Concrete protection

Deterioration of concrete structures in aggressive environments is a major problem today. Coating the surfaces, and thereby leaving the risk of cracks in the coating, has until now solved the problems with the durability of concrete structures.

A new concept now improves the ability of the concrete to resist the ingress in aggressive elements. It deals with the problems instead of only treating the symptoms.

Road constructions are increasingly exposed to chloride ingress as a result of the steadily increasing use of de-icing salts. A weak concrete cover allows ingress of chloride ions causing corrosion of the steel reinforcement. As a result of the corrosion, the steel expands, causing spalling of the concrete structure. Enormous sums are spent on repairing concrete structures, which have been damaged as a result of the environment they are exposed to.

Insufficient focus on the importance of the concrete cover is often the main reason for deterioration of a concrete structure. The concrete cover is the outer centimeter of a construction that protects the steel reinforcement bars from aggressive elements slowly diffusing through the concrete. However, research shows that the concrete cover is often of a lower quality than the rest of the construction because the concrete surface is cast against impermeable formwork.

With the introduction of the Controlled Permeability Formwork (CPF) liner concept, focus is directed to the concrete cover. A CPF liner consists of a filter and a drainage layer. The filter faces the freshly placed concrete and the drainage layer faces the formwork. The liner is either tensioned or glued on the formwork before casting.

The concept was invented in Japan and later adopted by European manufacturers. Among the leading manufacturers of CPF in Europe are Fibertex A/S of Denmark, which markets the product under the name Formtex, and DuPont of Luxembourg.

It is internationally acknowledged fact that the lower the water/cement ratio, the stronger, denser and more penetration-resistant is the concrete.

The function of the non-woven CPF liner is to drain surplus water and air from the surface of freshly placed concrete. When water is drained off the surface, the water/cement ratio in the concrete surface is reduced and this improves the strength and durability of the concrete cover.

Several independent test reports have documented the effect of using CPF when casting concrete. CPF form liners have found a variety of uses in the European and Middle East markets. The primary usage area is concrete structures placed in aggressive environments such as road constructions, parapets, drinking water tanks, waste-water tanks, dams and marine constructions.

The CPF liner concept is gaining ground and usage is increasing steadily.

The available documentation material supports the concept as a sound alternative to the more expensive surface treatment in achieving an acceptable lifetime in aggressive environments.

Innovative backfill for bridge

New research by the Transport Research Laboratory has identified the loading requirements and investigated various compressible materials which may be suitable for use as innovative structural backfill behind integral bridge abutments.

Investigations have confirmed, that most bridge deck expansion joints leak and contribute more than any other factor to corrosion of the deck by de-icing salts. For this reason joint-free integral bridges are more durable and cheaper to maintain.

However, thermal expansion and contraction of an integral deck may lead to the development of very high soil pressures behind the abutments. Traditionally thorough compaction of high quality granular backfill has been used behind bridge abutments to avoid settlement of the carriageway. In an integral bridge, however, better quality backfill accentuates the risk of high soil pressures developing.

Integral bridge design has therefore to accommodate or avoid the high forces and bending moments that may develop in the structure. One method of avoiding high soil pressures on the abutments is to use an elastic cushioning layer, thus allowing more economical design of new integral bridges. In addition, the method may also provide an economical conversion of existing conventional bridges into integral structures as part of the need to reduce long term maintenance costs. The TRL has also developed a laboratory test to quantify the capillary suction properties of concrete.

Corrosion of reinforcement caused by chloride penetration of concrete cover is a problem which affects many bridges worldwide. In many cases the corrosion starts much earlier in the life of a bridge than expected. This suggests that factors other than the historically accepted properties of concrete strength and permeability need to be taken into account.

Research on chloride ingress into structural concrete, carried out for a number of years at TRL, has shown that capillary suction is an important mechanism by which chloride is absorbed. The absorption is fast-approximately one million times faster than the movement of chloride through concrete by permeability processes. TRL say current guidance on the design of concrete mixes for durability does not take capillary suction into account. The test procedure uses cubes of concrete placed on foam, saturated with either water or salt solution, and the quantity of liquid absorbed is determined by measuring the weight change of the cubes. In order to reproduce, to some extent, the conditions experienced on site, the cubes are subjected to a cyclic wetting and drying regime, rather than to a continuous period of wetting.

Distances of penetration of the liquid can be calculated if the porosity of the concrete is known, and chloride concentrations are determined by drilling the cubes to different depths and analyzing the drilling dust for chloride content. Research aimed at reducing the time taken to carry out a test is currently in progress.