<p>  </p> <p>The use of headed reinforcement in concrete has found an increasing interest in construction applications. From shear reinforcement in walls to longitudinal reinforcement in beams and columns, there is a growing need to understand the behavior of headed rebars. A headed rebar is a deformed bar with a head attached to its end and while similar anchorage devices such as headed studs and hooked rebars are well established in theory with design equations developed, headed reinforcement lack this level of knowledge and hence, their application in industry is limited.</p> <p>Current code provisions such as fib Model Code 2010 allow the design of headed rebars as (1) a hooked bar, (2) a headed stud, and (3) using experimental results. Moreover, ACI 318-19 only contains a design equation for the development length of headed rebars but not its capacity. While the literature has justified the approximation of the capacity of headed rebars with hooked bars through a multitude of studies comparing both anchorage devices. Such a justification is not well-founded for headed studs due to a scarcity of studies comparing headed rebars to headed studs. Moreover, there is a lack of design equations accurately predicting the behavior of headed rebars in several parameters. All these issues emanate from the complexity of headed rebars due to their joint mechanism of anchorage coming from both resistance along the rebar deformations and bearing on the head.</p> <p>This study aims to better understand the behavior of headed bars by numerically analyzing the influence of different parameters on their performance. Furthermore, direct comparisons are made between headed reinforcement, headed studs, and straight bars to segregate the effect of the bond along the shaft and the bearing at the head on the behavior of headed bars. </p> <p>The parameters included in this study are embedment depth, edge distance, and concrete compressive strength. The numerical models are verified using a 3D non-linear finite element software MASA (Macroscopic Space Analysis) which employs the microplane model with relaxed kinematic constraint as the constitutive laws of concrete. Two numerical approaches, which differ only in the interface properties between the head and concrete, are validated against experimental results before carrying out the parametric study. Several properties including head, concrete, and bond stresses, along with ultimate capacities and crack patterns are extracted from the models and analyzed. Moreover, the load-displacement graphs of headed rebars, studs, and straight rebars are compared and contrasted. Assessments and theories about the discrepancies between the behavior of headed studs and rebars are stipulated. Finally, potential methods for formulating design equations are proposed for future studies.</p>

<p>Recent experiments have highlighted the efficacy of post-installed reinforcement in enhancing the capacity of groups of  bonded anchors undergoing concrete breakout failure mode. This technique is particularly useful to enhance the performance of anchorages installed in members of limited dimensions such as beams and columns. This thesis presents the results of corresponding numerical investigations on bonded anchor groups in concrete strengthened with post-installed supplementary reinforcement subjected to tension loads. The study is conducted using the 3D Finite Element (FE) approach. The constitutive law of concrete is the microplane model with relaxed kinematic constraint. The interface between anchor or reinforcement and concrete is modeled with two-node bar elements, which are assigned with corresponding bond-stress slip characteristics. The proposed FE approach is validated against experimental results available in the literature by comparing load-displacement behavior and failure mode. </p> <p>The validation incorporates anchor groups with different configurations of post-installed supplementary reinforcing steel bars. The numerical investigations provide a deeper insight into the detailed behavior of anchor groups with post-installed reinforcement through the visualization of crack patterns, stress flows, and strain development. The results show that the post-installed rebars can lead to a significant increase in the performance of post-installed anchorages, and the load increase depends on the number and arrangement of rebars and the failure mode of the system. </p> <p>Lastly, the thesis presents a parametric study on strengthened anchor groups with post-installed rebars in narrow reinforced concrete (RC) members under various configurations. These simulations mimic anchorages used for seismic retrofitting beam-column joints in RC structures using a fully fastened haunch retrofit solution. Due to the limited width and depth of beams and columns, the capacity of the anchorages is often the weakest link in such retrofitting methods. The results from the FE study indicate that the post-installed supplementary reinforcement can be an efficient solution for upgrading the performance of post-installed anchorages in such retrofitting techniques.</p>

Various anchorage systems including both cast-in-place and post-installed anchors have been developed for fastening both non-structural and structural components to concrete structures. The need for increased flexibility in the design of new structures and strengthening of existing concrete structures has led to increased use of various metallic anchors in practice. Although millions of fasteners are used each year in the construction industry around the world, knowledge of the fastening technology remains poor. In a sustainable society, buildings and structures must, from time to time, be adjusted to meet new demands. Loads on structures must, in general, be increased to comply with new demands, and the structural components and the structural connections must also be upgraded. From the structural connection point of view, the adequacy of the current fastenings for the intended increased load must be determined, and inadequate fastenings must either be replaced or upgraded. The current design models are generally believed to be conservative, although the extent of this behavior is not very clear. To address these issues, the current models must be refined to allow the design of new fastenings and also the assessment of current anchorage systems in practice. The research presented in this thesis consists of numerical and experimental studies of the load-carrying capacity of anchors in concrete structures. Two different types of anchors were studied: (I) cast-in-place headed anchors, and (II) post-installed adhesive anchors. This research focused particularly on the tensile load-carrying capacity of cast-in-place headed anchors and also on the sustained tension loading performance of post-installed adhesive anchors. The overall objective of this research was to provide knowledge for the development of improved methods of designing new fastening systems and assessing the current anchorage systems in practice. For the cast-in-place headed anchors (I), the influence of various parameters including the size of anchor head, thickness of concrete member, amount of orthogonal surface reinforcement, presence of concrete cracks, concrete compressive strength, and addition of steel fibers to concrete were studied. Among these parameters, the influence of the anchor head size, member thickness, surface reinforcement, and cracked concrete was initially evaluated via numerical analysis of headed anchors at various embedment depths. Although these parameters have considerable influence on the anchorage capacity and performance, this influence is not explicitly considered by the current design models. The numerical results showed that the tensile breakout capacity of headed anchors increases with increasing member thickness and/or increasing size of the anchor head or the use of orthogonal surface reinforcement. However, their capacity decreased considerably in cracked concrete. Based on the numerical results, the current theoretical model for the tensile breakout capacity of headed anchors was extended by incorporating several modification factors that take the influence of the investigated parameters into account. In addition, a supplementary experimental study was performed to verify the numerically obtained findings and the proposed refined model. The experimental results corresponded closely to the numerical results, both in terms of failure load and failure pattern, thereby confirming the validity of the proposed model. The validity of the model was further confirmed through experimental results reported in the literature. Additional experiments were performed to determine the influence of the concrete compressive strength and the addition of steel fiber to concrete on the anchorage capacity and performance. These experiments showed that the anchorage capacity and stiffness increase considerably with increasing concrete compressive strength, but the ductility of the anchor decreases. However, the anchorage capacity and ductility increased significantly with the addition of steel fibers to the concrete mixture. The test results also revealed that the tensile breakout capacity of headed anchors in steel fiber-reinforced concrete is significantly underestimated by the current design model. The long-term performance and creep behavior of the post-installed headed anchors (II) was evaluated from the results of long-time tests on adhesive anchors under sustained loads. In this experimental study, adhesive anchors of various sizes were subjected to various sustained load levels for up to 28 years. The anchors were also exposed to several in-service conditions including indoor temperature, variations in the outdoor temperature and humidity, wetness (i.e., water on the surface of concrete), and the presence of salt (setting accelerant) additives in the concrete. Among the tested in-service conditions, variations in the outdoor temperature and humidity had the most adverse effect on the long-term sustained loading performance of the anchors. Based on the test results, recommendations were proposed for maximum sustained load levels under various conditions. The anchors tested under indoor conditions could carry sustained loads of up to 47% of their mean ultimate short-term capacities. However, compared with these anchors, the anchors tested under outdoor conditions exhibited larger creep deformation and failure occurred at sustained loads higher than 23% of their mean ultimate short-term capacities. Salt additives in concrete and wet conditions had negligible influence on the long-term performance of the anchors, although the wet condition resulted in progressive corrosion of the steel. Based on the experimental results, the suitability of the current testing and approval provisions for qualifying adhesive anchors subjected to long-term sustained tensile loads was evaluated. The evaluations revealed that the current approval provisions are not necessarily reliable for qualifying adhesive anchors for long-term sustained loading applications. Recommendations were given for modifying the current provisions to ensure safe long-term performance of adhesive anchors under sustained loads.