Q: What can I do about moisture problems in a new concrete building?
A: Moisture problems can occur in tight concrete buildings without adequate ventilation that haven’t gone through one or two winter heating seasons to allow water to evaporate from the concrete. Unfortunately, air conditioning isn’t a very efficient way to remove the moisture because cool air won’t hold as much moisture as warmer air. Thus, turning down the thermostat to cool the building even more isn’t likely to help. In fact, it can even make the problem worse by increasing condensation on the cooler surfaces. The air conditioner may also be oversized which prevents it from cycling on frequently enough to remove the moisture.
Two steps will help to solve the problem. Use dehumidifiers with air conditioning set at moderate levels (75°F) during the first two summers of operation to drop the indoor relative humidity below 80%. Dehumidifiers are a more efficient way of removing moisture than reducing air temperature via air conditioning.
Second, ventilate the building as much as possible when heating and air conditioning aren’t being used. The tighter the building, the harder it is to get rid of excess moisture. Install an air exchanger to provide fresh air when the heating and cooling systems are not operating. Also, use bath and kitchen exhaust fans to remove moisture produced by occupants. Many builders now install ventilation ducts in walk in closets to increase ventilation and decrease moisture problems. If not, keep the closets open while you are experiencing problems.
Another solution is to turn on the heating system after the building has been completed for two weeks or so, regardless of the season. During this period ventilation should also be kept high. In a house with forced air, the fan should be on continuously. While it’s true that external heat drives some moisture further into the concrete (toward the cold side), the heat also reduces relative humidity in the building and allows water near the interior concrete surfaces to evaporate. The obvious downside to this approach is an uncomfortably high temperature in the building while finish work is being done, or delayed use of the building if heating is turned on after completion but before occupancy.
Condensation problems are often made worse when vinyl or other impermeable wall coverings are placed over the concrete at an early age. Evidence of moisture problems will be visible under the wall covering. Always discourage the use of impermeable wall coverings in hot and humid climates (such as on the Gulf coast). To help prevent this problem in moderate climates, delay applying the wall covering until the building has gone through two winters with the heating on and two summers with indoor relative humidity less than 80%.
Q: What causes concrete to crack?
A: Unexpected cracking of concrete is a frequent cause of complaints. Cracking can be the result of one or a combination of factors, such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. Cracking can be significantly reduced when the causes are taken into account and preventative steps are utilised.
Crazing is a pattern of fine cracks that do not penetrate much below the surface and are usually a cosmetic problem only. They are barely visible, except when the concrete is drying after the surface has been wet.
Plastic Shrinkage Cracking: When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth. These cracks are often fairly wide at the surface.
Drying Shrinkage: Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. Restraint to shrinkage, provided by the subgrade, reinforcement or another part of the structure, causes tensile stresses to develop in the hardened concrete. Restraint to drying shrinkage is the most common cause of concrete cracking. In many applications, drying shrinkage cracking is inevitable. Therefore, contraction (control) joints are placed in concrete to predetermine the location of drying shrinkage cracks.
D-cracking is a form of freeze-thaw deterioration that has been observed in some pavements after three or more years of service. Due to the natural accumulation of water in the base and subbase of pavements, the aggregate may eventually become saturated. Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate at the bottom of the slab and progresses upward until it reaches the wearing surface. D-cracking usually starts near pavement joints.
Alkali-aggregate reaction: Alkali-aggregate reactivity is a type of concrete deterioration that occurs when the active mineral constituents of some aggregates react with the alkali hydroxides in the concrete. Alkali-aggregate reactivity occurs in two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).
Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints or displacement of different portions of a structure.
Thermal cracks: Temperature rise (especially significant in mass concrete) results from the heat of hydration of cementitious materials. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and centre is too great. The width and depth of cracks depends upon the temperature differential, physical properties of the concrete and the reinforcing steel.
Loss of support beneath concrete structures, usually caused by settling or washout of soils and subbase materials, can cause a variety of problems in concrete structures, from cracking and performance problems to structural failure. Loss of support can also occur during construction due to inadequate formwork support or premature removal of forms.
Corrosion: Corrosion of reinforcing steel and other embedded metals is one of the leading causes of deterioration of concrete. When steel corrodes, the resulting rust occupies a greater volume than steel. The expansion creates tensile stresses in the concrete, which can eventually cause cracking and spalling.
Cracking in concrete can be reduced significantly or eliminated by observing the following practices:
1. Use proper subgrade preparation, including uniform support and proper subbase material at adequate moisture content.
2. Minimise the mix water content by maximising the size and amount of coarse aggregate and use low-shrinkage aggregate.
3. Use the lowest amount of mix water required for workability; do not permit overly wet consistencies.
4. Avoid calcium chloride admixtures.
5. Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks.
6. Provide contraction joints at reasonable intervals, 30 times the slab thickness.
7. Provide isolation joints to prevent restraint from adjoining elements of a structure.
8. Prevent extreme changes in temperature.
9. To minimise cracking on top of vapour barriers, use a 100mm (4in) thick layer of slightly damp, compactable, drainable fill, choked off with fine-grade material. If concrete must be placed directly on polyethylene sheet or other vapour barriers, use a mix with low water content.
10. Properly place, consolidate, finish and cure the concrete.
11. Avoid using excessive amounts of cementitious materials.
12. Consider using a shrinkage-reducing admixture to reduce drying shrinkage, which may reduce shrinkage cracking.
13. Consider using synthetic fibres to help control plastic shrinkage cracks.
Q: Does muriatic acid darken concrete?
A: Calcium hydroxide deposits may lighten the colour of concrete that is cured with water present on the surface. The calcium hydroxide is water soluble, but when exposed to air it’s converted to insoluble calcium carbonate. If an acid wash removes the calcium carbonate deposits, the darker underlying concrete will be exposed. The colour difference will be especially noticeable if hard troweling darkened the concrete surface or if a calcium chloride admixture was used in the concrete.
This kind of surface discoloration is discussed in PCA Research Department Bulletin RX203, Surface Discoloration of Concrete Flatwork. Treating the entire sidewalk with muriatic acid might enable you to match the darker coloured area where the stain was removed. However, some experimentation would be needed to develop a procedure that minimises colour variations. More information can be found in the Research Department Bulletin RX203.
Q: How can I remove graffiti from concrete?
A: Many products are suitable for removing spray-paints and felt-tip markings from concrete surfaces. These products are generally effective for removing crayon, chalk, and lipstick also. The manufacturer’s directions should always be followed. If satisfactory results are not obtained with the first remover tested, a second or third attempt with other products should be made. A single product may not remove multiple types of stains.
Several proprietary chemical strippers are available, many of which contain a citrus-based solvent, methylene chloride or potassium hydroxide. Citrus-based solvents are the least aggressive and may not work on certain paints, but they are the safest to use and often have less stringent disposal requirements. For best results, allow products based on potassium hydroxide to soak into the concrete surface for several hours before rinsing. These products also require a subsequent application of an acid neutraliser.
Abrasive cleaning can remove graffiti, but it will also remove the outer layer of concrete, making it more vulnerable to weathering. The cleaned area may also look different from the rest of the surface. After removal of the graffiti or before a structure is used, a graffiti barrier coating or sealer should be applied. The surface treatment should keep graffiti out of the concrete pores and on the surface for easy removal.
Q: How can I repair a pitted concrete platform surface?
A: The two most important issues are (1) the preparation of the slab to achieve an effective bond and (2) choosing a repair material that is compatible with the service condition or climate. Resurfacing Concrete Floors, IS144 is a good resource for the preparation of the slab surface.
In general, the slab surface must be clean, hard and all unsound material must be removed. This will typically require high pressure water blasting (3000psi or more), sandblasting or shot blasting. The slab surface may be dry or damp but not wet prior to placing the repair material. For a thin application, curing is most critical to produce a durable surface.
You can use traditional concrete or mortar for the repair down to a thickness of about 1in or a thin specialty topping for thinner sections.
Q: What causes random concrete cracks and can they be avoided?
A: Random cracks in concrete slabs are most frequently caused by one of the following mechanisms:
Settlement of the soils supporting the concrete slab
Restraint of horizontal movement due to fixed foundation elements
Overloading, applying a load larger than what the slab was designed to support
Restrained drying shrinkage of the slab
Settlement cracking takes place when the soils or fill beneath the slab have not been adequately compacted to provide a consistent level of support for the slab to limit the bending stresses, which crack the concrete. Settlement can be controlled with consistent preparation (compaction) of the base supporting the slab.
Slabs placed against fixed foundation elements (frost foundations, light standards etc.) produce cracks caused by bending forces, as the slab moves on the surface while the fixed foundation does not. Placing isolation joint material between the slab and the fixed foundation to allow the elements to move independently controls this mechanism, thus limiting the bending stresses and subsequent cracking.
Overload cracking is easily controlled with proper thickness design of the slab considering the largest load that may be applied to its surface.
Cracks due to restrained drying shrinkage are caused by the tensile stresses that build in the matrix of the slab as the concrete gives up moisture over time and is prevented from shrinking by the soils beneath it. The most common strategy for dealing with this type of random cracking is to provide closely spaced contraction joints (weakened planes) to predetermine where the concrete should crack. Smooth dowels and dowel plates are another common material used to provide good structural performance at working contraction joints; used more often with slabs greater than 150mm (6in) in thickness that will receive substantial loads the dowels provide load transfer across working contraction joints. Slabs with properly spaced contraction joints should typically limit the occurrence of random cracks to no more than 3% of the panels in the slab.
Keep in mind that a crack is not a failure of the concrete, but rather the normal behaviour of the material. It is also common to use steel reinforcement in a slab to hold cracks tightly to assure good structural performance. Tight joints provide good load transfer and maintain equal elevation across cracks, which is the measure of the structural performance of any slab. It should be recognised that the use of steel reinforcement may actually increase the potential for random cracks to occur, because the cracks are held tightly and thus do not allow for the full relaxation of tensile stresses in the slab.
Q: What causes early stiffening and what is the difference between false set and flash set?
A: Early stiffening is a premature loss of workability or plasticity of cement paste, mortar or concrete. This includes both false and flash set.
False set (plaster set) is evidenced by a significant loss of plasticity without the evolution of much heat shortly after mixing. From a placing and handling standpoint, false set tendencies in cement cause no difficulty if the concrete is simply mixed for a longer period of time or remixed without additional water before being transported or placed. False set occurs when too much gypsum dehydrates in the cement mill forming too much plaster (some plaster in the cement is desirable). This leads to stiffening due to the rapid reformation of secondary gypsum with interlocking needle-like crystals. Additional mixing without water breaks up these crystals to restore workability. Ettringite precipitation can also contribute to false set.
Flash set (quick set) is evidenced by a rapid and early loss of workability in paste, mortar or concrete. It is usually accompanied by the evolution of considerable heat, resulting primarily from the rapid reaction of aluminates. If the proper amount or form of calcium sulphate is not available to control the calcium aluminate hydration, rapid stiffening takes place. Flash set cannot be dispelled, nor can the plasticity be regained by further mixing without the addition of water.
Proper stiffening results from the careful balance of the sulphate and aluminate compounds, as well as the temperature and fineness of the materials (which control the chemical reaction rates).
A balance among the ions in plastic concrete is necessary to prevent early stiffening. The tendency for early stiffening may therefore be attributed not only to individual cementitious materials, but also to interactions between the various cementitious materials and chemical admixtures. For example, some Class C fly ashes contain significant amounts of aluminate phases and may disturb the balance because the cement sulphates may not be sufficient to account for them, leading to early stiffening. Some chemical admixtures, particularly Type A water reducers, may also disturb the ion balance with the same result.
Cements are tested for early stiffening using ASTM C451 (paste method) (AASHTO T 186), and ASTM C359 (mortar method) (AASHTO T 185), which use the penetration techniques of the Vicat apparatus. However, these tests do not address all of the mixing, placing, temperature and field conditions that can cause early stiffening. To detect early stiffening caused by interactions between concrete ingredients, the use of conduction calorimetry has been suggested.
Q: When you find lot of cracks on the completed apartment wall, how will you arrest them?
A: Use filling adhesive materials.
Use stitching and grouting method to heal cracks.
Q: How can you tell if you’re getting the amount of concrete you’re paying for?
A: The real indicator is the yield, or the actual volume produced based on the actual batch quantities of cement, water and aggregates. The unit weight test can be used to determine the yield of a sample of the ready mixed concrete as delivered. It’s a simple calculation that requires the unit weight of all materials batched. The total weight information may be shown on the delivery ticket or the producer can provide it. Many concrete producers actually over yield by about 0.5% to make sure they aren’t short-changing their customers. But other producers may not even realise that a mix designed for 1yd3 might only produce 26.5ft3 or 98% of what they designed.