Rehabilitation of Reinforced Concrete Deep Beams Using Carbon Fiber Reinforced Polymers ( CFRP )

Reinforced concrete structures that incorporates deep beams are generally susceptible to deterioration due to weathering effects and sulphur attacks, under-design in the detailing of concrete cover and/or reinforcement, and construction errors. In lieu of demolishing and replacing these structures, rehabilitation and strengthening using carbon fiber composites becomes a cost-effective v iable alternative. Recent advances in research and innovation have introduced concrete repair and strengthening systems that are primarily based on fiber reinforced polymer composites. These systems have offered engineers the opportunity to provide additional stability to the structural elements in question and to restore the damaged portions back to their original load carrying capacity. This paper investigates the effect of Carbon Fiber Reinforced Polymer (CFRP) composites in enhancing the flexural performance of damaged rein forced concrete deep beams. Two types of CFRP composites and epoxy were used in the experimental investigation carried out and as described by this paper: 1) h igh strength carbon fiber reinforced polymer (CFRP) plates, commercially known as MBrace Laminate, that are bonded using an epoxy resin specifically suited for the installation and used to strengthen existing structural members; and, 2) MBrace Fiber 230/4900, a 100% solids, low viscosity epoxy material that is used to encapsulate MBrace carbon, glass, and aramid fiber fabrics so that when it cures, it provides a high performance FRP sheet. Test samples were div ided into four groups: A control group, and three rehabilitated test groups with CRFP fibers, where the main variab le among them was the percent length of CRFP used along the bottom beam ext reme surface between supports (i.e, for two of the groups reinforced with MBrace laminates), and the use of MBrace Fiber 230/4500 CRFP sheets on the 4th beam along its vertical sides as well as the bottom ext reme face between supports. All beams had similar cross -sectional dimensions and reinforcement, and were designed to fail in flexure rather than shear. The results show that CFRP composites, both laminated and sheet type, have increased the load carrying capacity in comparison to the control specimen, where observations were recorded pertaining to the delayed formation of vertical flexural cracks at the section of maximum moment, and diagonal shear cracks at beam ends. The increase in the load carrying capacity varied among the three rehabilitated test group beams, with the 4 th group showing the highest ultimate load carrying capacity when compared to the control specimen.


Introduction
Reinforced Concrete Deep Beams (RCDBs) are considered one of the most important structural elements in civil engineering practice and are widely used in different types of structures, such as tall buildings, offshore structures, foundations, bridges, transfer girders, and shear walls (A l-Sarraf et al., 2011;Attarde and Barbat, 2015;Attarde and Parbat, 2016;Kong, 2006;and Suresh and Kulkarni, 2016).
Deep beams are popular for the structural horizontal elements of long spans where the number of intermediate supports are limited by architectural or navigational reasons.Because of the low span to depth ratio of deep beams when co mpared to normal flexura l elements, Hooke's Law wh ich states that plain sections remain p lain before and after bending no longer holds, and their analysis, design, thereby further complicating design and detailing.As a result, the significance of shear deformat ions in deep beams become mo re pronounced when compared to their shallow counterparts (Al-Sarraf et al., 2011;Attarde and Parbat, 2016;and Nawy, 2009).
For deep beams built outdoors, deterioration and loss of member carrying capacity can result fro m exposure to detrimental environ mental conditions during their service life, under-design, incorrect construction practices, or even the desire to increase the load carrying capacity of a perfectly sound structural system (Dav id et al., 1998).Cracking and spalling of the concrete cover is considered one of the most primary reasons for the deterioration phenomenon, as cracking and spalling will cause loss in carrying capacity as well as allow fo r corrosion and exposure to adverse environmental conditions long term (Ahmad, et al., 2012;and Kim and Yun, 2011).
Several techniques have been reported in the technical literature, including the injection of poly mer modified mortar (PMM) into the cracks in normal beams (Ahmad et al, 2012), or, in the case of deep beams, the use of external carbon fiber reinforced poly mer (CFRP) (Obaidat et al, 2011), as well as the use of unbonded post-tensioned carbon fiber reinforced polymer rods (Burningham et al, 2015).
For deep reinforced concrete beams when compared to regular shallo w beams both hav ing the same shear and flexu ral reinforcements, shear failure is most likely to occur in deep beams rather than in regular beams.Thus, retrofitting of deep beams with shear deficiencies using externally bonded reinforcement such as carbon fiber reinforced poly mer (CFRP) becomes an excellent solution.The advantages of using CRFP are nu merous and include their relat ively high tensile strength, lightweight, excellent corrosive resistance, high durability, ease of installation, and above all, the significant increase in the flexural and shear load carrying capacity of the structural member once the CRFP fibers are bonded to the exterior surface of the deep beam members (Ahmad i, 2013;Ali et al., 2013;Benjeddou et al., 2007;David et al., 1998).
This investigation deals with the use of CRFP fibers in fu ll or part ially along the extreme bottom concrete surface of the deep beam, as well as investigating the effect of using U type jacketing along the beam sides and extreme beam surface.The investigation compares the load vs. deflection relationship of the CRFP strengthened beams relative to the control beam and analyses the development of crack location and width along the members, with special emphasis on the type of failure at u ltimate no minal resistance (i.e., pure flexu re, pure shear, or flexural-shear).

Experimental Program
Experimental tests were conducted on a series of ten simply supported RCDBs under one concentrated load to investigate the use of externally bonded CFRP to restore and repair damaged RC DBs.Construction of all the test specimens was conducted according to ACI318M-14, ASTM C293-02 and ACI 440.2R-08 specifications.
Tests were carried out for: 1 Undamaged reinforced concrete deep beams subjected to one concentrated load (control specimens).
2 Damaged reinforced concrete deep beams to 60% of average u ltimate load of control beams; then rehabilitated with CFRP and subjected to one concentrated load.
The test specimens were div ided into four groups: a control test group and three rehabilitated test groups with CFRP fibers.Table 1 summarizes the details of each test specimen.A ready mix concrete with an average value of cubic compressive strength 23.8 (MPa) was used for casting the RCDBs.

CFRP Material and Rehabilitated Schemes
During the investigation of this study, two types of CFRP and epo xy resin were used during the rehabilitation procedure and they were M Brace Laminate with MasterBrace A DH 2200 and M Brace Fiber 230/ 4900 with MasterBrace SAT 4500.Typical properties of the CFRP materials are illustrated in Table 2.As a first step in rehabilitation procedure, all RCDBs were cleaned from any improper things like dust, laitance and loose material with a suitable sandpaper followed by washing them with water and then drying to ensure that RCDBs were clean.The next step was painting the required surface of RCDBs with a thin layer of epoxy resin approximately 3 mm followed by attaching CFRP laminate or sheet according to the adopted scheme and finally painting another layer of epoxy resin onto the CFRP with adequate pressure by a suitable hammer to remove any entrapped air bubbles.The RCDBs were cured for a minimum period in winter of fourteen days at room temperature.Epoxy is considered an essential part during rehabilitation procedure; for this reason two types of epoxy were used for CFRP material to avoid any debonding between the concrete surface and CFRP.Both types of epoxy resin in this study consisted of two components; a resin and a hardener.Both types of CFRP materials and Epoxy resin are shown in Figure 2.

Mbrace Laminate MasterBrace ADH 2200
MBrace Fiber 230/4900 MasterBrace SAT 4500 The second and third test groups were enhanced with two strips of MBrace laminate (each strip 100 mm wide) bonded externally to the bottom side of the beam and covering the full width of the beam between the supports in the 2 nd test group and with a length equal to 400 mm fro m left and right side of centre line of the beam in the 3 rd test group (i.e. with total length equal to 800 mm), wh ile 4 th test group was enhanced with a U-shape of MBrace Fiber 230/4900 covering full width of the beam between the supports and covering half of the depth fro m both sides of the beam.Schemes of CFRP materials of three rehabilitated test groups are illustrated in the Figure 3.During testing procedure, vertical mid-span deflection as well as formation of cracks with their propagation were recorded at each loading stage.

Results and Discussion
In the following sections, results of the control beam group will be introduced first and t hen the results of the rehabilitated beam groups will be co mpared with the control beam group in a table format.Also plots of load versus vertical mid-span deflection and failure mode at ultimate load capacity will be presented.

Ultimate Strength
Results of the control beam group with respect to the format ion of the first vertical hair line flexural and shear cracks, the increased width of vert ical flexu ral cracks, the ultimate load capacity and the failu re mode at u ltimate load capacity in addition to a comparison between their results are presented.Initially, hair line vertical flexural cracks were observed at the mid -span of the beams and by increasing the load, diagonal shear cracks in both shear spans and fro m both supports were started to appear, and when reach ing high load values, vertical flexural cracks increased in width.Table 3 summarizes test results of control beam group.According to Table 3 and Figure 5, there are some differences in the results between the two control beams, and this can be attributed to the difference in the testing machines with different load rates and the vibration process during casting might not have been the same between the two control beams since the vibration was manually undertaken due to the small spacing between stirrups.

Failure Modes
Before reaching the ultimate capacity, obvious widening in vertical flexural cracks at the mid -span of the beam with a rapid increase in the deflection values were observed; the latter indicates that yielding of the longitudinal reinforcement started at this stage.Both control beams failed in a ductile manner as expected.Figure 6 illustrates pure flexural failure of both control beams.

CB1 CB2
Figure 6.Pure Flexural Failure of CB1 and CB2 at Ultimate Load Capacity

Results of the Rehabilitated Beam Groups
In this section, the results of the rehabilitated beam groups are presented in a co mparison with the control beams group.Fro m the eight rehabilitated beams, two beams (RB1-2 and RB2-2) are excluded fro m the results due to a sudden tilt in the hydraulic jack and the specimen.Figure 7 illustrates tilting of the two excluded specimens.
Figure 7. Tilting of Specimens The rehabilitated beams were tested under one concentrated load and in two phases; phase one (pre -cracked phase) and phase two (rehabilitated phase).Beams in pre-cracked phase were tested until the load was reached an approximate value 300 kN (i.e.60% of the average ult imate load capacity of the control beams); and then the beams were removed and rehabilitated with MBrace laminate and MBrace Fiber 230/4900.

Results of Rehabilitated Beams in Rehabilitated Phase
In this phase, testing was continued after repairing the beams with MBrace Laminate and MBrace 230/4900.

Ultimate Strength
Table 4 summarizes test results of six rehabilitated beams with respect to the appearance of first vertical hair line flexu ral cracks, diagonal shear cracks in both shear spans and at both supports, increased width of flexural cracks, enhancements in the ultimate load capacity and the failure mode at ultimate load capacity.
According to Tables 3 and 4, there are some differences in the results between the control beam group and rehabilitated beam groups and also between beams of each group, these differences can be attributed t o the difference in testing machines with different load rates and to the external attachment of CFRP to rehabilitated beams.All increased percentages of rehabilitation was with respect to the average ultimate load capacity of control beams.It is clear that there was no improvement in the percentage of rehabilitation in the 3 rd test group; which means that scheme of CFRP used in this series is not effective in this test group in enhancing ultimate load capacity of rehabilitated beam with respect to the average ultimate load capacity of control beams.Another issue regarding the enhancement of ultimate load capacity, it is obvious fro m the test results that the enhancement of CFRP to the load carrying capacity ranged from 2.9% to 13.1% of rehabilitated beams in the 2 nd and 4 th test group.
% of Reh.9.6 2.9 0.47 3.1 13.1 6.8According to Figure 10, the response of the load-deflection curve of the 3 rd test group was appro ximately the same as the average response of the control beams, which indicates that scheme of CFRP used in this case has no significant effect in the 3 rd test group.

Failure Modes
In this section the failure mode of each rehabilitated beam is presented.The first beam of the 2 nd test group failed in pure flexu re with debonding of laminate CFRP whilst the second beam failed in pure flexu re with an additional peeling of the concrete from both supports.Beams of the 3 rd test group failed in pure flexure with peeling of the concrete from both ends of the laminate CFRP.Regarding the 4 th test group, both beams failed in pure flexu re with rupture and partial delamination of CFRP sheet.When failure of RB3 -1 and RB3-2 was reached at ultimate load capacity; the CFRP s heet was removed to see exact failure mode o f these beams.Figure 11 shows the failure modes of the rehabilitated test groups.

Summery
Figure 12 illustrates the enhancement in ultimate load capacity of each rehabilitated beam with respect to the average ultimate load capacity of control beams.
Referring to Figure 12, it is obvious that the results of rehabilitation in the 2 nd and 4 th test groups are close to each other, but with taking into the account the high cost of CFRP material, scheme of CFRP material in the 4 th test group is more effective over the 2 nd test group.

Conclusions
According to the experimental results of retrofit deep beams by using CFRP, the following conclusions were arrived at: 1. Externally bonded CFRP to pre-cracked beams delays the occurrence of diagonal shear cracks at both span ends as well as enabling an increased width of the vertical flexural cracks of rehabilitated beams to be acceptable compared to the control beams.
2. Both CFRP laminates and sheet retrofitted deep beams showed a significant increase in the ult imate load carrying capacity in reference to the control beam.The enhancement magnitude of retro fitted de ep beam ranged from 2.9% to 13.1%.
3. Tests showed that the 2 nd group using CRFP laminates showed a remarkable load carry ing capacity increase when compared to the 3 rd test group.
4. When varying the length of laminate and keeping all other variab les unchanged (i.e., the 2 nd and 3 rd groups), the use of full length between supports proved to be more effective in increasing the load carrying capacity when compared to the use of partial length laminates.
5. External bonding of CFRP to pre -cracked beams enhances the load carrying capacity when co mpared to control beams.Enhancement of load carrying capacity in the 2 nd test group (i.e., the laminate group) was 9.6% for RB1-1 and 2.9% for RB1-3, while for the 4 th test group (i.e., the sheet wrapped group) was 13.1% for RB3-1 and 6.8% for RB3-2.
6.According to the results of this study, the more effective scheme of CFRP in enhancing flexural performance of simply supported RCDBs is CFRP wrapped sheets (i.e. the 4 th test group).

Recommendations and Future Works
▪ Regarding test results of this study, more researches and studies are required for rehabilitation of RCDBs with more schemes of CFRP laminates and sheets in order to improve flexu ral performance of RCDB in old, new, and existing buildings and bridges.
▪ More schemes of CFRP co mposites are required for rehabilitation of RCDBs to improve flexural performance.External attachment of CFRP is required not only fro m the tension face of the beam, but also fro m both sides of the beam in order to improve flexural performance and to avoid debonding of CFRP composites.
▪ Finite element analysis is recommended for rehabilitation of RCDBs with CFRP composites by using ANSYS or ABAQUS software.

2. 1
Test SpecimensAll tested RCDBs had cross -sectional dimensions of 200 mm width, 500 mm depth and 1800 mm length.The ten test specimens were designed to fail in flexu re rather than shear.Design of test specimens was according to ACI318M-14, ASTM C293-02 and ACI 440.2R-08 specifications.The type of reinforcement used in this test for all test specimens was high yield strength deformed bars of grade 60 (ksi) wh ich have min imu m y ield strength of 420 (MPa).Two type of deformed bar reinforcement were used in RCDBs; and they are 2Փ14 as min imu m longitudinal top and bottom reinforcement; 25Փ12 closed stirrups and three layers of 2Փ12 as horizontal reinforcement.Dimensions and reinforcement details of test specimens are illustrated in Figure1.

Figure 1 .
Figure 1.Dimension and Reinforcement Details of Test Specimens (All dimensions are in millimeters)

Figure 2 .
Figure 2. Types of CFRP Materials and Epoxy Resin

(
Figure 3. Schemes of CFRP Materials for Rehabilitated Test Groups (All dimensions are in millimeters)

Figure 4
Figure 4 illustrates test setup of specimens at both Universities.

Figure 4 .
Figure 4. Test Setup (all dimensions are in millimeters) Figure 5.Comparison of Load versus Vertical Mid-Span Deflection between CB1 and CB2

3 Figure 8 .
Figure8shows the crack patterns (first vertical hairline flexural cracks at mid -span of the beam and diagonal shear cracks in both left and right shear spans as well as diagonal shear cracks at one or both supports in some specimens) of six rehabilitated beams.

Figure 9 .
Figure 9.Comparison of Load versus Vertical Mid-Span Deflection between Rehabilitated Beams for Each Test Group with peeling off concrete from both ends of laminate CFRP Pure flexural failure with rupture and partial delamination of sheet CFRP 3.2.2.2 Load versus Vertical Mid-Span Deflection This section will introduce a comparison in the response of load -deflection curve between control beams and rehabilitated beams of each group.

Figure 10 .
Figure 10.Comparison of Load versus Vertical Mid-Span Deflection between the Control Beam Group and the Rehabilitated Beam Groups

Figure 11 .
Figure 11.Failure Modes of Rehabilitated Test Groups

Table 1 .
Details of the test specimens

Table 2 .
Typical Properties of MBrace Laminate and MBrace Fiber

Table 3 .
Test Results of Control Beam Group

Table 4 .
Test Results of Six Rehabilitated Beams