Post-installed Reinforcement Research Articles

Mechanical performance of CRTS II slab tracks in reinforced-unreinforced transition zone in extreme heat events

Anchors have been widely utilized to reinforce the CRTS II slab tracks. The mechanical performance of the tracks in the reinforced-unreinforced transition zone exposed to extreme heat waves can be very complex due to the difference in mechanical properties between the two kinds of track segments. This paper investigates the damage behavior of the tracks in the reinforced-unreinforced transition zone in extreme heat events. A finite element model of the CRTS II slab track in the transition zone has been established and validated, in which the nonlinear mechanical properties of track materials and interfaces are considered by applying the Concrete Damaged Plasticity model and the Cohesive Zone Model respectively. The nonlinear temperature field in the track structure in extreme heat events is applied to the model. Structural damage of the track, including slab-end arching and interfacial failure, in the transition zone under extreme heat is thoroughly investigated. In particular, the effects of joint use of post-installed reinforcement anchors and other track maintenance measures like interface adhesives and concrete joint restoration are studied. The following conclusions are drawn: (1) The more reinforced track-slabs in the transition zone, the milder the track diseases including slab-end arching and interfacial gapping. (2) Joint use of anchor installation and other maintenance measures such as interface adhesives and concrete joint restoration shows a better effect on mitigating track damage than using post-installed reinforcement anchors solely. (3) Concrete joint restoration overweighs interface adhesives in terms of reducing slab-end arching and interfacial damage when they are jointly used with post-installed reinforcement anchors. (4) In the transition zone, track damage surges when the air temperature increases from 42 ℃ to 47 ℃, which is the temperature range for an extreme heat wave. Monitoring of track state in the transition zones is necessary during extreme heat waves. The novel findings are expected to provide some insights into the mechanical performance of slab tracks and help to prevent potential damage to the tracks in the transition zone, which is a common and practical issue in railway engineering.

Structural behaviour of post-installed reinforcement for 3D concrete printed shells – A case study on water tanks

Incorporating sufficient reinforcement to ensure a ductile structural behaviour is a persisting challenge in digitally fabricated concrete structures. This paper investigates the structural performance of a reinforcement approach for 3D concrete printed elements, consisting of an unreinforced 3D printed concrete shell and a sprayed shell reinforced with a conventional reinforcing mesh for application in water tanks. Four reinforced concrete elements produced with this approach were tested in direct tension and compared to a reference test of a monolithic specimen to analyse the behaviour of circular water tanks under hoop stresses. Two eccentric reinforcement arrangements and two different printing patterns were investigated. Despite the testing setup not perfectly representing the actual behaviour of circular water tanks, in which shell deformations are kinematically restrained, the feasibility of the fabrication method could be examined. The results did not show significant differences in the behaviour of the different fabrication methods, with similar ductility as expected in a conventionally reinforced shell. The eccentric reinforcement caused the crack formation to originate on the surface close to the reinforcement, accompanied by out-of-plane deformations. The cracks on the far side of the reinforcement opened suddenly and reduced the out-of-plane deformations. The predictions with models neglecting the eccentricity of the reinforcement overestimated the crack opening. The best predictions were obtained from the tension chord model by only considering the concrete area defined by twice the mechanical cover of the reinforcement.

Anchorage behavior of Fe-SMA rebars Post-Installed into concrete

The unique self-prestressing behavior exhibited by Iron-based shape memory alloys (Fe-SMAs) coupled with the ease of application makes them suitable for numerous structural applications requiring post-installed reinforcement. However, the practical realization of Fe-SMA rebars as post-installed reinforcement systems entails an understanding of their bond behavior with concrete to ensure the use of adequate anchorage length for prestress transfer and prevention of premature failure. The bond behavior of cast-in-place and near-surface mounted Fe-SMA rebars with concrete has been investigated in detail in a few recent studies. However, no study has so far investigated the bond behavior of Fe-SMA rebars post-installed into concrete. To address this important knowledge gap, this paper presents an extensive experimental campaign aimed at evaluating the anchorage of post-installed Fe-SMA rebars in concrete. The experimental campaign comprised 23 specimens, with the variable parameters of the study including the bond length, state of Fe-SMA rebar (non-activated or activated), drill-hole size, strength of the adjacent concrete, type of rebar (plain, threaded, or deformed), and the type of loading history (monotonic or cyclic). The experimental results showed that for bond lengths ≥ 5db, the ribbed Fe-SMA rebars primarily fail in a pull-out failure mode and exhibit large inelastic rebar strains and load capacity at bond failure. The paper is concluded with the comparison of anchorage length predictions of conventional design models with the results of the current experimental testing campaign. The comparison demonstrates that the anchorage length recommendations of existing design standards are generally conservative for Fe-SMA rebars.

Prevention of concrete breakout failure of expansion anchor in tension by post-installed reinforcement: Discrete analysis and experiment

Concrete breakout failures with acutely damaged concrete are one of the main problems caused by expansion anchors under tensile loading. In this study, the prevention of concrete breakout is investigated by post-installed reinforcement (PRs). Experimental and numerical studies were conducted to look into the effect of the PRs on the expansion anchors subjected to tensile loads. The 3D Rigid Body Spring Model (RBSM), which is based on discrete analysis at the mesoscale, was used as supporting numerical research. On pullout capacity, displacement, failure mechanism, and concrete breakout geometry, both experimental and numerical analyses were undertaken. The numerical analysis also looked at internal stress, strain, and concrete cracking. In general, the anchors with the PRs had steady displacement (ductile) and less brittle failure compared to the anchors without the PRs. The significant finding was in the failure mode, where severe concrete damage and concrete breakout failures were prevented by the PRs. The internal tensile stress of concrete was concentrated around the wedges of the anchors (interlocked area). The contribution of the PRs was shown by high strain values at around their mid-length. Additionally, due to the PRs, concrete cracking was less, mostly generated in the region between an anchor and its PRs. The utilization of PRs could be a method to protect concrete from breakout failure.

Enhanced service life of existing infrastructure buildings by use of concrete screw anchors as post-installed shear reinforcement

In the last decades, traffic on railways and roads has increased significantly in Central Europe. On the other hand, the existing infrastructure mainly origins from the late 1950s to the 1970s. This applies also for the existing bridge and tunnel infrastructure which is now up to 60 years old. Due to this age and new standards in the current codes, quite often the amount of shear reinforcement does not fulfil the current design code rules. As a replacement of existing structures is often highly unsustainable, methods to improve the shear strength are needed. Therefore, a new system for post-installed shear reinforcement by the use of concrete screws was developed at the University of Innsbruck in the last years. The idea is to use concrete screw anchors - already known as anchoring element - aspost-installed reinforcement. In over 60 shear tests with girders and slabs the excellent applicability of concrete screw anchors as post-installed shear reinforcement has been proven. Also shear tests with cyclic loading were performed prove the use for dynamically loaded structures, such as bridges. Based on these test results a design approach was determined to meet the Eurocode-2 assessment standards.Several pilot-projects with concrete screws have been realised in Austria and Germany. For example a rail-road-bridge over a German highway with a remaining service life of zero years was strengthened to a new service life of over 20 years. Also a city-centre tunnel in Munich which was erected below a historic building is currently strengthened under ongoing traffic in the tunnel using concrete screw anchors as post-installed reinforcement.

Structural Behaviour of Post-Installed Reinforcement for 3d Concrete Printed Shells – a Case Study on Water Tanks

Structural Behaviour of Post-Installed Reinforcement for 3d Concrete Printed Shells – a Case Study on Water Tanks

Strengthening of beams or slabs using temperature induction

AbstractSeveral aged reinforced concrete (RC) structures need to be strengthened. A typical method is to subsequently add rebar into slots filled with bonding mortar. However, this kind of strengthening only acts for live loads, not for dead loads, if no prestressing or prior lifting is applied. To overcome this deficiency, a novel method is presented. It activates the postinstalled reinforcement for all types of loading by temporarily inducing temperature from the outside. Doing so, statically indeterminate structures are locally relieved from stresses by constraint moments from systematically induced vertical temperature gradients during strengthening. To set the size of the gradients, the nonlinear material behavior, especially the significant reductions in constraint forces due to softening, is taken into account. If tempering is stopped after strengthening, stresses from dead loads redistribute over the newly formed total cross section. The method is theoretically derived, discussed, and verified in the lab on a two‐span RC girder. It turns out appropriate to enhance the bending resistance of beams or slabs.

Bonded anchors with post-installed supplementary reinforcement under tension loading – Experimental investigations

This paper presents the results of comprehensive and systematic experimental investigations carried out on anchorages (single anchors and groups) of bonded anchors supplemented with post-installed reinforcement subjected to tension loading. Until now, the use of supplementary reinforcement is allowed in the codes only for anchorages with cast-in headed anchors supplemented with cast-in reinforcement. In this work, tension tests are performed on single anchors and anchor groups with different amounts and arrangements of post-installed reinforcement. Epoxy based mortar was used for installation of both anchors and the reinforcement. It was found that the post-installed supplementary reinforcement can significantly increase the capacity of anchorages undergoing concrete cone breakout failure. Due to high bond strength of the mortar, even relatively short bond lengths within the breakout body were sufficient to activate the reinforcement and contribute significantly towards the resistance. The ultimate load was limited either by reinforcement yielding (for low amounts of reinforcement) or strut failure (for moderate to high amounts of reinforcement). Not just the amount but also the placement and arrangement of reinforcement plays an important role in the tension behavior of anchorages with post-installed supplementary reinforcement. At peak load, both the concrete as well as reinforcement contribute significantly to the resistance of the anchorage.

Design Recommendations for Bonded Anchors under Fire Conditions Using the Resistance Integration Method

Fire design of cast-in place and post-installed anchors in concrete under fire is covered by EN 1992-4, Annex D, allowing steel- and concrete-related failure modes of anchors to be calculated. This informative annex of EN 1992-4 is limited to cast-in place or mechanical anchors, whereas post-installed adhesive anchors remain out of its scope. This paper presents a study of the applicability of the more flexible resistance integration method (RIM), proposed originally for the design of the pull-out resistance of post-installed reinforcement (PIR) by Pinoteau, on bonded anchors in uncracked concrete. This method is validated from a comparison of test results obtained from two research projects conducted at CSTB and TU Kaiserslautern on bonded anchors in uncracked concrete under ISO 834-1 fire conditions. The data considered include tests conducted on anchor sizes from M8-M30 using three different adhesives (two epoxy adhesives and one cementitious mortar). Design of the pull-out resistance under fire using RIM requires numerical calculation of temperature profiles considering models of concrete and steel elements; different assumptions about modeling these elements can produce vastly different end results. Finally, recommendations for assessment procedures for bonded anchors under fire conditions are provided as entry data for design.

Post-installed anchors in concrete strengthened by post-installed reinforcement under tensile load

This research discusses post-installed anchors strengthened by post-installed reinforcement (PRs) under tensile loading. Both types of adhesive and expansion anchors were tested under tensile loading. The PRs are reinforcing bars that were installed in a vertical and an inclined direction around a single anchor, as different samples separately. In general, the results showed that the PRs in the vertical direction were more effective than those in the inclined direction. It was found that the tensile capacity of the adhesive anchors significantly improved using the PRs. Although, no major enhancement was recorded in the tensile capacity of the expansion anchors. For both types of anchors, the PRs prevented severe concrete breakout failure. Moreover, with the PRs, a higher ductile behavior was observed at both types of anchors. Employing PRs could be a feasible and practical method to strengthen anchorage systems in concrete and prevent severe damages in concrete substrates.

Interfacial failure and arching of the CRTS II slab track reinforced by post-installed reinforcement bars due to thermal effects

To prevent damage of the China Railway Track System (CRTS) II slab track induced by thermal effects, post-installed reinforcement (PIR) bars between the prefabricated slabs and the concrete base have been utilized in the track. This work presents a study on the interfacial failure and arching of the CRTS II slab track reinforced by PIR bars due to the effects of temperature change. To this end, a novel finite element model of the track was established, in which the non-linear interfacial interaction between the track slabs and the cement asphalt (CA) mortar layer was simulated by a cohesive zone model, the non-linear constitutive material properties of track concrete were represented by a concrete damaged plasticity (CDP) model, and the non-linear bond-slip relationship between PIR bars and track concrete was characterized by non-linear spring elements. Comparison between simulation results and experimental data indicates that the proposed modelling technique is capable of evaluating the performance of the track reinforced by PIR bars. The combined action of insufficient bond strength between PIR bars and concrete, and insufficient bond strength between track slabs and CA mortar layer, was analyzed. Numerical analysis results show: (1) The temperature rise of interface failure between the concrete slabs and the CA mortar layer can be improved by 6 °C after PIR bars installation. (2) The PIR bars are effective in reducing arching displacement of slabs caused by temperature rise in both lateral direction and longitudinal direction of track slabs. (3) Both PIR bond strength and interface bond strength influence the maximum arching displacement of track slabs significantly.

Experimental study on bond performance of advanced material based on composite Portland cements developed for anchoring systems with post-installed reinforcement bars in concrete

Abstract This paper presents an assessment study on the bond performance of new advanced mortars based on composite Portland cements. These developed mortars are used as anchoring material for post-installed reinforcement steel bars in concrete. The anchoring mortars were developed by laboratory studies in the recent years and they met several performance criteria regarding the properties in plastic state as consistency, cohesiveness, stability and in hardened state as strength, rigidity, deformation volume and bond to support. Anchoring systems with post-installed reinforcement steel bars are obtained through installation of reinforcement bars into drill holes in hardened concrete and filled with flowable mortars which may be polymer or cement-based mortars. Generally, post-installed reinforcement bars are used to connect layers of new concrete to old concrete and also new members to old concrete members. So far, the most employed anchoring material is the polymer-based chemical resin which have some undisputed advantages, but the durability and long-term performance along the expected service life is questionable. Recently, inorganic-injection-type anchors have been developed in Japan. These anchors are composed of cement, aggregate and additive agents. In this study, the employed anchoring-mortars are exclusively based on hydraulic binder, namely composite Portland cements which are included in the European class of cement CEM II. These composite cements contain supplementary cementing materials (SCM) as fly-ash or limestone which are combined with Portland clinker at the manufacture. The bond strength of the reinforcement steel bars was assessed based on the prescriptions provided in the European assessment document EAD 330087 and EN 1881. These two technical norms are limited only to deformed reinforcement bars with characteristics in conformity with EN 10080 and designed based on EN 1992-1-1. Within this study, the shear load on the reinforcement bars post-installed into hardened concrete is not considered.

Post-installed reinforcing bars – Requirements for their reliable use

Post-installed steel reinforcing bar connections carried out with anchoring mortars sufficiently qualified to fulfill the requirements for safety, serviceability, and durability of concrete structures are capable of developing bond behavior equivalent to cast-in-place steel reinforcing bars. Therefore, their design can be performed according to reinforced concrete design provisions. Post-installed anchorage technology offers fast and convenient solution to introduce new reinforcement in an existing reinforced concrete (RC) structural system, and hence serves as effective method for strengthening and upgrade of existing structures. The behavior of post-installed reinforcement depends on the existing condition of the concrete member. This paper addresses the aspects related to base material, installation, environment, and load conditions in service. This paper also comments on the qualification of anchoring mortars and provides information on the design of post-installed reinforcing bars as reinforcement (RC design perspective) or as anchorage (adhesive anchor design perspective) in the light of corresponding provisions necessary to achieve the same safety level in service compared to connections with cast-in steel reinforcing bars.

Structural Analysis and Design of Reinforced Concrete Bridge Corbels

Post-installed systems for the anchorage of safety barriers to bridge corbels are widely used today thanks to their flexibility and easiness of installation. Because of commonly found in situ boundary constraints, however, the design requirements for post-installed fasteners and rebars are frequently not satisfied or only partially satisfied. This paper assesses the mechanical response of a corbel where an innovative solution concerning the placement of post-installed reinforcement in reinforced concrete members was suggested. With reference to the refurbishment of bridge curbs, which usually requires concrete removal in the damaged top layers, the proposed method was based on the introduction of additional U-shaped post-installed rebars connecting the existing portion of the corbel to the newly cast top layer, in order to allow the transfer of the tension pull-out force exerted by the posts restraining the safety barrier. The layout investigated in this paper consisted of three anchors connecting the baseplate of the post supporting the safety barrier to the corbel (a layout commonly found in Italy). These anchors transfer the external actions (bending moment and shear) to the corbel thanks to the formation of a strut-and-tie system where the U-shaped rebars and the existing reinforcement play a crucial role. A strut-and-tie model of the corbel was presented to allow the use of a simplified approach to assess the safety of the corbel. The tests on real-scale specimens were also modeled numerically and additional models were considered to evaluate the effect of characteristics parameters (i.e., size of the corbel, existing shear reinforcement, etc.) on the overall response of the corbel.

Discrete mesoscale analysis of adhesive anchors under tensile load taking into account post-installed reinforcement

The utilization of post-installed anchors is growing fast to connect both structural and non-structural members. Adhesive anchors are one type of post-installed anchors which are formed as threaded bars. They can be post-placed into concrete with the help of epoxy adhesive. On the one hand, improvement of the performance of post-installed anchors, unlike cast-in-place anchors, has not drawn attention much. On the other hand, due to the complexity of their performance, a numerical study can be a supportive method for the experimental study. This paper addresses the effect of post-installed reinforcement (PR) on the adhesive anchors under tensile loading in both experimental and numerical studies. Reinforcing steel bars as post-installed reinforcement (fully embedded in concrete) were used to ameliorate the performance of the adhesive anchors under tensile force. Rigid Body Spring Model (RBSM) was developed as a simulation that is based on discrete analysis at mesoscale. Pull-out capacity, displacement, failure mode, and concrete cone geometry were measured by the experiments and the RBSM simulation. In addition, internal stress, strain, cracking patterns were investigated by the RBSM simulation. Moreover, some comparisons were conducted through the existing standard. In general, PR assist the anchors to have higher pull-out capacities, lower displacement as well as leading smaller size of concrete cone failure. Due to PR, the internal tensile stress of concrete changed to compressive stress resisting greater pull-out force. The higher strain is obtained at near the top of the anchors and at the mid-span of PR. Furthermore, cracking patterns in concrete were mostly within the area between the anchor and PR. Post-installed reinforcement can be considered as a method to improve the performance of adhesive anchors under tensile loading. This is validated by the RBSM simulation, which agrees with the experimental results.

Evaluation of design concepts for post-installed punching shear retrofitting

Punching shear is a brittle and sudden mode of failure observed in reinforced concrete slabs structures. This type of failure typically occurs around the slab-column connections, due to transverse forces being highly concentrated in these areas, thereby causing the column to punch through the slab. Several methods have been proposed for retrofitting and strengthening existing flat slabs against punching shear failure using different reinforcement-types. One of these methods relies on post-installed shear reinforcement for existing flat slab systems. The estimation of the punching shear resistance depends on several variables (geometry, mechanical, material properties, etc.) with some level of uncertainty. This study aims to assess the safety and economic performance of different methods for the design of post-installed reinforcement in an existing flat slab structure endangered by punching shear, using probabilistic analysis. The methods considered include the design procedure according to the American Concrete Institute (ACI) design code, the Canadian Standards Association (CSA) design code, the European Eurocode 2 (EC2) design code and analytical method based on the Critical Shear Crack Theory (CSCT). The probabilistic analysis was conducted based on the Monte Carlo simulation technique implemented using a MATLAB code developed in this study. The reliability index obtained for EC2 design procedure was found to be close to the EN 1990 target reliability level. CSCT and CSA, on the other hand, have a low-reliability index for most of the cases investigated.

Effect of Epoxy-based Adhesives and Embedded Length in the Bond Strength of Post-installed Reinforcement in Concrete

Effect of Epoxy-based Adhesives and Embedded Length in the Bond Strength of Post-installed Reinforcement in Concrete

Mix Design Approach of Supplementary Cementitious Materials Portland Cement-based Mortars for Anchoring Post-installed Reinforcement Bars into Hardened Concrete

Post-installed anchorages are used extensively in civil and structural engineering worldwide in both structural and nonstructural concrete constructions. The most used anchoring material is the polymer-based chemical resin which have some undisputed advantages, but the long-term performance over the expected service life is under concern.The paper presents a mix design approach of an alternative anchoring material which consist in well suited mortars with advanced characteristics in the fresh and hardened state. The anchoring material is a supplementary cementitious material Portland cement-based mortar used as structural bonding material for fixing reinforcing steel bars in hardened concrete.A series of standardized tests were performed within the experimental setup with the objective of assessing the performance of the mortars in terms of consistence, cohesiveness, strength and adherence.

Mesh reinforcing method for 3D Concrete Printing

Abstract 3D Concrete Printing (3DcP) is an emerging technology with promises to transform concrete construction using additive manufacturing technologies. One of the major challenges for 3DcP technology is the reinforcement method, particularly in the interlayer direction (across the layers). Previously developed methods include (1) pre-installed reinforcement where the reinforcement is placed or 3D printed before the concrete printing and (2) post-installed reinforcement where reinforcement is inserted and grouted after the 3D printing of concrete with holes for the reinforcements. A new in-process method is presented in this paper to embed mesh reinforcement at the same time when concrete layers are printed. Reinforcements in each layer are lapped in the interlayer (across the layer) direction to simulate a continuous reinforcement. Tests and calculations show that lapped mesh reinforcement was effective in functioning as a continuous reinforcement.

Local strengthening of anchorages with post-installed (supplementary) reinforcement

The existing anchorages in reinforced concrete structures may need strengthening due to an increase in the applied load during its lifetime. In certain cases, due to limited dimensions of the structural member (e.g. concrete slab or beam), the size of the anchorages that can be used is also limited and the standard design of anchorages may not be enough to provide required load-carrying capacity. In these above-mentioned cases, a method to strengthen the anchorage may be needed. In the present work, it is attempted to develop a method for strengthening of anchorages under tension loads by using post-installed reinforcing bars. Tests were performed on anchorages using bonded anchors (single anchors) without and with different configurations of post-installed reinforcement for strengthening. The main objectives were to investigate the influence of the reinforcement arrangement on the ultimate load capacity and on the load-displacement behavior of the anchorages. The bonded anchors were selected considering the ease of installation and freedom to choose the test parameters. The tests were performed on anchorages away from the edge. The test parameters were determined in a way that in the case of reference tests, concrete breakout was the dominant failure mode. The reinforcement was placed relatively close (with a distance of approx. 0.4·hef) to the anchor. The results clearly show that the post-installed reinforcement can result in a considerable increase in the load and deformation capacity of the anchorage. Depending on the amount and arrangement of the reinforcement, a change in the failure mode from concrete cone breakout to strut-failure could be observed. It was also shown that for the same amount of reinforcement provided, the arrangement of the reinforcement had a considerable influence on the effectiveness of the strengthening. To understand the mechanics better, in certain tests, strain gauges were applied on the reinforcing bars, which showed the interaction between the contribution of concrete and that of reinforcement in resisting the applied loads.