Date: January 1, 2010

Guide to Admixtures

Below are the most commonly used admixtures, there are many others that can enhance concrete properties. Do not hesitate to consult with us to find out which admixtures are appropriate for your next job.
 
Air-Entraining Admixtures
 
These produce microscopic air bubbles when the concrete is mixed.
 
Advantages:
 
  • Improve the concrete’s resistance to damage caused by freeze-thaw cycles and deicing salts.
  • Improve workability
  • Reduce bleed water
  • Reduce the segregation of concrete mixtures 
Keep in Mind:
 
  • For exterior flatwork that is subject to freeze-thaw cycles or to deicer salt applications, specify a normal air content of 4% to 7% of the concrete volume depending on the size of the coarse aggregate—the smaller the aggregate, the more air entrainment is needed.
  • Air entrainment should be avoided for concrete flatwork that will have a smooth troweled finish.
  • In high cement content concretes, entrained air will reduce strength by about 5% for each 1% of air added. In low cement concretes, adding air has less effect and may even make the concrete stronger.
Water Reducers
 
These admixtures disperse the cement particles for a more efficient use of cement.
 
Advantages:
 
  • Lower the water content in the concrete while increasing the strength
  • Allow for a higher slump without adding water
  • Make concrete easier to pump
  • Offset  water demand in hot weather
  • Improve the finishability of concrete flatwork
Retarders
 
These admixtures delay the initial setting of concrete by an hour or more,
 
Advantages:
 
  • Counter the rapid setting caused by high temperatures
  • Allow more time for placing and finishing 
  • Sometimes function as water reducers
Accelerators
 
These admixtures reduce the initial set time of concrete and give higher early strength.
 
Advantages:
 
  • Make fresh concrete stronger to resist damage by freezing in cold weather
  • Used in fast track construction requiring early form removal or a quick opening to traffic.
  • Chloride vs. non chloride
Keep in Mind:
 
Calcium chloride is an effective and economical accelerator, available as a liquid or in flake form. Very limited use for reinforced concrete because the chloride can corrode the reinforcing steel.
 
High Range Water-Reducers (HRWR)
 
  • Often called superplasticizers
  • Reduce the water content between 12 and 25%.
  • Increase strength and reduce permeability of concrete by , or increase the slump to produce “flowing” concrete without adding water
  • Essential for high strength and high performance concrete mixtures containing silica fume.
  • Can sometimes be added on the job site
  • Sometimes wears off in 30 to 45 minutes