Setting Time of Concrete:
First of all, understand the fact that setting time of concrete and setting time of cement are two different parameters. For finding out setting time of cement we use standard Vicat apparatus and this test is performed in a laboratory in control of temperature. Both the final and initial setting of cement indicates the quality of cement.
Setting time of concrete completely differs from setting time of cement. Setting time of cement does not match or compare cement setting time with which the concrete is made. The concrete setting time mostly depends upon the w/c ratio, temperature conditions, type of cement, use of mineral admixture, use of plasticizer, in particular, retarding plasticizer. The significance of setting parameter of concrete is more important for site engineers than setting time of cement. For keeping the concrete we use retarding plasticizers, which increases setting time and the duration up to which concrete remains in the plastic condition is of special interest.
Read More: WORKABILITY OF CONCRETE AND FACTORS AFFECTING IT.
Method of Testing:
The concrete setting time is determined by using a penetrometer test. This test is conducted as per IS 8142 of 1976 and ASTM C – 403.
In this test, an apparatus used is having a minimum lateral dimension about 150mm and depth is also 150mm. To perform this test there is 6 plungers or penetration needle with bearing areas of 645, 323, 161, 65, 32 and 16 mm2.
Each needle is placed at a distance of 25 mm from the bearing area. A device with hydraulic force is provided to measure the force required to cause penetration of the needle.
Procedure setting time of concrete test:
The test procedure involves,
- Taking a sufficient quantity of fresh concrete mix sample and sieves it through 4.75 mm sieve.
- The mortar sample passed through the sieve is collected.
- This mortar is then compacted by rodding, tapping, rocking or by vibrating.
- Level the surface and keep it covered to prevent the loss of moisture.
- Remove bleeding water, if any, using a pipette. Insert a needle of appropriate size, depending upon the degree of the setting of the mortar in the following manner.
- Bring the bearing surface of the needle in contact with the mortar surface.
- Gradually and uniformly apply a vertical force downwards on the apparatus until the needle penetrates to a depth of 25 ± 1.5 on, as indicated by the scribe mark.
- The time is taken to penetrate 25 mm depth could be about 10 seconds.
- Record the force required to produce 25 mm penetration and the time of inserting from the to water is added to the cement.
- Calculate the penetration resistance by dividing the recorded force by the bearing area of the needle. This is the penetration resistance.
For the subsequent penetration avoid the area where the mortar has been disturbed. The clear distance should be two times the diameter of the bearing area. The needle is inserted at least 25 mm away from the wall of the container.
Plotting of Graph:
- Plot a graph of penetration resistance as ordinate and elapsed time as abscissa.
- Test conducted must determine 6 penetration resistances.
- Continue the tests until one penetration resistance of at least 27.6 MPa is reached.
- Plot these penetration resistance values on the graph and connect each point.
- Now draw a horizontal line from penetration resistance equal to 3.5 MPa.
- The point of intersection of this with the smooth curve is read on the x-axis which gives the initial setting time.
- Similarly, a horizontal line is drawn from the penetration resistance of 27.6 MPa and point it cuts the smooth curve is read on the x-axis which gives the final set.