Graciela Marta Giaccio

The fracture energy (G F) of high-strength concretes

This paper presents results for the fracture energy of concrete (GF) obtained from a wide range of high-strength concretes. Strength levels up to 100 MPa, aggregate type and aggregate surface texture were included as variables. The determination ofGF was performed according to the recommendation of the RILEM 50-FMC Committee. Compressive and tensile strengths and the modulus of elasticity are also presented. Measured values ofGF are compared with those proposed in the last CEB Model Code.ResumeOn analyse les résultats de l’énergie de rupture de bétons présentant des niveaux de résistance à la compression allant jusqu’à 100 MPa. On a déterminé l’énergie de ruptureGF selon la recommandation de la Commission Technique RILEM 50-FMC. Les principales variables étudiées étaient les suivantes: niveaux de résistance, types et textures des granulats. On analyse les propriétés des mélanges obtenus avec différents types de gros granulats et une distribution granulométrique égale: trois granulats concassés (basalte, granit et calcaire) et deux graviers. On a mesuré aussi la résistance à la compression, le module d’élasticité et la résistance à la traction par fendage et par flexion de chaque béton. On a vérifié l’influence de la dimension des granulats sur l’énergie de ruptureGF, laquelle augmente encore avec la résistance du béton dans les domaines de haute résistance. Pour les bétons avec des granulats de 16 mm, on a mesuré des valeurs de l’énergie de ruptureGF proches de 200 N m−1. Les résultats obtenus sont comparés avec les valeurs indiquées dans le dernier Model Code du CEB.

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.

Dimensional stability of materials based on Portland cement at the early stages

In this work two fiber optic sensing techniques are used to study the dimensional stability in fresh state of different cementitious materials. A conventional Portland cement mortar and two commercial grouts were selected. The measurements were performed by using a Bragg grating embedded in the material and a non-contact Fizeau interferometer. The first technique was applied in a horizontal sample scheme, and the second one, by using a vertical configuration. In addition, a mechanical length comparator was used in the first case in order to compare the results. The evolution with time of the dimensional changes of the samples and the analysis of the observed behavior are included.

Effect of Beam Width on the Creep Behaviour of Cracked Fibre Reinforced Concrete

Many efforts have been made to develop a creep testing procedure for fibre concrete in cracked state. Among several proposals the use of a bending test seems a promissory alternative. The “creep testing procedure” includes a pre-cracking process up to some established crack width, the creep test itself applying permanent loads and finally, a bending test to evaluate the residual strength properties after creep. This paper compares the results of creep tests performed on beams of different widths (50, 100 and 150 mm). The use of thin fibre reinforced concrete specimens should be of interest for applications such as the reparation of concrete structures or the protection of elements exposed to extreme actions. A conventional fibre concrete incorporating 40 kg/m3 of hooked-end steel fibres with a 51.3 MPa compressive strength, class 4a, was used. The specimens were pre-cracked up to 0.5 mm and the sustained bending stress was equal to 60 % of the stress f R1 of each prism. The results show that the specimen width has a minor effect on the results of creep tests in cracked state but the variability increases when the width decreases.

Fracture Energy of Rice-Husk Ash Concrete

As with other mineral admixtures, the use of rice-husk ash leads to an improvement of the concrete internal structure, reducing the pore size and particularly an improvement in the interface bond. In this sense it can be assumed that the failure mechanism can be modified, and the concrete will exhibit a more brittle behavior. That has a special interest in high-strength concrete and in the design of large concrete structures. This paper focuses on the fracture behavior of rice-husk ash concrete. A wide range of concrete strengths are analyzed including normal and high-strength mixtures. The flexural behavior was analyzed following the general guidelines of the RILEM 50-FMC using a center-point loading arranged on notched beams of 400 mm span, measuring deflections and the crack mouth opening displacement (CMOD). In addition, the compressive strength and the elastic modulus were measured on standard cylinders. The effects of water-cementitious material ratio and the age of testing on the strength, energy of fracture and the characteristic length on concretes with and without rice-husk ash incorporation are discussed.