What is a concrete admixture and when should I use one in NB?

A concrete admixture is a chemical added to the mix — beyond cement, water, and aggregates — to modify the concrete's properties for specific conditions or performance goals. In New Brunswick, admixtures are practical tools for managing cold-weather pours, improving workability, accelerating or retarding set time, and extending durability.

The ready-mix plant adds admixtures during batching, though some (like calcium chloride in small quantities) can be added at the site. Here are the most relevant admixture types for NB concrete:

Accelerating admixtures speed up the hydration reaction and strength gain. The most common is calcium chloride (CaCl₂), used at 1–2% by weight of cement. In NB, accelerators are most useful for late-season pours (October–November) when you need the concrete to reach handling strength before temperatures drop overnight. Non-chloride accelerators are used when chloride ions could cause corrosion of rebar — typically in structural and foundation applications. Note: calcium chloride is not recommended in reinforced concrete because it promotes rebar corrosion over time.

Water-reducing admixtures (plasticizers) allow a workable, flowing concrete at a lower water-to-cement ratio than would otherwise be possible. This is the right solution when a contractor needs better workability on a hot summer day in Moncton without adding water to the truck. Normal-range water reducers reduce water demand by 5–10%; high-range water reducers (superplasticizers) can achieve 15–30% water reduction. The result is concrete that flows like a higher-slump mix but has the durability of a stiff, dry mix.

Retarding admixtures slow the hydration reaction, giving the concrete more time before it stiffens. Useful in hot July and August weather in NB when high temperatures cause the concrete to set too quickly, reducing the finishing window. Also used for large pours where the work takes several hours.

Air-entraining admixtures are added at the batch plant to introduce the protective air bubble network essential for all exterior NB concrete. This is not optional — it is standard practice for any outdoor pour in New Brunswick.

Shrinkage-reducing admixtures (SRAs) reduce drying shrinkage and can be useful in flatwork where crack control is important. More common in commercial applications but available for residential work.

For most residential NB homeowners, the practical takeaway is: specify air-entrained mix as the baseline, ask for an accelerating admixture if pouring in cool fall weather, and trust your contractor to specify a plasticizer if workability is needed rather than adding water at the site. Admixtures are the professional's toolkit — knowing they exist helps you have informed conversations with your contractor.