5 Starting point for foaming
Figure B.3 – Set-up for foaming
The linear expansion on setting shall be determined in continuous damp air storage by means of a simple extensometer described in B.11.2.2 with the neat plaster gauged to a standard final coat consistence. The plaster shall be stabilised before test by the method described in B.11.2.5.
The extensometer has an open V-shaped cradle closed at one end by a fixed plate and at the other by a movable partition carried on the stem of an ordinary watch pattern dial micrometer gauge reading to 1/100 mm (see Fig. B.4). The cradle is of brass or bronze 100 mm long, about 60 mm wide and 25 mm deep with a rounded bottom. The take-up or returning spring shall be light and the movement free.
To prevent the plaster sticking to the sides of the cradle, grease before use and line internally with thin non-absorbent paper having a glazed surface. Renew the paper lining for each test. Fill the gauged plaster into the cradle while the movable plate is held against the end, and stoke off smooth and level with the top of the cradle.
A convenient quantity of plaster to use is about 200 g. Gauge this with water in the manner and to the standard final coat consistence described in B.11.2.6. Fill the gauged plaster immediately into the cradle of the extensometer and adjust the zero point as described in B.11.2.3. Place the extensometer in the damp closet and note the zero reading. Leave it undisturbed for 24 hours and then take the final reading. Calculate the percentage linear expansion
Expose the plaster for 3-4 days in a layer not more than 12 mm in thickness to an atmosphere of 65 ± 3 % relative humidity at a temperature of 20 ± 5 °C with vigorous air circulation over the specimen throughout this period.