Raúl Zerbino

Uniaxial tension test for steel fibre reinforced concrete: a parametric study

Abstract A RILEM Draft Recommendation was proposed in 2001 for obtaining the stress versus crack opening (σ–w) response of steel fibre reinforced concrete through a uniaxial tension test. The present study analyses the robustness of the recommended test through a parametric study. Furthermore, the methodology is extrapolated to cores extracted from cast elements. Also, the effect of the coring direction with respect to the preferential fibre orientation caused by the compaction procedure is examined. The study demonstrates that the test is robust and representative of the material response, and could be used for determining the σ–w relation of the material that may be needed for comparing the performance of different fibres or for providing input for finite element analysis. No significant influence of the characteristics of the specimen or problems of instability due to the loss of control were encountered. There is some relative rotation between the crack faces but its influence on the σ–w response is expected to be negligible. The parameters obtained from the tests exhibit coefficients of variation of up to 30%, which is mainly attributed to the randomness of the number of fibres bridging the crack, considering the relatively small cross-section of the specimen.

Shear Failure of Steel Fiber-Reinforced Concrete Based on Push-Off Tests

Steel fiber-reinforced concrete (SFRC) uses fibers as shear reinforcement, a successful approach to coping with the brittle nature of shear failure. This article reports on a study undertaken to characterize the failure and toughness of SFRC subjected to direct shear loading at the material level. The study used a push-off test on a double-notched prism to quantify the shear stress-displacement behavior of SFRC. The shear stress-slip response (obtained experimentally) can be used to calculate toughness-based parameters, which can be employed in structural design. The authors note that the test can be performed in a stable manner for steel fiber-reinforced concrete, permitting the determination of the pre- and post-peak responses and, consequently, characterizing the shear stress that can be transferred across an open crack. The authors conclude that significant improvements in the ductility of concrete during shear failure and some increase in the shear strength are achieved through the incorporation of steel fibers in both normal- and high-strength concretes.

Long-Term Behavior of Cracked Steel Fiber-Reinforced Concrete Beams under Sustained Loading

The mechanical behavior of steel fiber reinforced concrete (SFRC) has been widely studied. In many cases, fibers are incorporated into concrete to improve the service life of structures by means of the three-dimensional (3-D) crack control capacity of this type of reinforcement. The residual (postcracking) capacity and long-term (creep) behavior of fiber-reinforced concrete is of utmost importance. This paper presents an experimental study on the behavior of SFRC beams, cracked and then subjected to long-term loading. The cracked beams were then placed in creep frames and subjected to different levels of load. The crack opening under constant gravity load was electronically measured over a period of approximately 21 months by means of displacement transducers. The concept of the crack-opening rate under long-term loading is introduced, and the conditions for a long-term stable response are discussed.

Characterization of granulated and pelletized blast furnace slag

Abstract The objective of this study is the characterization of slags from different sources by their chemical composition, glass content, rate and total heat of hydration and compressive strength development with a view to establish a relationship between some of their properties and compressive strength. Binders incorporating 50 per cent slag present the same classification derived from total heat evolved as from slag activity indexes.

Hospital waste ashes in Portland cement mortars

Nowadays, most concretes incorporate mineral additions such as pozzolans, fly ash, silica fume, blast furnace slag, and calcareous filler among others. Although the technological and economical benefits were the main reasons for the use of mineral additions, the prevention of environmental contamination by means of proper waste disposal becomes a priority. The chance of incorporating hospital waste ashes in Portland cement-based materials is presented here. Ash characterization was performed by chemical analysis, X-ray diffraction, radioactive material detection, and fineness and density tests. Conduction calorimetry and setting time tests were developed on pastes including ash contents from 0% to 100%. Mortars were prepared including ash contents up to 50% of cement. The results of setting time, temperature development, flexural and compressive strengths, water absorption, density, and leachability are analyzed. Results indicate that Portland cement systems could become an alternative for the disposal of this type of ashes.

EFFECTS OF HIGH TEMPERATURE ON RESIDUAL, MECHANICAL, AND TRANSPORT PROPERTIES OF CONCRETE

This paper presents an extensive analysis of the physical and mechanical properties of normal and high-strength concretes exposed to temperatures up to 700 deg C. Ultrasonic pulse velocity, static and dynamic modulus of elasticity, and strength and deformations under compressive loading were measured. In addition, flexural and splitting tensile strength and the fracture energy were also determined. Other tests for finding the water permeability, water penetration, and absorption were performed on concrete slices in order to analyze the differences in the physical properties of the cover and bulk concrete.

Development of Alkali-Silica Reaction under Compressive Loading and Its Effects on Concrete Behavior

The behavior of a reference concrete is compared to that of concrete strongly affected, under long-term compressive loads, by alkali-silica reaction (ASR) in this paper. Saturated condition cylinder storage took place at temperatures of 21 and 38 degrees C (70 and 100 degrees F). There was periodic measurement of concrete strains on a specimen set that remained unloaded and one loaded up to 40% of its compressive strength. There was also measurement of concrete prism expansion evolution when those prisms were in compliance with ASTM C1293 and stored under the same conditions as the cylinders. No significant differences were shown in the results in deformations under loads over time between specimens damaged by ASR and sound specimens; nevertheless, in concrete under sustained loads, ASR crack development changed. There was modification of residual mechanical properties as a consequence.

FAILURE MECHANISM OF CONCRETE, COMBINED EFFECTS OF COARSE AGGREGATES AND SPECIMEN GEOMETRY

Concrete compressive strength is mostly evaluated by tests performed on cylindrical specimens with slenderness ratio two. Nevertheless, specimens with lower slenderness ratio are also used (drilled cores, cubes, etc.). Tests performed on cubes are affected by the multiaxial stress field induced by the reduced slenderness ratio. Then, the crack propagation is modified depending on the characteristics of the composite material. This paper analyzes some phenomenological aspects of the evaluation of concrete compressive strength related to the effect of coarse aggregates on the initiation and propagation of cracks. The influences of strength level and microcracking are also discussed.

Non-Destructive Tests for the Evaluation of Concrete Exposed to High Temperatures

This paper presents a discussion on the use of non-destructive tests (NDT) for the evaluation of concretes damaged by exposure to high temperatures. The main analysis is based on the relationships between strength, modulus of elasticity and ultrasonic pulse velocity. Rebound hammer, break-off, and resonant frequency were also used. Tests were performed on concretes, prepared with different types of coarse aggregates and cements, with strength levels between 20 and 60 MPa. Exposure variables included maximum temperature (150°C to 700°C), time of exposure and the cooling rate. The obtained results indicate that, contrary to what happens with strength, the ultrasonic pulse velocity is a very good tool for the estimation of the static modulus of elasticity of thermally damaged concretes.

BREAK-OFF TEST FOR HIGH-STRENGTH CONCRETE

High-strength concrete is one of the new developments in concrete technology. In addition to compressive strength levels above 60 MPa, high workability, improvements in early age performance and durability appear to be its main advantages. This paper studies the suitability of the break-off test to evaluate strength development of high-strength concrete. Regression equations between break-off pressure and compressive strength were calculated. The effects of different variables such as age, type of coarse aggregate, and strength level are discussed. The obtained results indicate that the break-off test can be used satisfactorily to evaluate the quality of concretes with compressive strength levels up to 100 MPa.