Types of Rubble Masonry: Coursed Rubble Masonry & uncoursed Rubble Masonry

What Is Rubble Masonry?

Rubble masonry is a type of construction technique that involves the use of undressed, roughly squared stones or irregularly shaped stones that are arranged together with mortar to form walls, columns, or other structures.

The stones used in rubble masonry are usually of different sizes and shapes, and they are not cut or shaped to fit perfectly into place.

Instead, they are laid in a random pattern, and the gaps between the stones are filled with mortar. Rubble masonry is often used in building structures that require a more rustic, natural look, such as retaining walls, garden walls, and historic buildings.

The Strength of Rubble Masonry Depends on

• Physical characteristics of the stone.
• Quality of mortar used.
• Usage of long trough stones.
• Proper mortar filling in the voids after placing stones; especially in joints.
• Workmanship
• Climatic conditions

Historical Application

The use of stone masonry was practiced in earlier times as natural stone is available in our country in abundance.

Many temples made out of stone still exist in various places around the country like Ellora caves.

Its usage can be detected in medieval cathedral walls and some historic buildings. The cathedral walls have an outer facing of ashlar while the inner hearting comprises dry rubble masonry.

Romans even used masonry in defensive walls during the medieval period. Its best example Colosseum in Rome, Italy. But, even during that period, brick construction enjoyed a preference.

Read More: Brick Masonry Vs Stone Masonry – Which Is Better?

Type of Rubble Masonry

There are several types of rubble masonry techniques, each with its own unique characteristics and uses.

Some of the most common types of rubble masonry include random rubble masonry, coursed rubble masonry, dry rubble masonry, polygonal rubble masonry, and squared rubble masonry.

The choice of which type of rubble masonry to use depends on various factors such as the nature of the construction project, the desired aesthetic appeal, and the available resources.

1. Uncoursed rubble masonry
2. Coursed rubble masonry
3. Built to regular courses
4. Polygon rubble masonry
5. Flint rubble masonry
6. Dry rubble masonry

1. Uncoursed Rubble Masonry

In this type of masonry, stones are not arranged in a course, i.e. the stones are not placed in a horizontal layer rather just stacked over one another. That’s why it is called uncoursed rubble masonry. 

Stones are utilized as obtained from the quarry, thus, making it the cheapest masonry construction as it saves the cost of dressing the stones.

But, as the stones are undressed, they vary in shape and size to a large extent. This results in poor bonding between the stones.

This makes it the weakest masonry in terms of strength.  Large stones are laid first followed by the filling of smaller stones in the voids.

Although the stones are used directly from the quarry without dressing, their projecting inconvenient corners are knocked off so that the bushing is 

• Not more than 40 mm – for exposed walls without plaster
• Not more than 10 mm – for walls to be plastered

i) Uncoursed Random Rubble Masonry

The stone blocks are selected by a mason at random and placed roughly in a course so that a good bonding is achieved. Bigger stone blocks are laid at quoins & jambs to increase its strength. The weak corners and edges are removed with the help of a scabbling or spalling hammer.

As stones are used directly from the quarry, this masonry has varying appearances. The mason handpicks the blocks and places them to achieve a natural aesthetic appeal. 

The minimum thickness that can be constructed with great care in ordinary buildings is  225 mm (9”). And if the construction is to be done with ease, 300 mm minimum thickness can be achieved.

Application

• In-wall construction in ordinary buildings of low height.

ii) Uncoursed Square Rubble Masonry

The faces of the stones are given a straight finish with the help of a hammer.

The stones are laid along their natural beds, but in a random way without forming a course. 

The voids are filled with mortar and stone chips. Care should be taken that chips are not laid on the bedding. Larger stones are placed at quoins. 

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2. Coursed Rubble Masonry

In this type of masonry, stones are arranged in a course, i.e. the stones are placed in a horizontal layer. That’s why it is called coursed rubble masonry. 

i) Coursed Random Rubble Masonry

This type of masonry has hammer-dressed stones placed in a level course. The height of the stones should be more than their breadth, while its size varies between 5-10 cm.

The stones are laid in layers of equal height. If the smaller stones are encountered, then two or three stones are stacked one above another to obtain the height of the course in which it is placed.

Such stones are placed in the intermediate courses and avoided at corners, bedding, and other critical locations.

To ensure sufficient strength of the masonry, 14th of the face stones are tailed into the work for a length of at least twice their height. And all the remaining ones should be tailed into for the length of their height.

In addition to this, for stability, a header course is introduced at regular intervals. 

Application

This masonry is suitable in hilly areas where stones are available in abundance at a cheaper rate. It is used in the construction of

• Low height walls of public buildings, residential buildings, etc.
• Boundary walls.

ii) Coursed Square Rubble Masonry

It is a superior variety of rubble masonry.

In this type of masonry, stones are chisel-dressed into an almost square shape and arranged in courses.

These chisel-dressed square stone pieces are arranged in the facing while the backing constitutes of uncoursed random rubble masonry.

Though the height of the courses may vary, the height of stones in each course is almost kept equal. The height of the course is kept between 100-300 mm. The construction of backing and facing shall be done simultaneously.

The component – joint is the most crucial one in any construction. In this masonry, joints are to be kept uniform. Also, small chips of stones are used to avoid thick mortar joints.

A rockface appearance is attained by pitching the face of the stones. Or it can be dressed to give a smooth finish. Or smaller stones can be introduced in the same course, at intervals to produce a chequered effect.

I.S.: 1957 further classifies Coursed square rubble masonry into two types:

• First sort: Stones are squared on all joints. Bed joints are chisel dressed up to 80 mm into the face. Height of the courses should be minimum 150 mm. Also, the courses should be regular.
• Second sort: All requirements are the same as the first sort except that gap between joints should not be more than 10 mm and usage of chips should not exceed 15% of the total quantity of stones.

Application

This masonry is extensively utilized in hilly areas where good quality stones are easily available for the construction of,

• Public buildings
• Hospital
• Schools

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3. Built to Regular Coursed Masonry

I.S. code refers to this type of masonry as Brought to the course.  In this type of masonry, stones of uniform height are laid in horizontal layers not less than 130 mm in height.

The stone beds are hammer/chisel dressed to at least 10 cm depth from the face to facilitate proper bonding. The vertical joints in the consecutive courses should not be allowed to coincide.

The construction pattern of this type of wall resembles that of uncoursed random rubble masonry. The major difference is that this masonry is roughly levelled up to courses at irregular intervals unlike the uncoursed random rubble masonry, where the stone pieces are not laid in courses.

The intervals in this masonry are decided on the basis of locality and the type of stones used. This interval is generally in-between 300-900 mm. 

4. Polygon Rubble Masonry

In this type of masonry, the stones are roughly hammer-pitched into irregular polygonal shapes. The stones used here are mostly without prominent stratification.

These stones are embedded in the facing to show the irregularity of face joints in all directions for aesthetic appeal.

It is mostly similar to Course Rubble Masonry of the first sort.

Arrangement of Stones

• Long joints in facing are avoided.
• Joints are broken as much as possible so that they do not coincide vertically.

(Strength of the masonry is reduced to a great extent when joints of the adjacent courses are placed one above another). Small pieces of stones should be avoided in providing support to facing stones.

5. Flint Rubble Masonry

This type of masonry is generally put to use where flint/cobble is available in abundance. These stones are 8-15 cm thick, 15-30 cm long.

These stones are hard but brittle. They are laid in the facing, either coursed or uncoursed.

The brittleness of these stones reduces the strength of this masonry, which is supplemented by placing a Lacing course of thin stones/bricks at 1-2 m intervals in the vertical direction.

6. Dry Rubble Masonry

It is constructed in the form of random rubble masonry without mortar as its name suggests.

This is achieved by filling up the hollow spaces around the stones placed by smaller stone pieces and so on until the stones are tightly packed.

This is cheaper as the cost of mortar is nullified. But, at the same time, skilled manpower is required to manage the stones alone without mortar.

The height of the stone pieces should be,

• In no case greater than 6 m
• If it is greater than 3 m, three adjacent courses are laid & at 3 m intervals in square rubble masonry.

Applications          

• Non-load bearing walls- retaining walls
• Pitching of earthen dams & canal slopes

Factor Affecting Rubble Masonry:

Several factors can influence the construction and performance of rubble masonry and this factor include,

Stone Quality: The quality and characteristics of the stones used in rubble masonry play a crucial role in its durability and strength, stones should be strong, durable and free from cracks or weakness, choosing stones with appropriates resistance and hardness to weathering ensure the long term integrity of the masonry.

Quality of mortar: The quality of mortar used in rubble masonry is vital for its strength and durability, the mortar should have appropriate proportions of cement, sand, admixture and water.

Size and Shape of Stones: The size and Shape of Stones used in rubble masonry affects its stability and appearance, stones of uniform shape and size make it easier to achieve a stable and aesthetically pleasing structure, irregularly shape stones require careful placement to ensure stability and uniformity in the masonry.

Construction techniques: the construction technique employed in rubble masonry impacts its overall quality, proper laying stones, ensuring adequate compaction of mortar are crucial aspects of construction techniques.

Different Types of Stone Rubble Masonry and Their Applications

Masonry Type	Dressing/Finish	Course	Application
1. Uncoursed rubble random	No dressing; Directly from QUARRY	Laid randomly	In-wall construction of low heights
i) Uncoursed Square Rubble	straight finish	Randomly, but on natural beds	In hilly areas, where these stones are available in abundance
2. Coursed Random Rubble	Hammer dressed	Placed in a levelled course	Low height walls of public buildings, residential buildings, etc, Boundary walls
i) Coursed Square Rubble	Chisel-dressed into an almost square shape	Carefully arranged in courses	Construction of Public buildings, Hospital, Schools
3. Built to regular course	Hammer/chisel dressed to at least 10 cm depth from the face	Roughly levelled up to courses at irregular intervals	In-wall construction
4. Polygonal rubble masonry	roughly hammer-pitched into an irregular polygonal shape	Laid in course; similar to coursed rubble masonry	Walls of forts, as in Great Wall of India(Rajasthan)Walls of houses
5. Flint rubble masonry	Chiselled & carved in such a way to improve aesthetics of the building	Laid in the facing, either coursed or uncoursed	Construction of churches, houses, etc.Townhall walls
6. Dry rubble masonry	Either used directly or chisel dressed into the desired shape depending upon the aesthetic required	Laid randomly; similar to Uncoursed random rubble masonry WITHOUT mortar	Non-load bearing walls- retaining walls, Compound walls, Pitching of earthen dams & canal slopes

Conclusion:

In conclusion, the two main types of rubble masonry, coursed and uncoursed, offer distinct characteristics and applications in construction. Coursed rubble masonry involves carefully arranging stones in horizontal courses, creating a more uniform and aesthetically pleasing appearance.

It is commonly used in buildings where a neat and regular finish is desired. On the other hand, uncoursed rubble masonry involves irregularly shaped stones laid without specific alignment, resulting in a more rustic and natural look.

Uncoursed rubble masonry is often utilized in non-load-bearing walls or structures where strength and uniformity are not primary concerns.

The choice between coursed and uncoursed rubble masonry depends on factors such as design intent, available materials, and desired aesthetic appeal, ensuring that the appropriate type is selected to meet the specific construction requirements.

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