In this paper we derive the asymptotic properties of within groups (WG), GMM, and LIML estimators for an autoregressive model with random effects when both T and N tend to infinity. GMM and LIML are consistent and asymptotically equivalent to the WG estimator. When T /N → 0 the fixed T results for GMM and LIML remain valid, but WG, although consistent, has an asymptotic bias in its asymptotic distribution. When T /N tends to a positive constant, the WG, GMM, and LIML estimators exhibit negative asymptotic biases of order 1/T , 1/N , and 1/ 2N − T , respectively. In addition, the crude GMM estimator that neglects the autocorrelation in first differenced errors is inconsistent as T /N → c > 0, despite being consistent for fixed T . Finally, we discuss the properties of a random effects pseudo MLE with unrestricted initial conditions when both T and N tend to infinity.

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We model earnings processes allowing for lots of heterogeneity across agents. We also introduce an extension to the linear ARMA model which allows the initial convergence in the long run to be different from that implied by the conventional ARMA model. This is particularly important for unit root tests, which are actually tests of a composite of two independent hypotheses. We fit to a variety of statistics including most of those considered by previous investigators. We use a sample drawn from the Panel Study of Income Dynamics (PSID), and focus on white males with a high‐school degree. Despite this observable homogeneity, we find more latent heterogeneity than previous investigators. We show that allowance for heterogeneity makes substantial differences to estimates of model parameters and to outcomes of interest. Additionally, we find strong evidence against the hypothesis that any worker has a unit root.

Nowadays, one of the most popular ways to get a more sustainable cement industry is using additions as cement replacement. However, there are many civil engineering applications in which the use of sustainable cements is not extended yet, such as special foundations, and particularly micropiles, even though the standards do not restrict the cement type to use. These elements are frequently exposed to the sulphates present in soils. The purpose of this research is to study the effects in the very long-term (until 600 days) of sulphate attack in the microstructure of micropiles grouts, prepared with ordinary Portland cement, fly ash and slag commercial cements, continuing a previous work, in which these effects were studied in the short-term. The microstructure changes have been analysed with the non-destructive impedance spectroscopy technique, mercury intrusion porosimetry and the “Wenner” resistivity test. The mass variation and the compressive strength have also been studied. The impedance spectroscopy has been the most sensitive technique for following the sulphate attack process. Considering the results obtained, micropiles grouts with slag and fly ash, exposed to an aggressive medium with high content of sulphates, have shown good behaviour in the very long-term (600 days) compared to grouts made with OPC.

The microstructural effects of exposure to a 15% magnesium sulphate, 15% sodium sulphate, and mixed solutions were observed in mortars with and without silica fume, using the non-destructive impedance spectroscopy technique. The non-destructive "Wenner" resistivity test and the classical mercury intrusion porosimetry were used as contrast techniques. The compressive strength of the mortars was also studied. In view of the results obtained, impedance spectroscopy was the most sensitive technique for detecting changes in the porous network of the studied mortars. The addition of silica fume results in a more refined microstructure and a higher compressive strength in mortars exposed to aggressive sulphate solutions.

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This paper presents a study on the possibility of using fly ash cement as grouts for micropiles. 19 This type of special geotechnical work is commonly used for many applications. Generally, 20 micropiles grouts are prepared using Portland cement, although the standards do not restrict 21 the cement type to use, as long as they achieve a strength requirement. In this research, fly 22 ash cement grouts made with w:c ratios 0.40, 0.45, 0.50 and 0.55 were studied from 2 up to 23 90 days of age. Their microstructure was characterized using the non-destructive impedance 24 spectroscopy technique, electrical resistivity, and mercury intrusion porosimetry. Their 25 2 durability properties have been studied by determining the water penetration under pressure, 26 and the chloride diffusion coefficient. The compressive strength was also measured and 27 determined, and a maximum water:cement ratio, different for each cement type was obtained. 28All the results were compared to those obtained for Portland cement grouts. The results 29 obtained confirm that the performance of micropiles made using fly ash cement grouts is 30 adequate, and as it is well know the cements with mineral admixtures provide environmental 31 benefits, so the use of cement including fly ash will contribute to the sustainability, with 32 similar properties to those given by OPC. 33Keywords: micropiles, special geotechnical works, fly ash, durability, microstructure, 34 impedance spectroscopy, water:cement ratio. 35 1.-INTRODUCTION 36In the field of geological engineering, the use of special geotechnical works has become very 37 important. Some of the most commonly used special geotechnical works for civil engineering 38 structures and for building foundations are piles, micropiles, soil anchors and jet grouting 39injections. There are great differences between those types of works and one of these 40 differences is related to the material in which the steel reinforcement elements are embedded. 41In the case of the piles, concrete is usually used. However, for micropiles, soil anchors and jet 42 grouting injections, the reinforcement elements are embedded in cement grouts, although 43 mortars might also be used. This fact is very important, because the behaviour of the cement 44 grouts and mortars shows many differences compared to concrete. For example, in general 45 the porosity of hardened grouts is greater than the porosity of concretes [1], [2], and it could 46 influence the durability and mechanical properties of the elements of each particular special 47 geotechnical work. But on the other hand, a higher amount of cement might improve the 48 durability of this type of elements. So, a different performance could be expected if the 49 material used to protect the reinforcement steel elements is cement grout or concrete, as it is 50 3 usual for the majority of civil engineering structures. Furthermore, the uncertainties can 51 increase as a function of the cement type used, especially if it is used a sustainable cement, 52 which incorporates som...

One way to reach a more sustainable cement industry is replacing clinker by additions, such as waste brick powder. The objective of this research is to analyse the influence in the long-term (until 400 days) of waste brick powder in the microstructure, durability and mechanical properties of mortars which incorporate up to 20% of this addition as a clinker replacement. The microstructure has been studied with the non-destructive impedance spectroscopy technique. According to the obtained results, mortars with 10% and 20% of brick powder, showed good service properties in the long-term, even better than those made with ordinary Portland cement.