Erfarenheter från Södra Länken och andratunnelprojekt har visat på ett behov av bättrekontroll över krympegenskaper hos sprutbetong, främst för att undvika sprickbildningi samband med dräner för vatten- och frostsäkring. Två delprojekt har genomförts medsyfte att bättre förstå de grundläggande mekanismerna för krympning hos sprutbetong ochundersöka möjligheter att minska denna medbibehållen sprutbarhet. Effekten av fiberinblandning som kan bidra till att fördela krympningenpå flera fina sprickor och därmed undvika korrosion på stålfibrer undersöktes också. Undersökningarna visar att man i laboratorieskalakan åstadkomma en sprutbar betong med lägrecementinnehåll än vid vanlig proportionering,vilket begränsar krympningen. En begränsadförsöksserie visade att risken för sprickbildningmärkbart kan reduceras genom inblandning aven kombination av stål- och glasfibrer.
Continuous fibre wrapping technique is becoming a common repair and strengthening technique for concrete structures. In this method continuous fibre sheets are bonded to the surface of a concrete structure using organic resins. The main disadvantages of this method are: (i) the use of resins (organic compounds) which are hazardous for the manual worker, and (ii) the low permeability of this type of organic materials. These problems could be solved if a cement-based matrix is used instead of epoxy matrix. In the current development stage one of the limiting parameter in a cementitious carbon fibre composite is the bond between the fibres and the cementitious matrix. The main objective of the study presented in this paper was to improve the bond between the carbon fibres and the cementitious matrix. Two possible methods were studied: (i) the design of a complex binder based on Portland cement with polymer and silica fume additions and (ii) surface treatment of the carbon fibres prior to their casting in the cementitious composite.
This study investigates shrinkage of accelerated shotcrete (sprayed concrete), especially in the case of shotcrete sprayed on drains, a part of a tunnel lining not continuously bonded to the rock. One of the goals is to find methods of avoiding shotcrete shrinkage cracks in such drain structures. If cracks yet develop the crack distribution is of great importance, i.e. several fine cracks instead of one wide. By using both steel and glass fibres this may be achieved. A newly developed test set-up for shrinking, end-restrained shotcrete slabs is also presented and evaluated. The performed tests show that the addition of very fine glass fibres could be a solution to the cracking problem. The newly developed test equipment using concrete interacting with an instrumented granite slab represents a realistic way of testing restrained shrinkage. The on-going research focuses on the optimization of the glass fibre addition and the understanding of the interaction between shrinkage and creep of shotcrete.
This study investigated the shrinkage of shotcrete (sprayed concrete), especially the case of shotcrete sprayed on drains – a part of tunnel lining not continuously bonded to the rock. A newly developed method for testing the shrinkage of end-restrained shotcrete slabs is presented and evaluated. The test setup, which is designed to capture in situ behaviour, consists of shotcrete sprayed on an instrumented granite slab partly covered with a plastic sheet. The primary goal was to practically use and evaluate the test method with shotcreted test samples. Results from restrained shrinkage tests are presented along with results for free shrinkage. It is shown that the method realistically captures the behaviour of shotcrete drains on hard rock in situ. In addition, the corresponding compressive strength and flexural crack strength as functions of shotcrete age are also reported. The test method can be used for an evaluation of different solutions for avoiding shrinkage cracks in shotcreted soft drains, or in shotcrete that is fully bonded to a rock surface, with respect to preventing cracking or distributing the shrinkage strain into several fine cracks instead of one wide crack.
This study investigates early age bond strength of shotcrete (sprayed concrete), in the case of shotcrete sprayed on hard rock. Shotcrete differs from ordinary, cast concrete through the application technique and the addition of set accelerators which give immediate stiffening. The bond between shotcrete and rock is one of the most important properties. During the very first time after spraying the physical properties and the bond to the rock depend on the set accelerator and the micro structure that is formed. In this work a laboratory test method for measuring early bond strength for very young or early age shotcrete is presented. The newly developed method was tested and evaluated and proved that it can be used for bond strength testing already from a couple of hours after shotcreting.
Shotcrete is commonly used in Swedish tunnels. In this paper the author will discuss problems with curing, bond, residual strength, fibre content, spraying, quality control and testing, that have been encountered over the last 25 years. Some additional problems that have been observed include the following. Improper curing can cause detrimental cracking of a shotcrete lining. Poor cleaning can cause low bond strength between the shotcrete and rock. Poor control of the fibre content can cause low residual strengths. Optimistic spraying under difficult conditions might also lead to porous concrete. Quality control systems sometimes exist only in theory. The relevance of tests as performed is difficult to judge. These mistakes are costly and unnecessary since education and discipline can help to avoid most of them.
Shelters and some drifts in mines should be designed for rock burst induced by detonations. Absorption of released momentum is studied experimentally for both steel fibre and mesh reinforced shotcrete as well as rock bolts anchoring the shotcrete. Even if the shotcrete exhibits a bonding failure primarily, it cannot absorb a significant momentum because the failure mechanism is local. A rock bolt can absorb the momentum if it plasticizes over a reasonable length. A bolt design is demonstrated which gives the bolt such properties.
In this paper, laboratory tests to failure of ten large deep beams with I-shaped cross-sections are presented. All beams had the same geometry with a shear span-to-depth ratio of 1.25 but differed in the amount of the vertical and horizontal web reinforcement. The presented results from the measurements consist of load-deformation curves, crack widths and crack patterns and strain distribution near the supports. The ultimate loads for these beams have been calculated with two strut-and-tie models and one truss model. The first strut-and-tie model calculates the tensile contribution of both reinforcement and concrete and takes into account their influence on the principal tensile stress. The second strut-and-tie model is a modification of the first one where the stress distribution along the strut is redefined. The third method is the truss model that is incorporated in a Design Code. The truss model gave the best result for the beams with a higher reinforcement ratio that exhibited in a shear compressive failure. The diagonal tensile failure that occurred in the beams with a small amount of web reinforcement was best captured with the modified strut-and-tie model.
In this paper, analyses based on laboratory tests of ten large deep beams with I-shaped cross-sections loaded to failure are presented. All beams had the same geometry with a shear span-to-depth ratio of 1.25, but differed in the amount of the vertical and horizontal web reinforcement. All beam tests resulted in shear failure, either diagonal tensile failure or shear compressive failure, depending on the amount of reinforcement. The diagonal tensile failure is generally considered to be the most difficult failure to treat numerically. In this study different material models incorporated in commercial numerical analysis tools are studied. Material models based on fracture mechanics with either rotated or fixed crack directions as well as a plasticity-based model are used in the analyses. The analyses show that the plasticity-based model in Abaqus gives good agreement with the experiments regarding crack pattern, load-displacement response and estimated crack widths. The models based on fracture mechanics in Atena and Response tend to give too stiff behaviour in the load-displacement response, but generally give a good estimation of the load capacity. The analyses performed with Atena gave good estimations of the crack pattern, and the models with a fixed crack direction also gave good estimates of the crack width.
Sustainable development requirements in society are presenting specifically expressed challenges to building and construction. These challenges are recognised and met by the building and construction sector on an overall level, but only slowly penetrate into the discussions and operation of daily construction work. An international trend towards increased demands on service life planning, in Europe e.g. expressed by the Construction Products Directive, resulting in an adaptation of standards and codes might promote a change in the construction industry. The article presents the history and background of sustainable construction, pictures the development and state of relevant directives and standards, and couples sustainable construction to the trend towards performance based thinking in construction. The rapid development of IT-based life cycle design and management tools is briefly commented. Is the building and construction sector mature enough to meet the challenges and to take this road to the future? There are market arguments and business possibilities, but also a daily practice providing examples of counteraction
One of the most common deterioration mechanisms in concrete structures is reinforcement corrosion caused by chlorides. An often used repair strategy is to remove the damaged concrete and sometimes also undamaged concrete and replace with a repair concrete. The chloride contaminated undamaged concrete and the repair concrete have to be compatible in order to achieve a durable system. This laboratory study has investigated 13-year-old reinforced concrete specimens with both substrate concrete with mixed-in chlorides and an initially chloride free repair concrete. The main objective was to study chloride transport from the contaminated substrate concrete into the repair concrete and establish chloride profiles across the interfacial region and interfacial zone between the two materials. Another objective was to evaluate the location of reinforcement corrosion in the interfacial zone, in the substrate concrete and in the repair concrete. The main results from this laboratory investigation show that reinforcement corrosion occurs in and near the interfacial zone between chloride contaminated and repair concrete. It was found that the corrosion occurs in local areas with passive steel areas between, i.e. macrocell corrosion. The chlorides are transported from the contaminated substrate concrete into the repair concrete. This investigation indicates that there is a risk for reinforcement corrosion around a patch repair when the substrate concrete has chloride contents exceeding 1.0 wt% by weight of cement.