Placing and compaction

Frequently asked questions

What does a vibrator actually do, and how far does it reach?

It liquefies the concrete around it briefly so entrapped air can rise out and the mix can close around the reinforcement — and it does that over a RADIUS OF ACTION that is a property of the poker, not of the operator's patience. The radius depends on the vibrator's head diameter and frequency and on the mix, and insertions have to overlap it, which is why guides give a spacing rather than an instruction to move it about. Three errors follow from not knowing the radius. Spacing the insertions too far apart leaves unconsolidated wedges between them, which surface as honeycombing at the form face. Leaving the poker in too long lets the heavy aggregate settle and the paste rise, segregating the very concrete being compacted. And using the poker to push concrete sideways along the form — the most common misuse — moves the paste further than the stones, so the material that arrives at the far end is not the mix that left the truck.

Why is adding water on site so damaging?

Because strength is governed by the ratio of water to cement, and water added at the truck raises that ratio for the whole load without adding any cement to match. The rule of thumb that circulates on site — a bucket or two will not matter — is wrong by a margin that is easy to miss: a modest addition to a full load can cost a substantial fraction of the design strength, and it also increases shrinkage, increases permeability, and lowers durability in ways no cube test taken before the addition will show. The legitimate answer to concrete that is too stiff to place is a PLASTICISER, which increases workability without increasing water, added under control and recorded. That is also why a delivery ticket records the water added and why a mix that has been re-tempered on site is, strictly, a different concrete from the one specified — and why the supplier's liability usually ends at the moment somebody puts a hose in the drum.

If self-compacting concrete needs no vibration, why not use it everywhere?

Because it moves the difficulty rather than removing it. SCC is designed to flow into place and de-air under its own weight, which is transformative in congested reinforcement, in thin sections and where vibration is impractical or would disturb something — and it is measured by SLUMP FLOW, the spread diameter of a slumped cone, rather than by how far the cone subsided. The costs are real. It is more expensive, because the flow comes from admixtures and a higher fines content rather than from water. It is far more sensitive to variation: a small change in aggregate moisture can take it from flowing correctly to segregating, so it needs tighter batching control than a conventional mix. And it exerts close to full hydrostatic pressure on formwork, because it does not arch or stiffen as it fills — so forms designed for a conventional pour rate can be badly under-designed for it, which is why SCC formwork pressure is calculated separately rather than assumed.