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

Ultra high performance fiber reinforced concrete as strengthening material

The safety of structures is of high importance affecting people’s lives. Structural evaluation, and possibly intervention, is considered necessary for old structures, structures which have been affected by accidental actions and also for structures in high seismic risk areas. Research should now focus on the development of new sustainable techniques which increase the safety of existing structures, and at the same time minimize the necessary to build new structures and to consume new materials and resources. The present research investigates the effectiveness of an advanced material, such as Ultra High Performance Fiber Reinforced Concrete (UHPFRC), for the strengthening of existing Reinforced Concrete (RC) structures. For this reason, an extensive experimental study on the properties of the material and the application for strengthening of RC beams has been conducted. More specifically, parameters such the effect of fiber content on the performance, the workability and the cost of the material have been investigated first. Based on the analysis, an optimum mixture design has been selected and has been applied for strengthening of RC beams using different configurations. The results indicated that the strengthening with UHPFRC is a well promising technique and the performance of the strengthened elements has been increased in all the examined cases

Developments in the use of Ultra High Performance Fiber Reinforced Concrete as strengthening material

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is a novel material which has been developed the last few decades and has been applied in applications that require high strength, ductility and durability. Recently, the material has been applied in strengthening applications. The present study aims to investigate new techniques for the application of UHPFRC as strengthening material and to provide an insight into the parameters affecting the performance of elements strengthened with UHPFRC. The present research investigates for the first time the effectiveness of the use of dowels at the interface between UHPFRC and concrete to improve the connection between these two materials. Additionally, the effectiveness of the use of UHPFRC jackets for the strengthening of Reinforced Concrete (RC) beams has been examined. In the present research, a systematic experimental study has been conducted together with numerical study. The results demonstrate that both examined techniques are effective and should be taken into consideration when UHPFRC is applied for strengthening applications. The dowels result in better bonding at the interface and can delay the formation of cracks in the post elastic phase, leading to reduced interface slip values and subsequent enhanced load bearing capacity. This technique should be taken into consideration to eliminate the risk of premature de-bonding of the strengthening layer. The construction of UHPFRC jackets on the other hand, results in a dramatic increase of the stiffness and the load carrying capacity of the strengthened elements and should be preferred in cases of heavily damaged RC members

Investigation of key parameters affecting the use of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) as strengthening material

The use of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) for strengthening applications is gaining increasing attention due to its advantageous mechanical and durability characteristics. However, the lack of design guidelines and code recommendations hinders the extensive use of UHPFRC in strengthening applications. The aim of the present study is to offer valuable insights regarding the effect of critical parameters on the performance of UHPFRC as a strengthening material. In this current research, experimental and numerical results have been used to evaluate the effectiveness of UHPFRC layers for the structural upgrade of existing Reinforced Concrete (RC) beams. Critical parameters such as the thickness of the additional layers, the connection at the old-to-new concrete interface, the mechanical performance of UHPFRC, and the amount of additional longitudinal reinforcement have been studied, and recommendations for the effective design of the strengthened elements, considering both performance and cost, are presented. The results of the present study indicate that the addition of steel bars with different diameters in the layer increased the load-carrying capacity by up to 183%. On the contrary, an increase of 5.8% in the load-carrying capacity was achieved when the RC beams had 30 mm layers, and an 18.1% increment was achieved for layers with a higher thickness of 70 mm. The assumption of perfect connection at the interface, which can be achieved with dowels at the interface, resulted in further enhancement of the load carrying capacity, which was found to be equal to 11.8% for the 30 mm layer and 35.3% for the 70 mm layer. An increment of the UHPFRC tensile strength of the strengthening layer from 11.5 MPa to 14 MPa resulted in a further increment of the load-carrying capacity by 8%

Experimental and numerical study of the performance of Ultra High Performance Fiber Reinforced Concrete for the flexural strengthening of full scale Reinforced Concrete members

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is a cementitious material with enhanced mechanical characteristics. The superior mechanical properties of UHPFRC compared to conventional concrete, as well as the ease of preparation and application, make the use of this material attractive for strengthening applications. In the present study, an extensive experimental investigation on full scale Reinforced Concrete (RC) beams strengthened with UHPFRC layers has been conducted. Additional UHPFRC layers with and without steel bars have been added to the RC beams and the efficiency of the examined technique has been examined through flexural tests. An additional investigation has been conducted on the interface characteristics between UHPFRC and concrete through push-off tests. Finally, finite element analysis has been conducted and crucial parameters of the examined technique have been investigated. The results of the present study indicated that the strengthening with UHPFRC layers is a well promising technique, as in all the examined cases the performance of the strengthened elements was improved. Also, a good interface connection between UHPFRC and concrete was identified, with low measurements of slips at the interfac