How soon after a manhole bottom is poured can you break out any laterals?

How soon after a manhole bottom is poured can you break out any laterals?

• A. 6 hours
• B. 12 hours if calcium chloride was added to mix
• C. 24 hours
• D. 48 hours

Answer: (B)

The key idea is how accelerators affect early strength of concrete. Calcium chloride speeds up the hydration process, so the concrete develops enough strength to remove forms and break out laterals sooner than with a plain mix. When calcium chloride is added, about 12 hours after pouring the manhole bottom is typically feasible for breaking out the laterals, because the mix has gained the necessary early strength to avoid damage. If no accelerator is used, you’d generally have to wait much longer—often 24 hours or more—until the concrete is strong enough to safely break out. The 6-hour timeframe is usually too soon, and 24 or 48 hours would be appropriate only without an accelerator or under different curing conditions.

The key idea is how accelerators affect early strength of concrete. Calcium chloride speeds up the hydration process, so the concrete develops enough strength to remove forms and break out laterals sooner than with a plain mix. When calcium chloride is added, about 12 hours after pouring the manhole bottom is typically feasible for breaking out the laterals, because the mix has gained the necessary early strength to avoid damage. If no accelerator is used, you’d generally have to wait much longer—often 24 hours or more—until the concrete is strong enough to safely break out. The 6-hour timeframe is usually too soon, and 24 or 48 hours would be appropriate only without an accelerator or under different curing conditions.