Stone masonry is the process of building structures with the help of stone walls, which can be with or without the use of mortar. It is a craft of arranging and shaping stones to form a structure. This process of construction has been employed for thousands of years and several famous historical sites have been built using it. There are several types of stone masonry and one of them is random rubble stone masonry. Here we take a look at what random rubble stone masonry is, how it is built, its applications, advantages, disadvantages, and more.
What is Random Rubble Stone Masonry?
Random rubble stone masonry is the process of constructing a structure/wall using either undressed or hammer-dressed stones. The stones used for the construction are not uniformly shaped and are of irregular size and joints. The random rubble masonry is divided into two types- uncoursed random rubble masonry & coursed random rubble masonry.
Uncoursed Random Rubble Masonry
Uncoursed random rubble masonry is laid without forming the courses and is laid roughly with stones of different shapes and sizes. The projecting corners of the stones are sculpted off before laying and the joints are filled and flushed. This is one of the roughest and cheapest forms of stone masonry.
Coursed Random Rubble Masonry
These types of stone masonry are laid in levelled courses at equal intervals with stones being hammer-dressed. These stones are laid with headers having equal heights, however these stones could be of different sizes. All the courses differ in height and size.
Materials used in Random Rubble Masonry
Stones
Stones that are hard, sound, and durable are chosen for random rubble masonry. The stones shall not be less than 15 cm in size. The stones that have a rounded surface are generally not suitable. Stones that have a broader face are used for bedding to give stability. The stones are hammer-dressed before being laid.
Mortar
Cement mortar or lime mortar can be used for binding the joints and stones together. Mortar should be of specified composition. Cement mortars can be from a 1:3 to 1:6 ratio & lime mortar can be of 1:2 to 1:3 ratio. Mortars are prepared dry and mixed with water to bring them to a workable consistency. Sometimes, random rubble masonry does not utilise mortar, but instead relies on dry stacking of stones to erect walls.
Hammer/Chisel
The hammer or chisel or both are used to dress the stones to obtain suitable shape and size before being laid. It gives the necessary shape for stones that have uneven finishes and sets the platform for binding with the overlying horizontal course.
Construction Process in Random Rubble Masonry
Choosing the Stones
The stones can be excavated from the ground or can be selected from the quarries. They generally need to have uniform colour, quality, and texture, and should be clear from impurities. Stones that have a round face should not be selected. The selected tones should be tested in the lab and should pass the compression and water absorption test. The stones should possess a compressive strength of not less than 300 kg/cm² and should not absorb water more than 5%. The size of the stones should be not less than 15 cm and not more than 3/4th of the thickness of the wall.
Dressing
Each stone is hammer-dressed on the face to remove uneven edges and sides, making it suitable to be laid close to neighbouring stones. The dressing process removes the bushes of more than 40 mm on the exposed face and 10 mm on the plastered face.
Preparing the Mortar
A mixture of cement, sand, or lime is mixed to form a dry mixture of standard ratio. The dry mixture is then mixed with water to bring it to a consistency for binding.
Laying Stones
All the stones are watered before laying. The wet stones are laid close to the neighbouring stones. Ensure that the mortar joints that bind the stones are not more than 20 mm. The joints are monitored properly and a wooden mallet is used to position the stone firmly. The levels of the horizontal course can be adjusted using stones of similar height and mortar. Quoins can be used on alternate courses of the same height. After each layering, the structure is checked with a plumbing bob, to ensure the right shape.
Joints
Stones are laid in a way that all the joints are fully packed with mortar and stones. The face joints should not be more than 20 mm thick and can be flushed and finished. The top portion of the structure is finished with mortar and small stone pieces.
Curing
The masonry work that uses cement is constantly kept moist for a minimum period of one week, for complete curing. The structure should be protected from rain, as it may damage the construction due to mortar movement and other damages.
Advantages and Disadvantages of Random Rubble Masonry
Advantages
- Random rubble masonry is environmentally friendly, as it does not require manufactured materials such as bricks, cement, and steel.
- They are cost-effective as the completed masonry work doesn’t need dressing and can be left exposed.
- This masonry work doesn’t need too many skilled labourers and expensive equipment throughout the process.
- They can withstand heavy loads and are long-lasting.
- They can resist extreme weather conditions.
- Random rubble masonry provides a rustic, natural and aesthetically superior look.
Disadvantages
- They can be subjected to water damage, as water can easily penetrate through the structure and wash out the mortar and fine materials.
- If the mortar is poorly mixed and used for the random rubble masonry, they will reduce the strength of the structure, and even make the structure collapse.
- Cracks can develop within the structure if it goes through excessive stress due to external factors.
- This masonry works cannot be used near sea shores, as the salt content in the soil and air could damage the structure by crystallising the stones.
Applications
- These masonry works can be used for the construction of foundations, boundary walls, retaining walls, outer walls, godowns, residential buildings, and more.
- They are used for historical buildings such as cathedrals.
- They are used in eco-friendly buildings for the construction of walls and pavements.
- They can be used in the foundations of low-rise buildings up to two floors high.
Conclusion
Random rubble stone masonry is a basic, economical, and environmentally friendly masonry process used in the construction process. It has been in practice for thousands of years and is known for its high strength and durability. Such structures are eco-friendly, as they don’t require any manufactured materials that use high energy. Although they have numerous applications and advantages, the built form attains all the structural properties only if the masonry work is done accurately with careful placement of stones and proper sealing of joints.