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Concrete and Concreting. >> >> concrete additives
- concrete Additives
- Admixtures added to a normal concrete mix either before or during the mixing process to alter the characteristics of the concrete.
The ancient Romans knew that by adding ox blood to their concrete they increased it's ability to resist frost. In the modern era the use of concrete additives has increased so much that what follows here is really just a very brief attempt to cover a complex subject. Note that for the most part any or all of the admixtures here work in mixes for concrete, mortar and in various grouts.
- Cement types
- Not really concrete additives, because they totally replace the normal Portland Cement in a mix, I go into more detail on these on the next page in this series.
- Cement Replacements
- Quite often either the whole of the cement content of a mix, or part of it is replaced by non cement based materials. Again not true admixtures so I have a separate page after the next one.
- Accelerators (Anti-freeze agents)
- These additives are designed to speed up the rate of hydration in the cement. This could be because of cold weather and the risk of freezing or that there may be a need for early initial strength to speed up the process and re-use formwork. Calcium chloride can be used in mass concrete, but because of it's potential to promote corrosion in rebar it has been replaced by various nitrates, sulphates, carbonates or urea and polyglycolester.
- Retarders
- Retarding admixtures are a must have where I live in the tropics. The hot conditions during concrete pours make them essential to control setting times. The slowing down process also reduces heat of hydration.
- Surface Retarders
- Not true admixtures, but rather added to the surface of poured concrete to slow down the rate of setting of the surface to allow for finishes like exposed aggregate. Applied with simple spray equipment they are easy to use. A cheap and effective alternative to store bought products is normal household sugar.
- Air Entraining Agents
- It has been found that the entrainment of a small amount of air into concrete, between 4% to 8%, gives better workability, reduced segregation, reduced permeability and increased freeze and thaw resistance. They also increase the workability of the mix. Must be used with caution as too much can cause strength loss in the concrete.
- Water Reducers
- Used to increase the strength of concrete without decreasing the slump or workability of the mix. Also used to increase workability without loss of strength.
- Plasticisers
- Chemical agents added to a mix to increase it's workability. these range from highly complex industrial products for concrete to a bricklayer or plasterer adding lime to a sand and cement mortar. There are also ranges of synthetic lime and upon occasion even humble washing up detergent can be used (but not recommended) to increase workability.
- Superplasticisers (High range water reducers)
- These are organic chemical admixtures. Developed in Japan and Germany in the 1960's. Like the water reducers they allow for high slump concrete to be as strong as a normal lower slump concrete. They are split into four groups.
- Sulfonated melamine-formaldehyde condensates (SMF)
- Sulfonated naphthalene-formaldehyde condensates (SNF)
- Modified lignosulfonates (MLS)
- Polycarboxylate derivatives
It would be fair to say that without Superplasticisers we would not be using concrete pumps to move and place concrete today. They also improve the ability to place and compact concrete in areas of heavy reinforcement or as corefill in hollow concrete block construction. In essence they allow the concrete to flow easily.
- Water Repellents or water proofing agents
- Even the best of normal concrete is porous and can absorb water. Additives based on stearic acid and other chemicals are said to be hydrophobic or water repelling. They coat the pores in the concrete and fill any voids with a water resistant layer.
There are many more concrete additives that apply not only to concrete but to to mortars and grouts. Furthermore many are used in combination to create concrete with even more different qualities.
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Adam Smith 1723-1790
"When we build, let us think that we build for ever."John Ruskin 1819-1900
