A: Durability is a property of concrete, which determines its life span and how vulnerable the concrete and steel is to atmospheric chemical attacks.
Q: How do I prevent my concrete from losing strength and durability?
A: After the truck mixes the concrete to the designed workability, you should not request any additional water in the concrete. If you think you will need a very soft (flowing) concrete when casting your structure, this should be brought to the attention of the Readymix sales representative when placing the order.
Q: How does rain affect fresh concrete?
A: Rainfall during placement of concrete flatwork can present challenges to achieving a quality concrete. Potential outcomes range from no damage to a weakened non-durable surface. Only time will tell at which end of the range your specific situation will fall. Descriptions of a best-case scenario and a worst-case scenario follow:
Best case: The concrete is protected as much as possible from the falling rain. After the rain has stopped, the water that has fallen on the surface is allowed to evaporate the same way bleed water must be allowed to evaporate from the original concrete mixture prior to proceeding with finishing operations. To substantially change the water-cement ratio (w/c) of the concrete at the surface of the slab, energy must be added to the system, typically, in the form of troweling passes with excess water on the concrete surface. If the water is allowed to evaporate, the w/c remains reasonably low; since the w/c governs the strength of the concrete, there is no substantial damage to the finished surface. In extreme cases, it is not uncommon to physically remove excess water from the slab surface by dragging a garden hose or a broom across the concrete surface to lower the volume of water that must evaporate. With proper timing and process, the durability of the concrete is not affected.
Worst case: The concrete is not protected from the rain; the water is not allowed to evaporate from the slab surface and multiple passes of the floats and trowels used to finish the surface are made with the surface moisture in place. The energy supplied by the finishing operations mixes the excess water into the slab surface creating a high w/c ratio in the near surface of the concrete, reducing its strength and thus its durability. In the worst situation, the damage to the concrete surface is readily apparent since the texture of the surface is easily damaged or removed after the initial curing period. (If the surface is dusty after 14 days of curing, there is likely to be a problem) If the surface strength is only slightly affected, the long term durability of the concrete may be reduced as evidenced by a general loss of the surface mortar (scaling) after the concrete has been through a winter season of freezing and thawing cycles; however, the concrete strength and durability below the surface would not be affected.
In most cases concrete is warranted for one year, which will allow you to assess the potential durability of the concrete surface. In instances similar to this, many contractors are willing to extend the warranty for an additional period of time (an extra year or two) to settle the doubt.
Q: Is there an admixture that prevents scaling?
A: While there are a number of chemical admixtures and supplementary cementitious materials that can improve the quality of concrete, there is no substitute for quality concrete when the issue is spalling or scaling. The most important factors in achieving durable exterior concrete surfaces are appropriate cement content, air-entraining admixture (to produce the proper air content for the aggregate size), proper placement, consolidation, finishing and curing.
Q: What effects do deicers have on concrete?
A: Sodium chloride: Sodium chloride has little or no effect on properly air-entrained concrete but will damage plants and corrode metal.
Calcium chloride: Calcium chloride in weak solutions has little chemical effect on concrete or vegetation but does corrode metal; however, strong calcium chloride solutions can chemically attack concrete. The reaction is accelerated by high temperature
Magnesium chloride: A PCA literature search found three references comparing the effects of magnesium chloride with sodium chloride and other deicers on the scaling resistance of concrete. Unfortunately, the cited studies provide conflicting results.
The abstract from a German field study (Leiser 1967) states, “concrete surfaces were only slightly affected (by magnesium chloride lye) and that the solution is less harmful than granulated salt.” However, two recent studies found magnesium chloride to be more aggressive than sodium chloride.
In the first study (Cody 1996), concrete containing dolomite coarse aggregate was cored from five highway pavements. Small blocks were cut from the cores and subjected to wet-dry and freeze-thaw cycles in 0.75M and 3.0M solutions of NaCl, CaCl2, and MgCl2. Magnesium chloride was the most destructive deicer, producing severe deterioration under almost all of the experimental conditions. Calcium chloride was the next most destructive salt. Sodium chloride was relatively benign. In the second study (Lee 2000), the researchers again found magnesium chloride to be significantly more aggressive than sodium chloride in wet-dry and freeze-thaw conditions.
In both of these studies, the authors concluded that the major cause of deterioration by magnesium-based deicers was the formation of non-cohesive magnesium silicate hydrates (MSH), produced by the reaction of dissolved magnesium with calcium silicate hydrates of the cement. Because MSH does not form strong bonds with aggregate particles, these phases cause loss of cohesion in Portland cement paste and will promote crumbling. A common finding of the above research is that all deicers can aggravate scaling, emphasising the need for placing high-quality, air-entrained concrete in deicer environments.
Urea does not chemically damage concrete, vegetation or metal.
Ammonium: Deicers containing ammonium nitrate and ammonium sulphate should be prohibited because they rapidly attack and disintegrate concrete. Deicers used in low concentrations (2%–4% by weight) can cause more surface scaling than higher concentrations or no deicer at all.
Q: What causes efflorescence and how can it be avoided?
A: Efflorescence is a type of discoloration. It is a deposit, usually white in colour that occasionally develops on the surface of concrete, often just after a structure is completed. Although unattractive, efflorescence is usually harmless. In rare cases excessive efflorescence, within the pores of the material, can cause expansion that may disrupt the surface.
Efflorescence is caused by a combination of circumstances: soluble salts in the material, moisture to dissolve the salts and vapour transmission or hydrostatic pressure that moves the solution toward the surface. Water in moist, hardened concrete dissolves soluble salts. This salt-water solution migrates to the surface by vapour transmission or hydraulic pressure where the water evaporates, leaving the salt deposit at the surface. Particularly, temperature, humidity and wind affect efflorescence. In the summer, even after long periods of rain, moisture evaporates so quickly that comparatively small amounts of salt are brought to the surface.
Moisture testing to determine the vapour pressure at the slab surface will tell you how much moisture is moving through the slab. A common value of vapour pressure acceptable for moisture sensitive floor coverings is 3–5lb/1000sqft/24 hours. The Calcium Chloride Vapour Pressure Test is commonly used. Testing of the soils and concrete would identify the source of the soluble salts. A look at the drainage, irrigation systems, accommodation of the building runoff (downspout drops etc.) and ground waters, may give some valuable clues as to the source of moisture that drives this process
These types of problems can be very complex to resolve. One possible strategy would be to install a French drain system, which over time will lower the moisture content of the soil under the slab. With lower moisture content under the slab, the transmission of water through the slab will slow or nearly cease. Without the moisture, the salts are no longer transported to the slab surface and the process should stop. Avoid adding additional water to the system. In general, any wet process cleanup converts the build-up to a solution, which is re-deposited onto the concrete surface to reappear when the concrete dries. In many cases the use of a dry method cleanup will help to reduce or prevent a re-occurrence of efflorescence.
Q: What is ettringite and does it or the sulphate in cement contribute to expansion and disintegration of Portland cement concrete?
A: Ettringite, calcium sulfoaluminate, is found in all Portland cement concretes and is commonly referenced in petrographic reports. Calcium sulphate sources, such as gypsum are added to Portland cement to prevent rapid setting and improve strength development. Sulphate is also present in supplementary cementitious materials and admixtures. Gypsum and other sulphate compounds react with calcium aluminate in the cement to form ettringite within the first few hours after mixing with water. Essentially, all of the sulphur in the cement is normally consumed to form ettringite within 24 hours.
The formation of ettringite results in a volume increase in the fresh, plastic concrete. Due to the concrete’s plastic condition, this expansion is harmless and unnoticed. If concrete is exposed to water for long periods of time (many years), the ettringite slowly dissolves, reforming in less confined locations. Upon microscopic examination, harmless white needle-like crystals of ettringite can be observed lining air voids.
Any form of attack or disintegration of concrete by freeze-thaw action, alkali-silica reactivity (ASR) or other means, accelerates the rate at which ettringite leaves its original location in the paste, to go into solution and recrystallise in larger spaces. Both, water and space must be present for the crystals to form. The space is often provided by cracks that form due to damage caused by frost action, ASR, drying shrinkage or other mechanisms. Ettringite crystals in air voids and cracks are typically 2–4µm in cross section and 20–30µm long. Under conditions of extreme deterioration, the white ettringite crystals appear to completely fill voids or cracks. However, ettringite, found in its preferred state as large needle-like crystals, should not be interpreted as causing the expansion of deteriorating concrete.
Another term used in petrographic reports is Delayed Ettringite Formation (DEF). This refers to a condition usually associated with heat-treated concrete. Certain concretes of particular chemical makeup, which have been exposed to temperatures over about 70°C (158°F) during curing, can undergo expansion and cracking caused by later ettringite formation. This can occur because the high temperature decomposes any initial ettringite formed and holds the sulphate and alumina tightly in the calcium silicate hydrate (C-S-H) gel of the cement paste. The normal formation of ettringite is thus impeded.
In the presence of moisture, sulphate and alumina desorb from the confines of the C-S-H to form ettringite in cooled and hardened concrete. After months or years of desorption, ettringite forms in confined locations within the paste. Since the concrete is rigid and if there are insufficient voids to accommodate the ettringite volume increase, expansion and cracks can occur. In addition, some of the initial ettringite formed before heating may be converted to monosulfoaluminate at high temperatures and upon cooling, revert back to ettringite. Because ettringite takes up more space than monosulfoaluminate from which it forms, the transformation is an expansive reaction.
Only extreme cases of DEF result in cracking, and often DEF is associated with other deterioration mechanisms. Air voids can help relieve the stress by providing a location for the delayed ettringite to form. Finally, some petrographers or concrete technologists use the term ‘secondary ettringite’ to refer to both DEF and harmless ettringite found lining voids (often listed under secondary deposits in petrographic reports).
Q: Can water cause deterioration of concrete?
A: Generally, the combination of water and favourable temperatures increases the strength of concrete throughout its life cycle. However, water also can act as the transport system for nearly all mechanisms aggressive to concrete. Some examples are:
- Porous, water-saturated concrete that does not have adequate strength and entrained air is prone to scaling, which is a deterioration mechanism caused by freezing of water in concrete
- Water can carry aggressive chemicals into the concrete surface such as acids, sulphates or chlorides
- Concrete that contains alkali-reactive aggregates is subject to deleterious expansion from water
- Water that passes over the surface of concrete with a high velocity can erode the surface over time
Exposure to water is typically beneficial to concrete, but there are circumstances in which it can also contribute to the deterioration of concrete. Service environment conditions are key to determining whether water will have beneficial or deleterious effects.
Q: How do you diagnose damage from alkali-silica reaction (ASR) in a concrete structure?
A: The diagnosis of ASR in a concrete structure requires a combination of recognition of the visual symptoms of ASR, appropriate testing to verify the presence of ASR gel and deterioration of the concrete in the structure. Typical visual symptoms include unusual expansion of the concrete evidenced by longitudinal cracks, map cracking (random cracking pattern), closed joints, spalled surfaces, displacement of adjacent structural components, pop-outs, efflorescence and/or discoloration (darkened or blotchy areas). If site inspections reveal one or more of these visual symptoms, it may be appropriate to sample and test the concrete to verify the presence of ASR gel. In addition, the source of any other deterioration mechanisms should be noted and the structure should be evaluated for soundness. Appropriate testing can be performed using one of the following methods:
- Petrographic Analysis (ASTM C 856 or AASHTO T 299)
- Uranyl-Acetate Treatment (discussed in the Annex to ASTM C 856 or AASHTO T 299)
- Los Alamos Staining Method (Powers 1999)
Q: Can lightweight concrete be used for bridge construction?
A: Lightweight concrete can be and has been used for bridge construction for decades. There are many examples of lightweight concrete bridge constructions in different environments:
- The upper deck of the Oakland Bay Bridge constructed in 1936 (still in service)
- William Preston Lane, Jr. Bridge, Chesapeake Bay, Maryland, constructed in 1952 (still in service)
Of course any concrete mixture considered for a specific application should always be evaluated for the service environment in which it will be placed. Concrete used in bridge constructions in regions that have severe weather typically require low permeability, to provide protection for the steel reinforcement in the structure as the use of deicing chemicals increases the risk of corrosion of any steel reinforcement. Corrosion of the steel is a primary concern for the long-term durability of this type of structure. Low permeability and high strength are closely related concrete properties; as strength increases permeability decreases.
Concrete strengths used in bridges typically equal or exceed 5000psi and some bridge specifications require permeability testing using the ‘Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration,’ ASTM C1202. These minimum precautions should be implemented regardless of the type of concrete being considered.
In summary, lightweight concrete that has been properly tested to assure that it provides the required properties for the specific structure in question can be used for bridge construction.
Q: How do you protect a concrete surface from aggressive materials like acids?
A: Many materials have no effect on concrete. However, there are some aggressive materials, such as most acids that can have a deteriorating effect on concrete. The first line of defence against chemical attack is to use quality concrete with maximum chemical resistance, followed by the application of protective treatments to keep corrosive substances from contacting the concrete. Principles and practices that improve the chemical resistance of concrete include using a low water-cement ratio, selecting a suitable cement type (such as sulphate-resistant cement to prevent sulphate attack), using suitable aggregates, water and air-entrainment. A large number of chemical formulations are available, as sealers and coatings, to protect concrete from a variety of environments; detailed recommendations should be requested from manufacturers, formulators or material suppliers.
Q. How does blended cement affect durability?
A: Pozzolan combines with lime and alkalis in the cement and when water is mixed forms compounds which contribute to strength, impermeability and sulphate resistance It also contributes to workability, reduced bleeding and controls destructive expansion from alkali-aggregate reaction; the leaching of free lime is also reduced.
Q: Can concrete surfaces scratch or crack?
A: Concrete surfaces share many characteristics of other stone surfaces like marble or limestone and require similar care. They are very durable, but NOT indestructible. The surface can chip or crack if struck by a heavy object. The use of hot pads and cutting boards is recommended to protect the surface from extreme temperature changes and scratches. Concrete may experience small hairline cracks or crazing (similar to that which occurs on the surfaces of porcelain objects) that will not affect its strength or durability. Use of sealants makes the concrete surface stain resistant, but not stain proof. Prompt cleanup of spills, particularly acids such as citrus juice and wine, and the periodic application of a light coat of wax are required. Concrete’s natural tendency to acquire a patina or personality over time, is seen by many as a desirable characteristic.