A unified approach for determining the strength of FRC members subjected to torsion—Part I: Experimental investigation

The strength and behavior of fiber reinforced concrete (FRC) members subjected to torsion has received little attention in the literature. The primary objective of including fibers in concrete is to bridge cracks once they form, and in doing so, provide some post-cracking resistance to the otherwise brittle concrete. This and the accompanying paper that follows present the results of a comprehensive experimental and analytical study aimed at describing the behavior and strength of FRC members subjected to torsion. In this paper, results are presented on large scale pure torsion tests which have been conducted on eighteen 2.7 m long by 0.3 m wide by 0.3 m high beams with varying transverse and longitudinal reinforcement ratios along with varying steel fiber types and dosages. The results of this study demonstrates that the addition of steel fibers significantly increases the stiffness, rigidity and the maximum resisting torque and maximum twist when compared to the same specimen without fibers. The addition of fibers substantially reduced crack widths and crack spacings induced by torsion. The complementary behavior of specimens containing fibers and stirrups is explored along with a critical discussion on members containing low amounts of conventional longitudinal and/or transverse reinforcement

Retrofitting RC infills by a glass fiber mesh reinforced overlay and steel dowels: experimental and numerical study

Many existing Reinforced Concrete (RC) buildings infilled with masonry panels are vulnerable to seismic actions due to lack of proper seismic detailing. For buildings located in low-moderate seismicity regions, a possible retrofitting option could be a reinforced mortar coating applied on the building facades in order to significantly increase its seismic resistance. The present paper studies thin glass fiber mesh reinforced mortar overlays for strengthening RC weak frames infilled with hollow brick masonry. Attention is devoted to the steel dowel connection between the masonry panel and the RC frame to reduce shear sliding. An experimental evaluation of the effectiveness of this enhanced technique is presented and compared against previous results in which shear sliding significantly limited the structural response. In addition, a finite element model has been developed and calibrated to properly simulate the response of the test specimens

Behavior of lightly reinforced fiber reinforced concrete panels under pure shear loading

The load-deformation response of Fiber Reinforced Concrete (FRC) elements subjected to pure shear is still matter of strong debate within the scientific community. In this paper, the tests on six fiber reinforced concrete panels under pure shear are presented and discussed. The tests were conducted under displacement control and a peculiar loading frame was designed to ensure that a pure shear state of stress was established. Steel fibers were added in relatively low amounts (20 and 50 kg/m3), and two steel reinforcements (0.21% and 0.74%) were selected, aiming at simulating lightly reinforced elements. A critical discussion on the influence of fibers on both global and local behavior (tension stiffening, cracking formation and propagation, post-cracking stiffness and residual strength) is presented. Finally, a novel crack spacing formulation, extended to FRC, is proposed and compared against available experimental data

Innovative applications of Steel Fibre Reinforced Concrete in precast structures: non-linear finite element analyses for reinforcement optimization

Steel Fibre Reinforced Concrete (SFRC) has emerged as a practical and innovative material whose employment allows a partial or total substitution of conventional reinforcement with significant advantages in terms of cost and production time saving. Since ‘70s, several researches has shown the important improvement of concrete durability and mechanical properties resulting from the use of steel fibres. Non-linear numerical analyses may be performed to exploit the mechanical properties of SFRC in order to find the best combination of steel fibres and conventional reinforcement (optimized reinforcement). This paper presents some examples of reinforcement optimization developed through non-linear finite element analyses of precast elements in which conventional reinforcement is usually adopted

Experimental tests on fiber-reinforced alkali-activated concrete beams under flexure: Some considerations on the behavior at ultimate and serviceability conditions

Alkali-activated concrete (AAC) is an alternative concrete typology whose innovative feature, compared to ordinary concrete, is represented by the use of fly ash as a total replacement of Portland cement. Fly ash combined with an alkaline solution and cured at high temperature reacts to form a geopolymeric binder. The growing interest in using AACs for structural applications comes from the need of reducing the global demand of Portland cement, whose production is responsible for about 9% of global anthropogenic CO2 emissions. Some research studies carried out in the last few years have proved the ability of AAC to replace ordinary Portland cement concrete in different structural applications including the construction of beams and panels. On the contrary, few experimental results concerning the structural effectiveness of fiber-reinforced AAC are currently available. The present paper presents the results of an experimental program carried out to investigate the flexural behavior of full-scale AAC beams reinforced with conventional steel rebars, in combination with fibers uniformly spread within the concrete matrix. The experimental study included two beams containing 25 kg/m3 (0.3% in volume) of high-strength steel fibers and two beams reinforced with 3 kg/m3 (0.3% in volume) of synthetic fibers. A reference beam not containing fibers was also tested. The discussion of the experimental results focuses on some aspects significant for the structural behavior at ultimate limit states (ULS) and serviceability limit states (SLS). The discussion includes considerations on the flexural capacity and ductility of the test specimens. About the behavior at the SLS, the influence of fiber addition on the tension stiffening mechanism is discussed, together with the evolution of post-cracking stiffness and of the mean crack spacing. The latter is compared with the analytical predictions provided by different formulations developed over the past 40 years and adopted by European standards

In-plane cyclic tests on hollow clay brick masonry infills retrofitted by glass fiber mesh reinforced mortar coating

Most of the Reinforced Concrete (RC) buildings constructed in the second part of the last century were generally designed by neglecting the interaction between the RC frame and the masonry infill walls. On the contrary, even in case of lateral loads caused by moderate seismic events, infills generally lead to an important increment of RC frames stiffness by acting as diagonal struts. Therefore, by considering the low dissipative capacity of the frames (without seismic detailing), infills becomes real structural elements that might govern the failure mechanism of the whole building. In view of this, a possible way for strengthening low rising RC buildings placed in low seismicity areas may be based on techniques aiming at improving the capacity of the masonry infills and exploiting the “box behavior” of the building. The present research aims at proposing the use of thin AR-glass fiber mesh reinforced mortar overlays for strengthening the infills located along the perimeter walls of the RC buildings. The main purpose of the retrofitting method is to achieve an improved capacity and ductility of the entire building by exploiting the effect of the strengthening layers applied on the external side of the perimeter walls. In order to assess the effectiveness of the proposed technique, a series of quasi-static cyclic tests on full scale infills made of hollow clay units with horizontal cores were carried out. A special RC frame provided with steel hinges connected at the columns edges was built to simulate the behavior of the frame when the ultimate mechanism is fully activated. A total of three specimens including a not strengthened wall as well as a strengthened and a repaired panel were tested. The latter proved that, compared to the unstrengthened panel, the reinforced mortar coating was able to improve the stiffness as well as the capacity of the masonry infills

Design and Verification of Elevated Slabs Made with Hybrid Reinforced Concrete: Case Studies

The literature has recently reported many real applications with the use of Fiber Reinforced Concrete (FRC) for the construction of elevated slabs. Most of these case studies suggests the use of steel fibers as the only reinforcement in addition to the Anti-Progressive Collapse (APC) used to prevent brittle failure mechanisms. However, this reinforcement arrangement does not usually represent an optimized design solution as high amounts of fibers (Vf > 70 kg/m3) are often required for reaching the minimum capacity to resist design loads as well as to the internal actions due to restrained shrinkage. This paper focuses on the design of FRC elevated slabs by using the most recent design provisions reported in the fib Model Code 2010. A simplified design procedure based on a consolidated design practice is described. The use of a properly designed combination of fibers and conventional reinforcement, referred to as Hybrid Reinforcement, is proposed. Different case studies taken from the literature are considered to prove the effectiveness of the proposed design approach

Predicting the Residual Flexural Strength of Concrete Reinforced with Hooked-End Steel Fibers: New Empirical Equations

To characterize the tensile behavior of Steel Fiber Reinforced Concrete (SFRC), international codes generally adopt performance-based approaches that require to perform either indirect or direct tensile tests on concrete samples. The fib Model Code 2010 recommends to assess the tensile performance of SFRC by a Three Point Bending test on a notched beam able to provide a series of residual strengths corresponding to different crack mouth openings detected at midspan. Therefore, when designing SFRC structures, the tensile properties considered in the safety verifications must be checked by laboratory tests involving a suitable number of beam samples. On the contrary, especially in case of preliminary structure sizing, designers need simple tools for estimating the potential tensile performance of concrete by starting from its basic properties. The present paper proposes two equations for predicting the residual strengths fR1 and fR3 included in most of the equations reported by the fib Model Code 2010 for safety verification of SFRC members. The concrete compressive strength, the fiber aspect ratio and volume fraction are the only three parameters included in the formulations. The assessment of the model effectiveness has been based on a statistical analysis including the adoption of a modified Demerit Point classification method. The good predicted performance of the proposed equations has been also proved by comparison with other similar models reported by literature

Steel fibers for replacing minimum reinforcement in beams under torsion

This paper concerns an investigation on six large-scale Steel Fiber Reinforced Concrete (SFRC) beams tested in pure torsion. All beams had longitudinal rebars to facilitate the well-known space truss resisting mechanism. However, in order to promote economic use of the material, the transverse reinforcement (i.e. stirrups/links) was varied in the six large scale beams. The latter contained either no stirrups, or the minimum amount of transverse reinforcement (according to Eurocode 2), or hooked-end steel fibers (25 or 50 kg/m3). Material characterization were also carried out to determine the performance parameters of SFRC. The results of this study show that SFRC with a post-cracking performance class greater than 2c (according to Model Code 2010) is able to completely substitute the minimum reinforcement required for resisting torsion. In fact, the addition of steel fibers contributes to significantly increase the maximum resisting torque and maximum twist when compared to the same specimen without fibers. Moreover, SFRC provides a rather high post-cracking stiffness and a steadier development of the cracking process as compared to classical RC elements. This phenomenon improves beam behavior at serviceability limit state. The experimental results are critically discussed and compared to available analytical models as well as with other tests available into the literature

Flexural Design of Elevated Slabs Made of FRC According to fib Model Code 2010: A Case Study

Elevated concrete slabs are typically used as columns/piles supported floors in multi-story buildings or industrial facilities. The need to reduce construction time and costs has favoured the use of ever more advanced materials, like Steel Fiber Reinforced Concrete, as an alternative to conventional reinforced concrete. Many research studies and on-site applications have proven that steel fibers can be successfully used to totally substitute the main flexural reinforcement generally placed in conventional reinforced concrete slabs. However, in order to ensure the required minimum structural performance both at ultimate and serviceability loading conditions, the total removal of rebars requires the use of very high steel fiber contents (>70 kg/m3). However, the use of a proper combination of fibers and conventional rebars, generally known as Hybrid Reinforced Concrete (HRC), may represent a feasible solution to get the required structural performance by minimizing, at the same time, the total amount of reinforcement (fibers+rebars). This paper focuses on the design of HRC elevated slabs according to the design provisions for FRC structures reported by the fib Model Code 2010 and recently introduced also by the Italian structural design code (NTC 2018). A simplified design procedure based on a consolidated design practice is proposed. Emphasis is given to the use of HRC for optimizing the slab reinforcement. A case study of an elevated slab made with synthetic Fiber Reinforced Concrete is used to show the effectiveness of the design procedure