Concrete FAQ

	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 center 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. Minimize the mix water content by maximizing 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 minimize cracking on top of vapor barriers, use a 100-mm thick (4-in.) layer of slightly damp, compactible, drainable fill choked off with fine-grade material. If concrete must be placed directly on polyethylene sheet or other vapor barriers, use a mix with a 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 fibers to help control plastic shrinkage cracks.

We used muriatic acid to remove some stains from a concrete sidewalk that was several weeks old. The stains are gone but now the cleaned portion of the sidewalk is darker than the rest of the sidewalk concrete. What caused the darkening and what can be done to correct it?

Calcium hydroxide deposits may lighten the color 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 color difference will be especially noticeable if the concrete surface was darkened by hard troweling or if a calcium chloride admixture was used in the concrete.

Treating the entire sidewalk with muriatic acid might enable you to match the darker colored area where the stain was removed. However, some experimentation would be needed to develop a procedure that minimizes color variations.

In this case, the craze cracking provides an additional clue. When concrete isn’t struck off and floated correctly, low spots or birdbaths may be present. Puddles of bleedwater collect in these low spots. Then inexperienced finishers sometimes dust the bleedwater surface with dry cement so they can finish the concrete faster. This has two possible effects:

• It may lower the water cement ratio, thus darkening the concrete.
• It may increase the shrinkage of the high-cement-content layer, thus creating craze cracking.

This may have happened in your driveway. The only way to tell for sure is to take a core and have it examined by a petrographer.

1. concrete that is exposed to multiple freezing and thawing cycles when the concrete is saturated with water


2. concrete that is exposed to deicing chemicals

In either case the concrete surface is disrupted by tensile forces that develop as the result of water near the surface which expands as it freezes. For an existing driveway that is showing signs of this deterioration mechanism, it is recommended that the concrete surface be sealed with a high quality breathable sealer, such as silane, siloxane, or methyl methacrylate. Treating the surface with a sealer may help to limit the amount of moisture available in the concrete, potentially slowing or stopping the deterioration.

In extreme cases it may be appropriate to prepare the concrete for resurfacing and apply a new wearing surface of mortar. Since it is difficult if not impossible to match the color of the existing concrete, this method of repair requires that full panels (all of the area bounded by joints or driveway edges) of the surface be repaired as one application and in some cases the full driveway may need to be resurfaced to achieve a consistent color with the repair.

Left unprotected, concrete that is vulnerable to scaling may have a shortened service life. However, surface scaling of concrete is a gradual deterioration mechanism and when addressed with good remediation strategies the service life of the concrete can be improved. With reasonable maintenance there is good reason to believe that the concrete will be serviceable for many years (decades) to come.

Random cracks in concrete slabs are most frequently caused 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 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. This mechanism is controlled by placing isolation joint material between the slab and the fixed foundation to allow the elements to move independently, thus limits 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 150 mm (6 in.) 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 of itself is not a failure of the concrete, but rather the normal behavior 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 recognized 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.

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 will 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 added 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 sulfate 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 sulfate 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 sulfates 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.

Conduction Calorimeter

As an example, the moisture content of a concrete mixture with 334 kg/m³ (564 lb/yd³) of cement and a w/c of 0.45 and in a service environment with a 50% relative humidity could be estimated as follows:

Total water content:
334 kg cement/m³ times 0.45 w/c ~ 150 kg water/m³
(564 lb cement/yd³ times 0.45 w/c ~ 254 lb water/yd³ )

Chemically bound water at 0.24 w/c:
334 kg cement/m³ times 0.24 w/c ~ 80 kg water/m³
(564 lb cement/yd³ times 0.24 ~ 135 lb water/yd³ )

Moisture content:
150 kg water/m³ - 80 kg water/m³times .50 relative humidity ~ 35 kg water/m³
(254 lb water/yd³ – 135 lb water/yd3 times .50 relative humidity ~ 60 lb water/yd³)

In reality the relative humidity of the concrete will only reach 50% at the near surface of the concrete and the moisture gradient with depth will increase toward 100% relative humidity so this method of estimation would typically overstate the quantity of moisture available to leave the concrete due to the initial mixing water in the mixture.

This is only an estimate of the moisture available to leave the concrete, but may help to give some perspective to the limited amount of water that the concrete can contribute when considering the drying time of hardened concrete.

Relaltive Humidity Profile