Grade 10 building construction – Concreting Quiz
1. What are the basic ingredients of concrete?
Concrete is made by mixing cement (the binder), fine aggregate (sand), coarse aggregate (stones) and water.
2. What is the purpose of the slump test on fresh concrete?
The slump test assesses how workable or fluid fresh concrete is, helping to judge if it can be placed and compacted properly.
3. Which factor most reduces the strength of hardened concrete?
Too much water weakens the cement paste and increases porosity, reducing the final strength of concrete.
4. What is the main reason for curing concrete after placing?
Curing keeps concrete moist and at suitable temperature so the hydration reaction can continue, producing strength and durability.
5. Which tool is used to compact fresh concrete by removing air pockets?
A concrete vibrator consolidates fresh concrete by removing trapped air and improving density and bond with reinforcement.
6. What is 'bleeding' in fresh concrete?
Bleeding is when water rises to the surface of newly placed concrete; excessive bleeding can weaken the surface layer.
7. Which aggregate size is generally used for concrete in beams and slabs?
Coarse aggregates of about 10–20 mm are commonly used in beams and slabs to give strength while allowing good compaction.
8. What does 'grade of concrete M20' mean?
M20 denotes a mix with a characteristic 28-day compressive strength of 20 N/mm² (megapascals).
9. What is batch mixing of concrete?
Batching means measuring the amounts of cement, aggregates and water for each mix so the correct proportions are used.
10. Why is proper compaction important when placing concrete?
Compaction removes entrapped air, reduces voids and improves strength and durability and bond with steel.
11. What is segregation in concrete?
Segregation occurs when heavier aggregates separate from the paste, causing non-uniform concrete and weak zones.
12. Which of the following is an example of a cementitious admixture used to accelerate setting?
Calcium chloride is a common accelerator that speeds up cement hydration and setting time (used carefully because it can corrode steel).
13. When should formwork be removed from a concrete column safely?
Formwork is removed according to strength development and recommended time to prevent damage; immediate removal would be unsafe.
14. What does 'finishing' concrete mean?
Finishing involves operations like levelling, floating and trowelling to achieve the desired surface appearance and texture.
15. Which method is commonly used to cure small concrete elements on site in Kenya?
Maintaining moisture on the surface using wet coverings or ponding ensures proper hydration and strength development.
16. What is the main role of reinforcement (steel bars) in reinforced concrete?
Concrete is strong in compression but weak in tension; steel reinforcement resists tensile stresses and prevents cracking.
17. Why must aggregates be clean and free from dirt and organic matter?
Clay, silt or organic matter can prevent proper bonding between cement paste and aggregates, reducing strength and durability.
18. What is the effect of using crushed angular aggregates instead of rounded river stones?
Angular crushed aggregates provide improved mechanical interlock and bond but can increase water demand and reduce workability.
19. Which test measures the strength of concrete cubes after curing?
Concrete cubes are crushed in a compression testing machine to determine the 28-day compressive strength of the concrete.
20. What is hydration in the context of concrete?
Hydration is the chemical process where cement reacts with water to form hardened cement paste, binding aggregates into concrete.
21. What should a site worker do to prevent honeycombing in a concrete element?
Honeycombing (voids near the surface) results from poor compaction or segregation; good compaction and correct mix reduce it.
22. Which is a common way to transport fresh concrete on a small Kenyan building site?
On small sites, wheelbarrows or buckets are practical for moving fresh concrete short distances while preventing segregation.
23. Why should concrete be placed continuously within permitted time when forming a single element?
If placement is interrupted, the earlier concrete may set and form a cold joint that weakens the structure unless treated properly.
24. What is an admixture in concrete?
Admixtures are chemicals (plasticizers, retarders, accelerators) that change workability, setting time or durability when added to concrete.
25. Which practice helps to reduce shrinkage cracks in concrete?
Controlling water content and keeping concrete moist during curing reduces rapid drying and shrinkage that cause cracks.
26. What is a 'cold joint' in concrete construction?
A cold joint occurs when there is a time gap between pours so the earlier concrete has hardened and creates a poor bond with the new pour.