Prismatic Specimens Research Articles

An experimental investigation on freezing and thawing durability of high performance fiber reinforced concrete (HPFRC)

This study presents the results of an experimental investigation on the durability of a commercial High Performance Fiber Reinforced Concrete (HPFRC) subjected to rapid freeze/thaw cycles. High-performance fiber reinforced concrete (HPFRC) is currently being used in the construction industry both for strengthening existing members and for controlling opening cracks in new ones. In literature there are many studies focused on the evaluation of the HPFRC mechanical properties varying the matrix composition and/or the type and quantity of the fibers but a lack of knowledge exists for what concerns its durability. Durability of concrete may be defined as the ability of concrete to resist weathering conditions, chemical attack and abrasion while maintaining its desired engineering properties. In particular, durability can be defined by the percentage ratio of the dynamic modulus of elasticity after a number of freeze and thaw cycles to the corresponding value before the freeze and thaw cycles. In this paper, three different concrete mixtures with various fiber volume fraction, i.e. 0%, 1.25% and 2.50%, were examined. Seventy-five freeze-thaw cycles were performed according to UNI 7087-2017 on five prismatic specimens per each concrete mixture while other three specimens were selected as reference samples. In order to evaluate the durability factor of the three mixture, the resonant frequencies (transversal, longitudinal and torsional) and the dynamic moduli of elasticity were measured using the Impact Resonance Method (IRM) provided by the ASTM C215.

Evaluating the Effect of the Compressive Strength Development Factor on Concrete Creep Deformation in Bridge Construction

Abstract Excessive midspan deflection is often observed in large-span prestressed concrete girder bridges. In particular, the deformation caused by shrinkage and creep of concrete is an important part of deflection that often exceeds expectations. To achieve creep deformation control, extending the curing time and postponing the loading age are frequently adopted during construction. This article proposed an approach based on creep experiments with prismatic and beam specimens and viscoelastic model-based finite element analyses to evaluate the effect of the concrete strength development factor on the ultimate creep coefficient in the Comité Euro-International du Béton-Fédération internationale de la précontrainte (CEB-FIP) model. The concrete strength development factor ξcc(t/tu) is defined as the ratio of the mean compressive strength at the loading time to the maximum mean compressive strength obtained in the test. The strategies for targeted regulation and control of ξcc(t/tu) were discussed by establishing an artificial neural networks model for strength prediction from a database provided in our previous work. Uniaxial compressive prismatic specimens and pure bending beam specimens with a span of 5.0 m were used in the experiment for creep behavior observation. A viscoelastic finite element analysis (FEA) model was established based on solidification theory. The FEA model results were verified with measured data for capturing the creep behavior under ambient conditions to obtain reliable long-term creep deformation predictions. Finally, the proposed method was validated with the Xincheng bridge construction for the optimistic loading age determination, and the results indicated good feasibility in girder deflection control.

Avaluación de la Recuperación de Estructuras de Concreto Armado por Remoción sólo de la capa Afectada del Recubrimento de la Armadura.

En esta investigación se utilizaron especímenes prismáticos (CPP) con dimensiones de 45x80x90 mm moldeados con concreto, mortero estructural, mortero polimérico y graute. Cada CPP tiene dos barras de acero CA 60 Ø 5.0 mm insertadas. Las barras se usaron en tres condiciones: pintura protectora a base de zinc, pintura protectora a base de nitrito y sin protección. Las propiedades físicas de los materiales, como la resistencia a la compresión, la absorción de agua y la resistividad eléctrica, se midieron en muestras cilíndricas (CPC). Los CPP se sometieron a ciclos de secado durante cinco días en un horno a 50 ºC, seguidos de dos días de humectación por inmersión parcial en solución de NaCl al 3,5%. Después de cada ciclo, los potenciales de corrosión se midieron con equipos con voltímetro de alta impedancia y semicélula de cobre / sulfato de cobre. Así, se obtuvieron datos que permitieron comparar la eficiencia del concreto, del mortero estructural, mortero polimérico e del graute contra el ataque del cloruro. Así, se busca descubrir cual material tiene un rendimiento mejor. Esta pesquisa está en andamiento y se concluirá en 16/12/2019.

"CAD-on" Interfaces - Fracture Mechanics Characterization.

To apply fracture mechanics methodology to determine the interfacial fracture toughness of the interfaces present in "CAD-on" crowns consisting of CAD/CAM milled lithium disilicate veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal framework. The notchless triangular prism specimen fracture toughness test was used to determine interfacial fracture toughness. Four groups, each consisting of (6 × 6 × 6 × 12) mm prisms (n = 22), were produced. Half-size [(6 × 6 × 6 × 6) mm] specimens of IPS e.max CAD and IPS e.max ZirCAD were approximated under vibration with Crystal Connect fusing glass and sintered according to manufacturer's guidelines to obtain the following three interfaces: (1) e.max CAD/Crystal Connect/e.max CAD (Group I); (2) Zir CAD/Crystal Connect/Zir CAD (Group II); and (3) Zir CAD/Crystal Connect/e.max CAD (Group III). For Group IV (control, based on the "press-on" veneering technique), half-size [(6 × 6 × 6 × 6) mm] IPS e.max ZirCAD prisms were coated with ZirLiner and pressed with IPS e.max ZirPress ingots to obtain (6 × 6 × 6 × 12) mm prisms. All specimens were tested using a computer controlled material testing machine. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α = 0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured surfaces were characterized under a scanning electron microscope. All experimental groups demonstrated a cohesive mode of failure in the fusing glass layer. The number and size of defects appeared to correlate with the variability of fracture toughness values. There were no significant differences between the fracture toughness of the "CAD-on" interfaces (p = 0.052). The results suggested that the fracture toughness of Crystal Connect limited the interfacial fracture toughness values. The "CAD-on" fracture toughness value (Group III) was significantly greater than that of the ZirPress "press-on" control (Group IV) (p < 0.001). The "CAD-on" process results in stronger bonding between veneer and framework, compared to conventional veneering. The clinical use of "CAD-on" crowns could therefore be advocated. The selection of any restorative material requires a thorough analysis of advantages, limitations and results from clinical studies to inform the clinical decision in a case-by-case approach.

Incorporation of unserviceable tire waste in red ceramic

A spent tire without the legal possibility of running is classified as unserviceable tire waste. In this work, incorporation of rubber tire waste (RTW) in clay ceramics used in the production of bricks and roofing tiles was for the first time evaluated. The raw materials were initially characterized in terms of its mineralogical and chemical composition by X-ray diffraction (XRD) and X-ray fluorescence (XRF), respectively. Physical and mechanical properties of RTW incorporated ceramics such as bulk density, water absorption, linear shrinkage, flexural and compressive strength were determined. Besides, thermal analyses by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out in the plain RTW. Different ceramic compositions were prepared with amounts of 0.5; 1; 1.5 and 2 wt% RTW incorporated into the clay ceramic. Prismatic specimens were prepared by uniaxial pressing and then sintered at 850 and 950 °C. The results indicate that the investigated waste contributes to a saving in energy during the ceramic sintering due to its high amount of carbon. An increase in the ceramic porosity above 1.5 wt% of RTW was responsible for increasing the water absorption. Within standard deviations, both the shrinkage and strength were not impaired. The clay ceramic with 1 wt% of RTW sintered at 950 °C was the most suitable condition observed. Hence, these results showed a viable alternative to RTW, turning it into an environmentally friendly by-product for civil construction materials.

Study of the suitability of a new structural concrete manufactured with carbon fiber reinforced lightweight aggregates sintered from wastes

The suitability of three new lightweight aggregates containing carbon fiber residues (CAs) as components in structural lightweight concrete has been studied. Prismatic concrete specimens were prepared using these CAs as a coarse fraction. Additional specimens of normal-weight aggregate, commercial lightweight aggregate and mortar were prepared for comparison. The CA-concrete samples (CACs) have yielded compressive strength values between 35 and 55 MPa as well as low density and thermal conductivity results. Furthermore, the CACs have displayed the highest ratios of mechanical strength over density and the thermal conductivity, which means that there is a better balance between their mechanical and physical properties than in the other samples studied. These results indicate that the new CAs could have great potential for use in structural lightweight concrete, also complying with the principles of the Circular Economy.

Stress distribution and fracture evolution around a trapezoidal cavity in sandstone loaded in compression

To understand the failure mechanism of trapezoidal opening in rock engineering, the fracture evolution of rock containing a cavity under compression was investigated analytically and experimentally in this study. Firstly, the complex variable theory was employed to find the general solution for rock stress around a trapezoidal cavity under biaxial stresses. Secondly, a series of uniaxial compression tests were carried out on prismatic sandstone specimens with a circular or a trapezoidal cavity. During the tests, the acoustic emission (AE) and digital image correlation (DIC) techniques were jointly applied to record and analyze the damage and fracturing behavior. The results showed that both the cavity shape and lateral stress ratio have a significant effect on the stress distribution around the cavity. The fracture evolution starts from primary tensile cracks through compressive fractures and secondary tensile cracks to shear cracks, which can be well explained by the stress distribution. The primary and secondary tensile cracks are not responsible for the ultimate macro failure, which is dominated by the shear cracks. Compared to the specimens with a circular cavity, the crack initiation stress of the specimens containing a trapezoidal cavity is large, but the crack closure and the crack damage stresses are small, indicating relatively poor mechanical properties.

The feasibility of estimation of mechanical properties of limestone concrete after fire using nondestructive methods

The feasibility of estimation of mechanical properties of limestone concrete after exposure to high temperatures, using nondestructive methods, was investigated. Experimental study was carried out on two concrete mixtures that differed in water to cement ratio (w/c). After standard curing time, specimens were exposed to various temperature levels, i.e. 20 °C, 200 °C, 400 °C, 600 °C or 800 °C. Basic mechanical properties of concrete specimens were determined prior heating at ambient temperature. Once the specimens were cooled down to the ambient temperature, various nondestructive tests including ultrasonic (US) method, determination of rebound number, and resonant frequency method were performed. Further on, specimens were visually inspected to assess the damage of the concrete surface. To determine the residual mechanical properties of specimens after exposure to high temperature, the compressive and flexural strengths were determined on cubic and prismatic specimens, respectively. The main goal of presented research was the estimation of the compressive and flexural strengths of limestone concrete after fire based on developed relationships between results of destructive and nondestructive measurements performed on limestone concrete mixtures 1 and 2. The best relationship for estimation of compressive strength was determined based on measured fundamental torsional frequency, whereas the best relationship for estimation of flexural strength was determined based on combination of fundamental flexural frequency and US pulse velocity, both measured on prisms. The analysis of variance (ANOVA) showed that the influence of temperature on experimental results, obtained from destructive and nondestructive tests, is highly statistically significant for both mixtures. Posteriori test revealed that the majority of the measured quantities were significantly different on the temperature interval between 400 °C and 600 °C.

Flexural behavior of reinforced concrete beams with high performance fiber reinforced cementitious composites

This article presents an experimental study on the flexural performance of reinforced concrete (RC) beams with fiber reinforced cementitious composites (FRCC) and hybrid fiber reinforced cementitious composites (HFRCC) in the hinge portion. Beam specimens with moderate confinement were used in the study and tested under monotonic loading. Seven diverse types of FRCC including hybrid composites using fibers in different profiles and in different volumes are employed in this study. Companion specimens such as cylindrical specimens and prism specimens are also used to study the physical properties of composites employed. The moment-curvature, stiffness behavior, ductility, crack pattern and modified flexural damage ratio are the main factors considered in this study to observe the efficacy of the employed hybrid composites. The experimental outputs demonstrate the improved post yield behavior with less rate of stiffness degradation and better damage tolerance capacity than conventional technique.

Dynamic properties of lightweight foamed glass and their effect on railway vibration

Abstract Deformation properties of lightweight coarse grained material from recycled foamed glass have been determined from large-scale triaxial tests on prismatic specimens with dimensions 40 cm × 40 cm × 80 cm. Deformations were measured locally using vertical and horizontal local deformation transducers. Monotonic and cyclic loading at small to medium strain range were conducted. Three load sequences representing the expected conditions of use of lightweight material as vibration-reducing material in railway geotechnics have been used. Results indicate strong effect of brittle cellular structure of tested material as well as confining pressure dependency of elastic threshold shear strain and damping ratio. The results were used to assess the applicability of empirical formulas for shear modulus of granular materials to lightweight foamed glass. The parameters determined from the laboratory tests were further used in numerical analysis of railway dynamic response. The results of the numerical simulations show that replacement of fill in track embankment by lightweight material could improve the dynamic response of the track in reducing the vibration.

Investigation of cracking behavior and mechanism of sandstone specimens with a hole under compression

In this paper, the cracking behaviour of rocks with a hole loaded in compression was investigated both experimentally and theoretically. First, a series of uniaxial compression tests on prismatic red sandstone specimens containing a circular or a rectangular hole were performed using a rock mechanics testing system in conjunction with digital image correlation (DIC) and acoustic emission (AE) systems. Afterwards, the theoretical solutions of stress for a circular and a rectangular hole under different stress conditions were solved by combining the complex variable method and the Boxʹs complex algorithm. Experimental results show that the mechanical properties of rock specimens are significantly degraded by the hole and influenced by its shape. Crack evolution proceeds from primary tensile cracks through secondary tensile cracks and slabbing fractures to shear cracks, which can be visually displayed by the real-time deformation fields. The failure of specimens containing a hole under uniaxial loading is induced by the coalescence of the shear cracks and slabbing fracturing zones. Besides, several critical stresses associated with cracking behaviour were estimated based on a proposed method and the AE measurement. Theoretical investigation indicates that both hole shape and lateral stress ratio are important factors influencing the stress distribution, and the initiation mechanisms of different cracks can be well revealed according to the stress states around the holes under uniaxial compressive loads.

Enhancing the out-of-plane behaviour of unreinforced masonry walls under impact loading through the use of partially bonded layers of engineered cementitious composite

Published experimental work reveals that the out-of-plane behaviour of unreinforced masonry walls under impact loading can be significantly enhanced through the use of engineered cementitious composite layers fully bonded to the surface of the masonry. The disadvantage of this method is associated with the localised cracking exhibited by the engineered cementitious composite layer close to the joints forming between bricks. This cracking is associated with the bond developing between the masonry and the engineered cementitious composite layer and does not allow the latter layer to achieve its full potential, thus resulting in its premature failure. In an attempt to address this problem, a series of drop-weight tests were carried on masonry prismatic specimens strengthened with a layer of engineered cementitious composite partially bonded to the surface of the masonry acting in tension. The latter prismatic specimens consist of a stack of bricks connected with mortar joints. The specimens are considered to provide a simplistic representation of a vertical strip of a masonry wall subjected to out-of-plane actions associated with impact or blast loading. Analysis of the test data reveals that under impact loading, the specimens retrofitted with partially bonded engineered cementitious composite layers can exhibit a more ductile performance compared to that exhibited by the same specimens when strengthened with fully bonded layers of engineered cementitious composite. This is attributed to the fact that along its unbonded length, the engineered cementitious composite layer is subjected to purely uniaxial tension (free from any interaction with the surface of the masonry), allowing for the development of multiple uniformly distributed fine cracks.

The Physically Nonlinear Model of an Elastic Material and Its Identification

This work is devoted to the new variant of relations between the energetically conjugated Hencky strain tensor and corotational Kirchhoff stress tensor. The elastic energy is represented as a third-order polynomial of the Hencky tensor containing five material constants. Unlike the Almansi tensor in the Murnaghan model, the Hencky tensor allows assigning a clear physical meaning to material constants. Linear part of the constitutive relation represents the Hencky model and contains the bulk modulus and the shear modulus. The two extra constants express nonlinear effects at a purely volumetric strain and a purely isochoric strain, whereas the third constant takes into account the possible deviation from the similarity of the deviators of the Hencky stress and strain tensors. The resulting relations are naturally generalized for incompressible materials. In this case, the overall number of constants decreases from five to two. The designed test unit was used for a compression test of prismatic specimens made of incompressible material. The proposed version of the relations is in good agreement with the experimental data on the compression of rubber samples.

Residual properties of recycled concrete after exposure to high temperatures

This paper presents the results of an experimental study on the residual properties of recycled concrete after exposure to high temperatures. Three parameters were considered in this test: (a) high temperatures (i.e. 20, 200, 300, 400, 500, 600, 700 and 800°C); (b) recycled coarse aggregate replacement percentages (i.e. 0, 30, 50, 70 and 100%); (c) recycled coarse aggregate types (gravel and pebble). A total of 144 standard prism specimens (150 mm × 150 mm × 300 mm) of recycled concrete were prepared and subjected to different high temperatures for 1 h; after cooling down to ambient temperature, the following basic physical and mechanical properties were then tested and discussed: (a) surface change and mass loss ratio; (b) compressive strength; (c) compressive stress–strain curve; (d) elasticity modulus. Based on the mass loss ratio, a residual strength assessment method for recycled concrete after exposure to high temperatures was put forward.

Degradation Behavior of Concrete after Freeze-Thaw Cycles and Then Exposure to High Temperatures

Concrete behavior usually degrades due to freeze-thaw cycles, fire, or both. Existing studies on the degradation behavior of concrete due to exposure to high temperatures were primarily focused on unfrozen concrete. In this paper, the degradation behavior of damaged concrete, after different freeze-thaw cycles (25, 35, 45, and 55 cycles), exposure temperatures (20°C, 300°C, 400°C, and 500°C), and cooling methods (water-cooled and air-cooled), was tested with seventy-five prism specimens. The degradation behavior of the damaged concrete, such as surface characteristics, weight loss, compressive strength, peak strains, and elastic modulus, was studied and analyzed. Results show that (i) the surface color of the concrete does not change significantly throughout the test. As the number of freeze-thaw cycles and temperatures increase, the weight loss of the concrete specimens increases gradually. (ii) After freeze-thaw cycles, the relative strengths and elastic modulus of the concrete specimens significantly degrade compared with those of the unfrozen ones at same temperatures. (c) At elevated number of freeze-thaw cycles and exposure temperatures, the peak strain of the concrete increases gradually. (d) Cooling methods have different effects on the degradation of concrete under different number of freeze-thaw cycles. Finally, a uniaxial compression constitutive model for concrete after freeze-thaw cycles and then exposure to high temperatures was established and a good agreement was observed with test results.

COMPARISON OF INFLUENCE OF THERMAL CYCLES ON THERMO-ACTIVATED STRUCTURES WITH RESPECT TO PREDICTED MODELS ESTABLISHED IN THE MODEL CODE 2010

Global warming and increasing CO2 emissions make it necessary to reduce energy consumption while maintaining thermal comfort. The current trend advocates the combination of energy conservation strategies and energy efficiency technologies and therefore thermoactive structures play an important role. The Model Code 2010 establishes a model of the effect of temperature on the compressive, flexural and tensile strength for normal concretes, but this model only takes into account the effect of temperature, thus neglecting the age of the concrete, as well as the effect produced by thermal cycles over time. This article examines the behaviour of concrete subjected to thermal cycles, thus imitating the process of thermal loading and unloading in thermoactivated structures throughout its period of use. Cylindrical specimens have been made to study compression behaviour and tensile strength, and prismatic specimens have been made to determine flexural strength. The results obtained have been compared with the models established in the 2010 model code, and large differences can be observed between them. It can be concluded that the models established in the model code do not describe the behaviour of the concrete subjected to thermal cycles. Keywords: Plain concrete, temperature-related &amp; thermal effects, testing, thermoactive

A DIC-Based Study on Compressive Responses of Concrete after Exposure to Elevated Temperatures.

This paper provides an experimental investigation on the cracking process and residual mechanical properties of concrete after exposure to elevated temperatures. A total of 36 standard concrete prism specimens were tested after exposure to high temperatures of up to 600 °C. The failure modes, cracking process, residual mechanical properties, deformation characteristics and the strain distribution on the surface during the loading procedure were presented. The influences of exposure temperature and water–cement ratio (w/c) were interpreted. The digital image correlation (DIC) method was applied to quantitatively and visually characterize the development of cracking and relative displacement on the concrete surface. The findings suggest that the residual compressive strength and elastic modulus of the concrete decreases gradually with the increasing temperature, especially in the specimens with lower w/c ratio. The DIC technique provides an effective means to measure very precise and detailed information, including the crack opening and distribution of strain on the concrete surface.

A Determination of fracture toughness in plain concrete specimens by R curve

Since the last two centuries, concrete has become the number one building material in the world. Therefore, we are interested in studying the mechanical behavior at failure of this material and calculate its toughness. In this paper, we trace the R-curve to determine the critical stress intensity factor and the crack evolution by specifying the three stages: crack stopping stage, stable propagation stage and an unstable propagation stage. The R-curve was obtained from the curve connecting the crack mouth opening to the applied load for prismatic specimens normalized by ASTM and notched laterally (SENB). These specimens are made from concrete with 25 MPa compressive strength and subjected to three-point bending. The dimensionless ratio of the initial notch length and the specimen width is between 0.3 and 0.6. During loading, we can observe an elastic deformation followed by a significant plastic deformation, the average value of the critical stress intensity factor found is 1.05 MPa.m1/2.

Behaviour of oil palm broom fibres (OPBF) reinforced concrete

Recent concerns on global environmental sustainability and climate change have favoured the use of natural fibres as alternatives to traditional reinforcing steel in concrete due to their low carbon footprint. Oil palm broom fibres (OPBF) which are the ribs of oil palm leaflets have average length, cross-sectional area, density and tensile strength of 900 mm, 1.837 mm2, 0.84 g/cm3 and 389 MPa respectively, hence making the fibres superior to steel in terms of strength-to-weight ratio. A study on the effect of OPBF on some mechanical properties of concrete mix of 1:1.5:3 of cement, sand and gravel respectively, was carried out. Short discrete OPBF of 50 mm length were incorporated in the fresh concrete mix at 0.5%, 1%, 1.5%, 2%, 3% and 4% of total aggregate volume. A total of two hundred and seventy-six (276) samples of 150 cubic, 63 cylindrical and 63 prismatic specimens were cast to assess compressive strength of OPBF-concrete at 7, 14, 28, 56 and 112 days, and flexural and split-tensile strengths at 28, 56 and 112 days. Bond strength between OPBF and concrete matrix was also investigated through pull-out tests. Flexural strength and splitting tensile strength of concrete were slightly improved for 0.5–1% OPBF inclusion at 28 days but do not show marked improvements at 112 days. Failure of hardened OPBF-concrete was characterised by more fibre-pull-outs than fibre-fracture but with an enhanced ductility of the composite at failure. The post-yield behaviour of OPBF-concrete shows enhanced energy absorption capacity of between 70 and 320% at 112 days, thereby possessing potential application in low-cost earthquake resistant housing.

Manufacturing of lightweight aggregates from biomass fly ash, beer bagasse, Zn-rich industrial sludge and clay by slow firing

A 70:30 black-white clay mixture (C) has been blended with different proportions of three wastes: bagasse from beer production (BG), a flux sludge used in galvanization (FS) and fly ash enriched to 60% with K2O from a biomass plant (FA). A complete characterization of the raw materials was performed. Mixtures of C and FA for 15, 17.5 and 20 wt% of K2O were blended with water, shaped into prismatic specimens, oven-dried, muffle-sintered at 1000–1200 °C, and finally crushed into lightweight aggregates (LWAs). The amounts of FA corresponding to K2O = 20% for a temperature, T, of 1000 °C (LWA variety C20K-1000) yielded the best technological properties: oven-dry density of 1.22 g/cm3, water absorption of 39.8% and open porosity of 48.6%. Therefore, other specimens containing BG and FS were sintered according to the same protocol considering K2O = 20% and T = 1000 °C. White aggregates were also obtained at 1000 °C (C-1000 variety) from C-mixture (without FS, FA and BG). The addition of BG (5,7 and 10 wt%) did not mean any improvement with respect to C20K-1000, and in the case of FS (25–40%), the development of red color was fostered, which could be interesting for gardening or ornamental purposes. A leaching test conducted on the aggregate with 40 wt% FS showed that the concentration of heavy metals was below the permitted environmental thresholds. Although the leachable Zn concentration of the aggregate (205 ppm) was reduced by 85% with respect to the original residue, it is still high enough for crops that demand this metal. The outcome of this investigation proves the usefulness of recycling agricultural and industrial wastes to obtain high quality LWAs for agriculture or gardening.