Punching shear strength of recycled aggregate-steel fibrous concrete slabs with and without strengthening

Abstract

This paper presents an experimental study on the punching shear behavior of reinforced concrete flat slabs fabricated using recycled coarse aggregate (RCA) in addition to the presence of steel fibers strengthened by carbon fiber reinforced polymer (CFRP). The ratios of replacement natural coarse aggregate by RCA were (0, 35, 55, and 75%) and the volume fraction of the steel fiber used in this study was 1.2%. Two strengthening methods were used, namely, externally bonded reinforcement (EBR) and externally bonded reinforcement on groove (EBROG). 16 square slabs were cast with overall dimensions of 800 mm by 800 mm by 80 mm thickness. The slabs were designed to fail in punching shear only. All the slabs were simply supported on four edges and tested for punching via a vertical load applied through the square central column. The test results showed that the mechanical properties, ultimate loads, and first crack loads decreased with increasing proportion of RCA replacement, while the incorporation of steel fibers improved the compressive strength and ultimate loads by the ranges of 12%–17% and 16%–25%, respectively. Strengthened slabs lead to considerable improvements in the punching behavior of slabs, with an increase in punching load ranging from 22–30% compared to unstrengthened slabs when using the EBR technique, while the increase in the punching loads ranged between 40 and 55% when using EBROG strengthening technique. However, using half amount of CFRP with the EBROG method led to an increase in punching shear of 20–28%. This research indicates the particular improvement offered by the EBROG with regard to the punching shear capacity of fibrous recycled coarse aggregate concrete (RCAC) slabs compared to the EBR method. In addition, the mode of failure of reinforced concrete (RC) slabs changed from one of debonding failure to concrete cover separation when using the EBROG method.

Keywords

Punching shear carbon fiber reinforced polymer grooving method strengthening recycled coarse aggregate steel fiber

Introduction

Structural concrete is one of the most extensively used construction materials worldwide, and natural aggregate is one of the important materials used in the production of concrete. The building industries and concrete producers have come to the realization that they will need to use available aggregate instead of seeking the optimal aggregate for made an appropriate concrete for all purposes. Recycled coarse aggregate concrete (RCAC) has been recognized as a potential new type of concrete, in which broken pieces of waste concrete are used as aggregate. The RILEM publication in 1994 was one of the most significant developments in promoting the use of recycled concrete aggregate in new concrete (RILEM, 1994). The use of RCA is important in the sense of preserving our living environment and conserving natural resources (Hansen and Narud, 1983; Mahmoud et al., 2018; Rao et al., 2012). RCAC, however, has low strength, a low elastic modulus, poor workability, high water absorption, high shrinkage, and creep (Francesconi and Pani, 2016; Mahmoud et al., 2018; Rao et al., 2012); on the other hand, it is well-known for its low thermal conductivity and low specific gravity, which decrease the structure’s self-weight (Francesconi and Pani, 2016; Rao et al., 2012).

Concrete material is strong in compression while being very weak in tension, so it tends to be a somewhat brittle material. Punching shear failure is an undesirable failure mode of reinforced concrete flat slabs subjected to a concentrated load, which occurs suddenly with a small displacement (Xiao et al., 2018). There are different methods to strengthen the punching shear capacity studied by researchers including installing the beam at the tensile side of the slab near the column, putting the stud in a direction perpendicular to the slab plane (Kuang and Morley, 1993), the use of metal sheets at the sides of the slab (Ebead and Marzouk, 2002), and the use of various FRP composites for strengthening purposes (Azizi and Talaeitaba, 2019; Binici and Bayrak, 2003; Harajli and Soudki, 2003; Soudki et al., 2012). Rapid separation from the concrete surface during load bearing is a big issue in the use of FRP materials for strengthening concrete structures (premature failure due to debonding. Despite the concrete surface preparation having issues including implementation costs, environmental contamination, and conventional surface preparation is usually unable to fully eliminate the debonding problem. Therefore, it is advisable to develop alternative methods of surface preparation such as using the grooving technique to delay or prevent debonding (Mostofinejad and Mahmoudabadi, 2010). The new grooving method is introduced as a suitable approach to replacing surface preparation and is followed by good experimental results that the new externally bonded reinforcement on grooves (EBROG) has developed and been evaluated to install the composite used to strengthen the beam (Mostofinejad et al., 2014; Mostofinejad and Kashani, 2013; Mostofinejad and Mahmoudabadi, 2010). Adding steel fibers to the concrete mix is one of the approaches to improving the punching shear of RC slabs (Abdel-Rahman et al., 2018; Gołdyn et al., 2018; Maya et al., 2012; Xiao et al., 2018). Fibers are generally short, discontinuous, and randomly distributed throughout the concrete to produce a composite construction material known as fiber reinforced concrete (FRC). Steel fiber (SF) is the most popular type of fiber used in reinforced concrete (Behbahani et al., 2012). Steel fiber works by bridging the cracks in the concrete structure and leads to increased punching shear, moment resistance, flexural stiffness, ductility, and ultimate shear strength, and reduced crack spacing and crack widths (Abdel-Rahman et al., 2018; Mashrei et al., 2018).

The increase in the RCA replacement ratio leads to decreased punching shear strength, first cracking load, ultimate load, hardness, and energy absorption (Mahmoud et al., 2018; Xiao et al., 2018). However, the deflection of slabs increases with increasing proportion of recycled coarse aggregate added (Rao et al., 2012). Studies demonstrate that the addition of steel fibers is an effective and viable alternative to improving the shear behavior deformation ability of concrete slabs (Bhan and Kaur, 2018). Through experimental studies, the strength has been found to decrease with increasing proportion of replacement coarse aggregate, and an increase in strength was observed as the volumetric ratio of steel fibers increased. The replacement of 50% of natural coarse aggregate (NCA) with RCA and addition of 1.0% of steel fibers gives similar behavior to NCA (Bhan and Kaur, 2018).

Research significance

Little research is available in the literature on the punching shear behavior of RCAC slabs. This reality significantly limits the production and implementation of RCAC. As well as future studies on the topic of adding steel fiber to RCAC slabs to enhance the punching shear performance of slabs are still required. Also, reviewing the existing literature reveals that no previous studies have investigated the option of strengthening the RCA-steel fibrous concrete slabs using the grooving method. Therefore, the main objective of this study is to assess and investigate the positive effects of two strengthening techniques; externally bonded reinforcement (EBR) and externally bonded on the groove (EBROG) on punching shear behavior of RCA-steel fibrous concrete slabs. However, the effect of RCA replacement ratios and presence of steel fiber on the compressive strength and stress–strain relationship of concrete and on punching shear behavior of slabs are considered and discussed in this study.

Experimental program

A total of sixteen two-way slabs supported on four edges were loaded up to failure in punching shear with dimensions of 800 mm×800 mm and a thickness of 80 mm. The reinforcement ratio was constant for all slabs, which was 1.4%. Three concrete cubes of 150 mm×150 mm×150 mm and two cylinders of (300 × 150 mm) were cast for each slab specimen at the same time as slabs were cast to estimate the concrete compressive strength $(f_{cu})$ and to determine the stress–strain relationship, respectively. Figure 1 shows the dimensions and reinforcement details of the slabs. These slab specimens were divided into four groups, G1, G2, G3, and G4. The first group, G1, consists of four slab specimens with different proportions of recycled coarse aggregate (0, 35, 55, and 75%). The second group, G2, consists of four slab specimens, three of them fabricated with proportions of RAC of 35, 55, and 75% in addition to the presence of steel fibers with hooked-ends with a volume fraction of Vf = 1.2%. The last slab specimen was fabricated with 55% proportion of RAC and a mix of steel fibers (i.e., 0.6% hooked-end and 0.6% crimped type). The third group, G3, consists of two slab specimens fabricated with 55% RCA and a mix of steel fibers (Vf = 1.2%) and strengthened by orthogonal and skewed-shape CFRP using externally bonded reinforcement (EBR). The final group, G4, consists of six specimens with 55% RCA and a mix of steel fibers (Vf = 1.2%) and strengthened by CFRP using externally bonded reinforcement on groove (EBROG) with different groove shapes and techniques. The specimen descriptions are summarized in Table 1.

Figure 1.

Details of the specimen.

Table 1.

Details of the fabricated slab specimens.

Group	Specimen	Replacement of RCA %	Steel fiber %	Groove	Number and scheme of CFRP strips
G1	SN	0	0.0	—	—
	SR35-0	35	0.0	—	—
	SR55-0	55	0.0	—	—
	SR75-0	75	0.0	—	—
G2	SR35-F	35	1.2	—	—
	SR55-F	55	1.2	—	—
	SR75-F	75	1.2	—	—
	SR55-FM	55	1.2	—	—
G3	SR55-FM-O	55	1.2	—	4-Orthogonal strips
G3	SR55-FM-S	55	1.2	—	4-Skewed strips
G4	SR55-FM-GO	55	1.2	Grooves in one direction	4-Orthogonal strips
	SR55-FM-GS	55	1.2	Grooves in one direction	4-Skewed strips
	SR55-FM-GOH	55	1.2	Grooves in one direction	4-Orthogonal strips (half Strips)
	SR55-FM-GSH	55	1.2	Grooves in one direction	4-Skewed strips (half strips)
	SR55-FM-GOH-2D	55	12	Grooves in two directions	4-Orthogonal strips (half Strips)
	SR55-F-GO	55	1.2	Grooves in one direction	4-Orthogonal strips

SN: Slab with normal aggregate; SR: slab with recycled aggregate; F: hooked-end steel fiber; FM: mix of two types of fiber (50% hooked-end and 50% crimped); O: Orthogonal; S: Skewed strips; G: grooves; 2D: grooves in two directions; Half Strips: half area of CFRP strip

Materials

The materials used in this work are Portland cement, fine aggregate, natural coarse aggregate, and recycled coarse aggregate. All materials were tested according to ASTM standard specifications. The recycled aggregate was prepared manually by crushing and sieving many concrete cubes with the compressive strength ranging from 30–35 MPa at28 days. Deformed steel bars (Ø10 mm) were used as the main form of reinforcement of the slabs. The tensile test was conducted according to ASTM A615 (ASTM-A615/A615M:2015 (2015). The proportion of steel used throughout this study was 1.4%, with two types of steel fibers used (hooked-end and crimped). Figure 2 shows the shape of steel fibers used in this study.

Figure 2.

Shape of steel fibers. a. Hooked-end; b. Crimped.

Unidirectional CFRP sheet and epoxy as an adhesive material were used in this study. Table 2 shows the properties of the materials. The properties of CFRP sheet, adhesive material (epoxy), and steel fiber are presented as per the manufacturers’ recommendations.

Table 2.

The properties of the various materials used.

Material	Property	Value
Steel bar	Bar diameter (mm)	10
	Yield stress (MPa)	693
	Ultimate stress (MPa)	833
	Elongation (%)	11.5
CFRP sheet	Thickness (mm/ply)	0.167
	Tensile strength (MPa)	3500
	Modulus of elasticity (GPa)	220
Epoxy resin (Sikadur®-330)	Tensile strength (MPa)	30
	Tensile modulus (MPa)	4500
	Flexural modulus (MPa)	3800
Steel fiber		Hooked-end	Crimped
	Diameter (mm)	0.9	1
	Lengths (mm)	50	50
	Aspect ratios	55.5	50
	Specific gravity	2.5	2.5
	Tensile strength (MPa)	1150	1242
	Young’s modulus (GPa)	200	200
RCA	Water absorption (%)	4.95
RCA	Density (kg/m³)	1466

Mix of proportions of RCAC

Table 3 shows the proportions of two mixes, namely, the control concrete mix (without RCA and steel fiber), and the RCA concrete mix with and without steel fiber. The control mix was designed to achieve a cube compressive strength of 36 MPa tested according to BS EN 12,390 (BS EN 12,390: 2002 (2002)). Due to the high water absorption capacity of the RCA, additional water (about 5% of the recycled aggregate weight over the quantity of water used in the control mix depending on water absorption test of RCA as presented in Table 2) was added to ensure the slump stay in the range between 60–80 mm. The slump of the control mix was 80 mm and the average slump of RCA concrete mixes was 75 mm. Curing and storage of slabs, cubes, and cylinders were performed at the same time. All specimens were covered with wet material and plastic sheets on top to prevent moisture loss. The slabs were kept moist at all times during the curing period. The formwork was stripped at the end of 2 days, and the slabs were stored in the laboratory until testing.

Table 3.

Concrete mixes proportion

Mix	Material	Cement	Fine aggregate	Coarse aggregate	Water	Steel fiber
Concrete mix without steel fiber	Content (Kg/m³)	400	765	1040	210	0
Concrete mix with steel fiber	Content (Kg/m³)	400	765	1040	210	1.2%

Strengthening procedure

The strengthening of the slab specimens was carried out after the completion of the 28 days of the casting and curing processes. As mentioned previously, the specimens were divided into four groups. The specimens in groups G1 and G2 were unstrengthened, while the specimens in group G3 were strengthened by CFRP using the conventional externally bonded reinforcement (EBR) method. In this method, the tension face of the slabs was ground with a grinder machine, cleaned using an electric air blower, and washed using water. The CFRP sheets were cut to 650 mm in length and 80 mm in width. After the concrete face was prepared, epoxy (Sikadur®-330) was mixed and added to the specified concrete face and to the CFRP sheet. Lastly, the CFRP sheets were adhered to the concrete surface using adhesive material (epoxy), and the specimens then cured for 5 days before testing. The specimens in group G4 were strengthened by CFRP using the EBROG method. The dimension of the grooves were 10 mm wide and 8 mm deep, and were formed using a grinder machine according to the specified shapes, as presented in Table 1 and Figure 3. In this method, the grooves were fully filled by epoxy (Sikadur®-330), with additional epoxy (Sikadur®-330) added to the specified concrete face and to the CFRP sheet. Finally, the CFRP sheets were carefully adhered to the concrete surface. Table 1 and Figure 4 give an overview of the characteristics and shapes of the specimens, mixing, and application of epoxy.

Figure 3.

Steps of works. a. Work grooves; b. Cleaning; c. epoxy mixing; d. Filling the grooves with epoxy; e. The CFRP adhesive.

Figure 4.

Shape of strengthen for specimens and grooves. a. SR55-FM-O; b. SR55-FM-S; c. SR55-FM-GO, SR55-F-GO; d. SR55-FM-GS; e. SR55-FM-GOH; f. SR55-FM-GSH; g. SR55-FM-GOH-2D.

Test setup

The tests were carried out at the Civil Engineering Department of the University of Thi-Qar. A very rigid steel frame consisting of H-sections, a 40 mm wide hollow structural section (HSS) was used as the base to support the slab specimens. The load was applied via a universal machine with capacity of 190 kN. Rigid steel box (60 mm × 60 mm) was used to transverse the applied load from machine to sabs. One dial gauge was used to record the mid-span deflection of the slabs; the gauge was installed on the bottom surface of the slab. Figure 5 shows the test setup of the slabs.

Figure 5.

Test setup.

Results and discussion

All slab, cube, and cylinder specimens were tested 28 days after casting.

Mechanical properties of hardened concrete

Compressive strength

The average compressive strengths of cubes for all mixes used to cast the slabs are presented in Table 4.

Table 4.

The average compressive strength test for cubes.

Mix ID	Average compressive strength (MPa) f_cu	The (increase/decrease) in compressive strength (%)	Density (kg/m3)
SN-0	38.37	—	2429.92
SR-35-0	34.72	−9.50	2301.04
SR-55-0	31.02	−19.10	2281.48
SR-75-0	29.58	−22.90	2290.67
SR-35-F	39.21	+12.90	2427.85
SR-55-F	36.33	+17.09	2376.89
SR-75-F	33.25	+12.40	2355.55
SR-55-FM	34.72	+11.90	2383.11

It can be seen from Table 4 that the compressive strength of concrete cubes generally decreased with increasing the replacement ratio of RCA. It is clear that the compressive strength of RCAC with percentage of replacement of 35%, 55%, and 75% and without steel fiber were decreased by (9.5, 19.1, and 22.9%), respectively, in comparison with compressive strength of normal concrete. In contrast, adding the steel fiber (Hook-end/1.2%) to these mixes has been improved the compressive strength by (12.9, 17.09, and 12.4%), respectively. Adding mix of two types of steel fibers (hooked-end and crimped) has been improved the concrete mix with 55% RCA by 11.9%. Therefore, it can be concluded that the presence of steel fibers has a significant effect in improving the compressive strength of RCA. This may be attributed to the function of steel fibers in bridging the cracks, which has limited the initiation and spread of cracks. Noticeably, the effect of the hooked-end steel fibers on compressive strength was more than the effect of the crimped steel fibers, this may be attributed to the different geometry (shape and aspect ratio) of the two types. The aspect ratio of hooked-end fiber is more than the aspect ratio of crimped steel also, the hoked end my lead to prevent the slippage between the concrete and the steel fiber

Stress–strain curve

Figure 6 shows the stress–strain relationships of NC and RCAC mixes with various recycled coarse aggregate ratios (35%, 55%, and 75%) with a 1.2% volume fraction of steel fiber. Figure 7 shows the test setup for the cylinders to estimate their stress–strain curves and failure patterns. All samples were tested under uniaxial compression.

Figure 6.

The stress–strain relationships. a. NC and RCAC without steel fibers; b. RCAC with steel fibers; c. NC and RCAC with and without steel fibers.

Figure 7.

Cylinder failure mode pattern. a. SN; b. SR-35-0; c. SR-55-0; d. SR-75-0; e. SR-35-F; f. SR-55-F; g. SR-75-F; h. SR-55-FM.

Several points can be noticed from the results of the tests, which are as follows:

From Figure 6, the elastic behavior of RCAC was slightly different from that of normal concrete. RCAC showed high strain capacity, which means high ductility. The maximum strains for RCA types were in the range of 0.004–0.007, while the maximum strain of NC is 0.004. However, adding steel fibers to the RCAC mixes increases the strain in the elastic stage.

The results showed that the moduli of elasticity of the RCAC for all replacement percentages used in this study were lower than the modulus of elasticity for normal concrete mix (NC) and this result is in agreement with the results reported in references (Altaee and Khudair, 2020; Sryh and Forth, 2015). The static modulus of elasticity is calculated via equation (1) ASTM-C469-02, 2002 to ASTM-C469-14, 2014. Table 5 reports the various moduli of elasticity. The decrease in modulus of elasticity for RCA compared with normal concrete may be due to the presence of cement mortar on the surface of the recycled coarse aggregate (Sryh and Forth, 2015). Adding the steel fibers to the recycled aggregate concrete led to an increase in the moduli of elasticity. This can be attributed to increasing the bond strength of concrete, compressive strength, and ductility.

Ec = \frac{(S 2 - S 1)}{(ε 2 - 0.00005)}

(1)

where

Ec = static modulus of elasticity, MPa

S2 = stress corresponding to 40% of ultimate load, MPa

S1 = stress corresponding to a longitudinal strain (0.00,005), MPa and

ε2= longitudinal strain produced by stress S2.

Table 5.

The average compressive strength of cylinders and modulus of elasticity.

Mix ID	Average compressive strength fc (MPa)	Modulus of elasticity (MPa)
SN	25.36	23,703.60
SR-35-0	23.45	15,665.94
SR-55-0	22.49	14,313.03
SR-75-0	17.38	13,819.01
SR-35-F	30.15	16,032.37
SR-55-F	25.93	14,514.81
SR-75-F	25.09	15,054.12
SR-55-FM	25.35	13,706.73

Also, it may be noted from Figure 7 that the shear or the cone-shear type of failure in the cylinders occurred in normal concrete and recycled aggregate concrete without steel fibers, while RCAC with steel fiber cylinders were failed by cone-shear with more longitudinal cracks near the failure area and more lateral expansion in the middle of the sample. This may be attributed to an increase in the ductility of the concrete due to the use of the steel fibers.

Slabs tested under monotonic loading

The visual first punching crack

The visual first crack load denotes the emergence of visual cracks around the center at the bottom of slab surfaces as a circular or closed shape under load. The visual first punching crack loads for the various slabs are summarized in Table 6. It can be noted from the results of the slabs that the recycled coarse aggregate replacement ratios (35, 55, and 75%) led to decrease in the first crack load by 5.9, 6.86, and 11.60%, respectively. Adding the steel fibers improved the first crack load by 9–26% compared to the first crack load of the slabs without steel fibers, and ranged between 3 to 17% when compared with SN slab. However, when strengthening the slabs by CFRP sheets, the first crack loads increased by 69–78% compared to the same slab (SR55-FM) without strengthening and ranged between 98 and 111% when compared with the SN-slabs. Using CFRP to strengthen the slabs led to an improvement in the first crack load for both techniques (EBR and EBROG). The increase in the first crack load in presence of steel fiber can be attributed to the combination of two advantages of the steel fibers which are high tensile strength and working to bridge the cracks and prevent them from expanding during the loading progress. In the same manner, the strengthening slabs by CFRP using EBR and EBROG have improved the first crack load, this is due to the contribution of CFRP in providing confinement to the concrete and letting to delay the presence of cracks in the tension zone.

Table 6.

The visual first punching crack load.

Group	Specimen	First crack load (kN)	The (increase/decrease) in first crack load strength (%)
G1	SN	33.09	—
	SR35-0	31.13	−5.92
	SR55-0	30.82	−6.86
	SR75-0	29.25	−11.60
G2	SR35-F	34.05	+2.90
	SR55-F	38.83	+17.35
	SR75-F	36.61	+10.64
	SR55-FM	38.96	+17.74
G3	SR55-FM-O	66.06	+99.64
G3	SR55-FM-S	69.00	+108.50
G4	SR55-FM-GO	69.85	+111.00
	SR55-FM-GS	68.72	+107.68
	SR55-FM-GOH	65.50	+98.00
	SR55-FM-GSH	47.28	+42.88
	SR55-FM-GOH-2D	67.30	+103.38
	SR55-FH-GO	66.26	100.00

Cracking pattern and failure modes

The cracks pattern and modes of failure for all tested slabs are shown in Figure 8 and reported in Table 7. As the load gradually increased, the cracks visually appeared on the tensile area of the slab. When the load continued to increase, the cracks spread diagonally from the column toward the slab’s corners. As the slabs began to fail, a noisy sound was heard and the cut cone-shaped section of the slab around load was pushed downward; this behavior was reported in Refs. (Kuang and Morley, 1993; Xiao et al., 2018). Also, similar failure modes in recycled concrete slabs were reported in Refs. (Mahmoud et al., 2018; Xiao et al., 2018). Figure 8 shows that as RCA replacement percentage increased, the slab surface integrity was reduced. However, with the presence of steel fiber and CRFP, the slab surface and the cut cone integrity were both significantly improved.

Figure 8.

The Failure mode of slab specimens. a. SN; b. SR35; c. SR55; d. SR75; e. SR35-F; f. SR55-F; g. SR75-F; h. SR55-FM; i. SR55-FM-O; j. SR55-FM-S; k. SR55-FM-GO; l. SR55-FM-GS; m. SR55-FM-GOH; n. SR55-FM-GSH; o. SR55-FM-GOH-2D; p. SR55-FH-GO.

Table 7.

The test results of all tested slabs.

Group	Specimen	Ultimate load (kN)	The (increase//decrease) in load capacity (%)	Ultimate deflection (mm)	Mode of failure
G1	SN	94.46	—	9.08	Punching shear
	SR35-0	86.96	−7.91	10.60	Punching shear
	SR55-0	85.83	−9.10	9.17	Punching shear
	SR75-0	84.85	−10.20	9.71	Punching shear
G2	SR35-F	108.62	+15.02	14.02	Punching shear
	SR55-F	106.75	+13.01	10.92	Punching shear
	SR75-F	98.70	+4.50	9.45	Punching shear
	SR55-FM	102.90	+9.00	11.14	Punching shear
G3	SR55-FM-O	125.71	+33.03	9.13	Punching shear/debonding
G3	SR55-FM-S	133.44	+41.30	10.96	Punching shear/debonding
G4	SR55-FM-GO	159.22	+68.61	12.42	Punching shear/cover separation
	SR55-FM-GS	157.15	+66.42	11.43	Punching shear/cover separation
	SR55-FM-GOH	129.46	+37.01	10.16	Punching shear/cover separation
	SR55-FM-GSH	131.70	+39.40	11.07	Punching shear/cover separation
	SR55-FM-GOH -2D	123.62	+30.81	9.75	Punching shear/cover separation
	SR55-F-GO	143.30	+51.70	9.14	Punching shear/cover separation

The failure modes of the slabs strengthened by EBR technique were debonding of CFRP failure. Debonding failure is a major problem with the EBR method (Belarbi and Acun, 2013; Chen and Teng, 2003). In contrast, the failure mode of slabs strengthened via the EBROG technique was concrete cover separation. This is in agreement with results presented in Refs. Belarbi and Acun, 2013; Mostofinejad et al., 2014; Mostofinejad and Kashani, 2013; Mostofinejad and Mahmoudabadi, 2010, who demonstrated that the mode of failure of RC slabs changed from debonding failure to concrete cover separation when using the EBROG method. It was noted that the use of these methods considerably increases the punching shear capacity by eliminate the debonding of CFRP.

Punching load

Table 7 shows that the ultimate punching load is reduced with increasing proportion of RCA replacement. The reduction in the punching load was 7.9, 9.1, and 10.2%, for recycled aggregates replacement percentages of 35, 55, and 75%, respectively, in comparison with the ultimate punching load of the SN slab. When adding steel fibers (hooked-end or mix of hooked-end and crimped) to the mixture, the punching ultimate loads were improved. The increases in the ultimate load were 15.0, 13.0, 4.5, and 9.0% for the recycled aggregates replacement percentages of 35, 55, 75, and 55% (mix), respectively, compared to punching load of SN slab and 24.9, 24.4, 16.3, and 19.9% compared to the punching loads of slabs with the same percentages of RCA but without steel fibers. The increase in the load-carrying capacity can be attributed to the combination of two advantages of steel fibers which are high tensile strength and working to bridge the cracks and prevent them from expanding during the loading progress. It is noteworthy that the effect of hooked-end steel fibers on the punching load was more than the crimped steel fiber, as mentioned earlier in Compressive strength.

Table 7 also shows that the RCAC-fibrous slabs strengthened by CFRP have higher loads in comparison with unstrengthened identical slab (SR55-FM) or in comparison with the SN slab. The increase in the load-carrying capacity of slabs strengthened via the EBR method ranging between 22 and 30% in comparison with slab SR55-FM and ranged between 33 and 41.3% compared with the SN slab. In contrast, the ultimate loads of slabs strengthened via EBROG were significantly increased, where the increase in the ultimate load of these slabs ranged between 40 and 55% in comparison with SR55-FM and between 51.7 and 68.6% in comparison with the SN slab. However, using half amount of CFRP used in the previous slabs to strengthen other slabs via the groove technique improved the punching capacity in range between 20 and 28% in comparison with SR55-FM and ranged between 31 and 40% in comparison with the SN slab, that is, approximately the same percentages for slabs strengthened with double amount of CFRP using the EBR method. These results engorge to use half amount of CFRP with EBROG instead of used double amount of CFRP with EBR method, as well as change the mode of failure from debonding in EBR to concrete cover separation in EBROG, as previously discussed in Stress–strain curve. Finally, it can be concluded that the EBROG strengthening technique is particularly effective at improving the punching shear capacity of slabs. It is clear from Table 7 that slab SR55-FM-GO (slab strengthening with EBROG using orthogonal CFRP strips) had the highest ultimate load among the strengthened slab specimens. These results indicate that the strengthening techniques have a considerable effect on the ultimate punching loads of RCA-steel fibrous concrete slabs. EBROG method performs better at transferring stresses between CFRP materials and the concrete substrate due to increase the contact area between CFRP and concrete surface due to presence of grooves, as a result the slabs strengthened by EBROG method may be carried higher loads than the beams strengthened by EBR method. In other side, the effect of the direction of CFRP on the punching shear behavior of RCA-steel fibrous concrete slabs has been investigated and presented in Table 7. It clear that the effect of diagonal direction of CFRP as presented in specimen SR55-FM-S is more effective than the orthogonal direction as worked in specimen SR55-FM-O, these specimens strengthened by EBR method. The percentage of increase in the ultimate load for the SR55-FM-O and SR55-FM-S specimens were 33% and 41.3%, respectively, in comparison unstrengthened slab SR55-FM. However, no significant effect of the direction of CFRP on the ultimate loads of specimens strengthened by grooves method. Specimens, SR55-FM-GO and SR55-FM-GS, had the increase in the ultimate load of 68.61 and 66.42% over the SN slab, respectively. It can be concluded that EBROG grooves method, may eliminate the effect of the direction of CFRP, because the grooves work on restrict the spread of cracks in both the diagonal (skewed) and orthogonal strips. While in EBR method, the diagonal direction of CFRP which is perpendicular to the direction of cracks, normally formed starting from the column corners and extended toward the slab corners. Therefore, the diagonal direction of CFRP in EBR method is more effective in prevents cracks spread.

Load–deflection curve

16 slabs were tested until failure. The punching shear load at appropriate intervals and the corresponding deflections at the centers of the slabs were recorded to plot the associated load–deflection (P–Δ) curves to reflect their structural behavior, the punching shear capacity, ductility, and deformation of slabs.

Figure 9 shows the load–deflection relationships for slabs in all groups. The slabs were essentially in an elastic state during the initial loads. The slabs’ deflection curves started to diverge from the load axis after reaching the cracking load, with linear relationships. When the load reached about (0.85–0.9) Pu, the deflection curves started to tend toward the horizontal axis. The punching cone created, when the failure load was achieved, and the slab’s carrying capacity decreased considerably.

Figure 9.

Load–deflection relationship for all slabs. a. Load–deflection relationship of group G1; b. Load–deflection relationship of group G1and G2; c. Load–deflection relationship of the fibrous-slabs strengthened by CFRP sheets using the EBR; d. Comparison of load–deflection relationships of slabs strengthened by EBR and EBROG; e. Comparison of load–deflection relationships for slabs with and without strengthening.

It can be seen from Figure 9 that:

The load–deflection curves for the RCAC slabs showed greater ductility than the SN slab. Figure 9(a) clearly shows the comparison of the behavior of slab specimens with different RCA ratios. It may be noted that the slabs with RCA replacement experienced larger deflections than that of the SN slab. The increase in the deflections for slabs with RCA replacement may be due to the lower moduli of elasticity of the RCAC slabs.

Figure 9(b) shows the load–deflection curves of RCAC slabs with and without steel fibers. It is clear that the presence of steel fibers led to greater deformation at failure, and the number of cracks in the punching shear cone clearly increases.

Also, the results showed that the punching shear failure becomes obvious with increasing proportion of RCA replacement. As a result, the steel fiber worked to improve the slab punching shear performance and the negative effects of proportion of RCA replacement were reduced; at the same time, using steel fiber in the mix enhanced the stiffness, ductility, and deformation ability at failure.

Figure 9(c) shows the load deflection of the slabs strengthened by CFRP sheets using the EBR method in the presence of steel fibers and control slab (SR55-FM and SR55-0). It is clear that the behavior of the slabs that had been strengthened exhibited more ductile behavior than SR55-0, in addition to the higher loads and displacements in comparison to slabs SR55-FM and SR55-0.

When using half the amount of CFRP with the groove technique, almost identical punching shear failures and deflections in comparison to the EBR method were achieved. In addition, exhibited ductile behavior can be seen in Figure 9(d). This indicates that the EBROG is very effective in improving the punching shear of slabs, it is allowed to use half amount of CFRP that is used in EBR method and provide approximately the same ultimate load and deflection with more ductile behavior.

Slabs strengthened by the grooving technique could undergo greater deflection and withstand higher loads than other slabs. Slab SR55-FM-GO showed the highest deflection among the test specimens. Slab SR55-FM-GO had the highest deflection among the strengthened specimens, and also the highest punching shear load failure. Figure 9(e) clearly illustrates that slab strengthening via the EBROG method tended to present ductile behavior.

Effect of strengthening technique

Externally Bounded Reinforcement (EBR) and Externally Bounded Reinforcement on Grooves (EBROG) have been investigated to assess the punching shear behavior of RCA-steel fibrous reinforced concrete slabs. An examination of Table 7 reveals that the enhancements in the load-carrying capacity of slabs strengthened via the EBR and EBROG techniques ranged from 22–30% and from 40–55% in comparison with SR55-FM, respectively. These results indicate that the efficiency of the EBROG method is greater than for the EBR method in terms of increased punching shear capacity of the strengthened slab. Figure 10 shows the effect of the strengthening method on the load–deflection relationship.

Figure 10.

Load–deflection relationship for strengthening slabs.

Conclusions

From the results of this study, the following conclusions can be drawn:

Replacement of normal aggregate in concrete mixes by recycled coarse aggregate (RCA) led to decrease in the compressive strength by 9.5, 19.1, and 22.9% for replacement ratios of 35, 55, and 75%, respectively. However, adding steel fibers to the above mixes of RCAC increased the compressive strength by 12.9, 17.09, and 12.4%, respectively.

Generally, the punching shear load, deflection, and first crack load decreased with increasing replacement ratio of recycled coarse aggregates; however, they improved by adding steel fibers to the concrete mixes.

As the RCA content increased, the ultimate punching load decreased by 7.9, 9.1, and 10.2% with replacement ratios of 35, 55, and 75%, respectively. The addition of steel fibers improved the punching shear capacity by about 16–25% as well as the ductility of slabs were also improved.

The application of an external layer of longitudinal CFRP sheets using EBR technique increased the load-carrying capacity of slabs by 22%–30%, while strengthening slabs by EBROG technique led to increase in the load-carrying capacity by 40%–55%

The results showed that the ductility of orthogonal or skewed CFRP-strengthened slabs was increased over the control slabs.

By applying orthogonal or skewed grooves using EBROG method, the premature debonding of CFRP layers was prevented and higher values of peak loads and displacements were achieved. Using half the amount of CFRP gave almost similar ultimate loads to those found for the EBR method with double amount of CFRP. This is evidence of the particular efficiency of this method.

The failure modes of all slabs strengthened by grooving technique were mainly concrete cover separation, while the failure mode of the slabs strengthened by CFRP using traditional method (EBR) was CFRP-debonding failure. Concrete cover separation can be viewed as a positive sign of the effectiveness of the grooving technique as this mode of failure indicates that the CFRP is well agglutinated to the surface of concrete and is in a full effect to transform the mode of failure to concrete cover separation instead of debonding failure mode.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Ali M Saadoon

Mohammed A Mashrei

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