IMI Technical Services staff of architects, engineers, and construction professionals stand willing and able to entertain your technical questions — about masonry, of course.
For FREE expert assistance, call: 1-800-IMI-0988
Autoclaved Aerated Concrete
(AAC)
A: First discovered in Sweden in 1914, Autoclaved Aerated Concrete (AAC) masonry units are made by combining aluminum powder, cement, lime and water with finely ground sand, which causes dramatic expansion. They are dried in a mold, cured in a pressurized steam chamber (autoclave) and cut into modular dimensions. A popular alternative to scarce wood in Europe, it is now produced in over 200 plants in 35 countries, including four in the U.S. The lightweight units (typically produced in 12" x 24" blocks or larger) allow more square feet of wall to be erected more rapidly. They also have superior fire resistance (four hours per 4" thickness) and exceptional thermal and sound absorption benefits. Balanced
Design A: Fire safety is a concern in all building design and construction, but it is especially critical in multiple occupancy residences like dormitories and senior housing. An accidental fire started in one living space can quickly spread to the adjoining residences and impact many lives. The balanced design concept means using a four step system to save lives and property: In a balanced design, these four strategies are used together as a system, not traded off one for another. Active systems such as sprinklers and alarms work well, but only as long as the supporting mechanical systems work. They can’t provide the structural integrity needed for safe evacuation and fire fighting. They are clearly an integral part of the early warning and suppression equation, but passive systems like compartmentation with masonry walls work 24 hours a day, regardless of the mechanical systems. Masonry walls contain the fire and noxious gases and remain structurally stable—enhancing the evacuation of residents and the safety of fire fighters. Blending active and passive systems, rather than trading them off each other, is the balanced approach to fire safety. Prevention—to reduce the chance of having a fire through housekeeping, education, and building layout. Detection & Alarm—Smoke detectors and other warning devices to catch a fire when it is small, and hopefully keep it from growing into a larger one. Suppression—Using sprinklers, fire extinguishers or other suppression systems to assist in putting out the fire quickly and effectively. Strategic Compartmentation—Using building construction features like masonry fire walls to isolate and contain the spread of fire and toxic smoke while maintaining the structural integrity of the building. This is necessary to allow for the safe evacuation of the residents and to provide the fire fighters with sound and safe areas from which to fight the fire. Below
grade brick veneers
A: Good question. Brick and other masonry materials are porous and ideally should remain above grade. If your brick veneer comes in contact with the earth, your wall may experience unsightly stains caused by “rising damp” — the wicking of moisture and impurities. However, we realize some designers do not want to “see” bare foundation walls around their new buildings. In this case, you should install through-wall flashing with a drip edge to stop rising damp. Remember, your flashing material will now need to be supported so it does not sag and create an internal gutter. We recommend grouting the space under the flashing for the dual purpose of flashing support and added stability for the sub-grade wall. Block
wetness A: Currently,
there are no accepted industry standards for determining “how
wet is too wet.” But let’s use plain old
common sense. Then there’s weather, one of the most critical, variable, and uncontrollable factors in a construction project. Complicated? Yes! This is where skilled craftworkers come in. Mortar can be adjusted to accommodate some moisture variations in units or humidity, as can the spread length of the bed joint.
Cavity
Inserts
A: The idea of drainage wall or “cavity” inserts is not new. Contractors have been asked to install pea gravel into the base of a cavity wall (or single wythe wall) to help water reach weeps. However, depending on your geography and methods, that has limitations. Recently, the market is flush with good “new” open cell inserts including — but not limited to — base inserts of synthetic woven plastic (which keep mortar dropping at different levels), full height cavity inserts, galvanized metal screens spanning the air space above the weeps, and full height drainage boards. Weeps and wall vents serve two purposes. First, they let water drain from the wall. Secondly, they let air into the wall cavity to promote rapid drying of the wall. So, Yes, good cavity inserts allow weeps and vents to do their jobs. But, having a cavity insert device is no substitute for good workmanship. A craftworker cannot be any less careful when managing mortar droppings. Contact a local IMI training program for proven mortar dropping control strategies, at 1-800-JOBS-0988. ceramic tile floor preparations
A: Contrary to popular belief, acid is not the correct cleaner to use when preparing a dirty floor for the installation of ceramic tile. Dilute acid cleaners have long been used in the masonry industry to clean Portland cement haze from brick and tile surfaces after grouting. Acid cleaners do not remove organic compounds such as oil or grease, and tend to leave an acid residue behind. If not removed properly, that can affect the bond of Portland cement- based adhesives. Use a degreasing detergent to clean the floor. Use of a wetvac for final cleanup will help to insure that no residue is left that can affect the bond of the adhesive. Clean
cavities
A: “Clean but not pristine,” says it best when it comes to debris in the cavity. The key word in your question is “realistically.” It tells me you recognize that masonry is a material laid by hand in field conditions, during all kinds of weather and site situations. Keeping every single mortar dropping or protrusion out of the cavity is not cost effective and it is not necessary. The drainage cavity should be a nominal 2“ clear to allow for proper construction of the wall and to minimize mortar fins, droppings, and bridging. The purpose of the cavity is to drain any water before it can penetrate to the interior. Some mortar droppings and protrusions are to be expected, but the key point is the performance of the cavity: is it draining the water to the flashing? If it is draining, then the cavity is clean enough. You can work out guidelines for this with the job site sample panel prior to the project start so that everyone is clear about expectations.
Q: Can I use anti-freeze to keep mortar going in cold weather? A: “Heat, don’t treat,” is a good rule of thumb when dealing with mortar in cold weather. Accelerators can be used, but must be non-corrosive and non-chloride. Depending on the temperatures, there is a host of recommendations
for cold weather masonry. In Michigan,
“It was also great public relations for masonry performance in winter weather,” says President Dan Schiffer. “We were the star of the show on this large project (653,000 block, 1.5 million brick). Listening to your people in the field and being willing to try new ideas is a great way to beat Old Man Winter, and to prove that masonry works even on wintertime projects.” Cold
Weather Masonry
A: A.
With proper protection, masonry can be installed in virtually
any weather condition, but there are a couple of key
things to remember. Second, mortar temperature must be kept between 40o and 120o F, and the units above 20o F. Units with ice or snow must not be laid.
A third requirement is covering unfinished work at the end of the day (and not just in cold weather). A note on additives: Use of additives that could affect curing time requires prior approval. Some additives can be highly corrosive. Concrete – mesh
vs. fiber
A: Let’s start with the basics. Concrete with “fiber” in the mix generally does not require wire reinforcement. Proponents say it is more ductile and less subject to cracking. Control joints are grooved or saw cut, same as for non-fiber concrete. The surface finish will show the fibers. Concrete with steel reinforcement is still
widely used. It works only when imbedded during the
pour. It is either “pulled
up” into the pour, or reinforcing material is sandwiched
between pour sections. Cutting
Masonry A: The proper approach, in terms of both material and worker safety, is to cut wet. Some firms’ specifications take an overly cautious approach, due to a fear of material shrinkage. Shrinkage occurs as concrete masonry units dry, hence cautions about not laying wet block. IMI’s Construction Guide on architectural specifications says: cut wet. The amount of water added to the unit during site cutting is insignificant in terms of shrinkage, but a very real measure to protect the health of the workers cutting it. Wet cutting reduces the risk of air-born silica particulates, which have been linked to silicosis — a debilitating lung disease.
Fire
Safety A: Fire safety is a concern in all building design and construction, but it is especially critical in multiple occupancy residences like dormitories and senior housing. An accidental fire started in one living space can quickly spread to the adjoining residences and impact many lives. The balanced design concept means using a four step system to save lives and property: In a balanced design, these four strategies are used together as a system, not traded off one for another. Active systems such as sprinklers and alarms work well, but only as long as the supporting mechanical systems work. They can’t provide the structural integrity needed for safe evacuation and fire fighting. They are clearly an integral part of the early warning and suppression equation, but passive systems like compartmentation with masonry walls work 24 hours a day, regardless of the mechanical systems. Masonry walls contain the fire and noxious gases and remain structurally stable—enhancing the evacuation of residents and the safety of fire fighters. Blending active and passive systems, rather than trading them off each other, is the balanced approach to fire safety. Prevention—to reduce the chance of having a fire through housekeeping, education, and building layout. Detection & Alarm—Smoke detectors and other warning devices to catch a fire when it is small, and hopefully keep it from growing into a larger one. Suppression—Using sprinklers, fire extinguishers or other suppression systems to assist in putting out the fire quickly and effectively. Strategic Compartmentation—Using building construction features like masonry fire walls to isolate and contain the spread of fire and toxic smoke while maintaining the structural integrity of the building. This is necessary to allow for the safe evacuation of the residents and to provide the fire fighters with sound and safe areas from which to fight the fire. Flashing
- Single-Wythe walls
A: Architectural concrete masonry units have an irregular surface. That gives them their architectural interest, but also makes it difficult for the top cap to conform to the surface and to seal out wind-driven rain. Using flashing material under a metal cap (usually a self-adhering flexible product) is good practice. It helps prevent moisture that may penetrate the metal cap from entering the core spaces in the block. Another good practice is to use a standard concrete masonry unit for the top course, instead of the architectural unit. That gives the vertical legs of the metal top cap a regular, vertical surface to conform to, and it is much easier to caulk under the cap’s drip edge, forming a seal between cap and masonry. Even a half-high unit can be used, most of which gets covered by the top cap, avoiding visual distraction. And don’t forget two other important steps: detail a drip edge on the cap to direct the water away from the wall surface, and include proper laps on both the cap and the flashing. Fly
A: Yes. In 1999, fly ash was added to the material list within ASTM C476-99 Standard Specification for Grout for Masonry. This means it is an approved material in grout. What is not obvious in this standard is the maximum amount of fly ash that is permissible. That is 40%, when used with Portland cement. The mix cannot exceed 40%, if approved by the specifier. While this isn’t clear from an initial reading of the ASTM C476 standard, it is the correct interpretation. IMI
Educational Programs A: We’re glad you asked. Here is a list of IMI educational programs coming in 2004. We can also tailor programs to a firm’s specific needs. Just call 1-800-IMI-0988.
IMI 2004 Educational Programs (click
for more information) Interior
handset stone applications
A: The common — and most cost effective — method of hanging interior stone is the “spot and tie” method. In this application, usually copper or other # 9 non-corrosive wire is used to secure the stone to the back-up wall laterally. A gypsum casting plaster spot is applied to the connection to ensure alignment. The first course of stone is secured by a gravity anchor or relief angle to carry the load of the stone above. The following courses of stone are drilled on the edges to accept the wire. The wire is inserted into the hole and a wood shim is forced into the hole to secure the wire. The wire is then tied back through a hole to the back-up wall. The stone is checked for level and plum. A “spot” of plaster is applied to encase the wire and immobilize the tie back. Masonry Grout [see fly ash] Mortar
Joints – tooling
A: Tooling has several purposes. Aesthetically, it produces the joint profile, which is made when the jointer or “tool” is struck against the mortar before the joint has set. This profile is usually concave, but it can have other configurations, such as a grapevine or “v” shape. Functionally, proper tooling increases the water penetration resistance of the wall. The tool compresses the mortar against the unit, helping seal any slight separation cracks that might form at that interface. The action of jointer on the unset mortar brings the fines to the surface. This creates an exterior skin more resistant to water infiltration. Timing is key. Too early and the joint won’t seal, too late and it can pull the mortar away. Tooling is done when the joint is “thumbprint” hard. For color consistency, joints should be tooled at the same moisture level. The sequence of striking must be consistent — head, then bed joints or vice versa. A good BAC craftworker is trained in proper tooling techniques.
Q: What is the industry standard for mortar joint thickness in a concrete block wall? A: The MSJC 2002 masonry code, “Specification Section”, page S-21, states that the allowable mortar joint tolerance for the bed joint is +/- 1/8 in. (3.2mm) — head joints, –1/4 in. (6.4 mm) to + 3/8 in. (9.5 mm). Based on a typically specified 3/8 in. (9.6 mm) joint, this means a bed joint can be as small as 1/4 in. (6.4 mm) and as big as 1/2 in (12.8 mm). A head joint can be as small as 1/8 in. (3.2mm) and as big as 3/4 in (19.2 mm). These standards were based primarily on the ability of mortar to do its job properly. Too little or too much mortar may case inadequate bond and/or compressive strength or potential cracking in the joint itself. Either scenario may also contribute to water penetration problems.
This liberal joint tolerance range may allow skilled craftworkers to overcome non-modular dimensions and dimensional variation of masonry units. Be aware that designers have the option of writing more strict joint tolerances based on other accepted guidelines (such as ASTM C-90) or aesthetics. Some write specs to prohibit a thick head joint next to a thin one, even if it is structurally acceptable. The time to pay attention is during the bidding phase, before the job begins, in the sample panel or during early site inspections — not during the punch list phase of a project. Mortar
Testing A: Mortar testing is one of the most misunderstood things in masonry construction. Although the tests may seem relatively straightforward, the interpretation of results is often used incorrectly. There are two ASTM standards that relate to this issue, ASTM C 270 (Specification for Mortar for Unit Masonry) and ASTM C 780 (Test Method for Pre-construction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry). These two standards should NOT be used in combination. C 270 relates to the mortar specification before construction begins, and values are based on lab conditions. C780 relates to mortars prepared and sampled in the field, and only gives methods of testing, not compliance values. Bottom line: It is best not to do any mortar testing. Widely varying test results and difficulty interpreting them often just add confusion. The better way to ensure proper mortar is to observe the mixing. If forced to have some tests, ASTM C 780 can be used as a quality control measure. Take a baseline value at the beginning of the project, and then compare subsequent tests to that (not to values found in ASTM C 270). Prism testing of an entire assembly could be done the same way. This information can be used to make judgments about the quality of the masonry.
Q: An architect has designed a steel stud parapet, but I don’t feel comfortable in building one. What can I do to convince the architect to change his design? A: You are right to be concerned. Parapets are one of the most exposed elements on a building, not only the front face, but also the top and back of the wall. They are subjected to high winds, wind-driven rain, and temperature extremes. They also lack the weight of a wall above, which results in more unrestrained movement. The use of a steel stud backing behind a brick veneer in a parapet is risky and not recommended in most cases. There is a higher chance of moisture and therefore, corrosion, with moisture coming from many sources. Roof systems are prone to failure and leaks over time. Copings may also fail and may not be able to handle the differential movement between the studs and the brick. Steel is a great conductor of heat, and studs that run from a floor below up past the roof are subjected to different temperatures, increasing the chances for condensation. Steel studs are also more flexible than a masonry backup. The deflection may allow cracks to form in the masonry veneer, which in turn result in more moisture penetration.
Q: We are restoring brick facing on a 150-year-old building. It was covered with a simulated stone (cement) facing over wire mesh. Removing the stone facing uncovered numerous indentations in the brickwork, caused by the nailing of the wire mesh. How can we patch the brick? A: There is one material consistently suitable for patching brick, especially to match color, texture and physical properties. Cathedral Stone Products’ Jahn M100 is compatible with oven-fired materials such as brick and raw terra cotta. But the key is proper laboratory analysis, which the company requires. For product information, contact Cathedral Stone Products at 1-800-684-0901 or drude@jahnmortars.com. IMI is the only licensed Jahn certification provider. Contact Ed Bellucci at 1-800-JOBS-IMI or ebellucci@imiweb.org. Prestressed
masonry
A: Prestressed masonry is a very viable new masonry system that everyone from designers to craftworkers could stand to learn more about. It is masonry construction using either concrete or clay units, which has compression added. This method of reinforcing masonry can afford an economical alternative. For more information on design and installation, see IMI’s new Masonry Construction Guide on Prestressed Masonry. It covers frequently asked questions, construction sequence, and plenty of illustrations. That means it has a lot to offer everybody: architects, structural engineers, contractors and craftworkers. For copies of this or other IMI Construction Guides, call 1-800-IMI-0988.
Q: We plan to hang stone panels for the exterior face of our building. The back up system will be concrete masonry. What things should we consider when selecting an anchoring system? A: Each project has its own specific requirements, but these are the general properties you want in a stone panel anchoring system:
Tooling [see Mortar joints – tooling] topTuckpointing
A: Portland-lime
mortars are stronger, but in this case, that is not a
desirable characteristic. If the mortar is harder than
the unit, the unit will crack or deteriorate as the building
tries to move. The general rule is: never replace a mortar
with one of higher compressive strength. Generally speaking, with old buildings, the brick units are much softer and lower in compressive strength. Mortars used then were sand-lime based and softer. That’s why sand-lime or lime putty mortars are generally preferred for historic structures. top |
