Johan Silfwerbrand

Estimation of Chloride Ingress in Uncracked and Cracked Concrete Using Measured Surface Concentrations

The service life of repaired concrete bridge decks with chloride-initiated corrosion is estimated using real environmental loads. Major field data are presented from measurements of the surface concentration (SC) of chloride ions in concrete overlays in deicing environments. The diffusion coefficients (DC) of the overlay and the old concrete were estimated with different methods, used thereafter with the measured SC and the measured concrete covers (lower 5% percentile), to predict the service life with numerical methods. Numerical methods were used due to the varying SC, and because the concrete overlay and the old concrete bridge deck had different DC, and because the composite deck could hardly be treated as a semi-infinite body. The effect of chloride ions that accumulated at the soffit had to be considered for thin bridge decks. Both uncracked and cracked concrete were considered. The effect of the SC type on the service life was limited. Stopping the use of deicing agents was found to be beneficial for up to 30 to 40 years. Cracks increased the chloride ingress rate, and the effect increased considerably with high DC in the old concrete.

The real service life and repair costs for bridge edge beams

Bridges rarely just break down. It is the different structural members of the bridge that slowly degrade the whole structure. One of the most damaged structural members of a bridge is the edge beam ...

Bonded Concrete Overlays – Research Needs

Bonded concrete overlays are frequently used as a repair method. Three special issues are connected to bonded overlays: (i) bond between substrate concrete and overlay, (ii) stresses due to differential shrinkage, and (iii) durability of the repaired structure. Good bond has an uppermost importance for obtaining monolithic behaviour and, consequently, maintaining the strength and stiffness of the original structure. A good bond also prevents salt and contaminated water to flow through the interface having access to both the substrate and the overlay. Absence of micro cracking, absence of laitance layer, cleanliness, compaction, and curing are the main factors affecting bond. Numerous filed tests show that these factors are more important than other factors, e.g., surface roughness, surface preparation, traffic vibrations, stress state, and environmental conditions. However, there are still questions on how some of these factors affect the bond and on the understanding of the phenomena behind. Differential shrinkage, i.e., shrinkage difference between the new-cast concrete overlay and the previously shrunk concrete substrate leads theoretically to the development of shrinkage stresses of such a magnitude that the overlay would crack. Field and laboratory tests show, however, that this cracking must not be inevitable. Two beneficial and crack-reducing factors are restraint less than complete and concrete creep. Numerous investigators have tried to develop models for determination of the stresses, but the “final” theory accepted by the entire concrete society is still missing. The durability of the repaired concrete structure is dependent on the durability of the substrate concrete, the durability of the overlay, and the durability of the bond. The two first aspects have been investigated thoroughly during the latest decades. Here, the focus is on the third aspect, i.e., the bond development. Whereas Swedish investigations show an increased bond with time, some North American studies show the opposite. These differences are analysed in the paper.

Bonded Concrete Overlays for Repairing Concrete Structures

Abstract: The bonded concrete overlay is an important and frequently used repair method. The aim is either to replace deteriorated concrete or to increase the cross-section and, hence, the load-carrying capacity. Since the bonded overlay and the substrate structure act monolithically, the overlay contributes substantially to both load-carrying capacity and structural stiffness. A prerequisite for obtaining monolithic action is to provide good bond between substrate and overlay. The measures start with the selection of removal method. The chapter discusses more than 20 factors influencing bond and identifies the most important ones. Based on these factors and some case studies presented, the chapter is finalized by providing recommendations for obtaining a good result.

Long term performance of water repellent treatment : water absorption tests of field objects in Stockholm

In Sweden and many other countries the chloride ingress from deicing salts and seawater often sets the limit of service life for an exposed structure. Water repellent agents, today mainly consisting of alkylalkoxysilanes, are often used on concrete to prolong the service life of the structure. This is accomplished by protecting the reinforced concrete from chloride ingress and/or by changing the moisture content inside. Several investigations and field experiments show a significant decrease in water absorption between samples treated with a water repellent agent and untreated samples. These experiments are in most cases carried out just a few weeks or months after the treatment but what happens after a few years? Should the treatment be repeated or is one single treatment enough? The water uptake of 27 different concrete structures treated with water repellents in Stockholm were tested by Nyman and Leon seven years ago to see if the water repellent effect was still there a few years after the treatment. It was the case in most of the structures but since no measurements were done right after the treatment it is not possible to say if there is an aging effect or not. In December 2006 new cores were drilled at the same places and the same testing procedure was repeated. The results indicate that most of the treatments are still functioning well.

Penetration Depth for Water Repellent Agents in Concrete as a Function of Humidity, Porosity and Time / Eindringtiefe von Hydrophobierungsmitteln in Beton in Abhängigkeit des Feuchtigkeitsgehaltes, der Porosität und der Zeit

Water repellent agents are used on concrete to change the conditions for moisture transport and fixation and thereby protect the concrete and the reinforcement bars from, e.g., chlorides, frost damage and alkali silica reactions. The most frequently discussed topic regarding water repellent treatments is the penetration depth of the agent. What is required and what is needed to achieve it? The effective penetration depth of a water repellent agent is defined as the distance from the surface to the sharp line between dry and wet concrete after it has been sprayed with water. During the past decade several papers have been published where factors having a major influence on the penetration depth for different water repellent agents have been investigated. The conclusions that can be drawn from these papers are that the three most important factors are time, porosity and degree of saturation. The time referred to is the duration of contact between the water repellent agent and the concrete surface. The porosity and degree of saturation refer to the concrete pore system and the amount of moisture inside the concrete at the time of the impregnation. There is, however, a lack of investigations quantifying the influence of these factors. This paper presents an empirical equation that gives an idea on how much these factors affect the efficient penetration depth of the water repellent agent. The equation is based on 300 new tests described and analysed in the paper. It is clear that the polymerisation rate of the water repellent agent affects the penetration depth. A fast reaction has the effect of slowing down the penetration.

Safety Levels in Concrete Slabs-on-Grade

Concrete pavements and industrial concrete floors are two examples on slabs-on-grade. None of them is considered as a load-carrying structure and is therefore not designed according to codes for st ...

Is Debonding in Concrete Pavements Unavoidable

Concrete overlays constitute one of the most frequent measures to repair and strengthen concrete bridge decks, concrete pavements, and industrial concrete floors. Good, secure, and durable bond between overlay and substrate provides the prerequisite for monolithic action and improves the durability. 30 years of Swedish research on bonded overlays thought beam tests, slab tests, and field measurements shows that a good bond can be obtained if some important demands are fulfilled. The Swedish research also includes long-term studies on repaired concrete bridge decks. These studies show that the bond is of the same magnitude ten years after concrete repair independent of structural system of the bridge, climate zone, traffic volume, or use of de-icing salt. Laboratory studies show that good bond is also able to resist fatigue loading. This paper summarizes these research activities and provides heavy arguments for answering the question in the heading negatively. Debonding is not unavoidable.

Reducing Crack Risk in Industrial Concrete Floors

Cracking and curling are two important problems in industrial concrete floors. In many practical cases, it is easier to design the concrete floor slab for mechanical loads than for shrinkage stress ...

Improving Preventive Bridge Maintenance

During the last decade, the Swedish Road Administration (SRA) has transferred resources from corrective to preventive bridge maintenance. Presently, 10 to 15 percent of the budget is devoted to pre ...