Low-grade concrete strengthened in flexure by means of inorganic based composites

April 25th, 2012
Reinforced concrete is a basic material in the modern Western society. Blocks of flats, pavements, flyovers, underpasses, and even benches... are some of the elements made of reinforced concrete. Annual concrete production is measured by means of big figures and it is certainly difficult to imagine another product capable of replacing it in a short term.

Over the years, concrete structures lose their initial conditions and develop a series of pathologies that affect their mechanical behaviour and shorten their life. This negative effect is especially noticed in the material known as low-grade concrete. This term includes those concretes which were the result of design and execution mistakes, quality defects of materials, misuse or bad maintenance. The progressive decay of the concrete conditions would lead to the demolition of certain structures.

In a time influenced by an economical crisis that affects principally to the macro-sector of the Construction, it is necessary to look for new market niches. Rehabilitation is a business with broad possibilities. Hence, it is convenient to stimulate the study and development of innovative, easy-to-apply, effective, economic and sustainable solutions.

Within the scope of the European Doctoral Thesis Flexural strengthening of low grade concrete through the use of new cement-based composite materials, developed in Tecnalia, an innovative composite for low-grade RC strengthening in flexure has been studied. The presented material might be an alternative technique to the traditional retrofitting systems. The study aimed to deepen the knowledge of the effectiveness of the composite based on technical textiles embedded in an inorganic cementitious matrix (mortar slightly modified with polymers), known as Textile Reinforced Mortar (TRM). Likewise, a brief study was conducted in order to define the properties of low-grade concrete, analyse its pathologies and understand its origin through the analysis of real cases, the revision of the bibliography and the evaluation of the historical evolution of the material.

Textile-Reinforced Mortar (TRM)

There are several methods to strengthen reinforced concrete elements in flexure. The increase of the cross section by adding more reinforced concrete or the use of fibro-plastic composites (FRP) are the most common solution among designers. Nevertheless, these methods have numerous drawbacks in certain situations. TRM is presented as an alternative to overcome those problems. Its use is justified by the physical and chemical characteristics of the TRM constituent materials (resistance to high temperatures, permeability to water vapour, flexibility, etc.). Therefore, this composite is a compatible and easy-to-install technology for structural RC elements strengthening. The competitive cost of this system in comparison with the above mentioned methods should also be highlighted.

The developed work has been approached from an integral perspective of this strengthening solution in its application to concrete. In a first part, the materials involved in the study were characterized, from reinforced concrete (concrete and steel) to TRM (mortar and textiles). Three different materials were used as TRM internal reinforcement: carbon, basalt and drawn steel wire. The proposed strengthened system was further researched in a second stage by carrying out uniaxial tensile tests on TRM specimens and bond tests. The third part covers the experimental campaign on thirty six one-third scale beams (1.5m span) and sixteen full-scale beams (4.5m span) strengthened with different materials and techniques (use of mechanical anchors). In order to simulate the behaviour of low-grade concrete, the tested beams were made of low-quality concrete.

Subsequently, numerical approaches were carried out to evaluate the effect of the strengthening solution. Firstly, a mathematical model was developed to simulate the tensile behaviour of TRM, and it was validated in a numerical application developed in Microsoft Excel software to verify the strengthening effect of TRM. Secondly, the composite tensile behaviour was numerically simulated by means of the Finite Element Method (FEM), its strengthening effect was also checked with satisfactory results.

The experimental results obtained in this PhD thesis have revealed the mechanical effectiveness of TRM in the flexural strengthening of reinforced concrete beams. Likewise, it was possible to simulate correctly the effect of the composite material by means of simple applications that will facilitate the design of future retrofitting actions. In conclusion, it has been shown that exist enough reasons to present this strengthening solution as an optimum alternative to the traditional methods.

Provided by Elhuyar Fundazioa, Spain

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