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Concrete & Masonryaka: concrete additiveaka: chemical admixture

Admixture

In Plain English

A chemical added to concrete mix to change how it behaves — making it set faster, slower, easier to pour, or stronger.

Definition

A material other than water, aggregates, cementitious materials, and fiber reinforcement that is added to concrete before or during mixing to modify its properties. Admixtures can accelerate or retard set time, increase workability, reduce water demand, entrain air, or improve durability. Common types include water reducers (plasticizers), accelerators, retarders, and corrosion inhibitors.

Why It Matters in Bidding

Admixtures are a small line item that can carry outsized schedule and cost consequences. Specs or field conditions like hot weather, long haul times, or fast-track pours often dictate accelerators, retarders, or water reducers that the estimator must capture in the mix design and unit price. Overlooking required admixtures invites rejected loads, cold-joint risk, and strength failures that trigger change orders and delay.

Example

Bidding a winter foundation pour, the estimator confirms the concrete quote includes a non-chloride accelerator and cold-weather protection so the mix reaches strength on schedule without corroding the embedded rebar.

Related Terms

Water-Cement RatioPortland CementFly AshAir Entrainment

Frequently Asked Questions

Per cubic yard the admixture cost is modest, but the conditions requiring them, such as cold-weather accelerators or extended-haul retarders, often come bundled with added protection, testing, or labor. Estimators confirm whether the mix design specified in the documents includes the needed admixtures so the sub's unit price reflects actual placement conditions rather than a generic base mix.
Accelerators speed setting and early strength in cold weather or where fast form-stripping or traffic is needed. Retarders slow setting in hot weather, on long hauls, or for large placements requiring extended workability to avoid cold joints. The project's pour schedule and ambient conditions, not just the spec, often determine which the contractor must include.
The structural engineer sets performance requirements such as strength, durability, and exposure class, and the concrete supplier's mix design selects admixtures to meet them, typically submitted for approval before placement. Contractors verify the approved mix matches field conditions and pricing, since added admixtures discovered after award commonly become change-order disputes.

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