Verify of Anchorage Length in Reinforced Concrete - Coursework Example

Introduction In the design of reinforced concrete structures, it is required that the reinforcement be fully anchored so as to ensure the bar is developed to its yield stress. The sufficient amount of bar surface area must be exposed for full development and steel-concrete compatibility can be ensured through the use of chemical adhesion, friction and mechanical interlock via deformations and ribs on the bar. This therefore means that there is a minimum development length of the reinforcing bar that is required in anchorage of to the concrete bar before any compatibility can be achieved. Along this development length, the bar force that can be achieved increases gradually to reach full force in the bar. With a low confinement, this may result into a long development length. This has resulted in many experiments being carried out in order to find out ways in which the length of the development length can be minimized so as to save on expenses incurred with use of long steel bars and also eliminate structural stability issues. The length of the development bar makes it even more challenging task when designing structures in areas characterized by high seismic activity. This is due to the fact that the task of reinforcing steel for structures and connections is very difficult in these areas due to high levels of steel congestion. Moreover, the use of the conventional hooked bar anchorages may be unfeasible and impractical. Therefore, the alternative here is the use of a headed reinforcement which allows for very small development lengths hence can reduce congestion without comprising the integrity of the structure. This therefore makes the designing and detailing of the structure easier and more efficient. This idea has been utilized in offshore drilling platforms and on-shore projects like in bridge applications. For reinforced concrete structures subjected to moderate loading, the bond stress capacity of the system exceeds the demand and there is relatively little movement between the reinforcing steel and the surrounding concrete. However, for systems subjected to lot of loading, there may be need to exceed the capacity of localized bond which may also result in damage during movement between the reinforcing steel and concrete. For the reinforced beam connections subjected to loading due to seismic forces, there may be transfer of force and anchorage mechanisms within the vicinity of the joint typically result in severe localized bond demand. Various experiments involving beam connections that are subject to loading due to an earthquake indicates the response can be determined by analyzing the bond response. This technology is so unique since it relies on the fact that heads on the bar ca be used to provide a means of developing a bond strength through a separate load path that is provides in the headed bar through means of concrete bearing on the head itself. The exact proportion of the load that is taken through such mechanisms is uncertain i.e. evidence on literature states that around 75% of the load is taken through the bearing of the head with the remaining 25% being taken through the conventional bond strength. This therefore results into many factors and expressions that describes the development length of straight deformed bars may also apply to headed reinforcement as well. While there has been extensive research has been done on the headed bars, the research being carried out in this paper focusses on gaining new insights to the anchorage behavior of these bars, their appropriate length and well as develop expressions that can be used to describe the appropriate development length. Methodology A. General A test program consisting of seventy beam-end tests was conducted in order to investigate the development length of a headed reinforcement. Majority of the specimens used for testing involved head test bars while the rest included beam-end bars consisting of straight bars and those with 1800 hooks are the end. Therefore, the essence of this methodology section is to provide a vivid description of the variables considered in tests program and also explain the beam-end tests’ configuration tests. In addition to that, the material properties, fabrication of the specimen and the testing procedure are also discussed. B. Parameters to be tested The test analysis was divided into four batches of beam-end tests. The first batch was taken as the base for determining the best suited parameters for use in further investigation and also to plan the rest of the analysis. Due to material and logistic issue, only three specimens for each variable were fabricated and tested so as to ensure the credibility and reliability of the results. Some aspects of the experiment were kept constant throughout the testing duration and this included the constant test bar size, embedded length and the general strength of the concrete. Three primary variables were investigated throughout the testing procedures and the included: The concrete cover: This included beam-end specimens with both two and three bar diameters respectively, measured from the edge of the bar of cover was investigated. Around 0.5” of the test bar closest to the front was covered with a polyvinyl chloride (PVC) tube. Transverse Reinforcement: The spacing patterns and various quantities of the transverse reinforcement were used as an additional means of ensuring the test bar is confined in the specimen. In total, four stirrup spacing patterns were to be observed throughout the experiment. The reinforcing bar exposure: There are known effects of allowing the test bar to bond to the concrete. Therefore, in some specimens, the test bar was covered with a polyvinyl chloride (PVC) tube hence eliminating the bonding of the concrete to the test bar. It is important to note that inn tests of the former, around 0.5” of the test bar closest to the front was covered with a polyvinyl chloride (PVC) tube. This length is known as the lead length and is important is preventing a localized cone-type failure of the concrete at the loaded end of the specimen. C. The specimens for testing The materials required for testing included; Concrete: The concrete applied in this case was an air-free having nominal strengths of 30-25MPa that was provided by a local ready mix plant. Concrete mixes consisting of commercial cement, river sand and a nominal size of crushed limestone aggregate. Steel Reinforcement: The main test bars for the specimens were fabricated from graded steel with a metric size designation. Other specimen were fabricated from steel of a lower grade than that used by the principal test bars. Some reinforcement needed bending and this service were provided by a local steel fabricator. Headed test bars were fabricates using friction welding procedures in conformance with the specification for welded headed bar. Fabrication of the specimen The forms were fabricated from 0.75” plywood consisting of dry strips of polymeric layer aimed at protecting and sealing the wood from the concrete during placement. The formwork that did not get constructed from this special grade of wood was given protective coats of polyurethane so as to provide the dry-strip characteristic. Also, all forms of edges and joints were caulked and sealed to prevent leakage during casting. The formwork was calibrated such that the specimens will have an overall size of 9” x 18” x 24”. The depth for the specimens with three bar diameters of cover was increased by 1” so as to accommodate additional cover. Therefore the overall required dimensions of the specimens are 9” x 18” x 24” and all test bars are nominal in 1” in diameter and are cast 15” up from the bottom of the specimen. It is worth noting that the specimens are casts in an inverted position as compared to the testing position. Cast at the same height as the test bar, a section of 1” diameter steel conduit was placed adjacent to the test bar and extended to the back end of the specimen in order to provide a way of accessing the unloaded portion of the test bar. For the specimens that had hooked test bars, the steel conduit was cut off to fit the bend of the hook and sealed to the bar to prevent seepage into the conduit during the placement of the concrete. In specimens with headed test bars, a piece of steel, 1.75” long of piece of conduit with a diameter of 1.5” is fixed to the back of the head itself using an epoxy bonding agent. The piece of conduit that is 1” long is the fitted inside this 1.75” long piece and allowed to juxtapose the head with the outer piece being covered with clay to prevent its bonding to the concrete. This connection was also sealed to prevent seepage into the conduit. The unloaded ends provide a flat vertical surface in the specimens that have straight test bars in specimens with headed test bars. This is important since it ensures effective measuring of the unloaded-end slip using a single spring loaded linear variable differential transformer (LVDT) to contact the test bar’s unloaded end. However, it was found that the 1800 hooked test bar did not provide consequential ability hence some modifications had to be made. A 0.037’ steel wire was epoxied to the point on the test bar that marked this starting point of the hook. This steel wire was then fed through the steel conduit out to the back end of the specimen and left for the latter to the LVDT. The PVC bond breaking pipes had inside diameters equal to the test bar diameter and were used to control both the lead length and bonded length within the specimen. The lengths among each of the specimens was found to be varying. A number of pieces of reinforcing bar makes up the rest of the steel configuration. Three transverse 5” bars were used together with the specimen. One of them acted as a means of support to the test bar and the rest two were used in aiding the movement of the specimen. In addition to that, four double legged closed loop stirrups were oriented parallel to the flexure steel and provided shear reinforcement and give further means of confinement. These stirrups are positioned between the flexure reinforcement and are looped around the test bar. Curing and concrete placement Using two separate lifts which vibrated in six points evenly placed, the concrete was cast in the beam-end specimens. All the specimens were coved with wet burlap and sheets of plastic once the concrete has set up. Curing was done in this same way until the strengths of the ballast reached 20.7MPa. The forms were then removed and the specimens inverted to their test position and then left to cure until the concrete reached its designated test strength. Consequently, test cylinders measuring around 6”x12” were combined with steel and plastic molds and cast. They were then curd the same manner as the rest of the specimens. The compressive strength of the concrete was tested for seven days after pouring and then monitored until the strength asymptotically reached a value at which now it was appropriate to carry out the test procedure. Evaluation of Specimens with different Anchorage Lengths This experimental set up involves the use of longitudinal beams that are reinforced ad anchored in joints consisting of interior beam columns. The prototype specimen for this study consists of a single deformed reinforcing bar with 1” nominal diameter, anchored with in a reinforced anchorage block with a 15” in that is perpendicular with a longitudinal reinforcement and parallel to the anchored bar. The application of the load is done either as tension on the protruding end of the bar or as tension and compression on opposite ends of the bar. Load is reacted through frictional forces on the surface of the specimen and through bearing of embedded anchors at a prescribed distance from the longitudinal bar. The complete test program comprises seventeen specimens. The will vary from the anchorage length, the diameter of the bar, the volume of steel and the pattern of the load. In this case, the stress of the bar is defined by the load applied at each end of the bar and the slip is defined by considering the movement of the concrete block’s face in relative to the protruding end. D. The procedure Using the apparatus available, the beam-end specimens were tested. Both the specimen and test apparatus were secured to a structural floor using a two wide-range sections of four tie-down rods. This was followed by adding a load to the test bar by two fifty ton hollow-core jacks through two 1” in diameter load rods instrumented as load cells. An Amsler hydraulic pump was used to power the jacks hence providing a load to the test bar at the rate of 27kN per minute. The load was applied to the yokes via the load rods and was then transferred to the test bar via a steel wedge-grip assembly. This means that the tensile force acting on the test bar is cancelled out by an opposite compressive force that is applied by a bearing pad that is fixed rigidly to the apparatus of the frame. Around 3.5” of the lower part of the specimen’s front surface is occupied by the bearing pad and measures around 13.75” from the center of the bearing pad to that of the test bar. In order to measure the slip at the loaded end, two spring-loaded LVDTs with a range of 1” were attached to an aluminum block that was mounted to the test bar. A single LVDT that has a range of 1” was inserted through the steel conduit. A protruding steel cable wire was tied to the LVDT and the LVDT itself attached to the end of the steel conduit in cases where there was need to measure the hooked specimen. In order to measure the splitting crack widths resulting from the tests, an additional LVDT of a range of 5” was placed across the top surface. However, the LVDT was no used during the testing of headed specimens with no transverse reinforcement. Therefore using a data acquisition device, data from the load cells and the LVDTs were retrieved and processed using special computer programs for management and interpretation. Conclusion In accordance to the test procedures carried out, insights have been gained on the methods of anchoring steel reinforcing bars. In addition to that, it has provide a means of developing design guidelines for the development of the headed bars as well as contributing to the ACI building code. Also, the experimental investigations provide data for development, calibration and verification of a model to represent load transfer between reinforcing steel and concrete. The results the analytical investigations provide insight into model development and implementation within the framework of the finite element method. The experimental procedure consisted of evaluating the performance of beam-end test specimens. Most of the specimens consisted of the headed test bars with inclusion of straight and 1800 hooked test bars. Although the strength of the concrete was held constant ad test bars limited to 1” lengths, variables such as cover, bonded length and transverse reinforcement were controlled carefully and their effect on the test results investigates. The bond studies from previous experimental procedures indicate that the load transferred between the reinforced steel and concrete is idealized to be occurring due to friction resulting from interaction of lugs in both the concrete and the reinforcing bar. This interaction mechanically is used to describe the concrete’s bearing on lugs of steel that normally constitutes of crushed concrete. This crushed concrete is wedged at the front end of the lugs. Mechanical interaction is the dominant mechanism of load transfer at small to moderate slip levels and results in load transfer in the direction both parallel and perpendicular to the bar axis. Friction is the dominant mode of load transfer at extreme slip levels and immediately upon load reversal. Identification of these mechanisms supports calibration, description and implementation of a model that defines general bond response. After an evaluation of the test results, the bond strength and development length of straight deformed bars were compared. Based on these comparisons, best-fit lines were developed and were the applied in obtaining the design equations to describe the light characteristics of headed bars. These expressions form the basis that help in establishing the headed bar development length criterion to be considered in future practices. The specimens tested in the experimental procedure included headed deformed bars, headed smooth bars, straight deformed bars and hooked bars (both smooth and deformed). The outcome of the results shows that the headed bar anchorage performs almost identically in terms of the anchorage length to a hooked bar. Both the anchorages in both set ups are able to develop a reinforcing bar is a significantly shorter distance than a straight deformed bar. Another trend from the experimental set up is the confinement either in form of additional cover or stirrups. This acts to assist the anchorage of both hooks and headed bars. These confinement is so efficient that the headed bar can be developed effectively and entirely by the head with no assistance from the deformations on the bar. Therefore, for a designer using either cover or small number of stirrups, it would be possible to anchor a reinforcement bar by the action of the head alone and also be able to have the bar be applied in carrying the loads. The results of the experimental anchorage length investigations indicate that desired length is determined by a number of parameters. These parameters include characteristics of the component design and construction such as the property of the material used, the ratio of steel used and the thickness of the concrete used to cover the anchored reinforced bar. Within the context of this paper, the system parameters that are considered to determine bond response include concrete compressive strength and reinforcing bar diameter. These parameters are considered to be having a reduced effect on the bar deformation pattern, the spacing and the volume of reinforcement transverse to the anchored bar. Therefore, in order to verify the anchorage length in reinforced concrete, a design expressions is developed together with the design guidelines. It was found out that the development length of a standard headed bar with two bar diameters of cover can be predicted using the following equation; [Id / db] = [700 / (f c1/2)] The result if these equation can be compared with the coefficient of [1200 / (f c1/2)] applicable in a hooked anchorage. This therefore means that a headed bar can be anchored in 7/12 distance of that of a hooked bar. In case a stirrup or additional cover is provided, the anchorage length is reduced and will normally approach zero for a fully confined headed bar. Previous investigation of bond zone modeling suggests a number of plausible bond zone idealizations. Bond zone response may be modeled at several scales. For the current investigation bond response is represented at the scale of the reinforcing bar. This modeling scale appears to provide both a representation of bond zone response that is sufficient for accurate representation of structural component behavior as well as a global model that can be analyzed and solved using a typical finite element program and computer. The physical bond zone may be represented by one-, two- and three-dimensional elements. Additionally, bond response may be characterized as a one-, two- or three- dimensional phenomenon. Here the bond zone is defined to include the concrete and reinforcing steel in the vicinity of the reinforcing bar, because the response of concrete material and that of reinforced steel is determined by the response of the bond. However, a two dimensional element that has a finite length with zero width is used to represent the bond zone. This therefore provides a relation in continuity between the concrete and the reinforcing bars. The response of the bond zone is also considered and included in analyzing the slip and stress behaviors and also including the radial stress. In addition to that, the analysis of radial deformation versus bond element response simply provide continuity of the global model. This idealization of the bond zone and bond response allows for representation of only bond response within the bond element, representation of the effect of concrete and steel material behavior on bond response and representation of concrete and steel material response using the previously defined material models. The proposed anchorage length is verified through comparison with experimental data. The bond stress versus slip history as computed using the proposed model and as observed are compared for a series of specimens tested by a number of different researchers. The results of these comparisons indicate that the proposed model represents well bond zone behavior for a variety of slip histories, concrete and steel stress states and bond zone configurations Bibliography [1.] C.B. Wilby. Fire Design of Concrete Structures - Materials, Structures and Modelling. Lausanne, TX: Fédération international du béton (fib, 2007. Print. [2.] Eligehausen, Rolf, Rainer Mallée, and John F. Silva. Anchorage in Concrete Construction. Berlin: Ernst & Sohn, 2006. Print. [3.] McGinley, T J, and B S. Choo. Reinforced Concrete: Design Theory and Examples. London: E & FN Spon, 1990. Print. [4.] Newby, Frank. Early Reinforced Concrete. Aldershot: Ashgate, 2001. Print. Read More