Introducing chemical admixtures to concrete at the mixing stage enables some of the properties of the mix to be modified. The most common reasons for using admixtures in concrete are to increase workability without changing water content and to reduce water content without changing workability. This article examines how to achieve the best results.
Introducing chemical admixtures to concrete at the mixing stage enables some of the properties of the mix to be modified. The most common reasons for using admixtures in concrete are to increase workability without changing water content and to reduce water content without changing workability.
Antony Offenburg, Mapei’s product manager for admixtures, says chemical admixtures are generally used to enable better fluidity of the concrete at similar water contents, or to modify the concrete to allow for aggregate deficiencies.
He advises that the less amount of water used to provide a certain amount of workability, the higher the strength of that concrete. “The admixtures attach to the cement particles, and through a mechanism of electrostatic repulsion will provide its own fluidity though the cement particles and require less water to attain the required workability. This makes the concrete easier to move, place and compact, while providing higher strengths and better durability.”
Offenburg says if there is a lack of fine material in the aggregates, the use of rheology-modifying admixtures will compensate for this and allow good-quality concrete to be produced despite this physical problem. “Through the use of surface-retarding admixtures, very aesthetic concrete with exposed aggregates can be produced with relative ease.”
Today, ready-mix producers face even more demands from their customers, who are increasingly expecting concrete that is aesthetically pleasing, more sustainable, more durable, easier to place and safer to use.
Types of chemical admixtures
Admixtures are normally provided as water-based solutions and can be added to the concrete at up to 5% on cement weight, although many types are added at less than 2,0% and the majority at less than 1,0%. Concrete designers can use various types of admixtures to get different results in changing the properties of concrete.
When added to a concrete mix, plasticisers are absorbed on the surface of the binder particles, causing them to repel each other and de-flocculate. According to the Cement and Concrete Institute (C&CI), this results in improved workability and provides a more even distribution of the binder particles through the mix. “The main types of plasticisers are lignosulphonic acids and their salts, hydroxylated carboxylic acids and their salts, and modifications of both,” the institute states.
The typical dosage of a plasticiser varies from 200ml to 450ml per 100kg of cementitious material. Plasticisers usually increase the slump of concrete with the given water content. This admixture can reduce the water requirement of a concrete mix for a given workability, as a rule-of-thumb, by about 10%. The addition of a plasticiser makes it possible to achieve a given strength with lower cement content and may improve pumpability.
The C&CI says a number of plasticisers contain a retarder and can cause problems if overdosed. “While some plasticisers entrain varying amounts of air, others are reasonably consistent in the amount of air they entrain. Where plasticisers are used to increase workability, the shrinkage and creep will invariably be increased.”
These admixtures are chemically distinct from normal plasticisers and although their action is basically the same, it is more marked. “When these admixtures are used to produce flowing concrete, a rapid loss of workability can be expected and therefore they should be added just prior to placing.”
Super-plasticisers are usually chemical compounds such as sulphonated melamine formaldehyde, sulphonated naphthalene formaldehyde, modified lignosulphonates and polycarboxylate-based materials. The normal dosage of a super-plasticiser is between 750ml and 2 500ml per 100kg of cementitious material.
The C&CI states that this admixture is used to its best advantage in areas of congested reinforcement and where a self-levelling consistency facilitates placing. It also works well for high-strength concretes by decreasing the water and cement ratio as a result of reducing the water content by 15% to 25%.
The C&CI also says that special mixes must be designed for super-plasticisers and their use must be carefully controlled. “The effect of a super-plasticiser will last between 30 minutes to six hours, depending on the admixture used.”
An air-entraining agent introduces air in the form of minute bubbles distributed uniformly throughout the cement paste. “The main types include salts of wood resins, animal or vegetable fats and oils, and sulphonated hydrocarbons,” the C&CI says. The typical dosage for air-entraining agents is between 50ml and 150ml per 100kg of cementitious material.
This admixture can be used where improved resistance of hardened concrete to damage from freezing and thawing is required. It also delivers on improved workability, especially in harsh or lean mixes and reduces bleeding and segregation, especially when a mix lacks fines.
The C&CI adds that air entrainment may reduce the strength of concrete and overdosing can cause a major loss of strength. “As a general rule, 1% air may cause a strength loss of 5%. It is important that mixes should be specially designed for air entrainment and that the percentage of air entrained during construction should be monitored.”
Different types and sources of cement/cement extenders may result in the entrainment of different amounts of air for the same dose and mix proportions. “A change in cementitious content, in the grading or proportions of the fine fractions of sand will normally alter the volume of air entrained,” the C&CI continues.
The amount of air entrained may depend on the source and grading of sand in concrete. “Forced-action mixers entrain larger volumes of air than other types,” the institute says, adding that increasing ambient temperature tends to reduce the volume of air entrained. The use of ground-granulated blast-furnace slag and fly-ash tends to reduce the amount of air entrained.”
These admixtures speed up the chemical reaction of the cement and water, and thereby accelerate the rate of setting and/or early gain in strength of concrete. “Among the main types of accelerators are chloride-based, non-chloride based and shotcrete accelerators,” says the C&CI.
One should use 500ml to 2 000ml chloride-based accelerators per 100kg cementitious material. The dosage for non-chloride based accelerators is 500ml to 2 000ml per 100kg of cementitious material. Shotcrete accelerators react almost instantaneously, causing stiffening, rapid set and rapid hardening of the shotcrete.
The C&CI explains that where rapid setting and high early strengths are required, one can make use of accelerators. It is also an admixture that could be utilised effectively where rapid turnover of moulds or formwork is required and concreting takes place under very cold conditions.
All chloride-based accelerators promote corrosion of reinforcing steel and should not be used in reinforced concrete, water-retaining structures, pre-stressed concrete and steam-cured concrete.
The C&CI mentions that overdosing with these materials can cause instant setting of the concrete, resulting in equipment damage. “Accelerators work more effectively at lower ambient temperatures.”
These admixtures slow down the chemical reaction of the cement and water, leading to longer setting times and slower initial strength gain. “The most common retarders are hydroxylated carboxylic acids, lignins, sugar and some phosphates,” the C&CI says. Typical dosages for retarders are between 150ml and 500ml per 100kg cementitious material.
The C&CI states that admixture can be used when placing concrete in hot weather, particularly when the concrete is pumped. It is also good to add to the mix to prevent cold joints due to duration of placing and in concrete which has to be transported for a long time.
Also, retardation can last for days if a mix is overdosed beyond the limit recommended by the supplier. “Retarders often increase plastic shrinkage and plastic settlement cracking and a delayed addition of retarders can also result in extended retardation,” the institute says.
Innovation in the industry
In addition, Offenburg says innovation in the concrete sector is driven mainly by the cement producers and additive companies. “To date there has been significant development of new brands from all major cement companies to better serve the markets and product development is focused on the various different users of their cement.”
He adds that performance enhancement and the increased use of recycled materials used as extenders have allowed a reduced carbon footprint per kilogram of cement, and the additive companies play an important role in this process.
“Aggregate producers are also challenged to produce good-quality materials in an industry where environmental sustainability, cost control and end-user specification are their primary drivers.”
He also says the focus for the foreseeable future is a split between cost reduction and CO2 footprint, or a combination of these, to ensure both the environmental and financial sustainability of the concrete industry and the applications of the materials in varying scales, from the large construction companies to the smaller end-users.
Finally, a research paper named Structural concrete and sustainability, produced by Dr Graham Grieve and Santie Gouws, states that many different combinations of constituents are permitted to be used in South Africa.
According to the paper, extenders available commercially as separate products in this country are fly-ash, ground-granulated blast-furnace slag, corex slag and silica fume, all of which have to comply with the requirements of the relevant part of the SANS 1491 standards.
These cement extenders are all by-products from other industries, and contribute certain technical benefits to hardened concrete which can potentially be used to render concrete more environmentally sustainable.
Environmental sustainability comes about because these materials are derived from the waste streams of other industries, and only contribute a small amount of additional embedded energy through their processing and transport requirements. “The use of slag or fly-ash will therefore tend to significantly improve the sustainability of a concrete mix,” the paper states.
Structural concrete is typically proportioned to meet a 28-day characteristic strength required by the designer. “When cement extenders are used in a concrete mix, the general trend is for the rate of compressive strength gain to be slowed, and this effect is more pronounced as the extender content of the concrete increases as a proportion of the total binder content,” the research paper notes.
Full acknowledgement and thanks are given to the C&CI, Sarma, Aspasa, researchspace.csir.co.za and Mapei for the information given to write this article.