A-research within the field of concrete
By A-research, we mean systematic and methodical searching for new knowledge and new ideas that can conceivably form the basis for application. The A-research is open, which means that the research results are available to everyone. The main part of the Swedish Cement and Concrete Research Institute’s (CBI) A-research is financed by the Consortium for the Financing of Fundamental Concrete Research (the Consortium), and the government through the Institute’s strategic competence funds (SC funds). The Institute’s doctoral projects are also counted as A-research. Four doctoral projects are currently ongoing, two of which are part-financed by the Consortium.
The ongoing Consortium projects have been divided into two sub-areas by their reference group: sustainability (S) and production (P). Sustainability covers the projects on SCC’s durability, crushed aggregate concrete, nanocrete, CO2 cycle and three studies on new material. Production concerns different aspects of self-compacting concrete (SCC): simulation, variation stability (robustness), static stability (resistance to segregation) and production technology (field experiments), early hydration and concrete floors.
The purpose of the SCC projects is to increase the understanding of the performance of self-compacting concrete at the fresh stage and its durability in a Swedish environment. They examine salt frost resistance, resistance to chloride penetration and risks of what is known as thaumasite formation (a form of sulphate attack that can affect concrete containing limestone aggregate in case high sulphate levels are present) in self-compacting concrete containing limestone filler. Definite objectives at the fresh stage concern determining form pressure and developing tools for the analysis of forms and casting processes. The work with this project has so far resulted in one doctoral thesis. The project has identified SCC’s sensitivity to sub-material variations as an important research area and a project entitled Variation Stable SCC was begun during the autumn of 2006. The word “robustness” is used on an international basis. The overall objective of the latter-mentioned project is to increase the share of self-compacting concrete in Sweden. One of the factors that is judged to most affect (limit) the use thereof is variations in the concrete at the fresh stage, and the purpose of the project is thereby to determine the parameters and mechanisms that affect these variations. Also ongoing is a project concerning SCC’s static stability. Here, stability is used to mean the opposite of segregation. The literature study carried our earlier has studied factors that affect the stability. Experiments have been conducted and are currently analyzed.
In line with natural gravel being phased out and replaced by gravel from crushed rock, we need to develop means of optimally proportioning the concrete. This is where the Crushed Aggregate Concrete project comes in, which involves cooperation with several other players within the building sector. The reason for this is a greater understanding of the physical factors - particle form and size distribution - and surface chemical interactions that take place in cement-based suspensions such as micro mortar and concrete. The project was part of the national MinBaS programme that was completed during 2005 and, during 2006, also resulted in a licentiate thesis. It is now continuing in the MinBaS II programme and is also supported by the Swedish energy authority. The aim of this project is to increase the understanding of the physical and surface chemical interactions that take place in cement-based suspensions such as micro mortar and concrete. By physical characteristics we mean particle form and size distribution. By the surface chemical we primarily mean the interaction of the superplasticiser with the various constituents in the micro mortar, i.e. cement, filler and fine aggregate. Less expensive alternative analysis methodology and aggregate quality assurance methods are also needed in order to be able to produce a good concrete that can be used by the industry concerned.
The purpose of the Early Hydration project is to increase the understanding of the system that controls the early sustainability growth. In the long run, we want to create opportunities to differentiate between the business hours whose desired length depends on the distance between the ready mix plant and the casting site. Comprehensive experiments with different methods have been carried out with promising results. A series of experiments has been carried out using calorimetry with different salts to find out how they affect the course of hydration. These experiments showed that Ca, Sr and Ba salts, all of which can be part of the structure of C-S-H, reduced the business hours. Li-salts also have an accelerating effect. Anions such as chloride produced a more intensive acceleration period. A series of experiments has also shown that the addition of nanosilica (ultrafine silica) leads to both a reduction in the business hours and an acceleration of the hydration. The effect is accentuated by LiOH.
The floor project aims to find new solutions to avoid uncontrolled crack formation, which is one of the greatest problems with industrial concrete floors. A comprehensive state-of-the-art report was published in 2006 and also internationally in 2007. Various concepts such as shrinkage reducers, optimisation of the aggregate curve, shrinkage-compensating concrete, rockfill concrete and vacuum treatment have been identified as interesting to pursue. Material experiments with different concrete recipes are currently analyzed in which we are coordinating the study of factors that affect the risk of shrinkage cracks by measuring free shrinkage, restrained shrinkage (by ring tests), strength modulus of elasticity and tensile creep.
Ultraviolet light and titanium dioxide can have a self-cleaning effect on various types of surface. Air contaminants are principally broken down into carbon dioxide and water. The main aim of the Tiofield project is to develop a concrete with photocatalytic titanium dioxide that has self-cleaning and air-cleaning properties and to see how such a product can be implemented in concrete structures. In order to achieve these aims, several sub-objectives must be fulfilled. Some of these sub-objectives are to increase the understanding of the way in which photocatalytic titanium dioxide functions in the presence of cement, the way in which concrete characteristics are affected when titanium dioxide is used, and the way in which concrete containing titanium dioxide can be produced rationally.
Shotcrete is used primarily as rock reinforcement. The environment is moist, which means that many assume that the shrinkage can be disregarded. This is not the case, however. Shotcrete is essentially heavily prone to shrinkage due to a high cement content. It also appears as though the shrinkage has increased when there has been a changeover from water glass to alkali-free accelerators. Problems with shrinkage cracks arise primarily when shotcrete is sprayed on drains. CBI is studying the mechanisms that control hardening and shrinkage. The aim is to develop a shotcrete that shrinks less and that is less prone to shrinkage. The aids are fibres, shrinkage reducers and lower cement content.
The environmental issues are becoming more and more important in society as they are within the field of concrete. In cooperation with Cementa, the Swedish Environmental Research Institute and Lund Institute of Technology, the Swedish Cement and Concrete Research Institute (CBI) is running a project concerning the cycle of carbon dioxide in cement and concrete. The aim is to examine how much atmospheric carbon dioxide concrete recaptures upon carbonation. The project includes both chemical aspects of the carbonation of the various ingredients in concrete paste and quantitative measurements of the carbonation of actual concrete structures, particularly interior structures where measurement results are lacking. During 2008, three literature studies were carried out on concrete with lower sustainability, energy storage and self-healing concrete. Lower sustainability could be accepted as satisfactory for many purposes while savings in cement and other natural resources can also be enjoyed. Here, we study both the potential of this “15 MPa concrete” and how it can best be produced. Concrete is heavy and chemically inert, and it ought therefore to be possible to use concrete for energy storage and for balancing the temperature between day and night even more than we do today. Self-healing, closure, is the phenomenon whereby cracks in concrete can heal through different substances being added to the concrete, some of which are very sophisticated while others are simpler; but if it is possible to achieve the result through modest expenses, there are opportunities to extend the lifetime of the concrete structure and thereby save natural resources.