What is the correct slump for concrete in NB flatwork?

The correct slump for concrete flatwork in New Brunswick — driveways, patios, sidewalks, and garage floors — is 100–125 mm (4–5 inches). This range provides good workability for placing and finishing while maintaining the low water-to-cement ratio required for durable NB concrete.

Slump is a measure of concrete consistency and workability — how readily it flows and responds to finishing. The slump test is simple: fresh concrete is placed in a cone-shaped mould, the cone is lifted, and the distance the concrete slumps downward is measured. More slump means more fluid and workable; less slump means stiffer and drier.

Why slump matters for durability: The relationship between slump, water content, and concrete strength is direct. Higher slump generally means more water in the mix. Every litre of extra water added per cubic metre of concrete reduces compressive strength by roughly 5 MPa and increases shrinkage cracking. In NB's freeze-thaw environment, a weaker, more porous concrete fails faster — spalling, scaling, and cracking within a few years instead of decades.

The most common and damaging mistake on NB concrete pours is adding water to the truck at the jobsite to increase slump because the concrete seems stiff and hard to work. This practice — called re-tempering — is tempting but destructive. If you need more workability without adding water, the right solution is to specify a water-reducing admixture (plasticizer) at the batch plant. Plasticizers allow a 125–150 mm slump at the same water content as a 100 mm slump mix — better workability with no strength penalty.

Slump can decrease in transit. Ready-mix trucks leaving a plant in Moncton, Fredericton, or Saint John at 125 mm slump may arrive at your site with 100 mm slump, particularly on a hot July day or a long drive. This is normal and acceptable. Do not let the driver add water to compensate. If slump has dropped significantly below 75 mm and the concrete is genuinely difficult to place, discuss options with the plant — but the answer is not water at the jobsite.

For foundation walls and columns where the concrete must flow around dense rebar cages, a higher slump (150 mm / 6 inches) is sometimes specified, but this requires using a superplasticizer to achieve the slump without excess water.

Always confirm the required slump with your concrete contractor when reviewing the mix design. It is a basic quality-control checkpoint that serious contractors discuss as a matter of course.