The water retention capacity of mortar is the ability of mortar to retain water in general against suction and evaporation. It is indirectly a measure of the workability of the mortar.
The equipment used for this test consists of a water aspirator or other source of vacuum that is controlled by means of a three-way stopcock to a funnel, on which a perforated dish rests. The perforated dish must be made of metal, not attacked by masonry mortar. The base of the dish should have a metal thickness of 1.7 to 1.9 mm and will conform to the outline shown in the figure below. The minimum inner diameter of the stopcock bore should be 4 mm.
A mercury manometer, as shown in fig, indicates vacuum. A synthetic rubber gasket shall be permanently sealed at the top of the funnel and lightly coated with petroleum or mild cup grease during testing to ensure a seal between the funnel and the dish. Care will be taken to ensure that no holes in the perforated dish are clogged by the grease used on the rubber gasket. Rigid filter paper of an equivalent grade of Karl Schleicher & Schuel filter paper No. 576 or Whatman No. 50 filter paper shall be used. It should be of such diameter that it lies flat and completely covers the bottom of the dish.
Adjust the mercury relief column so that a vacuum of 5 cm is maintained as measured on the manometer. Place the perforated dish on the greased gasket of the funnel. Place a wet filter paper in the bottom of the dish. Turn the stopcock to apply vacuum to the funnel and check the equipment for leaks and to determine that the required suction has been achieved. 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 performing the flow test, return the mortar to the mixing bowl on the flow table and remix the entire batch at medium speed for 15 seconds.
- Immediately after remixing the mortar, fill the perforated dish with mortar slightly above the rim. Tamp the mortar with a tamper 15 times. Ten of the tamping strokes shall be applied at approximately uniform spacing adjacent to the rim of the dish and along the long axis of the tamping face placed at right angles to the radius of the dish. The remaining five tamping strokes will be applied at random points distributed in the central area of the dish. The tamping pressure will be sufficient to ensure the filling of the dish.
- When tamping is complete, the top of the mortar should be slightly above the rim of the dish. Smooth the mortar by pulling the flat side of the straight edge (with the leading edge raised slightly) across the top of the dish. Then cut the mortar onto a flat surface by pulling the straight edge with a cutting motion across the top of the dish in two cutting strokes, starting each cut near the center of the dish.
- If the mortar is pulled away from the edge of the dish during the process of pulling the straight edge across the dish, use a tamper to gently press the mortar back into contact with the edge of the dish.
- Rotate the stopcock to apply vacuum to the funnel. The time from the start of mixing the cement and water to applying the vacuum should not exceed 8 minutes.
- Quickly turn the stopcock to expose the funnel to atmospheric pressure after suctioning for 60 seconds. Immediately remove the perforated dish from the funnel, touch it to a damp cloth to remove the water droplets, and set the dish on the table.
- Then, using a bowl scraper, pour the batter into the dish for 15 seconds and mix.
- When mixing is complete, place the mortar in the dish for 15 seconds. When mixing is complete, place the mortar into the flow mold and set the flow.
- The entire operation shall be carried out without interruption and as soon as possible, and shall be completed within an elapsed time of 11 minutes after the commencement of mixing of cement and water for the first flow determination.
Calculate the water retention value for 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 the Preparation and Use of Masonry Mortars