The initial setting is defined as the loss of plasticity or the onset of rigidity (stiffening or consolidating) in fresh concrete. The final setting is defined as the onset point of strength. It is different from hardening, which describes the development of useful and measurable strength. Setting precedes hardening, although both are controlled by the continuing hydration of the cement. Setting time of cement is found out by a standard Vicat apparatus in laboratory conditions. Setting time, both initial and final indicate the quality of cement.
Setting time of concrete differs widely from setting time of cement. The setting time of concrete depends upon the w/c ratio, temperature conditions, type of cement, use of mineral admixture and use of plasticizers in particular retarding plasticizer. The setting parameter of concrete is more of practical significance for site engineers than setting time of cement. For cement paste, it uses the samples made of the water amount needed for consistency. For concrete, it uses the sieved mortar from a concrete with different water/cement or water/binder ratios. Moreover, for cement paste, it measures the penetration depth of the Vicat needle, 1 mm in diameter, under a constant weight. For concrete, it measures the resistance of the mortar to a rod under an action of the load.
The setting time of concrete is found by pentrometer test. This method of test is covered by IS 8142 of 1976 and ASTM C – 403. The procedure given below may also be applied to prepared mortar and grouts. The apparatus consists of a container which should have minimum lateral dimension of 150 mm and minimum depth of 150 mm. There are six penetration needles with bearing areas of 645, 323, 161, 65, 32 and 16 mm2. Each needle stem is scribed circumferentially at a distance of 25 mm from the bearing area.
A device is provided to measure the force required to cause penetration of the needle. The test procedure involves the collection of representative sample of concrete in sufficient quantity and sieve it through 4.75 mm sieve and the resulting mortar is filled in the container. Compact the mortar 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, by means of pipette. Insert a needle of appropriate size, depending upon the degree of setting of the mortar in the following manner.
Bring the bearing surface of 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 mm, as indicated by the scribe mark. The time 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 time water is added to 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. Needle is inserted at least 25 mm away from the wall of container. A setup for concrete setting time measurement is shown in Fig. 1.
Plot a graph of penetration resistance as ordinate and elapsed time as abscissa. Not less than six penetration resistance determination is made. Continue the tests until one penetration resistance of at least 27.6 MPa is reached. Connect the various point by a smooth curve. From penetration resistance equal to 3.5 MPa, draw a horizontal line. 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. A typical graph is shown in Fig. 2.
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