Water retentivity of mortar is the ability of mortars to retain water against suction and evaporation in general. It is indirectly a measure of the workability of mortars.
The apparatus used for this test consists of a water aspirator or other source of vacuum controlled by way of a three way stopcock to a funnel upon which rests a perforated dish. The perforated dish shall be made of metal, not attacked by masonry mortars. The metal in the base of the dish shall have a thickness of 1.7 to 1.9 mm and shall conform to the outline shown in figure below. The bore of the stopcock shall have a minimum inside diameter of 4 mm.
A mercury manometer, connected as shown in Fig, indicates the vacuum. A synthetic rubber gasket shall be permanently sealed to the top of the funnel and shall be lightly coated with petroleum or light cup grease during the test to ensure a seal between the funnel and the dish. Care shall be taken to ensure that none of the holes in the perforated dish is clogged from the grease used on the rubber gasket. Hardened filter paper of a grade equivalent to Carl Schleicher and Schuell filter paper No.576 or to Whatman No.50 filter paper shall be used. It shall be of such diameter that it will lie flat and completely cover the bottom of the dish.
Adjust the mercury relief column so as to maintain a vacuum of 5cm as measured on the manometer. Seat the perforated dish on the greased gasket of the funnel. Place a wetted filter paper in the bottom of the dish. Turn the stopcock to apply the vacuum to the funnel and check the apparatus for leaks and to determine that the required suction is obtained. Then turn the stopcock to shut off the vacuum from the funnel.
Mix the mortar to a consistency to give a flow of 110 to 115.
Immediately after making the flow test, return the mortar on the flow table to the mixing bowl and remix the entire batch for 15 sec at medium speed.
Immediately after remixing of the mortar, fill the perforated dish with the mortar to slightly above the rim. Tamp the mortar 15 times with the tamper. Ten of the temping strokes shall be applied at approximately uniform spacing adjacent to the rim of the dish and with the long axis of the tamping face held at right angles to the radius of the dish. The remaining five tamping strokes shall be applied at random points distributed over the central area of the dish. The tamping pressure shall be just sufficient to ensure filling of the dish.
On completion of tamping, the top of the mortar should extend slightly above the rim of the dish. Smooth off the mortar by drawing the flat side of the straight edge (with the leading edge slightly raised) across the top of the dish. Then cut off the mortar to a plane surface flush with the rim of the dish by drawing the straight edge with a sawing motion across the top of the dish in two cutting strokes, starting each cut from near the center of the dish.
If the mortar is pulled away from the side of the dish during the process of drawing the straight edge across the dish, gently press the mortar back into contact with the side of the dish using the tamper.
Turn the stopcock to apply the vacuum to the funnel. The time elapsed from the start of mixing the cement and water to the time of applying the vaccum shall not exceed 8 min.
After suction for 60 sec quickly turn the stopcock to expose the funnel to atmospheric pressure. Immediately slide the perforated dish off from the funnel, touch it momentarily on damp cloth to remove droplets of water and set the dish on the table.
Then, using the bowl scraper, flow and mix the mortar in the dish for 15 sec.
Upon completion of mixing, place the mortar in the dish for 15 sec. Upon completion of mixing, place the mortar in the flow mould and determine the flow.
The entire operation shall be carried out without interruption and as quickly as possible, and shall be completed within an elapsed time of 11 min after the start of mixing the cement and water for the first flow determination.
Calculate the water retention value for the mortar as follows;
Water retention value = A/B x 100
A = flow after suction and
B = flow immediately after mixing
IS: 2250 – 1981- CODE OF PRACTICE FOR PREPARATION AND USE OF MASONRY MORTARS