Fresh Concrete: Workability, Placement, and Consolidation
This domain tests whether you understand workability as a rheological property rather than a single number, and whether your placement and consolidation decisions would leave the hardened concrete free of voids, cold joints and segregation.
Measuring workability
- Slump (ASTM C143) measures yield stress only and is unreliable above about 190 mm; high-flow mixes need slump flow and T50 (ASTM C1611), J-ring or L-box for passing ability.
- Vebe time suits very dry mixes where slump cannot discriminate.
- Bleeding (ASTM C232) and segregation are stability failures, not workability failures, and are aggravated by excess water and poor grading.
Placement
- Limit free fall to about 1.5 m; use trunks or chutes below that to prevent segregation.
- Place in horizontal lifts of roughly 300 to 500 mm for vibrated work and place each lift before the previous one takes initial set, otherwise a cold joint forms.
- Never retemper concrete that has started to set, and never add water to regain slump; adjust the batch at the plant instead.
Consolidation
Internal vibrators need spacing no greater than about 1.5 times the radius of action, must penetrate 150 mm into the previous lift, and must be withdrawn slowly. Vibrators are not devices for moving concrete horizontally.
Worked example
A 100 mm vibrator head has a radius of action of about 400 mm in a 200 mm-deep wall lift, so spacing should not exceed 600 mm, giving a triangular grid near 500 mm. Each insertion lasts 5 to 15 seconds, until air bubbles stop and the surface turns glossy; continuing beyond that segregates the mix.
Traps
Treating slump as workability; assuming 2 L/m3 of added water gains only slump and not permeability and strength loss; vibrating a cold joint back to life; using immersion vibration to spread a heap.