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There is no unified method for deriving the tensile properties of fiber-reinforced ultra-high-performance cementitious composites (UHPCC). This study compares the most common material tests based on a large series of laboratory tests performed on a self-developed UHPCC mixture. The cementitious matrix, with a compressive strength of over 150 MPa and a matrix tensile strength of 8–10 MPa, was reinforced with 2% by volume of 15 mm long and 0.2 mm diameter straight high-strength steel microfibers. Over 100 uniaxial tensile tests were performed on three test configurations using cylindrical cores drilled out from larger prismatic specimens in three perpendicular directions. In addition to uniaxial tests, flexural tests on prismatic elements and flexural tests on thin plates were conducted, and the tensile properties were derived through digital image correlation (DIC) measurements and inverse analysis. Furthermore, splitting tensile tests on cylindrical specimens were employed to ascertain the tensile properties of the matrix. The outcomes of the diverse laboratory tests are presented and discussed in detail. The relationships between crack width and deflection in the context of flexural tests were developed and presented. In conjunction with compression tests and modulus of elasticity tests, the constitutive law is presented for the investigated materials. [ABSTRACT FROM AUTHOR]

The probabilistic evaluation of the capacity and response of structural systems is often done by probabilistic simulation. This approach relies on a considerable number of deterministic analyses to obtain an accurate estimation of the desired statistical characteristics. However, the need of performing a large number of analyses hinders the application of probabilistic simulation in practice, especially when the deterministic calculation of the structural response is computationally expensive in itself (e.g. nonlinear finite element analysis). To increase the appeal of probabilistic simulation, the number of analyses should be very small (i.e. ten or less). This study shows how different sampling techniques can be adopted to select a very small sample subset of analyses to be run, and how they can yield accurate results in estimating the shear capacity of prestressed reinforced concrete beams. Based on the results of the study, the Fractile Based Sampling (FBS) method emerges as a more promising sampling strategy than other sampling techniques, like Latin Hypercube Sampling (LHS). For the same small number of samples, the results show that FBS is preferable over LHS and other sampling techniques for capturing both the general distribution of the response and the tails of the distribution, which shows its usefulness in assessing probability of failure and reliability. [ABSTRACT FROM AUTHOR]

The use of textile reinforced concrete (TRC) as a shear strengthening solution for reinforced concrete (RC) structures is a viable and effective strategy. The low tensile strength of the cementitious matrix used to produce TRC, however, can strongly limit the performance achievable by such material. Using fiber/textile reinforced concrete (F/TRC), that is, TRC where short‐dispersed fibers were admixed to the cementitious matrix, has proven to overcome such limitations. An experimental campaign, aimed at assessing the influence of the fiber‐reinforced cementitious matrix, the number of layers, and their anchoring, was performed at the structural laboratory of Carinthia University of Applied Sciences. F/TRC strengthening configurations were tested on real‐scale T‐beams without internal transversal reinforcement. The results highlight the contribution of the cementitious matrix and hint at the possibility of simply summing its contribution to the one of the textile fabrics, while appropriate end‐anchorage seems to improve the performance of both elements. [ABSTRACT FROM AUTHOR]

The demand of effective and low-invasive methods for strengthening existing concrete structures is continuously increasing due to the ageing infrastructure. High performance materials can beneficially be applied to provide sustainable and robust strengthening solutions. The present article presents some exemplarily selected methods and innovative approaches for subsequent strengthening of existing RC structures. Main results of experimental investigations are summarized and future research topics identified.

Recently, innovations in textile-reinforced concrete (TRC), such as the use of basalt textile fabrics, the use of high-performance concrete (HPC) matrices, and the admixture of short fibers in a cementitious matrix, have led to a new material called fiber/textile-reinforced concrete (F/TRC), which represents a promising solution for TRC. Although these materials are used in retrofit applications, experimental investigations about the performance of basalt and carbon TRC and F/TRC with HPC matrices number, to the best of the authors' knowledge, only a few. Therefore, an experimental investigation was conducted on 24 specimens tested under the uniaxial tensile, in which the main variables studied were the use of HPC matrices, different materials of textile fabric (basalt and carbon), the presence or absence of short steel fibers, and the overlap length of the textile fabric. From the test results, it can be seen that the mode of failure of the specimens is mainly governed by the type of textile fabric. Carbon-retrofitted specimens showed higher post-elastic displacement compared with those retrofitted with basalt textile fabrics. Short steel fibers mainly affected the load level of first cracking and ultimate tensile strength. [ABSTRACT FROM AUTHOR]

The use of textile-reinforced concrete (TRC) in retrofit applications has proven to be an effective and viable solution. Many parameters, however, can affect the outcome, strongly inhibiting its potential. An experimental campaign was performed to assess the efficiency of TRC in strengthening reinforced concrete (RC) beams against bending with different varying parameters such as cementitious materials, number of textile layers, mechanical anchorages, and addition of short dispersed steel fibers. Results show how each parameter affects both the capacity increase and the failure mode, pointing out the beneficial effect of introducing both anchorage systems and short dispersed steel fibers. [ABSTRACT FROM AUTHOR]

A series of flexural tests were performed in order to investigate the effect of steel fiber reinforcement (SFR) in textile-reinforced concrete (TRC) plates. Some of the specimens were reinforced only with textile, some of them only with fibers, and some of them were provided with both textile and fiber reinforcement. The concrete matrix was a self-developed ultrahigh performance concrete (UHPC) mixture with a compression strength over 160 MPa. The tensile strength of the used textiles was around 1500 MPa for glass fiber textile and over 3000 MPa for carbon fiber textile. In case of fiber reinforcement, the concrete was reinforced with 2 vol% of 15 mm long and 0.2 mm diameter plain high strength steel fibers. The dimensions of the rectangular plate test specimens were 700 × 150 × 30 mm. The plate specimens were tested in a symmetric four-point bending setup with a universal testing machine. The tests were monitored using a photogrammetric measurement system with digital image correlation (DIC). The paper presents and evaluates the test results, analyses the crack patterns and crack development, and compares the failure modes. The results showed a general advantageous mechanical behavior of specimens reinforced with the combination of fibers and textiles in comparison to the specimens reinforced with only fiber or textile reinforcement. [ABSTRACT FROM AUTHOR]

High strength concrete overlays – Part 2: Member tests Small scale and member tests with different specimen geometries and test setups were performed to investigate the adhesive bond along interfaces between normal strength concrete and high strength overlay. Part 1 reports about the small scale tests, while this part 2 focuses on slab tests. Apart from standard application of normal and high strength concrete layers, two slabs were subject to cyclic loading during casting the concrete overlay. In general, also the member tests confirm the improvement of the bond properties with the high strength concrete overlay. All slabs with normal strength overlay concrete depicted some limited degradation along the shear interface in the tests, while members reinforced with high strength concrete showed hardly any degradation along the joint except for the slabs that underwent cyclic loading during overlay casting. [ABSTRACT FROM AUTHOR]

High strength concrete overlays – Part 1: small scale tests Small scale and member tests with different specimen geometries and test setups were performed to investigate the adhesive bond along interfaces between normal strength concrete and high strength overlay. While small scale tests were in the focus of the first series of the research project, in the second series member tests were performed as well as series of small scale tests with an optimized recipe for the high strength concrete layer (providing better workability and higher compressive strength). In comparison small scale tests with normal strength overlay were performed. In general the small scale tests provide consistent results with rather low scatter for same test‐setups and roughness category. The improved mix proportion for the high strength concrete had a significant effect on the bond strength results. In general the bond strength values were significantly higher than observed with normal strength overlay. [ABSTRACT FROM AUTHOR]

The effectiveness and durability of a concrete repair or retrofit is mostly affected by its bond and compatibility to the existing substrate. Although our understanding of concrete-concrete bond has advanced greatly, there remains a major uncertainty in the adoption of a proper methodology to assess the quality of bond, comprising the accuracy of reproducing the stresses that the interface undergoes in a structural application, the problem of disturbed stress paths, size & rate effects, drilling-induced damage, and the possibility of conversions between shear and tensile bond. A discussion on comparability of bond tests in tension and shear is presented. The study is based on a round-robin-like test program in two independent labs in Canada and Austria (UBC, CUAS) and encompasses normal strength, high strength, and fiber reinforced concretes. The raw data comparison is complemented with an investigation of semi-empirical methods and predictive models, conversions between shear and tension coefficients, and suitability/limitations of various methodologies for assessing the effects of fiber reinforcement on bond. [ABSTRACT FROM AUTHOR]

Strengthening methods for shear‐deficient bridges were developed and tested in cooperation with the HILTI Corporation and SIKA Austria. In order to identify the strengthening effect in shear, a number of 12 T‐shaped RC‐beams with a length of 5.38 m were designed. In the experimental campaign two different types of strengthening systems were applied and subjected to shear loading until failure. The successful use of a modern digital image correlation system provided the basis for an in‐depth analysis of failure mode and crack pattern as well as shear degradation at each load stage. Finally, the applicability of existing shear design models with consideration of the strengthening elements analogously to cast‐in shear reinforcement was evaluated. [ABSTRACT FROM AUTHOR]

Zur Beurteilung der möglichen Tragfähigkeitssteigerung von Flachdecken durch UHPC‐Verbundeinbauteile wurden neun Durchstanzversuche an Flachdeckenauschnitten im Bereich einer Innenstütze durchgeführt. Alle Versuchskörper waren mit Doppelkopfankern als Durchstanzbewehrung versehen. In acht Platten war zusätzlich zu den Doppelkopfankern ein Einbauteil aus faserbewehrtem UHPC vorhanden, um die Druckzone am Stützenanschnitt zu verstärken. Dabei wurden UHPC‐Einbauteile mit zwei unterschiedlichen Außendurchmessern verwendet. Weiterhin wurden einige Elemente durch die Anordnung von Fugen mehrteilig ausgeführt. Um einen nachträglichen Einbau der Doppelkopfanker zu ermöglichen, wurde ein Teil der Einbauteile zusätzlich mit Ankergassen ausgeführt. In den Versuchen mit UHPC‐Einbauteilen wurden signifikant höhere Bruchlasten erreicht als bei der Referenzplatte, die nur mit Doppelkopfankern als Durchstanzbewehrung versehen war. In den Versuchen mit den kleineren UHPC‐Einbauteilen konnte ein Durchstanzen außerhalb als Versagensursache benannt werden, wohingegen bei den größeren Einbauteilen eine deutliche Schädigung auch im Bereich des UHPC zu beobachten war. Enhancement of punching shear capacity of flat slabs by built‐in UHPC composite elements A total of nine punching shear tests on interior column‐slab connections was performed to investigate the potential enhancement of the shear punching capacity in flat slabs. All test specimens included double‐headed studs as punching shear reinforcement. Eight slabs were additionally reinforced in the vicinity of the column by an UHPC unit. These elements were built in around the columns in the slab compression zone, replacing there the normal strength concrete (NSC). UHPC units with two different diameters were applied. Several of these elements were provided with joints and channels for the double‐headed studs. The slabs including an UHPC element reached significantly higher failure loads than the test specimens without such an element. In the slabs including the smaller UHPC units, a clear punching shear failure outside the elements was observed. In the case of the larger UHPC units, significant cracks occurred inside the elements, indicating the initiation of a failure of the UHPC. [ABSTRACT FROM AUTHOR]

When reinforcing bars are post-installed in holes drilled in cured concrete, adhesive mortars are used to create a bond between concrete and bars. Appropriate adhesives can develop higher bond strengths than standard ribbed bars cast into concrete. A detailed design concept for the anchorage length of reinforcing bars has been developed by taking into account splitting/spalling of the concrete and pullout. Pullout and splitting tests on reinforcing bars set in concrete were carried out with different adhesive mortars and with varying concrete strengths and concrete covers. When higher bond strengths than those recommended for cast-in reinforcement are taken into account, it is important to check deformations and crack widths at the serviceability limit state (SLS) separately. For this reason, structural tests on slabs and corbels were carried out. Moreover, pullout tests on post-installed reinforcing bars were performed in order to measure displacements at service load level. [ABSTRACT FROM AUTHOR]

Interface shear transfer between differently aged concretes is a topic that crops up frequently and in different situations in structural design. In the fib Model Code for Concrete Structures 2010 the fundamental basics of concrete-to-concrete load transfer are given in section 6.3 and the corresponding design rules in 7.3.3.6. The different potential mechanisms contributing to the shear resistance along the interface, i.e. adhesive bond, aggregate interlock, friction and dowel action, are thus combined and their relationship taken into account by interaction factors. This article summarizes the most important results from past and ongoing studies and presents the background to the theory forming the design basis of fib Model Code 2010, the 'extended shear friction theory' (ESF). [ABSTRACT FROM AUTHOR]

The increasing demand on the performance of existing structures, together with their degradation, is among the main drivers towards the development of innovative strengthening solutions. While such solutions are generally aimed at increasing the load-bearing capacity of structural elements, serviceability limit states also play an important role in ensuring the performance and durability of the structure. An experimental campaign was performed to assess the cracking behaviour of reinforced concrete beams strengthened with different typologies of Textile-Reinforced Concrete. The specimens were monitored using Digital Image Correlation (DIC) technology in order to obtain a quantitative evaluation of the evolution of the crack pattern throughout the whole test. Results show the beneficial effects of this retrofitting strategy both at ultimate limit states and serviceability limit states, provide detailed insights on the progression of damage in the specimens and highlight how different parameters impact the cracking behaviour of the tested elements. [ABSTRACT FROM AUTHOR]

Maintenance of reinforced concrete structures is a prevailing topic, especially with regard to lifeline structures and bridges, many of which are now designed with a service life beyond 100 years. Reinforcement made of ordinary (carbon) steel may corrode in aggressive environments. Stainless steel, being much more resistant to corrosion, is a valid solution to facilitate the protection of the works, increasing the service life and reducing the need for repair and maintenance. Despite the potential for stainless steel to reduce maintenance costs, studies investigating the influence of stainless steel on the behavior of reinforced concrete structures are limited. This study investigated the bond behavior of stainless steel rebars by means of experimental tests on reinforced concrete specimens with different concrete cover thicknesses, concrete strengths, and bar diameters. In each case, identical specimens with carbon steel reinforcement were tested for comparison. The failure modes of the specimens were examined, and a bond stress–slip relationship for stainless steel bars was established. This research shows that the bond behavior of stainless steel rebars is comparable to that of carbon steel bars. [ABSTRACT FROM AUTHOR]