Structural strengthening of RC columns by means of high performance concrete layers

In the last two decades the retrofit of columns with Textile Reinforced Concrete (TRC) gained interest due to the possibilities of being used as an alternative to the more common Fibre Reinforced Polymer (FRP). The rising of this material is mainly due to the advantages related to the use of an inorganic matrix instead of the organic matrix of the FRP. In fact, the epoxy-based resin of FRP is responsible for some of the main drawbacks of the composite such as non-applicability on wet surfaces, low performances at high temperature, being hazardous for manual workers and potential incompatible with substrate materials. On the other hand, TRC is characterized by some advantages which are in common with FRP, such as high strength to weight ratio and the minimal impact on the original geometry. Recently, the development of this composite material involved the use of short fibres admixed in the cementitious matrix, which led to a new material called Fibre/Textile Reinforced Concrete (F/TRC). It has been observed that the addition of short fibres improves the performances of the cementitious matrix by increasing the first crack stress, the ultimate stress and the cracking control. During the development of the textile fabrics, many different materials were used. Among the large number of materials available in the market, basalt is gaining interest due to its mechanical performances and low environmental impact, if compared to the more common materials used in TRC, such as glass and carbon. In this research work an extensive experimental campaign is carried out by performing uniaxial compressive tests on 90 short columns. The columns are reinforced by TRC layers applied on their lateral surface. The goal of the research is to assess the efficiency of the proposed strengthening technique in enhancing the columns’ axial strength, through the confinement. Two main topics are addressed. The use of basalt textile as a possible alternative to the more common materials adopted in TRC, and the comparison of its behaviour to that of carbon textile, and the effect, on confinement of the columns, of admixing short steel fibres in the cementitious matrix of the TRC. A large number of variables are investigated in this work: • Geometry of the cross section. • Size of the cross section. • Presence of steel reinforcement. • Stirrups spacing. • Number of TRC layers. • Material of the textile fabric. • Load introduction layout. • Concrete compressive strength class. • Type of cementitious matrix adopted in the retrofit. • Presence of short steel fibres admixed in the cementitious matrix. Furthermore, characterization tests of the materials were executed, such as uniaxial tensile test and bond tests. A wide discussion of the test’s results is provided. The main finding are: 1. TRC retrofit improves the performances of RC columns in both load bearing and axial displacement capacities; 2. The adoption of short steel fibres admixed in the cementitious matrix significantly increases the maximum load reached by retrofitted columns; 3. Columns strengthened with basalt and carbon textile fabrics reach similar values of maximum load. 4. Therefore it is concluded that basalt could be a valuable alternative to carbon in the retrofit of columns with TRC and F/TRC.