Today’s architect knows that successful and repeatable sustainable design is a complex undertaking dependant on the collaboration of best-in-class design- build teams. What many architects may not realize, however, is that extending the traditional definition of this team to include building product manufacturers may provide new opportunities to access world-class research and development in the area of sustainable construction practices.
Specifically, chemists are continuously discovering ways to improve building products and, at the same time, providing the industry with much-needed metrics for defining sustainability including third-party verified analysis of environmental impacts and life-cycle assessments. The result is that today’s construction professionals can now access a vast portfolio of sustainable building solutions that were unimaginable just 10 or 20 years ago.
The following article will address the sustainability benefits of building envelope elements that have been advanced by chemistry.
Design professionals play a major role in slowing, stopping and perhaps someday reversing, the negative impact buildings have on the health of the planet. The factoids and statistics are plentiful.
According to www.epa.gov/greenbuilding, buildings accounted for 72 percent of total U.S. electricity consumption in 2006, and this number will rise to 75 percent by 2025. Building occupants also use 13 percent of the total water consumed in the United States per day. In addition, building-related construction and demolition debris totals approximately 160 million tons per year, accounting for nearly 26 percent of total non-industrial waste generation in the U.S.Commercial buildings alone use 20 percent of our energy and cause 17 percent of the annual greenhouse gas (GHG) emissions in North America, according to ENERGY STAR data. Residences use another 20 percent of our energy and emit a similar amount of GHGs - namely 1,270 megatons. Let’s see: 1,270 million tons or 1,270,000,000 tons or 2,540,000,000,000 pounds - that’s 2.54 quadrillion pounds of harmful gases. Combining residences and commercial buildings, that’s more than 5 quadrillion (16 zeros!) pounds of foul air every year. Let’s get to work!
However, on the bright side, a properly designed, high-performance building envelope can reduce a structure’s energy consumption by as much as 40 percent, according to “Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use,” National Institute of Standards and Technology. And as architects, we know a 40 percent savings is very possible: some of us are doing that and better every day.
Even these few but dramatic facts make it clear that improved performance of the building envelope is critical to reducing our collective carbon footprint and making our built environment more sustainable - albeit one building at a time. Each building (envelope) we create is a functional system of construction components, which - at a minimum - must protect its inhabitants against sun, rain, snow, hail, wind, dust, pollutants, allergens and pests, all while provide structural integrity as well. In addition, sustainability remains an overarching imperative that a high-performance building must also address through the components of its building envelope.
Design-build teams are faced with countless decisions and held responsible for the long-term consequences. Many of those decisions benefit from discoveries in chemistry, which are then adopted and adapted by building product manufacturers, who then create and supply increasingly sustainable products or assemblies. This article looks at the performance benefits that advancements in chemistry bring to each of six building envelope components:
3. Wall Systems
5. Air and Weather Barriers
<<1>> High-Performance Foam Insulation Materials
All can agree that a high-performance, sustainable building is dependant on a high-performance building envelope. Optimizing exterior and interior walls, roofs and foundations with plastic foam insulation provides many benefits for reducing energy demand, improving indoor air quality, reducing moisture and pollution infiltration and extending the service life of the building as a whole. We will look at three plastic foam insulating materials in the section below, including:
• Spray-applied polyurethane foam (SPF) insulation, air barrier and roofing systems (closed and open cell)
• Extruded polyurethane (PU)
• Expandable polystyrene (EPS)
Spray-Applied Polyurethane Foam (SPF)
SPF is used in a number of ways in construction. SPF is a two-component product that is manufactured onsite, but engineered in the molecular level to optimize performance for a specific application. By varying key components, the finished product can be modified to meet specific performance requirements for roofing applications, insulating air barrier systems, adhesive applications or wall insulation.
SPF insulation has two basic formulations: open-cell (low-density (LD) 8 kg/m3to 12 kg/m3(0.5 to 0.7 pcf) and closed-cell (medium-density (MD) 24 kg/m3to 48 kg/m3(1.5 pcf to 3 pcf). (A third formulation, high-density (HD), is used for roofing applications.) For each cell type, the liquid - comprised of isocyanate and a formulated resin - is mixed in the spray gun and then propelled by a blowing agent - water for open-cell and a chemical blowing agent for closed cell. In both cases, the formulations have no ozone-depleting blowing agents, formaldehyde or any ozone depleting chemicals, and they emit low to no VOCs. The formulated resin portion of any SPF product uses a very small amount of a renewable resource like soy oil or sucrose-based oil, but in all cases that natural component is negligible. The catalyst starts when the pressurized mix comes out as a liquid, rises as a foam and can expand up to 30 times. The major differences are:
• Open-Cell SPF is soft to the touch, has an R-value around 3.5 per inch, and is usually less expensive than closed-cell foam. It is water blown in place. An open-cell foam is defined as having around 60 percent of the cells within the material open (think popped bubbles), making it useful to absorb sound, but should not be used as a vapor or water barrier.
• Closed-Cell SPF is defined as having greater than 90 percent of the cells within the material closed and is chemically blown (with zero–ozone-depleting blowing agents), expands rapidly and significantly upon application, fills all voids, and sets up hard, forming a rigid foam plastic. It chemically bonds to the surface to which it is sprayed. It is an excellent air, moisture and vapor barrier, but not a sound absorber. It is denser, heavier, and offers the superior insulating performance of the two with an R-value around 6.5 per inch of thickness. It is usually more expensive.
In addition, SPF (and any closed-cell plastic foam insulation material) is FEMA-approved for flood prone areas. Because it is rigid and monolithic, its increased strength makes it an appropriate material in hurricane zones. The National Home Builders Association’s SPF research determined that it provides significant racking resistance in stick built projects, providing some structural benefit as well.
SPF is an efficient insulation material for roof and wall insulation (as we will discuss shortly), insulated windows and doors, and air barrier continuity components (sealants) - all contributors to reduced air leakage (infiltration or exfiltration), which accounts for 25 percent to 40 percent of (wasted) energy use according to ENERGY STAR studies.
As a high-performance insulation material, SPF can reduce the amount of fossil fuels needed to heat and cool buildings, reducing the resulting greenhouse gas emissions, and may require less energy to produce than some other insulation products. Plus, the amount of energy required to transport and install SPF is also minimized in comparison to other alternatives, according to a life-cycle Eco-Efficiency Analysis.
Finally, SPF is durable and maintains its physical properties over time. In a vertical wall application, it should last the lifetime of the building. In a low-slope roofing application, its lifespan can be renewed indefinitely with simple recoats (read more about SPF roofing below). It contributes little to the waste stream, and in a single product (depending on the formula and application, as well as local code requirements) can do the job of three or four products - insulation, air barrier, sealant, vapor barrier and weather barrier. Spray-applied polyurethane foam insulates and eliminates thermal bridging through fasteners or gaps in decking.
Polyurethane PU foam has an R-value of 6.7 per inch of material and is a closed-cell foam that uses a zero-ozone-depleting chemical blowing agent. It exhibits most the performance characteristics and benefits of closed cell SPF, but is engineered to be poured-in-place rather than spray applied. Rigid closed cell PU can be found in a variety of densities and is a key component used in Structural Insulated Panels (SIPs) for walls, floors, and roofs, as well as Insulating Concrete Forms (ICFs), insulated masonry block, entry doors, garage doors, exterior shutters, insulated siding and many insulation applications in civil engineering and infrastructure projects. PU is a key ingredient in OEM products such as weather stripping, gasketing and door sweeps. Outside of insulation, PU can be found in interior and exterior moldings and trim, ceiling medallions, decking, railings and porch posts, architectural columns, louver gable vents and composite board stock.
Expandable Polystyrene (EPS)
EPS offers an R-value of 3.5 per inch and is traditionally used for board stock insulation, and in wall systems such as SIPs, ICFs and Exterior Insulated Finishing Systems (EIFS). Patented in 1950, EPS is a cost-effective and easy-to-handle insulation material. As a closed-cell plastic foam insulation material, it is FEMA approved for use in flood zones.
A recent development in EPS is the advent of graphite-enhanced formulations. Graphite-enhanced EPS features microscopic flakes of graphite that work as small mirrors to reflect heat back into the environment and increase insulation value up to 20 percent versus standard EPS, reducing the thickness of insulation needed for equal results. In retrofit applications, the insulation is attached to gypsum board and installed on the interior side of exterior walls to provide a one-step process for both interior finishing and enhanced insulation performance. For new construction, graphite-enhanced EPS is gaining popularity for SIPs, ICFs and EIFS.
Energy efficiency is the major environmental and financial benefit these high-performance insulation solutions provide. Insulating R-values and resulting performance can be twice as effective as traditional materials.
1. Stop thermal bridging, which occurs when materials that are poor insulators come in contact with each other, allowing heat to flow through the path created. Insulation around or beside a bridge is of little help in preventing heat loss or gain due to thermal bridging. The bridging can be improved or eliminated by creative design to “dismantle the bridge” or by using materials with better insulating properties in the bridging configuration itself or by using more effective insulation products to block or isolate the heat/cold from reaching the bridge.
2. Reduce or eliminate convective loops, which are wall and ceiling cavities that act as room-sized heat exchangers, relentlessly pumping heat out of a space (to an attic or basement) even if there is no direct air leakage from indoors to outdoors. Convective loop heat losses occur in buildings within and at the top or bottom of uninsulated, wood-framed or metal stud interior partition walls and, of course, in hollow core masonry walls. They can be eliminated as well with proper application of SPF insulation that seals the cracks and crevices and inefficient wall assemblies that facilitate convective loops.
3. Likewise, uncontrolled air leakage can be reduced or eliminated with proper specification and application of high-performance insulation products like those discussed here.
Keeping precipitation out the building is the priority for any roof. Built-up roofs (BURs) - in various configurations and with numerous material combinations - have been on that job many decades. Other systems common for low-slope applications, including metal roofs, membrane roofing materials and spray-applied polyurethane foam (SPF) may be considered comparatively new. And vegetative roofs are enjoying resurgence as concerns for the environment grow. Let’s look at these last two broad roof categories, which have some important similarities, and better understand what chemistry has added to the mix of high-performance, more sustainable roofs.
Whether a roof has a natural, vegetative top layer or a synthetic, man-made look, an important element in each case is “what’s underneath?” Drainage and insulation can be handled in a number of ways, of course, but in many cases these two considerations get resolved or refined between the structure supporting of the roof and its waterproofing layer. Drainage is key to managing the precipitation that lands on the roof, and insulation helps manage the impact of the outside air and elements on the comfort of the inhabitants inside. In many cases, the insulation material used and the benefits it provides are essential to the roof system and overall building envelope performance.
Most low-slope roofing systems employ board stock insulation materials made from either expandable polystyrene (EPS) or extruded polystyrene (XPS). With an SPF roofing system, the system is comprised of a material known best for its insulation properties that also offers other qualities needed for a high-performance roof.
SPF Roofing Systems
As previously described, SPF is an insulation material commonly found in vertical wall applications that is also used in low-slope roof applications. It is unique from all other roofing systems. SPF is a generic chemical-basedproduct that has been adopted and adapted by countless building product manufacturers, all over the U.S. and the world. SPF is certainly not a new product, but its benefits are many and have stood the test of time.
As most architects and builders know, SPF insulation is rigid, lightweight, wind resistant and effective in extreme temperatures and weather conditions. SPF insulation has the highest R-value per square inch of any commercially available insulation material. It is a very cost effective and sustainable means to rescue a failing roof because it can be installed directly to the existing substrate without tear-off in about 95 percent of retrofit situations (according to Spray Polyurethane Foam Alliance Life Cycle Cost Study, Michelsen Technologies, LLC, 2004), provided that the materials from the original roof are structurally sound and have not taken on too much water. This practice can divert thousands of tons of waste from the landfill, while also affording a fast installation with limited disruption to occupants.
SPF is ideal for (very) low-slope roofs because its application and finished thickness can be very well controlled to provide slope-to-drain and avoid low spots (ponding). At the other extreme, SPF is ideal for domes, odd shaped roofs, or roofs with many openings for skylights, etc., because it can be applied to virtually any configuration with the same high-performance results. In fact, SPF insulated roofs paid for themselves through energy savings in 4.5 years on average, based on a Texas A&M study entitled “Energy Data Measuring Cost Saving on Campus SPF Roofs Compared to BUR Roofs,” Gerald Scott, PE, 1985.
SPF roofing systems also offer industry leading wind uplift resistance because there are no edges or seams for the wind to grab and pull away from the structure. Its composition allows it to withstand hailstorms and windborne debris while offering a 20-year life expectancy with limited maintenance requirements.
To protect the foam from the elements, and particularly from degradation due to exposure to UV rays, SPF roofing requires a top coating. Budget, climate, aesthetics and the choice of installer often determine which choice is best. Aggregate granules can be incorporated into the coating layer to provide a non-slip surface for those conducting roof inspections or servicing rooftop mechanical equipment. Three elastomeric coatings most frequently used as coatings for SPF roofs are:
• Urethane, typically the longest lasting of the choices;
• Silicone, which holds up well to high impacts/hail; or
• Acrylic, which resists dirt and tends to stay whiter longer are usually used.
All three of these high albedo-coatings (very reflective) do the following:
1. Reduce the absorption of solar energy;
2. Reduce surface temperatures;
3. Reduce heat transfer into the building; and
4. Help to reduce urban heat island effect and smog.
And to maximize cooling energy savings, these coatings typically have:
5. High solar reflectivity;
6. High infrared emissivity; and
7. Retain these properties for many years.
Elastomeric coatings tend to last between 10 and 15 years, depending on weather conditions and the amount of foot traffic they are exposed to during that time span. At the end of the coating’s service life, the SPF roof can be renewed by simply removing the coating and a minimal (1.4 inches to .5 inch) layer of the SPF (known as scarfing). A new layer of SPF (.5 inch to 1 inch) is applied, followed by a fresh coating. This practice can be repeated almost indefinitely, giving SPF roofing one of the longest life expectancies available for low-slope roofing.
A vegetative roof is first and foremost a roof, and therefore, has to get the waterproofing right. It is vital that the membrane underneath the plantings is durable and withstands the test of time. Again chemistry has made a significant contribution to the waterproofing component of a vegetative assembly. Done properly, a vegetative roof can double or even triple the useful life of the waterproofing system (roof) underneath it.
The chemical ingredients found in acceptable membrane roofing types for vegetative roofing according to the Federal Green Construction Guide for Specifiers, Section 07 55 63 (Section 07530)– Vegetated Protected Membrane Roofing,published by the Whole Building Design Guide, include:
• Thermoplastic polyolefin (TPO)
• Polyisobutylene (PIB)
• Polyethylene terephthalate (PET)
• High-density polyethylene (DHPE)
• Low-density polyethylene (LDPE)
• Polyvinyl chloride (PVC)
These chemical compounds are key ingredients in membrane materials, but they are not the membranes themselves. Performance of the final membrane product depends on each manufacturer’s unique recipe - even for comparable products based on the same chemical ingredients. For example, TPO is chlorine-free and offers high breaking and tearing strength and puncture resistance; PIB is gas-impermeable, requires no vulcanization, is elastic and most importantly, retains flexibility at low temperatures; and PVC provides excellent weathering characteristics, high tensile strength and long-term flexibility with excellent resistance to harsh chemicals and industrial pollutants.
Each of these membrane roof materials ensure the numerous benefits of vegetative roofs are realized:
• Reduced heat island effect
• Replacement of the green footprint lost when the building was constructed
• Control and retention of stormwater runoff
• Clean outside air with less CO2 and other harmful contaminates
All of these high-performance waterproofing membranes provide a robust, long-lasting solution and are recommended according to the Whole Building Design Guide.
<<3>> Wall Systems
Metal stud walls in commercial buildings and stick-built homes have been around a long time, but they are losing favor to more advanced wall systems that take advantage of leading edge chemistry and innovation by building product manufacturers to offer impressive environmental benefits among many others.
The following three wall systems can provide a more sustainable, high-performance building envelope than conventional construction techniques, especially when employed in concert with the other building envelope elements discussed here.
• Structural Insulated Panels (SIPs)
• Exterior Insulated Finishing Systems (EIFS)
• Insulated Concrete Forms (ICFs)
The common denominator for each of these wall systems is the insulation component expandable polystyrene (EPS). Polyurethane (PU) is an alternative to EPS, but is used much less frequently because it is typically more expensive for the same dimensions due to its higher R-value of 6.5/inch versus the EPS R-Value of 4/inch.
SIPs are essentially a “wall sandwich” with structural skin on either side of an EPS or PU insulating core. Suitable for residential and light commercial construction, SIPs are used for wall and roof applications.
SIPs begin life in a factory. CAD drawings of the structure to be built are converted to shop drawings, which are then plugged directly into computer numerical control (CNC) fabrication machines or are used to measure and cut the panels by hand. Special channels (chases) are cut into the foam to allow for the electrical wiring, and the insulation core is recessed around the edges to accept the connection splines or dimensional lumber used during construction.
Most SIPs feature OSB as the skins or facers that encompass the inner foam insulation core of the panel. But other skins are available, including:
• Fiber cement
• Fiber reinforced concrete
• Gypsum board
Regardless of the structural skins, those skins are rarely the final façade for the building. Standard panels are available in 4-by-8-foot or 8-by-24-foot configurations. Thicknesses typically range from 4.5 inches to 12.25 inches but custom sizes and thicknesses are also available. Insulating air sealants - single- and plural-component polyurethane foams - are commonly used with SIPs to help join panels together quickly, ensure air barrier continuity, increase energy efficiency and increase structural strength.
A recent Time & Motion study conducted by Reed Construction Data RSMeans Business Solutions showed that utilizing SIPs reduced installation time by 130 labor hours. When compared to RSMeans labor hours for a conventionally framed home, this is equivalent to time savings of approximately 55 percent. (For more information, see BASF Corporation Time & Motion Study,November 2006, submitted to BuildingInsight, LLC, conducted by Reed Construction Data/RSMeans.)
EIFShave survived some well-publicized problems from its past, such as leaks due to poor installation, which were often due to poor installation training. Well-tested master specifications for architects and aggressive contractor/installer training have made EIFS an extremely popular wall panel system today. Many manufacturers also offer moisture-drainage EIFS systems that include a drainage channel built into the insulation or adhesive elements of the system. Their popularity also stems from the incredible design flexibility they offer the architect. They are attached to the exterior façade and can be very impact and damage resistant based on the grade of mesh used.
<<5>> Air & Weather Barriers
Air and weather barriers fall into three classifications:
1. Water-resistive barriers are designed to keep liquid water out of walls. These barriers can be permeable or impermeable.
2. Air barriers are intended to seal the roof, walls and foundation so that air does not infiltrate or exfiltrate. The No. 2 source of water intrusion into a building (behind bulk water intrusion such as rain or melting ice) is humidity traveling with air. Air barriers can also be vapor permeable or impermeable.
3. Vapor barriers prevent moisture from permeating through the barrier material into the wall assembly. Vapor barriers prevent heat and water from traveling from high concentration to low concentration areas.
Air and weather barriers are important, primarily, because there is so much air and moisture flowing in and around most buildings and houses. During the winter months, water leaks, humidity from the clothes washer, showers and steam from cooking - even breathing - all create warm, moist air that will find the path of least resistance to where there is no heat and moisture. It can and does go anywhere and will take any circuitous route. Stack effect (the rule that hot air rises), wind effect caused by external elements and mechanical effect (often positive pressure in the form of make-up air) all exacerbate air migration rates. The taller the structure, the greater impact these effects can have.
As the air passes from a warmer to a cooler space it cools, and the moisture content drops off onto wood or metal or any other condensing surface, sometimes inside a wall or unsealed attic space. Can lights (housed in the ceiling) are a primary path for this migrating air traveling to the attic of a house. Wherever there is condensation, mold can result. Summer months have everything flowing in reverse and the basement and crawl space under a house come into play. Vapor barriers do stop the flow of moisture - when they are used - but may not prevent uncontrolled air migration.
When moisture goes where it shouldn’t, durability of building materials can be affected. Many materials are susceptible to rot, and many fibrous insulation materials can act as sponges that hold moisture. Indoor air quality is affected if air movement is not controlled: pollens get inside. The room over the garage is frequently cold, with the introduction of exhaust fumes and other VOCs into the living space posing a health risk to inhabitants. Air leakage can also cause back drafting and CO problems in a house.
Energy usage goes up since as much as 40 percent of it lost to air leakage and convection looping. Energy costs - to heat and cool and condition the air as well as GHG emissions - go up as well. In fact, a groundbreaking report from the National Institute of Standards and Technology (NIST), “Investigation of the Impact of Commercial Building Envelope Air tightness on HVAC Energy Use,” indicates that continuous air barrier systems can reduce air infiltration by more than 60 percent and energy consumption by up to 40 percent compared to buildings with typical air leakage rates.
Air barrier performance is defined by the air leakage rate - L/(s•m²•Pa) - the rate of airflow (L/s) driven through a unit surface area (m²) of an assembly by a unit static pressure difference (Pa) across the assembly. Several materials qualify as air barrier materials. Two high-performance air/weather barrier materials include:
A one-component, fluid-applied, vapor-permeable, air/water-resistive barrier membrane can be spray, roller, brush or trowel applied directly to above-grade wall substrates. One-component barriers resist air/water flow at 0.0049 l/m2 @ 75 Pa.
A two-component, closed-cell SPF (spray polyurethane foam) air/weather barrier can be spray applied at a rate of .5 inch to 2 inches maximum per pass with typical installations being at a maximum thickness of 4 inches and resulting R-values at 6.7 per inch of applied material. This material offers an air leakage rate of 0.000025 l/m2 @ 75 Pa. The two-component air/weather barrier system can be applied to glazing, metal, wood and concrete block among others. In some jurisdictions, SPF qualifies to do three jobs in one application - air barrier, vapor barrier and insulation.
Properly installed/applied, air and weather barriers can:
• eliminate uncontrolled air leakage;
• reduce HVAC requirements;
• reduce energy use by 3 percent-36 percent, depending on the building design and proper installation of the complete, continuous air barrier system;
• last for the life of the structures, as they are designed to do;
• prevent moisture intrusion due to air infiltration and exfiltration; and
• stop moisture-vapor transfer, condensing, mold and mildew within the wall cavity.
Code & Application Ambiguity
Interestingly, there is no air barrier requirement for commercial buildings in most states and codes. Although European countries have air barrier requirements in their building codes, Massachusetts was the first state to require an air barrier by code, but not until after 2000. And only a half dozen states have followed suit since then. LEED does not address air barriers either. Yet air barriers can save up to 40 percent of the energy a building uses.
House wrap is commonly used for residential construction and is effective if installed properly. Continuity of the airtight barrier is key - nailing an air-impermeable barrier to the structure is problematic, since any punctures/holes makes the barrier less than airtight. Consequently, fluid or spray applied air-impermeable barriers are best or a “stick-on” application with overlapping seams can work well too.
The location of the air barrier varies by climate, but is typically applied on the interior of a house. Conversely, air barriers are usually applied between the structure and the cladding on the outside of a commercial property.
Before giving any air barrier assembly its seal of approval, the Air Barrier Association of America (ABAA) is requiring manufacturers to prove their technologies can pass the ASTM E 2357 Air Leakage of Air Barrier Assemblies test. The purpose of this assessment is to determine whether air-barrier products, when used with other typical wall components, collectively function as an air-barrier assembly.
The test is performed on an 8-by-8-foot wall mock-up that includes typical wall penetrations - a window, galvanized duct, PVC pipe, post-applied brick tie-ins, electrical junction boxes, roof and concrete-foundation interfaces. The air barrier assembly is applied to the wall, complete with flashing and sealing materials applied around all penetrations and at air barrier joints in specified locations.
The wall specimen is then mounted in a sealed test chamber with an air supply that allows application and measurement of both positive and negative air-pressure differentials across the wall structure. The assembly then undergoes a “conditioning” process where the assembly is subjected to both positive and negative loads to confirm that the various materials will actually work together to provide an airtight seal. According to the E 2357, the air leakage of an approved assembly must not to exceed 0.2L/(s•m2) @ 75Pa. (0.04 cfm/ft2 @ 1.57 psf). Based on the results, the air barrier assembly is assigned an air leakage rating.
The benefits to air/weather barriers are many, including:
1. Reducing the footprint of a building through reduced energy use.
2. Using recycled or bio-based materials in its manufacture.
3. Low or no VOCs.
4. Engineered to last the lifetime of the structure, minimizing resources used.
The last of the six building envelope elements to discuss is concrete. On the surface, concrete seems pretty straight forward, but on closer examination, chemistry has probably done more to manipulate and improve the sustainable aspects of concrete than any other element discussed.
Admixtures are additives that are engineered to enhance specific performance attributes and they fall into these categories:
• Water Reducers - usually reduce the required water content for a concrete mixture by about 5 to 10 percent.
• Plasticizers - also known as superplasticizers or high-range water reducers (HRWR) reduce water content by 12 to 30 percent and increase flow rates.
• Accelerators - increase the rate of early strength development; reduce the time required for proper curing and protection, and speed up the start of finishing operations.
• Retarders - slow the setting rate of concrete and are used to counteract the accelerating effect of hot weather on concrete setting.
• Corrosion-inhibiting - used to slow corrosion of reinforcing steel in concrete.
Specific admixture formulations vary depending on the characteristics required. The performance priorities for concrete used in the construction of a major bridge are different from those for concrete used to lay the foundation of a house. Climactic conditions, setting times, the type of equipment used and the availability of local Portland cement or local sand all have an impact on the admixtures needed to produce the performance characteristics desired. Different admixtures can be used for ready-mix, pre-cast, masonry, paving and underground applications.
One development in concrete admixtures that achieves new levels of performance, economics and sustainability is an environmentally friendly, cost-effective concrete with optimized proportions in which supplementary cementitious materials, non-cementitious fillers, or both, are used with special high-range water-reducing admixtures and/or workability-retaining admixtures to meet or exceed performance targets.
Relative to a baseline reference mix, it attains desired setting characteristics, strength, durability, and if needed, a higher slump at a reduced cost to the producer. A simple equation sums up the recipe and its results. (See graph to the left).
Another innovation gaining ground because of its sustainability benefits are pervious pavement technologies - including pervious concrete and pervious asphalt - which play a significant role by providing uniform distribution of runoff into vegetated areas to keep the water from directly entering the storm-drain network, reducing runoff volume and promoting distributed infiltration. Pervious concrete helps to recharge groundwater, maintain aquifer levels, provide nourishment for trees and plants, reduce untreated runoff to storm sewers and eliminate hydrocarbon pollution from asphalt pavements and sealers.
Pervious concrete is a mix of Portland cement, coarse aggregate, water and admixtures, as well as recycled materials. Some also use a urethane-based adhesive/coating. Because there is little or no sand in the mix, the pore structure contains many voids that allow water and air to pass through. Chemical admixtures are used to enhance these formulations to improve ease of installation, including increased working time with improved concrete flow. The admixtures also increase compressive strength.
The role of chemistry in advancing high-performance building envelopes is encouraging. It is good to know that other professionals - chemists and building product manufacturers, in particular - are deeply committed to finding new and more sustainable ways to refine and reinvent every aspect of architecture and construction, just as design professionals are. The fundamental building blocks of the built environment discussed here are, at once, plain and sexy - the same yet entirely different, basis stuff and also totally exotic.
As a fitting example of what these building envelope elements can contribute to more sustainable homes, in this case, can be observed at a demonstration project in Canada, completed in 2007 (http://www.cmhc.ca/en/corp/nero/nere/2007/2007-11-09-1400.cfm). Alouette Homes lead one of 12 teams that designed and built a home in Eastman, Quebec, Canada - as participants in the Canadian Mortgage and Housing Corporation (CHMC) EQuilibrium housing initiative. Their demonstration house consumes only 10 percent of the energy used in a standard house with the same surface area. That’s a 90 percent savings! Meaningful change is very possible.
The CMHC’s EQuilibrium initiative demonstrates a new approach to housing in Canada, and represents a fundamental change in the way Canadians will think about their homes in the future. It strives to balance their housing needs with those of the environment. It brings together - under one roof - the principles of occupant health and comfort, energy efficiency, renewable energy production, resource and water conservation, and reduced environmental impact and pollutant emissions. EQuilibrium also refers to energy use and generation - striving for a net-zero house, a sustainable home.
This article has been approved by AIA for 1.0 learning unit and USGBC for 1 GBCI CE hour towards LEED Professional credentialing maintenance.
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