The bond between concrete and reinforcing bars is a very crucial parameter in engineering applications. It is this bond that determines the capability of reinforced concrete to perform its intended function effectively. A structure built using reinforced concrete can only perform as expected if the bond is strong enough to provide desired structural strength and stability. Therefore it is important to understand how the two materials, i.e. concrete and reinforcing bars, combines to form that crucial bond. Reinforced concrete refers to concrete made up the concrete and reinforcing bars (also known as rebars). Concrete is strong in compression and therefore has adequate capacity to support compression loads or resist compression force. The same concrete is weak in tension thus cannot resist significant tensile forces. On the other hand, reinforcing bars are strong in tension and therefore have adequate capacity to support tension loads or resist tensile forces. The reinforcing bars are also weak in compression meaning that they cannot support substantial compression loads.
When reinforcing bars are embedded in concrete, the homogenous material formed is reinforced concrete[1]. Reinforce concrete is also referred to a RCC or reinforced cement concrete[2]. In this material, the concrete contributes its high resistance to tensile forces and the reinforcing bars contribute its high ability to resist tensile forces. As a result of this, the reinforced concrete has great ability to resist both compression and tensile forces. When concrete combines with reinforcing bars, a strong and efficient bond is formed between them[3]. Reinforcing bars have ridges (surface deformations) that enhances the bond even further. Because of the strong bond, concrete is able to transfer stresses to the reinforcing bars effectively and the reinforcing bars also transfer stresses to the concrete. Concrete has brittle properties and therefore it easily cracks and breaks when subjected to tensile forces or various climate variations[4]. Reinforcing bars are very useful in reducing this cracking and breaking because they absorb the tension that could otherwise weaken the concrete[5].
The bond formed between concrete and reinforcing bars helps the reinforced concrete to act jointly without slipping when the structure is subjected to loading[6]. When the bond is perfectly formed, the plane section of the structural member remains unchanged even when the member bends. This implies that the member retains its structural stability even after bending. Measurement of this bond is done by determining bond stress, which is shear force acting on a reinforcement bar’s unit surface area. The bond stress is the resistance provided by the bond formed between the concrete and reinforcing bars. This bond stress can also be referred to as the shear stress created between the concrete and reinforcing bars embedded in it.
Concrete bond mechanism comprises of 3 main components: chemical adhesion, mechanical interlock and friction between the concrete and reinforcing bars[7]. Chemical adhesion is determined by the chemical reaction that occurs between the reinforcing bars and concrete, mechanical interlock is dependent on geometry of reinforcing bars and the deformation that occurs on their surface, and friction is influenced by the surface roughness. Despite these 3 components making up concrete bond mechanics, they also act independently thus they do not affect each other. The bond gets created at different stages as concrete interacts with reinforcing bars when the structure is subjected to tensile loading[8].
The bonding between concrete and reinforcing bars is a generally complex process[9]. At the start of loading process, bond stress is generally low as concrete remains uncracked nor unbroken and no slip has occurred in the reinforcing bars. At this stage, it is chemical adhesion that significantly controls the bond stress[10]. This implies that bonding mechanism or strength at this point is largely influenced by the chemical reaction between concrete and reinforcing bars[11]. A small percentage of bond stress is also contributed by microchemical interaction taking place between the surface of concrete and that of microscopic rough reinforcing bars[12].
When load increases, chemical adhesion gets broken down creating room for slip to occur[13]. Mechanical interlock that is caused by the reinforcing bars ribs’ wedging action resists separation of concrete and reinforcing bars. Radial cracks starts developing at the reinforcing bars ribs’ tips, also creating room for slip. Longitudinal cracks also develop and start spreading. With application of more load, wedging action increases as concrete crushes at the tip of reinforcing bars’ ribs. In this regard, wedging action component that is parallel to reinforcing bars is determined by the reinforcing bars’ rib pattern whereas the external component is determined by hoop stress that is delivered by adjacent confinement.
When the amount of traverse reinforcement (also known as links or stirrups) is at zero or low level, reinforced concrete structure can fail splitting following propagation of split cracks in the concrete cover reaching its external surface. When amount of traverse reinforcement is at medium level, splitting failure and pullout failure may be experienced because different phases of bond damage are occurring simultaneously along the bond length. If traverse reinforcement is at high level or if concrete cover is large, there is confinement of splitting cracks thus they can only occur around the reinforcement bar. In addition, splitting is only restricted to the primary crack near the reinforcement bar.
Bond performance is influenced by a wide range of factors[14]. Some of the most significant factors are:
This is sometimes generally termed as geometry of the reinforcing bars. Various studies have been carried out to determine the relationship between bar diameter and bond strength between concrete and reinforcing bars. Majority of the studies have shown that there is no significant influence that bar diameter has on bond performance. Some studies have shown that slight increase in bar diameter increases bond performance by a very small margin but not the maximum bond strength. From conclusions made from majority of studies, bond performance reduces with increase in bar diameter[15]. One of the reasons for this is because an increase in bar diameter increases frictional bond strength, which reduces bond performance.
Concrete strength, commonly known as compressive strength, is a very important parameter influencing bond strength[16]. Bond mechanism depends on how stress is transferred from the reinforcing bars to the concrete through shear interfacial and compression forces. Thus tensile strength and uniaxial strength of concrete play a key role in splitting and pullout failures respectively. Generally, it has been found that bond performance of reinforced concrete increases with increase in compressive strength of the concrete[17]. However, this proportional variation is only up to a certain amount of compressive strength beyond which bond performance will remain constant. Th concrete strength is also affected by various factors, including: porosity of concrete, water-cement ratio, aggregate soundness, bond strength between aggregate and cement paste, cement composition, and method of preparation[18]. Therefore in order to improve bond performance, it is important to ensure that the concrete is made using quality cement, aggregates and sand; suitable cement-water ratio; appropriate preparation, pouring and compacting procedure; and appropriate curing[19].
Confinement is a very important parameter that influences efficiency of the bond and control of cracks in concrete. Proper analysis and design of reinforced concreted structural elements should ensure that adequate reinforcement area is provided to allow the reinforcing bars resist internal tensile forces as designed and expected[20]. The reinforcing bars should be embedded in concrete at an appropriate distance on all sides of the section of the structure so as to develop maximum tension resistance at that particular section[21]. There are two categories of confinement: active confinement and passive confinement. Active confinement is developed when loads act on the element directly from the support or at column-beam connection joint at right angle to the reinforcing bar. On the other hand, passive confinement develops as a result of clamping action caused by a particular percentage of concrete cover at right angle to the stirrups or reinforcing bars. Generally, the effectiveness of active confinement is greater because it does not have a direct relationship with actual bond strength unlike passive confinement. Therefore confinement orientation also influences bond performance.
Surface roughness of reinforcing bars influences strength of reinforced concrete and by large bond performance[22]. In this context, surface roughness basically means deformation pattern. Reinforcing bars with a particular deformation pattern have high roughness. This roughness increases the effectiveness of combination between concrete and reinforcing bars. As a result of this, compressive strength increases. When compressive strength increases, it means that bond performance of the reinforced concrete also increases. Therefore deformed reinforcing bars improves bond performance. Nevertheless, the deformation in the reinforcing bars should not be too much to cause longitudinal cracking. The deformation should only be optimum to anchor and transmit forces. In this regard, it is also important to determine optimum deformation patterns that the reinforcing bars should have.
The type of concrete mix used in the reinforced concrete also influences bond strength. There are two main types of concrete mix: normal concrete and fibre reinforced concrete. Normal concrete is simply concrete that is made up of cement, aggregates, sand and water, without fibre. This concrete can fail easily especially when subjected to tensile forces because it is only strong in compression but weak in tension. Addition of fibre in reinforced concrete significantly increases compressive and tensile strength of concrete. The fibre increases capacity of energy absorption, toughness and flexural and tensile strength. The fibre creates a confining effect on the reinforcement and binds the concrete in lateral direction. All these increases strength of concrete. Once the strength is increases, bond performance also increases. Hence fibre reinforced concrete has higher bond performance than normal concrete.
Reinforced concrete structures experience different load events, depending on their application or location. These load variations expose them to a variety of possible failures[23]. There are 4 main types of bond failure: bar failure, concrete pullout failure, bar pullout and concrete splitting failure. Bar failure occurs when ultimate strength of reinforcing bar is relatively low thus the failure happens before either concrete splitting failure or bar pullout failure. Concrete splitting failure occurs when reinforcing bars develop longitudinal cracks. At this point, the bond is not able to function as expected, causing the specimen to split when radial cracks propagate and reach concrete surface’s external surface. Bar pullout failure occurs when the bond fails to function as anticipated as a result of local failure occurring in the concrete near the reinforcing bars. This happens because of the shear that is developed by the reinforcing bar’s ribs. Concrete pullout failure occurs when the bond between concrete and reinforcing bars exceeds concrete’s shear strength. With this, the failure occurs only in the concrete itself but there is a layer of concrete that is retained around then reinforcing bar even when it is drawn or pulled out.
There are different types of bond failure modes that can be experienced in reinforced concrete. Some of these are:
This failure occurs when tensile loading on the reinforced concrete structure increases. With the increased tensile loading on the reinforcing bar, ultimate bond strength is attained as the concrete teeth slips between the ribs. When the slip propagates, the concrete teeth also become more damaged until it is completely sheared off, resulting to pullout failure.
This failure mode occurs when tensile stresses present in concrete exceeds maximum tensile strength of the reinforced concrete component causing longitudinal cracks[24]. The cracks may cause the surrounding concrete to start splitting, resulting to a sudden decrease in bond stress. The primary causes of this failure are insufficient confinement of the reinforcing bars and inadequate concrete cover.
This failure occurs when reinforcing bars reach yielding point causing deflection of the reinforced concrete structure in a ductile way. When reinforcing bars yield, it becomes very easy for concrete to crush. The yielding causes formation of flexural cracks in concrete[25]. Increase in loading propagates these cracks causing concrete to crush and the structure to fail.
It occurs when tensile stresses in reinforced concrete are very high causing formation and propagation of flexural shear cracks. These cracks reduce the ability of the reinforced concrete structure to facilitate shear transfer mechanisms. Reduced shear transfer ability makes it difficult to attain static equilibrium. This results into relative displacement between concrete and reinforcing bars, which causes shear failure.
This bond failure mode occurs when the amount of reinforcement is lower than what is expected in a balanced reinforced concrete element. As a result of this, neutral axis is pushed upward to as to create equilibrium condition i.e. for tension force to be equal to compression force. This moves compressive force’s centre of gravity upwards. When bending moment continues to increase, reinforcement becomes strained exceeding its yield point[26]. Further loading increases strain in the element. When the reinforcement yields, it cannot sustain any extra stress with increasing strain and total tensile force remains constant. On the other hand, additional strain increases concrete’s compressive stresses. This shifts neutral axis upwards and compressive forces’ centre of gravity also moves upwards so as to maintain an equilibrium condition. The shifting of neutral axis continues until when concrete attains maximum strain then crushes. This failure mode occurs in under-reinforced concrete element[27]. Its occurrence is attributed to sustained yielding of reinforcing bars that cause greater strains in concrete. Yielding of reinforcing bars causes formation of cracks indicating that the section may collapse in the future. This means that the structure does not fail suddenly and measures can be taken to rectify the cracks once they develop.
This bond failure occurs when the amount of reinforcement is higher than what is expected in a balanced reinforced concrete element. This pushes the neutral axis downwards and strain present in reinforcing bars remains within elastic region. With increased loading on the element, stress and strain of reinforcing bars continue increasing together with tensile force. However, increase in stress of concrete is relatively slower. Therefore balance between compression and tension forces can be achieved by increasing concrete area that is resisting compression. As a result of this, the neutral axis is pushed downwards further until concrete attains maximum strain. This kind of element is said to be over-reinforced and the associated failure is referred to as compression failure. This is a brittle failure and should be avoided. When it occurs, the structure fails suddenly without any prior warning. It is exhibited by direct collapse of concrete with reinforcement remaining in position.
Reinforced concrete’s structural behavior depends on the interaction between concrete and reinforcing bars. The effectiveness of this interaction depends on various factors including contact area, concrete cover, reinforcing bar diameter and surface roughness of reinforcing bars[28]. It is the reinforcement that transfers tensile forces present in area of bending and separating cracks into concrete. This is done through bond action. Therefore bond stress has a significant impact on structural behavior of a reinforced concrete member[29].
The bond determines how effective reinforcing bars have been embedded in the concrete. If the bars are effectively embedded to create a strong bond, it means that the reinforced concrete structure is able to resist both compression and tension forces. The concrete and reinforcing bars will be responsible for resisting compression and tension forces respectively. This high bond strength improves the structural behavior of a reinforced concrete structure because it is able to perform its intended function effectively.
On the other hand, a decrease in bond strength creates room for defects thus compromising structural behavior and soundness of a reinforced concrete structure[30]. If the bond is weak, it means that the interaction between concrete and reinforcing bars is not adequate to enable the structure perform its intended function. A weak bond results to crack generation. Once the structure starts developing cracks, further loading will propagate the cracks thus compromising the soundness of the structure.
In general, bond is proportional to structural behavior. It is important to ensure that all factors affecting bond are considered when constructing a reinforced concrete structure. For the structure to perform as expected, the bond must be strong. This means that the concrete must have adequate compressive strength and the reinforcing bars should have properties that make them suitable for the intended application[31]. The designs of concrete mix and reinforcing bars must be done appropriately by following appropriate engineering codes and standards[32]. This will ensure that there is proper load transfer between concrete and reinforcing bars[33]. It is also worth noting that structural behavior is affected throughout the lifecycle of the structure. This necessitates consideration of factors that may affect bond such as corrosion[34]. The primary effect of corrosion is on reinforcing bars[35]. However, deterioration of reinforcing bars affects the bond meaning that the concrete gets affected. For instance, longitudinal and transversal cracks can easily evolve and form in corroded bars[36]. So during design, appropriate factor should be considered to ensure structural soundness of the reinforced concrete structure throughout its service life.
There are various experimental studies that have been conducted to analyze bond mechanism in reinforced concrete structures. A study was carried out to determine bond properties of textile reinforced concrete. The study found that bond strength between concrete and textile increases with increase in bond length. This is because increase in bond length increases loading and concrete workability and strength[37]. Another study was performed to establish the relationship between flexural behavior and bonding strength which found that bond strength increases when interlayer thickness and holding processing time are increased and decreases when temperature increases[38].
There are several other studies that have been conducted to investigate the bond between reinforcing bars and concrete. The two main tests used across most studies are: pullout tests and bond beam tests[39].
This is the conventional bond test. It is very easy to implement and basically involves pulling the reinforcing bar directly out of the bonded concrete sample. The experiment starts by preparing pullout samples. The samples are concrete cubes each bonded with a centrally placed reinforcing bar of standard bond length. The bar’s ribs are also protected by a plastic sleeve. The tests can also be performed on samples with different rib deformation patterns.
The actual pullout test is performed using servo-hydraulic universal testing machine that has a maximum capacity of about 60kN. It starts by placing the sample on a square steel plate that is secured by a stationary head. To ensure uniform distribution of stress on face of the sample, a rubber sheet is usually placed between the plate and sample. Tension force is then applied on the reinforcing bar’s longer end that is gripped by the machine’s moving head. Slip is measured by 2 linear variable differential transformer (LVDT) transducers. The transducers are fixed on a cross yoke fixed on the reinforcing bar. Application of load is controlled using a stroke at an increasing rate (mm/sec) until when bond failure happens. Slip of the reinforcing bar is recorded at predetermined loading increments from start to end. The values obtained are then used to calculate bond stress and establish the relationship between slip and load, which helps in understanding bond mechanism and structural behavior of reinforced concrete elements under varied loading.
If the bond is uniform, bond stress is determined using equation 1 below[40]
Where τ = pullout bond stress, P = applied load, L = embedded bar length, and d = embedded bar diameter.
Alternatively, bond stress can also be calculated using equation 2 below:
Where τ = pullout bond stress, P = applied load, fcm = target concrete strength class, fc = average concrete strength of test samples, d = embedded bar diameter (5d = L (bond length)).
This test is more advantageous that pullout test because of its capacity to capture actual bonding and de-bonding mechanisms of reinforced concrete samples. The experiment starts by preparing samples. Ribbed bars are embedded in a concrete beam and protected with plastic sleeves. The test is performed using a 4-column hydraulic universal testing machine that has a maximum capacity of about 250kN. The procedure starts by placing the beam on roller supports positioned on top of stationary head then placing a T-shaped hinge of equal width with that of the beam in compression zone at mid span of the beam. Load is applied on the beam gradually and constantly under the control of stroke until when bond failure happens. Slip is measured using 2 LDVT transducers fixed on cross yoke that is mounted on each side of the reinforcing bar. When bond failure is observed on side of the beam, the test has to be stopped. A clamp is then fixed so as to stop the reinforcing bar from slipping then the test is continued until the other side of the beam also fails. The results obtained are used to calculate bond stress and analyze bond mechanism of a reinforced concrete element.
Bond stress in this test is calculated using equation 3 below
Where τ = bond stress and σs = (for bar diameter, d < 16mm); Fa = total force applied on the test beam and An = surface area of the test beam.
The role of concrete in engineering field, specifically structural applications, cannot be overemphasized. Reinforced concrete is one material that transformed the construction industry during the 19th century and from then, it has remained one of the commonest building materials worldwide. Reinforced concrete is simply concrete that embedded with reinforcement[41], which can be reinforcing bars, rods, mesh or fibre. The reinforcement mainly absorbs tensile and shear stresses present in a concrete structure. This is very important in increasing structural stability of a concrete structure because concrete alone has the capacity to absorb compressive stresses but not tensile stresses. For this reason, reinforcement fills up the weakness of plain concrete, which makes the structure stronger and more stable.
The commonly used reinforcing material is steel. The main characteristics that make reinforcing steel a suitable material are high tensile and compressive strength and ductility[42]. On the other hand, plain concrete is characterized by high compressive strength and low tensile strength. When the two materials are combined, they form a structure that has high tensile and compressive strength. But this depends on bond strength or performance, which is also influenced by several factors discussed above.
If the concrete used is of adequate compressive strength and the reinforcing bars are of suitable diameter, has proper deformation pattern, appropriate confinement and adequate concrete cover, bond stress or performance is expected to be high. These characteristics ensure that the two materials are firmly combined, making it difficult for the reinforcing bars to pullout or the concrete to disintegrate.
When force is applied on a reinforced concrete structure, its bond stress is expected to increase with increasing load. At the start, the increase is very rapid but starts slowing down after some time. This continues until the structure attains maximum bond stress, which occurs at maximum load. At this point, slip is very small and changes slightly at almost constant bond stress. When maximum bond stress is attained, it means that the bond has reached its maximum capacity to perform as designed. Further loading is likely to cause failure that can happen in form of pullout or splitting. If the excess form exceeds the maximum tensile strength of the structure longitudinal cracks will be formed. The cracks cause disintegration of surrounding concrete, reducing bond strength abruptly. This kind of failure is what is referred to as splitting. Increased tensile loading can also cause formation of slips. If the slips propagates, the reinforcing bar can pullout leading to failure of the reinforced concrete structure. An important thing to note is that the failure generally depends on the bond between the concrete and reinforcing bars. If the bond is very strong, failure cannot occur easily. This is because strong bond means increased structural stability and soundness of the reinforced concrete structure, and vice versa.
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