PROBLEMS DUE TO WEATHERING OF STRUCTURES
AND DESIGN FOR WEATHERING
Design for Weathering Buildings Using Architectural Precast ConcreteThe Prestressed Concrete Institute (PCI) has published the attached report entitled, "Design for Weathering of Buildings Using Architectural Precast Concrete."Prestressed Concrete Institute BrochureAlthough the major topics in this article are directed toward precast concrete, the article also includes information on glass, glass cleaning, and details to reduce building rundown onto glass.PPG does not recommend glass cleaning methods provided under "Cleaning." PPG's specific glass cleaning recommendations are provided in the Technical Service Memo, "Residue on Glass" dated November 30, 1981. Design for Weathering of Buildings Using Architectural Precast ConcreteA primary consideration in the architectural design of buildings should be weathering i.e., the changes in appearance with the passage of time. Weathering affects all exposed surfaces and cannot be ignored. The action of weather may enhance or detract from the visual appearance of a building or may have only little effect. The final measure of weathering effects is the degree to which it changes the original building appearance.Visual changes occur when dirt or air pollutants combine with wind and rain to interact with wall materials. The run-off water may become unevenly concentrated because of facade geometry and details. The manner in which water is shed depends primarily on the sectional profiles of the vertical and horizontal discontinuities designed into the wall. Designers acquired, through the years, ways of handling traditional building materials in order to control the water flow down specific parts of a structure - copings, drip molds, gargoyles, window sills, and plinth details. However, many of these useful and relevant details have been discarded as superfluous decoration. For architectural precast concrete (as well as all other building materials), the awareness of weathering should be reflected in the design of wall elements and the integration of windows to control water migration and collect and remove water run-off. Staining that occurs through differential surface absorption and uneven concentrations of dirt due to water run-off are considered the most common weathering problems. Many of the effects of weathering can be predicted by studying local conditions and/or existing buildings in the area. With proper attention to the cause and effect of weathering, potentially detrimental results can be eliminated or at least minimized. Precast concrete will become dirty when exposed to the atmosphere just like any other material. Fortunately, with architectural precast concrete, the designer can choose shapes, textures, and details to counteract any negative effects of weathering. Although, regular cleaning of a building may make detailing a less critical factor, maintenance costs should be balanced against initial design costs. The major contributing factors to the weathering of precast concrete are: Dirt in the atmosphere. Atmospheric dirt or air pollutants include smoke or other gas, liquid droplets, grit, ash, soot, organic tars, and dust. Deposits washed onto the precast concrete from an adjacent surface or material. Water flowing over copper, bronze, weathering steels, or sheet metal and then over concrete may create stains over a period of years. Maintenance procedures, such as window cleaning, can produce dirt markings on precast concrete unless care is exercised. Chemical action. The concentration of corrosive elements such as sulfur dioxide is high in some urban environments. Sulfur dioxide when dissolved in rainwater produces dilute sulfurous and sulfuric acids. Acids etch the cementrich paste and the carbonated concrete surface. This etching action can produce a gradual change in color from the original surface, as the fine aggregate becomes exposed. Except for areas with unusually high concentrations of corrosive elements, the designer need not concern himself with this problem, beyond specifying concrete strengths and durabilities normally associated with architectural precast concrete. Soft aggregates such as some limestones and marbles should be avoided in corrosive atmospheres. Surface deposits from the panel materials such as efflorescence. Efflorescence is a crystalline deposit usually white in color, caused by soluble alkali salts that migrate to the concrete surface with evaporating water. Dark surfaces tend to show efflorescence more than light or white backgrounds. Organic growths (algae, lichen, etc.) although uncommon, may occur with high ambient humidity and temperature. Surface changes in the material. The cement-rich film on smooth finished concrete may develop surface crazing. Although crazing has no structural or durability significance, its presence may be accentuated visually when dirt settles in the minute cracks. Crazing generally will not appear if the outer cement skin has been removed by a surface finishing technique to expose the aggregate. Rust stains may result when reactive iron pyrites or other ferrous materials exist in the aggregates. Such aggregates should not be used for exposed aggregate panels. Rust stains caused by corroding reinforcing bars should not occur if adequate concrete cover is provided. Rust stains caused by corroding reinforcing bars should not occur if adequate concrete cover is provided. Rust stains due to corrosion of hardware should not occur if the hardware has adequate concrete cover or is protectively coated. Foreign matter deposited on the concrete surfaces during construction
such as oil, grease, mud, grout, plaster, and fireproofing.
Rain EffectsInitially, rain is a cleansing agent for the building's surfaces. However, at some point on the building, the run-off will pick up dirt and it becomes a soiling agent. The preferred lines of water flow must therefore be arranged so that, at the point where the water is expected to become a soiling agent, it will not detract from the finishes or forms of the building elements. (Dirt will drop out of the run-off water when the water flow velocity is decreased; for example, when the run-off is allowed to fan out.)The amount of rain water, and the velocity and angle at which it strikes, can be markedly different on each side of a building and at different heights. Therefore, it is not reasonable to expect equal weathering of all parts. The influence of tall or massive buildings, projections, courts, or passages on prevailing winds can cause wind eddies to upset the natural flow of air and rain. This makes the effect of rain water even more difficult to predict. Fig. 1 shows the volume of rain assumed to hit a building surface depending on the orientation of the surface. The rain, is assumed to be 10° to the vertical. However, the variability of rain under actual conditions makes all but a general prediction
difficult. Small forward-sloping surfaces usually weather cleaner. Large areas may begin to collect dirt at the lower end unless the angle is steep. With heavy rain, the dirt on horizontal surfaces and surfaces that have little slope may be partially washed off, streaking the surfaces below. In the case of light rain or drizzle, the dirt may collect and slowly flow down other surfaces in the general direction of the water flow resulting in pronounced random streaking. Backward-sloping surfaces collect little or no rain but are likely to be subject to a partial, nonuniform water flow from above which may carry dirt and cause serious streaking. Often backward-sloping surfaces are seen in shadow. In this case, the accumulation of dirt is not particularly noticeable if the dirt is acquired evenly without disfiguring streaks. The migration of run-off water is affected by: The location and concentration of rain deposit. The properties of water in contact with materials, especially surface tension. The forces of wind and gravity. The porosity, texture and geometry of the building surface. The designer should attempt to anticipate and plan for water flow over an exterior wall, particularly wherever there is a variation in the wall. Rain deposit is usually concentrated at the top of exposed building faces. Surface tension causes droplets of water to coalesce on non-porous surfaces such as glass and metal and to drain in irregular streams. Surface tension also allows flows to take place along the underside of horizontal surfaces and contributes to the concentration of water streams at sharp outside corners. The facade geometry of buildings is usually responsible for local concentrations of run-off. Such concentrations lead to the characteristic marking patterns frequently observed on building surfaces. A fairly even accumulation of dirt occurs on vertical surfaces. Where some feature causes a heavy concentration of water a lighter, cleaner streak (white-washing) is produced across the general pattern. Horizontal or near-horizontal surfaces tend to have the most dirt accumulation. Run-off then carries the dirt to adjacent vertical surfaces to produce dark streaks (dirt-washing) across the general pattern. On any building there may be combinations of dirt accumulation, dirt-washing, and whitewashing, Fig. 2. New buildings may show dirt-washing at locations of concentrated run-off when their over-all surfaces are still quite clean. Later the same areas may exhibit white-washing, after adjacent surfaces have been darkened by dirt accumulation. When run-off reaches a discontinuity the water may bead and drip free. This may increase or decrease the run-off concentration, affecting both its ability to carry suspended dirt particles, and its subsequent drying behavior. Such changes of flow concentration may disfigure the building surfaces. Certain building details, such as horizontal returns to vertical surfaces, tend to concentrate the flow of water on adjacent walls. Dirt marking often occurs when such concentrated flow is later dispersed on a vertical surface. The resultant slow running patterns of water become areas for subsequent adherence of airborne dirt. Mullions and other vertical elements meeting a horizontal element will often cause a concentration of flow that results in uneven weathering, Fig. 3. Vertical surfaces can be protected and weathering minimized by providing steeply sloping overhangs with drips. These tend to reduce dirt accumulation and the washing of dirt onto the vertical surfaces below. The intersection of horizontal and vertical projecting elements almost always creates dirt streaks. Such streaks run back from the edge of exposed columns and below the ends of horizontal elements even when they are steeply sloped at the top surface. To avoid such streaks stop the horizontal element short of the column. This confines run-off to the horizontal element and permits unimpeded washing of the column. Channeling of the column faces also will help to prevent water from running back along the edges.
Water flowing laterally or diagonally downward on a surface will concentrate where it encounters vertical projections or recesses. The secondary airflow due to wind is also important. It concentrates run-off at the outside corners of the building at columns, and at inside corners of vertical projections. Surface tension contributes to this effect by preventing flow back from vertical edges of small elements such as window mullions, often concentrating the flow at the corners. The porosity of the building surface influences wetting. Porous surfaces become wet rapidly and stay wet longer than non-porous materials. Surface texture affects the distribution of the run-off and the length of time required for the water to drain. Irregular and concentrated streams tend to form on smooth or lightly textured materials. A uniformly distributed broken flow is more likely to occur over heavily textured materials, Fig. 4. Glass areas cause build-up of water flow. Because glass is a non-absorbent
material, the flow rate of water down its surface is fast and there is
little time lag in its throw-off. By contrast rain water flowing down an
adjacent concrete wall surface will be slower (depending on the surface
texture) and its throw-off will be less complete. As a result, there is
a concentration of water at the bottom of each window - the very thing
the designer must guard against if differential patterning is to be avoided.
This flow must be dissipated, breaking up its concentration. Further, there
is always a tendency for water flow to be in greater volume at the edges
of the glass (the smallest amount of wind tends to drive rain toward the
edges of the glass).
Design of Concrete for WeatheringSince concrete qualities will influence the degree to which staining of concrete surface occurs the effects can be predicted and controlled.The duration of wet conditions and the penetration of water, dirt, acidic rainwater, carbon dioxide, and sulfur dioxide are directly related to the absorption of the concrete surface. Absorption and penetration also create difficulties in cleaning such surfaces to restore them to the original appearance. Low absorption can be achieved by producing a high density concrete. Concrete density is influenced by: (1) mold design, (2) concrete curing (3) concrete consolidation, and (4) mix proportions. Absorption should also be uniform, since differential surface absorption can cause blotches or streaking. Concrete mixes should be designed or evaluated for each individual project
with respect to strength and absorption. A water absorption test of the
proposed facing mixes may provide an early indication of the weathering
properties. (5) For architectural precast concrete, typically proportioned
for a 28 day strength of 5000 psi, water absorption should not be a problem.
To achieve good weathering characteristics, the maximum water absorption
for the concrete at 28 days should not exceed 5% by weight.
Architectural DetailsCareful attention should be given to the effects of exterior details on the weathering pattern of the building. In anticipating water flow, it is important that the water flow be traced to the final drainage point or to ground level. Particular consideration should be given to sloping surfaces, projections, water drips, parapet and roof edges, surface finish including color, openings in walls, and joints (or grooves).Water Drips Water drips may stop the streaking of a backward-sloping face when the drips are placed close to the forward edge. When placed under horizontal projections, drips will prevent streaking on vertical surfaces. Water will leave a drip at its lowest point and it is important to follow its course thereafter. Small chips and cracks may concentrate the flow, so that water will bridge drip details and allow wetting of the surface below. If dirty water falls onto other surfaces the problem may be merely relocated. However, if the wind tends to spread the water out on the surface below, uniformity of weathering may be obtained. To avoid streaks on the sides of window panels, the drip may be stopped short of the sides. Often recesses or grooves are incorporated in the side walls to further direct the water. These drips also prevent water run-off (after a storm) from slowly running over the window glass, a primary cause of glass streaking. Water flow may be evenly distributed or deliberately directed toward the center of the panel, Fig. 5. In the latter case, the panel should have a dark, rough textured surface slope below the window to break and mask the water's dirtying effect. The drip section should be designed in relation to the slope of the concrete surface, Fig. 6. To avoid a weakened section do not locate the drip too close to the edge of the precast unit. If no drip devices or projections are provided on a building face, run-off water can flow over the wall materials and windows for the total height of the building. Dirt may be deposited in sufficient quantities to cause disfiguring stains on wall units and, in a short period, may streak and stain the glass, Fig. 7. A sealant bead applied to precast units after erection, on plastic drips glued to the concrete, are remedial solutions used with varying success depending on their care in application.
Parapet and Roof Edges Parapet and roof edges should be designed to avoid run-off from flat roofs. A parapet of sufficient height (8 to 12 in.) will normally prevent water blowing from the roof over onto the face of the building. The top of the parapet should slope backwards for the major portion of its width, Fig. 8. Flashing needs to project at least 1 in. beyond vertical wall surfaces and have a proper drip device to throw run-off water clear. Projections of less than 1 in. will permit water to either flow back or be blown back against wall surface. The choice of flashing material and/or its treatment against corrosion should be based on preventing potential staining of the precast surface. Water may be directed from low roofs by scuppers. These should have sufficient projection to prevent the water hitting lower surfaces, Fig. 9. Surface Finish Concrete surface finishes vary considerably in their ability to take up and release dirt under weathering conditions. They should therefore be chosen for these so-called 'self-cleansing' properties. But the question of color and texture has an esthetic significance greater than considerations of weathering alone. The surface of smooth precast concrete is hard and impervious and easily streaked by rain; the weathering pattern being determined by the shape and accuracy of the units and the joints which are particularly vulnerable. Any irregularity in a smooth unit will be exaggerated by weathering patterns. Honed or polished surfaces have good weathering characteristics and are less susceptible to surface streaking. Smooth concrete may be susceptible to surface crazing when exposed to wetting and drying cycles. This is a surface phenomenon and does not affect structural properties or durability. In dirty atmospheres, crazing is accentuated by dirt collecting in the tiny lines. This will appear more in white finishes and horizontal surfaces. Textured finishes accumulate more dirt, but they can maintain a satisfactory appearance. The aggregate tends to break up and distribute water run-off and reduce streaking, Figs. 4 and 10. Rounded aggregates are largely self-cleaning. Angular aggregates of rough texture tend to collect dirt. However, dirt absorption is generally confined to the matrix. For this reason, as well as for architectural appearance, the area of exposed matrix between the pieces of stone should be minimized. A darker matrix will reduce the effect of atmospheric pollution. Extreme color differences between aggregates and matrix will create uniformity problems. For example, large-size exposed aggregates of light color provide a heavily textured surface that may seem to be very dirty after a time because the matrix becomes very dark and the high spots of the aggregate are washed clean. Medium textured finishes may still allow water to run or be wind-driven into streams to cause irregular streaks, but vertical ribs or flutes will help to control the runoff and prevent it from spreading horizontally. The dirt collects in the hollows helping to emphasize the shadow and, therefore, the texture itself. The use of appropriate, dark colors and rough textured surfaces can help to mask the effect of dirt deposits. The over-all darkening in tone that takes place is unlikely to be objectionable unless streaking occurs. In some cases, uniformly colored light surfaces contrasted with uniformly colored dark surfaces may be used to accentuate the depth of relief on a building face. Joints Joints are important features in creating weathering patterns. In addition to leading the water down the joints (real or false) the designer should determine where the water will finally emerge. Edges of joints should have a reasonable radius (chamfer) to reduce the possibility of chipping. Chips disrupt water flow and concentrate dirt. Non-staining elastomeric type joint sealants should be selected to prevent the possibility of bleeding and heavy dirt accumulation, common problems with mastics. Also, care should be taken to avoid sealants that collect dirt as a result of very slow cure or inherent static conditions. Two-stage joints must be vented and drained to the outside. Vent tubes should be 3/8 to 1/4 in. inside diameter polyvinyl chloride or other non-staining materials. Vents should be located at the junction of the horizontal and vertical joints and project at least 1/4 in. beyond the sealant. Therefore, if any moisture does come out of the vent tube, it will run down the face of the joint sealant and not over the face of the panels. Fig. 11 shows an elevation where some of the vertical joints, into which water is channeled, discharge this water over a vertical concrete surface. The water should be directed all the way until it reaches the ground or a drainage system.
Adjacent precast units should have faces aligned within accepted industry tolerances. Any discrepancy may pass undetected on a new building, but weathering will eventually emphasize the error with uneven staining of adjacent units. It is helpful when the architect, engineer, manufacturer, and erector understand all requirements in this respect and cooperate in the solving of problems. Openings in Walls - (Windows) Openings in walls that form the windows of a building help in breaking the surfaces but contribute their own weathering problems. Individual windows in a wall of architectural concrete need to be designed with two principles in mind: (1) to contain the water flow within the window area and, (2) to disperse it at the bottom of the window in such a manner that it is spread, not concentrated. There are many different ways of detailing windows, depending to some extent on their shape and the degree with which they interrupt water flow, Figs. 6 and 12. One detailing approach is to box the whole window out from the general wall face, Fig. 12b. If the cut back at the base of the window is, as near as possible, perpendicular to the wall, the detail can be very effective. The window jamb should be detailed to avoid water being blown around the reveal where it can streak the wall at the side of the window projections. A projecting window is more expensive than a simple opening, but it does avoid unacceptable staining. A coffered window profile minimizes staining or streaking. Here the window is set in a deep coffer whose margins project slightly in front of the general wall surface, Fig. 12c. The splayed lower plane of the coffer may streak, particularly in the corners, but these streaks will emphasize the modeling of the feature. If the coffer is reasonably deep there is no danger of water being carried off the edges of the window. The easiest windows to design are vertical slot windows. These interrupt the water flow least of all. The important consideration is the water directed to the base of the window. If the water volume is considerable (a very high window) and the windows are not kept clean, a large volume of dirt will be directed to the sill. The dirt presents a problem if under-window or spandrel panels are light colored concrete. Figs. 12e and 13 picture a solution where concrete scuppers, cast integrally with the window panels, direct all water from the windows away from the building. The walls of the building in Fig. 13 are all precast with brick-faced concrete panels between the windows.
Design details at the base of a continuous horizontal strip of glazing is as important as that at the bottom of an individual window. The taller the area of glazing, the more dirty water is involved. This water should be thrown clear of the building. Figs. 12e and 14 show a concrete panel shaped to prevent water from running over the face of the panel beneath the window. The water is directed instead to joint features along the windows, or to tubes which take it to the nearby joints or an internal drainage system. (Tubes may create problems during freezing temperatures.)
For continuous horizontal windows, a concealed gutter may be used. The gutters collect the water from the line of glazing and discharge it through a series of scuppers, Figs. 12e and 15. This is an expensive way of overcoming the problem. In a high building, or with high wind velocity, this solution is ineffective. Glass Staining or EtchingWhen damp material is in contact with or applied to glass, the glass surface may undergo subtle changes in the contact area. If the material in contact with the glass is inert and moistureproof, the glass surface will be protected by the material from changes caused by exposure to moisture. Later, if the contacting material is removed, a differential surface change may become quite visible and unattractive under some lighting and viewing conditions, even though the change is slight. Finely divided damp materials, e.g., dirt and dust, in contact with glass cause the glass constituents to dissolve slightly and be redeposited at the evaporating edge of the material resulting in "staining."When glass (sodium calcium silicate) is exposed to moisture, a minute amount of the glass will dissolve. (Glass will ordinarily lose some of its sodium by dissolution in water but the calcium then stops most of the dissolution. In polluted atmospheres, the acids -SOx or NOx - will attack the calcium and permit further dissolution.) If the dissolved material is washed away little change can be seen by the human eye. But when the solution remains on the glass, atmospheric carbonation of the alkali and alkaline earth silicates causes a subsequent deposit of silica gel. The gel on aging and exposure to atmospheric acids becomes difficult to remove. (Alkaline washing compounds will dissolve some of the silica gel leaving the glass again susceptible to moisture.) When this happens uniformly, the eye does not detect the differences. The silica gel deposit or the glass etch depth need not be thicker than a wavelength of light for the eye to detect it. All materials which tend to increase the length of time moisture is in contact with glass during wet-dry cycles are likely to speed staining. Dirt accumulation on glass, for example holds water on the glass longer causing moisture attack. Once the silica gel builds up and becomes hard, it retains moisture and also causes run-off water to flow along the same paths. The process of surface corrosion then becomes self-perpetuating. Frequent washing of the windows tends to remove the gel before it becomes hard, minimizing staining and etching. Directed slow water run-off and the resultant dirt accumulation cause the glass to be attacked nonuniformly and eventually, the cycle of water drying, gel forming acid atmosphere attack, and alkali washing compounds, causes in-depth glass dissolution and no amount of cleaning or buffing will remove the stain or etch. Staining will be more noticeable on tinted heat-absorbing glass because of the greater contrast between the light color of the stain or etch and the darker color of the glass. There is no known difference in the composition of tinted glasses, which contributes to this staining as compared to clear glass. The usual explanation for the etching of glass in concrete structures is that concrete contributes alkaline materials to the run-off water. Hydration of cement results in the formation of hydrated calcium silicates. Ca(OH)2, and aluminates with the remaining internal water becoming highly alkaline. It is well known that alkali, meaning high pH material, will attack glass. What is not well known, is that atmospheric acids (NOx, SOx, and COx,) can quickly neutralize these alkalies from concrete to produce neutral salts of calcium, sodium or potassium. Of these salts, only the carbonates of sodium and potassium are truly alkaline. However, even these salts are quickly converted to the bicarbonates which are only very weakly alkaline. The atmosphere is usually very acid in the larger cities, Fig. 16, therefore, very little if any alkali (high pH) material will be leached more than a few millimeters away from its source except in the case of very young concrete (less than 28 days). Rainfall can permeate concrete having high absorption and cause efflorescence. The efflorescening salts are usually neutralized by the carbon dioxide in the air before they can go very far. The leaching of concrete (efflorescence) ceases in 1 to 2 years because the surface lime is mostly carbonated in place and the interior lime cannot be reached.
In addition, chemical reaction of the cement compounds with sulfur and nitrogen oxides in the air occurs with the subsequent precipitation by evaporation of solutions containing the reaction products, such as gypsum (CaSO4 . 2H2O). The transference to and deposition of these materials on the window glass by rain water can result in surface staining and etching if they are allowed to remain on the glass for a period of time. (The gypsum acts in the same way as dirt in causing a stain.) The time period for a stain to result depends to some degree on the ambient temperatures with warmer temperatures causing the stain to occur sooner. Weathering steels, bronze. limestone or aluminum curtain wall buildings as well as precast concrete buildings may experience staining of window glass, Fig. 17. Analysis of powder scraped from glass stains on both a metal faced and concrete faced building show that a good portion of the stain is composed of gypsum (CaSO4 . 2H2O) indicating that Sox from the atmosphere also plays a role in staining. The calcium on the metal building had to come from airborne dust or from the glass.
The plasticity of concrete lends itself to use in many shapes which may not incorporate proper water run-off in the design. In addition, the rough surface textures of exposed aggregate concrete increase water retention and results in slow water run-off. When uniform wetting of windows occurs, staining of glass generally does not occur. However, when differential wetting of the windows occurs from a slow run-off of rain water, such as by dripping, stains will occur regardless of the construction material. Building details can reduce the amount of water discharged to the glass. Concrete frames at window heads should, wherever possible, be designed so that they do not splay down and back toward the glass unless drip details are incorporated into the frames. Without drip details, a direct slow wash down of the glass should be anticipated. The introduction of edge drips and a second drip or gutter serve as a line of defense against slow run-off. This can be accomplished by having a cast-in drip in the concrete or by the use of extrusions (either aluminum or neoprene) across the head of the window which have either an integral gutter or extended drip lip of at least 1 in., Fig. 18. Protection of WorkWeathering may also affect precast units during storage. If stacked in a position different from their final orientation, the panels may have to be protected to avoid streaking which may show as horizontal lines on the finished building. Protection against such weathering should be left to the precaster, but may influence the design of the unit.Rain water, or water from hoses used during the construction of the building can cause discoloration of exposed precast concrete by washing across building materials (such as steel, concrete, or wood) and then across the precast. The General Contractor should provide and maintain temporary protection to prevent damage or staining of exposed precast concrete during construction operations after installation. Particular care should be taken to avoid washing over the precast by jobsite water. Precast units and adjacent materials such as glass and aluminum should
be protected from damage by field welding or torch cutting operations.
Non-combustible shields (asbestos sheets) should be provided during these
operations. To minimize staining, all loose slag and debris should be removed
when welding is complete. All welds and exposed or accessible steel anchorage
devices should be pointed with a rust inhibitive primer, or in cases of
galvanized plates, a cold galvanized coating containing 95% zinc. Such
protection should be applied immediately after cutting or welding.
CleaningAll precast concrete should be furnished to the jobsite clean and acceptable condition. As erection of exposed cast work progresses, all dirt, mortar, plaster, grout stains, or other construction droppings should be removed by brushing or water washing. The precast units should be given a final cleaning only after all installation procedures, including joint treatment, are completed and at least 3 to 7 days after patching.If at all possible, cleaning of concrete should be done when the temperature and humidity allow rapid drying. Long drying periods increase the possibility of efflorescence and discoloration. Before cleaning, a small area should be cleaned and checked to be certain there are no adverse effects before proceeding. A suggested order for testing appropriate cleaning procedures for removal of dirt stains and efflorescence from precast concrete is:
Since the use of a dilute solution of muriatic acid (5 to 10%) may slightly change the color and texture of panel and thus affect the appearance of the finish, the entire wall should be treated to avoid discoloration or a mottled effect. A more dilute solution (2%) may be necessary to prevent surface etching that reveals the aggregate. Hydrochloric (muriatic) acid may leave a yellow stain on white concrete and therefore 3% phosphoric acid is preferable on white concrete. Rubber gloves, glasses, and other protective clothing must be worn by workmen using acid solutions or strong detergents. All precautions on labels should be observed because these cleaning agents can affect eyes, skin, and breathing. A mild soap, detergent, or perhaps a slightly acidic cleaning solution should be used to clean the glass. Harsh cleaners and abrasives, particularly those of an alkaline character are not recommended. If a light stain or etch remains, it may be removed with a slurry of cerium oxide and water or 4F pumice and water mixed to a paste consistency. As an alternative 4F pumice plus Windex may be rubbed on the glass applying light pressure with a clean damp cloth, using 10 to 20 strokes. If the above does not remove the stain, a motor driven lambs wool buffing head dampened lightly with a 25% solution of hydrochloric acid and sprinkled with 4F pumice should be carefully applied to the stained area. This is followed with another lambs wool head dampened with Windex plus a 5 to 10% solution of ammonium hydroxide. This solution can be sprayed on the glass or the buffing head and is used to neutralize the acid on the glass. The window is then rinsed with clean water. Household ammonia should not be used in place of ammonium hydroxide solution as some brands have been known to contain caustic alkali which can etch the glass. Careful handling of the acid solution is required since acid can irritate skin, and damage eyes and clothing. Great skill is required in use of buffing heads to avoid creation of "bulls-eyes" and other non-uniform surface effects. The hydrocholoric acid cleaning process will not remove a silica gel
deposit. Polishing with levigated alumina or a dilute (0.25 to 2%) solution
of either hydroflouric acid or sodium or potassium acid flouride will remove
the silica deposit. This treatment removes the silica deposit and at the
same time etches other areas so that the eye sees a uniform surface. However,
if the etch is already deep, the acid will not remove the streak because
it is already a different plane and reflects light at a different angle.
The hydroflouric acid or flouride treatment should be tested on several
panes in different locations before proceeding with the whole job. In some
cases, replacing the glass may be more economical than stain removal.
Proper precautions should be taken by workmen using these acid treatments
as the acids are highly corrosive, and in the case of the flourides, severe
eye or skin damage may occur.
Clear Surface CoatingsClear surface coatings or sealers may be considered for the possible improvement of weathering characteristics. The quality of concrete normally achieved in architectural precast concrete, even with the minimum practical thickness, does not need sealers for waterproofing.Sealers may be applied for the following reasons: The prime justification for their use will appear as a potential improvement of weathering qualities in urban or industrial areas to reduce attack of the concrete surface by airborne industrial chemicals.
A careful evaluation should be made before deciding on the type of sealers. This should include consultation with the local precasters. Suggested sealers should be tested on reasonably sized samples of varying age, and their performance verified over a suitable period of exposure or usage based on prior experience under similar exposure conditions. Any coating used should be guaranteed by the supplier or applicator of the sealer not to stain, soil, darken, or discolor the precast finish. Also, some clear coatings (sealers) may cause sealants to stain the concrete. Consult manufacturers of both sealants and coatings or pretest before applying the coating. The type of solvent used, as well as the solids, can affect the resulting color of the concrete surface so neither the type nor source of sealer should be changed during the life of the project. Generally, sealers having higher solid contents tend to produce darker surfaces with glossy effects. The amount of color change is dependent upon the materials in the sealer and their reaction with the materials in the concrete. Sealers consisting mainly of the methyl methacrylate form of acrylic resins having a high viscosity and high solids content produce the most durable finishes. Surface coatings should never be applied until all cleaning has been completed. In cases where the panels have been coated at the manufacturing plant and additional cleaning is required, it may be necessary to recoat those particular panels. Sealers should be applied in accordance with each manufacturer's written
recommendations. Generally, good airless spray equipment should be used
to apply the sealer uniformly on the surface and prevent surface rundown.
Two coats are usually required to provide a uniform coating, because the
first coating is absorbed into the concrete. The second coat does not penetrate
as much and provides a more uniform surface color. Care should be taken
to keep the sealer off glass surfaces.
SummaryThe effects of weathering on precast concrete walls are similar to those of brick or stone buildings. The small scale pattern of brick and stone, however, may not always show the normal accumulation of dirt to the same extent as large or plain surfaces of concrete.With precast concrete, the designer can choose shapes, textures and details to counteract the negative effects of weatering. The water flow over the wall should be traced, particularly, wherever discontinuities exist. Proper design will also reduce the possibility of window glass etching. Proper design and detailing for weathering of precast concrete units increase in importance whenever the atmosphere, is dirty or polluted, and where one or more of the following characteristics is present:
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