# Shear Performance of Concrete Beams with a Maximum Size of Recycled Concrete Aggregate

Authors: Brian Jacks Odero; Raphael N. Mutuku; Timothy Nyomboi; Zachary Abiero Gariy
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The use of concrete mixes with recycled materials has gained traction in research communities. However, the major drawback of the recycled concrete aggregate (RCA) has been the contribution of the cement paste mortar attached to the surface. The quantity of surface-attached mortar on the virgin aggregate dominates and defines the shear performance of concrete beams. Concrete shear failure is usually abrupt and catastrophic, especially in concrete beams without shear reinforcement. The effectiveness of the maximum size of RCA for the performance of untreated and treated with pozzolanic cement and rice husk ash (RHA) slurry was investigated. A total of sixteen (16) beams containing RCA of four (4) maximum sizes, 10, 14, 20, and 25 mm, were prepared and tested after 28 days. The beams were loaded in shear using shear span-depth ratios of 1.25 and 2.50. The results showed that the shear strength of the beams increases as the aggregate size increases to a maximum size of 20 mm, beyond which the shear strength reduces. It was attributed to the attached mortar, which increased as the size of the aggregate increased, creating a weak zone for shear failure in the concrete. In addition, RCA pretreatment using slurry-containing cement and RHA strengthened the attached mortar on the aggregate surface. It resulted in a dense and more compact aggregate that slightly improved the shear strength by about 2.2% for beams containing 20 mm RCA size. The ultimate shear strength of the experimental work in the study was predicted using eight (8) selected models from the literature. Models that account for the aggregate size and shear span to depth ratio resulted in shear strength ratios of 1.29, 0.91, 1.08, and 1.20 for Bažant and Becq-Giraudon, 2002, Zsutty, 1968, Xu et al., 2012, and Gastebled and May, 2001, which are almost similar outcomes to the experimental results.


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## Body

## 1. Introduction

Quite an amount of research has been done in an attempt to explain and highlight the problem of concrete shear [1–3]. The tractions have been so because of the unpredictable catastrophic nature of the occurrence of shear failure in reinforced concrete RC beams [4–6]. Some of the reasons for shear failure in beams have been attributed to the quality, and properties of the structural materials, methodology, and load subjected to structural members [1, 5, 6]. According to Ajamu and Ige [7], concrete is generally known to be a composite material consisting of cement, water, and aggregates with different surface textures. The study also acknowledges that over 75% of concrete by volume is taken by aggregate, within which coarse aggregate proportion occupies over 60% [7].Given the general trends around the world to create sustainability, the environment and economic construction industry is not exceptional. It brings to fore the utilization of recycled concrete aggregates (RCA) as a full substitute for natural coarse aggregates (NCA). RCA is known to possess preexisting macrocracks or microcracks that are probably majorly caused by the production method in the event of getting the right size. As the process is mechanically labor-intensive, it is likely to introduce more cracks and mortar, especially for small size aggregates. The aggregate is composed of the surface-attached mortar of varying quantities, and their sizes may be cumbersome and costly to remove [8, 9]. The study by Butler et al. [10] concluded that the contents of RCA make the process of assessing the failure mechanism fairly complex. They explained this to fracture energy that facilitates the propagation of failure crack within RCA concrete influenced by factors such as concrete mortar strength, overall aggregate strength (RCA), mortar-aggregate bond quality, coarse aggregate shape, RCA content, and volume of deleterious materials.Malešev et al. [11] concluded that the quality and properties of the recycled aggregate depend more on the quantity and quality of the cement mortar attached to the grain of the recycled aggregate. The study by Malešev et al. [11] noted that the amount of water absorbed within the grains of coarse RCA increases as the quantity of adhered mortar increases. In fact, Krezel et al. [12] found out that recycled concrete aggregate consists of 24% by volume of cement paste residue, and the fine content is approximately 2%, which can have a significant impact on the behavior of concrete made from such aggregate.The shear transfer mechanism in concrete beams without stirrups is by four main components, including dowel action, aggregate interlock, uncracked compression zone, and residual strength. Aggregate interlock takes up the proportion of 35–50% by shear resistance of beams without stirrups [7, 13, 14]. According to Tirassa et al., [15] shear failure is mitigated by steel reinforcement normally designed and arranged to counter crack opening and thus to enable the efficient transfer of tensile forces within the beam. Tirassa et al. [15] noted that for concrete coarse RCA without stirrups, the shear transfer is very critical because the aggregates are expected to serve this function without the assistance of reinforcement. Aggregate interlock resistance in shear is brought about by the frictional sliding effect and the tensile stress on the particles. If the shear stress is more than the aggregate, it will peel off from the surface of the stone, leaving behind a smooth surface. Both Trost et al. [16] and Jelić et al. [17] further emphasized that the aggregate interlock component to a large extent depends on cracked surface roughness and compressive strength of concrete. Therefore, for beams with RCA, it will be expected to perform that function. Other studies have recommended surface modification before the aggregate is put to structural action.The effectiveness of aggregate interlock is influenced by the specific quantities of the aggregates, which encompass the size effect, type of aggregate, and concrete strength fundamentally. Again, the size of aggregates is defined by the mixed design process to achieve or meet strength and durability requirements, e.g., 10, 14, 20, 30, and 40 mm.Yi et al. [18] concluded that for normal concrete, the maximum aggregate size has a minimal effect on the concrete tensile strength, however, the shear capacity of beams without shear stirrups is improved with an increase in aggregate size. For normal concrete, the ITZ is the weakest link [15, 19–26] in the concrete matrix, and it is essentially determined by the maximum size of the coarse aggregate [27]. The bigger the size of aggregate, the higher the shear resistance offered by aggregate interlock [27].Mohammed et al. [28] concluded that the size of RCA of 5–14 mm has almost similar performance as the same size of natural coarse aggregate (NCA). The study further suggests that for this to be achieved, it would be crucial to use high-quality RCA with minimal impurities [28].The major components of the hydration of cement and water in the concrete are C–H and C–S–H. The cement-based reaction and the presence of C–H in the composite matrix of C–S–H sheet helps maintain the volume stability of the binding paste. If leached out, it can cause potential volumetric shrinkage and eventual structural collapse [29]. C–H maintains a high pH in interstitial water to help for the continued hydration process. The diffusion of C–H to concrete surface can initiate carbonation by CO₂ in the air, thus protecting the reinforcement steel that is lost from carbonation-induced corrosion [30, 31]. The other way to make C–H more beneficial is transforming it to additional C–S–H from pozzolanic reaction, thus increasing the mechanical strength, durability, and sustainability of the cement paste in concrete [30, 32, 33]. However, excess C–H crystals, by being an efflorescence material, would be easily diffused to surfaces and crack voids, leaving behind ducts and routes within the concrete matrix [34–36]. One of the main solutions proposed by Shahbazpanahi et al. [23] to decrease C–H crystals is the introduction of silica-based pozzolanic material to consume C–H and form more C–S–H.However, coarse RCA is characterized by more than one ITZ in RCA [11, 37]. This area was found to be highly porous and mainly composed of a high amount of calcium hydroxide (C–H), other hydration products (calcium hydrate, ettringite), and unhydrated cement grains [38]. Li et al. and Liang et al. [39, 40], while focusing on the interface bond between the new concrete mortar and the surface of RCA, recommended surface pretreatment, i.e., coating RCA with cement slurry plus silica solution, and found positive results with the compressive strength of concrete.Pozzolanic pretreatment with slurry-containing cement and RHA for the optimum coarse RCA size will guide its adoption for structural concrete beams without shear stirrups. Furthermore, to improve the aggregate performance in beams, this study proposes a surface treatment mechanism for RCAs through a targeted approach to improve the physical and mechanical features of the aggregate. Finally, the study uses experimental investigation to contribute to the critical question of shear resistance of concrete beams containing RCA without shear reinforcement.


## 2. Materials and Methods

### 2.1. Material Properties

#### 2.1.1. Cement

In this study, ordinary Portland cement of grade 42.5N Type CEM I, used for concrete mixing, was sourced from local hardware, based in Juja, Kenya. The results of the chemical analysis done in accordance with BS EN 196–2 [41] show that the proportion of calcium oxide, CaO, is 82.6%, which is dominant, followed by silicon oxide, SiO₂, at 7.1%, while others constitute 3%.


#### 2.1.2. Fine Aggregates

River sand was purchased from sand suppliers from River Ewaso Nyiro, Urchers Post, Isiolo County, Kenya. Natural fine aggregates constitute those fractions from 150μm to 5 mm for standard sieves. The natural fine aggregates were well-graded, conforming to the limiting requirements for fine grading stipulated in BS 882 [42]. The fineness modulus was found to be 2.73 within the range of 2.8–1.5 for medium-fine sand, as described in BS EN 12620 [43].


#### 2.1.3. Coarse Aggregate

The size of natural coarse aggregate (NCA) used in this research in the preparation of parent concrete for RCA ranged between 5 and 20 mm. The NCA was sourced from local suppliers Warren Concrete Limited (K) in Kiambu County, Kenya. One of the challenges with recycled concrete aggregates (RCA) is the determination of the consistent quality of the source. NCA was used to design class C30 concrete, then used to prepare cubes, cylinders, and beams, which were tested and again recrushed into desired aggregates, with a maximum size of 20 mm, to maintain and ascertain uniformity of RCA source. RCA used in this research were taken from laboratory-tested concrete samples of known mix proportions and compressive strength, as shown in Table1. The waste concretes were manually crushed using a small claw hammer crusher into recycled concrete aggregates. Water absorption increases with the size of aggregates because of an increase in adhered mortar on the RCA surface [37].Table 1
Mix proportion and compressive strength of parent concrete used to produce RCA.
SpecificationsValuesWater -to- cement ratio0.54Cement (kg/m³)389Water (kg/m³)210Fine aggregates (kg/m³)645Coarse aggregates (kg/m³)1052Average compressive strength MPa34 ± 4.49


### 2.2. Physical Properties of Coarse RCA

The waste concrete from laboratory test cubes was crushed to produce the recycled coarse aggregates (RCAs) used in recycled concrete. From Table2, the coarse RCA was well-graded, which is within the ASTM: C33/C33M-13 [44]. Water absorption showed an increasing trend as the aggregate size increased from 10, 14, 20, and 25 mm. It supports the argument that as the size increase, the water absorption of aggregate also increases accordingly because of increased fractured surfaces of RCA. The RCA maximum sizes change as 10, 14, 20, and 25 mm. Hence, the properties of aggregate change are demarcated in Table 3. These aggregates were properly sieved to conform to BS 882 [42] requirement before being put to use.Table 2
Grading of coarse RCA based on maximum sizes.
S/No.Sieve size (mm)PassingLimits5–10 mm5–14 mm5–20 mm5–25 mm5–10 mm5–14 mm5–20 mm5–25 mm130———100———100–100225——10092.4——100–10090–100320—10092.753.1—100–10090–10035–7041410095.554.838.9100–10090–10040–8025–5551089.362.434.517.185–10050–8530–6010–406518.36.77.62.110–300–100–100–572.367.301.200–10—0—81.182.7—0—0–5———9Pan————————Table 3
Physical and mechanical properties of coarse aggregates.
PropertiesR-S25R-S20R-S14R-S10Specific gravity (SSD condition)2.392.402.422.38Specific gravity (oven-dry condition)2.242.252.282.22Water absorption (%)6.556.446.147.41Surface mortar (%)10.8010.2719.0726.00R: recycled concrete aggregate and S: maximum aggregate size.


### 2.3. Properties of Pozzolanic Cement and Rice Husk Ash

In this research, pozzolanic cement (PC) and RHA were used to treat the surface of recycled concrete aggregate (RCA) in a slurried form. RHA was purchased from Mwea Area, Kirinyaga County, Kenya. RHA was prepared by burning rice husks for 2 hrs. at 450°C. RHA was first sieved through a 0.30 mm sieve size. The chemical composition of the pozzolanic cement and rice husk ash was determined by the X-ray fluorescence (XRF) technique, and the results are as detailed in Table4. Table 4 shows that RHA has a high content of amorphous silicon oxide, SiO₂ (at 88.18%), while the pozzolanic cement (CEM II) has high calcium oxide, CaO, (at 54.95%).Table 4
Chemical analysis of RHA pozzolan and pozzolanic cement.
ContentPC (%)RHA (%)Silicon dioxide, SiO₂29.36 ± 0.0188.18 ± 0.09Potassium oxide, K₂O2.62 ± 0.013.85 ± 0.01Calcium oxide, CaO54.95 ± 0.032.94 ± 0.02Aluminium oxide, Al₂O₃4.62 ± 0.01—Iron oxide, Fe₂O₃4.74 ± 0.020.92 ± 0.01Others3.71 ± 0.014.11 ± 0.01∗LOI (%)4.19 ± 0.024.59 ± 0.01∗LOI = Loss of ignition according to BS EN 196–2 [41].


### 2.4. Surface Treatment Method

The effect of coarse RCA in concrete beams was assessed using the aggregates in untreated and treated states. The coarse RCA was categorized into four (4) different maximum sizes of 10, 14, 20, and 25 mm. Coarse RCAs were treated using pozzolanic slurries containing 15% pozzolanic cement and 5% rice husk ash, as shown in Table5. About 20% of treatment water was used to premix the surface of RCA to achieve an SSD state, and the remaining 80% of water was used to prepare the treatment slurry. The moistened RCA was introduced into the slurry mixing bath and then gently stirred for 5 minutes before leaving the mix to soak for 15 minutes. It is to allow the proper coating of the RCA surface. The aggregates were removed and allowed to air-dry at room temperature of 20°C for three (3) days. Sieve size 5 mm was finally used to remove finer particles, leaving behind coarse aggregate to prepare concrete mixes for beams.Table 5
Proportion of treatment slurries.
Aggregate typeAggregate max sizeNotationPozzolanconcentrationBinder (kg/m³)RCA (kg/m³)Water (kg/m³)Binder to waterratioPCRHARecycled concrete aggregate10R-S10——————14R-S14——————20R-S20——————25R-S25——————10TR-S1020%1551001002 : 1014TR-S1420%1551001002 : 1020TR-S2020%1551001002 : 1025TR-S2520%1551001002 : 10Figure1 shows the two (2) categories of coarse aggregates. It can be seen that the RCA contained surface-attached mortar containing several voids and cracks (see Figure 1(a)). Specifically, Figure 1(b) represents the treated RCA with 15% cement and 5% rice husk ash. The treated RCA depicts a more homogeneous surface and shape of RCA.Figure 1
Coarse aggregates. (a) Recycled concrete aggregate. (b) Treated recycled concrete aggregate.
(a)(b)


### 2.5. Mix Design and Proportions for Concrete Mix

Two mix types were prepared for each RCA source from ordinary Portland cement. A control mix of concrete from the course natural aggregate of same companion RCA that was well-proportioned in accordance to BRE mixes was obtained from Meru. The mix proportion was done using BRE for all constituents, as shown in Table6. The virgin aggregate in the RCA in each mix was of the same kind, depending on the RCA source, which was natural gravel from the river. The summary of mix proportions used is shown in Table 6.Table 6
Mix proportions.
ContentsR-S10TR-S10R-S14TR-S14R-S20TR-S20R-S25TR-S25Cement (kg/m³)419419382382382382345346Water (kg/m³)230230210210210210190190FA (kg/m³)602645634667674726675689CA (kg/m³)941100899110421054113610561077w/c ratio0.550.550.550.550.550.550.550.55FA: fine aggregate, CA: coarse aggregate, R: recycled concrete aggregate, S: maximum size, and TR: treated recycled concrete aggregate.


### 2.6. Preparation of Beams

All beams are 200 × 300 mm cross-sections, allowing a concrete cover depth of all specimens limited to 20 mm, and a total length of 1840 mm, as shown in Figure2. The beams were simply supported at 120 mm from the edge to prevent the tensile bond failure of the steel rebar, and the load was applied through the actuator head at varying distances from supports. The beams have a clear span between supports 1600 mm long and other geometrical details, as shown in Table 7. Longitudinal reinforcement steel was 2T12 (with cross-sectional area of 226 mm2) with a reinforcement ratio of 0.43% (ρs = 0.43%) and Young’s modulus, Es, of 201.4 N/mm2. The beams were designed to fail in shear, specifically concrete shear. As previously mentioned, there were two series of beams loaded with varying shear arm ratios, i.e., a/d = 1.25 and 2.5. The shear resistance of beams was determined using four-point bending (FPB) until the ultimate failure point.Figure 2
Schematic views of the beam geometry and details. (a) Beam schematic view. (b) Section details.
(a)(b)Table 7
Details of concrete beam for shear failure test.
Beam IDb (mm)d (mm)h (mm)a (mm)L (mm)a/dρs (%)T/R-S10 ∼ 1.25200266300332.516001.250.43T/R-S14 ∼ 1.25200266300332.516001.250.43T/R-S20 ∼ 1.25200266300332.516001.250.43T/R-S25 ∼ 1.25200266300332.516001.250.43T/R-S10 ∼ 2.50200266300665.016002.500.43T/R-S14 ∼ 2.50200266300665.016002.500.43T/R-S20 ∼ 2.50200266300665.016002.500.43T/R-S25 ∼ 2.50200266300665.016002.500.43


### 2.7. Material Test Results for Hardened Properties

Table8 details fresh and hardened properties of concrete that include slump, compressive strength, and tensile strength. It can be seen that there is a significant change in the slump. Six cylinders of a diameter of 100 mm and a depth of 200 mm for tensile strength test and six cubes of 150 × 150 mm for compressive strength test were cast with the beams for each category of aggregate sizes. The cubes and cylinder samples were also tested after 28 days together with concrete beams.Table 8
Fresh and hardened concrete properties.
MixSlump (mm)Compressive strength,fc (MPa)Tensile strength,ft (MPa)R-S1047.530.372.21R-S145627.952.28R-S203929.712.26R-S2538.529.032.15TR-S104431.672.38TR-S1441.7531.212.64TR-S2044.533.602.70TR-S2539.429.742.65


#### 2.7.1. Slump

The effect of RCA maximum size was investigated with respect to the workability of the concrete mix. The result shows that the maximum aggregate size has a significant influence on the concrete mix. The demand for water for workability between 30 and 60 mm was increased as the size increased. At the same time, the water demand increased with the use of pozzolans in the concrete mix.


#### 2.7.2. Compressive Strength of Concrete

R-S10 could have shown strength because of the increased compaction of the aggregate within the mix. The main reason for almost the same strength as the original natural aggregate could be attributed to the fact that the recycled concrete aggregates have high angularity index, which implies the aggregate had a rougher surface that resulted in the increased resistance to force and deformation. Furthermore, the use of CEM I (C42.5) could also be a contributing factor to high-strength development and gain of RCA. The maximum size of RCA increases the strength of the concrete mix. The treatment of aggregates also showed improved strength, with R-S14 showing the highest strength capacity.


#### 2.7.3. Tensile Strength of Concrete

The effect of aggregate size was investigated using the split tensile strength using the universal testing machines. Tensile strengths for the cylinder samples were averagely about 8% of cube compressive strengths for concrete containing untreated and treated RCA. According to Akçaoǧlu et al., [45] tensile strength is also reliant on aggregate size, and the loss in tensile strength with increasing aggregate size becomes greater in high-strength concrete. However, a similar study by McNeil and Kang [46] concluded that splitting tensile strength is less influenced by RCA content and maximum size than compressive strength compared to samples containing NCA.


## 2.1. Material Properties

### 2.1.1. Cement

In this study, ordinary Portland cement of grade 42.5N Type CEM I, used for concrete mixing, was sourced from local hardware, based in Juja, Kenya. The results of the chemical analysis done in accordance with BS EN 196–2 [41] show that the proportion of calcium oxide, CaO, is 82.6%, which is dominant, followed by silicon oxide, SiO₂, at 7.1%, while others constitute 3%.


### 2.1.2. Fine Aggregates

River sand was purchased from sand suppliers from River Ewaso Nyiro, Urchers Post, Isiolo County, Kenya. Natural fine aggregates constitute those fractions from 150μm to 5 mm for standard sieves. The natural fine aggregates were well-graded, conforming to the limiting requirements for fine grading stipulated in BS 882 [42]. The fineness modulus was found to be 2.73 within the range of 2.8–1.5 for medium-fine sand, as described in BS EN 12620 [43].


### 2.1.3. Coarse Aggregate

The size of natural coarse aggregate (NCA) used in this research in the preparation of parent concrete for RCA ranged between 5 and 20 mm. The NCA was sourced from local suppliers Warren Concrete Limited (K) in Kiambu County, Kenya. One of the challenges with recycled concrete aggregates (RCA) is the determination of the consistent quality of the source. NCA was used to design class C30 concrete, then used to prepare cubes, cylinders, and beams, which were tested and again recrushed into desired aggregates, with a maximum size of 20 mm, to maintain and ascertain uniformity of RCA source. RCA used in this research were taken from laboratory-tested concrete samples of known mix proportions and compressive strength, as shown in Table1. The waste concretes were manually crushed using a small claw hammer crusher into recycled concrete aggregates. Water absorption increases with the size of aggregates because of an increase in adhered mortar on the RCA surface [37].Table 1
Mix proportion and compressive strength of parent concrete used to produce RCA.
SpecificationsValuesWater -to- cement ratio0.54Cement (kg/m³)389Water (kg/m³)210Fine aggregates (kg/m³)645Coarse aggregates (kg/m³)1052Average compressive strength MPa34 ± 4.49


## 2.1.1. Cement

In this study, ordinary Portland cement of grade 42.5N Type CEM I, used for concrete mixing, was sourced from local hardware, based in Juja, Kenya. The results of the chemical analysis done in accordance with BS EN 196–2 [41] show that the proportion of calcium oxide, CaO, is 82.6%, which is dominant, followed by silicon oxide, SiO₂, at 7.1%, while others constitute 3%.


## 2.1.2. Fine Aggregates

River sand was purchased from sand suppliers from River Ewaso Nyiro, Urchers Post, Isiolo County, Kenya. Natural fine aggregates constitute those fractions from 150μm to 5 mm for standard sieves. The natural fine aggregates were well-graded, conforming to the limiting requirements for fine grading stipulated in BS 882 [42]. The fineness modulus was found to be 2.73 within the range of 2.8–1.5 for medium-fine sand, as described in BS EN 12620 [43].


## 2.1.3. Coarse Aggregate

The size of natural coarse aggregate (NCA) used in this research in the preparation of parent concrete for RCA ranged between 5 and 20 mm. The NCA was sourced from local suppliers Warren Concrete Limited (K) in Kiambu County, Kenya. One of the challenges with recycled concrete aggregates (RCA) is the determination of the consistent quality of the source. NCA was used to design class C30 concrete, then used to prepare cubes, cylinders, and beams, which were tested and again recrushed into desired aggregates, with a maximum size of 20 mm, to maintain and ascertain uniformity of RCA source. RCA used in this research were taken from laboratory-tested concrete samples of known mix proportions and compressive strength, as shown in Table1. The waste concretes were manually crushed using a small claw hammer crusher into recycled concrete aggregates. Water absorption increases with the size of aggregates because of an increase in adhered mortar on the RCA surface [37].Table 1
Mix proportion and compressive strength of parent concrete used to produce RCA.
SpecificationsValuesWater -to- cement ratio0.54Cement (kg/m³)389Water (kg/m³)210Fine aggregates (kg/m³)645Coarse aggregates (kg/m³)1052Average compressive strength MPa34 ± 4.49


## 2.2. Physical Properties of Coarse RCA

The waste concrete from laboratory test cubes was crushed to produce the recycled coarse aggregates (RCAs) used in recycled concrete. From Table2, the coarse RCA was well-graded, which is within the ASTM: C33/C33M-13 [44]. Water absorption showed an increasing trend as the aggregate size increased from 10, 14, 20, and 25 mm. It supports the argument that as the size increase, the water absorption of aggregate also increases accordingly because of increased fractured surfaces of RCA. The RCA maximum sizes change as 10, 14, 20, and 25 mm. Hence, the properties of aggregate change are demarcated in Table 3. These aggregates were properly sieved to conform to BS 882 [42] requirement before being put to use.Table 2
Grading of coarse RCA based on maximum sizes.
S/No.Sieve size (mm)PassingLimits5–10 mm5–14 mm5–20 mm5–25 mm5–10 mm5–14 mm5–20 mm5–25 mm130———100———100–100225——10092.4——100–10090–100320—10092.753.1—100–10090–10035–7041410095.554.838.9100–10090–10040–8025–5551089.362.434.517.185–10050–8530–6010–406518.36.77.62.110–300–100–100–572.367.301.200–10—0—81.182.7—0—0–5———9Pan————————Table 3
Physical and mechanical properties of coarse aggregates.
PropertiesR-S25R-S20R-S14R-S10Specific gravity (SSD condition)2.392.402.422.38Specific gravity (oven-dry condition)2.242.252.282.22Water absorption (%)6.556.446.147.41Surface mortar (%)10.8010.2719.0726.00R: recycled concrete aggregate and S: maximum aggregate size.


## 2.3. Properties of Pozzolanic Cement and Rice Husk Ash

In this research, pozzolanic cement (PC) and RHA were used to treat the surface of recycled concrete aggregate (RCA) in a slurried form. RHA was purchased from Mwea Area, Kirinyaga County, Kenya. RHA was prepared by burning rice husks for 2 hrs. at 450°C. RHA was first sieved through a 0.30 mm sieve size. The chemical composition of the pozzolanic cement and rice husk ash was determined by the X-ray fluorescence (XRF) technique, and the results are as detailed in Table4. Table 4 shows that RHA has a high content of amorphous silicon oxide, SiO₂ (at 88.18%), while the pozzolanic cement (CEM II) has high calcium oxide, CaO, (at 54.95%).Table 4
Chemical analysis of RHA pozzolan and pozzolanic cement.
ContentPC (%)RHA (%)Silicon dioxide, SiO₂29.36 ± 0.0188.18 ± 0.09Potassium oxide, K₂O2.62 ± 0.013.85 ± 0.01Calcium oxide, CaO54.95 ± 0.032.94 ± 0.02Aluminium oxide, Al₂O₃4.62 ± 0.01—Iron oxide, Fe₂O₃4.74 ± 0.020.92 ± 0.01Others3.71 ± 0.014.11 ± 0.01∗LOI (%)4.19 ± 0.024.59 ± 0.01∗LOI = Loss of ignition according to BS EN 196–2 [41].


## 2.4. Surface Treatment Method

The effect of coarse RCA in concrete beams was assessed using the aggregates in untreated and treated states. The coarse RCA was categorized into four (4) different maximum sizes of 10, 14, 20, and 25 mm. Coarse RCAs were treated using pozzolanic slurries containing 15% pozzolanic cement and 5% rice husk ash, as shown in Table5. About 20% of treatment water was used to premix the surface of RCA to achieve an SSD state, and the remaining 80% of water was used to prepare the treatment slurry. The moistened RCA was introduced into the slurry mixing bath and then gently stirred for 5 minutes before leaving the mix to soak for 15 minutes. It is to allow the proper coating of the RCA surface. The aggregates were removed and allowed to air-dry at room temperature of 20°C for three (3) days. Sieve size 5 mm was finally used to remove finer particles, leaving behind coarse aggregate to prepare concrete mixes for beams.Table 5
Proportion of treatment slurries.
Aggregate typeAggregate max sizeNotationPozzolanconcentrationBinder (kg/m³)RCA (kg/m³)Water (kg/m³)Binder to waterratioPCRHARecycled concrete aggregate10R-S10——————14R-S14——————20R-S20——————25R-S25——————10TR-S1020%1551001002 : 1014TR-S1420%1551001002 : 1020TR-S2020%1551001002 : 1025TR-S2520%1551001002 : 10Figure1 shows the two (2) categories of coarse aggregates. It can be seen that the RCA contained surface-attached mortar containing several voids and cracks (see Figure 1(a)). Specifically, Figure 1(b) represents the treated RCA with 15% cement and 5% rice husk ash. The treated RCA depicts a more homogeneous surface and shape of RCA.Figure 1
Coarse aggregates. (a) Recycled concrete aggregate. (b) Treated recycled concrete aggregate.
(a)(b)


## 2.5. Mix Design and Proportions for Concrete Mix

Two mix types were prepared for each RCA source from ordinary Portland cement. A control mix of concrete from the course natural aggregate of same companion RCA that was well-proportioned in accordance to BRE mixes was obtained from Meru. The mix proportion was done using BRE for all constituents, as shown in Table6. The virgin aggregate in the RCA in each mix was of the same kind, depending on the RCA source, which was natural gravel from the river. The summary of mix proportions used is shown in Table 6.Table 6
Mix proportions.
ContentsR-S10TR-S10R-S14TR-S14R-S20TR-S20R-S25TR-S25Cement (kg/m³)419419382382382382345346Water (kg/m³)230230210210210210190190FA (kg/m³)602645634667674726675689CA (kg/m³)941100899110421054113610561077w/c ratio0.550.550.550.550.550.550.550.55FA: fine aggregate, CA: coarse aggregate, R: recycled concrete aggregate, S: maximum size, and TR: treated recycled concrete aggregate.


## 2.6. Preparation of Beams

All beams are 200 × 300 mm cross-sections, allowing a concrete cover depth of all specimens limited to 20 mm, and a total length of 1840 mm, as shown in Figure2. The beams were simply supported at 120 mm from the edge to prevent the tensile bond failure of the steel rebar, and the load was applied through the actuator head at varying distances from supports. The beams have a clear span between supports 1600 mm long and other geometrical details, as shown in Table 7. Longitudinal reinforcement steel was 2T12 (with cross-sectional area of 226 mm2) with a reinforcement ratio of 0.43% (ρs = 0.43%) and Young’s modulus, Es, of 201.4 N/mm2. The beams were designed to fail in shear, specifically concrete shear. As previously mentioned, there were two series of beams loaded with varying shear arm ratios, i.e., a/d = 1.25 and 2.5. The shear resistance of beams was determined using four-point bending (FPB) until the ultimate failure point.Figure 2
Schematic views of the beam geometry and details. (a) Beam schematic view. (b) Section details.
(a)(b)Table 7
Details of concrete beam for shear failure test.
Beam IDb (mm)d (mm)h (mm)a (mm)L (mm)a/dρs (%)T/R-S10 ∼ 1.25200266300332.516001.250.43T/R-S14 ∼ 1.25200266300332.516001.250.43T/R-S20 ∼ 1.25200266300332.516001.250.43T/R-S25 ∼ 1.25200266300332.516001.250.43T/R-S10 ∼ 2.50200266300665.016002.500.43T/R-S14 ∼ 2.50200266300665.016002.500.43T/R-S20 ∼ 2.50200266300665.016002.500.43T/R-S25 ∼ 2.50200266300665.016002.500.43


## 2.7. Material Test Results for Hardened Properties

Table8 details fresh and hardened properties of concrete that include slump, compressive strength, and tensile strength. It can be seen that there is a significant change in the slump. Six cylinders of a diameter of 100 mm and a depth of 200 mm for tensile strength test and six cubes of 150 × 150 mm for compressive strength test were cast with the beams for each category of aggregate sizes. The cubes and cylinder samples were also tested after 28 days together with concrete beams.Table 8
Fresh and hardened concrete properties.
MixSlump (mm)Compressive strength,fc (MPa)Tensile strength,ft (MPa)R-S1047.530.372.21R-S145627.952.28R-S203929.712.26R-S2538.529.032.15TR-S104431.672.38TR-S1441.7531.212.64TR-S2044.533.602.70TR-S2539.429.742.65


### 2.7.1. Slump

The effect of RCA maximum size was investigated with respect to the workability of the concrete mix. The result shows that the maximum aggregate size has a significant influence on the concrete mix. The demand for water for workability between 30 and 60 mm was increased as the size increased. At the same time, the water demand increased with the use of pozzolans in the concrete mix.


### 2.7.2. Compressive Strength of Concrete

R-S10 could have shown strength because of the increased compaction of the aggregate within the mix. The main reason for almost the same strength as the original natural aggregate could be attributed to the fact that the recycled concrete aggregates have high angularity index, which implies the aggregate had a rougher surface that resulted in the increased resistance to force and deformation. Furthermore, the use of CEM I (C42.5) could also be a contributing factor to high-strength development and gain of RCA. The maximum size of RCA increases the strength of the concrete mix. The treatment of aggregates also showed improved strength, with R-S14 showing the highest strength capacity.


### 2.7.3. Tensile Strength of Concrete

The effect of aggregate size was investigated using the split tensile strength using the universal testing machines. Tensile strengths for the cylinder samples were averagely about 8% of cube compressive strengths for concrete containing untreated and treated RCA. According to Akçaoǧlu et al., [45] tensile strength is also reliant on aggregate size, and the loss in tensile strength with increasing aggregate size becomes greater in high-strength concrete. However, a similar study by McNeil and Kang [46] concluded that splitting tensile strength is less influenced by RCA content and maximum size than compressive strength compared to samples containing NCA.


## 2.7.1. Slump

The effect of RCA maximum size was investigated with respect to the workability of the concrete mix. The result shows that the maximum aggregate size has a significant influence on the concrete mix. The demand for water for workability between 30 and 60 mm was increased as the size increased. At the same time, the water demand increased with the use of pozzolans in the concrete mix.


## 2.7.2. Compressive Strength of Concrete

R-S10 could have shown strength because of the increased compaction of the aggregate within the mix. The main reason for almost the same strength as the original natural aggregate could be attributed to the fact that the recycled concrete aggregates have high angularity index, which implies the aggregate had a rougher surface that resulted in the increased resistance to force and deformation. Furthermore, the use of CEM I (C42.5) could also be a contributing factor to high-strength development and gain of RCA. The maximum size of RCA increases the strength of the concrete mix. The treatment of aggregates also showed improved strength, with R-S14 showing the highest strength capacity.


## 2.7.3. Tensile Strength of Concrete

The effect of aggregate size was investigated using the split tensile strength using the universal testing machines. Tensile strengths for the cylinder samples were averagely about 8% of cube compressive strengths for concrete containing untreated and treated RCA. According to Akçaoǧlu et al., [45] tensile strength is also reliant on aggregate size, and the loss in tensile strength with increasing aggregate size becomes greater in high-strength concrete. However, a similar study by McNeil and Kang [46] concluded that splitting tensile strength is less influenced by RCA content and maximum size than compressive strength compared to samples containing NCA.


## 3. Beam Shear Test Results and Discussions

### 3.1. Effect of Shear Span to Depth Ratio on Ultimate Shear Capacity

Shear arm length,a, defines the ratio of the moment resistance to shear force, Vc, for a concrete beam cross-section. For a given beam, the shear force capacity is inversely proportional to the shear arm length. Indeed, it was confirmed by the experimental results in this research, where an increase in the shear arm ratio from 1.25 to 2.5 reduced the shear force capacity by almost 50% of the beam capacity, as shown in Figure 3. This behavior was generally observed in all beams comprising the various sizes of coarse RCAs, both treated and untreated.Figure 3
Effect of shear arm ratio (a/d) on shear strength of beams.
(a)(b)As presented in Figure3(a), there was a notable reduction in the strength of beams containing an untreated aggregate by 63.1%, 50.0%, 50.3%, and 51.0% for beams marked R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25, respectively. R-S10 ∼ 1.25 resulted in the highest strength reduction (63.1%) because of the high composition of fines of hydrated paste that take up the portion of virgin aggregates. Again, this was followed by R-S25 ∼ 1.25, showing a 51% strength reduction, possibly because of increased hydrated paste around the virgin aggregate and the increasing number of weak links around RCA. Hawkins et al. [47] also confirmed an increase in the shear strength of a beam subjected to a shear arm ratio of less than 2.5. The author attributed this to the arch action effect, where the reduced length of the inclined strut within a beam increases the direct load transfer capacity to support [47].In the same way, concrete beams containing treated coarse aggregates (in Figure3(b)) marks TR-S10 ∼ 2.50, TR-S14 ∼ 2.50, TR-S20 ∼ 2.50, and TR-S25 ∼ 2.50; the reduction in strength because of shear span increase is 60.2%, 50.5%, 51.5% and 38%, respectively. Increment in shear span to depth ratio increases the lever arm, resulting in an increase in shear strength. This phenomenon was confirmed by Bhavani et al. [48]. Even though the reduction in the strength of the beams with treated aggregates is significantly similar to those of the untreated aggregates, the treatment resulted in comparatively higher values. For instance, TR-S20 ∼ 2.50 gave the highest strength of 2.01 N/mm2, indicating a 14% compared to the beam with untreated aggregates.Bhavani et al. [48] looked at various code provisions from multiple countries and concluded that the shear span to depth ratio decreases as there is an increase in the shear strength. Similarly, Kani [49], cited by Sarkhosh et al. [50], suggested that beams tested with shear span-depth ratio (a/d) less than 2.5 highly influences the shear resistance. It is because the applied shear force is transmitted directly to the support by compressive strut (arch action effect) within the concrete beam [49].


### 3.2. Effect of Aggregate Size on Ultimate Shear Capacity

The ultimate shear strength of slender beams without shear stirrups mainly depended on the concrete compressive strength and aggregate interlock between cracked surfaces [27]. Aggregate interlock is associated with type, and the maximum size is the greatest contributor to concrete shear capacity for beams without transverse shear reinforcement according to Fenwick and Pauley [51] as cited by Zhang et al. [26]. Figure 4 illustrates the effect of maximum coarse RCA size, untreated, and treated cases, on the shear strength of beams.Figure 4
Effect of maximum RCA size on shear strength (untreated RCA).Beams labelled R-S20 ∼ 1.25, containing untreated coarse RCA with a maximum size of 20 mm, yielded the highest shear strength, a value of 1.87 N/mm². However, there was a sharp decrease in shear strength to 1.47 N/mm² for beams containing RCA size 25 mm (marked R-S25 ∼ 1.25) . (a/d) as 1.25. The drop in shear strength from 1.76 N/mm2 to 1.54 N/mm2 for R-S10 ∼ 1.25 and R-S14 ∼ 1.25 can be explained by the fact that for R-S10 ∼ 1.25, the aggregate generally comprises small-sized crushed hydrated mortar. The small size increases the compaction of the aggregate particles in the concrete beam, leading to higher resistance to a shear load. Conversely, this trend is not true as aggregate size beyond 20 mm could be attributed to more adhered mortar attached to the surface of coarse RCA. On the other hand, in the beam marked R-S14 ∼ 1.25, the aggregate is more heterogeneous, as it is composed of hydrated cement pastes attached to the surface of the virgin aggregate, which results in a weaker ITZ that reduces shear resistance.A similar increasing trend is observed for beams tested with a shear span to depth ratio of 2.5, peaking for beams marked R-S20 ∼ 2.50 (0.93N/mm²). For beams tested witha/d as 1.25, the beam is subjected to more compression. Therefore, resistance is dependent on the packing density of the aggregates. The packing density of aggregates and particles increases as particle sizes reduces. A similar observation was made for the compressive strength of cube samples, where aggregate size 10 mm resulted in the highest compressive strength of 30.37 N/mm2 compared to other sizes. The general increment of ultimate shear capacity because of an increase in coarse aggregate size is consistent with the models proposed by Vecchio and Collins [52] and Bazant and Sun [53].Just like beams containing untreated RCA, in Figure4, the beams containing treated RCA showed inconsistent trends as the sizes increased from 10, 14, 20, and 25 mm when the beam was tested with a/d = 2.50. The previously observed lower strength can be explained by the packing nature of smaller-sized aggregates, leading to higher quantity of aggregates. A higher quantity of aggregates is expected in the cracked shear plane. It increases crack roughness and strengthens the aggregate interlock for shear resistance in beams [54]. The aggregate treatment increased shear resistance, with treated aggregate size 20 mm showing the highest strength of 2.01 N/mm2 and 0.95 N/mm2 for a/d = 1.25 and 2.5, respectively. A uniform increase in shear strength was observed for beams tested with a/d = 2.5 as the aggregate size increased from 10 to 20 mm, beyond which the trend dropped. Similarly, the strength development trend was observed for the compressive strength test, as shown in Figure 5. Furthermore, these findings are consistent with those of Bhavani et al. [48]; where the study emphasized that as the compressive strength increases, the corresponding shear strength increases to a certain limit of the concrete strength. In this study, concrete samples and beams containing 25 mm aggregate size showed reduced compressive strength and shear strength (see Figure 5), attributed to increased attached mortar around RCA.Figure 5
Effect of maximum RCA size on shear strength (treated RCA).Ignjatović et al. [55] affirmed that where the quality of parent RCA is higher than that of the new concrete, the shear resistance will be better. They attributed it to the increased angularity of RCA, which increased shear resistance in concrete. TR-S20 (with a/d as 1.25 and 2.50) gave the best shear strength performance, probably because of the pozzolanic treatment, leading to an increase in C–S–H that fills the voids and weak joints in RCA. Another author, Malešev et al. [11], argued that the increased thickness of ITZ implies better adhesion between aggregate grain and cement matrix, leading to better physical-mechanical properties of concrete.


### 3.3. Load-Deflection Response

Figure6 presents the relationship between shear load and midspan deflection of concrete beams with varying maximum sizes of untreated and treated coarse RCA. The response of concrete beams subjected to shear loading depicts a bilinear relationship. However, the initial stiffness is maintained, and then the stiffness of the beam decreases, following the second linearity pattern. Consequently, the initial crack for beams with untreated coarse RCAs (see Figure 6(a)) was recorded 18.93 kN (0.59 mm), 24.19 kN (0.34 mm), 16.08 kN (0.61 mm) and 13.82 kN (0.63 mm), for R-S25 ∼ 1.25, R-S20 ∼ 1.25, R-S14 ∼ 1.25, and R-S10 ∼ 1.25, respectively. The response of concrete beams subjected to shear loading depicts a bilinear relationship upto these points. However, the initial stiffness is maintained, and then the stiffness of the beam decreases, following the second linearity pattern.Figure 6
Load-deflection curves (a/d = 1.25).
(a)(b)Slightly better results were obtained in the experiment because of coarse aggregate treatment using 15% cement and 5% RHA. In Figure6(b), it is observed that all the beams depict a linear response between shear force and deformation until the occurrence of the initial crack at 31.86 kN (0.94 mm), 29.56 kN (0.40 mm), 33.73 kN (0.59 mm), and 34.22 kN (0.76 mm), for TR-S25 ∼ 1.25, TR-S20 ∼ 1.25, TR-S14 ∼ 1.25, and TR-S10 ∼ 1.25, respectively. The stiffness drops after the first diagonal cracking and decreases gradually with a further increase in deflection. The shear strength of the beam vanishes when the diagonal tension cracks the compression zone of concrete covering the flexural reinforcement splits.It confirms the suggestion by Akçaoǧ; lu et al. [45]. The study suggested that irregular aggregate shape and rough surface texture enhance mechanical interlocking, increasing the bond strength. However, smooth surface and rounded aggregates result in lower bonding with the concrete matrix [45]. Therefore, for the case of RCA-S10 ∼ 1.25 and RCA-S14 ∼ 1.25, there could be more cracking effect during the processing of the aggregates, resulting in low strength.Figure7 shows the load-deflection curves for all the tested beams with shear arm ratio a/d = 2.5. Figure 7(a) showed a steep slope of load–deflection, a straight line up to the respective elastic limit points, beyond which the beams depict different plastic deformations to an ultimate shear load. The first crack was initiated at an applied load of 18.93, 24.19, 20.61, and 13.82 kN for R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5, respectively. Beyond the elastic limit, the beams experience significant cracking, and the response of the beams becomes nonlinear until the ultimate load.Figure 7
Load–deflection curves (a/d = 2.5).
(a)(b)The response of the four (4) beams was almost similar, although the ultimate load values differed enormously among the tested beams. At final failure, the ultimate shear force of concrete beams containing untreated coarse RCA is 38.35, 49.56, 39.70, and 34.35 kN for beams marked R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5. However, the reduction in shear capacity for beams tested witha/d = 1.25 to 2.5 was overall between 40 and 60% for all beams. The reduction in shear capacity as the shear span ratio increases can be explained by the reduction in the moment of stiffness in the beam, given the increased shear arm length, leading to a reduction in the load carry capacity of the beam member. This observation is also consistent with the findings by Li et al., [56] who varied the shear arm length and recorded an almost 50% shear reduction for the shear span to depth ratio from 1.0 to 3.0.The beams also depicted a clear distinction between those containing treated and untreated coarse RCA (see Figure7(b)). The ultimate shear force for beams containing treated RCA is 42.48, 50.74, 44.25, and 41.89 kN, while the deflections for the beams are 3.85, 6.97, 5.30, and 3.37 mm for those marked as TR-S25 ∼ 2.5, TR-S20 ∼ 2.5, TR-S14 ∼ 2.5, and TR-S10 ∼ 2.5, respectively. Although there was a slight increase of 2.4% for beams made with treated 20 mm RCA, the beams showed the highest ultimate shear resistance and deflection, 50.74 kN and 6.97 mm, before the final collapse, which likely indicated increased stiffness within the aggregate interlock for the beams. The difference in the deflection behavior has been attributed to the low flexural stiffness modulus of the recycled concrete aggregate because of the reduced tensile strength of RCA. Other studies have also observed a similar trend and behavior of the beam [57]. In addition, the reduction in tensile strength can be attributed to decreased resistance to the fragmentation of granular skeleton of recycled concrete aggregate.


### 3.4. Strain in Longitudinal Reinforcement

The shear resistance in beams without transverse reinforcement is generally resisted by the aggregate interlock and dowel action mechanism [2]. The strain gauge attached to the bottom steel bars measured the change in strains in steel as the shear loads were applied during the test. Figure 8 illustrates the relationship between measured shear load versus longitudinal steel strains of concrete beams with varying maximum sizes of untreated and treated coarse RCA. The four (4) graphs of shear load vs reinforcement strain (see Figure 8) obey, to a large extent, the direct linear proportionality up to a certain limit where the beam experiences the first crack formation. However, significant differences are observed for beams with changes in maximum aggregate sizes, treatment modification to coarse RCA, and shear arm ratio a/d.Figure 8
Load–longitudinal steel strain curves (a/d = 1.25).
(a)(b)Figure8(a) represents the effect of maximum aggregate size on the shear strength of concrete beams containing untreated coarse RCA tested with a shear span to depth ratio, a/d, of 1.25. The curves show that the steel reinforcement actively works when the strain in the steel bars reaches 1.0–2.0 × 10−6. Direct proportionality between stress and strain was observed up to shear stress of 0.8, 0.16, 0.16, and 0.51 N/mm2 for R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25 marking crack initiation in the beams. Generally, after a shear stress exceeding about 0.8 N/mm2, the strain rate of steel was much faster. It is a typical effect of reinforced concrete elements because of the loss of the ability to transmit tension stresses through concrete in the cross-section that passes through the crack.Figure8(b) shows the stress-strain of reinforcement curves and highlights the contribution of RCA treatment and aggregate size on the strength of beams for shear span to depth a/d as 1.25. The yield strain is reached at higher shear stress than beams with untreated coarse RCAs shown in Figure 8(a). The results show that the shear stress of concrete beams containing treated RCA with 15% cement and 5% RHA slurry resulted in the shear stress of 0.73 N/mm2 (TR-S10 ∼ 1.25), 1.10 N/mm2 (TR-S14 ∼ 1.25), 0.91 N/mm2 (TR-S20 ∼ 1.25), and 0.78 N/mm2 (TR-S25 ∼ 1.25) for concrete beams tested a/d as 1.25. Considerable strain difference was found for beams at the same shear load. Changes in the slope are generally associated with the formation of shear cracks in concrete. Once cracking occurs, a redistribution of internal stresses occurs, resulting in higher strain in the reinforcement within the region of the cracked zone. It was believed to be only partially because of the changes in aggregate sizes and treatment of RCA.Generally, with the increase in shear arm ratio, 1.25 to 2.5, as shown in Figure9, the slope of the graphs was slightly increased. It could be attributed to increased tension because of increased moment tension contributed by increased shear arm length (a). The beams experienced more loads in flexure compared to beams with a/d as1.25, as the case in Figure 8.Figure 9
Load-longitudinal steel strain curves (a/d = 2.5).
(a)(b)Figure9 shows the distribution of the strain development for eight (8) beams done with treated coarse RCA. It is also useful to pay attention to the movement of the graph immediately after the crack (in the force range of 8–10 kN). For instance, for beams tested with a/d of 1.25, the increase in the steel strain is fast and occurs at a constant load value as compared to that with a/d of 2.5. It shows a faster development of crack formation for these beams of untreated coarse RCA as the beams lose their resistance as the loading progresses.In Figure9(a) with untreated coarse RCAs, the graphs showed a linear relationship between shear stress and strain in reinforcement up to a limit of yield point with values of 0.42 N/mm2 (R-S10 ∼ 2.5), 0.39 N/mm2 (R-S14 ∼ 2.5), 0.62 N/mm2 (R-S20 ∼ 2.5), and 0.53 N/mm2 (R-S25 ∼ 2.5). These showed a significant difference compared to beams with treated coarse RCAs, which were found to be 0.54 N/mm2 (TR-S10 ∼ 2.5), 0.61 N/mm2 (TR-S14 ∼ 2.5), 0.53 N/mm2 (TR-S20 ∼ 2.5), and 0.61 N/mm2 (TR-S25 ∼ 2.5), as shown in Figure 9(b).In tests, this increase in stress occurred within a very short duration. However, the behavior of the beams with treated coarse RCA is slightly different. After the crack formation, the steel gradually increases and is accompanied by a slight increase in the strength of the beam (larger for beams marked TR-S20 ∼ 2.5). The bond force between steel reinforcement and concrete and aggregate size influence the concretes’ ability to induce tensile stresses to reinforcement. A weak bond and poor-quality aggregate will result in failure manifested by crack formation at the bottom fiber of the beam [7] because the composite action between steel and concrete material will be reduced. For untreated RCA, the developed crack will cause the debonding of concrete and steel reinforcement. Eventually, the beam loses the stiffness property and collapses [7].


### 3.5. Strain in Concrete

The concrete strain development of the tested beams was evaluated using stress-strain curves shown in Figures10(a) and 10(b). Previous studies have shown that the coarse aggregate size normally influences the flexural and shear resistance of concrete beams [13]. This research investigated the size influence of aggregate size using untreated and treated coarse RCAs. Concrete compression strain is a critical parameter in determining the shear strength of beams at the compression zone, as shown in Figure 10. Generally, from Figures 10(a) and 10(b), the beams with untreated and treated coarse RCAs have a similar strain development pattern. In addition, it is observed that an initial higher slope up to the first (1st) crack emergence is followed by the reduced slope to the failure point of the beams (see Figures 10(a) and 10(b)).Figure 10
Load-concrete compression strain curves (a/d = 1.25).
(a)(b)Figure10(a), the first plot, represents concrete beams containing untreated RCA, tested with shear span to depth ratio a/d as 1.25. The relationship between shear force and concrete strain is generally linearly related until the first (1st) crack formation. Beyond 1st crack, as the loading continues, the slope is reduced till the final collapse of the beams. Interestingly, there was an insignificant difference in strain development slope in the beams with treated aggregates with a very slight difference after the crack initiation (see Figure 10(a)). The highest maximum measured shear and strain at the first (1st) crack failure point of the beams as 0.51 N/mm2 (73 × 10−6µm) was for the beam done with beam marked R-S20 ∼ 1.25. The lowest observed maximum shear stress and the strain 0.44 N/mm2 (163 × 10−6μm) for the sample marked R-S25 ∼ 1.25.The effect of aggregate treatment with 15% cement and 5% RHA slurry on different RCA sizes in concrete beams is shown in Figure10(b). It could be explained by an increase in the resistance of the compression zone for the beam subjected to the compression effect with a reduced quantity of shear fractured planes of concrete as the size of the aggregate increased. For beams with treated coarse RCAs (see Figure 10(b)), the beam marked TR-S20 ∼ 1.25 showed the highest maximum shear load and strain of 0.66 N/mm2 (184 × 10−6µm), while the lowest values 0.54 N/mm2 (171 × 10−6µm) was shown by sample marked TR-S10 ∼ 1.25. The treatment had more impact on the maximum aggregate size of 20 mm than others, and the treatment reduced the number of cracks in aggregate, increasing shear resistance by the compression zone of beams. It was explained by an increase in the resistance of the compression strut within the concrete beam—the treatment of aggregate improved concrete compressive strength by filling cracks within RCA.The results of concrete beams were tested in compression strain, where the shear arm ratioa/d = 2.5 is shown in Figures 11(a) and 11(b). Just like the results shown in Figure 10 above, the strain in concrete also showed a linear response with an increase in shear load up to the initiation of the first (1st) crack. However, for the series (in Figure 11) of beams, the maximum load before experiencing large strains in concrete was generally reduced.Figure 11
Load-concrete compression strain curves (a/d = 2.5).
(a)(b)The maximum values for beams containing untreated RCA are 0.21 N/mm2 (122 × 10−6 μm), 0.62 N/mm2 (103 × 10−6 μm), 0.23 N/mm2 (122 × 10−6 μm), and 0.18 N/mm2 (120 × 10−6 μm) for beams marked as R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5, respectively (see Figure 11). Comparatively, the beams made using treated coarse RCA resulted in the values of 0.29 N/mm2 (154 × 10−6 μm), 0.29 N/mm2 (124 × 10−6µm), 0.33 N/mm2 (130 × 10−6 μm), and 0.32 N/mm2 (155 × 10−6 μm) for beams marked as TR-S25 ∼ 2.5, TR-S20 ∼ 2.5, TR-S14 ∼ 2.5, and TR-S10 ∼ 2.5, respectively. When analyzing any strength level after a crack formation, it can be observed that in concrete beams made of 20 mm aggregates, the steel loads are significantly higher than in other sizes.The results indicate that the treatment of coarse RCAs contributes to the increase in concrete compression strain. It could be explained by the increase in the strength of the aggregate stiffness of the beam contributed by the increased strength of the aggregates as a result of cement plus RHA slurry treatment. The failure cracks around the aggregate and within the adhered mortar are minimized by the mechanism filling voids with C–S–H gel resulting from the reaction between pozzolans and C–H [58].


### 3.6. Cracking Propagation and Failure Patterns

Figure12 illustrates the cracking patterns for beams with shear arm ratio a/d = 1.25 for beams containing untreated and treated coarse RCA. The beam with shear arm ratio a/d = 1.25 showed a similar pattern where it was observed that the failure was more brittle and occurred abruptly with a bigger diagonal crack formed at a 45° angle of orientation. The number of flexural cracks was more for the beam with untreated RCA than those containing treated RCA (denoted as TR). The beams with untreated RCA generally display smoother crack low aggregate interlock, as confirmed by Fathifazl et al. [59].Figure 12
Crack patterns of the beams on shear failure (a/d = 1.25).From Figure12, the inclination of the critical diagonal crack is 42°, 38.5°, 53.5°, and 44° for TR-S10 ∼ 1.25, TR-S14 ∼ 1.25, TR-S20 ∼ 1.25, and TR-S25 ∼ 1.25, respectively, for beams containing treated RCA. For beams made with untreated RCA, the angle of critical crack was 42°, 39°, 52.5°, and 60.5° for R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25, respectively. For the beam marked TR-S20 ∼ 1.25, there was more flexural crack formation than in other beams. The redistribution of shear stress could explain it to other sections of the beams. However, in beams done with R-S10 ∼ 1.25, the were no other cracks apart from the major diagonal crack.The results imply that there was no effective redistribution of shear failure to other beam sections, which may be because of reduced sizes, meaning more failure planes within and around the aggregate. Generally, for the beam in the same category tested witha/d = 1.25, there was a delay before the formation of the critical failure crack but was eventually followed by a violent crack formation that marked the collapse of the beam [60]. It could be due to the high moment of rapture generated as the loading increases, causing more tension stresses within the concrete matrix. After the violent failure of the beam with a/d = 1.25, the longitudinal tension reinforcement acted as a tie.Figure13 highlights the cracking patterns for beams with shear arm ratio a/d = 2.5. The shear cracks are represented by the red dotted lines, representing the critical diagonal shear crack pattern resulting in shear failure, and a series of minor shear-flexure cracks were observed, which were distributed within the bottom section of the beam and areas of crushed concrete. The bottom tensile zones are characterized by more cracks for beams containing untreated RCA than those with treated RCA beams. It shows that the treatment enhances the strength of RCA, thus increasing shear resistance by aggregate interlock, making the entire beam to participate in the redistribution of stress.Figure 13
Crack patterns of the beams on shear failure (a/d = 2.5).Compared to the failure pattern in Figure13, the beams tested with a shear arm ratio, a/d, as 2.5 failed more in perfect diagonal shear while at the same time exhibiting few shear-flexure cracks before final failure is reached. The behavior could be explained by the increased compression strut length within the beams, and the crack path is delineated along the strut direction. Generally, for beams tested with a/d = 2.5, at the initial cracking stage, the RCA concrete beams exhibited a higher number of cracks than those with a/d = 1.25. A similar observation was made by Yang et al. [25]. It can be explained by inferior ITZ of coarse RCA being weaker than NCA.The inclination of the critical diagonal crack is 61.5°, 45°, 39.5°, and 51° for TR-S10 ∼ 2.50, TR-S14 ∼ 2.50, TR-S20 ∼ 2.50, and TR-S25 ∼ 2.50, respectively, which were measured at the midheight of the beam. Again, for those beams containing untreated coarse RCA, the orientation of the main diagonal crack was 51°, 48°, 68.5°, and 59° for R-S10 ∼ 2.50, R-S14 ∼ 2.50, R-S20 ∼ 2.50, and R-S25 ∼ 2.50, respectively. The average angle of shear cracks to the beam for beams with RCA was higher than that in TRCA beams. However, from the tests (beam witha/d = 2.5), the sections continued to resist the formation of the diagonal crack up to the failure point. It implies that when the shear aim, a/d, was increased, more of the section was subjected to tension. Hence, the section could resist the formation of diagonal cracks. In the beams indicated in Figure 13, the failure was predominantly by flexure compared to those in Figure 12.Prior to the formation of the final diagonal crack, the beams developed shear-flexure cracks first oriented almost 90° and evenly distributed within zone II and III of the beams. The orientation of cracks was such that for zone I (shear) near 45°, zone II (shear-flexure) cracks generated as they approached the compression zone. This zone II was characterized by a lot of branching effects or forming tributaries as the crack progresses past half the depth of the beam. A crack in zone III (flexure) more than 90° normal to the horizontal axis indicates flexural failure.


### 3.7. Comparison between Actual Test Results and Theoretical Predictions of Shear Strength

Figure14 shows the summary of shear strength ratio of experimental-to-ultimate shear strength (see Tables 9 and 10) as given by the eight (8) universal shear model expressions by ACI 318-19 [61], BS 8110-1 [62], EC2-1.1 [63], Fib Model Code 2010 [64], Bažant and Becq-Giraudon [65], Zsutty [66], Xu et al. [67], and Gastebled and May [68], as shown in Table 11. The average value ultimate shear strength ratios are 2.41, 2.24, and 1.92 for ACI 318-19 [61], BS 8110-1 [62], and EC2-1.1 [63], respectively. The three (3) codes show relatively low values of R2, probably because the parameters captured by the models may not accurately explain the relationship between the variables assessed in experimental data. The ACI 318-19 [61], BS 8110-1 [62], and EC2-1.1 [63] do not incorporate the concrete contribution in determining the ultimate shear force. It brings about conservative results compared to the experimental tests of shear-critical reinforced concrete beams. Some of the key parameters of the experimental beam tests without shear reinforcement containing RCA include maximum aggregate size, da, and shear span to depth ratio, a/d.Figure 14
The mean shear strength ratio according to empirical models.Table 9
Theoretical models of the ultimate shear strength given by different codes.
AuthorModel for ultimate shear strength (MPa)ExplanationACI 318-19 [61]vc=0.66λsβrρw1/3fc′β=0.8fck6.89100ρ>1BS 8110-1 [62]vc=0.79/γm100ρfc/251/3400/d1/4βr = 0.75 (if 50–100% replacement otherwise 0.9)EC2-1.1 [63]νc=CRd,ck100ρ1/fck1/3λs=2/1+0.004dfc = cylinder compressive strengthFib model code 2010 [64]νc=0.153d/a1/31+200/d100ρfc′1/3fck = characteristic compressive strength = fc/0.8Bažant and Becq-Giraudon [65]vc=0.54ρ1/3fc′+249ρ/a/d3∗1+5.08/do/1+d/2.5doCRd,c = (0.18 or 0.12) d = effective depth of beamsZsutty [66]νc=2.2fc′ρd/a1/3x2.5d/aa = shear spanXu et al. [67]νc=1.018/dd/as1/3ρ1/61−ρ2/30.0255fc′+1.024k=1+200/dGastebled and May [68]νc=1.109/dd/as1/3ρ1/61−ρ2/3fc′0.35Esγm = material safety factor for shear, taken as 1.25 ρs=As/bwdAs = section of longitudinal reinforcement do = maximum coarse aggregate sizeTable 10
Prediction of concrete ultimate shear strength by empirical models.
Beam IDdafca/dρsVUlt.VtestVEC2-03ACI318BS8110EC2-1.1FIPBazZsuttyXu et. alGast. and May(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)TR-S10-1.251031.671.250.43198.2499.12130.2828.4730.4135.5847.6457.98111.868.8161.5TR-S14-1.251431.211.250.43195.8897.94126.7328.2630.2635.4147.4157.56111.2568.3761.18TR-S20-1.252033.61.250.43213.58106.79129.3329.3231.0136.2948.5959.73114.0270.6662.78TR-S25-1.252529.741.250.43136.8868.44128.3427.5929.7834.8446.6556.19109.4866.9660.16TR-S10-2.501031.672.50.4383.7841.89132.1228.4730.4135.5837.8145.144.3754.6248.81TR-S14-2.501431.212.50.4388.5044.25131.4728.2630.2635.4137.6344.7744.1554.2748.56TR-S20-2.502033.62.50.43101.4850.74134.7529.3231.0136.2938.5646.4545.2556.0949.83TR-S25-2.502529.742.50.4384.9642.48129.3827.5929.7834.8437.0343.743.4553.1547.75R-S10-1.251030.371.250.43187.5693.78130.2827.8829.9935.0946.9856.78110.2567.5760.6R-S14-1.251427.951.250.43182.2491.12126.7326.7429.1734.1345.6954.47107.2465.2558.87R-S20-1.252029.711.250.43199.4299.71129.3327.5729.7734.8346.6356.16109.4466.9460.14R-S25-1.252529.031.250.43156.5878.29128.3427.2529.5434.5646.2755.52108.666.2859.65R-S10-2.501030.372.50.4369.0834.54132.1227.8829.9935.0937.2844.1643.7553.6348.1R-S14-2.501427.952.50.4381.4240.71131.4726.7429.1734.1336.2742.3742.5651.7946.72R-S20-2.502029.712.50.4399.1249.56134.7527.5729.7734.8337.0143.6843.4353.1347.73R-S25-2.502529.032.50.4376.7038.35129.3827.2529.5434.5636.7343.1843.152.6147.35Table 11
Summary of concrete and beam test results.
Model/codesVExp./VCal.ACI318-19BS8110EC2-03FIPBaz.ZsuttyXu et. alGast. and MayAverage2.412.241.921.551.290.911.081.20SD0.960.890.760.460.370.130.320.36CoV (%)39.639.739.829.628.614.329.529.6Min.1.241.150.980.930.780.630.640.72Max.3.643.442.942.201.791.141.511.70Counts1616161616161616UCL95%3.913.643.112.271.871.111.581.76LCL5%0.910.850.720.830.710.700.580.65No. unsafe001161386Note: CoV = Coefficient of variation, SD = Standard deviation, UCL 95% = Upper confidence level 95%, and LCL 5% = Lower confidence level 5%.The mean ratio and coefficient of variation of experimental to predicted strength are 1.55 (29.8%), 1.29 (28.6%), 0.91 (14.3%), 1.08 (29.5%) and 1.20 (29.6%) for Fib Model Code 2010 [64]; Bažant and Becq-Giraudon [65]; Zsutty [66]; Xu et al. [67] and Gastebled and May [68]; respectively. Again these models show the highest R2 values between 0.89 and 0.90, clearly showing a strong relationship between the variables tested and the expressions. Figure 14 shows the safe side for most analytical results based on Zsutty [66]. This is due to the ultimate shear carried by aggregate size by interlock in the shear strength contribution. The Bažant and Becq-Giraudon [65] model could give a lower bound for the shear capacity of available experimental data.It is interesting to note that both Bažant and Becq-Giraudon [65] model and Fib Model Code 2010 [64] considered the same factors in their models, but the Bažant and Becq-Giraudon [65] model overpredicted the shear capacity of the beams. The disparity could be the assumption of crack propagation through aggregate rather than around the aggregate, which is accounted for by a strain factor in the Fib Model Code 2010 [64]. The model by Bažant and Becq-Giraudon [65] accounted for the strength contribution due to beam action in RCA by considering the influence of the aggregate size, while the model expressed by Zsutty [66] accounted for the effect of longitudinal reinforcement and a/d ratio.


## 3.1. Effect of Shear Span to Depth Ratio on Ultimate Shear Capacity

Shear arm length,a, defines the ratio of the moment resistance to shear force, Vc, for a concrete beam cross-section. For a given beam, the shear force capacity is inversely proportional to the shear arm length. Indeed, it was confirmed by the experimental results in this research, where an increase in the shear arm ratio from 1.25 to 2.5 reduced the shear force capacity by almost 50% of the beam capacity, as shown in Figure 3. This behavior was generally observed in all beams comprising the various sizes of coarse RCAs, both treated and untreated.Figure 3
Effect of shear arm ratio (a/d) on shear strength of beams.
(a)(b)As presented in Figure3(a), there was a notable reduction in the strength of beams containing an untreated aggregate by 63.1%, 50.0%, 50.3%, and 51.0% for beams marked R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25, respectively. R-S10 ∼ 1.25 resulted in the highest strength reduction (63.1%) because of the high composition of fines of hydrated paste that take up the portion of virgin aggregates. Again, this was followed by R-S25 ∼ 1.25, showing a 51% strength reduction, possibly because of increased hydrated paste around the virgin aggregate and the increasing number of weak links around RCA. Hawkins et al. [47] also confirmed an increase in the shear strength of a beam subjected to a shear arm ratio of less than 2.5. The author attributed this to the arch action effect, where the reduced length of the inclined strut within a beam increases the direct load transfer capacity to support [47].In the same way, concrete beams containing treated coarse aggregates (in Figure3(b)) marks TR-S10 ∼ 2.50, TR-S14 ∼ 2.50, TR-S20 ∼ 2.50, and TR-S25 ∼ 2.50; the reduction in strength because of shear span increase is 60.2%, 50.5%, 51.5% and 38%, respectively. Increment in shear span to depth ratio increases the lever arm, resulting in an increase in shear strength. This phenomenon was confirmed by Bhavani et al. [48]. Even though the reduction in the strength of the beams with treated aggregates is significantly similar to those of the untreated aggregates, the treatment resulted in comparatively higher values. For instance, TR-S20 ∼ 2.50 gave the highest strength of 2.01 N/mm2, indicating a 14% compared to the beam with untreated aggregates.Bhavani et al. [48] looked at various code provisions from multiple countries and concluded that the shear span to depth ratio decreases as there is an increase in the shear strength. Similarly, Kani [49], cited by Sarkhosh et al. [50], suggested that beams tested with shear span-depth ratio (a/d) less than 2.5 highly influences the shear resistance. It is because the applied shear force is transmitted directly to the support by compressive strut (arch action effect) within the concrete beam [49].


## 3.2. Effect of Aggregate Size on Ultimate Shear Capacity

The ultimate shear strength of slender beams without shear stirrups mainly depended on the concrete compressive strength and aggregate interlock between cracked surfaces [27]. Aggregate interlock is associated with type, and the maximum size is the greatest contributor to concrete shear capacity for beams without transverse shear reinforcement according to Fenwick and Pauley [51] as cited by Zhang et al. [26]. Figure 4 illustrates the effect of maximum coarse RCA size, untreated, and treated cases, on the shear strength of beams.Figure 4
Effect of maximum RCA size on shear strength (untreated RCA).Beams labelled R-S20 ∼ 1.25, containing untreated coarse RCA with a maximum size of 20 mm, yielded the highest shear strength, a value of 1.87 N/mm². However, there was a sharp decrease in shear strength to 1.47 N/mm² for beams containing RCA size 25 mm (marked R-S25 ∼ 1.25) . (a/d) as 1.25. The drop in shear strength from 1.76 N/mm2 to 1.54 N/mm2 for R-S10 ∼ 1.25 and R-S14 ∼ 1.25 can be explained by the fact that for R-S10 ∼ 1.25, the aggregate generally comprises small-sized crushed hydrated mortar. The small size increases the compaction of the aggregate particles in the concrete beam, leading to higher resistance to a shear load. Conversely, this trend is not true as aggregate size beyond 20 mm could be attributed to more adhered mortar attached to the surface of coarse RCA. On the other hand, in the beam marked R-S14 ∼ 1.25, the aggregate is more heterogeneous, as it is composed of hydrated cement pastes attached to the surface of the virgin aggregate, which results in a weaker ITZ that reduces shear resistance.A similar increasing trend is observed for beams tested with a shear span to depth ratio of 2.5, peaking for beams marked R-S20 ∼ 2.50 (0.93N/mm²). For beams tested witha/d as 1.25, the beam is subjected to more compression. Therefore, resistance is dependent on the packing density of the aggregates. The packing density of aggregates and particles increases as particle sizes reduces. A similar observation was made for the compressive strength of cube samples, where aggregate size 10 mm resulted in the highest compressive strength of 30.37 N/mm2 compared to other sizes. The general increment of ultimate shear capacity because of an increase in coarse aggregate size is consistent with the models proposed by Vecchio and Collins [52] and Bazant and Sun [53].Just like beams containing untreated RCA, in Figure4, the beams containing treated RCA showed inconsistent trends as the sizes increased from 10, 14, 20, and 25 mm when the beam was tested with a/d = 2.50. The previously observed lower strength can be explained by the packing nature of smaller-sized aggregates, leading to higher quantity of aggregates. A higher quantity of aggregates is expected in the cracked shear plane. It increases crack roughness and strengthens the aggregate interlock for shear resistance in beams [54]. The aggregate treatment increased shear resistance, with treated aggregate size 20 mm showing the highest strength of 2.01 N/mm2 and 0.95 N/mm2 for a/d = 1.25 and 2.5, respectively. A uniform increase in shear strength was observed for beams tested with a/d = 2.5 as the aggregate size increased from 10 to 20 mm, beyond which the trend dropped. Similarly, the strength development trend was observed for the compressive strength test, as shown in Figure 5. Furthermore, these findings are consistent with those of Bhavani et al. [48]; where the study emphasized that as the compressive strength increases, the corresponding shear strength increases to a certain limit of the concrete strength. In this study, concrete samples and beams containing 25 mm aggregate size showed reduced compressive strength and shear strength (see Figure 5), attributed to increased attached mortar around RCA.Figure 5
Effect of maximum RCA size on shear strength (treated RCA).Ignjatović et al. [55] affirmed that where the quality of parent RCA is higher than that of the new concrete, the shear resistance will be better. They attributed it to the increased angularity of RCA, which increased shear resistance in concrete. TR-S20 (with a/d as 1.25 and 2.50) gave the best shear strength performance, probably because of the pozzolanic treatment, leading to an increase in C–S–H that fills the voids and weak joints in RCA. Another author, Malešev et al. [11], argued that the increased thickness of ITZ implies better adhesion between aggregate grain and cement matrix, leading to better physical-mechanical properties of concrete.


## 3.3. Load-Deflection Response

Figure6 presents the relationship between shear load and midspan deflection of concrete beams with varying maximum sizes of untreated and treated coarse RCA. The response of concrete beams subjected to shear loading depicts a bilinear relationship. However, the initial stiffness is maintained, and then the stiffness of the beam decreases, following the second linearity pattern. Consequently, the initial crack for beams with untreated coarse RCAs (see Figure 6(a)) was recorded 18.93 kN (0.59 mm), 24.19 kN (0.34 mm), 16.08 kN (0.61 mm) and 13.82 kN (0.63 mm), for R-S25 ∼ 1.25, R-S20 ∼ 1.25, R-S14 ∼ 1.25, and R-S10 ∼ 1.25, respectively. The response of concrete beams subjected to shear loading depicts a bilinear relationship upto these points. However, the initial stiffness is maintained, and then the stiffness of the beam decreases, following the second linearity pattern.Figure 6
Load-deflection curves (a/d = 1.25).
(a)(b)Slightly better results were obtained in the experiment because of coarse aggregate treatment using 15% cement and 5% RHA. In Figure6(b), it is observed that all the beams depict a linear response between shear force and deformation until the occurrence of the initial crack at 31.86 kN (0.94 mm), 29.56 kN (0.40 mm), 33.73 kN (0.59 mm), and 34.22 kN (0.76 mm), for TR-S25 ∼ 1.25, TR-S20 ∼ 1.25, TR-S14 ∼ 1.25, and TR-S10 ∼ 1.25, respectively. The stiffness drops after the first diagonal cracking and decreases gradually with a further increase in deflection. The shear strength of the beam vanishes when the diagonal tension cracks the compression zone of concrete covering the flexural reinforcement splits.It confirms the suggestion by Akçaoǧ; lu et al. [45]. The study suggested that irregular aggregate shape and rough surface texture enhance mechanical interlocking, increasing the bond strength. However, smooth surface and rounded aggregates result in lower bonding with the concrete matrix [45]. Therefore, for the case of RCA-S10 ∼ 1.25 and RCA-S14 ∼ 1.25, there could be more cracking effect during the processing of the aggregates, resulting in low strength.Figure7 shows the load-deflection curves for all the tested beams with shear arm ratio a/d = 2.5. Figure 7(a) showed a steep slope of load–deflection, a straight line up to the respective elastic limit points, beyond which the beams depict different plastic deformations to an ultimate shear load. The first crack was initiated at an applied load of 18.93, 24.19, 20.61, and 13.82 kN for R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5, respectively. Beyond the elastic limit, the beams experience significant cracking, and the response of the beams becomes nonlinear until the ultimate load.Figure 7
Load–deflection curves (a/d = 2.5).
(a)(b)The response of the four (4) beams was almost similar, although the ultimate load values differed enormously among the tested beams. At final failure, the ultimate shear force of concrete beams containing untreated coarse RCA is 38.35, 49.56, 39.70, and 34.35 kN for beams marked R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5. However, the reduction in shear capacity for beams tested witha/d = 1.25 to 2.5 was overall between 40 and 60% for all beams. The reduction in shear capacity as the shear span ratio increases can be explained by the reduction in the moment of stiffness in the beam, given the increased shear arm length, leading to a reduction in the load carry capacity of the beam member. This observation is also consistent with the findings by Li et al., [56] who varied the shear arm length and recorded an almost 50% shear reduction for the shear span to depth ratio from 1.0 to 3.0.The beams also depicted a clear distinction between those containing treated and untreated coarse RCA (see Figure7(b)). The ultimate shear force for beams containing treated RCA is 42.48, 50.74, 44.25, and 41.89 kN, while the deflections for the beams are 3.85, 6.97, 5.30, and 3.37 mm for those marked as TR-S25 ∼ 2.5, TR-S20 ∼ 2.5, TR-S14 ∼ 2.5, and TR-S10 ∼ 2.5, respectively. Although there was a slight increase of 2.4% for beams made with treated 20 mm RCA, the beams showed the highest ultimate shear resistance and deflection, 50.74 kN and 6.97 mm, before the final collapse, which likely indicated increased stiffness within the aggregate interlock for the beams. The difference in the deflection behavior has been attributed to the low flexural stiffness modulus of the recycled concrete aggregate because of the reduced tensile strength of RCA. Other studies have also observed a similar trend and behavior of the beam [57]. In addition, the reduction in tensile strength can be attributed to decreased resistance to the fragmentation of granular skeleton of recycled concrete aggregate.


## 3.4. Strain in Longitudinal Reinforcement

The shear resistance in beams without transverse reinforcement is generally resisted by the aggregate interlock and dowel action mechanism [2]. The strain gauge attached to the bottom steel bars measured the change in strains in steel as the shear loads were applied during the test. Figure 8 illustrates the relationship between measured shear load versus longitudinal steel strains of concrete beams with varying maximum sizes of untreated and treated coarse RCA. The four (4) graphs of shear load vs reinforcement strain (see Figure 8) obey, to a large extent, the direct linear proportionality up to a certain limit where the beam experiences the first crack formation. However, significant differences are observed for beams with changes in maximum aggregate sizes, treatment modification to coarse RCA, and shear arm ratio a/d.Figure 8
Load–longitudinal steel strain curves (a/d = 1.25).
(a)(b)Figure8(a) represents the effect of maximum aggregate size on the shear strength of concrete beams containing untreated coarse RCA tested with a shear span to depth ratio, a/d, of 1.25. The curves show that the steel reinforcement actively works when the strain in the steel bars reaches 1.0–2.0 × 10−6. Direct proportionality between stress and strain was observed up to shear stress of 0.8, 0.16, 0.16, and 0.51 N/mm2 for R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25 marking crack initiation in the beams. Generally, after a shear stress exceeding about 0.8 N/mm2, the strain rate of steel was much faster. It is a typical effect of reinforced concrete elements because of the loss of the ability to transmit tension stresses through concrete in the cross-section that passes through the crack.Figure8(b) shows the stress-strain of reinforcement curves and highlights the contribution of RCA treatment and aggregate size on the strength of beams for shear span to depth a/d as 1.25. The yield strain is reached at higher shear stress than beams with untreated coarse RCAs shown in Figure 8(a). The results show that the shear stress of concrete beams containing treated RCA with 15% cement and 5% RHA slurry resulted in the shear stress of 0.73 N/mm2 (TR-S10 ∼ 1.25), 1.10 N/mm2 (TR-S14 ∼ 1.25), 0.91 N/mm2 (TR-S20 ∼ 1.25), and 0.78 N/mm2 (TR-S25 ∼ 1.25) for concrete beams tested a/d as 1.25. Considerable strain difference was found for beams at the same shear load. Changes in the slope are generally associated with the formation of shear cracks in concrete. Once cracking occurs, a redistribution of internal stresses occurs, resulting in higher strain in the reinforcement within the region of the cracked zone. It was believed to be only partially because of the changes in aggregate sizes and treatment of RCA.Generally, with the increase in shear arm ratio, 1.25 to 2.5, as shown in Figure9, the slope of the graphs was slightly increased. It could be attributed to increased tension because of increased moment tension contributed by increased shear arm length (a). The beams experienced more loads in flexure compared to beams with a/d as1.25, as the case in Figure 8.Figure 9
Load-longitudinal steel strain curves (a/d = 2.5).
(a)(b)Figure9 shows the distribution of the strain development for eight (8) beams done with treated coarse RCA. It is also useful to pay attention to the movement of the graph immediately after the crack (in the force range of 8–10 kN). For instance, for beams tested with a/d of 1.25, the increase in the steel strain is fast and occurs at a constant load value as compared to that with a/d of 2.5. It shows a faster development of crack formation for these beams of untreated coarse RCA as the beams lose their resistance as the loading progresses.In Figure9(a) with untreated coarse RCAs, the graphs showed a linear relationship between shear stress and strain in reinforcement up to a limit of yield point with values of 0.42 N/mm2 (R-S10 ∼ 2.5), 0.39 N/mm2 (R-S14 ∼ 2.5), 0.62 N/mm2 (R-S20 ∼ 2.5), and 0.53 N/mm2 (R-S25 ∼ 2.5). These showed a significant difference compared to beams with treated coarse RCAs, which were found to be 0.54 N/mm2 (TR-S10 ∼ 2.5), 0.61 N/mm2 (TR-S14 ∼ 2.5), 0.53 N/mm2 (TR-S20 ∼ 2.5), and 0.61 N/mm2 (TR-S25 ∼ 2.5), as shown in Figure 9(b).In tests, this increase in stress occurred within a very short duration. However, the behavior of the beams with treated coarse RCA is slightly different. After the crack formation, the steel gradually increases and is accompanied by a slight increase in the strength of the beam (larger for beams marked TR-S20 ∼ 2.5). The bond force between steel reinforcement and concrete and aggregate size influence the concretes’ ability to induce tensile stresses to reinforcement. A weak bond and poor-quality aggregate will result in failure manifested by crack formation at the bottom fiber of the beam [7] because the composite action between steel and concrete material will be reduced. For untreated RCA, the developed crack will cause the debonding of concrete and steel reinforcement. Eventually, the beam loses the stiffness property and collapses [7].


## 3.5. Strain in Concrete

The concrete strain development of the tested beams was evaluated using stress-strain curves shown in Figures10(a) and 10(b). Previous studies have shown that the coarse aggregate size normally influences the flexural and shear resistance of concrete beams [13]. This research investigated the size influence of aggregate size using untreated and treated coarse RCAs. Concrete compression strain is a critical parameter in determining the shear strength of beams at the compression zone, as shown in Figure 10. Generally, from Figures 10(a) and 10(b), the beams with untreated and treated coarse RCAs have a similar strain development pattern. In addition, it is observed that an initial higher slope up to the first (1st) crack emergence is followed by the reduced slope to the failure point of the beams (see Figures 10(a) and 10(b)).Figure 10
Load-concrete compression strain curves (a/d = 1.25).
(a)(b)Figure10(a), the first plot, represents concrete beams containing untreated RCA, tested with shear span to depth ratio a/d as 1.25. The relationship between shear force and concrete strain is generally linearly related until the first (1st) crack formation. Beyond 1st crack, as the loading continues, the slope is reduced till the final collapse of the beams. Interestingly, there was an insignificant difference in strain development slope in the beams with treated aggregates with a very slight difference after the crack initiation (see Figure 10(a)). The highest maximum measured shear and strain at the first (1st) crack failure point of the beams as 0.51 N/mm2 (73 × 10−6µm) was for the beam done with beam marked R-S20 ∼ 1.25. The lowest observed maximum shear stress and the strain 0.44 N/mm2 (163 × 10−6μm) for the sample marked R-S25 ∼ 1.25.The effect of aggregate treatment with 15% cement and 5% RHA slurry on different RCA sizes in concrete beams is shown in Figure10(b). It could be explained by an increase in the resistance of the compression zone for the beam subjected to the compression effect with a reduced quantity of shear fractured planes of concrete as the size of the aggregate increased. For beams with treated coarse RCAs (see Figure 10(b)), the beam marked TR-S20 ∼ 1.25 showed the highest maximum shear load and strain of 0.66 N/mm2 (184 × 10−6µm), while the lowest values 0.54 N/mm2 (171 × 10−6µm) was shown by sample marked TR-S10 ∼ 1.25. The treatment had more impact on the maximum aggregate size of 20 mm than others, and the treatment reduced the number of cracks in aggregate, increasing shear resistance by the compression zone of beams. It was explained by an increase in the resistance of the compression strut within the concrete beam—the treatment of aggregate improved concrete compressive strength by filling cracks within RCA.The results of concrete beams were tested in compression strain, where the shear arm ratioa/d = 2.5 is shown in Figures 11(a) and 11(b). Just like the results shown in Figure 10 above, the strain in concrete also showed a linear response with an increase in shear load up to the initiation of the first (1st) crack. However, for the series (in Figure 11) of beams, the maximum load before experiencing large strains in concrete was generally reduced.Figure 11
Load-concrete compression strain curves (a/d = 2.5).
(a)(b)The maximum values for beams containing untreated RCA are 0.21 N/mm2 (122 × 10−6 μm), 0.62 N/mm2 (103 × 10−6 μm), 0.23 N/mm2 (122 × 10−6 μm), and 0.18 N/mm2 (120 × 10−6 μm) for beams marked as R-S25 ∼ 2.5, R-S20 ∼ 2.5, R-S14 ∼ 2.5, and R-S10 ∼ 2.5, respectively (see Figure 11). Comparatively, the beams made using treated coarse RCA resulted in the values of 0.29 N/mm2 (154 × 10−6 μm), 0.29 N/mm2 (124 × 10−6µm), 0.33 N/mm2 (130 × 10−6 μm), and 0.32 N/mm2 (155 × 10−6 μm) for beams marked as TR-S25 ∼ 2.5, TR-S20 ∼ 2.5, TR-S14 ∼ 2.5, and TR-S10 ∼ 2.5, respectively. When analyzing any strength level after a crack formation, it can be observed that in concrete beams made of 20 mm aggregates, the steel loads are significantly higher than in other sizes.The results indicate that the treatment of coarse RCAs contributes to the increase in concrete compression strain. It could be explained by the increase in the strength of the aggregate stiffness of the beam contributed by the increased strength of the aggregates as a result of cement plus RHA slurry treatment. The failure cracks around the aggregate and within the adhered mortar are minimized by the mechanism filling voids with C–S–H gel resulting from the reaction between pozzolans and C–H [58].


## 3.6. Cracking Propagation and Failure Patterns

Figure12 illustrates the cracking patterns for beams with shear arm ratio a/d = 1.25 for beams containing untreated and treated coarse RCA. The beam with shear arm ratio a/d = 1.25 showed a similar pattern where it was observed that the failure was more brittle and occurred abruptly with a bigger diagonal crack formed at a 45° angle of orientation. The number of flexural cracks was more for the beam with untreated RCA than those containing treated RCA (denoted as TR). The beams with untreated RCA generally display smoother crack low aggregate interlock, as confirmed by Fathifazl et al. [59].Figure 12
Crack patterns of the beams on shear failure (a/d = 1.25).From Figure12, the inclination of the critical diagonal crack is 42°, 38.5°, 53.5°, and 44° for TR-S10 ∼ 1.25, TR-S14 ∼ 1.25, TR-S20 ∼ 1.25, and TR-S25 ∼ 1.25, respectively, for beams containing treated RCA. For beams made with untreated RCA, the angle of critical crack was 42°, 39°, 52.5°, and 60.5° for R-S10 ∼ 1.25, R-S14 ∼ 1.25, R-S20 ∼ 1.25, and R-S25 ∼ 1.25, respectively. For the beam marked TR-S20 ∼ 1.25, there was more flexural crack formation than in other beams. The redistribution of shear stress could explain it to other sections of the beams. However, in beams done with R-S10 ∼ 1.25, the were no other cracks apart from the major diagonal crack.The results imply that there was no effective redistribution of shear failure to other beam sections, which may be because of reduced sizes, meaning more failure planes within and around the aggregate. Generally, for the beam in the same category tested witha/d = 1.25, there was a delay before the formation of the critical failure crack but was eventually followed by a violent crack formation that marked the collapse of the beam [60]. It could be due to the high moment of rapture generated as the loading increases, causing more tension stresses within the concrete matrix. After the violent failure of the beam with a/d = 1.25, the longitudinal tension reinforcement acted as a tie.Figure13 highlights the cracking patterns for beams with shear arm ratio a/d = 2.5. The shear cracks are represented by the red dotted lines, representing the critical diagonal shear crack pattern resulting in shear failure, and a series of minor shear-flexure cracks were observed, which were distributed within the bottom section of the beam and areas of crushed concrete. The bottom tensile zones are characterized by more cracks for beams containing untreated RCA than those with treated RCA beams. It shows that the treatment enhances the strength of RCA, thus increasing shear resistance by aggregate interlock, making the entire beam to participate in the redistribution of stress.Figure 13
Crack patterns of the beams on shear failure (a/d = 2.5).Compared to the failure pattern in Figure13, the beams tested with a shear arm ratio, a/d, as 2.5 failed more in perfect diagonal shear while at the same time exhibiting few shear-flexure cracks before final failure is reached. The behavior could be explained by the increased compression strut length within the beams, and the crack path is delineated along the strut direction. Generally, for beams tested with a/d = 2.5, at the initial cracking stage, the RCA concrete beams exhibited a higher number of cracks than those with a/d = 1.25. A similar observation was made by Yang et al. [25]. It can be explained by inferior ITZ of coarse RCA being weaker than NCA.The inclination of the critical diagonal crack is 61.5°, 45°, 39.5°, and 51° for TR-S10 ∼ 2.50, TR-S14 ∼ 2.50, TR-S20 ∼ 2.50, and TR-S25 ∼ 2.50, respectively, which were measured at the midheight of the beam. Again, for those beams containing untreated coarse RCA, the orientation of the main diagonal crack was 51°, 48°, 68.5°, and 59° for R-S10 ∼ 2.50, R-S14 ∼ 2.50, R-S20 ∼ 2.50, and R-S25 ∼ 2.50, respectively. The average angle of shear cracks to the beam for beams with RCA was higher than that in TRCA beams. However, from the tests (beam witha/d = 2.5), the sections continued to resist the formation of the diagonal crack up to the failure point. It implies that when the shear aim, a/d, was increased, more of the section was subjected to tension. Hence, the section could resist the formation of diagonal cracks. In the beams indicated in Figure 13, the failure was predominantly by flexure compared to those in Figure 12.Prior to the formation of the final diagonal crack, the beams developed shear-flexure cracks first oriented almost 90° and evenly distributed within zone II and III of the beams. The orientation of cracks was such that for zone I (shear) near 45°, zone II (shear-flexure) cracks generated as they approached the compression zone. This zone II was characterized by a lot of branching effects or forming tributaries as the crack progresses past half the depth of the beam. A crack in zone III (flexure) more than 90° normal to the horizontal axis indicates flexural failure.


## 3.7. Comparison between Actual Test Results and Theoretical Predictions of Shear Strength

Figure14 shows the summary of shear strength ratio of experimental-to-ultimate shear strength (see Tables 9 and 10) as given by the eight (8) universal shear model expressions by ACI 318-19 [61], BS 8110-1 [62], EC2-1.1 [63], Fib Model Code 2010 [64], Bažant and Becq-Giraudon [65], Zsutty [66], Xu et al. [67], and Gastebled and May [68], as shown in Table 11. The average value ultimate shear strength ratios are 2.41, 2.24, and 1.92 for ACI 318-19 [61], BS 8110-1 [62], and EC2-1.1 [63], respectively. The three (3) codes show relatively low values of R2, probably because the parameters captured by the models may not accurately explain the relationship between the variables assessed in experimental data. The ACI 318-19 [61], BS 8110-1 [62], and EC2-1.1 [63] do not incorporate the concrete contribution in determining the ultimate shear force. It brings about conservative results compared to the experimental tests of shear-critical reinforced concrete beams. Some of the key parameters of the experimental beam tests without shear reinforcement containing RCA include maximum aggregate size, da, and shear span to depth ratio, a/d.Figure 14
The mean shear strength ratio according to empirical models.Table 9
Theoretical models of the ultimate shear strength given by different codes.
AuthorModel for ultimate shear strength (MPa)ExplanationACI 318-19 [61]vc=0.66λsβrρw1/3fc′β=0.8fck6.89100ρ>1BS 8110-1 [62]vc=0.79/γm100ρfc/251/3400/d1/4βr = 0.75 (if 50–100% replacement otherwise 0.9)EC2-1.1 [63]νc=CRd,ck100ρ1/fck1/3λs=2/1+0.004dfc = cylinder compressive strengthFib model code 2010 [64]νc=0.153d/a1/31+200/d100ρfc′1/3fck = characteristic compressive strength = fc/0.8Bažant and Becq-Giraudon [65]vc=0.54ρ1/3fc′+249ρ/a/d3∗1+5.08/do/1+d/2.5doCRd,c = (0.18 or 0.12) d = effective depth of beamsZsutty [66]νc=2.2fc′ρd/a1/3x2.5d/aa = shear spanXu et al. [67]νc=1.018/dd/as1/3ρ1/61−ρ2/30.0255fc′+1.024k=1+200/dGastebled and May [68]νc=1.109/dd/as1/3ρ1/61−ρ2/3fc′0.35Esγm = material safety factor for shear, taken as 1.25 ρs=As/bwdAs = section of longitudinal reinforcement do = maximum coarse aggregate sizeTable 10
Prediction of concrete ultimate shear strength by empirical models.
Beam IDdafca/dρsVUlt.VtestVEC2-03ACI318BS8110EC2-1.1FIPBazZsuttyXu et. alGast. and May(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)TR-S10-1.251031.671.250.43198.2499.12130.2828.4730.4135.5847.6457.98111.868.8161.5TR-S14-1.251431.211.250.43195.8897.94126.7328.2630.2635.4147.4157.56111.2568.3761.18TR-S20-1.252033.61.250.43213.58106.79129.3329.3231.0136.2948.5959.73114.0270.6662.78TR-S25-1.252529.741.250.43136.8868.44128.3427.5929.7834.8446.6556.19109.4866.9660.16TR-S10-2.501031.672.50.4383.7841.89132.1228.4730.4135.5837.8145.144.3754.6248.81TR-S14-2.501431.212.50.4388.5044.25131.4728.2630.2635.4137.6344.7744.1554.2748.56TR-S20-2.502033.62.50.43101.4850.74134.7529.3231.0136.2938.5646.4545.2556.0949.83TR-S25-2.502529.742.50.4384.9642.48129.3827.5929.7834.8437.0343.743.4553.1547.75R-S10-1.251030.371.250.43187.5693.78130.2827.8829.9935.0946.9856.78110.2567.5760.6R-S14-1.251427.951.250.43182.2491.12126.7326.7429.1734.1345.6954.47107.2465.2558.87R-S20-1.252029.711.250.43199.4299.71129.3327.5729.7734.8346.6356.16109.4466.9460.14R-S25-1.252529.031.250.43156.5878.29128.3427.2529.5434.5646.2755.52108.666.2859.65R-S10-2.501030.372.50.4369.0834.54132.1227.8829.9935.0937.2844.1643.7553.6348.1R-S14-2.501427.952.50.4381.4240.71131.4726.7429.1734.1336.2742.3742.5651.7946.72R-S20-2.502029.712.50.4399.1249.56134.7527.5729.7734.8337.0143.6843.4353.1347.73R-S25-2.502529.032.50.4376.7038.35129.3827.2529.5434.5636.7343.1843.152.6147.35Table 11
Summary of concrete and beam test results.
Model/codesVExp./VCal.ACI318-19BS8110EC2-03FIPBaz.ZsuttyXu et. alGast. and MayAverage2.412.241.921.551.290.911.081.20SD0.960.890.760.460.370.130.320.36CoV (%)39.639.739.829.628.614.329.529.6Min.1.241.150.980.930.780.630.640.72Max.3.643.442.942.201.791.141.511.70Counts1616161616161616UCL95%3.913.643.112.271.871.111.581.76LCL5%0.910.850.720.830.710.700.580.65No. unsafe001161386Note: CoV = Coefficient of variation, SD = Standard deviation, UCL 95% = Upper confidence level 95%, and LCL 5% = Lower confidence level 5%.The mean ratio and coefficient of variation of experimental to predicted strength are 1.55 (29.8%), 1.29 (28.6%), 0.91 (14.3%), 1.08 (29.5%) and 1.20 (29.6%) for Fib Model Code 2010 [64]; Bažant and Becq-Giraudon [65]; Zsutty [66]; Xu et al. [67] and Gastebled and May [68]; respectively. Again these models show the highest R2 values between 0.89 and 0.90, clearly showing a strong relationship between the variables tested and the expressions. Figure 14 shows the safe side for most analytical results based on Zsutty [66]. This is due to the ultimate shear carried by aggregate size by interlock in the shear strength contribution. The Bažant and Becq-Giraudon [65] model could give a lower bound for the shear capacity of available experimental data.It is interesting to note that both Bažant and Becq-Giraudon [65] model and Fib Model Code 2010 [64] considered the same factors in their models, but the Bažant and Becq-Giraudon [65] model overpredicted the shear capacity of the beams. The disparity could be the assumption of crack propagation through aggregate rather than around the aggregate, which is accounted for by a strain factor in the Fib Model Code 2010 [64]. The model by Bažant and Becq-Giraudon [65] accounted for the strength contribution due to beam action in RCA by considering the influence of the aggregate size, while the model expressed by Zsutty [66] accounted for the effect of longitudinal reinforcement and a/d ratio.


## 4. Conclusion

This paper investigated the shear strength of concrete beams experimentally with full replacement of RCA as coarse aggregates using sixteen (16) RC beams without shear reinforcement. The main aim was to determine the contribution of coarse RCA based on aggregate size (10, 14, 20 and 25 mm) effect and pretreatment with cement plus RHA slurry (15% PC and 5% RHA) for use in concrete beams. The tests measured the mid-span load-displacement relationship and contribution of the maximum aggregate size and shear span to depth ratio. This comes from the understanding that the properties of coarse RCA are influenced by the quantity and strength of surface adhering mortar to the original natural aggregate. The obtained laboratory results were also compared with the outcome of the empirical expression from ACI 318-19 [61], BS 8110-1 [62], EC2-1.1 [63], Fib Model Code 2010 [64], Bažant and Becq-Giraudon [65], Zsutty [66], Xu et al. [67], and Gastebled and May [68]. The conclusions made were as follows:(i)
The ultimate shear strength of concrete beams with treated coarse RCA slightly increases by 21.5% (10 mm), 7.8% (14 mm), 2.2% (20 mm) and 11.1% (25 mm) compared with untreated RCA tested with shear span to depth ratio 2.5.(ii)
Shear span to depth ratio was found to significantly influence the shear strength of beams, notwithstanding the nature of RCA coarse RCA content used. For beams containing the treated aggregates, an increase in shear span to depth ratio from 1.25 to 2.50 resulted in a reduction of 60.2% (10 mm), 50.5% (14 mm), 51.5% (20 mm), and 38.0% (25 mm), respectively.(iii)
Shear strength of concrete beams depicts an increasing trend as maximum size increases, giving the highest value of 0.93 MPa and 0.95 MPa, respectively, which was observed for those beams containing untreated and treated 20 mm maximum aggregates size.(iv)
Among the models and empirical code expressions considered, the standard codes overestimated the shear strength of beams. This is because they focus on compressive strength rather than shear span ratio and aggregate size, which are crucial for the shear performance of beams without shear stirrups. The mean shear strength ratio and COV are 2.41 (39.6%), 2.24 (39.7%), 1.92 (39.8%) and 1.55 (29.8%) for ACI 318-19 [61], EC2-1.1 [63], BS 8110-1 [62], Fib Model Code 2010 [64], respectively. It implies that the prediction of the shear strength of the RCA members using the current provisions was conservative. However, Zsutty [66] and Xu et al. [67] significantly underestimate the shear performance of beams, giving a shear strength ratio and COV of 0.91 (14.3%) and 1.08 (29.5%), respectively.



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*Source: 6804155-2022-09-17.xml*