Creep and Shrinkage Behaviour of Disintegrated and Non-Disintegrated Cement Mortar

One way to prevent cement from ending up in landfills after its shelf life is to regain its activity and reuse it as a binder. As has been discovered, milling by planetary ball mill is not effective. Grinding by collision is considered a more efficient way to refine brittle material and, in the case of cement, to regain its activity. There has been considerable research regarding the partial replacement of cement using disintegrated cement in mortar or concrete in the past few decades. This article determines and compares the creep and shrinkage properties of cement mortar specimens made from old disintegrated, old non-disintegrated, and new non-disintegrated Portland cement. The tests show that the creep strains for old disintegrated and old non-disintegrated cement mortars are close, within a 2% margin of each other. However, the creep strains for new non-disintegrated cement mortar are 30% lower. Shrinkage for old disintegrated and non-disintegrated cement mortar is 20% lower than for new non-disintegrated cement mortar. The research shows that disintegration is a viable procedure to make old cement suitable for structural application from a long-term property standpoint. Additionally, it increases cement mortar compressive strength by 49% if the cement is disintegrated together with sand.

Modification of Variance-Based Sensitivity Indices for Stochastic Evaluation of Monitoring Measures

In complex engineering models, various uncertain parameters affect the computational results. Most of them can only be estimated or assumed quite generally. In such a context, measurements are interesting to determine the most decisive parameters accurately. While measurements can reduce parameters’ variance, structural monitoring might improve general assumptions on distributions and their characteristics. The decision on variables being measured often relies on experts’ practical experience. This paper introduces a method to stochastically estimate the potential benefits of measurements by modified sensitivity indices. They extend the established variance-based sensitivity indices originally suggested by Sobol’. They do not quantify the importance of a variable but the importance of its variance reduction. The numerical computation is presented and exemplified on a reference structure, a 50-year-old pre-stressed concrete bridge in Germany, where the prediction of the fatigue lifetime of the pre-stressing steel is of concern. Sensitivity evaluation yields six important parameters (e.g., shape of the S–N curve, temperature loads, creep, and shrinkage). However, taking into account individual monitoring measures and suited measurements identified by the modified sensitivity indices, creep and shrinkage, temperature loads, and the residual pre-strain of the tendons turn out to be most efficient. They grant the highest gains of accuracy with respect to the lifetime prediction.

PLAIN AND PVA FIBRE-REINFORCED GEOPOLYMER COMPACT TENSION SPECIMEN CRITICAL AREA SURFACE COMPOSITION ASSESSMENT

For more than 40 years, low calcium alkali-activated cement composite, or in other words, geopolymer, has been around. In recent years there has been increased interest in this material and its properties. It is mainly due to the claim that geopolymer is the cement of the future. This claim is based on environmental factors. For instance, the CO2 emissions for geopolymer binder can be up to 6 less than for Portland cement binder. Most of the researches regarding geopolymer composite properties examine only mechanical and long-term properties in compression. There has been a lack of long-term tests in tension due to difficulties in performing them. As the tensile stresses are an essential part of structure assessment, it is necessary to evaluate new material properties as thoroughly as possible. Due to the nature of geopolymer specimen hardening (polymerisation), there is a difference in modulus of elasticity development and shrinkage caused by binding that could have factors that regular Portland cement specimens do not.This article aims to evaluate the surface composition of plain and 1% PVA reinforced geopolymer compact tension specimens that have been subjected to creep and shrinkage tests. Specimen cross-section images were acquired using the scanning electron microscope (SEM). Using the quantitative image analysis method, amounts of cross-section composition elements are determined. Furthermore, the amount of cracks is determined and compared between plain and PVA fiber-reinforced specimens.It has been determined that even though 1% of PVA fibre-reinforced specimens have lower tensile strength, their creep and shrinkage strains are lower, and the number of microcracks at the notch base of the specimen. Still, it has to be acknowledged that the amount of air voids in all analysed specimens is relatively high.