Influence of matrix grade on the mechanical behaviour of fibre-reinforced concrete

Mechanical behaviour of fibre-reinforced concrete is influenced both by the properties of the fibres (geometry, aspect ratio, dosage) and the properties of the matrix (concrete grade, curing time, water-to-cement ratio). Many research studies have been published that focus on the influence of different kinds of fibres on the final properties of concrete. Against this the influence of the matrix grade on the mechanical properties of fibre-reinforced concrete is a topic that is almost unexplored. A mechanical characterisation of fibre-reinforced concrete in the hardened and fresh states is carried out, varying the concrete grade, the fibre dosage and the fibre type (steel and polyester). The results of the experimental research indicate the importance of the matrix grade on the bond between the steel fibre and the matrix and, consequently, on the mechanical performances of the composite material. Furthermore the concrete grade also influences the minimum volume fraction of polyester fibres affecting the matrix toughness

Ultrasonic pulse velocity test for non-destructive investigations of historical masonries: an experimental study of the effect of frequency and applied load on the response of a limestone

The ultrasonic pulse velocity (UPV) method can be conveniently used for non-destructive testing of physical–mechanical properties of the stones within historical masonry, as well as to check the state of damage and microcracking. Before to proceed with in situ measurements, it is important to assess the contribution that both intrinsic characteristics of the stones and external factors may give to the ultrasonic response. In this work the effect of different wave frequencies, sample geometry and application of a compression load on the response of a natural stone to UPV test has been investigated. An extensive experimental campaign in laboratory conditions was carried out on a soft limestone, used in the historical building heritage of the Southern Italy. A negligible UPV dispersion was found at the used frequencies of 1 MHz, 120 and 55 kHz when a compression load was not applied; the measured velocities were found to be influenced by the stone inhomogeneity rather than by the sample size. They showed a slight decrease and still negligible dispersion under load up to the visible damage. Dispersion increased with the cracking progression. This indicates that enhanced capability of UPV, in checking material quality and damage conditions, can be obtained by combining the use of different wave frequencies

Analytical prediction of crack width of FRC/RC beams under short and long term bending condition

It is well known that fibers are effective in modifying the cracking pattern development of concrete structural element, causing an higher number of cracks and, consequently, lower crack spacing values and narrower crack widths compared to the matrix alone. This effect could be exploited to improve durability of Reinforced Concrete (RC) structures, especially of those exposed to aggressive environments. The analytical prediction of crack width and spacing in Fiber Reinforced Concrete (FRC) structural elements in bending is still an open issue. A crack width relationship for RC elements with fibers similar to those developed for classical RC structural members would be desirable for designers. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new MC2010, embrace this idea. However further validation of these models by experimental results are still needed. On the other hand, the study of the influence of a sustained load on crack width in presence of the fiber reinforcement is a topic almost unexplored and important at the same time. In the present work, the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term flexural loading. A theoretical prediction of crack width and crack spacing was carried out according to different international design provisions. The analytical results are discussed and compared in order to highlight the differences between the models and to check the reliability of the theoretical predictions on the basis of the experimental data

Long term behavior of FRC flexural beams under sustained load

Early applications of Fiber Reinforced Concrete (FRC) mainly concerned structural elements where the convenience of using short fibers was found in the possibility of substituting conventional reinforcement. However, FRC toughness can be conveniently introduced into the engineering practice with another perspective, to take advantage of the crack control for enhancing structural durability. In this context, the research study presented herein shows the results of an experimental campaign aimed at investigating the influence of short fibers (steel and polyester) on the short and long-term behavior of Reinforced Concrete (RC) beams. The experimental program includes crack width and flexural displacement measurements as well as chemical tests, which gave evidence of the effectiveness of polymeric and steel fibers in improving long-term serviceability of RC beams. The results presented in the paper show that flexural displacements, crack widths and carbonation depth were reduced by the fibers addiction

Research of correlations between NDT and DT to assess mechanical properties of a soft stone in ancient masonry

A deep knowledge of the physical and mechanical properties of the constituent materials of ancient masonries is of crucial importance in the choice of the proper intervention technique. In case of historical buildings sustainable diagnostic procedures responding to the conservation constraints, should have the lowest degree of intrusion and the fullest respect for their physical integrity. The sample’s extraction from existing structures for laboratory tests is one of the major problems in the field of diagnosis of ancient buildings and this has moved the scientific community to propose alternative non-destructive techniques to evaluate the mechanical and physical properties of the building stones. In the present work non-destructive and destructive tests have been investigated as tools for assessing the compressive strength of “Lecce stone”, a soft calcarenitic stone used as traditional building materials in the Southern Italy. Ultrasonic pulse velocity (UPV), Schmidt hammer test and compressive tests on microcores have been compared with mechanical destructive tests on cubes in order to found correlations between the results. The final aim is to assess the reliability of the non-destructive investigated methods in describing the mechanical performance of the stone, reducing the use of destructive analyses on masonries

Analytical prediction of crack width of FRC/RC beams under short and long term bending condition

It is well known that fibers are effective in modifying the cracking pattern development of concrete structural element, causing an higher number of cracks and, consequently, lower crack spacing values and narrower crack widths compared to the matrix alone. This effect could be exploited to improve durability of Reinforced Concrete (RC) structures, especially of those exposed to aggressive environments. The analytical prediction of crack width and spacing in Fiber Reinforced Concrete (FRC) structural elements in bending is still an open issue. A crack width relationship for RC elements with fibers similar to those developed for classical RC structural members would be desirable for designers. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new MC2010, embrace this idea. However further validation of these models by experimental results are still needed. On the other hand, the study of the influence of a sustained load on crack width in presence of the fiber reinforcement is a topic almost unexplored and important at the same time. In the present work, the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term flexural loading. A theoretical prediction of crack width and crack spacing was carried out according to different international design provisions. The analytical results are discussed and compared in order to highlight the differences between the models and to check the reliability of the theoretical predictions on the basis of the experimental data

Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone

UPV as non-destructive technique can effectively contribute to the low invasive in situ analysis and diagnosis of masonry elements related to the conservation, rehabilitation and strengthening of the built heritage. The use of non-destructive and non-invasive techniques brings all the times many advantages in diagnostic activities on pre-existing buildings in terms of sustainability; moreover, it is a strong necessity with respect to the conservation constraints when dealing with the historical-architectural heritage. In this work laboratory experiments were carried out to investigate the effectiveness of ultrasonic pulse velocity (UPV) in evaluating physical and mechanical properties of Lecce stone, a soft and porous building limestone. UPV and selected physical-mechanical parameters such as density and uniaxial compressive strength (UCS) were determined. Factors such as anisotropy and water presence that induce variations on the ultrasonic velocity were also assessed. Correlations between the analysed parameters are presented and discussed. The presence of water greatly affected the values of the analysed parameters, leading to a decrease of UPV and to a strong reduction of the compressive strength. A discussion of the role of the water on these results is provided. Regression analysis showed a reliable linear correlation between UPV and compressive strength, which allows a reasonable estimation of the strength of Lecce stone by means of non-destructive testing methods such as the ultrasonic wave velocity. Low correlation between UPV and density was found, suggesting that other factors than density, related to the fabric and composition, also influence the response of the selected stone to the UPV. They have no influence on the UCS, that instead showed to be highly correlated with the packing density

Assessing the reliability of non-destructive and moderately invasive techniques for the evaluation of uniaxial compressive strength of stone masonry units

In this work Ultrasonic Pulse Velocity (UPV), Schmidt Hammer Rebound (SHR) test and strength assessment on microcores (UCSm) and standard cubic samples (UCSc) were used to detect the uniaxial compressive strength of stone masonry units. The analysis of the variability of the measurements allowed to investigate the significance of each test to differentiate the masonry blocks. The latter was evaluated by a Variability Index (VI), as the ratio between the variances at block scale and among the blocks. VI was found higher for UCSc and UPV than for UCSm and SHR measurements. A regression analysis aimed to the correlation of uniaxial compressive strengths evaluated by conventional destructive test on stone cubes with the other test results. The findings showed a good linear correlation among UCSc and UPV values (R2 = 0.83), thus supporting the reliability of UPV to screen the masonry units and to estimate their uniaxial compressive strength. The correlation of UCSc with UCSm was reasonable (R2 = 0.76), while it was low with SHR results; some limits related to the use of SHR and UCSm tests are also discussed

Cracking analysis of FRC beams under sustained long-term loading

Crack formation within concrete members undergoing flexural loading is a complex mechanism, which governs the serviceability and durability of concrete structures. As for reinforced concrete (RC) members, a number of works based on empirical or theoretical approaches are published in the scientific literature. All the models propose a formulation for the estimation of crack spacing and crack width taking into account several parameters. Mechanical properties of concrete matrix, reinforcement ratio, concrete cover, bar diameter and size effect are the most influencing parameters on the cracking pattern of RC members, while tension stiffening can be influential as well. In Fiber Reinforced Concrete (FRC) elements the presence of short fibers modifies the crack pattern within the members due to the development of a residual tensile stress and greater toughness. Normally the number of cracks within the length of FRC members is higher while the mean crack spacing and the crack width are lower. In fact the crack bridging effect of fibers consists in post-cracking stresses between the crack faces. Such mechanism is mainly governed by the interface bond between fiber and concrete matrix. Therefore, the volume fraction and the geometrical properties of fibers strongly influence the overall contribution in the cracking phenomena. A limited number of design codes have taken into account the modified behaviour of FRC members (especially in the case of steel fibers) by providing specific equations for crack width. This work presents the results of an experimental campaign on RC beams subjected to sustained service loads and environmental exposure for 72 months. In some beams, short steel or polyester fibers were added to the concrete matrix. The results presented in the paper show that the addition of fibres in concrete reduces both flexural displacements and crack widths, by modifying also the long-term behaviour of FRC members