Strengthening of existing reinforced concrete structures using ultra high performance fiber reinforced concrete

Most of the new Reinforced Concrete (RC) structures which are built nowadays have a high safety level. Nevertheless, we cannot claim the same for structures built in the past. Many of these were designed without any regulations, or are based on those which have proved to be inadequate. Additionally, it seems that many old structures have reached the end of their service life and, in many cases, were designed to carry loads significantly lower than the current needs specify. Therefore, the structural evaluation and intervention are considered necessary, so they can meet the same requirements as the structures which are built today. Existing techniques for the strengthening and retrofitting of RC structures present crucial disadvantages which are mainly related to the ease of application, the high cost, the time it takes to be applied, the relocation of the tenants during the application of the technique and the poor performance. Research is now focused on new techniques which combine strength, cost effectiveness and ease of application. The superior mechanical properties of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) compared to conventional concrete, together with the ease of preparation and application of the material, make the application of UHPFRC in the field of strengthening of RC structures attractive. The present research aims to investigate the effectiveness of UHPFRC as a strengthening material, and to examine if the material is able to increase the load carrying capacity of existing RC elements. This has been achieved through an extensive experimental and numerical investigation. The first part of the present research is focused on the experimental investigation of the properties of the material which are missing from the literature and the development of a mixture design which can be used for strengthening applications. The second part is focused on the realistic application of the material for the strengthening of existing RC elements using different strengthening configurations. Finally, in the last part, certain significant parameters of the examined technique, which are mainly related to the design of the technique, are investigated numerically. From the experimental and numerical investigation of the present research it was clear that UHPFRC is a material with enhanced properties and the strengthening with UHPFRC is a well promising technique. Therefore, in all the examined cases, the performance of the strengthened elements was improved. Finally, an important finding of the present research was that the bonding between UHPFRC and concrete is effective with low values of slip at the interface

Dowels effectiveness investigation between ultra high performance fiber reinforced concrete and reinforced concrete

In the present study, the performance of Reinforced Concrete (RC) beams which were strengthened with Ultra High Performance Fiber Reinforced Concrete (UHPFRC) and dowels at the interface was investigated. RC beams with a length of 2.2 m strengthened with UHPFRC layers at the tensile side. Before the application of the layers, the interface between RC and UHPFRC was roughened. During the testing, the interface slips between UHPFRC and RC were recorded using Linear Variable Differential Transformers (LVDTs). The beams were tested under four-point flexural test. The results of the present study indicated that the dowels at the interface reduce the slips at the interface, delay the formation of cracks and result in higher load carrying capacity

Fiber content and curing time effect on the tensile characteristics of ultra high performance fiber reinforced concrete

Ultra high performance fiber reinforced concrete (UHPFRC) is a concrete type with superior mechanical properties and of a relatively high tensile strength. The tensile stress–strain characteristics of UHPFRC are highly affected by the mixture design and the curing regime. In this study, an extensive experimental investigation has been conducted with direct tensile tests on a number of specimens that contained different percentages of steel fibers and different cement types were applied. Also, various curing regimes were investigated. Different models depending on the steel fiber amount were proposed for the simulation of the stress–strain and the stress-crack opening response of UHPFRC, while the fracture energy was also calculated for the different fiber contents. Finally, the effect of fiber content and curing time on the variation of the experimental results are discussed

Investigation of the effectiveness of dowels at the interface between reinforced concrete and ultra high performance fiber reinforced concrete

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is a material with unique properties in tension and compression and high energy absorption in the post-cracking region. The superior performance of UHPFRC allows the construction of thin elements with high strength and ductility. A promising application of UHPFRC is for strengthening of existing Reinforced Concrete (RC) structures. In the present study, UHPFRC layers have been applied for the strengthening of full-scale RC beams. For the improvement of the UHPFRC-to-RC interface connection, dowels have been placed in addition to the roughening of surface of the initial beam and the effectiveness of this technique has been evaluated. Flexural tests have been conducted and the interface slips have been recorded in addition to the load-mid span deflections and the load carrying capacity. The results showed that the dowels resulted in better bonding at the interface and delayed the formation of the cracks in the post elastic phase with lower values of slips and subsequent higher load carrying capacity. The main conclusion of this study is that the addition of dowels at the UHPFRC-to-RC interfaces is a technique which should be considered when UHPFRC layers are used for the structural upgrade of existing RC structures

Ultra-high-performance fiber-reinforced concrete under cyclic loading

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is a novel cementitious material with enhanced strength in tension and compression, and significantly high energy absorption in the postcracking region. The application of UHPFRC for the earthquake strengthening of existing structures could considerably improve the performance of existing structures due to its superior properties. There are published studies where the direct tensile and the flexural behaviors of UHPFRC have been investigated and the superior tensile strength and post-crack energy absorption have been highlighted. However, there are not any published studies on the performance of UHPFRC under cyclic loading. In this paper, the results of an extensive experimental program on UHPFRC under direct tensile cyclic loading are presented and a constitutive model for the response of UHPFRC under cyclic loading is proposed. The accuracy of the proposed model is validated using experimental results from various loading histories and for different percentages of fibers, and the reliability of the proposed model is highlighted

Combined non-destructive testing (NDT) method for the evaluation of the mechanical characteristics of ultra high performance fibre reinforced concrete (UHPFRC)

Ultra-High Performance Fibre Reinforced Concrete is a material which is becoming increasingly popular in structural applications, mainly due to its superior mechanical characteristics. The mechanical properties of this material are of high importance and the development of non-destructive techniques is vital for the evaluation of the mechanical characteristics of existing structures. In the current study, Ultra-High Performance Fibre Reinforced Concrete with different amounts of steel fibres has been examined. Compressive and tensile tests have been conducted alongside with Ultrasonic Pulse Velocity and Rebound Hammer measurements and the development of appropriate empirical non-destructive models has been examined

Combined non-destructive method for evaluating the mechanical performance of ultra high performance fibre reinforced concrete (UHPFRC)

Non Destructive Testing (NDT) of concrete is extensively used for the in-situ assessment of the mechanical characteristics of concrete. The main advantage of NDT techniques is that they are simple and quick, while the mechanical characteristics of concrete can be evaluated without causing any damage in the existing structures. However, the characteristics of various concrete types are highly affected each time by their mix composition. Hence, the reliability of NDT techniques is questionable and appropriate validation and calibration is required. In the current study, two different mixes have been examined, one representing Ultra High Performance Concrete (UHPC) without steel fibres, and another one with 3% steel fibres representing Ultra High Performance Fibre Reinforced Concrete (UHPFRC). Compressive tests have been conducted alongside with Ultrasonic Pulse Velocity (UPV) and Rebound Hammer (RH) measurements. The experimental results have been used for the development of a model for the evaluation of the mechanical characteristics of UHPFRC using the combined UPV and RH method (SonReb). The results of the current study highlight the efficiency and the reliability of SonReb method for the estimation of the compressive strength of UHPC and UHPFRC using RH and UPV results

Experimental and numerical investigation on the size effect of Ultrahigh-Performance Fibre-Reinforced Concrete (UHFRC)

In the last few years, there has been increasing interest in the use of Ultrahigh-Performance Fibre-Reinforced Concrete (UHPFRC) layers or jackets, which have been proved to be quite effective in strengthening applications. However, to facilitate the extensive use of UHPFRC in strengthening applications, reliable numerical models need to be developed. In the case of UHPFRC, it is common practice to perform either direct tensile or flexural tests to determine the UHPFRC tensile stress–strain models. However, the geometry of the specimens used for the material characterization is, in most cases, significantly different to the geometry of the layers used in strengthening applications which are normally of quite small thickness. Therefore, and since the material properties of UHPFRC are highly dependent on the dimensions of the examined specimens, the so called “size effect” needs to be considered for the development of an improved modelling approach. In this study, direct tensile tests have been used and a constitutive model for the tensile behaviour of UHPFRC is proposed, taking into consideration the size of the finite elements. The efficiency and reliability of the proposed approach has been validated using experimental data on prisms with different geometries, tested in flexure and in direct tension

Strengthening of reinforced concrete beams using ultra high performance fibre reinforced concrete (UHPFRC)

In this study the efficiency of the use of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) for the strengthening of existing Reinforced Concrete (RC) beams has been investigated. Experimental work has been conducted to determine UHPFRC material properties. Dog-bone shaped specimens have been tested under direct tensile loading, and standard cubes have been tested in compression. These results have been used for the development of a numerical model using Finite Element Method. The reliability of the numerical model has been validated using further experimental results of UHPFRC layers tested under flexural loading. Further numerical study has been conducted on full-scale beams strengthened with UHPFRC layers and jackets, and these results were compared to respective results of beams strengthened with conventional RC layers and with combination of UHPFRC and steel reinforcing bars. Superior performance was observed for strengthened beams with UHPFRC three side jackets, and the efficiency of this technique was highlighted by comparisons with other strengthening techniques