Setting time significance and test

Significance

• The initial setting time gives enough time to mix, transport, place and compact the concrete or mortar before it stiffens; a short value (less than 30 min) makes the work difficult.
• The final setting time shows when the concrete has hardened enough to bear loads, which helps decide when formwork can be removed, curing started and the work moved on.
• It checks quality: abnormal setting (flash set or false set) indicates deterioration, wrong gypsum content or storage defects.
• It shows whether the cement suits the work (for example, hot-weather work).

Test (Vicat apparatus, IS 4031 Part 5)

1. Take 400 g of cement and mix it with water equal to 0.85 P (P is the standard consistency percentage); gauge for 3-5 minutes, starting the stopwatch at the moment water is added.
2. Fill the Vicat mould resting on a glass plate and level the top.
3. Initial setting time: lower the 1 mm square needle gently onto the paste at regular intervals. When the needle stops 5 ± 0.5 mm from the bottom of the mould, the time elapsed since adding water is the initial setting time.
4. Final setting time: replace the needle with the needle having the annular collar (attachment). Final setting is reached when the needle makes an impression but the collar does not. The time is counted from adding water.
5. Keep the apparatus at 27 ± 2 C and 90% humidity, and do not let the needle fall on the same spot twice.