Concrete in which dry coarse aggregates are first packed to minimise voids and then cement-sand mortar is injected under pressure to fill the voids is known as:
Correct Answer: A. Pre-packed concrete
📚 Detailed Explanation: Pre-Packed Concrete — Aggregate First, Mortar Injected
Why A (Pre-packed concrete) is correct: Pre-packed concrete (also called pre-placed aggregate concrete or intrusion grouted concrete) is a special type of concrete in which coarse aggregate is first placed and compacted in the formwork, and then a fluid cement-sand mortar (sometimes with fine aggregates) is injected under pressure from the bottom to fill all the voids. The result is a very dense, low-shrinkage concrete with minimal segregation.
Construction Method for Pre-Packed Concrete
| Step | Operation |
|---|---|
| 1. Aggregate placement | Clean, well-graded coarse aggregate (typically 20–40 mm) placed and compacted in formwork; minimum void content achieved |
| 2. Injection pipe installation | Injection pipes placed at the bottom of the form; vent pipes at the top |
| 3. Mortar preparation | Cement-sand mortar (with plasticiser for fluidity) prepared; must be highly flowable to penetrate all voids |
| 4. Mortar injection | Mortar pumped under pressure from the bottom; displaces air upward; fills voids from bottom to top |
| 5. Curing | Standard moist curing; minimal formwork bleeding |
Types of Special Concrete
| Type | Method | Key Property | Application |
|---|---|---|---|
| Pre-packed concrete | Aggregate placed first; mortar injected under pressure | Very dense; low shrinkage; no segregation | Underwater; repairs; machine foundations; massive structures |
| Vacuum concrete | Standard concrete placed; excess water removed by vacuum mat applied to surface | Low w/c at surface; high surface strength | Industrial floors; pavements |
| No-fines concrete | Only coarse aggregate and cement; no sand | Very porous; good thermal insulation; lightweight | Walls; drainage layers; insulation |
| Aerated concrete | Aluminium powder added; gas bubbles form; or pre-formed foam added | Very lightweight (<1200 kg/m³); thermal insulation | Partition walls; infill; insulation panels |
- Pre-packed concrete: coarse aggregate packed first; cement-sand mortar injected under pressure to fill voids.
- Advantages: extremely dense, very low shrinkage, no segregation, excellent bond to existing structures.
- Ideal for: underwater repairs, massive foundations, structures where conventional placing is difficult.