Durability characteristics of Light weight concrete

As with normal weight concrete, durability of the structural lightweight concrete is directly affected by its permeability. In general, concrete permeability is affected by many factors such as w/b ratio, cement type, curing, maturity of concrete, etc. The permeability of concrete as a whole is considerably higher than that of its components, namely the mortar matrix and coarse aggregates. According to Mehta (1986), this is mainly the result of micro cracks caused by the elastic mismatch between these components responding differently to temperature changes, service loads and volume changes due to chemical reactions within concrete (as cited in ACI Committee 213, 2003).

Light weight concrete

Due to similar rigidities of mortar matrix and lightweight coarse aggregate (elastic compatibility), there are reduced number of micro cracks observed in contact zone of lightweight concrete compared to that of normal weight concrete, which in turn results in lower permeability. In addition to this, hygrol equilibrium and pozzolanic reaction are two factors also contributing to improvement of the contact zone in lightweight concrete (ACI Committee 213, 2003). 

Hygrol equilibrium can be defined as a state at which aggregate surface and mortar matrix have similar water concentration. In normal weight concrete, mixing water accumulates on the surface of the dense aggregate (wall effect) and increases local water-cement ratio, causing porous matrix at the contact zone. In contrast, porous surface of lightweight aggregate allows water transfer and thus avoids accumulation of water on aggregate surface. Therefore, hygrol equilibrium is reached and formation of weak zones caused by differential water concentration are prevented.

The pozzolanic reaction between silica rich surface of lightweight aggregate and calcium hydroxide formed by the hydration of Portland cement increases the density and strength of the interfacial transition zone.

For all these reasons, the contact zone in lightweight concrete is superior to that of normal weight concrete (ACI Committee 213, 2003) and thus less permeable. It should also be remembered that pore system in lightweight aggregates is generally discontinuous, therefore porosity of lightweight aggregates does not influence the permeability of concrete (Neville, 2003).

Since the permeability of the structural lightweight concrete is low, its durability to aggressive chemical solutions is usually quite satisfactory (Mehta & Monteiro, 2006). Sulphate containing groundwater and chlorides in sea water are some examples of the aggressive chemical solutions. Seawater also contains sulphates; however, productions of sulphate attack are soluble in sea water due to presence of chlorides. Therefore, sulphates in seawater do not cause deleterious levels of expansion (Holm & Bremner, 2000).

Another durability concern for concretes is alkali-aggregate reaction. There is no reported case of deleterious alkali-aggregate reaction in lightweight concrete with natural or manufactured lightweight aggregate (Holm & Bremner, 2000). Nevertheless, ACI Committee 213 (2003) recommends testing of natural aggregates against any potential for alkali-aggregate reaction or having a record of satisfactory service history.

Freezing-thawing resistance of structural lightweight concrete is superior to that of normal weight concrete provided that aggregates are unsaturated before mixing (Neville & Brooks, 2010). This performance is generally attributed to the porous structure of lightweight aggregates which act as pressure relief zones for increasing hydraulic pressure as the water freezes (Harrison, Dewar, & Brown, 2001). Air entrainment is especially beneficial when aggregates are close to saturation. Air entrained lightweight concrete shows similar resistance against freezing-thawing action to that of air-entrained normal weight concrete (Mehta & Monteiro, 2006).

Jensen at al. (1995) reported that even relatively low temperatures between 100 and 300oC may cause significant amount of reduction in compressive strength and elastic modulus of high-strength lightweight concretes and added that spalling depends largely on the moisture content. They also suggested that inclusion of 0.1 to 0.2 percent polypropylene fibres in lightweight concrete mixture results in significant reduction of spalling for lightweight concretes (as cited in Holm & Bremner, 2000). Reduction in spalling is attributed to release of steam pressure through the conduits developed by the melting of the polypropylene fibres (ACI Committee 213, 200).