Water related factors of deterioration to buildings may take the largest toll on structures which are snow, rain, moisture, internal condensation, and humidity. Biological factors include fungi, bacteria, and insects. Chemical contributors may include oxidizing agents, i.e bleach, reducing agents, i.e. sulfides, acids, i.e. bird droppings, bases, i.e lime, salts, i.e. chlorides, or even chemically neutral substances such as fat or oil. Solar radiation, air quality, freeze thaw effects and wind are other environmental contributors to building deterioration.
The majority of building envelope failures can be attributed to water, in one of its many forms (gas, liquid, and solid). The sources of water that could affect a building envelope include:
Water ingress and absorption. Water ingress is typically a function of moisture load and enclosure resistance. Most materials or systems have the capability to absorb some water for a defined period of time without degradation..If absorbed moisture is allowed to sufficiently dry prior to the period which degradation will occur, then these materials could achieve reasonable durability despite the absorption of water. Enclosure assemblies can show signs of water entry due to forces such as gravity, capillary action, or wind blown water. Material deterioration can occur if the water ingress cannot be managed or drained to the exterior, or if materials do not have the capability to store water without degradation.
Condensation. Condensation occurs on a surface with a temperature below the dew point of the air in which it exists. The likelihood or extent of condensation is related to the relative humidity of the air and material temperatures. Problematic condensation within building envelopes is often related to uncontrolled air leakage, vapor diffusion, rain penetration, or snow melt. Condensation is typically controlled through the careful design and installation of air and vapor barriers.
High RH levels. Although condensation is typically the result of high micro climate RH levels, situations can exist where materials are damaged due to sustained high RH levels without condensation (i.e. mold growth)
Deterioration factors in concrete
o Physical processes – freeze / thaw, abrasion, thermal cracking
o Carbonation and ingress of chlorides, leading to a risk of reinforcement corrosion in the presence of water and oxygen
o Chemical attack – includes the external attack of sulfates and acids, and internal attack of alkali aggregate reaction.
Deterioration factors in steel
o Corrosion is a major deterioration factor in steel, which need a combination of water and oxygen to corrode.
o Corrosion may be provoked by particularly aggressive environments.
Deterioration factors in timber
Main durability risk factors in timber are moisture, insects and fungi. From these, the following durability issues can arise:
o Deformation of members due to moisture movement
o Fungal decay (dry and wet rot) and insect attack ( wood boring beetles and termites)
o Structural performance phenomena can occur like reduction in strength and stiffness.
Air and Air Pollutants
Air and its components – oxygen, nitrogen, and other by-products can be an agent for deterioration, as well as a transportation mechanism. As a transportation mechanism, air can bring moisture, water, and pollutants to areas of the building envelope that would normally be protected from these agents. Moist air traveling through a building envelope can result in mold growth on organic materials or corrosion on metallic materials. Common air contaminants include chlorides in maritime climates, sulphur dioxide from vehicle emissions, hydrochloric acid near manufacturing plants, nitric acid from fossil fuel combustion, and chlorine in pool environments. Buildings located in environments with these high concentrations of reactive contaminants can experience more rapid degradation of a variety of building envelope components.
Wind plays an important role in the service life of building materials. Enclosure design requires consideration for peak loading as well as cyclical loads that cause shortened life from “overworked” materials. Wind loading can also cause depressurization of enclosure cavities, which can increase air leakage, water ingress, moisture movement and condensation. Wind pressures are also responsible for uplift on roofing assemblies and wind driven rain that can penetrate unprotected areas.
Biological and Ecological Agents
Molds or fungi, as well as rodents, insects, and birds can affect the service life of building materials. The presence of fungi, tempered air and moisture (typically above 22% moisture content in wood materials) can cause deterioration of organic materials and unacceptable occupant health conditions.
Insects, birds, and rodents can damage materials by digesting, gnawing, nesting or depositing corrosive droppings. Vegetation in the form of vertical vines or horizontal landscaping can significantly impact building fascades and structural elements due to root growth.
Extreme temperatures or temperature fluctuations can cause significant movement in materials like copper and vinyl, creating deformation of materials, and unintended gaps and hole at material junctures. Freezing temperature can lead to frost heaving, ice jacking, spalling of masonry and damage to brittle materials. Excessive heating of materials (i.e. metal flashing and roofing can lead to “bleeding” of materials onto finished cladding, and material cracking, bulging or ridging. Extremely hot temperature, such as those that might occur in building fires can have a multitude of detrimental effects with respect to service life. These temperatures can temporarily or permanently change the physical properties of materials, making them ineffective for their intended use.
Material selection and the assembled enclosure can be greatly influenced by UV radiation from the sun. When material absorbs radiation from the sun, energy is produced that can cause a chemical reaction and material property changes (i.e., becoming brittle, yellowing, chalking, or fading). Most assemblies with UV sensitive material require the use of a covering material (i.e., metal flashing over exposed roofing membrane), limiting building aesthetic and design options. Other materials have limited service life as a result of UV degradation (i.e., many sealant materials and water based paint finishes). Conversely, night sky radiation can also cause heat loss and problems with condensation and corrosion in some roofing assemblies (i.e. zinc roofing)
Chemical Reactions and Incompatibility
Although chemical reactions are not a specific environmental agent, they are typically coupled with environmental agents to cause damage (i.e. corrosion).
For example, galvanic corrosion is a typical problem with incompatibility between metals, or the use of pressure treated wood and zinc coated fasteners. Other compatibility issues include the use of various coatings, caulking, and membranes in contact with each other.
Positive Aspects of Agents Affecting Building Durability
There are some positive side effects that impact the service life due of building materials. For example, patina corrosion protects copper roofs, temperature shifts dry moist materials, landscaped or green roofs protect UV sensitive roofing materials, wind cools buildings, and water running over zinc strips minimizes algae growth.
HOW CAN DURABILITY AND SERVICE LIFE BE IMPROVED?
The main culprits that can rob durability are poor workmanship and lack of knowledge of the properties of materials. It is important to identify the problems that manifest themselves as shortcomings in our traditional materials and look for opportunities to improve material performance in housing and buildings. Explore new techniques, materials, components and systems that promise to improve durability while reducing life-cycle costs. Develop methods for accelerated assessment of materials, components and systems that reflect in-place builder installed performance. Greater attention should be paid to details which influence how a structure deals with water run off and drainage.
Building components require varying degrees of maintenance, repair and replacement during the life cycle of a building. The extent and intensity of maintenance, repair and replacement varies significantly, depending on how appropriately the service life of materials, assemblies, and systems are harmonized, and how accessible they are for periodic maintenance, and replacements.