strengthening of reinforced concrete …jestec.taylors.edu.my/vol 15 issue 4 august...

Journal of Engineering Science and Technology Vol. 15, No. 4 (2020) 2559 - 2579 © School of Engineering, Taylor’s University

2559

STRENGTHENING OF REINFORCED CONCRETE DEEP BEAMS WITH OPENINGS BY NEAR SURFACE MOUNTED STEEL BAR

HAYDER H. KAMONNA1, LUBNA R. ALKHATEEB1,*

1Department of Civil Engineering, University of Kufa, AL-Najaf, Iraq *Corresponding Author: [email protected]

Abstract

This research includes an experimental study on the strengthening of a reinforced concrete deep beam with openings by near surface mounted steel bar. The experimental work includes testing of thirteen simply supported reinforced concrete deep beams under two-point load. Four specimens were considered as control specimens, including one specimen without openings, whereas the other three specimens had two openings located symmetrically at each shear span. The other nine specimens were strengthened by using NSM steel bars in three different configurations around the openings. All the specimens had a cross-section of 200 mm×400 mm with 1500 mm in total length. The test variables are sizes of the openings, locations of the openings, bar diameter, and arrangement of strengthening bars around the openings. The test results showed that the presence of the openings in the beam led to a decrease in the ultimate load of about 49%, 56%, and 70% for specimens with square openings near loading points, square openings at the load path, and rectangular openings, respectively. The test results also showed that the ultimate load of specimens that was strengthened by vertical bars can improve up to14%. However, the ultimate load of specimens that was strengthened by both vertical and horizontal bars can improve up to 40%. While strengthening the specimens with a diamond strengthening scheme can upgrade the ultimate load up to 34%, changing the bar diameter slightly increases increase in the ultimate load to about 6%.

Keywords: Deep beam, Near surface mounted, Openings, Shear strengthening, Strengthening configuration.

2560 H. H. Kamonna and L. R. Alkhateeb

Journal of Engineering Science and Technology August 2020, Vol. 15(4)

1. Introduction

1.1. Deep beam and openings In the construction of tall buildings, the beams at the lower part carry larger loads compared with the beams in a normal building. The deep beam was created to solve these problems. The deep beam has a larger depth because it’s a load transferring structural component that is usually used to transfer a great amount of loads to the supports by compression struts joining the loads and supports [1]. ACI Code 318 [2] defines “deep beam” as a structural component in which either the concentrated load is applied in a span equal to or less than twice the depth, or a clear span of equal to or less than four times the depth. In many structures ranging from offshore gravity structures to tall building structures, deep beams have become a widespread structural element. They can be utilized as foundation beams, panel beams, and deep grid walls in offshore gravity-type concrete structures. Deep beams are usually utilized in engineering structures such as a bunkers and deep girders, water tanks when the walls act as vertical beams that extend between columns supports [3].

In a building, there are mechanical, electrical, and passageways of access that need to be considered during the construction process. A larger sized deep beam will cause more obstruction to these utilities. In some cases, these utilities have to penetrate the deep beams [1]. Sometimes, utility pipes and service ducts are put below the soffit of the beam and they are covered by a suspended ceiling because of an aesthetic requirement that generates dead space. In spite of passing the ducts and the pipes across the openings in the ground beams, will be able to decrease in dead space, which may lead to a more economical and compact design. The presence of the openings causes a nonlinear stress distribution along the depth of the deep beam and geometric discontinuity of the beam. The design of the deep beam with an opening is not covered by the present codes practice. The behaviour of RC deep beams with openings is unlike the behaviour of RC solid beams. Because of the formation of discontinuity regions, the behaviour of the beam will change from a simple behaviour to a complex one. Hence, the provision of openings in RC beams will lead to the decrease of the stiffness and strength of the deep beams and cause an increase in cracking and deflection. The behaviour of the deep beam with the opening greatly influences the size, shape, and position of the opening [4].

1.2. Near surface mounted The concrete structures need to be strengthened or rehabilitated in their life extension because of different factors such as raise service load, environmental impact, mistakes in construction and design, and mechanical damage. The near-surface mounted (NSM) and external bonded reinforcement (EBR) methods are considered the most common strengthening techniques. The externally bonded reinforcement technique includes externally strengthening materials such as fiber reinforcement polymer and steel plates. However, this method is more exposed to premature failures such as delamination, debonding of the longitudinal sheet, and other kinds of premature failure, which prohibit the strengthening component from obtaining the entire flexural capacity. The external bonded reinforcement sheet is more exposed to environmental, thermal, and mechanical damages. So, the NSM method is considered an efficient alternative to the EBR method [5].

Strengthening of Reinforced Concrete Deep Beams with Openings by . . . . 2561


The near surface mounted strengthening method is less prone to mechanical damage, vandalism and fire, and an accidental effect because the surrounding concrete protects the NSM reinforcement bars. This feature is especially necessary for this state because the location of the bars is very close to the surface, which makes them more prone to the environmental effect [6]. The NSM method was first used in Finland in 1940 to strengthen a bridge deck slap. This work was done by positioning the bars into the grooves that were created within the concrete face and filling them with epoxy [7].

In this study, steel bars are utilized as the strengthening material because of its economic feasibility. It provides a cost-effective solution, as well as the less costly availability of steel bars in the market. Steel bar have better long-term durability and good bond performance [5]. The test variables are the location of openings, the sizes of openings, and the effect of the strengthening configuration around openings and bar diameter.

1.3. Literature review From the past until the present day, different studies have been conducted to investigate the behaviour of RC deep beams with openings. Yang and Ashour [8] studied the behaviour of RC continuous deep beams with openings that are reinforced by different configurations of web reinforcement. The results revealed that the beams that have web reinforcement above and below the openings showed shear strength and ultimate load more than beams that have web reinforcement only above the openings. The beam capacity with openings positioned in exterior shear spans was 10 to 15% smaller than that of the solid deep beams; however, the shear strength of these beams dramatically decreased in range from 40 to 50% compared with solid deep beams, irrespective of shear span-to-overall depth ratio. On the other hand, for beams with openings located in interior shear spans, both the shear and load capacities sharply decreased compared with the solid deep beams. Yusof [9] investigated the behaviour of RC deep beams with a circular opening located near the loading point. The test variable is the size of circular openings. The sizes of circular openings considered in this study were Φ200 mm, and Φ250 mm. Results showed that providing openings with Φ200 mm led to a decrease in the capacity of the beam by 28.9% and providing openings with Φ250 mm led to a reduced capacity of the beam by 34.86%. So, increasing the sizes of circular openings caused a decrease in the capacity of the beam.

Different studies were carried out with the EBR method on the strengthening of the RC deep beam with an opening by using FRP as strengthening material. Abduljalil [10] presented an experimental investigation on the strengthening of RC deep beams with openings with EBR CFRP strips. The results of the experimental work showed that using externally bonded CFRP strips to strengthen openings improved the stiffness of the beams, reducing the diagonal crack width, and increasing the ultimate shear strength. Hussain and Pimanma [11] presented an experimental investigation on the behaviour of the RC deep beam with circular openings, which was strengthened by externally bonded sprayed fiber reinforced polymer composites (SFRP). The test variables were the direction of SFRP, the thickness of SFRP, and the sizes of the openings. Their results revealed that the use of SFRP to strengthen the openings improved the beam capacity from about 64% to 130%. It was found that the SFRP, which was installed on three sides (u-shaped), was more efficient than the two-sided SFRP in the strengthening of shears.



Many previous studies investigated the flexural and shear behaviour of RC beams that were strengthened by NSM FRP strips or bars. Husain et al. [6] presented a numerical investigation to study the flexural behaviour of RC beams that were strengthened by NSM FRP bars. Three kinds of FRP bars were used to strengthen the beams in the flexural: the GFRP bars, the CFRP bars, and the AFRP bars. The numerical study was conducted by using the ANSYS-2013 program. It concluded the following: increasing the diameter of NSM FRP bars led to the increase of initial stiffness and reduction in the ductility. Increasing the number of NSM FRP with the same reinforcement ratio slightly improved the beam capacity and decreased the ductility. Increasing the bond length of NSM FRP bars led to an increase in the deformation capacity, the initial stiffness, and the ultimate load. Samad et al. [12] used NSM CFRP anchor bars to strengthen the RC deep beams. It was found that the shear capacity of RC deep beams increased from about 17.3% to 25.5%, and the central deflection reduced from 6.4% to 15.1%. In addition, utilizing this method improved the flexural capacity of the beam under the same conditions. Nafzin and Prabhakaran [13] investigated the shear strengthening of RC beams by NSM steel bars, as well as the effect of the NSM bars diameter and the NSM bars direction on the capacity of the RC beam. They found that the increase in the diameter of the NSM steel bars led to the increase in the strength of the RC beam. The specimen strengthened by inclined NSM steel bars showed a more significant increase in the ultimate load compared with the specimen that strengthened by vertical NSM steel bars. Shakir and Kamonna [14] used NSM steel bars to strengthen high-strength RC corbels. They adopted different strengthening configurations. They considered two shear-span-to-depth ratios, which were 0.85 and 1.25. The failure loads were enhanced by about 54% and 41% for the shear-span-to-depth ratios of 0.85 and 1.25, respectively, compared with the specimens that were not strengthened. Abdzaid and Kamonna [15] investigated the flexural performance of continuous (RC) beams that were strengthened by NSM steel bars. The parameters that were studied included the development lengths, reinforcement area, and the types of strengthening material with end-anchors of (CFRP) sheets. There was a considerable increase of about 108% in failure load compared with the control beam.

From surveying the previous studies, one can observe that the literature on the strengthening of RC members by using low-cost NSM steel bars is limited. This technique is not used to strengthen the RC deep beam with openings. Most of the researchers have used CFRP materials as a strengthening material in the NSM method, despite their high cost. So, there is a need for a consensus that adopting steel bars in the NSM method instead of CFRP bars is more beneficial. This study aims to investigate the behaviour of RC deep beams with openings that are strengthened by NSM steel bars.

2. Materials Properties All the specimens were casted by using ordinary Portland cement that was manufactured by the Kufa Cement Factory. The chemical and physical properties of cement are listed in Tables 1 and 2. Black crushed gravel with a maximum size of 19mm was utilized as a coarse aggregate. Table 3 shows the results of the coarse aggregate’s sieve analysis. Natural sand from the sea of Najaf was used as a fine aggregate. Table 4 shows the results of the fine aggregate sieve analysis. Superplasticizer of about 0.1% of cement weight was added to reduce water in the mixture and to obtain good workability with a low-water-to-cement ratio. The water



that is suitable for drinking was used for the casting and curing process. Six cubes with the sizes of 150 mm×150 mm and three cylinders of 200 mm in height and a 100 mm diameter were cast to obtain the compressive and tensile strengths, respectively. The mixing proportion (cement, sand, and gravel) was 1:1.72:2.5 by weight [16]. Table 5 presents the proportions of the materials of the concrete mix that met the required strength. The average compressive and tensile strengths of the concrete were 34 MPa and 3.3 MPa, respectively. The properties of the steel reinforcement are listed in Table 6. Epoxy paste was used to bond the steel bar in the groove when adopting the NSM technique. The commercial name of the adopted epoxy is Sikadur-30LP. Table 7 shows the properties of the epoxy paste.

Table 1. Chemical properties of cement. Chemical compound Test result Iraqi Standard

(IQ. S No. 5/1984 SiO2 20.2% --------------------- Cao 60.1% --------------------- Fe2O3 3.48% ---------------------

L. S. F 0.9 Ranges (0.66-1.02 % for Portland cement types except Low heat cement

MgO 3.21% ≤5% for Portland cement SO3when C3A < than 5%

---------------------

≤2.5%

SO3 when C3A > 5%

1.8% ≤2.8%

Loss on Ignition 2.92% ≤4% Insoluble Residue 1% ≤1.5% C3S 41.3% ----------------------- C2S 26.841% ----------------------- C3A 9.79 --------------------- C4AF 10.6% ---------------------- M.A 1.7 -------------------- Free Lime 0.63% --------------------

Table 2. Physical properties of cement.

Physical property Result Limits of Iraqi Specification No. 5/1984

Initial Setting time (Vicat minute) 60 >45 minute

Final setting time (Vicat minute) 105 <600 minute

Compressive strength in age (3days) N/mm2 25.74 >15 N/mm2 Compressive strength in age (7days) N/mm2 29.87 >23 N/mm2

Table 3. Sieve analysis of fine aggregate. Sieve size (mm) Percent of passing, % (IQ. S NO 45/1984)

10 100 100 4.75 98 90-100 2.36 89 75-100 1.18 74 55-90 0.6 56 35-59 0.3 24 8-30

0.15 6 0-10



Table 4. Sieve analysis of coarse aggregate. Sieve size (mm) Percent of passing, % (IQ. S NO 45/1984)

20 100 100 14 99 95-100 10 36 30-60 5 1 0-10

Table 5. The quantities of the materials used in the concrete mix. Material Amount Cement(kg/m3) 410 Sand(kg/m3) 706 Gravel(kg/m3) 1025 Water(L/m3) 171.56 w/c 0.416 Superplasticizer 4

Table 6. Tensile test results of steel bars. Bar diameter Yield stress (MPa) Ultimate stress (MPa) Elongation Ø8 500 646.67 21% Ø10 607 736 12.5% Ø12 578 676 13.5% Ø16 602 689 15.5%

Table 7. Physical properties of epoxy paste.

Appearance Component (A): white, Component(B): black

Component (A+B): gray Mixing ratio A:B (3:1) Modulus of elasticity 10000 N/mm2 Compressive strength >85 N/mm2 Tensile strength >17 N/mm2 Shear strength ≈7 N/mm2 Tensile Adhesion strength For concrete>4 N/mm2 For steel>22

3. Experimental Work All the tested specimens had a cross section of 200 mm×400 mm with a total length of 1500 mm and a 1300 mm clear span. The shear-span-to-depth ratio was kept constant and equal to 1.1. The specimens were tested under two points loading up to failure. All the specimens were reinforced by 2Ø10 steel bars in the compression zone and 4Ø16 steel bars as tension reinforcement, which extended beyond their support by 327 mm with a 90o hook. The minimum shear reinforcement area according to ACI 318 M14 [2] code was adopted by using Ø8 stirrups at 200 mm c/c. The details of reinforcement for the solid beam are shown in Fig. 1.

Four specimens were considered as control specimens, which consisted of one without openings. That specimen was treated as a reference for the other unstrengthen specimens, whereas the other three specimens have two openings located at each shear span with different sizes and locations. The sizes of square and rectangular openings were 150 mm×150 mm and 250 mm×150 mm, respectively. The locations of the openings were chosen at the load path in the first case and near the loading point (the openings located at a distance of 50 mm from the loading point) in the



second case. The other nine specimens were strengthened by using NSM steel bars around openings in different configurations. The details of the three groups are shown in Figs. 2-4, and the specimens’ details are listed in Table 8.

Fig. 1. Dimensions and reinforcement details of control beam.

(a) BSOCN. (b) BSOCV10.

(c) BSOCVH12. (d) BSOCI10.

Fig. 2. Details of group one.



Fig. 3. Details of group two.



(a) BROCN. (b) BROCVH10.

(c) BROCVH12. (d) BROCI10.

Fig. 4. Details of group three.

Table 8. Specimen’s details.

Group No.

Beam name

Location of

opening

Shape of opening

Diameter of NSM steel bar

Details of opening

strengthening Solid beam CB --------- -------- ---------- -----------------

Group

one

BSOCN the opening centered at load path

Square

---------- ----------------- BSOCV10 Ø10 mm Vertical bars

BSOCVH12 Ø12 mm Horizontal and vertical

bars BSOCI10 Ø10 mm Inclined bars

Group two

BSONN The

opening near the

load

Square

----------- -------------------

BSONV10 Ø10 mm Vertical bars

BSONVH12 Ø12 mm Horizontal and vertical

bars BSONI10 Ø10 mm Inclined bars

Group three

BROCN

the opening centered at load path

Rectangular

----------- -----------------

BROCVH10 Ø10 mm Horizontal and vertical

bars

BROVH12 Ø12 mm Horizontal and vertical

bars BROI10 Ø10 mm Inclined bars



4. Installation of NSM Steel Bars The following steps were followed to achieve the strengthening process. In the beginning, a sketch was drawn to fix the boundaries of the desired grooves, which were needed for embedding the strengthening bars.

The sketch was made to ensure that the grooves would be drilled with the necessary shapes and dimensions, ensuring that the strengthening bars were kept away from the opening edges at a distance greater than 20 mm. The grooves were drilled in a depth and a width of about 1.5db (db. is the diameter of the bar). The grooves were made in the concrete with the required predetermined size by utilizing a special concrete saw with a diamond blade. After that, they were cleaned with water and dried by air from the dust and impurities.

Figure 5 shows the processes of groove drilling. The layer of epoxy paste that was added filled to half the depth of the groove. Then, the strengthening bar was placed gently and pressed slightly so that the epoxy paste would flow around the bar and fill completely between the bar and the sides of the groove [17]. After that, the second layer of epoxy paste was added in such a way that the bar was completely hidden, and the surface was levelled. Then the specimens were left for ten days to let the epoxy paste reach the desired strength. Figure 6 illustrates the specimens after completed the strengthening process.

(a) BSOCVH12. (b) BSOCV10.


Fig. 5. Grooves creation.



(a) BROCVH12. (b) BSOCV10.

(c) BROCI10.

Fig. 6. The embedding of NSM steel bars into the grooves by epoxy paste.

5. Test Setup The specimens were painted white in order to recognize the crack propagation before starting the test. The thirteen simply supported deep beams were tested up to failure by using a universal testing machine at the University of Kufa. The capacity of the machine was 2000 kN. All specimens were tested under two points -loading with a clear span of 1300 mm. To prevent local crushing of the concrete, bearing plates with dimensions of100 mm×200 mm×15 mm and rubber plates with dimensions of 100 mm × 200 mm × 6 mm were used at the loading points and supports. Steel shaft were put under bearing plate to obtain simply supported beam. The central deflection was measured by using a dial gage, which was put at the center of the beam’s bottom soffit. A blackboard pen was used to indicate the propagation of cracks on the specimens. The crack width was measured by using the crack meter (see Fig. 7 for the steps of the test setup).

6. Conducting the Test Before conducting the test, a data logger was run to capture data. The load was applied at a constant rate of about 5 kN/min. The deflection at mid-span was recorded at every 10 kN loading increment. At every 20 kN loading increment, the propagation of the developed cracks was marked by using a blackboard pen and the width of the major cracks were measured by using the crack meter. The specimens were loaded until the failure and the data were captured by the data logger and stored in the compute.



(a) Preparation of the specimen for testing.

(b) Loading points. (c) Supporting points.

Fig. 7. Test setup.

7. Test Results and Discussion

7.1. Influence of creating openings in the shear spans on the beam strength The effect of providing the openings in the shear spans of the beam is investigated in this section. The sizes of the openings were 150 mm ×150 mm and 250 mm×150 mm. The square openings were positioned at load paths in the BSOCN specimen and near the loading points in the BSONN specimen. BROCN contained rectangular openings centered at the shear spans. The test results showed that the openings that were provided in the beam led to a significant decrease in the carrying capacity of the beam. This is because the presence of the openings in the beam caused a significant decrease in the area of the concrete that resisted the applied stress, as well as the fact that the interruption of the load path with openings exposed the corners of the openings to a high concentrate of stress. So, the first cracks formed at the corners of the openings. This led to occur excessive cracking and deflection in the beam, causing a sharp decrease in shear capacity of the beam. The decrease in the ultimate load due to the presence of the openings about 49%, 56%, and 70% for BSONN, BSOCN, and BROCN, respectively. The openings located away from the load paths caused a smaller decrease in the beam strength compared with the case in which the openings were positioned at the load paths. It can be concluded that the openings, if they are necessary, should be placed away from the load paths. Test results also found that increasing the sizes of the openings caused a sharp reduction in the beam capacity. Increasing the sizes of the openings by about 67% led to a decrease in the cracking and ultimate loads by about 40% and 32%, respectively. The test results of the control specimens are listed in Table 9.



Table 9. Test results of the control specimens.

Specimen

Flexural Crack (kN)

First shear crack (kN)

Ultimate load (kN)

Decrease in

ultimate load

Failure mode

CB 140 230 630 --------------

Shear +comparison

BSOCN 160 100 280 56% Shear +comparison

BSONN 160 100 320 49% Shear +comparison

BROCN 140 60 190 70% shear

7.2. Effect of strengthening RC deep beam with openings by NSM steel bars

7.2.1. Strengthening by vertical bars The effect of using vertical strengthening bars to strengthen the openings on the cracking and ultimate load was investigated in this section. The test results showed that, using the vertical strengthening bars to strengthen the openings contributed to the distribution of the cracks in a large area of the beam instead of the concentration of them in area that surrounded the openings. This caused an increase in the ultimate loads. The test results listed in Table 10 show the 12% and 14% increases in the ultimate load for the BSOCV10 specimen and the BSONV10 specimen, respectively, compared with the unstrengthen specimens. However, there was no recorded improvement in the cracking load. This may be attributed to the fact that only the gross concrete section influenced the cracking load, and the gross concrete section remained unchanged.

7.2.2. Strengthening by vertical and horizontal bars In this section, the effect of using both vertical and horizontal steel bars to strengthen the openings was studied. The test results exhibited that the use of the vertical and horizontal strengthening scheme had a significant effect on the cracking and ultimate loads. The presence of this strengthening type around the opening prevented stress concentration at the corners of those openings and prevented the propagation of cracks from the openings’ corners to the loading and supporting points. These cracks were distributed around strengthening bars instead of their propagation from the corners of the openings. This led to an improvement in the first cracking load, a decrease in deflection, and an increase in the ultimate loads. The BSOCVH12 and BROCVH12 specimens showed an increase in the first cracking load by about 60% and 66%, respectively, compared with the unstrengthen specimens, whereas BSONVH12 did not show any improvement in the first cracking load. The test results listed in Table 10 shows that, the use of the vertical and horizontal bars to strengthen the openings led to an improvement in the ultimate load by about 5%, 35%, and 40% for BSONVH12, BSOCVH12, and BROCVH12, respectively, compared with the unstrengthen-specimens.



7.2.3. Strengthening by using diamond strengthening shape around openings This section will present the investigation into how diamond-shaped inclined bars are used to strengthen openings. The use of the diamond-strengthening scheme was found to improve the distribution of the stresses and prevent stresses from being concentrated at the corners of the openings. So, the first shear cracks were observed at the bottom face of the beam instead of the openings’ corners. Specimens BSOCI10 and BSONI10 did not show any improvement in the first cracking load. However, there were recorded increases of 20% and 40% in the first shear crack at the corners of the openings for BSOCI10 and BSONI10, respectively, compared with the unstrengthen specimens. BROCI10 showed an improvement in the first cracking load of about 30%. Besides that, there was a recorded increase in the first shear cracks at the corner of the openings of about 66% compared with unstrengthen specimens. The test results listed in Table 10 shows that, the use of the diamond-strengthening scheme improved the ultimate load by about 9.4% and 34% for BSONI10 and BROCI10, respectively, compared with unstrengthen specimens. However, this strengthening scheme did not provide any improvement in the failure load of BSOCI10.

The significant increase in the ultimate load of BROCI10 may be attributed to the interruption of the load path by the inclined strengthening bars. This led to arresting the development of the cracks and the distributing the stresses more efficiently while preventing them from becoming concentrated at the corners of the openings. The presence of the inclined strengthening bars around the openings prevented cracks from becoming propagated from the corners of the openings to the support and loading points, ensuring that the cracks were propagated around the strengthening bars instead of their propagation from the corners of the openings. So, this led to a decrease in deflection and a significant improvement in both the cracking load and ultimate load.

The inclined strengthening bars followed the same direction of the load path in BSOCI10 and BSONI10. The steel bars applied a compression force on the concrete cover, which caused the cover to be crushed. So, an insignificant increase in the ultimate loads was recorded. On the other hand, the openings partially interrupted the load path, thus causing a slight increase in the ultimate load that was recorded for BSONI10. The inclined strengthening bars in BSONI10 specimen interrupted the load with a small angle and did not follow exactly the same direction of the load path.

7.2.4. Effect of changing the diameter of the steel bars In order to investigate the effect of the change in bar diameter on the cracking load and ultimate load, the vertical and horizontal strengthening scheme was adopted for two specimens that had rectangular openings with the dimensions 250 mm×150 mm. The rectangular opening was strengthened by vertical and horizontal bars with a diameter of Ø10 mm in BROCVH10 and a diameter of Ø12 mm in BROCVH12. BROCVH12 showed improvement in the first cracking load of about 11% compared with BROCVH10. In terms of ultimate load, BROCVH12 gave a slight increase in the ultimate load of about 6% compared withBROCVH10. Changing bar diameter did not have a significant effect on the beam capacity. Test results of the all specimens are listed in Table 10.



Table 10. Test results of all specimens.

Specimen

First flexural crack (kN)

First Shear crack

at corner (kN)

Increase in First Shear

crack at corner (kN)

Ultimate

load (kN)

Increase in ultimate

load

Mode of failure

Test result of specimens in group one

BSOCN 160 100 --------- 280 ---------

Shear + compression

BSOCV10 140 100 --------- 315 12.5% Shear

BSOCVH12 160 160 60% 380 36%

Shear + compression

+ lateral bucking of steel bar

BSOCI10 90 120 20% 280 -----------

Shear + compression

Test result of specimens in group two

BSONN 160 100 ---------- 320 --------- Shear +

compression

BSONV10 180 100 ----------- 365 14%

Shear+ compression

BSONVH12 120 100 ---------- 335 5%

Shear+ compression

+ lateral bucking of steel bar

BSONI10 180 140 40% 350 9.4% Shear+

compression

Test result of specimens in group three BROCN 140 60 ------ 190 ------- Shear

BROCVH10 140 90 50% 250 32% Shear block

BROCVH12 160 100 66% 265 40% Shear+ shear block

BROCI10 140 100 66% 255 34% Shear+ shear block

7.3. Crack patterns Figure 8 shows crack patterns for the specimen without openings (the CB specimen). The flexural crack in this specimen formed at the bottom face of the beam. As the load increased, the first flexural crack developed in addition to appear new flexural cracks. Then, with a further increase in the load, the first diagonal crack was observed in the supporting point and propagated to the loading point. At



the later stage of loading, the propagation of flexural cracks stopped, and the shear crack propagated further up to failure.

In the BSOCN and BSONN specimens, the shear crack formed at the opposite corners of the openings. As the applied load was increased, the first observed shear cracks formed at the corners of the openings, which tended to propagate towards supporting and loading points. Then, new shear cracks were observed in the region below openings and tended to propagate to the lower inner corner of the openings. As the load was increased, further flexural cracks formed in the maximum moment region. At the final stage of loading, the shear cracks that were observed at the corners of openings propagated to the supporting and loading points and caused the failure of the beam. When the size of the openings increased from 150 mm×150 mm to 250 mm×150 mm, the distribution of cracks did not change. But the shear cracks that were observed at opposite corners of the openings formed at an earlier stage of loading compared with the case when the specimen had openings with smaller sizes. The crack patterns are more severe in the specimen with rectangular openings because of a higher decrease in the concrete body. As the applied load increased, the shear crack was observed clearly because it had greater width at the corners of the openings. This caused failure at an earlier stage of loading. Figures 9-11 show cracks patterns of group one, group two, and group three, respectively.

The distribution of the cracks in the specimens, which was strengthened by vertical strengthening bars, was similar to the cracks distribution in the unstrengthen specimens. The vertical strengthening bars contributed to distributing the cracks in a larger area of the concrete body. This caused greater number of cracks to form than those in the specimens without strengthening (the control specimen). Adopting horizontal and vertical strengthening bars contributed to the dispersion of the stresses and prevented them from becoming concentrated at the corners of the openings. The cracks were then distributed in a larger area and decreased in number and width compared with the control specimens. Horizontal and vertical strengthening bars also prevented the cracks from being propagated from the opposite corner of the openings to the supporting and loading points. Therefore, the cracks distributed around strengthening bars and in the maximum moment region. The diamond-strengthening scheme that surrounded the openings prevented the concentration of the stresses at the corners of the openings. So, the first crack was observed at the bottom face of the beam. The strengthening bars interrupted the load path, and as a result, the stresses dispersed and obstructed the cracks from propagating from the corners of the openings to the loading and supporting points. And this led to a formation of more cracks that had smaller width compared with the unstrengthen specimens. Therefore, these cracks distributed around the openings and in the maximum moment zone.

Fig. 8. Crack patterns for control specimen (CB specimen).





Fig. 9. Crack patterns for all specimens in group one.

(a) BSONN. (b) BSONV10.

(c) BSONVH12. (d) BSONI10.

Fig. 10. Crack patterns for all specimens in group two.

(a) BROCN. (b) BROCVH10.

(c) BROCVH12. (d) BROCI10.

Fig. 11. Crack patterns for all specimens in group three.



7.4. Load-mid span deflection relationships Load-deflection curves for the control specimens and those in group one, two, and three are shown in Figs. 12-15 respectively. At the early stage of loading, the beam had an elastic behaviour that was observed until the formation of the first cracks. Then, the curves began to drop and tended to be nonlinear. Later, several cracks formed and propagated. The stiffness of the beam decreased, and this can be noticed from the increase in the recorded deflection at the same increment of loading.

Figure 12, which represents the load-deflection curves for control specimens, shows that the deflection of BROCN, (rectangular openings) had increased more than that of the other control specimens, which caused a sharp decrease in beam stiffness.

Figures 13 and 14 illustrate the load-deflection curves for group one and group two, respectively. The stiffness of BSOCV10 and BSONV10, which were strengthened by vertical strengthening bars, did not improve. The deflections for the strengthened and control specimens that were recorded proved this point. This may be because the vertical strengthening bars did not obstruct the load path. So, several cracks were formed around the openings. Figures 13-15 of the load-deflection curves for group one, group two, and group three, respectively show that adopting the horizontal and vertical strengthening scheme decreased the deflections of about 12%, 25%, 28%, and 35% for BSONVH12, BROCVH12, BROCVH10, and BROCVH12, respectively, compared with the unstrengthen specimen. The horizontal and vertical strengthening scheme significantly improved the beam stiffness. On the other hand, the beam that was strengthened with a diamond-strengthening scheme exhibited good improvement in the beam stiffness beyond the cracking stage, such that the recorded deflections were reduced by about 21% and 27% for BSOCI10 and BROCI10, respectively. The deflection in BSONI10 did not show a considerable decrease. So, there was no significant improvement in the beam stiffness. This may be due to the presence of the openings near the maximum moment zone. This led to cause higher deflection value. So, the specimen did not exhibit any increase in the beam stiffness.

Fig. 12. Load-deflection

curves for all control specimens. Fig. 13. Load-deflection curves for all specimens in group one.



Fig. 14. Load-deflection curves for all specimens in group two.

Fig. 15. Load-deflection curves for all specimens in group three.

8. Conclusions This work includes conducting an experimental study on the behaviour of a RC deep beam with openings that are strengthened by NSM steel bars. From this study, the following conclusions can be drawn:

• The location of the openings had a significant effect on the ultimate load. Positioning the openings at load path caused a decrease in the ultimate load of about 12.5% compared with the openings that were located far away from the load path by about 75 mm. So, the behaviour of the beam depends mainly on the degree of interruption of the opening with a loading path.

• Increasing the size of the openings by about 67% led to a decrease in the cracking and the ultimate loads by about 40% and 32%, respectively.

• Creating openings in the beam caused a decrease in the failure load by about 49%, 56%, and 70% for the beam with a square openings positioned near the loading point, beam with square openings positioned at load path and beam with rectangular openings positioned at load path respectively.

• Providing openings away from the load path with small sizes is considered a more suitable choice than providing openings at the load path with large sizes.

• The vertical strengthening bars that were used to strengthen the openings didn’t cause any increase in the first cracking load. However, it improved the ultimate load by about 12% and 14% for the beam with square openings that was positioned at the load path, as well as the beam with square openings that was located near the load, respectively.

• Strengthening the openings with vertical and horizontal bars caused an increase in the beam capacity by about 5%, 35%, and 40% for the specimen with square openings near the load, for the specimen with square openings at the load path, and for the specimen with rectangular openings, respectively.

• Positioning the strengthening bars in an inclined direction around the openings improved the beam capacity by 9.4% and 35% for the beam with square openings positioned near the load and the beam with rectangular openings positioned at the load path, respectively.



• When the inclined bars did not extend to the top and bottom ends of the beam and were stopped at the top and bottom covers, the development the cracks were not interrupted by the strengthening bars. Therefore, the cracks developed outside of the strengthening region freely. So, in this case, the inclined strengthening scheme did not improve the ultimate load. When the inclined bar extended to the top and bottom ends of the beam, the inclined strengthening scheme significantly enhanced the ultimate load.

• Changing the diameter of strengthening bars from 10 mm to 12 mm slightly improved the ultimate load by about 6% and did not improve the beam stiffness.

Nomenclatures db Diameter of bar, mm Fcu Compressive strength of concrete (cube), MPa Fc' Compressive strength of concrete (cylinder), MPa Ft Tensile strength of concrete, MPa Fy Steel yield stress, MPa w/c Water to cement ratio Greek Symbols Ø Diameter of reinforcement bar, mm Abbreviations

10, 12 bar diameter Ø 10 mm and Ø12 mm respectively ACI American Concrete Institution AFRP Aramid Fiber Reinforced Polymer ASTM American Society for Testing and Materials B Beam C Control CFRP Carbon Fiber Reinforced Polymer EBR External Bonded Reinforcement FRP Fiber Reinforced Polymer GFRP Glass Fiber Reinforced Polymer I Inclined IQ. S Iraqi specification N Refer to beam without strengthening N Near load NSM Near Surface Method O Opening R Rectangular RC Reinforced concrete S Square STM Strut and Tie Method V Vertical VH Vertical and Horizontal



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