As per IS specifications, what should be the maximum final setting time for ordinary Portland cement?
⏱️ Initial vs. Final Setting Time
It's vital to distinguish between the two stages of setting:
- Initial Setting Time: This is the point when the cement paste begins to lose its plasticity and stiffen. It marks the end of the workable period for placing and compacting concrete. For OPC, the minimum initial setting time is 30 minutes.
- Final Setting Time: This is the point when the cement has hardened sufficiently to sustain some load. It marks the time after which construction activities can begin on the concrete surface. The IS code specifies a maximum limit for this.
📝 Detailed Analysis of the Options
(a) 30 minutes
This is the specified minimum initial setting time for Ordinary Portland Cement (OPC). It is the most common incorrect answer chosen by confusing initial and final setting times.
(b) 10 hours
This is the correct answer. According to IS specifications, the final setting time for Ordinary Portland Cement should not be more than 10 hours (or 600 minutes). This ensures that the concrete hardens within a reasonable timeframe, allowing for the construction schedule to proceed.
(c) 1 hour & (d) 6 hours
These values are incorrect. While the concrete will certainly be harder at 6 hours than at 1 hour, the maximum permissible time limit given by the standard is 10 hours.
📊 Setting Times for Different Cements (as per IS Codes)
| Type of Cement | Initial Setting Time (Minimum) | Final Setting Time (Maximum) |
|---|---|---|
| Ordinary Portland Cement (OPC) | 30 minutes | 10 hours (600 minutes) |
| Rapid Hardening Cement | 30 minutes | 10 hours (600 minutes) |
| Quick Setting Cement | 5 minutes | 30 minutes |
| Low Heat Cement | 60 minutes | 10 hours (600 minutes) |
💡 Study Tips
- The "30 and 600" Rule: For OPC, the two most important numbers to remember for setting time are 30 minutes (minimum initial) and 600 minutes / 10 hours (maximum final).
- Initial is a "Minimum": You need at least this much time to work with the concrete.
- Final is a "Maximum": The concrete must be hard by this time.
- Spot the Exception: Notice that Quick Setting Cement is the major exception to the 10-hour final set rule, as it's designed to harden extremely fast.