Masonry Structures

Notes

• Brick quality is judged by compressive strength, water absorption (not above 20 percent), efflorescence and a ringing sound when struck.
• Stone is durable and strong in compression, but heavy and weak in tension; it needs mortar and good bonding with through stones.
• Hollow blocks have cores (voids above 25 percent of the gross volume); they are light, reduce dead load and can be reinforced and grouted for seismic resistance.
• CEB is environment friendly, needs no firing, and has good thermal behaviour, but must be protected from water and erosion by plaster and projecting eaves.
• All masonry units are strong in compression but weak in tension and shear, so masonry is mainly used for gravity load-bearing walls.

Stone masonry is built from natural stone laid in mortar (or dry). It is widely used in hilly regions where stone is available locally.

Types

1. Rubble masonry: stones of irregular shape.
   • Random rubble (uncoursed): stones of different sizes placed without courses; gaps filled with small stones and mortar.
   • Coursed rubble: stones are roughly dressed and laid in courses of equal height.
2. Ashlar masonry: accurately dressed stone with fine joints; fine ashlar has joints up to 3 mm, rough tooled a little more. It is strong and neat but expensive.
3. Dry stone masonry: laid without mortar, using careful fitting; used for retaining walls and terrace walls.

 Random rubble        Coursed rubble        Ashlar
 +-/\--+--+--+        +-----+---+--+       +----+----+
 |  \/ |  /\ |        +--+-----+---+       +--+----+--+
 +--+--\/--+--        +-----+-----+--       +----+----+

Characteristics

• High compressive strength and durability, good fire resistance.
• Weak in tension, shear and bending, so walls are brittle and rely on weight and bonding.
• Large wall thickness (usually 450–600 mm), heavy mass and thus large seismic force.
• In rubble walls with mud mortar the inner core and the two leaves often separate in earthquakes.

Construction rules for good behaviour

• Use through stones (bondstones) extending the full wall thickness at about 1.2 m horizontally and in every course level, so that the two leaves act together.
• Limit wall height to about 8–10 times the thickness, and keep openings small and away from corners.
• Use cement or cement–lime mortar rather than mud wherever possible; fill the core carefully with small stones and mortar, not loose rubble.
• Provide horizontal bands of timber or reinforced concrete at plinth, lintel and roof levels, and corner stitching.