The durability of concrete products

There are many examples of pure lime mortar keeping its functional properties for 2000 to 3000 years, but there are also examples of Portland cement mortars that have crumbled within 10 years (Grunau, 1980). Some concrete buildings with Portland cement have stood undamaged for over 100 years.

Durability is clearly dependent on the quality of both workmanship and raw materials, as well as the proportions of the mix and the environment of the building. In recent years it has become evident that certain types of air pollution accelerates the decomposition of concrete. Carbon dioxide and sulphur dioxide, both of which occur in high concentrations around industrial areas and towns, are particularly damaging.

It has been proved that carbon dioxide can carbonize up to 40 mm into concrete. This means that part of the carbon dioxide originally released in the production of the cements is reabsorbed (carbonata-tion). But at the same time, concrete loses its alkaline properties as a result and reinforcements of steel can be subject to corrosion. The next phase of breakdown usually occurs quite quickly, and involves the slow loss of the concrete itself. A wide survey of the Finnish building stock revealed a series of cases where concrete buildings had to undergo extensive repairs within less than 10 years of their completion as a result of carbonatation and insufficient frost resistance (Lahdensivu etal., 2007). In the USA, one bridge per day is demolished as a result of such processes.

Much of today's concrete contains organic additives, and these types of concrete break down even more quickly. Mortars with organic resins have been seen to decay within two to four years (Grunau, 1980).

Geopolymeric cement has good resistance to aggressive urban air, and the majority of Portland pozzolana concrete mixes have a greater resistance to pollution than pure Portland concrete. There is no long-term experience of how lime sandstone and sulphur concrete last. The same can be said for lime concrete, which is seldom used in northern countries.

Concrete can be protected through construction detailing. The most important rule of thumb is to avoid details that are continually exposed to rain. This includes exposed horizontal surfaces, where soot and other polluting particles will settle as well, later to be washed down over the adjacent facades.

13.2.3 Recycling

13.7

Norwegian re-usable foundation system in concrete units. All the components are standardized and locked together internally with grooves and bolts. During demolition, the ties and pillars are lifted up, leaving only the bases of the pillars standing in the ground. The rest is quality-controlled on site and then transported directly to a new building site. Gaia Lista, 1996.

13.7

Norwegian re-usable foundation system in concrete units. All the components are standardized and locked together internally with grooves and bolts. During demolition, the ties and pillars are lifted up, leaving only the bases of the pillars standing in the ground. The rest is quality-controlled on site and then transported directly to a new building site. Gaia Lista, 1996.

The value of in situ concrete in terms of recycling is low. It can, however, be crushed and ground to aggregate. The majority has to be sorted and used as landfill. In theory, steel can also be recycled from reinforcement, though this is a complex process using machines for crushing the concrete, electromagnets for separating, etc. Until 1950, smooth circular steel bars were used which were much easier to remove from concrete. Fibre reinforcement has no recycling potential.

Constructions consisting of prefabricated components such as blocks and slabs have considerably better recycling possibilities. By using mechanical fixings or mortar joints that make easy dismantling possible, the whole component can be re-used.

The mortar used for construction with concrete blocks is usually produced with strong Portland cement. The construction is, therefore, very difficult to disassemble without destroying the blocks. Alternatives are the different lime mortars, mainly based on hydraulic lime. In some cases, weaker mortar may require compensation in terms of reinforcement. The end connections of larger concrete units like slabs, beams and columns are usually grouted. Floor slabs are often covered with a concrete topping or a cement screed. These constructions should be avoided and substituted with bolted connections, which make dismantling a lot easier without the risk of damaging the elements (Figure 13.7). Beams, columns and slabs reinforced with bars or wires cannot usually be cut and refitted in the same way as steel structures, and should therefore initially be produced in standardized dimensions. It is imperative that all steel reinforcement is well-covered by the concrete, to prevent corrosion. When recycling older concrete elements, one must analyse the material since it may contain asbestos fibres or even PCB which at one stage was used as additive in many concrete mixtures.

In Denmark and Sweden there are many examples of industrial and agricultural buildings almost entirely built of recycled concrete units. In former East Germany, prefabricated elements from large apartment blocks have been shown to be re-usable (Asam, 2007). For many years, the Netherlands government has subsidized development of re-usable concrete constructions and this has led to a number of new structural solutions.

Sulphur concrete can be melted back to its original state, and aggregate can be removed by sieving and possibly be re-used.

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