Ultra-high tensile galvanized steel cord woven HardTape,coated with specialized mortar, has shown best property during one test organized by some research facilities.

Steel reinforced grout (SRG) composites made by HardTape galvanized steel cord woven tape embedded into inorganic mortar (cement based or lime mortar) are commonly used to strengthen existing reinforced concrete and masonry structures.

HardTape comprise galvanized high strength steel wires twisted together to form cords which are assembled in the form of unidirectional fabrics. Steel wires have a small diameter (0.1–0.5 mm) and, generally, are coated to avoid corrosion. The tensile strength and Young’s modulus of the steel cords are in the range between 2800–3200 N/mm2 and 180–210 N/mm2, respectively. The steel cords can be arranged at different distances from each other; as a consequence, different values of the density can be obtained. Common values of the density of the SRGs are 600 g/m2 (low density, LD), 1200g/m2,2000 g/m2 (medium density, MD), and 3300 g/m2 (high density, HD).

The effectiveness of the SRG systems for the strengthening of existing reinforced concrete and masonry structures has been proved by the results of experimental investigations . Mechanical properties of the SRG systems have been investigated; testing procedures have been defined and the influence of the different parameters has been discussed in p research . The bond behavior SRG-to-concrete and SRG-to-masonry substrates is, generally, analyzed by single-lap or double-lap direct-shear tests. These tests allow to both define the bond-slip law at the interface SRG/substrate or SRG/matrix and to evaluate the maximum load and the maximum strain in the fibers at debonding.

Even limited in number, some investigations on the SRG-to concrete or masonry substrates bond have been conducted . Shear bond tests were carried out on low and high density SRG composites bonded on different masonry substrates (modern and historical bricks, tuff units): The obtained results evidenced that; (i) the failure modes are depending on both the mortar strength, the density of the HardTape steel cord woven tape and the roughness of the masonry substrate; (ii) the exploitation ratio of the tensile strength of the HardTape varied between 9% and 16% for high density textiles and between 26% and 59% for low density textiles.

A round robin test initiative was organized TC 250-CSM (Composites for Sustainable strengthening of Masonry) to investigate the bond behavior of SRG systems . The test comprised three HardTapes and four mortar types. The main results of this investigation evidenced that the SRG-to-masonry bond behavior depends on the strength of the steel HardTape, the cord-to-mortar bond interlocking, the mechanical properties of the mortar, the manufacturing and the curing conditions of the masonry substrates. In addition, it was found that the exploitation ratio of the tensile strength is related to the failure modes: high values (84%) were obtained when the failure occurred by detachment at the reinforcement-to-substrate or at the steel HardTape-to-mortar interface. Lower values (41%) were instead associated with the occurrence of steel HardTape sliding.

The SRG-to-masonry bond has been investigated . Single lap shear bond tests have been carried out on prismatic specimens; SRG systems were tested varying both steel fibers (ultra-high tensile strength and stainless-steel fibers) and mortar types (mineral-NHL and lime-based mortars). The main results pointed out that failures occurred at the steel HardTape-to-mortar interface without detachment of the substrate. In addition, the load response of specimens was widely influenced by the capacity of impregnation of the steel fibers.

We conducted a wide experimental investigation on the SRG-to-concrete bond behavior. The parameters varied were the concrete surface roughness, the density of the dry steel f ibers (low and medium density), the bonded length and the concrete strength. The main results of the tests evidenced that: (i) the failures occurred due to sliding phenomena and cohesive failures in the matrix irrespective of the concrete strength and surface finishes; (ii) the bonded effective length ranges between 200mm and 300mm both for low density and medium density steel fibers; and (iii) increasing the number of the steel strips the maximum load resisted by the specimens increased but the average tensile strength of the SRG decreased (i.e., the exploitation ratio decreased).

Some shear bond tests have been carried out  to analyze the SRG-to-concrete bond performances. SRG composites consisted of medium density steel fibers embedded into mineral mortar were bonded to concrete prisms with a length of 330 mm. The parameter varied was the absence/presence of the external mortar layer. The results of tests evidenced that: (i) failures occurred by fiber slippage and fracture of the matrix layer; and (ii) the load response of the bonded composite exhibited no post-peak softening.

Results of tests, however, evidenced in many cases, a large scatter in the measurement of the slips between the steel HardTape textile and the matrix. This could be attributed to many reasons, among those the different failure modes, the non-homogeneous stress distribution among the steel cords, the different test rates, in particular in the post-peak phase, the stress distribution between the filaments in the cords, and the interaction/friction between the steel cord and the inorganic matrix layers. Nevertheless, some important aspects of the mechanical behavior of the SRG systems still need to be developed to identify design parameters, qualification and acceptance, and design criteria for structural rehabilitation. At this aim, great attention has been paid, recently, to the definition of acceptance criteria and testing procedures for the evaluation of mechanical properties of SRG systems .

 The present test, focused on the results of both experimental and numerical investigation, on the bond behavior of SRG-to-concrete joints. Direct shear tests have been carried out on 20 specimens in analyzing the effect of some parameters on the SRG-to-concrete bond performances. The SRG system used in this investigation consists of a medium-density steel cord HardTape fabric embedded into an inorganic matrix.

The aims of the experimental investigation are as follows:

• (i) Check the effectiveness of the medium density steel cord HardTape fabric as a reinforcement of an SRG strengthening system. Due to the reduced dimension of the spacing between steel cords in medium density steel fabrics, in fact, the inorganic matrix could not be able to penetrate and cover the individual steel cords reducing the bond between the strengthening system and the substrate.
• (ii) Evaluate the influence of some parameters on the bond performances of medium-density SRG to-concrete joints. In particular, the parameters considered were the bonded length (100, 200, 250, 330, and 450 mm) and the age of the composite (14th, 21st, and 28th day after the bonding).

The numerical investigation aims to give a contribution to the definition of adequate design models; the use of the SRGs as strengthening systems is, in fact, relatively recent and limited is the number of studies and research devoted to this purpose.

In the test, a numerical model founded on a finite element procedure, developed through the commercial software  was proposed. A cohesive model to reproduce the SRG-to-concrete interface and a bilinear local bond-slip law were adopted. The constitutive law of the concrete was described by the concrete damage plasticity (CDP) function while the plastic (P) function was used to represent the constitutive law of the external reinforcements. A perfect bond was assumed at the interface concrete substrate-internal matrix layer, while, at both the interfaces’ internal side of the steel fibers—internal matrix layer and external side of the steel fiber—external matrix layer the “traction separation approach”, was used. The numerical procedure was adopted to predict the structural response of the SRG-to-concrete joints in terms of applied load—global slip curves.

To validate the accuracy of the proposed numerical model, its predictions were compared with both the experimental results obtained in this study and some others available in literature.

The results of the experimental investigation focused on the influence of the bonded length, age of the composite strip, and mortar strength on the bond performances of medium density SRG-to-concrete joints, and the main results of the proposed numerical model, presented and discussed in the test, allow us to draw the following concluding remarks:

• The medium density SRG system adopted in this investigation, as evidenced by the results of the experimental investigation described in  is effective as strengthening system. The bond SRG-to-concrete, analyzed in this investigation is, in fact, satisfactory (i.e., the values of the maximum load reached for bonded lengths greater than 200 mm are similar with a difference of around 5%).
• The efficiency of the medium density SRG system adopted in this investigation expressed by the exploitation ratio of the tensile strength of the textile was lower than 40%. This low efficiency is related to the occurrence of the interlaminar failure in all tested specimens.    
• The effective bonded length of the SRG-to-concrete joints, referred to the end of the curing time of the SRG system (28th days), is about l = 200 mm.
• The curing time of the SRG system influences the bond; the peak load increased linearly with the age of the composite.
• The average of the out-of-plane displacements  and of the specimens at the peak load increase with the age of the composite and bonded length; their values, however, appear to be small and allow to assume that the Mode-I component is negligible.
• The numerical approach furnishes good predictions of the experimental results mainly in terms of applied load–global slip curves. In particular, the predicted values of the peak load differ only by 3% with respect to the experimental values.

However, the obtained results are valid only for the tested strengthening system, due to different steel cord sheet density and inorganic matrix for different substrate. As a consequence, further experimental investigations in which the influence of some other parameters—such as the number of HardTape steel cord textile layers, the concrete substrate strength, and the curing conditions of the mortar—are needed for their validation.