Concrete FAQ

A contractor placed a concrete driveway for me last night. Before they could start the finishing work, it rained for several minutes. We covered everything we could but the concrete still got quite wet. In the end, they were able to finish (floating and edging), but I am really concerned how this will hold up long term.

A:Rainfall during placement of concrete flatwork can present challenges to achieving a quality concrete. Potential outcomes range from no damage to a weakened nondurable 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 just as bleed water from the original concrete mixture must be allowed to evaporate 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, and since 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 a 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, and in instances similar to this one many contractors are willing to extend the warranty for an additional period of time (an extra year or two) to settle the doubt.

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, and 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 that "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, emphasizing 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 sulfate should be prohibited because they rapidly attack and disintegrate concrete. Deicers used in low concentrations (2% to 4% by weight) can cause more surface scaling than higher concentrations or no deicer at all. 

Efflorescence is a type of discoloration. It is a deposit, usually white in color 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 vapor 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 vapor 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 vapor pressure at the slab surface will tell you how much moisture is moving through the slab. A common value of vapor pressure acceptable for moisture sensitive floor coverings is 3 to 5 lb./1000 sq.ft./24 hours. The Calcium Chloride Vapor Pressure Test is commonly used. Testing of the soils and concrete would identify the source of the soluble salts. and 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 is driving 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 buildup 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

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 can slowly dissolve and reform 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 recrystallizes in larger spaces such as voids or cracks. 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 two to four micrometers in cross section and 20 to 30 micrometers 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 sulfate 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, sulfate 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).

• 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, sulfates, 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

• 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)

• 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)