GLASS BLOCK, PRE-CAST CONCRETE & TERRA COTTA
Karen Warseck, AIA
asonry is a generic term which is loosely used to describe varied types of exterior building materials including brick, clay tile, stone, concrete block, glass block, adobe, precast concrete and terra cotta. While each of these materials differ from the other in form, color, composition, strength and utility, they share some basic similarities in construction ... and destruction.
Contemporary masonry walls generally tend to be designed and constructed in one of two ways: either a facing built into a backup wall, or as a curtain wall tied to a structural frame of steel or concrete. The masonry units themselves (specifically excluding glass block which will not be addressed in this article) tend to have porous surfaces which allow water to be absorbed into the wall. They share a common factor in that both water retention in the wall and various environmental stresses are the major causes of failure of masonry systems.
The ability of a masonry wall to withstand deterioration is a function of the interaction of the original design, the care in construction and the prompt and proper attention to water-entry problems. Water in the wall system usually is first noticed when it enters the interior wall and causes the wallcovering to stain, or paint to peel. The first thought of the property manager or owner is to call a contractor to put a coating on the exterior of the building to inhibit water entry into the wall system. This may be exactly the wrong thing to do, and may, in fact, be the cause of worsening deterioration. It may be likened to scratching an itch. Scratching may alleviate the problem temporarily, but it sure doesn't cure the itch, and may aggravate the situation by introducing an infection or spreading a rash. The cause of the problem must be found and remedied.
The natural tendency of the porous surface to absorb water is not, in itself, necessarily something to worry about. Normally during a rainstorm, the masonry will absorb some water, but very rarely will it penetrate through the masonry units themselves. Other factors are generally at work. Oftentimes, the leakage does not originate within the walls. Leaking roof and flashing details can allow water to migrate into the wall assembly and cause problems. Covers or copings on tops of parapet walls which have open joints or are poorly constructed or missing can also be a common point of leakage into walls. Failed caulking around windows and doors can allow seepage around these penetrations which may be blamed on the wall itself.
Even when it is the wall that is leaking, the porosity of the masonry material is usually not the cause. Water will find its way through the path of least resistance, usually open joints or cracks. Cracks may be caused by myriad reasons involving stress, movement and a failure to provide for them. However, the major cause of throughwall leakage is not as obvious - voids in the mortar and/or hairline cracks due to limited bonding between the mortar and the masonry unit itself.
Once the water is in the wall assembly, it becomes imperative to get it back out. If it has been absorbed into porous masonry, it will tend to evaporate naturally. If an impervious waterproofing coating has been applied to a surface, any water which has found its way into the wall assembly will remain trapped. If the water has condensed and accumulated inside the cavity between a veneer and back-up or curtain and interior wall, it will tend to trickle down the inside of the cavity and out weep holes. If these holes, designed to allow water to drain, are clogged or missing, any accumulated water will remain in the wall or make its way into the interior of the building. In either case, it can mean trouble.
When water is caught within the wall, it can cause steel structural members and masonry ties to rust. Rusting of the structure can lead to weakening of the entire building. Since masonry walls tend to be thinner over structural members, the expanding nature of the rust can cause the masonry to move out of the wall plane. Rusting of the masonry ties can cause them to fail. This will allow the exterior face of a cavity or curtain wall system to delaminate. If there is enough movement, the exterior wall may come loose from the structure and collapse.
Even if a water-resistant coating is used instead of a waterproof one, the trapped moisture may still cause problems. With a waterproof coating, the water cannot escape at all and can freeze within the wall causing chunks of masonry to fall off the surface. This is called "spalling." Even if moisture can escape through a "breathable" coating by evaporation, the salts acquired by the passage of the water through the construction materials (such as concrete or mortar) will crystallize just below the outer surface of the masonry. This condition, if left untreated, will allow the salts to build up and cause exfoliation or spalling.
Not only is spalled material extremely hazardous to anyone underneath it when it falls, but it can also contribute to spiralling deterioration. In brick masonry, for example, because of the way it is made, the surface is harder and less porous than the interior. When this protective surface is removed by spalling, the soft porous interior it exposed. Since it is more porous than the outer surface, more water can be absorbed and trapped in the wall. This water can freeze, causing further spalling and greater damage.
Thus, coatings, if used at all, should be applied as a preventive measure, rather than a rehabilitative one. The best method of making a watertight masonry wall is to first inspect the joints for hairline cracks. If these are present, cutting out the old mortar to a depth of ¾-inch and then repointing the masonry with a high lime content mortar will usually make the masonry walls watertight that is, providing the rest of the building is, too!
Details of design and care in construction play an integral part in the weather-tightness of a masonry wall. The proper mortar mix is essential and can significantly affect the flexibility of the joint. But even a proper mortar mix will not help if the mason is unskilled and leaves voids, or places masonry units in partially set mortar. The voids and hairline cracks caused by this will show up while the building is still new. Other serious damage due to design or construction flaws will not appear as quickly. These problems may take decades to manifest themselves, but the owner/ manager should be aware of their possibility and watch carefully for their symptoms - cracking and movement of the masonry units out of the wall plane.
Masonry, in general, is a stiff' material not prone to flexing. When used in conjunction with a material that is more flexible, the masonry will tend to crack long before its counterpart when subjected to movement or stress. So, for example, if a masonry curtain wall is tied to a flexible steel structure, unless adequate precautions are designed in and constructed properly - when the frame moves, the wall will crack. These precautions are in the form of "soft" joints. Soft joints are vertical cuts in the masonry wall, or horizontal joints below a bearing shelf angle, that are sealed with a material that will stretch or compress, depending on building movement. They will relieve the strain on the masonry due to building movement or environmental stress. Other less obvious reasons for including soft joints have to do with the interaction of dissimilar materials and/or thermal expansion. Concrete, for example, shrinks upon aging, but bricks absorb moisture and grow. The different properties of the materials will cause stresses which, if soft joints are not included in the building, will cause it to crack.
Shrinkage of a concrete structural frame can cause floor-to-floor heights of a building to decrease. Steel will expand and contract much more readily and in greater amounts than masonry. The side of a parapet wall facing the sun will expand more than the side away from the sun, causing the wall to bow. All these subtleties must be accounted for in the design, or else cracking or movement out of the wall plane will occur.
Other items which the owner/ manager should be aware of are the presence of efflorescence and staining, both of which can make a building look unattractive to tenants. Efflorescence is the result of salts having been dissolved in water, drawn to the exterior surface and deposited as a white, dusty substance. Unless the wall has been coated and the salts prohibited from reaching the exterior, efflorescence is not, in itself, harmful. It is, however, a sign that water has penetrated the wall. When the source of water penetration is discovered and rectified, the efflorescence will disappear. Washing the building without determining the cause of water penetration will be futile, since the water already in the wall system will continue to leach out the salt, and the efflorescence will reappear. We have already noted that dissolved salts deposited under the surface of coated masonry can cause spalling.
Staining has many causes, including simple dirt, soot, metal oxidation, organic matter and improper cleaning methods. Sometimes the discoloration is actually a part of the masonry itself. Dirt and soot are carried atmospherically, deposited on any flat surface and then washed over the edges, appearing in a flume pattern on the wall. Oxidation of metals will cause rusty, green or brownish stains in the mortar joints and sometimes over the face of the brick. Organic matter will be left after ivy is removed.
There are three different systems of cleaning a masonry wall that are in general use - water, chemical and abrasive. Water methods include low-pressure washes over an extended period of time, high-pressure washes and steam cleaning. Chemical methods include muriatic acid and other acid and/or detergent based proprietary formula cleaners that are used in conjunction with a water rinse or chemical neutralizer. Abrasive methods include sandblasting and grinding.
Inappropriate cleaning methods or products may actually do more harm than good. Each type of cleaning system - water, chemical, and abrasive -must be thoroughly investigated and tested on the building, if possible, to determine any detrimental effects the system may have. These tests should be done on a surface area large enough to adequately show the results, and, since different masonry materials will react in different ways to different cleaning methods and products, on each type of masonry existing on the building. Ideally, the composition of the masonry and mortar should be analyzed to determine if any chemical reactions will occur during cleaning.
In all cases, the masonry joints should be tight before proceeding with any cleaning method. If acids are allowed to enter the wall system, corrosion problems can occur in the masonry ties. If joints are loose, they can be destroyed by sandblasting. Water in the system can cause any of the problems noted earlier.
Many things should be considered before choosing the right cleaning methods and products including whether or not cleaning is a good idea. If the discoloration is due to weathering, cleaning the "dirty" area will mean removal of a portion of the masonry itself. Repaired areas are often stained to match the existing masonry, and cleaning can make these differences apparent. Acid cleaners, even in a dilute form, seriously deteriorate limestone and marble and should never be used. Harsh chemicals can etch glass, corrode aluminum and destroy plants surrounding the building.
Using high-pressure equipment to spray acid based cleaners may drive them too far into the masonry. This makes complete rinsing impossible, which can cause a white scum to form on the masonry surface. Sandblasting and other abrasive methods can severely damage brick masonry, mortar and stone. Scarred surfaces caused by blasting will accumulate dirt faster than smooth surfaces. Iron or copper in the water supply may stain, or in some cases, disfigure a surface cleaned by water washing. Any cleaning method which requires the use of water should never be undertaken when the possibility of freezing exists.
Periodic examinations of the building are the best way to ascertain if the building needs attention. The property manager or owner should be sure that each wall and parapet of the structure is inspected for cracks, movement out of plane, spalling, staining and efflorescence. This inspection may be made by the maintenance staff. If any or all of these symptoms are present, a professional scrutiny is warranted. Keep in mind, however, that a contractor - though quite knowledgeable in the application techniques of coatings and cleaners and the results they can have may be unaware of the underlying causes of the problems, and inadvertently make them worse. He also may have a possible conflict of interest, in that he may be more inclined to recommend a service which he performs, whether or not there may be something more appropriate. Representatives of manufacturers of coating and cleaning products may suffer from a similar potential conflict of interest. The best solution is to commission a specialist in rehabilitative architecture/engineering to survey the problems and recommend solutions that suit that particular project.
There is no one way to fix or clean every building. The objectivity of the consultant can help assure that the money spent to repair masonry damage will not be spent again five or ten years later when the price has escalated, and/or the damage compounded. The consultant can also provide alternatives and prioritize work to help fit an existing budget. In addition, the professional architect and engineer can prepare construction documents for repairs, or specifications for cleaning, that will allow meaningful competitive bidding by contractors. Apples to apples - so that the property manager/owner knows what he is getting for the price. And also, that he is getting the best price for a particular type of service.
Home | Articles
Karen Warseck, AIA. CSI, is president of Building Diagnostics® Associates, a Hollywood. Fla., firm that specializes in the analysis of roofing and waterproofing problems.
This article was reprinted from the January 1984 issue of Building Operating Management Magazine. For other great articles, log on to Facilities Net at http://www.facilitiesnet.com.