Concrete 186. Concrete is mainly used for foundations in transitional shelter and is often unreinforced. Concrete blocks are also commonly used for walling and are sometimes fixed with a light mortar to enable their reuse, resale or recycling. 187. Concrete results from the mixture of cement, sand, gravel and water. The gravel and sand aggregates are held together by cement, which is cured through a chemical reaction with water, achieving 80% strength within 30 days when cured correctly. 188. Table 5.1 shows commonly used concrete mix ratios by volume. Table 5.1 Concrete mix ratios
Table 5.1 Concrete mix ratios
Floor
Cement
Sand
Fine sand
Aggregate
1
2
0
4
Beams, columns
1
2
0
3
Waterproofing
4
5
1
10
Adapted from van Lengen, J., 2008. The Barefoot Architect: A Handbook for Green Building. Reinforced concrete
189. Concrete has an excellent capacity to withstand compressive forces, however, it has no tensile strength. This makes it particularly appropriate for use in columns but potentially problematic when used in beams or slabs due to the tension that occurs on the underside of these members. This may lead to severe complications, which could cause the concrete to crack or even break. To give concrete some tensile strength it is necessary to reinforce the concrete by using steel bars or meshes for example. The concrete is poured around the steel reinforcement and after curing, the concrete and reinforcement form a composite material with both compressive and tensile strength.
Prefabricated concrete building elements
190. The main use of prefabricated concrete in transitional shelter is in columns as part of a frame structure, used in areas where timber is difficult to source sustainably. Concrete building elements can also be prefabricated, rather than being cast in situ. These are faster to use and may be cheaper, however, considerable care must be taken with jointing. Prefabricated concrete is rarely appropriate in seismic areas. Adapted from MSF Spain, 2011. Small Constructions Manual, p.39.
! Attention: Steel and salt Beware of the need to protect steel elements in reinforced concrete against the effects of water salinity, especially in coastal areas. 177