Pick up the INCORRECT statement from the following about admixtures:
Correct Answer: D. Admixtures give high strength (INCORRECT statement)
📚 Detailed Explanation: Admixtures Do NOT Directly Give High Strength
Why D is the incorrect statement: Admixtures modify specific properties of concrete, but they do not directly confer high ultimate compressive strength. Strength is fundamentally determined by the water-cement ratio (Abrams' Law), the degree of compaction, and the quality of cement and aggregates. While some admixtures (like superplasticisers) allow a lower w/c ratio to be used which then improves strength, the admixture itself is not “giving” strength.
Analysis of Each Statement
| Statement | Correct? | Explanation |
|---|---|---|
| A. Admixtures accelerate hydration | ✓ Correct | Accelerating admixtures (CaCl2, carbonates, silicates) increase the rate of cement hydration; this is their defined function |
| B. Admixtures make concrete waterproof | ✓ Correct | Water-repelling/waterproofing admixtures (sodium silicate, stearates) fill capillary pores and reduce water absorption, making concrete effectively waterproof |
| C. Admixtures make concrete acid proof | ✓ Correct | Certain admixtures or cement types (e.g. high-alumina cement, acid-resistant admixtures) can provide resistance to mild acid attack |
| D. Admixtures give high strength | ✗ INCORRECT | Admixtures alone do not give high strength. Strength = f(w/c ratio, compaction, curing). Superplasticisers ENABLE low w/c by improving workability, but the strength gain comes from the lower w/c, not the admixture itself. |
What Actually Governs Concrete Strength
| Factor | Effect on Strength |
|---|---|
| Water-cement ratio (primary) | Lower w/c → higher strength (Abrams' Law: f ∝ 1/w/c) |
| Degree of compaction | Full compaction = maximum strength; 5% voids → 30%+ strength loss |
| Curing | Longer, moist curing → more hydration → higher strength |
| Cement type and grade | Higher-grade OPC or RHPC → higher early and 28-day strength |
| Aggregate quality | Strong, clean, well-graded aggregate; no deleterious materials |
- Admixtures do NOT directly confer high ultimate strength — this is the incorrect statement.
- Superplasticisers allow water reduction (enabling higher strength via lower w/c) but the admixture itself is not the source of strength.
- Admixtures modify workability, setting time, durability, and impermeability — not primarily strength.