Initial & Final Setting of Concrete

The cement paste’s initial setting time in concrete is when it begins to harden, and the cement paste’s ultimate setting time occurs when it has sufficiently hardened to leave an impression on the paste in the mold when a 1 mm needle is inserted, but no impression at all when a 5 mm needle is inserted.
Theoretically, the initial setting time of concrete is the interval of time between adding water to the cement and the point at which the needle in a 1 mm square part of the Vicat mold fails to pierce the cement paste, which is positioned between 5 and 7 mm from the mold’s bottom. The final setting time is the interval of time from the addition of water to the cement until the paste in the mold is punctured by a 1 mm needle but not by a 5 mm attachment.

The technical aspects of setting times for concrete

1. Cement must set at a rate that is neither too fast nor too sluggish. In the first scenario, there might not be enough time to move and position the concrete before it solidifies too much. In the second scenario, an excessively extended setup period may cause the work to proceed more slowly and may also delay the structure’s practical use due to insufficient strength at the intended age.
2. Hardening, which is the increase in mechanical strength following a particular level of resistance to the penetration of a certain attachment forced into it, should not be confused with setting.
3. Setting time is the amount of time needed for cement paste to harden to a specific consistency.
4. Partly associated with the first chemical reaction between cement and water, which produces the aluminum-silicate combination.
5. The paste begins to lose its flexibility during the initial setting time.
6. The initial setting time test is crucial for cement concrete transportation, placement, and compaction.
7. To postpone the hydration or hardening process, an initial setting time period is necessary.
The point at which the paste totally loses its flexibility is known as the final setting time.
It is the amount of time needed for the cement paste or cement concrete to reach the proper consistency and take on the shape of the mold that it is cast in.
10. Safe scaffolding or form removal is facilitated by determining the final setting time period.
11. The cement and water undergo an almost full primary chemical reaction during this time.

How to Determine the Final Setting of Concrete

The Required Equipment Are

1. Vicat’s apparatus
2. balance
3. Trowel
4. Glass plate
5. enamel tray
6. measuring cylinder
7. stop watch

Test Procedure

Preparation of First Step

1. Prior to beginning the test method, a consistency test should be conducted to determine the amount of water needed to give the paste a normal consistency (P).
2. Using 400 g of cement, make a neat cement paste by adding 0.85 P of water to the cement weight.
3. Gauge time is maintained at three to five minutes. As soon as the water is introduced to the cement, set the stopwatch. Note the time now (T1).
4. Pour the cement paste measured previously into the Vicat mold, which is set on a glass plate. Once the mold is fully filled, level the paste’s surface with the top of the mold by smoothing it off. The so-prepared cement block is referred to as the test block.

Initial Setting Times’ Test

1. Position the test block within the mold, resting it on the non-porous plate beneath the needle-bearing rod.
2. Gently lower the needle until it touches the test block’s surface, then quickly release it to let it pierce the test block.
3. The test block is entirely punctured by the needle at the beginning. Repeat this process, letting go of the needle fast every two minutes, until the needle is unable to penetrate the block for a distance of approximately 5 mm measured from the mold’s bottom. Take note of the time now (T2).

Final Setting Times’ Test

1. Use an annular attachment needle to replace the Vicat apparatus’s needle in order to ascertain the final setting time.
2. When the test block’s surface is gently touched with the final setting needle and the needle leaves an impression but the attachment does not, the cement is said to have finally set. Note the time now (T3).

Worksheets

Initial Setting Time = T2-T1
Final Setting Time = T3 – T1

Whereas
T1 = The initial time water is introduced to cement
T2 = Time at which the needle breaks through the mold between 5 and 7 mm from the bottom
T3 = The time it takes for the needle to leave an impression but the attachment doesn’t.

Standard Setting Time Specifications for Various Types of Cement

The setting time varies for each type of cement depending on the admixtures added during the manufacturing process. The table below enumerates standard setting times according to IS Codes.

Table 1: Setting Time for The Various Type of Cement

Necessary to Calculate The Cement’s Initial and Final Setting Times

The paste begins to lose its flexibility at the initial setting time. The test is crucial for cement concrete transportation, placement, and compaction. In order to postpone the hydration or hardening process, the initial setting time duration is also necessary.
The paste totally loses its flexibility at the final setting time.It is the amount of time needed for cement concrete or paste to adequately harden and take on the shape of the mold into which it is cast. Determining the ultimate setting time period makes it easier to remove scaffolding or forms safely. This is the time when the cement and water undergo their principal chemical reaction, almost to completion.