Lightweight concrete has many applications and hence its significant to know its durability properties and resistance in different environmental conditions. That is why the durability of lightweight concrete is discussed in the following sections.
Lightweight concrete according to American code is defined as concrete that its air-dry density is smaller than 1810 Kg/m3.
The definition of lightweight concrete in other counties might slightly be different, for instance, according to Norway Code the concrete is assumed to be lightweight concrete if its saturated surface dry density is equal or greater than 1800 Kg/m3 and its compression strength should not surpass 85 MPa.
Lightweight concrete is produced by blending cement, water, fine aggregate, and lightweight coarse aggregate for instance clay materials, slate materials, and expanded shale.
Fig.1: Lightweight Concrete Cube
Fig.2: Lightweight Aggregate Concrete Construction
Factors Affecting Durability of Lightweight Concrete and its Remedies
Following are the different durability properties of lightweight concrete and its remedies:
- Freezing and thawing resistance
- Lightweight aggregate resistance against chemical attack
- Abrasion resistance
- Corrosion resistance of reinforced lightweight concrete
Freezing and Thawing Resistance of Lightweight Concrete
The resistance of lightweight concrete against freezing and thawing effect is based on number of factor such as types of aggregates, proportions of concrete mixture, aggregate moisture content, and the percentage of air entrainment.
It is demonstrated through tests that, the durability of non-air entrained lightweight concrete under freezing and thawing condition is better than then non-air entrained normal weight concrete, specifically if natural fines are utilized. This is true for most of lightweight aggregate regardless of whether the aggregate is in pre-soaked or air dried situation.
Regarding air entrained performance under freezing and thawing conditions, the ability of lightweight concrete that is produced with lightweight aggregate in air dry condition is considerably greater than that of normal weight concrete.
The performance of both lightweight concrete made with lightweight aggregate in pre-soaked concrete and normal weight concrete are similar and do not show significant differences.
Freezing and thawing resistance of high strength lightweight concrete, which its compressive strength ranges between 54 and 73 MPa, is exceptional and substantially great.
Lightweight Aggregate Resistance Against Chemical Attack
Lightweight coarse aggregates are not likely to react with alkalis, therefore, chemical attacks might not influence lightweight concrete the same way as normal weight concrete. Lightweight concrete matrix usually has high cement content a d low free water to cement ratio which make the penetration difficult.
Dense fines are applied in the production of the majority of structural grades of lightweight aggregate concrete, that is why such concrete need to be investigated for possible reactivity.
Abrasion Resistance of Lightweight Concrete
Lightweight concrete resistance against abrasion is based on strength, hardness, toughness characteristics of aggregate and cement paste, and the bond between aggregate and cement paste. The resistance of lightweight concrete is enhanced as the properties are improved.
Lightweight aggregate resistance to abrasion is decreased substantially if the aggregate particles are exposed. So, it is recommended to blend low density coarse aggregate and natural fine aggregate, to provide concrete surface protection through surface treatments, and enhance matrix quality.
It is reported that, the resistance of lightweight concrete used in the construction of bridge deck subjected to hundred million crossings of vehicles show similar resistance to that of normal weight concrete.
It is advised to undertake certain level of restrictions in commercial utilization where steel wheeled industrial vehicles are employed although surface protection is usually provided in such applications.
Carbonation of Lightweight Concrete
Carbonation is the reaction between calcium hydroxide produced because of cement hydration and carbon dioxide in the atmosphere. This will create calcium carbonate that lead to reduce alkalinity, which naturally protect embedded reinforcement from corrosion in concrete.
Concrete protection degradation may be anticipated through PH in concrete and it is detrimental when it is reduced to around 9 from 13, because the reinforcement protection will be eliminated and steel bars are considerably vulnerable to corrosion.
Most of lightweight aggregates are porous that makes lightweight concrete more previous and permits gas diffusion for instance carbon dioxide.
This problem can be tackled provided that a good aggregate distribution is obtained and continuous paths though particles to the steel reinforcement is avoided to decrease carbonation rate.
The ability of lightweight concrete to withstand carbonation can be improved by providing thick concrete cover and increase the amount of cement content. It is shown through tests that the depth of carbonation is quite small if the amount of cement content is larger than 350 Kg/m3.
Corrosion Resistance of Reinforced Lightweight Concrete
Concrete alkalinity provides a protective layer to steel reinforcement and avoids steel corrosion. The alkalinity of concrete can be increased by using large percentage of cement and it is recommended to be greater than 350 Kg/m3 otherwise the steel reinforcements will be corroded quickly.
Apart from using large amount of cement, it is highly advantageous and beneficiary to compact the concrete adequately because it contributes to the concrete resistance against carbonation ingression. Providing large concrete cover is another technique to decrease carbonation rate.