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Frequently Asked QuestionsAnswers provided by Atkinson-Noland & Associates, Inc. and RMMI Technical Director Diane Travis, dianet@rmmi.org
Q. I am building a structure with Single Wythe CMU walls. Do I need flashing and a water repellent coating? A. Concrete block, particularly lightweight concrete block, is not weather tight if you do not apply a water repellent coating to it. Water repellent coatings usually last between 2 and 10 years, depending upon the exposure of the building and the competence of the applicator. As a rule of thumb, penetrating water repellents last longer because they are not as prone to ultraviolet degradation as surface coatings are. I recommend Silane and Siloxane-based coatings because of this longevity. If you want to read more about water repellent coatings for concrete block, go to the web site for the National Concrete Masonry Association at HYPERLINK "http://www.ncma.org/" www.ncma.org. Look for an icon labeled “E-Tek”. Click on this heading and it will take you to another web site where you can download technical articles. I suggest you download:
My favorite flashing/weep system for single wythe walls is called Cavity Vent. It is a strip of corrugated plastic that you install at the base of the wall, where the block meets the foundation. I like the product because it is fast and easy to install and it does not deteriorate over time. Structural engineers like it because it does not interfere with rebar placement and it does not cause shear plane weakness at the base of the wall. Go to HYPERLINK "http://www.masonrytechnology.com" www.masonrytechnology.com and click on the drawing of “Cavity Vent” to see more information about this system. Q. I am planning to build an anchored stone veneer wall with 4” deep rough, natural stones stacked in a random ashlar pattern. Should I leave an air space behind the stone or should I fill the gap solidly with mortar and grout? A. You need the air space to be solidly grouted to stabilize the uneven stones. The grout pack will shrink and pull away from the building paper (which you have stapled in front of the sheathing, right?). This gap will only be 1/16” wide or so, but it is a continuous slot where moisture can slip down the wall, hit the flashing and weep out of the wall. I recommend 100% cotton rope as weeps for this system. This system is drawn out in a book called Masonry Veneer, published by the Masonry Institute of America. Go to their web site at www.masonryinstitute.org to order a copy of the book ($20). You can also come by our office and look at the book. Q. Do I need weep holes or dampproofing for a retaining wall?
Q. When do I need to install relieving angles in a tall, brick-veneered building? A. For a detailed answer, go to the Brick Industry Association (BIA) web site at www.bia.org. Click on “Online Technical Notes” and download Tech Notes 18 and 18A. The section titled “Horizontal Expansion Joint” states that these joints are not necessary for buildings of less than three stories in height. BIA engineers maintain that you can go 30’-0” high before you need a relieving angle in a façade with high windows. You can go 38’-0” at a gable. The point about windows comes up because the differential movement between the brick veneer and the back-up system becomes most problematic at window openings. Q. How big should the air gap be in a cavity wall? A. According to the Brick Industry Association, air gaps should be:
The size of the air gap is measured from the back side of the brick to the facing edge of the sheathing or structural wall behind the veneer. If your wall has a big air gap, the ties that anchor the veneer in place tend to buckle when the wind pushes the exposed veneer with a lateral load. You can tolerate a wider air gap if you use stronger brick ties. You can also simply put in more brick ties or you can make the air gap smaller. If you are building a fence that is exposed to weather on both sides, you are building a “composite wall” not a “cavity wall”. You should sandwich the brick directly to the CMU with no air gap at all. If you are constructing the wall of a building with an inside face and an outside face, it should have an air gap, flashing and weep holes to direct moisture to the outside face of the wall. Q. How do I calculate the fire rating of my structural brick or brick veneer wall? I often get questions from architects, masons or suppliers who need to prove to code authorities that their walls are properly fire rated. If you are using a brick veneer assembly, there are several U.L. assemblies that will satisfy the code guys. You can find information about these assemblies on the UL website (www.ul.com) or in a UL Fire Rating Manual.
I also tell people to use Table 720.1(2) of the 2003 IBC for fire ratings. This table will tell you how many inches of brick you need to achieve a given fire rating. If you are using 75% solid brick (brick with small round core holes) you should consult the fourth line of this table 1-2.1. If you are using structural brick (brick with large holes designed to carry rebar and grout), you would use the third line of the chart (1-1.3). Q. How do I calculate the fire rating of my concrete block wall? A. I often get questions from architects, masons or suppliers who need to prove to code authorities that their walls are properly fire rated. The code guys are used to seeing U.L numbers for assemblies that have been tested at Underwriters’ Laboratory. Unfortunately, if you ask for these tested assemblies you must use concrete block units that are “classified” and have been tested by the U.L. folks. The closest source of classified CMU is Boise, Idaho. I do not recommend importing block from Idaho. Not only is it a waste of transportation money, the block can be cracked and damaged by the long trip. I tell people to use Table 7-B of the 1997 UBC or Table 719.1 of the 2000 IBC. These two tables give essentially the same information. They tell you how many inches of masonry you need to achieve a given fire rating. There is a slight twist to this chart. If you read the footnotes in the table you will find that the code is interested in “equivalent thickness” of the concrete block, not actual thickness. Equivalent thickness is essentially the solid thickness that would be obtained if you recast the unit using the same height, same length and same amount of concrete, but eliminated the core holes. You can contact the block supplier to get the equivalent thickness of his product or you can consult Tek Note 7-1a from the National Concrete Masonry Association. According to this Tek Note, typical equivalent thicknesses are:
If you fill all the core holes with a non-combustible material, you can count the actual thickness as the equivalent thickness. Acceptable fill materials are grout, sand, pea gravel, crushed stone, pumice, scoria, expanded shale, expanded fly ash or perlite. Your last challenge is to determine which category of concrete block you are using. The aggregate used in your block will define which classification you need to use. Again, contact the manufacturer and ask which aggregate is specified for your project. Pumice and expanded shale are the two most commonly used lightweight aggregates. Calcareous or siliceous gravel aggregates are used to manufacturer normal weight CMU. Using the chart in the Code you will see that all lightweight block mixes will give you
Normal weight block (also called sand-and-gravel block) is not as efficient at dissipating the heat of the fire. You need more thickness to achieve a given fire rating
If you need to achieve a 4-hour fire rating you can either use a 12” deep lightweight block or you can fill all the core holes with sand, grout or perlite. If you fill all the cells:
A. We like to give generic advice whenever possible but I do have some preferences. Do NOT use any flashing with vinyl in it. The high pH of wet mortar eats holes in vinyl. This eliminates any PVC flashing products from my recommended list. According to Architectural Graphics Standards, mortar is also corrosive to pure aluminum and pure zinc. Alloys with these materials do not seem to have trouble with corrosion. I recommend metal flashings only for the longest lasting buildings (like cathedrals and university buildings). Although the metal flashing is slightly more expensive than flexible flashings, the time required to cut it, fold it and weld it accounts for the real price jump between flexible flashings and metal ones. Flexible bitumen flashings are my personal favorite for Colorado’s climate. The material is cost effective, easy to work with, long lasting and tolerant of the high pH of mortar. The tar coating will even self-heal small holes and cracks once the sun comes out and heats the wall. The manufacturers tell you to peel off the backing and install the flashing sticky-side down on a clean, dry substrate. You must also hold the flashing back 1/2” from the face of the wall so that the sun doesn’t melt the tar and have it drip down the face of the building. If you want the flashing to continue to the face of the wall, you can either pair the bitumen flashing with a 1” wide galvanized metal drip edge or you can use Hyload flexible flashing, a bitumen-based flexible flashing system that comes with an integral drip edge. Go to their web site at www.hyloadflashing.com to see more. Q. The inspector didn't show up on time but I built the wall anyway. Now they want me to tear down the wall because they can't verify proper construction. What do I do? A. The inspector will be interested in verifying that reinforcing and grout are present as described in the design documents. This can be done without tearing down the wall. Reinforcing steel and grout can be detected nondestructively using metal detection devices and sounding. To determine if a cell is grouted, the simplest technique is to "sound" the cells with a small hammer. Empty cells, when knocked with a small hammer, will produce a sharp "thok" sound, because the face shell vibrates as a free plate. Grouted cells produce a dull, lower frequency "thump" sound, because the face shell is bound to the grout and cannot vibrate freely. The presence of grout should be verified by drilling into the masonry. The presence of steel can be sensed using metal detection devices known as "pachometers." Pachometers sense the size of a reinforcing bar if the depth is known, or the depth if the size of the bar is known. Pachometers can also detect wall ties, joint reinforcement, and lap splices in reinforcement. As with sounding, the results of pachometer investigation should be verified by drilling into the masonry and observing the steel directly. In addition to sounding, there are several technologically advanced methods for sensing the presence of steel and grout, including ultrasonic pulse velocity, impact-echo, radar reflections, and thermographic imaging with infrared cameras. These techniques can be efficient for inspection of large areas of wall. For many projects, however, sounding with hammers and the use of metal detectors are sufficient to provide the required information. To satisfy the building official, hire an engineering or testing firm experienced in this type of investigation. For most projects, a one-day site visit by an experienced expert will suffice to determine whether or not the wall was built according to specifications. Q. I have an old historic building that needs some joints repointed. What mortar should I use? A. Your old historic building needs a different mortar type than the mortar types used in modern masonry construction. Most historic buildings were constructed with bricks and mortar that are much softer and more permeable than modern bricks and mortar. The use of modern mortar to repoint soft masonry can result in long-term problems such as spalling and cracking of bricks, as well as moisture transmission problems. Prior to the introduction of Portland cement to the masonry industry in the early 1900s, all masonry was constructed with lime mortar. The lime used was not the same as the hydrated lime used in modern masonry, so historic mortars cannot be replicated by simply leaving out the Portland cement from your mortar recipe. There are several options for replicating the properties of old mortar, including mortar formulations with small amounts of Portland cement, and the use of small-batch craft limes available from Europe and some manufacturers in the United States. It is also important to use aggregates that match the original in sand type and in the aggregate particle size distribution, and to match the color. A mortar analysis that includes measurement of the aggregate gradation and the binder to aggregate ratio is helpful for designing an appropriate mix. Guidance for the design of a mortar mix appropriate for your building can be found from many sources, including the National Park Service Preservation Briefs (www.nps.org), the Historic Scotland (www.historic-scotland.gov.uk ), and the Association for Preservation Technology. The American Society for Testing and Materials (ASTM) provides ASTM C 270, Standard Specification for Masonry Mortar, the standard for modern masonry mortar. ASTM is currently working on a standard for restoration mortars. This new standard will provide a method for determining which properties are important for your project and how to design your mix to provide these properties. Look for this standard to be published as soon as 2006. Q. I am bidding a project that appears to have too much steel and grout in the wall. Is there any way to find out if this design is correct? A. Remember that the structural design of masonry walls depends on a lot of factors. Just because you built a similar building on the last project with different steel requirements does not mean the design is wrong - there may be fewer columns on the inside of the building, or maybe this design was based on higher wind speeds. That said, we have seen some details normally reserved for high seismic zones out west creep into some of our local designs. In earthquake-prone areas, the building codes actually require bond beams and reinforcement every 4' on center, as a minimum. Some engineers mistakenly use these details in Denver, even though we have a low seismic risk. Contact RMMI if you have concerns about a job's structural requirements. We can't value engineer the entire project, but could check a few wall sections to make sure it isn't over-designed. Q. The testing agency reported that one batch of their mortar samples from my job site did not meet the specified compressive strength. Do I have to take down this area of masonry and rebuild it? A. Mortar strength is frequently misunderstood and often over emphasized. Although mortar strength has an influence over the strength of the masonry assemblage (mortar and units together), the role it plays is minor. Rather, the strength of a masonry wall is largely determined by the strength of the units. With concrete masonry especially, a 50% decrease in mortar strength may result in less than a 10% decrease in the compressive strength of the masonry assemblage. Prism tests, therefore are more important tests for quality control of the finished product. If prism tests show adequate strength, there is no reason on demolish and rebuild an area of masonry. Q. My wall has a crack in it. Should I be concerned? A. That depends on where the crack is, and how it has been behaving. Some cracks are caused by simple masonry expansion or shrinkage and are normally not a cause for alarm. Other cracks, such as over a door or window opening, could lead to instability and can be very serious. Contact a structural engineer to help you out - especially if you have noticed the crack getting wider recently. Find out the cause of the crack before attempting any repairs. The type of repair the engineer chooses depends on if the crack is active or dormant . Active cracks open and close with regular cycles of temperature, loads, or foundation movement. Either cure the source of the movement and repoint the crack with mortar, or use a flexible crack filler such as sealant. You can also cut in a new movement joint at a convenient location, and then repoint the original crack. Dormant cracks that have not moved in several years are usually safe to grind out and repoint with mortar. Q. Which masonry tie is best for veneer construction? A. Any type of two-piece adjustable tie is appropriate for most masonry construction. There are many different configurations of masonry ties. As long as the tie allows for vertical movement (slip) but is rigid horizontally, it should be fine. Corrugated strap anchors are not appropriate for large panels of veneer masonry. The veneer is not structural. When the wind blows, the lateral load on the veneer is transmitted to the structural back-up wall behind it by the rigid masonry ties. As you can imagine, corrugated ties cannot transfer this lateral load. If you compress them, they just corrugate! The Brick Industry Association says that you shouldn’t use corrugated strap ties on any building “larger than a residential garage”. I translate this to mean that if you have a panel of brick or block veneer taller than 10 feet or longer than 24 feet, you should use two-piece adjustable ties. Q. What is the proper way to judge a masonry wall? A. Aesthetic Concerns distance of 20 feet under diffused lighting conditions. You should be standing directly in front of the wall, not sighting down the edge of it. These requirements are an extrapolation of the ASTM standards governing acceptable quality of brick and block. The standard asks for diffused lighting conditions because edge-lit exposure magnifies imperfections in any flat wall system (stucco, precast, masonry or metal panels). You are asked to stand 20 feet away from the wall so that you will view the wall as a whole, not placing extreme emphasis on each brick, block or mortar joint. Any imperfections that show up from this distance are significant enough to make it to the punch list. You can also use a mock-up panel as a standard for judging the finished masonry work. A mock-up panel should be erected at the beginning of the project. It should be at least 4 feet by 4 feet in size and should include all the different masonry materials that will be included in the final building. We recommend that the panel be built exactly as the final wall will be built, with insulation, wall anchors, weep holes and a least one corner. For an accurate test of the proposed cleaning methods prescribed for the building, you need to clean the mock-up panel with the same method which will be used to clean the whole building. The mock-up panel is a three-dimensional representation of an agreement between the owner, the architect, the general contractor and the mason contractor. This is the point where they officially work out the level of craftsmanship and appearance that will be acceptable in the final wall. A mock-up panel is considered to be part of the project documentation and is as legally enforceable as the drawings or the specifications. The mock-up panel should remain in place until the owner finally accepts the project. If necessary, you can move the panel but do not throw it away. Structural Concerns of mortar joints, allowable deviation from plumb, level and flat construction and allowable deviations from intended placement of masonry walls and piers. The standards listed under ACI 503.1 are more concerned with structural integrity of the building than with aesthetics. Water Penetration reviewing masonry construction for cracks needs to be done with a crack gauge held next to the wall surface. Mortar joints with large cracks must be ground out and repointed. The repointed joints may not match the rest of the wall for months, perhaps even as long as a year. Q. How do I know when my building needs repair? A. Masonry walls are amazingly resistant to the ravages of time. They do not fade, rust, rot or burn. Brick walls never need repainting, although concrete block ones need to have the water repellent coating renewed from time to time. The number one enemy of masonry in Colorado is water, coupled with our climate where the temperature often drops below freezing. When water freezes, it expands 9%. This expansion can crack even the sturdiest construction. We have over 150 freeze-thaw cycles a year in Denver. If a brick is saturated when the temperature dips below 32 degrees, this expansion causes hairline cracks that can lead to more significant problems down the road. My best advice is to keep your masonry dry. Make sure the gutters and downspouts are repaired so they can quickly flush water away from the wall. Verify that the parapet caps and the window sills are firm and tight and shed water well. The National Trust for Historic Preservation suggests that you take photographs of your building once a year--about 2 to 4 hours after a rain storm. These annual photographs will let you compare the condition of the building from year to year. If you see any puddles at the base of the building it means that you should re-grade your landscaping or re-slope the sidewalk. If you see big wet patches on the wall, this indicates that water is getting into the wall instead of being shed away. This greatly increases the likelihood that your wall will be wet one night when it freezes. You are likely to see these wet spots under window sills, under parapet caps and behind downspouts. If you suspect that the joints in a particular area of the wall are damaged, take a key and run it along a horizontal mortar joint, pressing firmly. If little flakes of white mortar pop off the wall, your mortar joints probably need repointing. You do not have to repoint the entire wall—just the damaged portion. Call Rocky Mountain Masonry Institute for our list of recommended restoration specialists. Show the mason your photographs. It will help him confirm the extent of the repair work. If you keep the wall dry, you should only need to repair your mortar joints every 20-30 years. Although brick is inherently water repellent, concrete block is not. It always needs a water repellent coating to keep moisture out. Since most of these repellents are clear, it is difficult to know when it is time to recoat. According to an article published in the April 1996 issue of Masonry Construction magazine, penetrating coatings made of Silane or Siloxane will last 10 years or more. Acrylic based coatings last 5-7 years and Urethane-based products last 1-3 years. The real determining factor for longevity is how well the coating was applied. A coating specialist can test the wall with a RILEM tube and can tell you if your wall needs recoating. The RILEM test tells you how much wind-driven rain your wall can resist. Call Bud Barr, Barr Sales Co. (303-753-6525) or Rio Grande Co. (303-825-2211) to find someone to test your wall. |
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