BEST PRACTICES FOR ACHIEVING IMPERVIOUS MASONRY
V T LONDHE FEBRUARY 1983–OCTOBER 1985
The stone shall be sand stone and shall be obtained form the quarries, appearing decay and defects like cavities, cracks, flaws, sand holes, injurious veins, patches of loose or soft materials and other similar defects that may adversely affect its strength and appearance. As far as possible stones shall be of uniform colour, quality or texture. Generally stone shall not contain crypts crystalline silica or chart, mix and other deleterious materials like iron – oxide organic impurities etc., Stones with round surface shall not be used. The compressive strength of common types of stones shall be 300 Kg. / Sq. cm and the percentage of water absorption shall generally not exceed 5%. For laterite this percentage is 12%.
Normally stones used should be small enough to be lifted and placed by hand. Unless otherwise indicated, the length of stones for stone masonry shall not exceed three times the height and the breadth or base shall not be greater than three fourth the thickness of a wall, or not less than 15 cm the height of stone may be upto 30 cm. Random rubble masonry shall be uncoursed or brought to the courses with stones of sizes as referred above an shapes picked up random from the stones brought from the approve quarry. Each stone shall be hammer dressed on the face, the sides and bed such that the bushing shall not project more than the 40 mm on the exposed face and 10 mm on the face to be plastered. The mortar used for joining shall be as specified. All stones shall be wetted before use. Each stone shall be placed close to the stones already laid so that the thickness of the mortar joints at the face is not more than 20 mm. Random rubble masonry brought to the course is similar to uncoursed random rubble masonry except that the courses are roughly leveled at intervals varying from 30 cm to 90 cm in height according to the size of stones used.
Each stone shall be hammer dressed on the face, the sides and the bed. Hammer dressing shall enable the stones to be laid close to neighboring stones such that the bushing in the face shall not project more than 40 mm on the exposed face and 10 mm on the face to be plastered.
The mortar used of joining shall be as specified.
All stones shall be wetted before use. Each stone shall be placed close to stones already laid so that the thickness of the mortar joints at the face is not more than 20 mm. Face stones shall be arranged suitably to stagger the vertical joints and long vertical joints shall be avoided. Stones for hearting or interior filling shall be hammered down with wooden mallet into the position firmly bedded in mortar. Chips or sprawls of stones may be used for filing of interstices between the adjacent stones in heartening and these shall not exceed 20% of the quantity of stone masonry. To form a bond between successive courses plum stones projecting vertically by about 15 to 20 cm shall be firmly embedded in the heartening at the interval of about one metre in every course. No hollow space shall be left any where in the masonry.The masonry work in wall shall be carried up true to plumb or to specified batter.
Random rubble masonry shall be brought to the level courses at plinth, window sills, lintel and roof levels. Leveling shall be done with concrete comprising of one part of the mortar as used for masonry and two parts of graded stone aggregate of 20 mm nominal size.The masonry in structure shall be carried uniformly. Where the masonry of one part is to be delayed, the work shall be raked back at an angle not steeper than 45o.
Bond or through stones running right through the thickness of walls, shall be provided in walls upto 60 cm thick and in case of walls above 60 cm thickness, a set of two or more bond stones overlapping each other by at least 15 cm shall be provided in a line from face of the wall to the back.
In case of highly absorbent types of stones (Porous lime stones and sand stone etc.) single piece bond stones may give rise to dampness. For all thicknesses of such walls, a set of two or more bond stones overlapping each other by at least 15 cm shall be provided length of each such bond stone shall not be less than two third of the thickness of the wall.
A very important observation for any masonry dam project is that the masonry should be executed out of very good stones of more or less of a cubical or cuboidal shape. The masonry would always be impervious particularly if every stone is vibrated manually with ¾-inch diameter steel rods so that no voids are left inside the masonry. This will result in the mortar consumption in the masonry to be of the order of 42% which as is actually required.
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