June-3rd July 2015, Empa – Swiss Federal Laboratories for Materials and Technology, Dübendorf, Switzerland)
  • COST Action TU1207 - Next Generation Design Guidelines for Composites in Construction (GENERAL MEETING, Discussions: Testing, Analysis & Design, Management Committee Meeting, ESR meeting) -Universitat Politècnica de Catalunya, School of Civil Engineering, Barcelona, Spain
  • fib 32. meeting (The International Federation for Structural Concrete) - Task Group 5.1, FRP (Fibre Reinforced Polymer) reinforcement for concrete structures, Universitat Politècnica de Catalunya, School of Civil Engineering, Barcelona, Spain
  • COST Action TU1207 - Next Generation Design Guidelines for Composites in Construction (GENERAL MEETING, Textile Reinforced Mortars, Industry Seminar, Management Committee Meeting, ESR meeting) –  from 04/04/2016 to 06/04/2016 Event Place: Sala Widowiskowo-Konferencyjna, Campus B al. Politechniki 3a, University of Technology (TUL), Lodz, Poland
  • fib 33. meeting (The International Federation for Structural Concrete) - Task Group 5.1, FRP (Fibre Reinforced Polymer) reinforcement for concrete structures, 6th April 2016, University of Technology (TUL), Lodz, Poland