Flexural behavior of recycled aggregate concrete beams strengthened with carbon fiber reinforced polymer

Abstract

Using recycled aggregate in the construction of reinforced concrete (RC) buildings has become essential to reduce the waste produced from demolished buildings. In this paper, the influence of using carbon fiber reinforced polymer (CFRP) in strengthening of RC beams with recycled coarse aggregate (RCA), as a partial/full replacement for natural coarse aggregate (NCA), was experimentally investigated. Twelve RC beams with various RCA ratios (0%, 30%, 70%, and 100%) were prepared and tested under four-point loading. Four beams without CFRP were tested till failure while the other eight beams were subjected to service loads then strengthened using CFRP and re-tested till failure. The cracking pattern, deflection, as well as strain in longitudinal and transversal reinforcement were recorded. The failure loads of the tested beams were compared to those predicted from various international design codes. Results showed that the flexural cracks appeared in specimens with 100% RCA earlier than those with NCA. Moreover, the increase in RCA% resulted in resisting tensile stresses by stirrups at a lower load value and achieving a more ductile behavior. Also, the experimental results showed that including RCA in beams slightly decreased the failure load for both strengthened and un-strengthened RC beams. Furthermore, beams with higher amount of RCA experienced more deflections than those with lower amount of RCA. Such behavior was found more prominent in the CFRP strengthened beams.

Keywords

recycled coarse aggregate reinforced concrete beams strengthening carbon fiber reinforced polymer

Introduction

It is evident that the rapid construction development consumes a massive amount of natural resources such as natural aggregate and produces a vast quantity of construction and demolition waste (Ahmed and Lim, 2020). Therefore, sustainability measures should be adopted to decrease the impact of the construction sector on the environment (Silva et al., 2017). About 80% of the concrete weight consists of natural coarse and fine aggregate (Katerusha, 2021). Replacing the natural aggregate with recycled aggregate in concrete reduces the negative impacts on the environment by reducing the use of natural resources and decreasing the waste quantities.

Extensive research was performed to determine the mechanical, physical, and chemical properties of recycled coarse aggregate (RCA) such as shape, specific gravity, strength, water absorption, and reaction with Alkali (Japanese Industrial Standards Committee, 2014; Nagataki et al., 2004). Results showed that RCA from crushed concrete has some inferior properties than those of natural coarse aggregate (NCA). For instance the presence of attached cement mortar in RCA results in higher water absorption than NCA which leads to higher porosity and lower strength concrete (Kang and Kee, 2017; Liu et al., 2021). The absorption capacity ranged between 0.8%–3.7% and 3.7%–8.7% for the NCA and RCA, respectively (Boyle, 2013; Wen et al., 2015). It was found that the specific gravity of NCA (2.4–2.9) is usually higher than that of RCA (2.1–2.4). Effect of using RCA on the compressive strength of concrete elements was investigated by many researchers (Bidabadi et al., 2020; P and N, 2021; Zhu et al., 2021). Some studies (Bidabadi et al., 2020; Zhu et al., 2021) showed that using the RCA decreases the concrete compressive strength up to 30%. While other studies (Li et al., 2017) proved that the compressive strength of concrete with RCA can be higher than that of concrete with NCA through the addition of nano materials or using other methods such as bacterial, micro-encapsulation, or sequential mixing approaches (Tam et al., 2020). Other studies (Ajdukiewicz and Kliszczewicz, 2002; Mi et al., 2020) proved that the compressive strengths of concrete with either RCA or NCA were comparable with differences not exceeding 17%. An experimental program by (Pereira et al., 2012) showed that the recycled aggregate concrete (RAC) may have worse performance compared to the natural aggregate concrete (NAC). However, adding super plasticizers to the RAC led to a mechanical performance better than NAC without additives. (Gholampour et al., 2017) developed a model to predict the compressive, tensile, and flexural strengths based on the utilized water/cement ratio and amount of RCA. (Xu et al., 2019) used the Grey System Theory to estimate the sensitivity of the mechanical properties of RAC. The mechanical properties of RAC were found strongly correlated with the concrete mixture proportions and the geometrical shape of aggregates. The replacement percentage of RCA had less effect on the properties of concrete. (Xu et al., 2021) and (Li et al., 2022) developed prediction models to estimate the uniaxial and triaxial compressive strength of concrete. Their models revealed that the RAC is more sensitive to lateral confinement compared to NAC.

McNeil and Kang (2013) studied the impact of using RCA on large scale RC beams. The authors found that the modulus of rupture of concrete with RCA was slightly less than that of concrete with NCA due to the presence of residual mortar which weakens the interfacial transition zone between mortar and aggregate. It was also found that the modulus of elasticity of RCA is lower than that of NCA which leads to a more ductile behavior. Furthermore, the compressive strength of recycled coarse aggregate concrete (RCAC) is influenced by the amount and properties of recycled aggregate. However, in case of tension failure and the concrete strength with RCA is similar to that with NCA. Also, the use of RCA in RC beams had insignificant effect on the ultimate moment.

Alqarni et al. (2020) investigated the effect of changing the properties of RCA on the shear behavior of RC beams. The study was implemented on eight RC beams incorporating four types of coarse aggregate (limestone, quartzite, scoria, and steel slag). The beams were tested under three-point loading configuration and were fabricated with/without the use of shear reinforcement. The experimental results showed that both beams with/without shear reinforcement experienced similar failure mode due to diagonal tension shear crack. However, aggregates with high abrasion resistance increased the ultimate shear strength even in beams with similar compressive strength. The beam strength increased when the abrasion resistance and density of aggregate increased.

Carbon fiber reinforced polymer (CFRP) strengthened beams commonly fail by early intermediate crack debonding (ICD) (Fu et al., 2017). This failure starts at the tension zone specifically at the toe of the major flexural crack. (Fu et al., 2018) implemented an experimental study to investigate the effect of using inclined CFRP U-jackets on the performance of RC beams. The experimental program was conducted through testing eight full-scale beams with various heights, widths, and inclination angles of the U-jacket CFRP. Performance of specimens was investigated in terms of mode of failure, deflections, and CFRP strains. The experimental results showed that a 45° inclined CFRP U-jacket shifted the failure mode of the tested beams from ICD to concrete crushing or rupture of the CFRP. Furthermore, using the 45° CFRP U-jacket led to an increase in the failure load and maximum strain of the CFRP plate with percentages of 55.8% and 134%, respectively, compared to the control specimen. The experimental program showed that specimens with a 45° U-jacket performed much better than their counterparts with 90° or 135° U-jacket in terms of rupture and concrete crushing. While the 90° U-jacket significantly improved the ductility of the tested specimens. The flexural performance of fiber RC beams strengthened using CFRP with/without preloading was investigated by (Helal et al., 2020). The results showed that strengthening led to an increase in the elastic stiffness of the beams by up to 65%.

Al-Saawani et al. (2020) investigated the effect of changing beam dimensions on the type of debonding failure (ICD or concrete cover separation). The authors estimated the ultimate capacity and failure mode of 14 CFRP strengthened beams with shear span-to-depth ratio (S/D) of 1.5–7. All beams were simply supported and tested under four-point loading protocol. Four beams were used as control specimens without CFRP while 10 beams were strengthened with CFRP sheets. The bottom surface of the beams was strengthened by CFRP sheets which were extended to the supports. The results showed that S/D had a significant effect on the mode of debonding failure. Beams with S/D < 3.0 experienced intermediate crack debonding while beams with S/D > 3.0 experienced concrete cover separation at failure.

Based on the reviewed literature, there is a lack of experiments on full-scale testing of CFRP strengthened RCAC beams. Also, some conflicting results were found regarding the compressive strength of RCAC and the effect of changing the amount of RCA ratio on the behavior of RC beams. In addition, no accurate analytical results using code provisions for strengthening of RCAC beams were found in the literature.

The performance of strengthened NCAC and RCAC beams is experimentally investigated in the current paper. The adequacy of various code provisions in predicting the failure loads for such beams is examined. The paper has three objectives: (1) experimentally study the flexural behavior and failure modes of RCAC beams strengthened with CFRP sheets, (2) assess the efficiency of strengthening of RCAC beams using CFRP sheets, and (3) evaluate the effect of using different amounts of RCA on the ultimate capacity, deflection, and steel strain in RC beams. The paper starts by providing a comprehensive description of the experimental program including the specimen design, properties of materials, fabrication procedure, as well as the test setup and measurements. Then, the experimental results and relevant discussions are presented to investigate the crack patterns, failure modes, load-deflection relationships, and load-strain relationships, for both longitudinal and transverse reinforcement. Finally, a comparison between the experimental results and the design provisions of the ACI 318-19 (ACI Committee 318, 2019), ACI 440.2R-17 (ACI Committee 440, 2017), Eurocode-2 (British Standards Institution, 2004), and AS-3600 (Standards Australia Limited, 2018) codes is carried out.

Experimental program

Specimen design

The experimental program in this research includes 12 RC beams which were constructed using four concrete mixes with variable NCA to RCA ratios. The beam designation is as follows: R refers to the RCA ratio and S stands for the strengthening scheme (None, Scheme 1, and Scheme 2), as shown in Table 1. Four beams were constructed without strengthening (control beams) while another eight beams were constructed with two CFRP strengthening schemes shown in Figure 1. The beams were designed for shear and flexure according to the ACI 318-19 code to attain flexural failure before shear failure.

Table 1.

Beam designation.

Beam designation	% Coarse aggregate		Strengthening
Beam designation	Natural (%)	Recycled (%)	Strengthening
R00-S0	100	0	None
R30-S0	70	30	None
R70-S0	30	70	None
R100-S0	0	100	None
R00-S1	100	0	Scheme 1
R30-S1	70	30	Scheme 1
R70-S1	30	70	Scheme 1
R100-S1	0	100	Scheme 1
R00-S2	100	0	Scheme 2
R30-S2	70	30	Scheme 2
R70-S2	30	70	Scheme 2
R100-S2	0	100	Scheme 2

Figure 1.

Beam strengthening schemes.

The beams had a rectangular cross-section of 150 mm width and 300 mm height and a total length of 2300 mm. The target compressive strength of concrete was 30 MPa which is commonly adopted in construction. The same longitudinal reinforcement ratio of 1.15% was used for all beams. Four high grade steel bars of 12 mm diameter were used in the tension side whereas two mild steel bars of 8 mm diameter were used in the compression side. The shear reinforcement consisted of 8 mm mild steel stirrups with a spacing of 100 mm. Control beams were loaded till failure, while the other eight beams were preloaded with a service load of 95 kN. The preloading stage was implemented in this research to resemble an actual beam that will be strengthened using CFRP after experiencing service loads.

Material properties

The 12 mm diameter longitudinal reinforcement has a yield strength of 500 MPa while the secondary 8 mm diameter longitudinal bars and shear reinforcement has a nominal yield strength of 300 MPa. All the reinforcing bars were obtained from the same batch of steel production. Both types of steel reinforcement were tested in the lab to verify that they meet the requirements of (ASTM A615, 2008). Table 2 shows the ductility requirements for the steel bars, whereas Table 3 shows the mechanical properties of the steel bars compared to the properties found in their data sheet.

Table 2.

Density and ductility requirements for steel bars.

Bar diameter	Bar designation*	Density (kN/m³)	Length before testing	Experimental test deformation		Data sheet min. Elongation (%)
Bar diameter	Bar designation*	Density (kN/m³)	Length before testing	Elongation (mm)	Elongation (%)	Data sheet min. Elongation (%)
8	B300D-P	73.53	80.00	16.00	20.00	19.00
12	B500DWR	73.70	120.00	20.00	16.67	13.00

Bar designation*: according to “ISO 6935-2:2007(E)”.

Table 3.

Mechanical properties of steel bars.

Bar diameter	Bar designation*	Experimental test strength			Data sheet strength
Bar diameter	Bar designation*	f_y (MPa)	f_u (MPa)	Tensile/yield	f_y (MPa)	f_u (MPa)	Tensile/yield
8	B300D-P	339.37	470.74	1.39	300	600 (max)	1.25
12	B500DWR	545.82	701.52	1.29	500–650	—	1.25

Bar designation*: according to “ISO 6935-2:2007(E)”.

The CFRP used in this research is (SikaWrap®-230 C), which is a unidirectional carbon fiber fabric, and it was installed on the concrete surface using an epoxy-based adhesive (Sikadur®-330). The properties of the CFRP are as follows: a nominal thickness of 0.129 mm, characteristic tensile strength of 3200 MPa, elastic modulus of 210,000 MPa, and ultimate tensile strain of 1.59%. These properties conform with the (D3039, 2017) standards. The Sikadur 330 adhesive had a modulus of elasticity of 3800 MPa, and a maximum elongation at break of 0.9%. To apply the strengthening schemes on the tested beams, they were inverted upside down to facilitate the strengthening process, as shown in Figure 2, then a layer of epoxy was applied, followed by the application of the CFRP. Finally, another layer of the epoxy adhesive was applied to ensure good bond between the CFRP and concrete.

Figure 2.

Beams after application of carbon fiber reinforced polymer and adhesive.

The specific gravity and water absorption of NCA and RCA were characterized based on the (ASTM C127, 2015) standards, while the bulk density of the fine aggregate, NCA, and RCA was determined based on the (ASTM C29, 2017) standards. The RCA was prepared by crushing concrete cubes, cylinders, and broken beams from the (Concrete Research Laboratory), Cairo university. Fine materials (below 0.15 mm), and large size particles (above 25 mm) were removed by mechanical sieving. The specific gravity, bulk density, and water absorption results, presented in Table 4, were used in the concrete mix design. Grading of the aggregate particles was determined based on the (ASTM C33, 2018) standards (after the RCA sieving process). It was observed that the RCA particles had a slightly smaller particle size than that of NCA particles. The water absorption of RCA was found significantly higher than that of NCA due to the old mortar particles attached to the RCA. Figure 3 shows samples of the fine aggregate, NCA, and RCA, while Figure 4 shows the sieve analysis for the three extracted samples.

Table 4.

Specific gravity and bulk density of aggregate.

Aggregate type	Specific gravity	Bulk density (kg/m³)	Water absorption (%)
Fine aggregate	2.66	1711	—
Natural coarse aggregate (NCA)	2.60	1528	0.5
Recycled coarse aggregate (RCA)	2.55	1497	8.5

Figure 3.

Aggregate particles.

Figure 4.

Sieve analysis test for the aggregate particles.

The concrete mixtures were designed with a target compressive strength of 30 MPa based on 150 mm cube specimens conforming with BS EN 206-1 (British Standards Institute, 2016). The mixes were based on standard mixes rich in coarse aggregate to develop a clear relationship between the use of RCA in concrete and the flexural capacity of beams. Concrete mix proportions are presented in Table 5.

Table 5.

Concrete mix design for 1 m³.

Concrete mix	Cement (kg/m³)	Water content		Coarse agg.		Fine aggregate	Fine agg/coarse agg.
		Water content		NCA	RCA	Fine aggregate
		kg/m³	W/c	kg/m³	kg/m³	kg/m³
R00	400	205	0.51	1120	—	650	0.58
R30	400	205	0.51	780	340	650	0.58
R70	400	205	0.51	340	780	650	0.58
R100	400	205	0.51	—	1120	650	0.58

Fabrication and curing procedures

The beam specimens were constructed, cured, and tested in the (Concrete Research Laboratory) of the Faculty of Engineering, Cairo University. The beam specimens were cast along with three standard cubes to assess the compressive strength, and three standard cylinders to determine the splitting tensile strength according to (ASTM C496, 2017). The beams, cubes, and cylinders were stored in a semi-controlled environment with a temperature range of 15–25°C after the removal of the formwork. All test specimens were subjected to the same fabrication and curing conditions for 28 days prior to testing as shown in Figure 5.

Figure 5.

Fabrication and curing of beams.

Test setup and measurements

Testing of the beams was carried out using a universal Shimadzu machine and a 500-ton hydraulic testing frame. A four-point bending setup was used in the testing procedure, as shown in Figure 6(a). The clear span between the two supports was 2000 mm. The load was applied in the middle of a rigid steel I-beam, rested on two rigid steel plates representing the loading points at a spacing of 666 mm. Deflection measurements were carried out through the use of three Linear Variable Differential Transformers (LVDTs) having a maximum measuring length of 100 mm. One LVDT was placed at the middle of the beam, and the other two were placed at 1/4 of the clear span, as shown in Figure 6(b). Three strain gauges were placed on each specimen, one was placed on the mid-length of the main steel reinforcement, the second gauge was placed on the stirrup at distance d/2 from the support, the third gauge was placed externally on top concrete surface at the beam mid-span, as shown in Figure 6(b). The steel strain gauges had a length of 10 mm, which were made by (KYOWA). The concrete strain gauges had a length of 60 mm and were manufactured by (Tokyo Measuring Instruments Lab).

Figure 6.

Measuring devices and test setup.

Code provisions

Control beams

The first cracking load and the load-carrying capacity of the control beam specimens were predicted using three code provisions; ACI 318-19 (ACI Committee 318, 2019), Eurocode 2 (British Standards Institution, 2004), and AS 3600-2018 (Standards Australia Limited, 2018) and compared to the experimental results. The cracking load was calculated based on the flexural tensile strength $(f_{t})$ using equations (1)–(3), according to the ACI 318-19 (ACI Committee 318, 2019), Eurocode 2 (British Standards Institution, 2004), and AS 3600-2018 (Standards Australia Limited, 2018) provisions, respectively, and equation (4)

f_{r} = 0.62 * λ * \sqrt{f_{c}^{'}}

(1)

f_{ctm} = 0.3 * f_{ck}^{(\frac{2}{3})}

(2)

f_{ct \cdot f} = 0.6 * \sqrt{f_{c}^{'}}

(3)

M_{cr} = \frac{f_{t} * I_{g}}{y_{t}}

(4)

where:

f_{c}^{'}

and

f_{ck}

are the concrete characteristic strength,

λ

equals one for normal weight concrete.

M_{cr}

denotes the predicted cracking moment. Regarding the load-capacity calculation, the neutral axis depth and the flexural strength were calculated using equations (5) and (6) (ACI 318-19 (ACI Committee 318, 2019)), equations (7) and (8) (Eurocode 2 (British Standards Institution, 2004)), and equations (9) and (10) (AS 3600-2018 (Standards Australia Limited, 2018)), respectively

β_{1} C = \frac{A_{s} f_{y} - A_{s}^{'} f_{y}^{'}}{0.85 * {fc}^{'} * b}

(5)

M_{u} = A_{s} f_{y} * (d - \frac{β_{1} C}{2}) + A_{s}^{'} f_{y}^{'} * (\frac{β_{1} C}{2} - d^{'})

(6)

λx = \frac{A_{s} f_{yk} - A_{s}^{'} f_{yk}^{'}}{η* f_{cd} *b}

(7)

M_{u} = A_{s} f_{yk} * (d - \frac{λx}{2}) + A_{s}^{'} f_{yk}^{'} * (\frac{λx}{2} - d^{'})

(8)

a = \frac{f_{y} * A_{s} - f_{y}^{'} * A_{s}^{'}}{α_{2} *f c^{'} *b}

(9)

M_{u} = A_{s} f_{y} * (d - \frac{a}{2}) + A_{s}^{'} f_{y}^{'} * (\frac{a}{2} - d^{'})

(10)

where: β₁ is the ratio between the depth of rectangular stress block to the depth of the neutral axis; C and x are the distance from top concrete fiber to the neutral axis (mm);

A_{s}

and

A_{s}^{'}

are the area of the main steel and secondary steel bars, respectively (mm);

f_{y}

(or

f_{yk}

) and

f_{y}^{'}

(or

f_{yk}^{'}

) are the main and secondary steel yield stress respectively (MPa); d is distance between maximum compression fiber to the center of tensile steel reinforcement (mm);

λ

is a factor defining effective height of concrete stress block;

η

is a factor defining effective strength of concrete stress block;

f_{cd}

is the concrete design compressive strength (taken equal to the concrete characteristic strength

f_{ck}

for comparison with experimental results).

α_{2}

is a factor for the effective strength of concrete stress block

(= 0.85 - 0.0015 f_{c}^{'})

Beams with CFRP strengthening

The ultimate flexural strengths for the CFRP strengthened beams were predict using equations (11) and (12) (ACI 440.2R17 (ACI Committee 440, 2017))

M_{u} = A_{s} f_{y} * (d - \frac{β_{1} C}{2}) + A_{s}^{'} f_{y}^{'} * (\frac{β_{1} C}{2} - d^{'}) + Ψ_{1} A_{f} f_{fe} (d_{f} - \frac{β_{1} C}{2})

(11)

f_{fe} = E_{f} * ε_{f}

(12)

where: ψ₁ is CFRP flexural strength reduction factor (taken equal to one to compare with experimental results);

A_{f}

d_{f}

, and

f_{fe}

are area (mm²), depth (mm), and stress (MPa) in the CFRP laminates.

E_{f}

and

ε_{f}

are modulus of elasticity and strain of CFRP laminates. To account for the possible debonding failure,

ε_{f}

was calculated using equations (13) and (14)

ε_{f} = ε_{cu} * (\frac{d_{f} - c}{c}) - ε_{bi} \leq ε_{fd}

(13)

ε_{fd} = 0.41 * \sqrt{\frac{f_{c}^{'}}{n E_{f} t_{f}}}

(14)

where:

ε_{cu}

is the maximum strain in concrete,

ε_{bi}

is the strain in the concrete during the installation of the CFRP (taken equal to 0, as the installation was done after removing the loads on the beams),

ε_{fd}

is the debonding strain of CFRP, n is the number of CFRP layers, and

t_{f}

is the thickness of the CFRP laminates.

Results and discussion

Mechanical behavior of concrete

The compressive strength and splitting tensile strength were evaluated using the average of three standard 150 × 150 × 150 mm cubes and three standard 150 × 300 mm cylindrical specimens respectively. Figure 7(a) shows a slight difference in compressive strength not exceeding ±7% when changing the amount of RCA in the concrete mixtures. Reduction in compressive strength not exceeding 30% was reported by several studies (Butler et al., 2011; Poon et al., 2004; Xiao et al., 2012). The small difference in the current study is attributed to the good quality of parent concrete, which has a great influence on the strength of recycled concrete (Katz, 2003). Figure 7(b) shows that the splitting tensile strength increased by 13%–15% when the amount of RCA was increased. This trend occurred due to the rough surface of RCA that tend to increase the splitting tensile strength, as proposed by Matias et al. (2013). The concrete density of both cube and cylinder specimens are shown in Figure 8. A slight decrease in density not exceeding 1.80% was noticed in RCA mixes. Also, cube and cylinder specimens were following the same trend and their differences did not exceed 2.48%.

Figure 7.

Strength of concrete mixtures.

Figure 8.

Density of concrete mixtures.

Behavior of concrete at preloading stage

The preloading stage was carried out to investigate the difference in performance for NCAC and RCAC beams before strengthening. Beams were preloaded to reach a value of 95 kN, which is approximately equal to 60% of the ultimate load predicted by ACI 318-19 (ACI Committee 318, 2019) provisions for the control beams. From Figure 9(a), one can observe that all beams experienced the same behavior until they reached approximately 25 kN. However, further loading showed that NCAC beams attained smaller mid-span deflections at any specific loading point. Regarding the RCAC beams, it was noted that the percentage of RCA had no significant effect on the resulting mid-span deflection. Furthermore, slightly lower longitudinal steel strain values were noticed in NCAC beams than those of RCAC beams, as shown in Figure 9(b). The concrete strain at the maximum compression zone was nearly the same for both NCAC and RCAC beams with an increase not exceeding 7.5% between each RCA increment by increasing the RCA%, as shown in Figure 9(c).

Figure 9.

Behavior of concrete beams at preloading stage.

Crack pattern and mode of failure

In terms of the crack pattern, two main differences were noticed between RCAC and NCAC beams: (1) cracks in RCAC beams were closer to each other compared to those in NCAC beams (2) cracks started to appear at an earlier loading stage in RCAC beams compared to NCAC beams. All control beams failed after yielding of the longitudinal tension reinforcement and a ductile behavior was achieved. Control beams (R00-S0, R30-S0, R70-S0, and R100-S0) exhibited concrete crushing at failure. Different failure modes were observed for Strengthening Scheme 1: R00-S1 and R30-S1 failed by debonding of CFRP, R70-S1 exhibited a shear failure, while R100-S1 experienced concrete cover separation. Most of the beams strengthened using Scheme 2 (R00-S2, R30-S2, and R70-S2) failed by concrete crushing at the compression zone, except for R100-S2 which failed by rupture of the longitudinal CFRP strip. Figure 10 shows the failure modes for all tested beams.

Figure 10.

Beam failure modes and failure point at final stage of loading.

Load-deflection relationships

Figure 11 shows the midspan deflection results for all the tested beams. Control beams experienced a linear elastic behavior until a load value of approximately 170 kN. Upon further application of load, the longitudinal steel reached its yield stress and the load deflection curve continued in a non-linear nearly horizontal behavior until failure, as shown in Figure 11(a). The results show that both strengthening schemes increased the linear part of the load-deflection curve by approximately 11% for Scheme 1 and 23% for Scheme 2, compared to that of the control beams. After yielding of the longitudinal reinforcement, the non-linear part of the load-deflection curve showed higher stiffness for both strengthening schemes, as presented in Figure 11. It can be noted that NCAC beams sustained higher ultimate loads than RCAC beams by 5.2% for control beams, 5.0% for Strengthening Scheme 1, and 2.9% for Strengthening Scheme 2, Figure 11.

Figure 11.

Deflection curves of the tested beams till failure.

Load-strain relationships

Figure 12(a) shows the strain results of the longitudinal reinforcing steel. One can notice that the steel yielded at a higher load value in CFRP strengthened beams compared to the control beams, exact values range from 7.4%–22.4% for scheme 1, and 23.5% ∼28.7% for scheme 2. No significant difference was noticed between the longitudinal steel strain for NCAC and RCAC beams.

Figure 12.

Load –strain relationships for all beams.

The recorded concrete strains for the tested beams are shown in Figure 12(b), and it can be observed that the modulus of elasticity increased for both the linear and non-linear parts of the curves for both strengthening schemes compared to the control beams. Also, control beams tended to reach higher concrete strain values than strengthened beams in all cases, except for R30-S0. Moreover, the concrete transition from linear to non-linear behavior occurred at a higher load value in strengthened beams. In addition, RCAC beams tended to reach higher maximum strain values than NCAC beams in most strengthening cases. No specific trend was observed for the reached maximum strain in concrete for the tested beams.

The stirrup strain presented in Figure 12(c) shows that the increase in RCA ratio caused the stirrups to carry tensile stresses at lower load values by about 10.3%–22.1% in case of control beams, and 8.2%–45% for CFRP strengthened beams, compared to NCAC beams.

Cracking and failure loads

The first cracking load was determined in the preloading stage (i.e., before applying CFRP strengthening) and plotted against RCA ratio for all beams in Figure 13. The average of three beams for each RCA% is shown in the figure. It can be observed that the cracks start at a lower load value when the RCA ratio increases. Such behavior may be explained by the lower modulus of elasticity of RCAC, which tends to attain higher deformations at the same load compared to NCAC.

Figure 13.

First cracking load for beams with different percentages of RCA.

A comparison between the ultimate load for the beams is presented in Figure 14(a) the results show that increasing the RCA ratio slightly decreases the ultimate load. Moreover, Strengthening Scheme 1 increased the ultimate load by 17.4%–28.2%, while Strengthening Scheme 2 increased the ultimate load by 35.3%–42.7% for all RCA ratios.

Figure 14.

Flexural test results.

Figure 14(b) shows that the control beams with higher amount of RCA replacement experienced larger deflections than the control beam with 0% RCA by 9.7%–18.1%. The same trend was noticed for the Scheme 2 strengthened beams with an increase percentage of 2.8%–6.8%. All the Scheme 1 beams showed the same trend with an increase percentage of 14.7%–20.9% compared to the 0% RCA Scheme 1 beam, except for the 70% RCA beam (R70-S1) which did not follow the same trend due to experiencing a sudden shear failure (shown in Figure 10(m)). Furthermore, Strengthening Scheme 1 experienced the lowest deflection, which may be attributed to the premature failure of Strengthening Scheme 1 caused by debonding of the CFRP, shear failure, or concrete cover spalling.

The ultimate load sustained by the beams is plotted against the different strengthening schemes using all RCA percentages in Figure 15. One can observe that strengthening increased the ultimate load carrying capacity for all the tested beams. Also, increasing the RCA ratio had an adverse effect on all beams except for the two control beams R30-S0 and R70-S0 which showed a favorable effect when the amount of RCA was increased from 30% to 70%. Moreover, NCAC beams sustained higher ultimate loads in all strengthening cases.

Figure 15.

Effect of strengthening for mixes with different RCA.

Table 6 provides a more detailed insight into the ultimate experimental load sustained by the tested beams. Compared to the ultimate loads of NCAC beams, RCAC beams in all strengthening cases sustained a lower ultimate load of 5.2%–11.7% (control beams), 5.0%–16.7% (Strengthening Scheme 1), and 2.9%–7.1% (Strengthening Scheme 2). Further, the minimum reduction in flexural strength caused by the addition of RCA in the beams was observed in Strengthening Scheme 2 (2.9%–7.1%), which indicates the effectiveness of this strengthening scheme for RCAC.

Table 6.

Experimental ultimate load for all the tested beams.

Beam designation	Experimental load (kN)	Decrease in ultimate load due to the presence of RCA	Increase in ultimate load due to strengthening
R00-S0	192.5	—	—
R30-S0	177.5	−7.8%	—
R70-S0	182.5	−5.2%	—
R100-S0	170.0	−11.7%	—
R00-S1	239.5	—	24.4%
R30-S1	227.5	−5.0%	28.2%
R70-S1	216.0	−9.8%	18.4%
R100-S1	199.5	−16.7%	17.4%
R00-S2	260.5	—	35.3%
R30-S2	253.0	−2.9%	42.5%
R70-S2	242.0	−7.1%	32.6%
R100-S2	242.5	−6.9%	42.6%

The use of strengthening scheme 1 increased the ultimate load carried by the beams with a range of 17.4%–28.2%, while strengthening scheme 2 increased the ultimate load by 32.6%–42.6%.

Comparison with code provisions

The experimental results for the tested control beams were compared to those obtained from the ACI 318-19 (ACI Committee 318, 2019), Eurocode 2 (British Standards Institution, 2004), and AS 3600-2018 (Standards Australia Limited, 2018) code provisions. The results are shown in Figure 16, one can observe that the predicted first cracking loads from the three code provisions are significantly lower than those obtained from the experiments. However, cracking load from the ACI 318-19 (ACI Committee 318, 2019) code was the closest to the corresponding experimental load, followed by that from the AS 3600-2018 (Standards Australia Limited, 2018) and Eurocode-2 (British Standards Institution, 2004) codes, Figure 16(a). The predicted ultimate load using the three code provisions are presented in Figure 16(b), where the Eurocode-2 (British Standards Institution, 2004) predicted the closest ultimate load to the experimental value with differences ranging between 0.1% and 12.3%. The ratio between the experimental ultimate load and those predicted by the ACI 318-19 (ACI Committee 318, 2019) code ranged between 2.3% and 14.7%. These ratios were 2.7%–15.2% for the results obtained from the AS 3600-2018 (Standards Australia Limited, 2018) code.

Figure 16.

Experimental and predicted load for control beams.

Table 7 presents a summary of the experimental results and the predicted values based on ACI 318-19 (ACI Committee 318, 2019) and ACI 440.2R-17 (ACI Committee 440, 2017), using the experimentally tested material properties. Moreover, the decrease in the load-carrying capacity of RCAC beams compared to NCAC beams is calculated for both strengthening schemes. The experimental failure loads of all beams were more than those predicted by code standards by 2.3%–40.1%. Therefore, the ACI guidelines can predict the flexural strength of NCAC and RCAC beams with a reasonable safety factor for both control and CFRP strengthened beams. Moreover, significantly higher experimental failure loads than the code predictions were obtained for beam with strengthening Scheme 2 (32.4%–40.1%) when compared to those with Scheme 1 (9.4%–28.8%). Although the noticeable increase in the experimental/predicted load ratios implies a more conservative design, the economical aspect is adversely affected. Therefore, it is recommended that the flexural strength parameters in the utilized code provisions should be revised to accommodate the change in the flexural strength due to utilizing RCA and CFRP strengthening sheets.

Table 7.

Experimental and predicted loads for tested beams.

Beam designation	Experimental load (kN)	Predicted load , * (kN)	% Experimental Load/predicted Load
R00-S0	192.50	167.85	14.7%
R30-S0	177.50	168.43	5.4%
R70-S0	182.50	166.41	9.7%
R100-S0	170.00	166.23	2.3%
R00-S1	239.50	185.91	28.8%
R30-S1	227.50	187.23	21.5%
R70-S1	216.00	182.79	18.2%
R100-S1	199.50	182.43	9.4%
R00-S2	260.50	185.91	40.1%
R30-S2	253.00	187.23	35.1%
R70-S2	242.00	182.79	32.4%
R100-S2	242.50	182.43	32.9%

*Predictions for control beams are based on ACI 318-19.

** Predictions for strengthened beams are based on ACI 440 2R-17.

Conclusions

Twelve RC beams with (0%, 30%, 70%, and 100%) RCA and two different CFRP strengthening schemes were tested to investigate their flexural behavior under two-point loading. Four beams were constructed and tested without preloading or strengthening and considered as control beams. The other eight beams were preloaded with a load equal to 60% of their ultimate capacity. Failure loads of the tested beams were compared to those predicted according to the provisions of the ACI 318-19 (ACI Committee 318, 2019), Eurocode 2 (British Standards Institution, 2004), and AS 3600-2018 (Standards Australia Limited, 2018) codes. The following conclusions can be drawn from the conducted study:

• Difference in compressive strength between the mixes with various RCA did not exceed 7%. While, the splitting tensile strength increased by 13%–15% when the amount of RCA was increased compared to the mixes with NCA.

• During pre-loading stage, percentage of RCA had insignificant effect on the resulting mid-span deflection. However, NCAC beams possessed a noticeable higher modulus of elasticity compared to that of beams with RCAC.

• Cracks started to appear in RCAC beams at a loading stage earlier than that of NCAC beams.

• Longitudinal steel yielded at a higher load level in CFRP strengthened beams compared to control beams by 7.4%–22.4% for beams with Scheme 1%, and 23.5% ∼28.7% for beams with Scheme 2.

• The modulus of elasticity of strengthened beams was higher than that of control beams for both the linear and non-linear parts of the load.

• Replacing NCA with RCA, and further increase in the RCA ratio in the beams caused the stirrups to carry tensile stresses at earlier loading stages for both control beams (−10.3% ∼ −22.1%), and CFRP strengthened beams (−8.2% ∼ −45%).

• Experimental failure loads of the control beams were higher than those predicted by the ACI 318-19 code (ACI Committee 318, 2019) by 2.3%–14.7% for all RCA ratios. These percentages were 0.1%–12.3% and 2.7%–15.2% for the Eurocode 2 (British Standards Institution, 2004) and AS 3600-2018 (Standards Australia Limited, 2018) codes, respectively.

• Experimental failure loads of beams with Strengthening Scheme 2 were 32.4%–40.1% higher than those predicted by ACI 440.2R-17 (ACI Committee 440, 2017) code.

• The experimental results show that the provisions of international codes can be used safely to predict the flexural ultimate capacity of RCA beams strengthened with CFRP.

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

Yasser M Mater

Ahmed A Elansary

Hany A Abdalla

References

ACI Committee 318 (2019) Building Code Requirements for Structural Concrete (ACI 318-19): An ACI Standard; Commentary on Building Code Requirements for Structural Concrete (ACI 318R-19). American Concrete Institute.

ACI Committee 440 (2017) Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. American Concrete Institute.

Ahmed

Lim

(2020) Production of sustainable and structural fiber reinforced recycled aggregate concrete with improved fracture properties: a review. Journal of Cleaner Production 279: 123832.

Ajdukiewicz

Kliszczewicz

(2002) Influence of recycled aggregates on mechanical properties of HS/HPC. Cement and Concrete Composites 24(2): 269–279.

Al-Saawani

El-Sayed

Al-Negheimish

, et al. (2020) Effect of shear-span/depth ratio on debonding failures of FRP-strengthened RC beams. Journal of Building Engineering 32: 101771.

Alqarni

Albidah

Alaskar

, et al. (2020) The effect of coarse aggregate characteristics on the shear behavior of reinforced concrete slender beams. Construction and Building Materials 264: 120189.

ASTM A615 (2008) Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement. West Conshohocken, PA: ASTM International.

ASTM C29 (2017) Standard Test Method for Bulk Density (“Unit Weight”) and Voids in Aggregate. West Conshohocken, PA: ASTM International.

ASTM C33 (2018) Standard Specification for Concrete Aggregates. West Conshohocken, PA: ASTM International.

10.

ASTM C127 (2015) Standard Test Method for Density, Relative Density (Specific Gravity), and Absorption of Coarse Aggregate. West Conshohocken, PA: ASTM International.

11.

ASTM C496 (2017) Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens. West Conshohocken, PA: ASTM International.

12.

Bidabadi

Akbari

Panahi

, et al. (2020) Optimum mix design of recycled concrete based on the fresh and hardened properties of concrete. Journal of Building Engineering 32: 101483.

13.

Boyle

(2013) Evaluation of recycled concrete for use as aggregates in new portland cement concrete pavements. (Doctoral dissertation, Washington State University) https://rex.libraries.wsu.edu/esploro/outputs/graduate/Evaluation-of-Recycled-Concrete-for-Use/99900525043001842

14.

British Standards Institute (2016) BS EN 206:2013+A1:2016 Concrete. Specification, Performance, Production and Conformity. London: BSi.

15.

British Standards Institution (2004) 1-1. Eurocode 2: Design of Concrete Structures–Part 1-1: General Rules and Rules for Buildings. London: BSi.

16.

Butler

West

Tighe

, et al. (2011) The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement. Cement and Concrete Research 41(10): 1037–1049.

17.

Concrete Research Laboratory . Available at: http://eng.cu.edu.eg/en/concrete-research-laboratory/.

18.

D3039 A (2017) ASTM D3039/D3039M-17, Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. West Conshohocken, PA: ASTM International.

19.

Chen

Teng

, et al. (2017) Mitigation of intermediate crack debonding in FRP-plated RC beams using FRP U-jackets. Composite Structures 176: 883–897.

20.

Tang

, et al. (2018) Inclined FRP U-jackets for enhancing structural performance of FRP-plated RC beams suffering from IC debonding. Composite Structures 200: 36–46.

21.

Gholampour

Gandomi

Ozbakkaloglu

, et al. (2017) New formulations for mechanical properties of recycled aggregate concrete using gene expression programming. Construction and Building Materials 130: 122–145.

22.

Helal

Yehia

Hawileh

, et al. (2020) Performance of preloaded CFRP-strengthened fiber reinforced concrete beams. Composite Structures 244: 112262.

23.

Japanese Industrial Standards Committee (2014) JIS A 5308 ready-mixed concrete.

24.

Kang

Kee

S-H

(2017) Improving the quality of mixed recycled coarse aggregates from construction and demolition waste using heavy media separation with Fe3O4 suspension. Advances in Materials Science and Engineering 2017: 1–12.

25.

Katerusha

(2021) Attitude towards sustainability, study contents and the use of recycled concrete in building construction-case study Germany and Switzerland. Journal of Cleaner Production 289: 125688.

26.

Katz

(2003) Properties of concrete made with recycled aggregate from partially hydrated old concrete. Cement and Concrete Research 33(5): 703–711.

27.

KYOWA . Available at: https://www.kyowa-ei.com/eng/product/category/strain_gages/kfgs/index.html.

28.

Dai

Zhan

, et al. (2022) Strength criterion of recycled aggregate concrete under triaxial compression: model calibration. Construction and Building Materials 320: 126201.

29.

Long

Tam

, et al. (2017) Effects of nano-particles on failure process and microstructural properties of recycled aggregate concrete. Construction and Building Materials 142: 42–50.

30.

Liu

Xing

Liu

, et al. (2021) Experimental on repair performance and complete stress-strain curve of self-healing recycled concrete under uniaxial loading. Construction and Building Materials 285: 122900.

31.

Matias

De Brito

Rosa

, et al. (2013) Mechanical properties of concrete produced with recycled coarse aggregates–influence of the use of superplasticizers. Construction and Building Materials 44: 101–109.

32.

McNeil

Kang

THK

(2013) Recycled concrete aggregates: a review. International Journal of Concrete Structures and Materials 7(1): 61–69.

33.

Pan

Liew

, et al. (2020) Utilizing recycled aggregate concrete in sustainable construction for a required compressive strength ratio. Journal of Cleaner Production 276: 124249.

34.

Nagataki

Gokce

Saeki

, et al. (2004) Assessment of recycling process induced damage sensitivity of recycled concrete aggregates. Cement and Concrete Research 34(6): 965–971.

35.

(2021) Prediction of compressive strength of recycled aggregate concrete using artificial neural network and cuckoo search method. Materials Today Proceedings 46: 8480–8488.

36.

Pereira

Evangelista

De Brito

, et al. (2012) The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates. Construction and Building Materials 28(1): 722–729.

37.

Poon

Shui

Lam

, et al. (2004) Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete. Cement and Concrete Research 34(1): 31–36.

38.

Sikadur®-330 . Available at: https://egy.sika.com/content/dam/dms/eg01/b/sikadur_-330.pdf.

39.

SikaWrap®-230 C . Available at: https://egy.sika.com/content/dam/dms/eg01/h/sikawrap_-230_c.pdf.

40.

Silva

De Brito

Dhir

, et al. (2017) Availability and processing of recycled aggregates within the construction and demolition supply chain: a review. Journal of Cleaner Production 143: 598–614.

41.

Standards Australia Limited (2018) Reinforced Concrete Design: In Accordance with AS 3600-2018. Sydney, NSW: Cement & Concrete Aggregates Australia: Standards Australia.

42.

Tam

Wattage

, et al. (2020) Methods to improve microstructural properties of recycled concrete aggregate: a critical review. Construction and Building Materials 270: 121490.

43.

Tokyo Measuring Instruments Lab . Available at: https://tml.jp/e/product/strain_gauge/concrete.html.

44.

Wen

McLean

Willoughby

, et al. (2015) Evaluation of recycled concrete as aggregates in new concrete pavements. Transportation Research Record 2508(1): 73–78.

45.

Xiao

Poon

CJSCTS

, et al. (2012) Recent studies on mechanical properties of recycled aggregate concrete in China—a review. Science China Technological Sciences 55(6): 1463–1480.

46.

Chen

Demartino

, et al. (2021) A Bayesian model updating approach applied to mechanical properties of recycled aggregate concrete under uniaxial or triaxial compression. Construction and Building Materials 301: 124274.

47.

Zhao

, et al. (2019) Parametric sensitivity analysis and modelling of mechanical properties of normal-and high-strength recycled aggregate concrete using grey theory, multiple nonlinear regression and artificial neural networks. Construction and Building Materials 211: 479–491.

48.

Zhu

Zhao

Dai

, et al. (2021) Prediction of compressive strength of recycled aggregate concrete based on gray correlation analysis. Construction and Building Materials 273: 121750.