Concrete Deficiencies – When do I need an inspection?

Concrete reinforced with steel rebar is one of the most common construction materials around. Concrete is cheap, easy to install and plentiful. Concrete structures have been known to last a long time and has been used in construction for thousands of years.

Concrete does require periodic inspections and maintenance. When engineers will most commonly perform a visual inspection of concrete. The engineer is looking for several deficiencies that may reveal that the concrete structure requires a rehabilitation. The most common resource for visual inspections of concrete is the Ontario Structural Inspection Manual (OSIM). This document is produced by the the Ministry of Transportation of Ontario (MTO). The document is written for bridges but can be referenced for other concrete structures as well.

The OSIM recommends that all bridges and retaining walls be inspected every two years. An acceptable standard for residential and commercial concrete structures would be every 5 years. Retaining walls, balconies and foundations that are in poor repair should be inspected more frequently until they are repaired or replaced.

OSIM groups common concrete defects as lists criteria for the severity. The follow defects are from the OSIM.

Corrosion of Reinforcement

Steel reinforcing (rebar) inside of concrete can rust and corrode due to the process known as electrolysis. As water and chloride ions infiltrate the concrete the reinforcement will break down due to corrosion.

Light – Light rust stain on the concrete surface;
Medium – Exposed reinforcement with uniform light rust. Loss of reinforcing steel section less than 10%;
Severe – Exposed reinforcement with heavy rusting and localized pitting. Loss of reinforcing steel section between 10% and 20%;
Very Severe – Exposed reinforcement with very heavy rusting and pitting. Loss of reinforcing steel section over 20%.

Delamination

Concrete that is separated from the main body but not completely is delaminated. Most commonly, as corroded rebar expands it separated the outer portion of the concrete. Delamination is most commonly detected by using a hammer to lightly tap the concrete and to listen for a distinct hollow sound. Most of the time delaminated concrete is not able to be visually observed.

Light – Delaminated area measuring less than 150 mm in any direction.
Medium – Delaminated area measuring 150 mm to 300 mm in any direction.
Severe – Delaminated area measuring 300 mm to 600 mm in any direction.
Very Severe- Delaminated area measuring more than 600 mm in any direction.

Spalling

Spalling is when large concrete pieces break completely away from the structure. Spalling is the next step after delamination where corroded rebars expands and breaks off concrete. Spalling may also be caused by impacts and overloaded concrete structures.

Light – Spalled area measuring less than 150 mm in any direction or less than 25 mm in depth.
Medium – Spalled area measuring between 150 mm to 300 mm in any direction or between 25 mm and 50 mm in depth.
Severe – Spalled area measuring between 300 mm to 600 mm in any direction or between 50 mm and 100 mm in depth.
Very Severe – Spalled area measuring more than 600 mm in any direction or greater than 100 mm in depth.

Cracking

Observations of cracks is the most common deficiency in concrete structures. A crack is a fracture within the cement and aggregate of the structure and may be a surface crack or completely through the member. Cracking is caused by a number of processes. Most small cracks are caused by shrinkage, as the concrete cures it shrinks and cracks slightly. Concrete performs poorly in tension, when it is being pulled apart and is susceptible to cracks anytime it is subjected to tensile loads. Generally small cracks less than 0.3mm is not of a structural concern and is attributed to the shrinkage of the concrete.

Hairline cracks – less than 0.1 mm wide.
Narrow cracks – 0.1 mm to 0.3 mm wide.
Medium cracks – 0.3 mm to 1.0 mm wide.
Wide cracks – greater than 1.0 mm wide.

Scaling

Scaling is the local flaking and cracks due to the freeze-thaw deterioration of concrete. This defect is most common in poorly finished concrete.

Light – Loss of surface mortar to a depth of up to 5 mm without exposure of coarse aggregate;
Medium – Loss of surface mortar to a depth of 6 to 10 mm with exposure of some coarse aggregates;
Severe – Loss of surface mortar to a depth of 11 mm to 20 mm with aggregate particles standing out from the concrete and a few completely lost.
Very Severe – Loss of surface mortar and aggregate particles to a depth greater than 20 mm.

Disintegration

Disintegration of concrete is the physical deterioration of concrete into small fragments and particles. This will most likely be observed on the exterior side of the structure where it is exposed to weather.

Light – Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium – Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of reinforcement;
Severe – Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of reinforcement over a large area.
Very Severe- Loss of section in excess of 100 mm deep and extending over a large area.

Erosion

Erosion is caused by sand and gravel particles suspended in water damaging submerged concrete. This will be common in concrete structures that are exposed to water and flow.

Light – Loss of section up to 25 mm in depth with some loss of coarse aggregate;
Medium – Loss of section between 25 mm and 50 mm deep with considerable loss of coarse aggregate and exposure of reinforcement;
Severe – Loss of section between 50 mm and 100 mm deep with substantial loss of coarse aggregate and exposure of reinforcement over a large area.
Very Severe – Loss of section is in excess of 100 mm deep and extending over a large area.

Further Inspections

Following the visual inspection the engineer may recommend that non-destructive testing techniques be applied. These tests can determine the strength of the concrete, degree of corrosion in the rebar or reveal other deficiencies.

Contact Us

Although this information comes from the Ontario Structural Inspection Manual and is predominately applied to bridges and culverts, this information can be used on any concrete structure. If you have a residential or commercial property with concrete foundations, retaining walls or structure and you notice some of these defects it is time to hire an engineer to do a review. It is important to properly maintain and inspection concrete before it gets too severe.

Reference: Ministry of Transportation Ontario, Ontario Structure Inspection Manual (OSIM). Government of Ontario, [PDF] April 2008

2 Comments

  1. Brendan SheltonAugust 22, 2019

    I appreciate you explaining what spalling is and how it is caused by corroded rebars or impacts to the structure. My son is building a new garage on his property and is trying to figure out how to get it to last for as long as possible. We will keep your article in mind as he makes a plan to accomplish this task.

    Reply
    1. IN EngineeringAugust 22, 2019

      You can use coated rebar in the slab which will present salt from your vehicles from corroding the rebar and causing spalling. This would only be needed if your municipality or county uses salt during the winter.

      Reply

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