Concrete Admixtures
Table of Contents
What are Concrete Admixtures?
Concrete admixtures are commonly used in reinforced concrete elements. Concrete admixtures or “additives” are added during or before the mixing of the concrete constituents (water, cement and aggregates). The admixtures alter the mechanical properties of the concrete to suit the site requirements or obtain certain properties.
The use of concrete admixtures can help reduce the costs of concrete construction and can be used as a measure to prevent failure when mixing the concrete. The term “additive” and “agent” is sometimes used in place of admixture by companies or producers.
Admixtures in concrete is not a new concept and the addition of materials to change concrete properties have been used throughout history. The Romans experimented and used natural materials, such as blood, pig’s fat and bones. However, these combinations can lead to a wide range of concrete, which may be weak or strong; but there are Roman bridges and buildings that have been standing and used for over 2000 years.
Engineers have developed standardized mixes and proportions when mixing water, cement, aggregates, and limiting the cement types and cover to reinforcement when designing and placing concrete, through experimentation, trial and error methods.
Types of Concrete Admixtures
The types of concrete admixtures used in the concrete mix can be categorized in the list below, which is also described in BS EN 934-2 (Admixtures for concrete, mortar and grout).
- Normal plasticising and water-reducing admixtures
- Air-entraining admixtures
- Superplasticising/ high-range water-reducing admixtures
- Accelerating admixtures: set and hardening types
- Retarding and retarding plasticising admixtures
Air entraining admixtures
Air entraining admixtures are resins or surfactant blends that forms small stable bubbles in the concrete. This controlled bubble of air is below 1mm diameter with a high proportion below 0.3mm.
The following advantages of air entraining admixture include the following:
- Increased resistance to freeze-thaw degradation
- Improved workability of concrete mix
- Improved consistence of concrete
- Increased cohesion of mix (less bleeding and segregation after pouring of concrete)
Concrete that is located externally and exposed to rainfall is affected greatly by the effects of freeze thaw attack. The water freezes in the capillary voids of the concrete causing cracking and spalling. This kind of deterioration in concrete buildings such as external/exposed columns needs to be checked/inspected regularly to prevent any atmospheric exposure to the reinforcement, which will cause corrosion.
The entrained air bubbles acts a pressure relief by allowing the water to freeze and expand in these areas, minimizing any cracking or spalling.
Air entrainment in the concrete mix is required for all freeze-thaw classes. The minimum air content by volume is 4.0%, 4.5%, 5.5%, or 6% with aggregate sizes of 40mm, 20mm, 14mm, and 10mm respectively. Exposure classes XF3 and XF4 require freeze-thaw resisting aggregates.
It is important to note that the strength of concrete correlates to the percentage of air-entrainment in the concrete. Producing concrete with higher strengths becomes difficult as approximately, every 1% of additional air entrained, concrete strength will fall by around 5%.
Normal plasticising and water-reducing admixtures
Water-reducing admixtures are incorporated in concrete mixes to reduce the amount of water to achieve the required consistence class, and concrete workability without affecting the air content or setting of concrete. Water-reducing admixtures are called “plasticising admixtures” when they are used to increase the consistence or workability of concrete.
When the water-cement ratio is low in concrete, this leads to greater compressive strength. Water-reducing admixtures allows the concrete strength to be increased whilst maintaining workability.
The following benefits of normal plasticising and water-reducing admixtures are listed below:
- Increases workability of concrete
- Higher strengths for reduced water content
- Same strength at a reduced cement content
- Reduces bleeding in concrete
Superplasticising/ high-range water-reducing admixtures
Superplasticisers are admixtures which is used to reduce the amount of water in a concrete mix in ranges between 5.0% to 40.0%, which is a significant amount.
Superplasticising admixtures are used in the manufacture of high-strength concrete, ready mix concrete, shotcrete and high-performance concrete. The concrete admixture is absorbed onto the cement particles, lowering the interparticle attraction and producing a more uniform dispersion of cement grains, which reduces the amount of water needed to achieve a given paste viscosity.
The characteristics of concrete is determined by the type of superplasticising admixture applied in the concrete mix. The type of admixture used, is determined by the specific job and required outcome.
Naphthalenes – This allows strength increase and water reduction in concrete.
Melamines – This allows good early strength gains in concrete. However, it can cause the mix to loose workability and is prone to bleed and segregation.
Polycarboxylate ethers – This is the more expensive superplasticiser to the other types. This admixture allows the maximum amount of water reduction and early age strength development, whilst maintaining excellent fluidity and flow retention.
The following benefits are listed below:
- Allows very high workability for a given water content.
- Allows high water reduction
- Helps concrete in difficult applications (sprayed concrete in tunnels)
- High early age concrete
- High-performance concrete
- Durable concrete
Accelerating admixtures: set and hardening types
Accelerating admixtures are used to either increase the rate of hardening and early age strength of concrete. It is also used to accelerate the setting and hardening of concrete. This admixture can be used in combination with superplasticisers to reduce the water-cement ratio to increase the compressive strength.
The hardening effect from accelerating admixtures on concrete increases the strength after setting, and the stiffening of concrete refers to the concrete setting. The admixtures increases the rate of hydration allowing early age strength development, which is very important and needs to be considered in early design stages or project specific jobs.
The materials used for concrete accelerators include the following:
- Calcium chloride
- Calcium nitrate and nitrate
- Sodium nitrate and nitrite
- Sodium thiocyanate
- Calcium formate
The benefits of accelerating admixtures are listed below:
- High early strength of concrete.
- Normal setting time and strength development of concrete at low temperatures.
- Reduced setting time of concrete.
- Faster construction leasing to savings on labour costs and reduced work programme.
- Able to remove formwork and moulds around concrete earlier, saving costs on labour and time. The formwork can be removed early due to early age strength.
- The reduction of early heat hydration minimizes thermal cracking in concrete.
Concrete accelerators are useful in emergency situations where concrete needs to be repaired immediately (concrete with exposed reinforcement). This is also useful for structures that need to be used quickly or the setting of concrete is required before the next phase of a project is started (i.e. Tunnel construction).
Retarding and retarding plasticising admixtures
Retarding admixtures are a type of concrete admixtures, which only delays the setting of the cement, and does not affect the other mechanical properties of concrete. A lot of retarding admixtures are used in combination with superplasticisers or as multifunction admixtures.
This concrete admixture allows the fresh concrete mix to stay in a fresh state longer, before the mix hardens on site. This is useful when a design is complex and requires a complex placement sequence, or there is a special architectural finish and prevents cold joints forming in concrete.
Cold joints are an issue in large mass concrete pours, due to successive pours and the initial set may start hardening before the second pour. This prevents the concrete to act monolithically and the retarding agent can prevent these kinds of mistakes/issues on site.
The mechanism of retardation in a concrete mix can be understood in the following method below:
- The formation of silicate hydrates in concrete slows down as the admixture adsorbs strongly on the cement surface.
The following advantages of retarding agents are summarized below:
- Prevention of cold joints between concrete pours and complex placement sequence
- Extended setting time of concrete
- Allows longer transportation time of concrete mix (mix stays fresh)