Verify of Anchorage Length in Reinforced Concrete - Coursework Example

Results: Bond behaviour In the construction of the reinforced concrete, efficient and reliable force transmission between concrete and reinforcement is needed for optimal design. The transmission of forces from the reinforcement to the adjacent concrete occurs for a deformed bar due to: Chemical adhesion that is between the concrete and the bar. Frictional forces resulting from the forces transversing to the surface of the bar, roughness of the interface and relative slip between the surrounding concrete and the bar. Bearing of the ribs or mechanical anchorage against the surface of the concrete. When the bar has initially slipped, force is transferred through bearing. Friction, on the other hand especially between the deformations (ribs) and then concrete plays an important role in force transfer, as illustrated by epoxy coatings, which lower friction coefficient and thus result in lesser bond capacities. Furthermore friction also plays another important role for the case of plain bars, with slip-induced friction coming as a result of the transverse stresses at the surface of the bar caused by small deviations in the shape of the bar and minor, even though surface roughness is significant. Plain bars with possibly low allowable bond stresses have been used for several years for reinforced concrete in places like North America and to-date are still utilized in some regions of the world. In a case where a deformed bar is moved with respect to the friction forces on the ribs, the barrel of the bar are finally mobilized. The compressive forces on the ribs raise the value of the frictional forces. The forces on the surface of the bar are balanced by compressive and shear stresses on the concrete contact surfaces, that are set into tensile stresses which do result in cracking in the planes that are either parallel or perpendicular to the reinforcement. Factors affecting bond A number of factors affect the bond between the concrete and reinforcing bars and. Bond behaviour, background research, and relationships existing between bond geometric and strength and material properties are presented under three major subject headings: concrete properties, structural characteristics and bar properties. The structural features addressed include concrete cover and the spacing of the bar, the bonded bar length, the degree of transverse reinforcement, use of noncontact lap splices and the bar casting position. Concrete cover and bar spacing Curves develop to be steeper and bond strength rises as bar and cover spacing increase. For great bar spacing and cover, there is possibility to come up with a pull out failure. For the case of bar spacing and smaller cover, there occurs a severe tensile failure impacting in lower bond strength. Splitting failures do occur within the bars, between the free surface and the bars, or possibly both. Pull-out-like failures do occur with certain splitting if the member has essential transverse reinforcement to restrain the anchored steel. Bond failures entailing split of the concrete bars which aren’t restrained by crosswise reinforcement, the load of peak level regulated by the reaction of the tensile concrete, in turn is influenced by on both energy dissipation capacity combined with the capacity of tensile, generally termed as fracture energy. Development and splice length When the splice length of a reinforcing bar increased there will be an increase in its bond capacity. Though the nature of bond failure results into a rise in strength which is not proportional to the rise in bonded length. For the case of anchored bars, the concrete longitudinal splitting initiates at a free surface or rather transverse flexural crack in which the bar is most highly stressed. In the case of spliced bars, splitting gets started at the ends of the splice, as it moves towards the centre. Transverse reinforcement Transverse reinforcement restrains spliced and developed bars by limiting the progress of splitting cracks and, therefore, increasing the bond force necessary to cause failure. A rise in the transverse reinforcement results in an increased bond force, eventually changing a splitting effect to the pull out failure. Extra transverse reinforcement, beyond that required to cause the transition from a splitting to the pull out failure, results to be progressively less effective, consequently providing no bond strength increase. Anchorage of tensile reinforced wall-columns This method would include an anchorage technique of tensile reinforcement bars within joint regions of a flat-beams and wall-column with no projections on the roof slab. According to (Rajagopalan, 2005, p. 89), a U-shaped tensile anchorage reinforcing bars would simplify the bar arrangement about the joint, since it is not necessary that the anchorage is positioned into the beams. The reinforcing bars for the U-shaped tensile anchorage can contribute to avert slippage of the reinforcement of the tensile from the joint areas. Supplemental stirrups are also important in that to prevent bond let-down of the flat-beams of longitudinal reinforcing bars. The bending moment created impact to the joint areas of a wall-column and the flat-beams is transformed into the wall-column’s shear force, and connections between deflection angle and shear force. The specimen FBI having the straight anchorage was lastly unsuccessful in shear at the joint regions with no slippage of tensile reinforcement of the particular regions. For the specimen FBI, the maximum strengths in every loading direction through the loading cycle of 1/50rad. FBU –the specimen with U-shaped anchorage was observed to have strength weakening with the slippage afore the shear failure. The specimen FBU had the maximum strengths at the loading cycle of 1/100rad. in either side of the loading directions. They were estimated to be 70% in values terms of the FBI specimen, the U-shaped tensile anchorage of reinforcing bars of the column of the wall consequently made the features of the joint regions worse. In either the case, the maximum strengths were less than the considered values. It is one of the reasons that adhesion of the longitudinal reinforcing bars of both specimens lost in the course of the loading tests. From another point of view, the slippage on the FBU2 specimen having the U-shaped anchorage, supplemental stirrups and supporting re-bars wasn’t observed in the course of the loading tests. The specimen FBU2 performed well though finally failed in shear within the joint regions. It is noted that adhesion of the beams’ longitudinal reinforcing bars was not lost through the loading tests by so conducting PC steel rods to improve and regenerate the adhesion properties. The peak strengths in every loading direction of the FBU2 specimen surpassed the calculated values and again surpassed the maximum strengths of FBI specimen whose shear failed at the joint regions. The specimen FBU2 sustained on 80% of the optimal strengths up until the drift of 1/25rad. in every loading direction. Slip Response with respect to bond stress Investigations about the bond stress with respect to slip response on a larger extent have widespread. Material and system parameters for particular investigations differ hugely; consequently, data delivered by the subsequent investigations differ. Though, wholly this bunch of information collected describes the fundamental features exhibited in a response known as bond-slip. Various distinct tentative investigations categorize information for definite illustration of bond response. Bond Failure Bond failure is most likely to happen near the ends of beams, that is, high flexural bond stresses combines with high local bond stresses. There are two forms in which bond failure may take, both of which is as a result of wedging action at the time when the bar is pulled to the concrete and usually it acts in concrete by shear crack. National Bureau of Standards, denoted as N.B.S and University of Texas investigation have indicated that bond failure will happen when bond force, U extents to a critical value. It`s worth noting that during failure, the force U is not dependent of bar size. The idea is consistent with notion of “wedge action” that the splitting force relies on the driving force, but not wedges width. Bond and Anchorage Reinforcement for concrete to improve on the strength of a segment in tension is depending on the compatibility of the two materials in acting together to resist the external load. The element of reinforcing, for example a reinforcing bar, goes the same deformation or strain just like the surrounding concrete so as to prevent the separation or the discontinuity of the two materials holding the load. The modulus of the ductility, elasticity, and the rupture or yield strength of the reinforcement considerably should be higher than those of the concrete to increase the capacity of the reinforced concrete segment to a meaningful level. Subsequently, materials such as aluminium, rubber, brass or bamboo aren’t appropriate for developing the bond or adhesion needed between the concrete and the reinforcement. The two materials, for example, steel and fibre glass possess the primary factors necessary: ductility, yield strength, and bond value. Discussion: The loading tests of the static cyclic on the three specimens of the T-shaped joint consisted of a flat-beams and wall-column was carried out on a test consideration of anchorage system of tensile reinforcing bars of the wall-column, for instance, U-shaped anchorage in the joint region, the straight anchorage on the slab of the roof, U-shaped anchorage with supplemental stirrups and supporting reinforcing bars. The straight anchorage test in conclusion failed in shear at joint regions of a flat-beams and wall-column. Nevertheless, the calculated value was higher than the maximum strength attained. This was because the adhesion of the flat- beams concrete and the longitudinal reinforcing bars was lost at the time of loading tests. Reinforcing tensile of the wall-column slipped from the joint regions in U-shaped anchorage system test having no additional reinforcing bars. When supporting reinforcing bars added to the supplemental stirrup and the U-shaped anchorage to link the longitudinal reinforcing bars, it made it possible to restrict the slippage of the reinforcement tensile and thus make the joint strength greater. Supporting the bars of reinforcement for the supplemental stirrup and U-shaped anchorage around the joint regions were important to increase the U-shaped anchorage arrangement performance. Bond Strength A study unravelled by (Jansze, 1997) became the very initial investigations to hint about the extrapolation of bond strength on the deformed reinforcement. An investigation by Tepfers proposed a model of analysis where the concrete that surrounds a specific reinforcing at a particular point tends to be considered, for instance as a thick-walled cylinder exposed to interior pressure and shear. The pin -pointed correlation depicts the inner pressure and shear corresponds correspondingly to the radial stresses and bond that advanced at the interface of the concrete-steel. Hence, the outward force transference at the interface of concrete-steel defines the stress of the tensile hoop established within the concrete that surrounds the bar, consequently the critical load. A recommendation by Tepfers holds that bond strength usually is as a result of the ability of the concrete neighbouring the reinforcement bars to hold the exerted hoop stresses. The proposed system failure modes are three and include: partially cracked-elastic, plastic, and elastic. The mode of elastic failure illustrates an arrangement where the concrete that surrounds the reinforcement bar displays a response that is linearly-elastic and the bond strength matches the concrete carrying an optimum tensile stress which equals that of the concrete tensile strength. The mode of failure that is partially cracked-elastic defines a system where radial cracks initiate at the interface of the concrete-steel, however do not transmit to the parts of the item. The concrete which is cracked is presumed to lack tensile strength though the bond strength matches that of the uncracked concrete holding an optimal stress that equals the strength of the tensile. The mode of plastic failure illustrates a system where entirely the whole concrete about the anchored bar is presumed to hold a tensile hoop stress which equalizes the strength of concrete tensile. The information brought forth by Tepfers is in support of the fact that bond strength is as a result of the hoop stresses established in the immediate concrete. The information also concurs with the inference that the model that is partially cracked elastic consequences into a lesser certain bond strength. Controlling bond strength Bond strength outcome from a combination of various parameters, which includes the mutual adhesion within the steel interfaces and the concrete with respect to the pressure of the hardened concrete alongside the steel bar or wire as a result of the concretes’ drying shrinkage. Furthermore, friction interlock felt between the bar surface projections or deformations and the concrete due to the minimal actions of the tensioned bar cause increased resistance to slippage. The total effect as a result of this is known as the bond. The major factors controlling the bond strength would include: Adhesion felt between the reinforcing elements and the concrete. Gripping effect due to the drying shrinkage of the adjacent concrete and also the shear interlock found between the surrounding concrete and the bar deformations. Interlock, frictional resistance and sliding as the reinforcing element is exposed to tensile stress. The impact of concrete quality and the concrete strength in compression and tension. Mechanical anchorage impact of the ends of bars over development splicing, hooks, length, and crossbars Shape, diameter and spacing of reinforcement as they impact on crack development. It is of note that the contributions of these factors individually are difficult to quantify or separate. Shear interlock, concrete quality and the shrinking confining effect are considered the major factors. Bond response that yields reinforcement experimental justification of the reinforced concrete elements gives backing of the suggestion that bond strength is decreased for reinforced concrete yielding in tension and raised for reinforcement which yields in compression. This perceived response partially is enlightened based on the Poisson effect that make the reinforcing bar’s diameter to shrink immediately when tensile yielding ensues hence letting steel to slip beyond the immediate concrete with ease thereby creating an expansion of the diameter of the reinforcing provided compressive yielding ensues. This advances the connectivity between surrounding concrete and the steel consequently cumulating the bond strength. Furthermore it is suggested that the transmission of tensile stress across to the adjacent concrete creates further global destruction to the immediate concrete as compared to the transmission of compressive stresses which contributes to decreased bond capacity from the perspective of tensile loading in comparison to compressive loading. Bond-Link methods Among the first bond models was suggested by Ngo and Scordelis (1967). They proposed that the global predetermined element model signifies a slightly reinforced concrete exposed to monotonically rising third-point loading but also entails elastic stress continuum features to denote the measurable behaviour of reinforcing steel and plain concrete. The damage of concrete is denoted by the overview of pre-defined cracks to the web of the system. The linkage concerning the concrete and the reinforcing steel is attained by the zero-length with a bond link of two-dimensions items that are called two orthogonal springs. The assumption is that bond-link element has neither length nor height and the springs are presumed to have relationships of deformation and linear elastic force (Eligehausen, Malleì & Silva, 2006, p. 184). Due to the fact that the reinforcing steel again is represented as an element of plane stress, while the orthogonal spring is representing the dowel action of the reinforcing steel; consequently is considered the most essential for extensively open cracks and important undertaking through crack faces. In more simplified terms their proposal entails a system of stiff bond response which is perpendicularly positioned with the reinforcing steel axis. It’s noted that the outcome for various analyses depicted that the deformation of global aspect experienced by the beam member proportionally relates to the rigidity of the bond-link element although the entire difference in deflection is nominal for a change of 40% in the stiffness of the bond. References List ELIGEHAUSEN, R., MALLÉE, R., & SILVA, J. F. (2006). Anchorage in Concrete construction. Berlin, Ernst & Sohn. JANSZE, W. (1997). Strengthening of reinforced concrete members in bending by externally bonded steel plates: design for beam shear and plate anchorage. Delft, Delft UniversityPress. RAJAGOPALAN, N. (2005). Prestressed concrete . Harrow, U.K., Alpha Science International. T. J. MacGinley, and B. S. CHOO. (1990). Reinforced concrete Design Theory and Examples. (Second Edition). Zekany, A. J., Neumann, S., Jirsa, J. O., and Breen, J. E.( 1981), “The Influence of Shear on Lapped Splices in Reinforced Concrete,” Research Report242-2, Center for Transportation Research, Bureau of Engineering Research, University of Texas at Austin, Tex., July, 88 pp. Zuo, J., and Darwin, D. ( 2000). “Splice Strength of Conventional and High Relative Rib Area Bars in Normal and High-Strength Concrete,” ACI Structural Journal, V. 97, No. 4, July-Aug., pp. 630-641. Menzel, C. A., and Woods, W. M. (1952). “An Investigation of Bond, Anchorage and Related Factors in Reinforced Concrete Beams,” Research Department Bulletin42, Portland Cement Association, Nov., 114 pp. Read More