COATINGS MANUAL HRSD APPENDIX E AVOIDING COATING FAILURES DUE TO CRACKING OF CONCRETE

Transcription

1 COATINGS MANUAL APPENDIX E AVOIDING COATING FAILURES DUE TO CRACKING OF CONCRETE The purposes of this section of the manual are as follows: To acquaint the reader with the major causes of cracking of concrete substrates, and To explain when cracking in concrete to be coated can be expected to actively move and propagate through coating systems, and To inform the reader of those situations when Design Resources should be contacted regarding special crack treatment details. Major Causes of Cracking in Concrete Substrates There are a number of common causes of cracking in concrete substrates. Plastic Shrinkage Cracking When fresh concrete is placed and is being finished (such as with floor slabs), much of the water (in it not necessary for the chemical reaction of hydration) comes out of the concrete at the exposed surface. This water is called bleed water. When new concrete is placed under hot weather conditions and/or where there is very low humidity and high wind velocities, rapid evaporation of water from the exposed surface of the concrete can occur. When the surface moisture evaporates faster than it can be replaced with bleed water, the surface of the concrete shrinks. Because this shrinkage is restrained by the wet concrete below the drying surface layer, tensile stresses are created in the weak, hardening concrete. The result is the formation of shallow cracks that vary both in length and width. Often this crack formation develops a random, polygonal pattern or are parallel to each other. These types of cracks are called Plastic Shrinkage Cracks. These cracks are generally quite wide at the surface and narrow towards the bottom. Plastic shrinkage cracking can vary in length from 3 or 4 inches to several feet. Crack frequency will vary widely with the spacing of cracks ranging from a few inches to feet apart. These cracks start out as shallow cracks, but if left unattended can progress to become full depth cracks. Generally, plastic shrinkage cracking is observed during placement and finishing operations and is corrected through finishing and curing. If not corrected, the cracking often remains shallow in depth as wet curing or other curing practices are implemented. This type of cracking typically does not continue to move with thermal changes in the concrete structure. This type of cracking generally requires surface repair, but does not typically require special crack detail treatment because it does not continue to move or propagate. Appendix E -1-

2 COATINGS MANUAL Watchout! Plastic shrinkage cracking may appear similar to cracking associated with small scale restrained drying shrinkage which can be problematic for coatings. The best way to distinguish between the two types of cracking is as follows: Chip into the cracks and look at them under a 10x lighted microscope to see if they continue below the upper inch or two. Detecting this is easier if you spray or pour a little isopropyl alcohol on the area of the crack. As it evaporates, it will take longer to dissipate from a crack making it appear darker. If the crack is shallow, it will likely have no detrimental effect on a subsequently applied coating. See Figure 1 which shows the typical pattern of plastic shrinkage cracks. Cracking Caused by Settlement Once concrete has been placed, vibrated, finished, and is curing, it continues to consolidate. As the plastic concrete hardens, it is often restrained in local zones by rebars, other concrete structures like footings, or by the formwork itself. This restraint often causes voids and/or cracks to occur adjacent to the element restraining the concrete. See Figure 2 which shows cracking caused by obstructed settlement. Settlement of hardened concrete can over time cause cracking also. In this case, the subbase soils may not have been sufficiently compacted or the foundation design did not allow for adequate subsidence. Settlement cracking in existing concrete structures that are several years old is generally nonactive in terms of future movement. Therefore, it does not propagate such movement through a coating system. Look to see if the crack is clean and open. If it is, settlement may still be continuing. If the crack is filled with dirt, dust, etc. and does not appear clean, the settlement has likely stopped and the crack will most likely not move in the future. Figure 3 shows structural settlement cracking. Settlement cracks that develop in new concrete structures during plastic consolidation generally are not thermally active and therefore do not pose a recracking problem for coatings. These types of cracks are, again, generally shallow and can be examined by chipping and magnified visual inspection. Restrained Drying Shrinkage Cracking Cracking of concrete is often the result of what is called restrained drying shrinkage. This occurs over a longer period of time than plastic drying shrinkage and can take up to a month or much longer to manifest itself as visible cracks. Drying shrinkage is the result of excessive and rapid moisture loss from the cement paste in the concrete and the resulting volume change. This volume change at the exposed surface causes shrinkage. The shrinkage is restrained by the coarse aggregate in the concrete and by rebars, subbase friction and by other surfaces on to which the concrete is placed. This restraint reduces the percentage of the shrinkage, but it also contributes to the formation of tensile stresses in the concrete as the shrinkage forces are restrained. Once the tensile stresses established exceed the tensile strength of the concrete at a given age, the concrete cracks. Remember that concrete is typically very strong in compression, but weak in tension. Its tensile strength is approximately 10% of its strength in compression. Appendix E -2-

3 COATINGS MANUAL Without the restraint imposed by rebars, subbase (soil) friction, coarse aggregates, and other factors, the concrete would still shrink, but not crack. For example, if fresh cement paste was placed on a highly polished glass surface, it would shrink, but with little or no restraint would not crack. Restrained drying shrinkage forms two different types of cracking. The first is surface crazing. Surface crazing has an alligator like pattern consisting of numerous, interconnected, shallow closely spaced, fine cracks. Figure 4 shows craze cracking caused by small scale drying shrinkage. Crazing is caused when the upper surface layer of concrete had a much higher water content in it than the lower interior concrete. A common example of crazing occurs when bleed water is overworked into the upper surface layer of concrete during finishing. When rapid moisture loss occurs in this scenario, the high percentage of shrinkage (due to higher relative water content) is restrained by the concrete below the surface layer. The result is the formation of craze cracking. This process is restrained drying shrinkage on a small scale. Craze cracking will become visibly apparent within a few days to a week of the concrete placement. Craze cracks do not typically move actively unless they occur in walls or slabs exposed to large temperature swings over short periods of time on a frequent basis. However in the case of thick elevated slabs or members or where concrete will be heavily loaded or thermally cycled, craze cracking can propagate to form full depth cracks. Again, the depth of these cracks should be examined as noted above. Restrained drying shrinkage also forms full depth cracks in concrete structures. These cracks are generally quite long and will be perpendicular to the longest span of concrete placed. Examples of full depth drying shrinkage cracks include cracks that run between building columns or walls where control joints were improperly cut (too late in concrete s cure time) or where control joints were not installed at all. Many other factors contribute to full depth drying shrinkage cracks. These include improper detailing of reinforcing steel, high water to cement ratios in the concrete, inadequate curing of the concrete, and many other factors. Full depth drying shrinkage cracks can take as long as one to three months or longer to develop after the concrete is placed. Once formed, these cracks almost always move slightly with thermal changes, on going settlement, etc. The risks associated with applying coatings over concrete that has excessive restrained drying shrinkage are two fold. First, there is a high likelihood that cracks will form after 28 days and after a coating has been applied as in the case in many new construction projects. Secondly, these cracks generally continue to move to some extent over time due to thermal changes, etc. In both cases, it is quite likely that the crack in the substrate will telescope or be reflected through the coating. Figure 5 shows typical full depth restrained drying shrinkage cracks. Appendix E -3-

4 Other Causes of Cracking in Concrete COATINGS MANUAL There are several other causes of cracking in concrete structures that can affect coating system performance. Again, the cracking that is typically of most concern is through depth cracking. This is because through-depth cracks are likely to move to some extent due to thermal or load related changes in the concrete structure over time. Cracks that are only at the surface or are shallow (approximately 1" or less in depth) generally do not relieve structural movement whatever the cause of the movement. Hence, shallow cracks in concrete rarely pose a reflective cracking problems for coating systems. Another cause of concrete cracking is related to thermal stresses. This is mostly associated with temperature differences within concrete during curing. When mass concrete structures are constructed, the heat associated with curing (called heat of hydration) often dissipates at different rates from one part of the concrete to another. This is especially true in large dams, columns, piers, beams, etc. The loss of heat of hydration at different rates causes volume changes. These volume changes cause the formation of tensile stresses in the concrete. Once these tensile stresses exceed the tensile strength of the concrete, the concrete will crack. Typically, the interior concrete in a large structure will have a greater temperature due to heat of hydration than the exterior concrete. The cracking associated with such differential temperature induced stresses will occur on the exterior of the concrete. Such cracking can be quite deep and will likely continue to show movement through seasonal thermal changes or from loading in or on the structure. Coating over such cracks without special treatment of the cracks is generally not recommended. See Figure 6 which shows thermal stress related cracks. Chemical reactions also promote cracking in concrete structures. These reactions include sulfate attack, alkali-silica reactions alkali-carbonate reactions, and carbonation reactions. Chemical reactions that cause concrete cracking can be the result of the materials used to make the concrete or may be promoted by substances with which the concrete comes in contact. Many chemicals are aggressive to concrete and can cause cracking along with the familiar dissolution of the concrete s hardened cement paste. Acids and hot alkaline (caustic) liquids are the most common corrodents. This appendix does not deal with these chemical reactions in detail, but you should inquire as to the source of chemicals promoting attack and cracking. In the case of materials used in concrete, cracking can develop as the result of gradual, slow expansive reactions that occur between the alkalies in the cement paste and active silica found in aggregates or between alkalies and certain carbonate aggregates. It is unusual for these types of reactions to occur today in new concrete structures because required testing typically prevents the use of such reactive materials as concrete aggregates. However, it is not unusual to find ongoing alkali-silica reactions or alkali-carbonate reactions in concrete structures that are 20 years old or older. Appendix E -4-

5 COATINGS MANUAL Alkali-Silica Reactions (or ASR) are characterized by the formation of a swelling silica gel which attracts water from other portions of the concrete matrix. This mechanism causes localized expansion and associated tensile stresses that cause cracking and disintegration of the concrete. ASR is not readily identifiable with the human eye, but the cracking it promotes is typically micro-cracking with accompanying disintegration. Hence, if you encounter a disintegrated older concrete surface, inquire if the concrete has been tested for ASR before recommending a coating system. See Figure 7 which shows ASR related concrete cracking. Alkali-Carbonate reactions occur when specific types of dolomitic limestone aggregates are used as aggregates for concrete. These argillaceous dolomites have a very fine grain structure. Alkali-carbonate reactions are characterized by a network type pattern of cracking. These reactions do not produce silica gel, but do promote gradual expansive degradation of concrete. Here again, if you are asked to coat a disintegrating structure with a network pattern of cracking present, ask to see if testing has been performed for alkali-carbonate reactions. If the testing shows this to be the case, any coating system applied over the concrete will ultimately fail due to reflective cracking or substrate deterioration. Both ASR and Alkali-Carbonate reactions are irreversible in concrete. Therefore, any coating system applied over concrete subjected to these reaction mechanisms will fail sooner or later. No guarantees should ever be contemplated in the case of coating systems for concrete afflicted with these mechanisms. See Figure 8 which shows an example of Alkali-Carbonate related cracking. Sulfate Attack. When concrete is routinely exposed to various substances, cracking can result also. For instance, sulfate bearing waters often promote expansive deterioration of concrete that is manifested by cracking. This is called sulfate attack. When sulfates react with the calcium hydroxide that makes up approximately 25% of cement paste, calcium sulfate or gypsum is formed. Gypsum then comes in contact with tricalcium aluminate in the hydrated cement paste. These two compounds react to form calcium sulfoaluminate with a subsequently large volume increase. This volume increase results in the creation of localized tensile stresses that are significantly higher than the tensile strength of the cement paste. The end product is closely spaced cracking and surface degradation. Sulfate attack can be stopped if the contaminated concrete is removed and the concrete is protected from sulfate exposure in the future. The best way to accomplish this protection is with the appropriate coating system. The lesson with sulfate attack is do not coat over concrete contaminated with sulfates. If you encounter concrete with frequent, closely spaced cracks, ask if the concrete is exposed to sulfate rich chemical solutions or to sulfate rich ground waters. If you re not sure, ask if the concrete can be tested for sulfate reactions. If it is determined to be sulfate attack, do not coat over the cracked concrete. The contaminated concrete must first be removed. See Figure 9 which shows typical sulfate attack related cracking of concrete. Carbonation Concrete will also crack due to reaction with liquid phase or gas phase (air) carbon dioxide. When the calcium hydroxide in concrete s cement paste reacts with CO 2, calcium carbonate is formed. Calcium carbonate has a smaller volume than calcium hydroxide. The resulting shrinkage often promotes surface crazing that is extremely fine and frequent. Carbonation related surface craze cracking is most common in new concrete that is not properly cured or when improperly vented combustion type heaters are used during cold weather concrete placements. Inadequate ventilation for these heaters causes excessive CO 2 gas exposure for the Appendix E -5-

6 COATINGS MANUAL new concrete. Carbonation of concrete can also occur in immersion service if the water has a high dissolved CO 2 concentration. When this occurs, the CO 2 is hydrated to form carbonic acid and other carbonates. The carbonic acid causes surface etching and microcracking of the concrete. Coating over carbonated concrete is not problematic provided the shallow surface crazing layer of concrete is removed during surface preparation. It must be removed because the concrete s ph may be too low and because the crazed surface layer is typically weak and will result in coating delamination. The best way to recognize carbonation is to rule out other chemical exposures, know the concrete is relatively new, and measure the surface ph using litmus paper. If the only exposure is ambient air or natural waters, and the ph is 7.0 to 10.0, it is most probable that carbonation is the mechanism that caused the surface crazing. See Figure 10 which shows carbonation related surface craze cracking of concrete. Weathering also can cause the cracking of concrete. The most common weathering mechanism that promotes crack formation is freezing and thawing. Generally speaking, freeze thaw related cracking is the result of the concrete having had a higher than appropriate water content and/or inadequate air entrainment. Freeze-thaw promoted cracking is typically a surface phenomenon and can be coated over with little threat of reflective cracking provided the coating system used is not too porous. See Figure 11 which shows typical freeze-thaw related cracking. Other weathering mechanisms can also cause concrete to crack. These include frequent wetting and drying and cyclic heating and cooling. Both mechanisms cause volume changes that can promote crack formation. The extent of the volume changes and integral exposure severities and frequencies determine the extent of cracking that occurs. Cracking from such causes can be very problematic for coating system performance especially in the case of heating and cooling. The reason is that severe heating/cooling exposures often promote through depth cracking of concrete. Wetting/drying exposures typically only promote shallow surface cracking and, as such, can generally be coated over successfully. This is especially effective in most cases because the coating system isolates the substrate from the crack causing mechanism, i.e. wetting. Rebar Corrosion Related Cracking Cracking is also caused when reinforcing steel in the concrete corrodes. Under the right conditions, rebar does not corrode in concrete because of the tightly adhering protective oxide coating that forms on the steel in the highly alkaline environment created by the hydrated cement paste. This resistance to corrosion is known as passive protection. Under many different chemical exposure conditions including carbonation, the alkaline environment of the cement paste is broken down and the rebar subsequently is exposed to moisture (electrolyte) and oxygen causing corrosion. This corrosion produces ferrous oxides and hydroxides which are larger in volume than the metallic iron. This growth in volume creates bursting stresses around the rebars creating radial cracking. These radial cracks produce splitting actions that propagate along the bars longitudinally once the concrete s tensile strength has been exceeded. This results in the common cracking you see in concrete structures exposed to chlorides from de-icing salts. The resulting cracks formed are parallel to the rebars. These types of cracks are characterized by the Appendix E -6-

7 COATINGS MANUAL presence of rust bleeding and by the hollow sound of the delaminated concrete over the rusting bars. Coating over such cracks is never recommended. The delaminated concrete and the corroding rebars should be repaired prior to coating the subject structure. Otherwise, the cracks will continue to lengthen and widen and be reflected through the coating system. See Figure 12 which shows rebar corrosion related cracking. Miscellaneous Causes of Cracking There are several other causes of cracking in concrete including overloading structures during construction, thermal shock of precast concrete members when embedded metal parts are welded, errors in design of reinforcing steel details, and externally overloading structural concrete elements in service, etc. In most cases, these types of cracks need to be repaired before the subject concrete can be coated. Otherwise, reflective cracking will occur through the coating system in most of these cases. One example of improper design caused cracking is worthy of mention herein. When concrete is placed as a floor slab (on grade or elevated) or a wall, inside and outside corners where the concrete terminates or is interrupted occur. At such locations, the concrete will shrink in two directions. Therefore, careful design attention must be paid to reinforcing steel detailing to resist shrinkage related cracking. The mechanism of the cracking is restrained drying shrinkage. If this attention to detail is not implemented, cracking typically occurs. These types of corners are called Reentrant corners. They provide a place where the concentration of shrinkage induced stresses occurs and initiates crack formation. There are many related factors to reentrant corner cracking including bending, loading (in plane), and volume changes. Coating over untreated reentrant corners is very risky because once these cracks have formed, they tend to continue to be thermally active and/or propagate in length in the future. Therefore, it is always recommended to treat these cracks especially prior to coating over them and/or after coating up to them. See Figure 13 through 16 which show typical examples of the other types of cracking of concrete discussed herein. Now that we have discussed the major causes of crack formation in concrete and examined when coating over various types of cracks is prudent or not, the next logical step is to capture what methods should be used to identify the cause of cracking in concrete structures to be coated. For this information, please refer to Spreadsheet A that follows this section of this Appendix. This spreadsheet also highlights those cases where you (the specifier) should contact the Technical Services Department of the Coating s Manufacturer or the resources listed for this Coatings Manual for assistance regarding special crack treatment details for coating systems. Appendix E -7-

8 Appendix E -8- COATINGS MANUAL

9 Appendix E -9- COATINGS MANUAL

10 Appendix E -10- COATINGS MANUAL

11 Appendix E -11- COATINGS MANUAL

12 Appendix E -12- COATINGS MANUAL

13 Appendix E -13- COATINGS MANUAL

14 Appendix E -14- COATINGS MANUAL

15 Appendix E -15- COATINGS MANUAL

16 Appendix E -16- COATINGS MANUAL

17 Appendix E -17- COATINGS MANUAL

18 Appendix E -18- COATINGS MANUAL

19 Appendix E -19- COATINGS MANUAL

20 Appendix E -20- COATINGS MANUAL

21 Appendix E -21- COATINGS MANUAL

22 Appendix E -22- COATINGS MANUAL

23 Appendix E -23- COATINGS MANUAL

24 COATINGS MANUAL SPREADSHEET A Evaluation Methods to Identify Causes of Concrete Cracking Cause of Cracking Evaluation Methods and Watchouts Coat or Do Not Coat Plastic Shrinkage Cracking Happens when concrete is wet. Coat over after normal resurfacing. Cracks are shallow - 1" or less in depth typically. Cracks are generally wide at upper surface and narrow towards bottom. Cracks vary in length and frequency. Does not generally progress into full depth cracks unless aggressive heating/cooling occurs in structure. Requires surface repair prior to coating, but does not need special treatment. Can appear as surface crazing like small scale drying shrinkage cracking, which happens later. Be careful to check depth of cracks and when cracking developed. Contact CSM Tech. Services or Resource Names for Special Crack Treatment Details Not necessary CSM Coating System Manufacturer Appendix E -24-

25 COATINGS MANUAL Settlement Cracking (Curing Phase) Look for cracking in newer concrete adjacent to an existing concrete element such as a footing that provided restraint to the cracked concrete during placement/curing. Usually diagonal cracking. Do Not Consult with CSM or Resources listed. If structure is 1 to 5 years old and crack is open and clean, it is likely that cracking is caused by active settlement. Settlement cracks that are serious are usually full depth cracks or very deep cracks. Check crack depth visually or with lighted field microscope. Appendix E -25-

26 COATINGS MANUAL Settlement Cracking (In Service) Usually related to inadequate or inconsistence subbase soil compaction, piling settlement, or differential footing settlement, etc. If crack is open and clean and structure is older than 5 years, inquire if any blasting, earthquakes, or major excavations have occurred near the structure in recent months or years. If the answer is yes, new settlement is occurring. If structure is older than 1 to 5 years and crack is not clean and open, (is filled with dirt, dust, etc.), settlement has stopped. If concrete structure is new, (few months old) and a diagonal crack is present, but is shallow, the crack was most likely caused during consolidation of the new concrete, in this case, 90% of the time, the crack will not continue to move or propagate; so coating over it once filled is ok. Chip crack to check depth with microscope or magnifier. Do Not Can coat over after surface cracks filled. Contact CSM or Resources listed. Appendix E -26-

27 COATINGS MANUAL Craze Cracking: From Restrained Drying Shrinkage. Surface of concrete will have an alligator type pattern consisting of numerous, interconnected, shallow closely spaced, fine (hair like) cracks. Will occur in concrete within a few days to a week after concrete placement. If a similar pattern is observed the very first day, it is plastic drying shrinkage related. (See above.) Chip the concrete to 1/2" to 1" depth and examine with a lighted microscope to determine the depth. If cracks are shallow (<1"), coating over the craze cracks will not be problematic. Spray apply isopropyl alcohol over area and let evaporate for a few minutes. Will take longer to dissipate from cracks which will aid you in determining depth. Coat over. Coat over. Not Necessary Not necessary if cracks are shallow. Appendix E -27-

28 COATINGS MANUAL Craze Cracking: From Restrained Drying Shrinkage. Craze cracking is often caused by Restrained Drying Shrinkage on a small scale. Typically, these cracks are very shallow. However, in the case of thick elevated slabs or members or thick ongrade slabs that will be heavily loaded or thermally cycled, craze cracking can propagate to form full depth cracks that can be thermally active, (will move in future). When craze cracking is found in such thick members or structures, use dye and water to check for full depth by ponding or using a full cup of dyed water to drain through the cracks. Do not coat over if cracks are shown to be full depth. If cracks are shallow, coat over with specified system. Discuss with CSM or Resources listed. Appendix E -28-

29 COATINGS MANUAL Full Scale Restrained Drying Shrinkage These cracks will be quite long and will be perpendicular to the longest unbroken (by joints) spans of concrete placed. Typically, these cracks will develop over several weeks to three months after concrete placement. Examples of this cracking include cracks that run between building columns or walls or where control joints were improperly cut (too late) or were not installed at all. These cracks are almost always full depth especially in thicker members. These cracks often show up after 28 days: Watchout: If coating over new concrete and these cracks develop, cracking will propagate through the coating. Do not coat over directly if not using either EC-1 or EC-2. Do not coat over directly if not using either EC-1 or EC-2. Consult with CSM or resources listed. Consult with CSM or resources listed. Appendix E -29-

30 COATINGS MANUAL Full Scale Restrained Drying Shrinkage Thermal Stresses During Concrete Curing in Massive Concrete Structures. Alkali-Silica Reactions If it is a cast in place wall application, check for crack location on both sides of the wall. Such cracks are likely to move in future. If there are no control or expansion joints in a slab or wall in spans longer than 40'- 0" or so, chip concrete to examine crack depth or look on both sides of walls to determine depth. If through depth or close to it, do not coat directly over such cracks. These cracks will generally be hairline to 1/2" in width and are found in large concrete structures such as foundations, large beams, and other massive members. These cracks can develop between a few days and 28 days after placement. Has been problem in NE, NW, & SW in U.S. Characterized by surface disintegration and numerous microcracks. Typically occurs in older (20 years or more) concrete structures. If suspected, testing must be performed by others to affirm or reject the mechanism. Mechanism is irreversible in concrete. Alkali-Carbonate Reactions Look for concrete disintegration accompanied by a network pattern of cracking on micro level. If suspected, testing must be performed by others to affirm or reject the mechanism. Do not coat over directly unless special crack treatment is performed and EC-1 or EC-2 are used. Do not coat over directly unless special crack treatment is performed and EC-1 or EC-2 are used. Do not coat over directly unless special crack treatment is performed and EC-1 or EC-2 are used. Do not coat over. Do no coat over. Consult with CSM or Resources listed. Consult with CSM or Resources listed. Consult with CSM or Resources listed. Consult with CSM or Resources listed. Consult with Resources listed for this document. Appendix E -30-

31 COATINGS MANUAL Will find in older structures where limestone is common locally in soils. Alkali-Carbonate Reactions Mechanism is irreversible in concrete. Sulfate Attack Look for expansive deterioration of concrete accompanied by cracking. Identify chemicals to which concrete is exposed by asking questions. Could be sulfuric acid (also causes sulfate attack) sodium sulfate, potassium sulfate, calcium sulfate or sulfate rich natural waters. If likely from exposure chemistry, ask that testing be performed to identify the sulfate reaction compounds and depth of contamination. Contaminated concrete must be removed if concrete is to effectively be resurfaced/ repaired and coated. Main reaction products are gypsum and ettringite. Do not coat over. Do not coat over unless testing/removal of contaminated concrete/concrete repairs are properly implemented. Consult with Resources listed. Appendix E -31-

32 COATINGS MANUAL Carbonation Weathering Freeze Thaw Starts out as surface craze cracking and formation of a soft, weak outer layer (powdery) of concrete. Ask about exposure conditions and check for other chemicals like sulfates. Occurs on new concrete open to air for long time or in new concrete exposed to poorly ventilated heaters when placed in winter. Scrape surface with knife, will remove powdery cement paste easily. Use litmus paper to measure ph of surface before and after scraping. Use distilled water with litmus paper. If ph is 7.0 to 10.0, it is probable on new concrete (never immersed) that carbonation is the cause. Cracking will be surface crazing and cracking that is shallow. Occurs in cold weather climates or where freeze-thaw cycles are combined with wet/dry weather cycles. Cracks will be shallow in depth. Coat over after surface prep. removes weak carbonated layer. Coat over once surface repaired or resurfaced. If uncertain, consult with CSM or Resources listed. Rule out all other chemical exposures. Weathering Heating/Cooling Look for shallow surface cracks unless heating/cooling is enhanced by exposure effects i.e. boilers, etc. Check depth of cracking as described elsewhere herein and define the heating/cooling temperatures and cycle frequencies. If deep cracking, do not coat over If shallow cracking, coat over after resurfacing and prep. If uncertain, consult with CSM or Resources listed. If uncertain, consult with CSM or Resources listed. Appendix E -32-

33 COATINGS MANUAL Reinforcing Steel Corrosion Construction Overloading Thermal shock in precast concrete from welding embedded clips or plates for connections. Errors in design of reinforcing steel details: Reentrant cracking at corners or at concrete thickness changes. Errors in design of reinforcing steel details: Reentrant cracking at corners or at concrete thickness changes. Look for cracking parallel to rebars with rust bleeding/ staining on concrete. Sound the concrete with a hammer to determine if concrete cover is hollow. It will likely be hollow. Cracking is jagged typically and occurred during initial construction. Crack proximity to metal embedments will be close and obvious. In floor slabs, reentrant cracking is typical at inside corners. In walls, it occurs at outside corners more often than not. In floors, beams, and footings, crackings at improperly reinforced concrete thickness changes is not uncommon. Look at structure and at drawings. Reentrant cracking is almost always diagonal to window or door openings or to outside corners in walls (between the horizontal and vertical perimeter of the corner). In floor slabs, it is diagonal between the two walls that are 90 o to one another. Never coat over. Do not coat over. Do no coat over. Do not coat over reentrant cracks or cracks at concrete thickness changes directly Do not coat over reentrant cracks or cracks at concrete thickness changes directly If uncertain, consult with CSM or Resources listed. Structural repairs will be req d. first. If uncertain, consult with CSM or Resources listed. Consult CSM or Resources listed. Consult CSM or Resources listed. Consult CSM or Resources listed. Appendix E -33-

34 COATINGS MANUAL External Overloading Structural cracks caused by overloading a structure are generally obvious as diagonal cracks at column-beam connections, punching through failures of floor slabs, or cracking is open and jagged. If you suspect external overloading at all, do not coat over. A Structural Engineer should evaluate the structure. Consult CSM or Resources listed. Appendix E -34-