Choosing the Right Admixture for Ready-Mix Concrete

Ready-mix concrete continues to be one of the strongest and most durable materials available for horizontal surfaces, such as parking lots and driveways.  Further, on-site mixed concrete is difficult to regulate since it isn’t made in controlled conditions, like that of a ready-mix plant.

On the other hand, choosing the right admixture for your ready-mix concrete recipe is critical for producing long-lasting concrete. Depending on the conditions in which you’ll be working with the concrete and the function the concrete will be serving, your choices for admixtures will narrow.

Admixtures are materials added to mix designs other than water, aggregate, hydraulic cement and reinforcing additives. Admixtures are added to batches right before mixing or while mixing occurs. Depending on the admixture chosen, the characteristics of the concrete will change.

Retarding admixtures

Often made of organic and inorganic materials, retarders serve to slow concrete’s setting rate. Materials used in retarders include calcium, sodium, ammonium, a host of acids, phosphates, fluorites and magnesium salts, among others.

Retarding admixtures offer a variety of benefits:

• Stronger than plain concrete
• Makes up for heat’s effect on concrete setting times
• Some types offer water-reducing capacity

Super plasticizers

Also known as high range water-reducing admixtures, these admixtures serve to increase concrete’s workability by raising the slump. While this type of admixture can be costlier than others, it enhances concrete in ways other admixtures cannot.

Mix designs using this admixture have several advantages:

• Shrinkage is not affected
• Usable in high doses
• Can resist freeze and thaw cycles

Air-entraining admixtures

While air bubbles may not seem ideal for concrete, there are many instances where highly porous hardened concrete is the perfect material for construction projects.

Use air-entraining admixtures if you need concrete with the following properties:

• Resists the stress of water in cold temperatures
• Resists drying and wetting cycles
• Good for quick repairs

Air-entraining admixtures are also compatible with most concrete admixtures.

Water-reducing admixtures

If you need stronger concrete or have little access to water, these admixtures reduce the water-cement ratio.

Water-reducing admixtures have the following characteristics:

• Require less water
• Workability is increased without strength loss
• Cement has lower CO₂ emissions

Accelerating admixtures

If the job requires concrete to set exponentially fast and offer higher strength in early stages, accelerating admixtures are the best additions to a cement mixture. Accelerating admixtures come with a downside: Steel reinforcements could corrode faster. This downside is dependent on the recipe as a whole, however.

Top benefits of accelerating admixtures:

• Increases the rate of early concrete strength development
• Useful for working with concrete in winter weather
• Increases the setting time of concrete

Corrosion-inhibiting admixtures

These admixtures are designed to slow corrosion of steel reinforcements in concrete. Further, concrete using corrosion-inhibiting admixtures does not require as much maintenance as concrete without it. This is a popular admixture for precast concrete applications, though there are many instances when it is useful for ready-mix concrete projects.

Ready-mix ordering tips

Finally, once you’ve decided on an admixture or you need more information before picking an admixture, you’ll speak with a representative of the ready-mix plant you’ve chosen. To ensure your end product is high quality, make sure you discuss these topics:

• How far the jobsite is from the ready-mix plant
• Where the mixing trucks should be on the jobsite
• What the job is like: incline, decline, basement area, flat space
• What the environment is like

Also, for great concrete mixes, you may want to make sure the plant you’ve chosen to work with is using a Rapid twin shaft mixer for their ready-mix concrete. These are currently the best mixers on the market and help ensure the end product is high quality and to design standards.