The construction industry depends largely on conventional materials which are cement, granite and sand for concrete production. The increasing demand for concrete in the construction industry using normal weight aggregates such as gravel and granite has led to the depletion of the natural granite deposits at an alarming rate (Emiero and Oyedepo, 2012; Alengaram et al., 2008), while efforts to maintain ecological balance has also been a major challenge. However, efforts to substitute granite as coarse aggregate has been a major challenge (Adom-Asamoah, and Afrifa, 2010).
Lightweight aggregate concrete (LWAC) has successfully been investigated and used for structural purposes for many years. This is due to the added benefits to the construction industry in its application. Its use results in higher strength to weight ratio, higher tensile strain capacity, superior heat and sound insulation characteristic, and lower dead loads with smaller sections for structural members (Nahhas, 2013; Yasar, 2003). This and other associated benefits make the use of lightweight concrete (LWC) one of the important strategies in reducing the cost of construction in developing and underdeveloped countries (Shafigh et al., 2010; Ndoke, 2006). Lightweight aggregates may be either artificial or natural aggregates that are produced from a wide variety of raw materials and production procedures. Depending on the production techniques and the source of origin, lightweight aggregates vary within broad limits (Chi et al., 2003). The use of different materials, ranging from industrial wastes to agricultural waste, and other naturally occurring lightweight aggregates for concrete production have been reported in literature. Palm kernel shells (PKS) are agricultural solid waste obtained from cracking the fruits of the palm tree. PKS are hard, stony, light and naturally sized, flaky and of irregular shapes depending on the cracking pattern of the nut. Large quantities of PKS are produced by the palm oil mill but only a fraction is used as fuel for steam boilers at palm oil mills. The greater proportion of this by-product is therefore disposed off in stockpiles in open fields which results in negative impact on the environment. Exploiting the use of PKS in construction not only maximise the use of oil palm, but also helps to preserve natural resources and maintain ecological balance. Studies (Okpala, 1990; Mannan and Ganapathy, 2002) have shown that the use of PKS as aggregates can produce structural concrete of compressive strength of about 20 N/mm2 at 28 days with a density in the range of 1800-1900 kg/m3. The structural behaviour in relation to flexure and bonding has been reported in addition to the mechanical properties of PKS concrete (Teo et al., 2006; Alengaram et al., 2010). Teo et al. (2006) reported that ultimate moments predicted using BS 8110 provides a conservative estimate for PKS concrete beams up to a reinforcement ratio of 3.14%. Deflections and crack widths at service loads were all reported to be within maximum allowable values stipulated by BS110. Teo et al. (2007) investigated the structural bond and durability properties of PKS aggregate concrete. The authors concluded that the experimental bond strength of PKS concrete was much higher than theoretical bond strength as stipulated in BS 8110. Alengaram et al. (2010) concluded that the bond between PKS and cement matrix in tension was weaker compared to the bond between crushed granite aggregates and cement matrix.
Shear behavior and design of reinforced lightweight aggregate concrete (LWAC) beams in shear is one of the most topical issues in the structural behavior of reinforced concrete due to its brittle nature (Russo et al., 2004). It is generally assumed that lightweight concrete members are weak in shear due to their lower tensile strength. The shear failure of reinforced concrete beams without web reinforcement is a distinctive case of failure which depends on various parameters such as shear span to effective depth ratio (a/d), tension steel ratio (ρ), type of aggregate, strength of concrete, type of loading, and support conditions (Sudheer et al., 2011). The amount of shear reinforcement has a direct relation on the behavior of structural RC members. This is because it is possible for the structure to fail in a brittle manner without any warning if the shear stress exceeds the shear carrying capacity (Jasim, 2009). Structural design codes have therefore made provisions for the determination of safe shear capacities of concrete and have become more stringent on prevention of sudden failure (Mansour et al., 2004; Yang et al., 2003). Jumaat et al. (2009) investigated the shear strength of four reinforced oil palm shell foamed concrete (OPSFC) beams with a target density of 1600 kg/m3 and a 28-day compressive strength of about 20N/mm2. It was concluded that OPSFC with shear reinforcement failed in flexure mode while beams without shear reinforcement failed in shear modes. Additionally, it was reported that OPSFC beams had 50% higher deflections compared to corresponding NWC beams at ultimate stage. The spacing of cracks in OPSFC concrete beams were all reported to be closer than those found in NWC beams. Setiawan and Saptono (2012) reported on the shear capacity of reinforced concrete beam with different cross section types of lateral reinforcement on minimum ratio. It was concluded that the various types of shear reinforcement cross sections had no significant differences for shear capacity of the concrete.
To fully understand the behaviour of PKS concrete in shear, it is of importance that the effect of shear reinforcement on the behaviour of PKS RC beams in shear be investigated. Knowledge of the post-peak deformation characteristics of shear-critical reinforced concrete members of PKS lightweight concrete is important to better understand the contribution of the shear (web) reinforcement and the failure mechanisms for structural applications. This study is therefore aimed at studying the effect of the amount of shear reinforcement on the shear strength of PKS and NWC RC beams. To this end, the code compliant behaviour of PKS RC beams in shear was investigated using BS 8110, ACI 318 and EC 2 to determine their suitability as shear models for LWC.
Prediction of shear capacity of RC beams
The behaviour of RC elements in flexure has been well understood such that their flexural strengths can be predicted with reasonable accuracy. Contrary to the flexural behaviour of structural members, sophisticated approaches have been proposed based on fracture mechanics, physical models of structural failure, and finite element analyses (Song et al., 2010; Oreta, 2004) to predict the behaviour of beams in shear. That notwithstanding, no single theory is available for estimating the precise shear strength of reinforced concrete elements (Bentz et al., 2006). This problem has been attributed to the complex nature associated with the shear transfer mechanisms especially after cracks are initiated (Song et al., 2010). As a result, the design and behavior of structural concrete to shear is an important and ongoing area of research in structural concrete. However, research has revealed that the resistance of reinforced concrete members to shear is the summation of several internal shear transfer mechanisms (NCHRP, 2005). Some (Hassan et al., 2008; Taylor, 1974) suggest that transfer mechanisms include shear in the uncracked compression zone of the beam (ranging between 20% and 40%), aggregate interlock mechanism or interface shear transfer (ranging between 35% and 50%), dowel action of the longitudinal reinforcement (ranging between 15% to 25%), whilst others Jung and Kim (2008) attribute the transfer to other factors such as residual tensile stresses across the crack, and the presence of shear reinforcement. It is therefore believed that a major component of the shear transfer in the fractured interface is generated from the frictional forces that develop across the diagonal shear cracks due to "aggregate interlock" which provides resistance against slip (Hassan et al., 2008). Generally, tensile stresses due to external loading will exceed the tensile capacity of the concrete, leading to flexure cracks. Once these cracks form, the beam no longer responds to loads in an elastics manner. Shear is then transferred through the intact concrete in the compression zone above the flexure cracks. If the shear span-to-depth ratio is small, arching action and a compression strut may also develop to carry part of the shear.
Most design codes however use empirical equations for simplicity. Some design codes like the ACI limits the shear capacity of a member to the onset of diagonal cracking of the beam, regardless of the ultimate shear load which may usually be higher than the cracking load. This is because the extent to which a member continues to resist shear after the onset of diagonal cracking is unpredictable and depends largely on the loading configurations and the material properties (Juan, 2011). Considering the complex nature of shear resistance, design codes consider the shear capacity of concrete members from two mechanisms: design shear capacity of the concrete without shear reinforcement, Vc and the contribution of shear reinforcement, Vs as shown in Table 1.
For lightweight aggregate concrete (LWAC) members, (BS 8110-2, 1985) adopts the same design parameters as that of normal weight concrete members for concrete grades greater than 25 MPa. Meanwhile the concrete’s design stress (Vc) is taken as 0.8 times the values for normal weight concrete if the grade of concrete is greater than 25 MPa. This factor is also imposed on the maximum limit of shear stress that a section can be subjected to. That is 0.63fcu or 4 MPa whichever is lower.
Materials and beam identification
Ordinary Portland cement with a 28-day compressive strength of 42.5N was used in the study. The fine aggregate used in the study was river sand. The coarse aggregates were crushed palm kernel shells (PKS) with a maximum size of 12.5 mm (for PKS concrete) and granite (for the normal weight concrete). The shells were flushed with water to remove dust and other impurities. The aggregates were oven dried and the physical properties were determined (Table 2). Due to the high water absorption properties of the PKS aggregates (Table 2), they were pre-soaked for 24 hours and subsequently air dried prior to its use. Mix proportions of the PKS concrete was obtained from trial mixes due to the comparatively different nature of the PKS aggregates (Table 2), to obtain a target design strength of 25 N/mm2. Meanwhile, the granite concrete was designed for a target strength of 25 N/mm2 using the UK’s Department of Environment (1998) mix design method. Sika viscocrete, high-range water reducing admixture (superplasticizer) was used to improve the workability of the PKSC mix since the water/cement (w/c) was kept low to obtain the target strength. The mix proportions of both PKSC and NWC are presented in Table 3.
Eight reinforced concrete beams (four PKS and four granite reinforced concrete beams) were cast and tested in the Civil Engineering Concrete Laboratory of KNUST. Four of these beams were made of PKS concrete (P0, P200, P250 and P300) and the other four, normal granite concrete (N0, N200, N250 and N300). All the beams were of the same length of 2000 mm with the same cross-sectional dimension of 110mm (width) and 225 mm (depth). Each beam is designated by a letter and a number. The letters (P or N) denotes the type of concrete whilst the number (0, 200, 250 or 300) denotes the spacing of shear stirrups (Table 4). Each concrete type had one beam (P0 and N0) without web reinforcement. The other three beams of each type of concrete (P200, P250, P300, N200, N250 and N300) had web reinforcement in the spacing of 200, 250 and 300 mm, respectively. Both categories of beams were provided with nominal hanger bars of 2R6. Deformed mild steel bars of mean yield strength 271.2 N/mm2 were used for the longitudinal reinforcement and shear reinforcement (stirrups). The geometrical properties and reinforcement details of the beams are shown in Figures 1 and 2. Companion concrete specimens of 150 × 150 × 150 mm and 100 × 100 × 500 mm were cast to study the compressive and flexural strengths respectively. Curing was done using hessian mat spread on the beams in the open temperature with regular watering until 28-days.
Testing of beams
The beams were simply supported on a stiff steel frame in the Civil Engineering Laboratory of the KNUST, Kumasi. A hydraulic actuator under crosshead displacement control was used to apply loads through a steel spreader beam. A four point loading configuration (Figure 3) was used for the test. The spreader beam had sufficient bending capacity and stiffness to avoid excessive deformation and yielding before failure of the test specimens.
Beam deflections at mid-span for a steady loading rate of 0.2 kN/s were measured with the aid of a dial gauge with a 0.001 mm accuracy fixed at the bottom of each beam. Cracks were marked on the sides of the specimens as they developed, in order to assess the first flexural and shear cracks, and crack widths at tension steel levels. Observation of cracking was performed visually while the crack propagation and crack pattern were marked by hand. Selected crack widths were measured using a crack microscope of optical magnification X10 and reading to 0.02 mm. Initiation and propagation of both flexural crack and shear cracks were closely observed and recorded against corresponding applied loads.
Properties of aggregates
The results of the physical properties of both PKS and crushed granite aggregates are presented in Table 2. The physical properties of the aggregates conformed to the minimum requirements of (BS 882, 1992), however, the PKS aggregates appear stronger than the granite aggregates used. The results of the densities and mechanical properties such as compressive and flexural strength properties of the PKS concrete and normal weight concrete beams tested at the age of 28 days are presented in Table 4. The results show that the compressive strength of PKS concretes are higher than the minimum required strength of 17 N/mm2 for structural lightweight concrete (ASTM C330, 1999). Considering the physical properties, the lower compressive strength could imply that the compressive strength of the PKS ultimately depends on the bond between the aggregates and the cement matrix. Thus, improving the bond between the PKS aggregates will result in increased compressive strength of the PKS concrete.
General behaviour of beams
Table 5 shows the values for cracking loads, ultimate loads and the maximum deflections. In all beam specimens, flexural tension cracks propagated first in the pure bending zone followed by shear cracks in the shear zone. Flexural cracks initiated at 10 kN which represent 20% of ultimate load for the palm kernel shell concrete beams without shear reinforcement (P0) compared to 14 kN (23% of ultimate load) for the normal weight concrete beams without shear reinforcement (N0). First flexural cracks varied from 12.9 to 16.7% of the ultimate for PKS concrete beams with shear reinforcement (P200, P250 and P300). First flexural cracks for the NWC beams with shear reinforcement (N200, N250 and N300) varied from 18.2 to 21.6% of the ultimate load. It is reported that the type of aggregate in concrete can influence the characteristics of the reinforced concrete members in shear (Adom-Asamoah et al., 2012). This was evident as failure loads of NWC beams were comparatively greater than those of PKS concrete beams (Table 4). In all beams, it was seen that increasing the amount of shear reinforcement (stirrups) led to an enhancement of cracking strength of the beams as observed by others researched on other non-conventional aggregates for concrete (Adom-Asamoah and Afrifa, 2013).
Load-deflection, cracking and failure behaviour
First flexural cracks were observed at lower loads among the PKS concrete beams when compared to the NWC beams. This indicates a lower bending strength of PKS concrete beams as shown by the results of the modulus of rupture test (Table 4). In all instances, the ultimate deflections for PKS concrete beams were higher than corresponding NWC concrete beams (Table 5). This may be attributed to the good energy absorbing quality of the PKS aggregates derived from the low AIV and ACV typical of PKS compared to NWC (Teo et al., 2007). The initial straight portion of the curve, before first crack, indicated that the pre-cracking stiffness of the beams were always higher than those of PKS independent of the type of beam and amount of shear reinforcement. At this stage, the beams behaved elastically until the onset of cracks, after which shear is transmitted through the concrete section above the flexural cracks. The onset of cracking in the concrete is thus controlled by the bending strength of the material itself and the compressive strength of the concrete. The beams without stirrups (P0 and N0) failed at a much lower load and had significantly less deflection than the beams with shear reinforcement (Table 5). Shear transfer through the shear reinforcement was activated once the concrete section was no longer able to resist the applied stresses. Consequently, the ultimate deflections were proportional to the amount of shear reinforcement (Figure 4).
The cracking loads and corresponding service loads and crack widths are summarized in Table 5. Generally, all beams failed in shear mode except P200 which failed as a result of flexural shear. Cracking of PKS beams were along the convex surfaces of the shells which shows a weak bond between the PKS aggregates and the cement matrix. The PKS concrete beams exhibited early flexural cracks compared to the companion NWC beams. This could be attributed to the lower modulus of rupture of the PKS beams (Table 4). Diagonal shear loads varied from 48.4 to 58.3% of ultimate loads for PKS concrete beams and 63.6 to 67.6% of ultimate loads for NWC for beams with shear reinforcement. The post-diagonal cracking shear resistance of PKS concrete beams without shear reinforcement were comparatively higher than that of corresponding NWC beams (Table 5). Diagonal shear cracks were observed at about 56% of the ultimate load for the PKS concrete as compared to 66.7% of ultimate loads for corresponding NWC beams. At the post-diagonal cracking, beams without shear reinforcement derive their shear resistance from aggregate interlock mechanisms, and the dowel action between the aggregates and the longitudinal reinforcement. The results show good aggregate interlock in the PKS beams compared to the NWC beams, thereby contributing to the higher post-diagonal cracking shear resistance. More so, it could be deduced from Table 2, the palm kernel shell concrete were able to absorb more loads even after cracking as compared to the normal weight concrete probably due to the lower ACV of 5.3%. In addition, after the post-diagonal cracking, the good bonding between concrete and the longitudinal reinforcement resulted in the transfer of stresses to the steel; leading to the utilization of the tensile strength of the steel to achieve equilibrium in the composite section (Nejadi and Gilbert, 2004).
The number of cracks in PKS concrete beam without shear reinforcement (P0) was 13 compared with 22 cracks in NWC (N0). The average number of cracks in the PKS beams with shear reinforcement (P200, P250 and P300) were greater than those of corresponding NWC beams. The amount of shear reinforcement was directly proportional to the number of cracks in both types of concrete specimens at failure. As observed by (Lim et al., 2006; Hassan et al., 2008) a higher number of cracks resulted in smaller crack widths for both types of concrete beams. It is also seen from Table 5 that PKS concrete beams developed smaller crack widths compared to that of corresponding NWC beams. The wider cracks in NWC can be attributed to physical characteristics of the normal weight aggregate as aggregate interlock plays a significant role in shear transfer across a diagonal crack (Adom-Asamoah and Afrifa, 2013). It was expected that the higher deflections in the PKS beams coupled with the smooth convex surfaces would result in wider crack openings along the convex surfaces. However, the required friction developed by the smooth crack surface of the PKS aggregate concrete for shear transfer, is lost at a lower crack width compared to normal weight concrete. Since the developed friction is low, the crack slips before yielding of the shear reinforcement (Duthinh, 1997), thereby leading to a brittle failure. This could have contributed to the higher post-diagonal cracking resistance but lower ultimate failure loads. Cracks occurring within concrete do not only reduce the stiffness of structural members (Duthinh, 1997) but also make it weak with age due to exposure of structural steel to the environment (Parghi et al., 2008). Therefore, crack widths have to be controlled to acceptable limits in reinforced concrete members for a variety of reasons, such as, control of deflection, corrosion protection, impermeability, maintenance of integrity and appearance of the structure.
Failure modes
Table 5 presents various modes of failure for all specimens. The mode of failure of the test specimens was brittle in nature with virtually no warning. With the exception of P200 which failed as result of concrete crushing, all beam specimens failed as a result of diagonal tension/flexural-shear. Ahmed et al. (1995) noted that the diagonal tension (shear) failure of concrete is brittle in nature and fails with little or no warning especially the failure of beams reinforced only with flexural (tensile) reinforcement. Typically, the concrete rupture completely with wide diagonal cracks which are widest in the flexural tensile zone of the beam. Ultimate failure of all beams occurred with the material rupturing along a fresh diagonal crack parallel and independent from earlier formed cracks. The beams remained relatively stable after the formation of the diagonal crack, which run between the inner edges of the supports (Sagaseta and Vollum, 2010). Diagonal cracks were far from the mid-span of the beam, and splitting the bond of the longitudinal reinforcement all the way to the point of loading. It was observed that initial diagonal tension cracking occurred at a low load in the PKS beams which is related to the flexural strength of the concrete (Juan, 2011). That notwithstanding, PKS concrete beams were able to develop sufficient shear capacity from other mechanisms to continue resisting increasing loads. In addition to the diagonal shear failure, the NWC showed bond failure at the tension side of the beams. This could be attributed to the high stress concentration near the support. It is reported that for shear span/effective depth (as/d) ratio greater than 1 but less than 2.5, the diagonal crack often forms independently and not as a result of flexural crack development (Sagaseta and Vollum, 2010; Kong and Evans, 1994).
Beam specimen P200 failed in a ductile manner and as result of concrete crushing/flexural shear (Figure 5). Vertical flexural cracks propagated from the tension side of the beam to the compression zone. Crushing of the concrete occurred with associated disintegration of the concrete cover in the compression zone. Once the concrete crushed, the beam continued to resist shear stresses through the shear reinforcement and the dowel action of the longitudinal reinforcement until the longitudinal reinforcement yielded. Specimen P200 contained the greatest amount of shear reinforcement for the PKS beams in this study. Given the low compressive strength (Table 3), P200 failed as a result of flexural shear. This implies that the increased shear capacity (resulting from high shear reinforcement ratio) of the PKS beams led to the flexural shear before reaching its full shear capacity. Jumaat et al. (2009) observed that oil palm shell foamed concrete (OPSFC) beams with 150 mm centre to centre web reinforcement failed in flexure compared to OPSFC beams without shear reinforcement which failed in shear modes. Thus increasing shear reinforcement in reinforced PKS concrete beams results in flexural failure mode instead of shear mode.
Shear resistance characteristics of PKS and NWC beams
To analyze and compare the shear strength of beams, the ultimate shear load (Vu) is normalized to account for the difference in compressive strength between SCC and NC. Since the shear strength is proportional to the square root of the compressive strength of concrete (fc) the normalized shear load (Vn) was determined as follows:
Normalized shear load and stress for all experimental PKSC/NWC beams are tabulated in Table 7.
In general, PKSC beams with shear reinforcement exhibited higher Vns compared to NWC beams with shear reinforcement. The normalized shear stress, Vns also decreased with increasing spacing of shear reinforcement for both NWC and PKSC beams. The normalized shear stress of NWC beams without shear reinforcement was however, found to be higher than corresponding PKSC beams with varying spacing of shear reinforcement (Figure 6).
Comparison of test results and some existing code shear models
Table 6 compares the experimental results and theoretical loads calculated using equations in Table 1. The performance of BS 8110, EC 2 and ACI 318-08 in predicting the ultimate shear load of both PKSC and NWC beams with and without shear reinforcement have also been presented. It is seen that ACI 318-08 under predicted the ultimate shear capacity of both PKSC and NWC beams irrespective of the amount of shear reinforcement. The ratio of V(ACI 318)/Vexp ranges between 67 and 76% for PKSC and NWC beams with shear reinforcement.
The study shows that the conservativeness of the ACI increases with decreasing amount of shear reinforcement, and can be used to safely to predict the ultimate shear resistance of both PKSC and NWC beams. The ratio of BS 8110 prediction to experimental values ranges between 84 to 87% for PKSC beams with shear reinforcement and ranges between 81 and 97% for NWC beams with shear reinforcement. It is seen that, both ACI 318 and BS 8110 are conservative, especially for the palm kernel shell concrete beams. However, the ACI is found to be more conservative than the BS 8110. The ratio of predicted EC 2 values to the experimental values ranges from 86 to 89% for the PKSC concrete beams and ranges from 95 to 99% for the NWC beams. EC 2 is found to closely predict the ultimate shear capacity of the PKSC and NWC beams compared to that of the ACI and the BS 8110. For NWC beams without shear reinforce-ment, the predictions are 70%, 76% and 82% of the shear capacity of the NWC beams for the ACI, BS 8110 and EC 2 respectively. The predictions of PKS beams without shear reinforcement are 72, 73 and 80% of the shear capacity of the NWC beams for the ACI, BS 8110 and EC 2, respectively.
The shear stress ratios of experimental values to BS 8110 predicted values range from 1.15 to 1.19 for PKSC beams with shear reinforcement while the ratio range from 1.04 to 1.23 for NWC beams with shear reinforcement. The shear stress ratios of the experimental values to EC2 predicted values range from 0.98 to 1.11 for PKSC beams without shear reinforcement. The variation of the shear stresses in relation to the amount of shear reinforcement was found to be non-proportional. That notwithstanding, the shear stresses decreased with decreasing amount of shear reinforcement. As seen from Figure 7, the experimental shear stress results were higher than the code predictions, indicating that the codes are more conservative with the design of reinforced PKSC beams.
The shear resistances of NWC and PKS concrete beams with and without shear reinforcement were described based on the experimental results. The results of these tests are discussed with particular attention to cracking loads, maximum crack width, deflection, ultimate loads and the effect of shear reinforcement on both PKS concrete and NWC beams. The experimental failure loads are also compared with the predictions of the methods contained in the BS 8110, ACI 318-08 and the EC 2. From the study, the following conclusions are drawn on the behaviour of PKS concrete beams in shear.
The general behaviour of PKS concrete beams in shear is comparable to that of an equivalent NWC beam. The addition of shear reinforcement showed a higher enhancement of the shear capacity of the PKS beams compared to corresponding NWC beams. The failure modes of PKS concrete beams were brittle in nature as a result of diagonal tension. The post-diagonal shear resistance of PKSC beams are higher than that of corresponding NWC beams, especially beams with shear reinforcement. Nevertheless, the ultimate shear strength of NWC beams were higher than that of corresponding PKS concrete beams. EC 2 gives the closest prediction of the shear capacity of PKSC beams compared to the shear models of BS 8110 and ACI 318-08 which tends to under-estimate the shear capacity of PKS concrete beams especially PKS beams without shear reinforcement. The results of the study show that PKS has good potential as a coarse aggregate for the production of structural lightweight concrete for structural application construction.
