Identify the INCORRECT statement regarding ready-mix concrete (RMC). Consider that ordinary Portland cement is used for making RMC.
- AReady-mix concrete in wet condition is transported by using a transit mixer from the batching plant to the construction site.
- BReady-mix concrete increases the speed of construction.
- CReady-mix concrete packed in bags should be added with a hydration activator while mixing with water.Correct
- DThe mix design of RMC is tailor made to suit the placing methods of the contractor.
Solution
Concept
OPC hydrates with plain water, so bagged ready-mix concrete does not need a hydration activator.
Step-by-step solution
1. Check A: wet RMC is carried from the batching plant to the site in a transit mixer. True. 2. Check B: RMC saves site batching time, so construction is faster. True. 3. Check C: OPC starts hydrating when water is added. No hydration activator is needed. False. 4. Check D: the RMC mix is designed to suit the contractor's placing method (pump, chute, etc.). True. 5. So the incorrect statement is C.
Explanation
Ordinary Portland cement reacts with water by itself, so no activator is needed for bagged dry ready-mix. Statements A, B and D are correct facts about RMC. So C is the incorrect statement.
Why the other options are wrong
A: correct, transit mixers carry wet RMC. B: correct, RMC speeds up work. D: correct, the mix is tailor-made to the placing method.
Shortcut
OPC needs only water to hydrate. Activators are for materials like slag-based or geopolymer binders, not OPC.
Common mistake
Thinking dry bagged concrete needs a special chemical to start hydration.
Correct answer: C) Ready-mix concrete packed in bags should be added with a hydration activator while mixing with water.