Cantilever balconies are commonly seen protruding from the façade of condominiums and apartments. However, this arrangement can lead to problems arising from heat transfer and condensation, resulting in mold growth.
Typically, steel framed cantilever balconies have beams that extend into the building and connect to the structure. Concrete balconies are usually cast integrally with the rest of the floor. During winter months, the exposed balcony structure becomes cold and when it meets the warm building interior, typically near a sliding glass door, thermal transfer increases, as does the possibility for condensation and mold growth. Building owners should be aware of this possibility and watch for condensation forming in the area under the carpet or wood flooring, near the balcony.
Fortunately, for new buildings with cantilever balconies, products are now being produced to prevent condensation and mold by inserting a thermal break (insulation) between the exterior and interior portions of these structures. These new balcony structural inserts can carry significant weight, while preventing interior heat transfer. The most common materials used for these products are stainless steel and fiberglass reinforced laminate composites. Although these are new products, they appear promising.
Unpredictable summer thunderstorms can wreak havoc on a concrete pour. Rainwater can cause a new concrete surface to become soft, which in turn decreases the abrasion resistance and strength of the concrete, while increasing the tendency for dusting and cracking to develop. The key to preventing damage to the concrete surface by a rainstorm is proper preparation and timing. Before a storm occurs, a protective enclosure can be built around the work site with wood and plastic sheeting. If you get caught without protection, once it starts raining, it’s best to wait, let the rain pass, and pull or push the surface water off the edge of the slab before completing finishing. Contractors should never work the rainwater into the freshly placed surface or broadcast dry cement on the wet surface in an attempt to soak up the water.
Just because it starts raining during or soon after a concrete pour, does not necessarily mean that your project is doomed. It all has to do with timing, and at what stage in the curing process the concrete is in. If the rain occurs when the concrete is fresh (about 2-4 hours after mixing), the surface should be protected from the rain. If the finishing process was recently completed, rainwater may not cause damage as long as it is not worked into the surface and the slab is left untouched. If the concrete has stiffened to the point where it is ready for grooving and grinding (typically 4-8 hours after mixing), damage due to rain is usually no longer a concern.
After a rainstorm, damage to the concrete should be assessed. A visual survey can be performed to note any obvious defects. A simple scratch test, using a screwdriver, can be performed to compare the relative surface scratch hardness of any areas in question to those slab sections known to be of good quality. A quantitative approach includes removing several core samples and checking them in a lab with an electronic microscope. (See our previous blog entry from February 17, 2012 about petrography)
If the surface quality of the concrete is found to be compromised, remedies are available. Isolated repairs can be made immediately after a storm by using the some of the same concrete that was used for the concrete placement itself. If small areas of a thin slab are damaged, it may be more economical to remove and replace full depth sections. If there are large areas in a thick slab with damage, a thin application of a repair mortar could be applied after the damaged concrete is removed.
It’s hard to find a building today without concrete surfaces stained by rust. Rust stains can adversely transform the aesthetics of a beautiful building. How can rust stains be removed? Let’s find out!
Once rust staining has occurred, it is important to remove the stains without altering the color or finish texture of the concrete. Two techniques which can be implemented are dry methods (i.e. sandblasting, wire brushing, grinding, etc.) and wet methods (i.e. waterblasting, chemicals, etc.). If surface texture is not a priority, the dry methods can be a quick and cost-effective way to remove stains. If the final finish is important, as is commonly the case with architectural concrete, chemical treatments are recommended.
Mild stains usually can be removed with an oxalic acid or phosphoric acid solution, applied to a saturated concrete surface. Deeper stains typically require a poultice, which absorbs the chemical solutions and then forms a paste over the stain. Older buildings require more attention with stain removal because the chemical treatments may remove other contaminants in the concrete, creating a lighter color than the adjacent concrete.
The rule of thumb when putting a cleaning solution on your stained carpet or clothes applies with concrete. Be sure to test different chemicals on small, inconspicuous areas to evaluate the treatment. Also, the longer you let a stain sit, the more difficult it is to remove, so seek help quickly when rust stains appear!
Some people may say that concrete is a paradoxical material: it is strong and yet fragile; it is mundane and yet remarkably versatile. But more often than not, this material is taken for granted as the surface of everyday elements such as streets and sidewalks.
As discussed in our September 26th, 2011 post “Curing Concrete in the Cold,” temperature plays a crucial role in the outcome of newly placed concrete. Curing concrete in temperatures above 80° Fahrenheit can be as challenging as doing so in temperatures below 35° Fahrenheit (see “Curing Concrete in the Cold ”)
Concrete cured at high temperatures will have a high rate of evaporation, causing uncontrolled thermal cracking, which in turn compromises the concrete strength and durability. Laboratory testing has proven that concrete improperly cured under high temperature conditions can lose as much as fifty percent (50%) of its service life. Concrete naturally produces heat as it is mixed and cured. So placing a material that has internal heat on a hot day is quite challenging.
The first step in ensuring adequate concrete curing and reducing the temperature of concrete are taken at the batch plant by adding ice as part of the batch water, using chilled batch water, or cooling the concrete with liquid nitrogen.
Then, it is up to the Contractor to ensure additional adequate conditions. The concrete placement should be scheduled as early in the day as possible to avoid the hottest part of the day. Advanced planning and timing should also be performed to avoid delays in delivery, placement, and finishing. If long haul times cannot be avoided, it is possible to include a retarder as part of the mix design to prevent fast setting. However, the amount of retarder is limited by the work intended, as elevated amounts of retarder will crust the top surface of a slab while the underlying concrete remains soft.
Prior to the placement, the forms, subgrade, and reinforcement need to be soaked to ensure that unsaturated materials do not absorb the moisture from the concrete mixture. Once the concrete is at the job site, water may be added to the mixture to adjust the slump only at the time of the truck arrival, and only if the mix design allows it. In the event that water is added, it shall not exceed the volume listed on the batch ticket provided with each truck load. Once the concrete is in the process of being placed, water must not be added to the mixture. Doing so will greatly compromise the concrete. To make things even more interesting, concrete must be placed within 90 minutes from the time it was mixed in the truck.
After the top surface of the concrete has been given a finish, moisture should be prevented from evaporating by covering the elements with soaked burlap or cotton rugs. It must be ensured that the coverings remain continuously wet so that they do not absorb water from the concrete during the first seven days after placement.
Who would have thought that such a dull material would require such a meticulous procedure to ensure it reaches its true potential?
Recently, we were asked to investigate a building where mold was growing in the walls. During the investigation, we came across a product that we had not encountered before. It was a sheetrock panel with a shiny aluminum backing.
These panels are made by laminating special kraft-backed aluminum foil to the back surface of regular drywall panels. After reviewing the product literature, the panels offer some desirable qualities as they are intended to create a vapor retarder that helps prevent interior moisture from entering via exterior walls, it helps maintain comfortable room humidity all year-round, and creates a comfortable temperature during the appropriate seasons.
After reviewing the product limitations, we found that this drywall was not ideal for our situation. This was primarily because the interior walls were decorated with vinyl wallpaper, which created a double vapor barrier on the interior wall. This condition trapped moisture and created a place for mold to grow. Unfortunately, because the foil backing was not visible, no one knew that a problem was created by adding the wallpaper. Sometimes even simple cosmetic changes can lead to unexpected consequences.
Wood, unlike other common building materials, relies on nature to develop its structural properties. Because timber is a natural material, it routinely undergoes testing to ensure that design values remain accurate.
Earlier this year the American Lumber Standards Committee (ALSC) approve new, reduced design values for southern pine lumber that are set to go into effect on June 1, 2012. The changes apply to No.2 and lower grades of visually graded southern and mixed southern pine with widths and thickness between two (2) and four (4) inches. These changes will affect 2x4s and 4x4s, which are the sizes most commonly used in light-frame construction. Future changes can be expected for all southern pine dimensional lumber once additional testing is complete. The reason for the reduction has not been fully explained, but , but a likely cause is forestry management practices that result in faster tree growth. Fast-growing trees produce wider growth rings, making the wood less dense than trees that develop at a slower rate.
The design values will be reduced for bending, tension, and compression by up to thirty-five percent according to the Southern Forest Products Association (SFPA). These changes result in shorted spans and smaller load capacities for the affected lumber. The new design values are to be used on all work permitted after the June 1, 2012 effective date, although since Building Codes are enforced at state or local levels, the exact dates of enforcement will vary.
This change to the design values and the expected changes for larger dimensional lumber must be taken into account when constructing anything of southern pine in the future. New design tables are available from the SFPA, as well as comparisons with other species. The changes also present the problem that repairing damaged lumber with new southern pine may require the use of a larger cross section, change in grade, or modification to spans to provide adequate support for the load.
An effective corrosion protection plan is an essential aspect of the design, repair or maintenance of reinforced concrete structures such as parking garages and balconies or steel building frames wrapped in masonry, which are exposed to the elements. Severe corrosion of the embedded steel must be avoided so that the structure maintains its full strength.
Corrosion of steel embedded in concrete or masonry is an electrochemical process that occurs in the presence of moisture and oxygen, which can accelerate if the structure is exposed to deicing salts or a salt-water environment. Once started, corrosion will continue until it is controlled.
To control corrosion, the steel can be coated with a protective material such as zinc, paint, or epoxy. Alternately, the electrochemical process can be mitigated with cathodic protection. Coatings generally only protect the steel until the barrier is breached, while cathodic protection provides a more active means to address corrosion.
In cathodic protection, zinc anodes may be attached directly to the steel to alter the electrochemistry and force the steel to become the cathode and be protected while the zinc anode “sacrifices” itself. Anodes can be used in new construction as well as in the repair of existing structures.
Another form of cathodic protection, applies a small electrical current directly to the steel, which prevents the electrical process of corrosion from occurring. This system requires a power supply, a sacrificial anode, and instrumentation for monitoring.
Selection of the appropriate level of corrosion protection for an existing structure is based on many factors. Among them are the level of corrosion damage, environment around the structure, potential for corrosion activity, the cost and design life of the corrosion protection system, and the expected service life of the structure.
While there is no one “most” important component of any given building envelope system, flashings are certainly critical parts. Whether it be roofing (flat or sloped), exterior cladding (brick, siding, EIFS, etc.) or windows and doors, flashings play a key role in the success of these systems.
Flashings can consist of metal, sheet materials, or even liquid-applied membranes and are typically located at key areas (window/door perimeters, wall transitions, penetrations through roofs or walls, etc.) that would otherwise be prone to leakage. Their primary functions are to keep water away from vulnerable components and/or redirect it away from interior spaces.
The cost of installing proper flashings is a relatively low percentage of an overall system. On the other hand, the effects and costs associated with missing or improper flashings can be extreme.
We regularly encounter the consequences when flashings are not included or are improperly installed. In addition to obvious problems (interior water penetration) rot or corrosion of framing and other concealed building elements can develop and progress unnoticed. Such damage can be extensive and even disturbing.
The accompanying photographs show what can happen when an EIFS assembly lacks flashings to collect and divert water away from internal elements. Repeated and prolonged wetting of the underlying wood components produced the severe damage you see and it went undetected until we opened the system up for inspection.
Buckled wood flooring, moldy carpets, peeling sheet flooring, and bubbles in liquid applied floors can all be signs of excessive or trapped moisture below the floor covering. Moisture permeating from concrete floor slabs affects the performance of flooring systems such as resilient and textile floor coverings and coatings, and can cause floor covering system failures such as, debonding and deterioration of finish flooring and coatings, as well as microbial growth. Manufacturers of such floor covering systems generally require moisture testing to be performed before installation on concrete. Omitting these tests when they are required by the Manufacturer may void the flooring system warranty. This is a relevant issue for new construction or any project where the flooring/coating manufacturer requires ASTM testing prior to the flooring installation.
All of the concrete slab moisture tests require proper surface preparation (removal of adhesives, previous coatings, surface impurities, etc.), followed by a waiting period (usually 72 hours) under typical ambient conditions. Three tests are required for the first 1,000 square feet of floor space, and one additional test is required for each additional 1,000 square feet. When the test results are compared to the flooring manufacturer’s installation requirements, they can indicate if the concrete slab is acceptable for the installation of resilient flooring.
Four different tests can be run on concrete slabs to check the moisture content. The ASTM F710 standard test method involves the measurement of the pH level of the concrete surface, which is required data for all three of the slab moisture testing methods. As Portland cement hydrates, calcium hydroxide and other alkaline hydroxides are formed. The pH of wet concrete is extremely alkaline, while the pH of a floor with at least a thin layer of carbonation is typically between 8 and 10.
ASTM F1869 measures the Moisture Vapor Emission Rate (MVER) of a concrete slab expressed in lbs/1000 ft2/24 hours. This is achieved by placing a dish of calcium chloride on the slab surface under a plastic cover, allowing the crystals to absorb moisture emitted from the slab over a 72 hour period, and weighing the dish after. Typically, flooring manufacturers require an MVER of 3 lbs/1000 ft2/24 hours or less before installation.
ASTM F2170 measures the relative humidity throughout the depth of the concrete slab as a percentage by drilling a small diameter hole into the slab. ASTM F2420 is another test that also measures the relative humidity of the slab as a percentage, but this uses an insulated hood positioned over a portion of the concrete slab. This method simulates the placement of resilient flooring on the concrete surface itself, but it is not commonly used in the United States.
So how long does it take for a concrete slab to dry out, and how soon should the slab moisture tests be performed? Unfortunately, the drying time for concrete slabs can vary greatly depending on atmospheric conditions and the mix design. However, a typical 4” thick slab with a water-cement ratio of 0.45 can take anywhere from 90 to 120 days to achieve an acceptable range. Slabs that contain lightweight aggregate or that only dry from one side (such as slabs installed over a moisture vapor retarder or on metal decking) may need a much longer drying time.
If the concrete slab does not meet the flooring manufacturer’s requirements for installation, it is advisable to wait at least 30 days before performing another set of tests. In the interim, you can perform a plastic sheet test per ASTM D4263 (commonly called a mat test), and if the slab appears dry, then a re-test using the more rigorous ASTM procedures, above, can be performed.
Please give us a call if you are experiencing flooring problems. We have a certified technician ready to help!!
We haven’t quite developed Superman-grade X-ray vision, but we’re getting close. Some would argue too close, given the invasive capabilities of equipment used for airport body scans; but that’s fodder for other blogs.
Radiography (erroneously referred to as X-ray) examination used to be the only reliable way of determining the presence and depth of reinforcing steel or conduits in concrete. That technology employs a radioactive source capable of penetrating concrete sufficient to project an image onto a photo/radiographic film or screen placed on the side opposite the source. X-rays project a high-energy beam of electrons with similar effect and the energy requirements make those systems essentially non-portable. Radioactive materials are inherently hazardous and their possession and use are tightly controlled by the Nuclear Regulatory Agency.
A safer and more accessible way to see inside concrete is the use of ground-penetrating-radar (GPR). GPR directs high-frequency, high-energy radio waves into the material to be examined (concrete, asphalt, soil, etc.) and the materials located therein will reflect the waves back to the source. Because the degree and nature of the reflections vary with density and other factors, an image of the scanned structure is created.
GPR has been around for awhile, but resolution issues limited its use in building diagnostics. Improved resolution and such innovations as 3-D imaging have elevated GPR’s use to preeminence in non-destructive testing of concrete and other building elements.