Design Considerations for Eco–Roof Waterproofing

eco roof waterproofing

Dr. C. S. Suryawanshi, Former Chief Engineer, (PWD) Mumbai.

It's a new world when it comes to commercial architecture and building construction. No longer is the focus only on expert craftsmanship, innovative design and attention to detail. Rather, all eyes are examining the process itself, the materials used, and most of all, the impact of construction on the environment. And there has been a quite a crowd jumping on the green bandwagon.

contemporary approaches to green roof technology began in the urban areas of Germany over 40 years ago. Because of ongoing water quality degradation and a limited existing infrastructure for the control of stormwater in these areas, few alternatives were available for improved stormwater management designs. Environmental and economic considerations helped spur the development of green roof systems that could provide the necessary stormwater treatment on-site. The paper discusses various design aspects related to materials properties, environment and plant requirements responsible for the waterproofing. The paper also out lines the various types of waterproofing systems useful for the purpose.

Historical Records

A relatively new phenomenon, green roofs first were developed in Germany in the 1960's and today, make up about 10 percent of all German roofs. The first green roofs in the United States made their appearance several decades later, but interest and popularity continues to grow as developers, building owners and government officials begin to see the environmental, economic, aesthetic, and social benefits of roof-top vegetation.

From the ancient Hanging Gardens of Babylon to the modern aesthetics of Le Corbusier's "New Architecture," integrating nature into the urban fabric has always been a very desirable amenity for people. The original inspiration for contemporary green roofs came from rugged Iceland, where sod roofs and walls have been used for hundreds of years. The sod roofs soon became popular throughout Scandinavia (Briggs, 2000). This Icelandic architectural style originated from a lack of natural resources, so people had to make do with the local materials of sod and stone (Briggs 2000). Roofs were usually made of turf, and the thick walls of the structures contained bottom layers of stone followed by cut blocks of sod alternating with strips of thin turf. Whenever possible, driftwood was included for timbers, as is the case in the church at Vidimyri, one of the six so-called sod churches that are still standing in Iceland. Built in 1834, it has been preserved as a monument and still functions as a parish church. Old timbers were always recycled whenever found in good condition.

Historically, engineered green roofs have originated in northern Europe, where sod roofs and walls have been utilized as construction materials for hundreds of years.

Concept of Green Building

Now a day the focus the world over is on constructing ‘Green Buildings,' which address environmentally sustainable issues in a holistic manner. In countries like the U.S the concept of green buildings is highly evolved, whereas in Canada and Brazil green buildings are quite prevalent. The concept is catching up even in China.

"It was around 2000 that the concept started gaining momentum across the world. In India it was going on in bits and pieces."

Minimize urban sprawl and needless destruction of valuable land, habitat and green space, which results from inefficient lowdensity development. Encourage higher density urban development, urban re-development and urban renewal, and brown field development as a means to preserve valuable green space.

Preserve key environmental assets through careful examination of each site. Engage in a design and construction process that minimizes site disturbance and which values, preserves and actually restores or regenerates valuable habitat, green space and associated eco-systems that are vital to sustaining life.

The ideal "green" project preserves and restores habitat that is vital for sustaining life and becomes a net producer and exporter of resources, materials, energy and water rather than being a net consumer.

A Green What?

Green roofs, sometimes also called vegetative roofs, are built on top of buildings and are generally planted with vegetation that requires minimal maintenance and watering. As development replaces land with buildings and parking lots, the amount of impervious surfaces grows. The idea of green roofs is to replace the green space that was lost with the new construction. Green roofs are similar to rooftop gardens except they are not maintained and they are neither ornamental nor recreational. Green roofs are practical and beneficial to the building and surrounding site.

Green roofs slow down and clean storm water runoff, which otherwise can exacerbate flooding and increase erosion. Green roofs absorb storm water and release it slowly over several hours. They can retain 60 to 100 percent of the storm water they receive. In addition, they last longer than standard roofs because they're protected from ultraviolet radiation and extreme temperature fluctuations. Green roofs also provide insulation to the building. In winter the roof stays warmer than the ground, thus warming the building, and in summer the plants cool the roof and divert heat from the building.

What is Green Roofing?

Simply put it is a green space on top of a building structure developed from soil and plantings. Unlike a roof garden that utilizes plantings in containers scattered over the roof area, today's green roof covers the waterproofing system entirely with soil and vegetation. Vegetated roofs, orgreen roofs as they are frequently called, have been in existence for centuries. Sod roofs have kept many homes warm in the winter and cool in the summer with their grass or plant layer placed atop of the sod base. This natural, environmentally friendly roof provided the basis of construction for the green roof assemblies used on commercial buildings and homes throughout the world today.

Composition of Green Roofs

Green roofs, also known as garden roofs and eco-roofs, are made from a layered structure of components. Covering the roof deck is a waterproofing membrane, often composed of rubberized asphalt, to guarantee a tight seal. The next layer is a protective root barrier, to prevent plant roots from penetrating the roofing membrane. This layer varies in strength based on the landscape design or selection of plants. On top of the root barrier is an insulation/air barrier, composed of extruded polystyrene or other insulation material. Depending on the needs of the vegetation, the insulation layer may be topped with an additional moisture-holding mat.

Next is the drainage/water storage and aeration layer. Using specially designed retention cups and channels made of recycled polyethylene, the drainage layer allows for effective, controlled runoff of excess water. A layer of filter fabric tops this, to filter soil fines and debris, allowing water to pass through to the drainage and aeration layer.

The top layers are the soil layer and the vegetation. Lightweight engineered soil provides a stable structure for the plants' root system and supplies nutrients, water and oxygen while remaining as light as possible to prevent excessive loading of the root structure.

Depending on building conditions, climate and anticipated use of the roof, a wide variety of typical landscape and garden plants are suitable. Plants with shallow root systems and resistance to direct radiation, drought, frost and wind are wellsuited to all types of green roof landscaping; but even perennial flowers, shrubs, small trees and sod grasses can be used for intensive roofing landscapes.

Green Roofs

The term "green roof" is generally used to represent an innovative yet established approach to urban design that uses living materials to make the urban environment more livable, efficient, and sustainable. Other common terms used to describe this approach are eco–roofs, and vegetated roofs. Green Roof Technology (GRT) is the system that is used to implement green roofs on a building. Green roofs are constructed using components that
  • Have the strength to bear the added weight;
  • Seal the roof against penetration by water, water vapor, and roots;
  • Retain enough moisture for the plants to survive periods of low precipitation, yet are capable of draining excess moisture when required;
  • Provide soil-like substrate material to support the plants;
  • Maintain a sustainable plant cover, appropriate for the climatic region;
  • Offer a number of hydrologic, atmospheric, thermal and social benefits for the building, people and the environment;
  • Protect the underlying components against ultraviolet and thermal degradation.

Types of Green Roofs

Green roofs are generally categorized as extensive or intensive. Extensive roofs are ideally suited for locations that will receive little maintenance or where structural capacities of the roof are a concern. Sedum, herbs, grasses and other vegetation that can withstand harsh conditions are recommended. Intensive roofs use plants that require regular maintenance, such as watering, fertilizing and mowing. These roofs must be structurally stronger, and often serve as pedestrian recreational areas. Several other variations include the shallow-intensive garden roof, which combines a lightweight roof assembly with slightly deeper soil to accommodate sod lawns and perennials, and sloped extensive applications, which can be applied to sloped roofs with a pitch of up to 45 degrees.

Intensive green roofs generally require more effort for the tending of plants, whereas the term extensive roofs call for a more passive approach. Intensive green roofs also emphasize the use of space and therefore raise higher aesthetic expectations than more functional extensive green roofs. Intensive green roofs generally need deeper substrate, more diverse plants including trees and shrubs, and proper watering schedules. Thus they involve higher costs (Dunnett and Kingsbury 2004; Peck et al. 1999). As in many design classifications, however, there are actually degrees of intensiveness in the approach to rooftop greening.

In order for plants to grow on roof tops, natural environmental conditions have to be recreated. This can easily be done by the installation of a series of functioning layers which, while retaining the necessary water to support the plants, allow excess water to drain off and protect the roof surface from plant roots and mechanical damage. A variety of systems are supplied by manufacturers which provide a stable roof-top environment for plant growth.

A typical system includes the following:
  • Vegetation layer: Low growing, stress tolerant alpine and herb species
  • Lightweight Soil: 50-100mm in depth
  • Filter Mat
  • Drainage Layer: Aggregate or plastic cups
  • Root Barrier
  • Waterproof Membrane

Waterproofing Aspects

There are so many myths about waterproofing. The people dealing with construction are normally different from people using such facility or maintaining them. Either of them passes on the blame on each other, and this ends in an unnecessary and expensive treatment to places, hardly susceptible to leakage, while real culprit is somewhere else, and gets altogether neglected. This may be the result of inadequacy of individual's knowledge of behavior of parent material so far its resistance to water ingress is concerned and Porosity of building materials, especially that of concrete, and knowledge of the waterproofing treatment adopted.

Water Flow–Drainage

Roof drainage is an instructive example of water flow. If water is to drain from the surface, there must be a high point and a low point. The rule for adequate drainage is that the fall (slope) must be at least 1 in 80 and that must be achieved. Creative pessimism dictates that the falls intended- i.e. shown on drawings or specified will not necessarily be obtained. Using normal methods there is a level of accuracy to which any part of a building can be built, so a further rule that has to be emerged. Double the fall to be obtained i.e. an achieved fall of 1 in 80 is allowed for by specifying,fall of 1 in 40. It is then for the contractor to take such precautions that will ensure that normal building accuracy (or special building accuracy, if that has been specified) is reached.

Air Pressure, Gradients, Moisture, and Wind

As moisture vapor causes a partial pressure, known, as vapor pressure there will be a gradient across a construction, whenever air on one side, contains more moisture than the other. The overall steepness of that gradient will depend on the difference in the vapor pressure on each side of the construction; the shape on the vapor-resisting properties of the materials that made up the construction.

The action of the wind on the external surface of a construction, results in an air pressure. As such, pressure inside is usually different. An air pressure gradient arises. A positive air pressure externally assists in driving rain through gaps in the construction. Provision for pressure equalization to take place is a form of balancing. Its achievement can help greatly, to reduce the risk of rainwater penetration through constructions.

Garden Roof System (GRS)

Garden roof systems (GRS) are specialized roofing systems that support vegetation growth on rooftops. GRS not only add aesthetic appeal to the unused roof space that is available in most urban areas; they also provide multiple benefits in an urban context. From a building's point of view, the plants and soil protect the roofing membrane from exposure to ultra violet radiation, extreme temperatures and physical damage, thus contributing positively to the roof's service life. GRS also reduce energy demand on space conditioning, and hence greenhouse gas (GHG) emissions, through direct shading of the roof, evapotranspiration and improved insulation values. If widely adopted, GRSs could reduce the urban heat island (an elevation of temperature relative to the surrounding rural or natural areas due to the high concentration of heat absorbing dark surfaces such rooftops and pavements) which would further lower energy consumption in the urban area. From a community's point of view, GRS can be used as a source control tool for the stormwater management strategy in the urban area. Part of the rain is stored in the growing medium temporarily, and to be taken up by the plants and returned to the atmosphere through evapotranspiration. This delays and reduces runoff and takes a load off the city's storm sewage system. The plants can also remove airborne pollutants and improve the air quality in the urban areas.

In addition to the roofing membranes, a GRS consists of several major components, namely, root resistance layer, drainage layer, filter membrane, growing medium and vegetation. The components act together to provide a suitable environment that supports plant growth while not compromising the waterproofing function of the roofing membrane. GRS can be installed on both conventional and protected membrane systems.

In the roofing industry, GRS is generally categorized into extensive and intensive by the weight of the system. Extensive GRS is lightweight, consisting shallow growing medium with small plants (e.g., sedums, herbs and grasses). These systems require very low maintenance. Intensive GRS is heavyweight and contains much garden soil. The greater soil depth allows growing of bigger plants such as shrubs and trees.

Design Criteria of Roof

The main criteria for selection of waterproofing material and movement control in roofs are:
  1. Integrity and durability of the weather proofing material and performance. Flat roof finishes are the most vulnerable due to low movement tolerance.
  2. Integrity of the roof structure.
  3. Interaction with supporting or adjoining elements of frameworks.
  4. Integrity of attached ceiling finishes if any and integrity of the supporting structures.

Failure Limits of Roof Structure

The failure limits of roof construction are therefore determined by the effect on the finishes or weatherproofing membrane and secondly by any detrimental effect on the supporting or adjoining structures as well as on the roof structure itself.

As in other elements, the amount of movement, which can be accommodated without damage or failure, depends not only on the strength and durability of the affected components, but also on the manner of their assembly. This can greatly affect the stress induced by movements and can reduce the effect of movements being transmitted between layers, particularly of roof finishes.

Mechanical Properties of Waterproof Roof Covering

The resistance to movements of various types of roof coverings varies greatly depending on their form material and assembly.

Pitched roof coverings being the overlap unit type can stand considerable movements without failure or causing loss of weather tightness.

Flat Roofs:

Flat roof coverings on the other hand being continuous; rely on their complete integrity from cracks to maintain weather resistance.

Effect of Layers

The mechanical strengths of the top (water excluding) layers of multi-layer coverings will depend on the amount of stress transmitted by the base fabric and the strength of this in turn, in the case of bituminous impregnatedcoated fabrics is the strength of the fabric itself.

Most traditional roofing felts suffer from the weakening effects of repeated loadings. Sudden movements are accommodated by elasticity and longer term movement cycles can also be accommodated by a certain amount of viscous flow. The stresses in the top layer also depends on the amount of stress relaxation which can take place between the layers in the time, i.e. the rate of straining effect. It also depends on the distance from the substructure or cause of movement (see Figure 5.1) and on the amount of slip due to partial bonding. Single layer roofing systems are naturally less able to reduce strains than partially bonded layers.

Advice of Structural Engineer

A structural engineer should always be consulted prior to roof garden landscape design and construction.

Rooftop structures must typically be able to support a dead load of 150psf to commodate the construction of a garden. The roof must be completely covered by an elastomeric material and protected by a concrete topping slab.

It is recommended that a completely new waterproofing layer be added to the existing structure to insure the longevity and integrity of the waterproofing system.

A waterproof topping coat of concrete should be used to protect the waterproofing.

Properties of Concrete

The ease with which a fluid can be flow through the matrix of a solid is believed to be the ermeability. It is obvious that the size and continuity of pores in any porous material will determine its permeability. Several theories attempt to relate the microstructural parameters of cement products with either diffusivity (The rate of diffusion of ions through water filled pores) or permeability.

Compared to 30 to 40 percent capillary porosity of typical cement paste in hardened Concrete, the volume of pores in most natural aggregate is usually 3 percent and rarely exceeds to percent. From permeability date of some natural rocks it appears that the coefficient of permeability of aggregate vary as that of hydrated cement paste of W/C ratios in the range of 0.38 to 0.71.

Permeability Aspect of Concrete

Permeability of concrete should not be confused with absorption. It is not a simple function of its porosity but depends on the size, distribution, and continuity of pores. The volume of pore space, as distinct from its Permeability, is measured by absorption and two quantities are not necessarily related. Dense concrete is said to possess low permeability. Unfortunately, so far, it has not been possible for concrete technologists, to set limits for permeability which could be subjected to practical tests, and whatever tests are available, and those are of academic importance.

In practice, it is wrong notion to understand permeability and absorption in the same sense. In fact, permeability of concrete is not a simple function of its porosity but varies with the size spread i.e. distribution and continuity of the pores. The size of capillary pores ranges up to 1.3 nm and that of gel pores is too smaller than that. The volume of pores space in concrete, as distinct from its permeability is measured by absorption and these two quantities are not necessarily co-related. In body of concrete, the capillary pore structure allows ingress of water. It is generally observed concrete gel has porosity of the order of 28% but its permeability is about 7x109m/s. The permeability of cement paste as a whole is 20-100 times greater than that of the gel itself. Most concrete Technologists believe that, permeability although a complex property of concrete, depends largely on.
  1. Quality of cement and aggregate.
  2. Quality and quantity of cement paste in concrete (and quality of cement paste depends on amount of cement, the W/C ratio of the mix and hydration of cement).
  3. Bond developed between paste and aggregate.
  4. Degree of compaction of concrete & standard of curing.
  5. Presence or absence of cracks or characteristics of cracking behavior.
  6. Characteristics of any admixture used in the mix.
Morphological Aspects of Concrete: From decades to gather compressive strength is reckoned as most valuable Engineering property of concrete. The science of RCC and PCC totally devolves on this one aspect only. The circumstances under differing environments recently recognized as harmful to concrete surface have compelled the attention to other characteristics such as durability, impermeability, and volume stability.

The ability of concrete to oppose weathering actions, chemical attack, abrasion and other conditions during its service life have come to be identified as "Durability." Whereas capability of concrete building products, components, assembly, or construction to perform the function/s intended in design and construction is valued on its service ability, these definitions are broad enough to embrace all practical aspects in general. However, strength parameter doesn't give true picture of quality of concrete since it is directly related to structure of cement paste. Strength, durability, serviceability and volume change of hardened cement paste that is important element in concrete appears to depend not so much on chemical composition as on the physical structure of the products of hydration of cement and on their relative volumetric proportions. In fact, it is the presence of flaws, discontinuities, and pores, which are of significance and bear an impact on strength. Since present day knowledge of field engineers in respect of this fundamental approach is inadequate, it is essential to relate strength to assessable parameters of the structure of hydrated cement paste. It will be seen that the primary factor in this is "porosity" i.e. the relative volume of pores or voids in the cement paste. Unfortunately the porosity, of the hydrated cement paste and micro cracking are difficult to assess and quantify in a manner, useful to the engineer to relate them to study the effects on strength factor. In fact, it is cement paste, whose structure is complex, which consists of several sources of flaws and discontinuities, despite proper compaction of concrete and even before the application of an external load up to 50% of the volume of Cement paste may consist of pores. Again, presence of aggregates either coarse or fine aggregates the position. The cracks from various sources, randomly distributed in the matrix vary in size and orientation. Consequently concrete renders itself weaker than the cement paste which it contains. The actual failure paths (when viewed under SE MICROSCOPE reveal) follow the interfaces of the largest aggregate particles, cut through the cement paste and occasionally also through aggregate particles.

Pore Size Distributions Throughout the hardened Cement paste there is a spread of whole range of pore sizes i.e. larger capillary pores and smaller ones of gel pores. When only partly hydrated paste contains interconnected system of capillary pores. Pastes that had been rapidly dried are noticed to be closest in structure when compared to undried pastes. The sorption test results indicated that their structures were dominated by platy particle of the formed shaped meso–pores and micro–pores. Slow drying produced small pores nearly cylindrical or spherical, has been established by various researchers. To get even distribution of pores it is essential to have sufficient higher degree of hydration of the capillary system to became segmented through partial blocking by newly developed cement gel, so that capillary pores get interconnected by the much smaller gel pores. Table 1.0 shows the min, period of curing required for capillary pores to become segmented. However, it is also established that finer the cement shorter is the period of curing necessary to produce degree of hydration at a given W/C ratio.

Moisture Movement & Creep

Materials exhibiting creep as well elastic characteristics are often grouped as visco-elastic. Most building materials when subjected to sustained load undergo an instantaneous (elastic) deformation followed by a time dependent deformation generally recognized as creep. Creep strain composes delayed elastic strain (eventually recoverable on removal of load) and viscous strain, which remains as a permanent one when load is removed. In porous materials capable of absorbing or giving off moisture, the creep strains are linked in with moisture movement, which leads progressive shrinkage or expansion depending upon the nature of material, its initial moisture content and external environment. The theory of creep and shrinkage in concrete based on the migration of moisture from the cement gel can be established by a simple phenomenon. When a concrete specimen is over dried, it expands, when exposed to the normal environment of say 60% humidity. The gel is so thirsty that it eagerly takes up moisture from the hour and in doing so expands, this expansion takes place even against a high compressive strain. In fact, all porous materials display above characteristics. In fact up to a certain level creep in concrete is proportional to stress give rise to the concept of "Specific Creep" i.e. creep strain per unit stress. This can be arrived at from the relation.

Specific creep c=t/(a + bt). Where, t=timing from loading, a,b=Constants determined by experiment

There are several important design and structural differences between ground level landscape development and rooftop developments. The following are the special construction requirements and considerations when developing a roof garden. Protection of the integrity of the roof and structure.

Protection of the Roof and Structure

The single most important element in rooftop garden construction is protecting the integrity of the roof and the structural components under the garden. For this reason there must be waterproofing of exceptional longevity to prevent damage and to reduce the possibility of long term expensive reconstruction. For this reason it is recommended a completely new waterproofing layer be added to the existing structure to insure the longevity and integrity of the waterproofing system.

Load Bearing Capacity

The structural engineer should verify the maximum load bearing capacity of the existing structure. These figures should be available from the records of the previous construction. Typically, a minimum additional dead load limit of 150 psf between columns is needed to accommodate the construction of a roof garden. Loads above columns and at the roof's edge can be considerably higher; however a structural engineer should be consulted to establish the load bearing capacity of those areas.

These higher load bearing areas should be used to accommodate larger specimen plantings and trees.

Waterproofing

As mention before, a completely new waterproofing system should be installed to protect the building's structure. There are several types of waterproofing systems available; however, elastomeric materials offer the greatest protection. Bituminous waterproofing should be avoided. Over time the organic components in bituminous waterproofing interact with the soils and the plant materials and therefore increase the likelihood of system failure.

A properly installed waterproofing system can last the lifetime of the building, however a single small leak may require the removal of the entire garden to find and repair the damage. Therefore, in order to insure the integrity of the waterproofing it is recommended that a protective topping coat of concrete be applied, as soon as possible, following the installation of the new waterproofing.

Recent Development in Waterproofing Methods

Demands for reliable waterproofing as well assured moisture and vapor leakage proofing methods and materials are being constantly felt. In many civil engineering constructions, traditional methods of waterproofing have not been found satisfactory. Consequently, repairs and renewals have to be taken up more often than they are required.

How Green Roofs Are Made

Green roofs are constructed in layers on top of the roof. The number of layers depends on the type and root depth of the plants selected, the slope of the roof, and the materials used in the layers. Layers can include, from the top down to the roof, the following: a filter fabric to hold the plants in place, Innovative roof garden systems the growth media and the plants, a drainage Polygum roof garden, root resistant membrane layer, a root barrier, an insulation layer, and a waterproofing layer. Sometimes more than one function is combined in a single layer.

The filter fabric holds the soil in place and prevents small soil particles from entering and clogging the drainage layer underneath. Generally, growth media is a mix of about two–thirds inorganic material (such as expanded slate or crushed clay) and one-third organic material (humus and topsoil).

This mixture provides essential drainage, soil air capacity, and organic nutrients.

The drainage layer carries away excess water and makes an extremely stable and pressureresistant sub-base. A root barrier prevents deep roots (in the case of trees, for example) from damaging the roof. The insulation layer is optional and prevents water stored in the green roof system from extracting heat in the winter or cool air in the summer. The waterproofing layer is critical and ensures that water doesn't seep into the roof.

System of Waterproofing

Waterproofing systems have been grouped into five types according to type of construction and characteristics of material of waterproofing. However following are widely favored.

a) Protective coatings–Recently Developed Systems

b) Waterproofing by injections.

The injection system of waterproofing by chemical grout has been found to be successful in stopping leakages completely even at a number of underground projects.

Recent Advancement

Polymer technology and its applications in waterproofing field are the modern techniques of waterproofing using newly developed products. Benefits derived from the use of polymer in concrete for waterproofing are multifold particularly with regard to its efficiency and reliability. New generation has successfully used it for stopping leakages in building constructions, tunnels; power houses underwater structures, dams, reservoirs, aqueducts, etc.

Classification of Water– proofing Method/ Systems

The waterproofing Methods/ systems can conveniently be classified according to five major characteristics of Building materials, to oppose or resist the ingress of moisture.

Hydrolithic Water-proofing

Hydrolithic action mostly uses water as a base media and provides protective coating over the porous surface. Dampness and efflorescence, which cause formation of powdery white deposits on the surface of brick walls and masonry constructions, are common problems in most of the buildings. The likely damages are mainly due to ingress of water into masonry. Protective surface coatings; used mainly for getting a better surface finish and appearance of the structure, also offer resistance to moisture ingress.

Various coating materials such as epoxies polyurethanes chlorinated rubber acrylates etc. are available. They form a film to oppose entry of water. Some coatings give good film but have little penetration with the result in the long run they loose adhesion with concrete.

Certain coatings are vapor permeable, which allow passage to gases and vapors but exclude passage of liquids, vapor, barrier. Coatings block passage of both vapors and liquids and are suitable for underground or water logged structures also.

Plastic emulsions are particularly water-soluble. Epoxy emulsions, Acrylates, Silicones etc, have been used to stop the efflorescence and dampness. A few paints based on acrylic emulsion show better results but have failed to arrest efflorescence completely though epoxy emulsion as well as silicones being low viscosity liquids was found useful because they could travel deep. Permeability of water vapor by pressure is reduced completely by formation of insoluble mass or plastic membrane like film into pores. Performance of these emulsions are superior and allow the concrete to breath, i.e. allow entrapped water vapor to permeate out without allowing diffusion of oxygen, carbon dioxide, chlorine ion or rain water from outside. After the application of the protective coating, an additional coating of polymer emulsion mixed cementitious mortar is preferable as it provides high impermeability.

Integral Waterproofing

Waterproofing by inclusion of integral waterproofing compound in concrete in one form or the other has been widely carried out for many years since past and they are generally believed to have no adverse effect on durability provided they don't contain chloride. Polymer based integral waterproofing cement compounds give good performance, and provides high durability to concrete.

There are a number of integral waterproofing compounds and liquids conforming to the requirements of Indian Standards Specifications I.S. 2645-1975 These products are of two types:

(i) Water repellant materials such as stearates or oleates, which through surface tension effects discourage the penetration of dampness into and through concrete.

(ii) Fine particulate materials often used in conjunction with water reducers, which partially block and reduce the size of the pores in concrete, and lower the permeability to water.

Neither type of materials make concrete truly waterproof. The former can be beneficial in discouraging rising damp and rain penetration, but it has little effect against an applied head of water. In some circumstances, it can reduce problems of efflorescence.

The latter can be effective in lowering the permeability of low quality concrete but is unlikely to cause any significant improvement in that of better quality concrete (with a water/cement ratio less than 0.6) so called waterproof concrete, i.e. that containing a waterproofing admixture is not an acceptable substitute for a dampproof membrane for slabs–on ground.

New generation waterproofing techniques include polymer-based integral waterproofing cement compounds. These products are water soluble and have been found to provide significant eduction in the permeability of concrete due to more effective dispersion of cement particles in the mix and cause substantially high reduction in water/cement ratio. Water permeability is reduced tremendously by more than 85 per–cent.

Integral Waterproofers

Integral cement waterproofing compounds are generally mixed with cement at a prescribed dosage rate. These are broadly classified into three groups; permeability reducers, water repellents or hydrophobers and polymer modifier for cement. Fine particulate materials like round sand, whitening, bentonite, flyash, colloidal silica and flurosilicates; salts of high sulphonic acids, detergents and sulphonated carbohydrates are mostly used as permeability reducers and their major role is to reduce the water –cement ratio and hence the permeability of mortar or concrete. Another class of chloride free integral waterproofers based on air entraining plasticizers, normal plasticizers, super plasticizers, and retarding plasticizers are the most modern types to make the concrete waterproof. Such materials are specially used for marine and super structures.

Water repellents or hydrophobers are generally soaps - water-soluble and sulphonium salts of fatty acids, butyl stearate, and selected petroleum products like mineral oils, waxes, and emulsified asphalts etc. The chemicals in such waterproofers form a thin hydrophobic layer within the network of cement mass by coating the cement particles.

Polymer modifier used for cement is organic polymers dispersed in water. These are now extensively used in the country these days as they impart better flexibility, reduce water permeability, increases tensile strength and bonding behavior of cement particles and hence provide excellent waterproofing properties. The materials used are discussed in detail later in the text.

Membrane Type of Waterproofing

It affords a highly impermeable layer for durable waterproofing and protection of roofs, terraces, balconies, sun shades etc. against extreme weathering conditions, irregular temperature variations, industrial pollution, rain etc. It may be, a modified polymer based durable and elastic coating in semi-paste state, which forms a seamless, continuous watertight flexible membrane that makes the treated surface impermeable to water.

Failure Limits of Weatherproof Membrane and Base Fabric

Failure in the function of the weatherproof membrane can take in the following forms:

  1. Failure to discharge rainwater by ponding
  2. Local movement blistering ripples cockling
  3. Slippage or creep.
  4. Rupture: Local cracking or holes
  5. Delamination or uplift
  6. Puncturing.
  7. Degradation of surface material.
(a) to (e) can be brought about or be aggravated by movements in the substrate or structure or in the weatherproofing layer itself.

Sheeting Materials–As Roofing Membrane

Single ply roofing membrane is latest addition in the area of waterproofing. The recent advances in polymer science have benefitted the roofing technology as it has resulted into the development of number of new roofing materials during the last 20 years or so. [Flexible PVC membrane is most popular in the thermoplastic category and EPDM (ethylene propylene diene monomers) is the most popular in the elastomeric category though polychloroprene, polyisobutylene and chlorosulphonated polyethylene elastomeric membranes are also being manufactured and used; as detailed later]. Single ply membrane can be used in loose laid (ballasted), adhered, or mechanically fastened systems with insulation atop or beneath the sheet. When loosely laid the membrane remains unattached to the substrate except at the perimeter of the roof and penetration such as vent pipes. Because it is free floating, it can accommodate movement of the substrate and small amounts of entrapped moisture. However, it should be weighted down with smooth river gravel, avers of concrete blocks to prevent wind uplift. The ballast provides also additional protection against attack by wind uplift. Some obvious limitations of this system are that it can only be used on flat roofs and only on buildings that can structurally support the weight of ballast, otherwise sagging and ponding may occur.

The membrane may be attached with adhesive used alone or in combination with mechanical fasteners. In these cases, ballast is not required. Fully adhered systems may be used to advantage for covering sloped roofs. Preparation of the deck to ensure having a clean smooth, stable surface with taped joints is essential to maintain good bond. Partial bonding with adhesives alone or in combination with mechanical fasteners allows greater movement of membrane than a fully adhered system. Depending on the type of membrane, joints are made between sheets by heat fusion, torching, solvent welding, and tacky tapes or with adhesives

The major advantages of such elastomeric membranes are.
  1. These are lightweight; weight is about 1.2 kg/m2 as against 9 kg/ m2 for a built-up-roofing membrane, or 4.5 kg/m2 for a single ply modified bitumen membrane, which makes them the obvious and preferred choice for lightweight constructions.
  2. Work with single ply roofing proceeds cleanly and quickly and large areas can be closed in under wider range of climatic condition. Because the sheet is lightweight, it is often possible to re-roof with minimum surface preparation and frequently without having to spend the extra time and money for removal and disposal of the old roof. Also, this permits undisturbed occupancy of the building reroofing.
  3. The single-ply roofing provides the architect with new degrees of freedom in color and design.
  4. The improved safety aspects of the single ply synthetic roofing systems are deserving of mention in regard to both the installation crews and the structure. The potential for burns and inhalation of hot bitumen fumes is eliminated. Fire hazards are also reduced.

Epoxy Based Coatings

One of the widely accepted polymeric materials by the construction industry is epoxy resin; as epoxy based products provide solution to various construction problems in the form of coatings, sealants, mortars, adhesives, injection grouts and so on. The increasing demand of epoxy based products; (growth rate of more than 20. percent per annum) is mainly due to their good mechanical strength, adhesion with different substrates and chemical resistance. Epoxy resins modified with other polymeric systems such as polysulphides, phenolic etc. and coal tar have also been recently developed to meet out the requirements of aggressive and industrial environment.

Epoxy based coatings are mostly used as waterproofing, damp proofing and protective coatings for internal applications. Water thinable epoxy coatings can be applied on wet surfaces for damp proofing.

Epoxy grouts, though costly as compared to other polymeric grouting materials, have an edge over other chemical grouts as these can restore structural integrity by bonding cracks together, even in the presence of water and have superior chemical resistance. Effectiveness of chemical grouting is increased manifold, if grouting is followed by the application of polymer liquid membrane compatible with the grouting polymer.

Polyurethane Based Coatings

Polyurethane based coatings and compounds are the most versatile materials which have ttracted the building industry because of their efficacy in solving the commonly faced problems related to the ingress of water and protection of structures. Polyurethane based products are available either in the form of one component or two component systems. Onecomponent systems are generally solvent based while two component systems can either be solvent based or solvent free. Solvent based polyurethanes are mostly used for waterproofing, dampproofing, flooring and anticorrosive coatings while the applications of solvent free polyurethanes are as sealants, chemical grouts, insulation etc. The superiority of polyurethanes over other liquid membranes are due to their high elasticity (can accommodate expansion and contraction of substrate due to temperature variations), strong adhesion to substrate, high abrasion and cracking resistance (film can withstand erosion due to rain and wind as well as movement of people without wear and tear), higher resistance to biological defacement and UV radiation. The occurrence of moisture curable polyurethanes is another significant addition in this field.

Moisture curable polyurethanes are used for various applications such as adhesives, sealants, and damp proofing of concrete structures. They provide durable and cost-effective solution for the protection of structures in damp conditions.

Keeping in view the properties of various coating systems, their applications have been broadly summonsed in Table-1.

Sheeting Materials- Single–Ply Synthetic Roofing Membranes

Single ply roofing membranes is another latest addition in the area of waterproofing. The recent advances in polymer science have benefited the roofing technology as has resulted into the development of number of new roofing materials during the last 20 years or so. Flexible PVC membrane is most popular in the thermoplastic category and EPDM (ethylene propylene diene monomers) is the most popular in the elastomeric category though polychloroprene, polyisobutylene, and chlorosulphonated polyethylene elastomeric membranes are also being manufactured and used.

Single ply membrane can be used in loose laid (ballasted), adhered, or mechanically fastened systems with insulation atop or beneath the sheet. When loosely laid, the membrane remains unattached to the substrate except at the perimeter of the roof and penetration such as vent pipes. Because it is free-floating, it can accommodate movement of the substrate and small amounts of entrapped moisture. However, it should be weighted down with smooth river gravel, pavers or concrete blocks to prevent wind uplift. The ballast also provides additional protection against attack by ultraviolet light and may prevent tear propagation by wind uplift. Some obvious limitations of this system are that it can only be used on flat roofs and only on buildings that can structurally support the weight of ballast, otherwise sagging and ponding may occur.

The membrane may be attached with adhesive used alone or in combination with mechanical fasteners. In these cases, ballast layer is not required. Fully adhered systems may be used to dvantage for covering sloped roofs. Preparation of the deck to ensure having a clean smooth, stable surface with taped joints is essential to maintain good bond. Partial bonding with adhesives alone or in combination with mechanical fasteners allows greater movement of the membrane than a fully adhered system.

Depending on the type of membrane, joints are made between sheets by heat fusion, torching, solvent welding, and tacky tapes or with adhesives. The major advantages of such electrometric membranes are:

(i)These are lightweight, weight is about 1.2 kg/ m2 as against 9 kg/m2 for a built-up-roofing membrane or 4.5 kg/m2 for a single ply modified bitumen membrane, which make them the obvious and preferred choice for lightweight constructions.

(ii) Work with single ply roofing proceeds cleanly and quickly and large areas can be closed in under wider range of climatic condition. Because the sheet is lightweight, it is often possible to re-roof with minimum surface preparation and frequently without having to spend the extra time and money for removal and disposal of the old roof. In addition, this permits undisturbed occupancy of the building during re-roofing.

(iii)The single-ply roofing provides the architect with new degrees of freedom in color and design.

(iv)The improved safety aspects of the single ply synthetic roofing systems are deserving of mention in regard to both the installation crews and the structure.

The potential for burns and inhalation of hot bitumen fumes is eliminated. Fire hazards are reduced.

Conclusion

The waterproofing systems described above have been successfully adopted abroad and research on improvement on the subject is in progress.

References

  • I.S. 3384–1965 Specification for bitumen primer for use in water– proofing and damp proofing.
  • I.S. 3067–1966 Code of practice for general design details and preparatory work for damp proofing and waterproofing of building.
  • Albrecht Dürr, Dachbegrünung: Ein Ökologischer Ausgleich translated: Green Roofs: An Ecological Balance. (Bauverlag, GmbH, Wiesbaden and Berlin, Germany 1995)
  • Briggs, Greg S. "Why Should You Care About Green Buildings?" Skill Ward Magnusson Barkshire, Inc. 15 Sept. 2000.
  • Canada's Office of Urban Agriculture. City Farmer Publication. Urban Agricultural Notes "Rooftop Gardens."
  • City of Olympia, Public Works Department, Water Resources Program, Impervious Surface Reduction Study, Final Report, (May 1995).
  • Daniels, Elizabeth. "Green Buildings Starts at the Top." Business & Industry Resource Venture 15 Sept. 2000.
  • Environment News Network (ENN). "Chicago Hopes Rooftop Garden Cools Air." United Press International. 17 May 2000. www.enn.com.
  • "Green Groweth the Rooftops." Environment Dec 2000: vol 42, p. 7.
  • Knepper, Claire A. "Gardens in the Sky." Journal of Property Management. Mar-Apr 2000, vol. 65.: 36-40.
  • Suryawanshi C.S (Dr), "Water– proofing of Civil Engineering Structures." Hand Book for Practising Engineers 2001.

NBMCW April 2008

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