Concrete Admixtures
CSI Division 03, Concrete
Chemical additives mixed into concrete at the batch plant or jobsite to modify how it behaves: accelerators to speed curing in cold weather, retarders to slow it in hot weather or for long pours, water reducers, and air-entraining agents for freeze-thaw resistance, each selected for the specific conditions and performance a given pour needs.
Why it matters
The wrong admixture, or the right one at the wrong dosage, can change a pour's working time and final strength enough to cause real problems, concrete that sets faster than the crew can finish it, or concrete that doesn't develop its design strength on the schedule the project needs.
On a real project
A contractor pouring a large reinforced slab in cold weather specifies a non-chloride accelerator to keep the concrete curing on schedule despite freezing overnight temperatures, avoiding the corrosion risk a calcium chloride accelerator would pose to the embedded rebar, rather than pouring a standard mix that could freeze before gaining enough strength.
Who this matters most to
A Structural Engineer approves which admixtures a mix design can use. A Concrete Worker or ready-mix supplier doses and places the mix in the field.
Where this goes wrong
A ready-mix supplier adds an accelerator to a large pour without confirming it against the structural engineer's approved mix design. The accelerated concrete sets faster than the finishing crew can work it, and several sections are left with a rough, improperly finished surface that has to be ground down.