Heritage is the result of activities of the past, which continue to be practiced today in modern times. Heritage is broadly classifiable into Tangible heritage, viz. Structures, sculptures, tools, etc., and Intangible heritage viz. folk traditions, performing arts, chanting of religious hymns, etc. (1) The various monuments spread out across the length and breadth of the nation are architectural testimonies to its diverse cultural history – a heritage that speaks of our glorious past. There is no doubt that preservation, conservation and restoration of the heritage buildings in India is of utmost importance (2). The preservation of our tangible heritage and heritage structures is fast gaining traction with bodies such as ASI and local Heritage Conservation Committees taking an active interest in the field.
One of the key problems arising in the field is finding materials compatible to the existing structure. Structures in India were mostly built in stone or brick masonry using locally available stones and mortars based on mud, clay and lime (2). Considering a dearth of the old material sources, craftsmanship and techniques, it becomes imperative to use new age available materials with properties matched to the ones used in the heritage structures. These new materials are capable of providing special advantages (ease of application, water and weathering resistance) while maintaining structural compatibility and aging characteristics of the old materials.
This paper reviews materials and methodologies available to conservation architects and engineers today to aid in durable preservation of our heritage structures. Some of the materials covered include cleaning agents, damp-proof courses, specialty mortars for joints and backfill, specialist renders and protective aids for Heritage Structures. The paper also highlights some case studies in India, as an example of use of these materials.
Rehabilitation of old masonry buildings can be classified also as Restoration of Monuments and historical buildings. In most of the cases the majority of construction material used are unconventional. These include natural stones, wood, lime mortars and plasters as compared to new materials (OPC cement based). In general the deterioration of natural materials is a process associated with environmental influences, entry of water into masonry leading to dampness, efflorescence deposits, biological growths like growth of plants etc. Further the attack of acid rains due to polluted environment has accelerated the process of deterioration. Figures 1 & 2 show efflorescence and rising dampness respectively.
Due to the fact that the materials of construction, used in heritage buildings, are highly absorbent, rising dampness is very common in areas where water table is high. The cause of deterioration of masonry materials is the crystallization of water-soluble salts in the pores of masonry and natural stones. When they are concentrated near the surface as efflorescence, they lead to decay. Figure 3 shows this mechanism. The removal of the efflorescence from the surface and substrates considerably increase the durability of affected structures. It should be noted that each dampness problem is unique and individual, therefore requires specific solutions. We associate dampness with the visual manifestation or discolouration, peeling and spalling plasters. However, by the time it is visually observed a lot of damage is already been done to the structure. Therefore, the first and foremost step in treatment of dampness should be professional diagnosis to determine the root cause of this occurrence and then proceed to solve the problem. The inspection should first identify the damp patches. Dampness initiates decay in the wood much before the wetness is observed by naked eye or touch. Dampness coupled with temperature variations and humidity promotes the growth of mould, fungi and mites. Dampness rectification should be priority in repairs.
Understanding the Structures
A majority of the heritage structures are built in clay brick or stone (locally available) masonry set in clay or lime mortar. Structurally, most of the buildings are load-bearing. It is very important to evaluate the building structure, prior to undertaking a restoration project. The types of masonry (bonds, single or double wall structures), additional supporting elements (metal straps, wooden beams or rafters, stone columns or masonry columns, arches) and decorative elements (lintels, cornices, statues, etc.) need to be evaluated in depth to understand their impact in the structure and the damage they have undergone.
If seen from a human body perspective, a masonry structure is like the spinal column, with the masonry blocks behaving as the rigid vertebrae and the softer pliable lime mortar joints behaving as the tissue surrounding the vertebrae, allowing for flexibility and movements. In combination, this system is capable of sustaining loads. This principle is shown in Figure 4. Therefore many a time it is noticed that old masonry structures crack / deteriorate even more (both through the joints as well as the stone / bricks) if their joints are filled with a mortar containing modern OPCs and Sand.
Understanding the Materials used
Masonry / load bearing structures would generally be built in locally available stone masonry or in clay brick masonry, set in clay, lime or mud mortars. For these materials it is vital to figure out some basic properties to better select materials for restoration (3). Some of these properties include:
Physical Properties: Modulus of elasticity, compressive strength, hardness / abrasion resistance, frost resistance, water vapour diffusion, swelling/shrinking, capillary absorption of water, porosity/pore distribution
Chemical Properties: Chemical composition, resistance to aging (colour)
Nature of Chemical attack on Substrate: Sulphates (gypsum), salts (nitrates, chlorides), water and acid soluble components in stone (from acid rain)
It is necessary to ascertain the physical and chemical properties of stone to evaluate which surface preparation methods would be best suited to the stone. In case of water damage, it is important to evaluate porosities of both the masonry block (brick or stone) to decide how to treat the substrate as well as to find the path of water movement through the structure. Understanding the elastic modulus and compressive strength of the blocks, can help us design mortars of matching elastic, thermal and strength properties. Understanding the chemical composition and nature of chemical attack, can help us devise the correct material systems to divert these aggressive influences away from the principle structure and help protect their durability.
Chemically as well as physically, it is not very difficult to re-create age old mortars. The materials used in original constructions consisted primarily of:
- Lime Mortars (can be recreated today with Naturally Hydraulic Lime or Artificial Hydraulic Lime)
- Clay-Lime Mortars (contained reactive aluminosilicates and lime, that provided hydraulicity)
- Cement or Cement Lime Mortars (In comparatively newer structures, say post 1900s)
The soft and porous properties of lime mortar provide certain advantages when working with softer building materials such as natural stone and terracotta. For this reason, while Portland cement continues to be commonly used in new constructions of brick and concrete construction, in the repair and restoration of brick and stone-built structures originally built using lime mortar, the use of Portland cement is not recommended. Once the materials are understood, the repair material system can easily be devised by selection of the correct material system
Investigation – Guidelines
First and foremost it should be checked that overall structure is structurally sound. Old buildings are normally designed with higher safety factors. To generalize, few columns can have cracks and deteriorated bases and bottom spalling. Rust formations are observed if old iron parts are embedded. The major problems are mostly on the southwest side due to the lashing rains and flooding at the bottom of the structure due to level differences between the new roads and the old structures. This gives rise to dampness, rising dampness as well as growth of fungus and vegetation. At the bottom portion there is rising dampness or condensation problem associated with biological growth.
Cracks are normally observed on masonry walls, verandah pillars etc. which are made of soft sand stone. In addition, the balustrades of stones or wood are mostly in deteriorated stage if not properly maintained. The roof waterproofing systems fail and the new waterproofing system installed are sometimes incompatible with materials like bitumen etc. Lime plasters are damaged and have to be replaced with waterproof plaster made of either lime or cement depending upon the limitations specified by conservation architects. Observation shows that the polluted atmosphere and heavy acid rains accelerate the attack on the structure.
- Testing And Investigation
- Following testing should be conducted before any repair program is undertaken.
- Visual observation and investigation.
- Determine whether the damage is structural or non-structural in each element.
- Determining the degree of deterioration of natural stone as well as porosity.
- Non-destructive tests can be conducted to determine existing strengths and elastic moduli.
Once the decision is been made about the selection of cleaning, job site test should be conducted on small batches to determine the effectiveness.
Utmost care should be exercised in design of suitable cleaning system for cleaning the existing stone masonry and facades. There are three basic methods of cleaning of masonry surfaces.
- Water/steam cleaning
- Abrasive cleaning
- Chemical Cleaning
Depending upon the degree of strength and porosity of the stones one should make a decision to use the combination of above methods. Please note if grit blasting is done and the stone is damaged the damage is irreversible and should be used only in cases where the stones are strong and non porous. Water or steam cleaning is a better alternative, so long as the pressure of the water is lesser than the softness of the stone. A chemical cleaning agent may be used after thorough testing, especially for removal of cement from the masonry or for removal of vegetation from the masonry surface.
Rehabilitation of old buildings with historic significance is different compared to normal repair of concrete structures as the materials used in construction are natural and the rehabilitation or restoration is to be done by maintaining the architectural aesthetics and ambience. One should be able to blend new materials and new structural concepts with old building materials and structural designs. It should be borne in mind that primary responsibility is not only to regain the architectural aesthetics but also to make the old buildings perform their new roles with total safety.
- The Material Systems
- The material systems available for heritage conservation include:
- Chemical Damp-proof Course for Brick Masonry
- Crystallization Based Coatings for Waterproofing
- Specialty Cement Free Mortars for Joint Filling, Structural Back Filling
- Specialty Cement Free Renders and Plasters
- Surface Consolidants
- Hydrophobic Impregnations
Chemical Damp-proofing Course
Damp proofing is defined by the American Society for Testing and Materials (ASTM) as a material that resists the passage of water with no hydro-static pressure and waterproofing as a treatment that resists the passage of water under pressure. A damp-proof course (DPC) is a barrier through the structure designed to prevent moisture rising by capillary action such as through a phenomenon known as rising damp. Rising damp is the effect of water rising from the ground into property. The damp proof course may be horizontal or vertical. A Chemical DPC layer is usually laid generally, 1 foot over the ground, regardless if the wall is a load bearing wall or a partition wall.
These materials are low viscosity materials to be injected into the brick-work under low pressure. These materials are of different types, silicifying chemical damp-proofing Course (DPC), hydrophobic injection, hydrophobic creams and nano-silica suspensions.
These materials are to be injected into the brick-work based on the dampness / moisture content in the brickwork. Silicifying Chemical DPC can be used upto 65% moisture content. Hydrophobic DPC liquids and Creams and the nano-silica suspensions can be used up to 95% moisture content in the brickwork. Figure 5 shows the application of Chemical DPC. These materials either end up silicifying the porous brick structure or completely hydrophobizing it, thereby preventing transport of water and salts through the brick work.
Crystallization based Coatings for Waterproofing
Especially in brick structures under plasters, it is better to use a crystallization-based coating. The crystallization-based coating permeates into the brick structure, thereby consolidating and closing all the surface pores. This creates a seamless covering over the masonry surface, rendering the masonry waterproof. Figure 6 shows application of the crystallization based coating.
Specialty Cement Free Mortars for Joint Filling, Structural Back Filling
OPC free mortars are best suited for use in restoration of heritage structures and are best for repointing or using as a flowing structural backfill. Lime mortars offer an advantage with softer types of masonry, where use of cement in many cases eventually results in cement breaking away some masonry blocks when it reaches the end of its life. The mortar is a sacrificial element which should be weaker than the bricks so it will crack before the bricks. It is less expensive to replace cracked mortar than cracked bricks. Under cracking conditions, Portland cement breaks, whereas lime often produces numerous microcracks if the amount of movement is small. These microcracks recrystallise through the action of ‘free lime’ effectively self-healing the affected area. Historic buildings are frequently constructed with relatively soft masonry units (e.g. soft brick and many types of stone), and minor movement in such buildings is quite common due to the nature of the foundations. This movement breaks the weakest part of the wall, and with Portland cement mortar this is usually the masonry. When lime mortar is used, the lime is the weaker element, and the mortar cracks in preference to the masonry. This results in much less damage, and is relatively simple to repair. Lime mortar is more porous than cement mortars, and it wicks any dampness in the wall to the surface where it evaporates. Thus any salt content in the water crystallises on the lime, damaging the lime and thus saving the masonry. Cement on the other hand evaporates water less than soft brick, so damp issues are liable to cause salt formation and spalling on brick surfaces and consequent disintegration of bricks. This damp evaporation ability is widely referred to as ‘breathability’.
These mortars are specially formulated with a variety of materials. Internationally they can be composed of Naturally Hydraulic Lime (NHL), calcareous or siliceous fillers, special pozzolans and specialty additives. It can also also be formulated from non-hydraulic lime with specialty additives and fillers. New generation additives help match the moduli and provide special properties to the mortar. These cement free mortars can be designed as a completely hydrophobic mortar, or as a mortar that would allow salt and moisture transport through it to prevent damage to the masonry.
Specialty Cement Free Renders and Plasters
The restoration of damp masonry in historic buildings is always a challenging task. Particularly in cases where only cement-free renders are permitted, it is inherently difficult to provide a solution that satisfies both the functional and the aesthetic requirements. Due to advancements in material technology, we are now in a position to provide heritage structures with a convincingly authentic appearance, while at the same time comprehensively satisfying the requirements of heritage conservation. The renders concerned are based on natural hydraulic lime, offering cement-free refurbishment for durable restoration results both inside and out.
Today cement-free moisture regulating renders can be formulated for damp, salt-laden surfaces. This breathable lime render can be applied directly onto damp masonry. Due to its unique pore geometry, the render can actually absorbs moisture and the deleterious salts dissolved within it, allowing them to be continuously transported to the render’s exposed surface. The moisture is released into the ambient air, leaving the salts to crystallize for easy removal. These historic renders also exhibit a high degree of self-supporting stability and serve as a mold inhibitor. Some of the properties demanded of these renders include :
- High level of porosity
- High absorption of salts
- High water vapor diffusion
- Low capillary suction
In terms of measurable parameters the renders can be characterized by the following requirements:
- Air void content, fresh mortar: > 25%
- Consistency: 17,0 + 0,5 cm
- Resistance against water vapour diffusion: < 12
- Water penetration after 24 h: < 5 mm
- Capillary suction w24: < 0,3 kg/m²
- Porosity of cured mortar: > 40%
- Compressive strength after 28 d: 1,5 – 5 N/mm²
These materials are specially designed siliceous materials that help bind and consolidate the friable materials, especially in old brickwork or sandstone or limestone blocks used in heritage structures. This helps to increase their load carrying capacity. These materials are generally low viscosity liquids that can be simply sprayed on to
When mineral building materials come into contact with water, they absorb an amount which depends on their porosity. Water is one of the main causes of structural damage such as cracks, frost damage, algal growth and salt efflorescence. Much of this damage can be prevented or significantly reduced by means of hydrophobic impregnation systems. This is a colorless, non-film-forming treatment of mineral substrates, particularly facades of fair-faced masonry. The result: capillary absorption of water is inhibited long term, yet the substrate’s breathability is not affected. These materials are based primarily on silicone technologies.
- Drastic reduction in water uptake (>80%)
- Retention of high water-vapor permeability
- Longlasting effects thanks to good penetration power and high UV resistance
- Adequate resistance to alkalis
- Low gloss and low tack
- Low environmental impact
- Sand-lime brick
- Mineral plaster
Keeping in mind the severe demands of materials for conservation of heritage structures, one does not need to always confine oneself to old-fashioned materials. Since the materials are similar in composition even today, we can engineer materials especially for use in restoration and conservation of heritage structures. Use of chemical DPCs, consolidants and hydrophobers, should become norm to protect the structures in the long run from the effects of water and the weather.
As far as restoring structural integrity of the structures goes, we can today engineer cement free mortars for use as both free flow structural backfill and as specialty mortars for joint-filling. These mortars can be matched to the original structure for compressive strength as well as e-modulus.
Also the materials available in the field of historic renders are very advanced today. The pore geometry of lime mortars can be engineered in such a way that they allow moisture to wick out from the structure and into the atmosphere to maintain integrity of the masonry underneath. Similarly these renders allow salt laden efflorescence to come to the surface of the render (from where it can be removed) thereby ensuring the salts do not destroy the masonry, nor do they destroy the beautiful rendered surface.
Artists should be specially invited for the cosmetic repairs and their art should be to recreate original designs, motifs, borders etc. New addition if should blend thoroughly well with the original architecture and can enhance it beauty and attraction of structure if it is resigned for change of use like a Hotel, Resort etc. Old and new materials should be blended judiciously and be pretested for compatibility. The decision of Conservation Architects and historians should be a base for design of preservation problems. Historic Preservations are preservations of our culture.
- Excerpts from MC-Bauchemie’s Documentation on Restoration of Heritage Structures