Air in concrete

1 What types of concrete products contain air?
2 Why trap air in concrete?
3 How is the air trapped into the concrete ?
- 3.1 Air entraining agents
- 3.2 Foaming agents
4 Related articles on Designing Buildings

[edit] What types of concrete products contain air?

There are many different concrete products that contain air, these vary in the formulas used, how the products are processed, how much air and how it is arranged. There are many terms, sometimes describing the same or similar products as well as variations designed to achieve different performance criteria, and some are given below;

Standard concrete
Aerated concrete (AC) or non-autoclaved aerated concrete (NAAC)
Autoclaved aerated concrete (AAC)
Reinforced autoclaved aerated concrete (RAAC)
Cellular concrete (CC)
Aerated cellular concrete (ACC)
Aerated lightweight concrete (ALC)
Foamed concrete.
Lightweight blocks.
Thermal blocks.
Breeze blocks.
Insulation blocks.
Aircrete.

[edit] Why trap air in concrete?

[edit] Standard concrete

Concrete compaction is an important part of pouring concrete, using tools such as a concrete vibrator helps agitate the mix in order to release excess trapped air. A general rule of thumb for a standard concrete mix is that strength will reduce by 6% for every 1% increase in trapped air. Despite agitation a standard concrete pour will normally contain around 1% air.

[edit] Entrapped air

When the air content within concrete is about 1% - 3% bubbles are relatively large in size, over around 1mm and randomly located throughout the mix. This is referred to as entrapped air, in most cases an unintended consequence of mixing concrete, with higher percentages resulting from insufficient agitation when the mix is poured. So if the strength of concrete decreases with air, why would a contractor or manufacturers want to trap air?

[edit] Entrained air

Concrete that contains uniformly distributed microscopic bubbles of less than 1% throughout, can be beneficial to the final product. This type of trapped air is called air entrainment and usually makes up between 3%-8% by volume of a standard finished concrete product, that may also contain aggregates This entrained air can can help improve the performance of concrete in a number of ways.

[edit] Even distribution

A certain amount of regular bubbles within a concrete mix increases the surface area of concrete (within the pour) which can help maintain the distribution of moisture more evenly throughout the concrete, which reduces the amount and the impacts of what is known as bleeding. Bleeding is where water within a concrete pour slowly rises to the surface, because it is the lightest of the materials (compared to sand, cement and aggregate). The higher the amount of bleeding the less evenly distributed the water is through out the pour and the less consistent the final product will be, increasing the surface area of concrete throughout the mix with entrained air reduces the bleed and keeps water evenly distributed throughout the mix so it dries more evenly and results in a more consistent final product.

[edit] Freeze thaw

Concrete installations in climates that at certain times of the year are prone to freeze may suffer from the freeze thaw effect as with other natural stones or rocks, which can over time lead to failure. Air entrainment can be beneficial in these climates because the tiny air bubbles distributed evenly throughout the concrete can act as expansion vessels during freezing temperatures when moisture absorbed during wet periods expands. Freeze thaw is a natural phenomenon that under normal conditions gradually creates cracks, splits and fault lines in natural rocks as water freezes, expands and then thaws, the same applies to concrete. The impacts of this can be significantly reduced through air entrainment thus improving the performance of concrete over time.

[edit] Workability

Finally because entrained air creates a more regular, even distribution of air throughout a mix the slurry is said to be more workable. It is also worth noting agitation is also required for entrained air concrete as it is poured as ready mixed products can contain higher percentages of air, particular in transportation. Furthermore changes in temperature can also impact the air content of a pre-prepared or ready mixed cement slurries with indications being that an increase of 30 degrees might decrease air content by up to 25%, whilst temperature drops of the same can increase the air volume by up to 40%. These levels might then be reduced or balanced by agitation as the cement is poured in its final location.

[edit] Cellular air

When the air trapped within cement increases significantly, with mixes containing anything above 25% and up to around 85% air by volume, the strength of concrete is reduced but other characteristics develop, which can be advantageous. This type of concrete is usually referred to as cellular or aerated concrete, and can be formed into lightweight, insulative, thermal or fire blocking products such as blocks or panels, but also used on bespoke formwork. Importantly the ingredients of this type of concrete differ from standard concrete, and it does not contain aggregate or only fine aggregates such as fine sand. The main advantages of higher levels of trapped air in concrete mixes are described below.

[edit] Material efficiency

Because up to 80% of the volume of cellular or aerated concrete is air, it significantly increases the volume of material achieved per gram of material invested. This can have environmental benefits both in terms of the carbon emissions associated with Portland cement because less is used, but also in relation to the negative impacts of sand mining for the same reason. These aspects can also impact costs efficiencies.

[edit] Thermal performance

The air cells within the concrete improve the thermal resistance of concrete, so much that they have some insulative properties. The thermal performance increases as the air percentage increases, which will also impact the strength of the block which will decrease. One other advantage is that such concrete products can retain certain levels of thermal mass as well as having insulative qualities which can help regulate the temperature of internal environments.

[edit] Weight and use

As cellular concrete is lighter it can reduce transport costs as well as having site safety benefits in being easier to work with. There is also some evidence to show that because of the lighter weight, buildings constructed in seismic areas will perform better under earthquake conditions as seismic transmission is reduce because of the weight difference. Because of the weight, larger blocks or panels can be used, which speeds the building process up.

[edit] Acoustic performance

In a similar way to thermal performance the closed cells of air within the concrete can have sound reducing qualities.

[edit] Fire resistance

The fire performance of any concrete product, including cellular concrete is generally good, with over 200mins for a 100mm wall and over double for a wall three times as thick.

[edit] Permeability

Because the many independent small bubbles or pores, are closed, water absorption and moisture conduction are generally slow, whilst the finish faces can easily be plastered, painted or tiled to act as a further seal.

[edit] How is the air trapped into the concrete ?

Admixtures are ingredients that are are added to a concrete mixture whilst it is being prepared and before pouring, as opposed to additives which are added to the cement when it is being manufactured. There are many different admixtures used in the concrete industry to perform different tasks, for example:

Water reducing admixtures come as standard, mid-range plasticisers (Calcium, sodium and ammonium lignosulphonates) and super plasticisers (poly carboxylate, multicarbovylatethers), reduce the water demand by 10% to 30%.
Retarding admixtures (or retarding plasticicers) slow down the rate of hydration such as calcium sulphate, gypsum, starch, cellulose, common sugars, and salts of acids.
Accelerating admixtures speed up the setting time of concrete, calcium chloride is one of the most common but they also include triethenolamine, calcium formate, silica fume, fine silica gels.
Pozzolanic admixtures are sometimes used to reduce cementitious material in concrete and can help reduce risks associated with certain unwanted chemical reactions such as sulfate attack, leaching and alkali aggregate reactions. Such admixtures can be artificial (fly ash, blast furnace slag, silica fume and rice husk ashes) or naturally occurring (clays, shale, volcanic tuffs, pumicite).

The admixtures above also tend to increase the air content within concrete as a side effect, except for accelerators and pozzolans. There is a long list of other admixtures used for damp proofing, colour adjustment and to combat specific durability issues and attacks including fungal growth, pests and corrosion, as well as to improve bonding. Air detraining additives are designed to help remove excess air from concrete, such as water insoluble alcohols, phosphate and silicones.

[edit] Air entraining agents

Air entraining admixtures help to increase air content, many of which are referred to as surfactants, which stands for surface active agents, and have a water repelling or hydrophobic chain (non-polar hydrocarbon) and water attractive or hydrophilic chain (anionic polar). Formally there are two main classifications of air-entraining admixtures which are wood-derived acid salts (vinsol resins and wood rosins) and synthetic resins (such as foam). Essentially these are either added to the initial mix and create gas pockets through the chemical reactions that occur as water is added and the concrete hardens, known as air entraining agents or the prepared slurry is mixed with them to introduce air pockets which might be called foaming agents (although some air entraining agents are also referred to as foaming agents).

One example of a foaming agent added to the initial mix is aluminium powder, it reacts with the calcium hydroxide formed on the hydration of cement to produce hydrogen gas bubbles. The agent is mixed with a fine aggregate (usually sand or fly ash), cement, lime, gypsum, and water and reacts on hydration creating air pockets. These types of air entrained concretes are normally referred to as cellular concrete or aerated concrete (AC). Variation on these types of products include autoclaved aerated concrete (AAC) which is essentially the same but once poured and setting, it is placed in an autoclave which applies steam and pressure, this speeds up the curing process and creates a stronger product. Another variation is reinforced autoclaved aerated concrete (AAC) which includes some form of reinforcement to improve the tensile strength of the product and always larger unit sizes to be formed.

[edit] Foaming agents

Whilst some foaming agents are used at the initial stages of the mix and create air pockets through chemical reactions during curing, foaming agents that produce foamed concrete (also called aircrete) tend to refer to a slightly different process. In many ways simpler but perhaps less controlled this process involves making a slurry with all the normal ingredients (minus larger aggregates) and then creating a stable foam separately containing air pockets, these two elements ae then mixed to create a foamed slurry mix. when this mix sets evenly the pockets of air incorporated into the foam become part of the concrete. Such products can be made with industrial foaming agents and foam tools but they can also be made with standard household foam liquids such as detergents and washing up liquid.