Q19: Addition of calcium chloride in concrete results in:
Difficulty: Medium
A. Increased strength
B. Reduction in curing period
C. Retardation of loss of moisture
D. All options are correct
Correct Answer: B. Reduction in curing periodSolution:Calcium chloride is an accelerator that speeds up cement hydration and causes concrete to harden rapidly. Because it accelerates the rate of strength gain, concrete reaches its minimum required strength sooner — thereby reducing the required curing period. It does not directly increase the ultimate 28-day strength, and it does not retard moisture loss (in fact, accelerated hydration consumes water faster).
Q20: Which of the following acts as retarder for the concrete?
Difficulty: Easy
A. Calcium chloride
B. Calcium lignosulphonate
C. Calcium stearate
D. Aluminium powder
Correct Answer: B. Calcium lignosulphonateSolution:Calcium lignosulphonate is a by-product of the paper-making (sulphite) process and is a well-known retarding admixture (also acts as a plasticiser/water reducer). It adsorbs onto cement particle surfaces and delays their hydration. Calcium chloride (A) is an accelerator. Calcium stearate (C) is a waterproofing agent. Aluminium powder (D) is a gas-forming agent used to make aerated concrete.
Q21: Admixtures which cause early setting and hardening of concrete are called:
Difficulty: Easy
A. Accelerators
B. Retarders
C. Air entraining agents
D. Workability admixtures
Correct Answer: A. AcceleratorsSolution:Accelerators are admixtures that increase the rate of cement hydration, causing concrete to set and harden earlier than normal. They are used in cold regions (to gain strength before frost), for emergency repairs, and in precast production. Examples include calcium chloride (CaCl₂), soluble carbonates, and silicates.
Q22: Calcium chloride added in concrete acts as:
Difficulty: Easy
A. Air entraining agent
B. Plasticizer
C. Retarder
D. Accelerator
Correct Answer: D. AcceleratorSolution:Calcium chloride (CaCl₂) is the most commonly used accelerating admixture in concrete. It increases the rate of hydration of cement compounds, reduces the initial and final setting times, and promotes rapid early strength gain. It is used in cold weather concreting and for reducing the curing period. However, it is restricted in reinforced concrete (max 2% by wt. cement) and completely prohibited in pre-stressed concrete.
Q23: The addition of CaCl₂ in concrete results in: (i) increased shrinkage (ii) decreased setting time (iii) decreased shrinkage (iv) increased setting time
Difficulty: Hard
A. Only (i) and (iv)
B. Only (iv)
C. Only (i)
D. Only (i) and (ii)
Correct Answer: D. Only (i) and (ii)Solution:CaCl₂ causes two specific effects: (i) increased shrinkage — CaCl₂ accelerates all hydration reactions including C₃S, which produces more Ca(OH)₂ and C-S-H, and the accelerated microstructure formation leads to greater drying shrinkage; and (ii) decreased setting time — CaCl₂ is an accelerator that reduces both initial and final setting times. Therefore only (i) and (ii) are correct.
Q24: The common admixture used to accelerate the initial set of concrete is:
Difficulty: Easy
A. Gypsum
B. Calcium chloride
C. Mixture of bitumen and inert material
D. By-product of bitumen
Correct Answer: B. Calcium chlorideSolution:Calcium chloride (CaCl₂) is the most commonly used accelerating admixture worldwide. It accelerates the initial set of concrete by speeding up cement hydration. It is particularly useful in cold weather to ensure concrete sets and gains strength before freezing temperatures damage it, and in emergency repairs to minimise downtime.
Q25: Which of the following is a limitation of lightweight concrete when compared to conventional concrete?
Difficulty: Medium
A. Increased permeability
B. Reduced density
C. Enhanced thermal property
D. Higher fire resistance
Correct Answer: A. Increased permeabilitySolution:Lightweight concrete is made using lightweight aggregates (expanded clay, pumice, fly ash pellets) or by introducing air voids. Its limitations include increased permeability compared to conventional concrete, because the porous, angular nature of lightweight aggregates and the deliberate void structure increase the capillary path for water ingress. Reduced density (B), enhanced thermal properties (C), and higher fire resistance (D) are actually advantages of lightweight concrete, not limitations.
Q26: When calcium sulphate attacks calcium aluminate hydrate present in concrete, it produces:
Difficulty: Hard
A. Asphalt
B. Anthracite
C. Ettringite
D. Calcium hydroxide
Correct Answer: C. EttringiteSolution:Sulfate attack occurs when sulphate ions (SO₄²⁻) in groundwater, soil, or seawater react with calcium aluminate hydrate (C₃A hydration product) in concrete to form ettringite (calcium sulpho-aluminate, 3CaO·Al₂O₃·3CaSO₄·32H₂O). Ettringite is highly expansive — its formation causes expansion, cracking, and eventual disintegration of the concrete. This is why low-C₃A cements (sulphate-resisting Portland cement, SRPC) are used in sulphate-rich environments.
Q27: Select the correct option regarding the Ultrasonic Pulse Velocity (UPV) test: 1. Used to measure strength of wet concrete 2. Used to estimate strength of finished concrete elements 3. It is a non-destructive test
Difficulty: Medium
A. Statements 1 and 2 are correct
B. Statements 2 and 3 are correct
C. Statement 3 only is correct
D. All three statements are correct
Correct Answer: B. Statements 2 and 3 are correctSolution:The UPV test is a non-destructive test (statement 3: correct) used to assess the quality, homogeneity, and estimated compressive strength of hardened (finished) concrete elements (statement 2: correct). Statement 1 is incorrect because UPV cannot meaningfully measure the strength of wet/fresh concrete — it is only applicable to hardened concrete where the ultrasonic pulse can travel through a solid matrix.
