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META TITLE: Visual Inspection Of Concrete: Sources Of Surface Defects

META DESCRIPTION:

Identify concrete surface damage and probable sources: Shrinkage (plastic, autogenous, drying) Cracking (plastic settlement, thermal, crazing)

A Visual Inspection Of Concrete: Investigating The Source (s) Of Surface Defects

Inspectors will rely on the guidance provided by the International Standards of Practice for inspecting commercial properties. In evaluating concrete problems, one of the important decisions inspectors must make is determining whether an issue is the result of conditions that have stabilized with a low chance of continuing to be a headache in the near or distant future, or are they likely to worsen over time and even cause the concrete wall or floor to fail.

Chemically, concrete is a complicated material, and a visual inspection will not always answer those questions. Basic knowledge of concrete mixes, installation, weather conditions, and other factors that can affect how it ages will give inspectors the best chance of making sound decisions and recommendations to their clients.

Investigating Reasons Why Cracks Have Formed On The Surface Of A Concrete Slab

In general, relatively small movements of formwork in the early stages of hardening will tend to cause cracks to form. Also, movement of the sub-grade (soil below ground level), such as settling or heaving, can crack concrete. This may happen as a result of changes in soil volume in response to changes in the soil’s moisture content, or it may be by subsidence. Subsidence is settling that can have a number of causes. Sub-surface mining, extraction of natural gas, the dissolution of limestone or conditions related to groundwater can all lead to soil settlement creating a void.

So if during the concrete inspection the source of cracking can be accurately identified then the inspector can assess whether the condition that caused the cracking has stabilized so that it is no longer likely to result in additional cracking or encourage the propagation of existing cracks. Many like those relating to concrete shrinkage are shallow in nature caused by forces that allow conditions to stabilize relatively quickly and do not lead to structural problems. Others, like those caused by soil subsidence or changes in soil volume, are stimulated by forces that can continue to affect concrete for a long time. This long-term instability can continue to result in serious structural problems over the long term.

Finally, cracks that appear before the concrete has hardened are called plastic cracks. Plastic cracks are typically due to poor mix design, placement practices or curing methods, and may also be triggered by settlement, construction movement, and excessively high rates of evaporation. Cracks that appear after concrete has hardened can have a variety of causes, and sometimes may be traced back to more than one source.

Let’s Take A Closer Look At Some Types Of Cracks

Plastic Settlement Cracking

This type of crack can be found on the surface of the concrete directly above aligned steel reinforcements that can allow corrosive agents, such as water and chloride solutions, to reach and corrode steel. Cracks resulting from plastic settlement do not always cause corrosion. If reinforcement steel has adequate concrete coverage, corrosive agents will not reach it. This condition is best recognized by the crack pattern, which typically reflects the even spacing of the reinforcement steel. When there is excessive water in the concrete mix Cracks can also form very quickly within minutes or hours after the pour.

Plastic Shrinkage

Plastic shrinkage is caused by excessively high rates of evaporation from the surface of the concrete and can form within the early hours of the drying process. The cracks can be diagonal or random on the surface of the concrete slab.

For hydration to take place, a ratio of only about 25% water-to-cement is needed. To improve workability, an extra amount of water is often added bringing up the level to around 45%. This surplus water forces cement particles apart, suspending them in water. Once the concrete has been placed, the heavier aggregate particles settle, and the weight of the mix forces excess water to the surface. This excess water is called bleed water. Once the bleed water has evaporated from the surface, the concrete will still be wet and the surplus water will continue to dry upward through evaporation from the surface, and downward through absorption by the sub-grade, unless the concrete is installed directly on a vapor barrier, such as plastic, in which case, all drying will be upward.

Autogenous Shrinkage

The presence of plastic shrinkage results from the loss of water to the atmosphere. Autogenous shrinkage on the other hand takes place with no loss of water to the atmosphere. Autogenous shrinkage is caused by internal drying, with water being absorbed by the components in the concrete. As the long-term chemical hydration process continues – and it can continue for many years — water in the pores within the cement paste is absorbed, and the pores are filled, to some degree, by materials produced during hydration. This process leads to decreased permeability and increased strength and durability of the cement paste. Absorption of water from the pores also causes shrinkage. Since there is no loss of water to one exposed surface, autogenous shrinkage is more uniform than plastic shrinkage. However, tensile stresses still develop, and embedded steel can cause anomalies in an area of concrete with relatively uniform stress. These anomalies can cause variations in stress within the concrete that are relieved by cracking. Autogenous shrinkage cracking will be shallow and is not a structural issue. The cracks may look similar to those formed during plastic shrinkage and are often propagations of those created during plastic shrinkage. Outside corners are also high-stress points.

A Re-Entrant Corner Crack

Re-entrant corners are high-stress areas prone to transverse cracking from plastic shrinkage. A re-entrant corner is where any inside corner forms an angle of less than 180 degrees into the body of the slab. As the concrete dries and shrinks, the wedge shape of the re-entrant corner encourages concrete to crack off at the point of the angle into the slab.

Drying Shrinkage

Drying shrinkage can appear after the concrete has hardened and some degree of bonding has developed between the cement paste and the aggregate. As concrete continues to dry, it will continue to shrink. Drying shrinkage can include losing moisture to the air and autogenous shrinking, as noted previously. The cracks will look similar to those formed during plastic shrinkage. Cracking during drying shrinkage may be the propagation of those that initially developed during plastic shrinkage.

Drying shrinkage cracks typically extend across the face of a wall or across a concrete floor, which is called transverse cracking. Wall shrinkage cracks are often diagonal and do not extend to the corners. They are typically shallow and linear, although such cracks are not always continuous.

Transverse shrinkage cracks appear where the tensile strength of concrete is lowest, such as where concrete is thinner at a control joint, or across an area of concrete with cracks on either side. Shrinkage cracks propagate from the tips, so concrete adjacent to the termination of a shrinkage crack is under more tensile stress than concrete in other parts of a slab.

Resistance To Shrinkage

As water evaporates from the exposed surface and is absorbed by the sub-grade, capillary force pulls water from the voids between the cement particles in the main body of the concrete, and the concrete will continue to shrink. Differential drying and shrinkage rates between concrete at the surface and the underlying concrete create tensile stresses. This phenomenon is called resistance to shrinkage, and this resistance, caused by moisture surface tension in capillaries in the concrete, called capillary stress, can exceed 400 pounds per square inch (psi) in normal concrete. In high-strength concrete, capillary stress can exceed 600 psi. The tensile stress created by resistance to shrinkage is relieved by cracking. Greater differences in shrinkage rates will create greater tensile stresses, with the increased likelihood of cracking. Hot and windy conditions increase surface evaporation, so the concrete is more likely to crack than if it’s placed during cool, calm and cloudy conditions.

Thermal Cracking

Variations in the temperature of concrete cause it to expand and contract. Significant differences in temperature between the outer and inner portions can cause concrete to crack. Especially when concrete is fairly new and has not gained much strength and is in an expanded condition from the heat generated during hydration, as it cools, cracks can develop. Cracking can be stimulated by temperature gradients that may be the result of differences in thickness, such as when the exterior cools faster than the interior, although this is more typical in massive structures. It may be caused by environmental conditions, such as cold weather. The forces at work are similar to resistance to shrinkage.

Crazing

Pattern cracking, also called map cracking and craze cracking, appears as a network of random cracks on the concrete’s surface. The cracking is usually shallow (less than 1/8-inch deep) and not a structural issue. It’s seldom a durability problem but more of a cosmetic one.

The area enclosed by pattern cracking may be anywhere from 1/2-inch to 4 inches across.

Pattern cracking can be caused by the following:

  • Excessive water in the mix
  • Over-vibration of the concrete, causing coarse aggregate to settle and cement paste to concentrate at the surface
  • Over-working the surface with a steel trowel during finishing
  • Performing finishing operations while bleed water is still on the surface
  • Sprinkling cement dust on the surface to soak up bleed water

Some Other Surface Damage Can Be Observed During A Visual Inspection Of Concrete

Scaling

Scaling is the shedding of flakes of hardened concrete at the surface. It can be caused by a number of conditions:

  • Exposure to freezing and thawing can cause scaling, which can be made worse by the application of de-icing salts.
  • Concrete that has been improperly cured or that has inadequate air entrainment will be less resistant to scaling caused by freezing.
  • Finishing operations started while bleed water is still on the surface can weaken the surface layer and cause dusting or scaling.

When concrete is placed during hot and dry conditions, the bleed water may appear to be gone, but the surface may still be actively bleeding. The bleed water may be evaporating as it reaches the surface. During such conditions, finishing operations may be started under the mistaken impression that the surface is finished bleeding.

  • Over-working the surface during finishing will reduce the air content of the surface concrete, leaving it weaker and more vulnerable to scaling due to freezing conditions.
  • Fertilizers, such as ammonium sulfate and ammonium nitrate, will chemically attack the concrete surface.
  • Poor drainage causes water to pool, and water containing de-icing salts can also lead to pooling on the surface for extended periods of time.

Small Patches Of Flaking

Smaller patches of damage have expanded to create a much larger area of concern. In addition to properly timing finishing operations, proper curing will help prevent dusting and scaling. In hot and dry environments, the sub-grade should be dampened before the concrete is placed, and the surface should be kept damp to keep it from drying too quickly. In cool and damp environments, a water-repelling sealer should be applied to keep the surface from absorbing too much water. Concrete is most fragile during the first year after placement, so de-icing chemicals should be avoided during that time, and the concrete should be protected from absorbing moisture just before freezing weather develops.

During inspections, the most common place to find this type of damage is parking garage floors. The undercarriage of vehicles can accumulate frozen slush from roadways that contains chloride solutions and be absorbed into the concrete. Poor finishing practices, such as over-working the concrete or working bleed water back into the surface, will leave the surface weak and more likely to flake. Concrete less than a year old that may be exposed to chlorides should have a sealer applied that is designed specifically for concrete to help prevent freeze damage. Sealers may have to be re-applied periodically, depending on the type of chemicals used on the roadways in a given area, as well as the climate zone. It is sometimes possible to remove a weak or damaged surface layer of concrete and apply a thin, bonded re-surfacing product based on Portland cement, latex-modified concrete or polymer-modified mortar. Inspectors can recommend that their clients investigate products or methods that have been used successfully in the area where the building is located.

In conclusion, the great challenges of inspecting concrete are the limitations of a visual inspection and the many variables that affect concrete. Inspectors who take the time to learn about the fundamentals of concrete and become familiar with the local factors that can affect it will better understand what problems to look for, where to look for them, how to recognize them and their potential for serious, long-term damage and seek the help of professional testing facilities such as the American Concrete Repair Institute and/or the International Concrete Repair Institute.