Effect of the curing conditions of concrete on the behaviour under freeze-thaw cycles*

The aim of this work is to relate the curing conditions of concrete and the addition of an air-entraining admixture with the damage caused by freeze–thaw cycles. In countries with a continental climate, the curing of concrete in summer is performed under climatic conditions of high temperature and low humidity, and during the winter the concrete suffers conditions of freeze–thaw, often accompanied by the use of de-icing salts. This paper shows the experimental results of the behaviour of concrete specimens cured under climatic summer conditions (high temperature and low humidity) and then subjected to freeze–thaw cycles. Curing of the specimens includes conditions of good and bad practice in relation to wetting and protection of the concrete. It also examines the effectiveness of using an air-entraining admixture in both cases. The experimental programme includes an evaluation of the mechanical properties of the concrete, the study of the cement hydration and the measurement of the volume and pore sizes of the concrete. These tests were performed before and after the application of the freeze–thaw cycles. The results obtained showed that the specimens without air-entraining admixture show a deterioration of mechanical properties after the freeze–thaw test. However, the inclusion of air bubbles benefits the behaviour of concrete against freeze–thaw cycles so even better mechanical properties after the test were observed. This anomalous behaviour is because the cement hydration process continues over the freeze–thaw tests, closing the pore structure. This aspect has been confirmed with the DTA and TG tests performed

Microstructure, magnetic and mechanical properties of Ni-Zn ferrites prepared by Powder Injection Moulding

Nowadays, the electronic industry demands small and complex parts as a consequence of the miniaturization of electronic devices. Powder injection moulding (PIM) is an emerging technique for the manufacturing of magnetic ceramics. In this paper, we analyze the sintering process, between 900 °C and 1300 °C, of Ni–Zn ferrites prepared by PIM. In particular, the densification behaviour, microstructure and mechanical properties of samples with toroidal and bar geometry were analyzed at different temperatures. Additionally, the magnetic behaviour (complex permeability and magnetic losses factor) of these compacts was compared with that of samples prepared by conventional powder compaction. Finally, the mechanical behaviour (elastic modulus, flexure strength and fracture toughness) was analyzed as a function of the powder loading of feedstock. The final microstructure of prepared samples was correlated with the macroscopic behaviour. A good agreement was established between the densities and population of defects found in the materials depending on the sintering conditions. In general, the final mechanical and magnetic properties of PIM samples were enhanced relative those obtained by uniaxial compaction.

NDT for concrete under accelerated freeze/thaw tests and surface scaling

Freezing of water or salt solution in concrete pores is a main cause for severe damage and significant reduction of the service life. Most of the freeze-thaw (F-T) accelerated tests measure the scaling of concrete by weighting. This paper presents complementary procedures based on the use of strain gages and ultrasonic pulse velocity (UPV) for measuring the deterioration of concrete due to freezing and thawing. These non-destructive testing (NDT) procedures are applied to two types of concretes, one susceptible to F-T damage and the other does not. The results show a good correlation between scaling and the measurements obtained with NDT. Showing NDT the advantage to detect before the damage and to perform continuous measurement

Study of the damage evolution of the concrete under freeze-thaw cycles using traditional and non-traditional techniques

Some experiments have been performed to investigate the cyclic freeze-thaw deterioration of concrete, using traditional and non-traditional techniques. Two concrete mixes, with different pore structure, were tested in order to compare the behavior of a freeze-thaw resistant concrete from one that is not. One of the concretes was air entrained, high content of cement and low w/c ratio, and the other one was a lower cement content and higher w/c ratio, without air-entraining agent. Concrete specimens were studied under cyclic freeze-thaw conditions according to UNE-CENT/TS 12390-9 test, using 3% NaCl solution as freezing medium (CDF test: Capillary Suction, De-icing agent and Freeze-thaw Test). The temperature and relative humidity were measured during the cycles inside the specimens using embedded sensors placed at different heights from the surface in contact with the de-icing agent solution. Strain gauges were used to measure the strain variations at the surface of the specimens. Also, measurements of ultrasonic pulse velocity through the concrete specimens were taken before, during, and after the freeze-thaw cycles. According to the CDF test, the failure of the non-air-entraining agent concrete was observed before 28 freeze-thaw cycles; contrariwise, the scaling of the air-entraining agent concrete was only 0.10 kg/m 2 after 28 cycles, versus 3.23 kg/m 2 in the deteriorated concrete, after 28 cycles. Similar behavior was observed on the strain measurements. The residual strain in the deteriorated concrete after 28 cycles was 1150 m versus 65 m, in the air-entraining agent concrete. By means of monitoring the changes of ultrasonic pulse velocity during the freeze-thaw cycles, the deterioration of the tested specimens were assessed

Hyperspectral images and polymerase chain reaction (PCR) for the detection of allergen (peanuts traces) in powder foods.

The addition of preservatives to some kind of foods may be a potential risk of spoilage, as it is the transformation of sorbate into the off-odour 1-3-pentadiene by certain microbial species. This is the case of the capacity of some strains of moulds and yeasts that are able to decarboxylate sorbic acid and transform it into 1-3 pentadiene, a volatile compound with an unpleasant petroleum odour. (Casas et al. 2004).

Sensing colour stability and mixtures of powder paprika using optical reflectance and image analysis.

The production and industry of paprika present several problems related to quality and to production costs. One of the main difficulties is to obtain an objective and quick method for predicting quality. Quality in powder paprika involves: quantity of carotenoids and the appearance and stability of colour. The method used currently for determining quality is the measurement of absorbance at 460 nm wavelength, of an acetone extract of carotenoids, but there is no information about the appearance of the paprika or the stability of its colour with time. " Another important problem is the presence of mixtures of powdered paprika produced in the Spanish region of "La Vera", which has a peculiar way of production, with a high '' quality and price, with other products of lower quality. It is necessary to obtain methods which are able to detect the fraud.

Characterization of spontaneous combustuion tendency of dried sewage sludge

The general purpose of this study was the determination of the safety conditions to avoid the presence of explosive atmospheres in the wastewater industry. Eight Spanish plants located in Madrid, Barcelona and Málaga were considered and several sludge samples were taken in different seasons. The base for the assessment of the spontaneous ignition behaviour of dust accumulations is the experimental determination of the self-ignition temperature under isothermal conditions. Self-ignition temperatures at four volumes were obtained for one sample of sewage sludge, allowing their extrapolation to large storage facilities. A simple test method, based also on an isothermal study of samples, is the UN classification of substances liable to spontaneous combustion. Two different samples were so tested, obtaining unlike results if transported in packages of different volumes. By means of thermogravimetric techniques it is possible to analyse the thermal susceptibility of dried sewage sludge. Apparent activation energy can be obtained from the rate of weight loss. It is also applied to the study of self-ignition susceptibility by modifying test conditions when oxygen stream is introduced. As a consequence of this oxidant contribution, sample behaviour can be very different during testing and a step drop or sudden loss of weight is observed at a characteristic temperature for every substance, associated to a rapid combustion. Plotting both the activation energy and the characteristic temperature, a map of self-ignition risk was obtained for 10 samples, showing different risk levels for samples taken in different locations and at different seasons. A prediction of the self-ignition risk level can be also determined.