Historical Masonry -Early history, types, Seismic Performance and Retrofitting

Historical structures and masonry monuments have always been important because of their historical significance and heritage value. These structures represent architectural art, culture, and heritage of ancient era and attract tourists from all over the world. This way, they contribute substantially in tourism business. Therefore, national and international government bodies should pay attention to preserve these structures. Safety of these structures from weathering, aging and earthquake has now become serious issue. Therefore, appropriate methodology is needed for analysis, retrofitting and renovation of old masonry structures

Early History of Masonry

Masonry is a type of construction, made of units laid in and bound together by mortar. Masonry construction is one of the oldest building techniques known to man which was developed in agricultural age (Neolithic age) approximately 10,000 B.C. At that time, People used to build their houses using stones and mud. Later, brick masonry and mortar were developed. Bricks were invented around 3500 BC, in Mesopotamia. The earliest brick masonry construction was found in Egypt at the period of Babylonian empire about 2500 B.C. From this time onward, masonry has been used extensively in building construction throughout the human civilization. Carrying along the simplicity and ancient history of construction process, masonry construction continues to be a widely popular in modern society as well.

 

Masonry Type

Stones, sun-dried clay and burnt clay bricks were basic construction materials in the majority of historical and old masonry structures. For the binding material, mud, lime, lime-surkhi and cement-sand mortar were mainly used. Based on the materials used in construction masonry can be characterize as stone and brick masonry.

   Stone masonry

In stone masonry, stones were used as construction materials unit. This type of masonry construction is practiced usually in those places where stones are easily available. Stone masonry can be seen in many historical structures worldwide. It is also very cost effective and affordable technique for housing construction and commonly used in developing countries. Stones are generally stiffer than burned clay bricks, and good in storing potential energy. The structural faults due to stress concentration usually initiate in mortar joints of the masonry. Originally, Lime mortar was used as binder in stone masonry construction because the ductility of lime mortar reduces stress concentration. Thickness of mortar bed joint was preferred as per the sizes of stone units. Generally, structures made of stone masonry use to under gone cracking due to their excessive self-weight or some settlements. According to Juhasova et al [2008], no significant improvement in behaviour of stone masonry was observed even after replacing lime with cement. Even, lack of ductility of cement mortars made it more vulnerable to earthquakes. Figure 1.1(a) shows photograph of stone masonry construction.

Brick masonry

In brick masonry, units are made of either sun-dried clay or burnt clay brick. Brick size may vary with place to place due to different manufacturing traditions. The strength of brick depends on firing temperature and quality of clay. There are quite variation of materials of brick and mortar with the locations. They usually made of local available materials. Basic materials used in mortar are sand, water and binding agent such as mud, clay, lime or cement etc. Brick construction technique is cheaper and widely used in housing construction. In the historical masonry structure, brick were often not of standard dimensions; therefore high mortar thickness was used to level the each course of masonry.  Figure 1.1(b) shows a photograph of Residency garden in Lucknow, which is a brick-masonry structure.

 

Performance Under Seismic Condition

Old and historical structure made of brick or stone masonry are highly vulnerable to earthquake. Historical masonry structures have shown poor performance in past earthquakes. According to a report of UNESCO over Bhuj earthquake in 2001, around 15500 out of 25000 historical monuments were severely damaged. The worst disaster due to earthquake occurred in 1976 in China, which caused deaths of  240,000 people and mostly deaths were occurred due to collapse of masonry buildings. The common reasons for poor seismic performance of old masonry structures are:

• Low tensile and shear strength of masonry units.
• Presence of voids, often concentrated in loose internal cores of stone masonry, which causes detachment of layer or out of plane expulsion during earthquake and result in brittle collapse.
• Lack of connections among structural members such as roof, wall and floor
• Presence of horizontal member (roof and floor) with poor in-plane stiffness.
• Aging and weathering of materials, which cause degradation of strength local and global stiffness.
• Absence of seismic design provision at that period, therefore these structures are not built for seismic load.
• Presence of multi leaf stone masonry with weakness of the internal layer and lack of interlocking among Wythe’s.

 

 

Retrofitting

Retrofitting term refers to addition of new elements in existing structure to increase the overall strength and life span. It consists of varieties of treatments such as preservation, restoration, rehabilitation and reconstruction. Preservation of structure means a process to sustain the existing form, integrity and materials of historical property. Rehabilitation refers to the process of creating a new application for any structure through repair, modification and additions while preserving the features conveying historical, cultural, or architectural values of structure. Restoration is the process of accurately restoring a property, as it existed at a particular period. Reconstruction is described as the act of replicating a property at a specific period.

Historical masonry structures made of bricks and stones deteriorate continuously due to aging, weathering, and impact of temperature, wind and rainwater. Nowadays, increasing air pollution, chemical and biological attacks have speeded up damage on these structures. Earthquake has also been a major threat for them. Therefore, there are extreme needs for retrofitting and strengthening program for old historical masonry structures.

Choosing the suitable intervention strategy is a great challenge involved in the retrofitting operation. For the preservation of historical structures, a structural assessment should also be carried out with architectural evaluations. Structural assessment includes the characterization of the structural materials, which is very helpful in determining the need and extent of structural rehabilitation. Improper material characterization may lead to wrong structural assessment therefore; material characterization is critical and extremely important step of retrofitting. Minimizing disturbance and damage to the surrounding building and providing temporary support are the major challenges in retrofitting of historical masonry structures.