NICET - Highway Construction Inspection Practice Exam

Which of the following describes the effect of excessive vibration on concrete?

1. Firming the mix

2. Creating a stronger slab

3. Causing laitance formation

4. Reducing curing time

The correct answer is: Causing laitance formation

Excessive vibration on concrete can lead to the formation of laitance, which is a weak, milky layer of cement and fine particles that can form on the surface of freshly placed concrete. This occurs because the vibration can cause the separation of aggregates and the finer constituents of the mix, allowing the cement paste to rise to the surface. As a result, the top layer of the concrete becomes less dense and ultimately weaker, compromising the overall strength and durability of the slab. This is distinct from other choices, which do not accurately represent the effects of excessive vibration. For example, while proper vibration can help in consolidating the mix and achieving a denser concrete, too much vibration leads to the opposite effect—disrupting the homogeneity of the mixture. Similarly, excessive vibration does not lead to a stronger slab, as it can negatively impact the integrity of the concrete, making it more susceptible to degradation over time. Additionally, curing time is not inherently reduced by excessive vibration; in fact, poor finishing due to laitance formation could compromise the moisture retention required for proper curing.