Concrete Placement, Consolidation, and Finishing
Placement, consolidation and finishing is the heart of the trade: getting concrete into the forms without segregation, removing air properly, and timing every finishing step to the concrete's stiffening.
Core rules
- Pre-pour check: forms, steel, embeds, vapour retarder, grade, equipment, weather, and test cylinders arranged.
- Free fall is limited to about 1.5 m; use chutes or drop pipes in deep walls. Place in horizontal lifts and keep concrete off the form faces where possible.
- Consolidate with an internal vibrator inserted vertically at spacing of about 1.5 times the radius of action, penetrating 100 to 150 mm into the previous lift, held 5 to 15 seconds until air stops and a thin sheen appears. Never use it as a spreader; over-vibration segregates.
- Strike off, then wait. Bull float after bleed water has left the surface; edge and groove while the concrete still takes an impression; trowel in stages as it sets.
- Control joints are cut to about one-quarter of slab thickness, with spacing commonly 24 to 36 times thickness, and panels kept roughly square. Saw as soon as the blade will not ravel the edge, often 4 to 12 hours, sooner in hot, windy weather.
- Isolation joints separate slabs from columns, walls and other fixed elements. Broom exterior slabs; avoid a hard steel-trowel finish on air-entrained exterior work.
Worked example
A 100 mm slab measures 6 m by 9 m. Joint spacing of 24 to 36 times thickness gives 2.4 to 3.6 m. Dividing the 6 m side into two 3 m panels and the 9 m side into three 3 m panels produces nine 3 m square panels, 30 times thickness, within range and with a 1:1 aspect ratio. Cut depth is one-quarter of 100 mm, so 25 mm. Saw within the first day, once the concrete can carry the saw without edge raveling.
Traps
Troweling while bleed water is still standing, which causes blisters, delamination and dusting. Over-vibrating or dragging the head sideways. Cutting joints too shallow, so a random crack appears beside them. Brooming too late, when the surface is too hard to texture.