Chloride Penetration - Diagnosis
Visual inspection, mechanical techniques, or chemical tests are used for diagnosing chloride induced corrosion of rebar in concrete. Usually, the easier the diagnosis, the higher degree of degradation. We can categorize these diagnostic methods into moderate and severe, corresponding to the degree of corrosion from chloride ingress.
Chloride Content Analysis – This test requires samples to be extracted from an existing structure and chloride content (by % weight of sample) to be determined. AASHTO T260 can be used. There are also several field test kits available for this analysis. Use caution when reporting data from field kits, because many kits give results significantly lower than AASHTO T260. Currently ACI 318-95 accepts 0.3 % water-soluble chlorides by weight, but new proposed limits reduce this amount to 0.08% water-soluble chlorides.
Schmidt Hammer – The Schmidt hammer is a non-destructive test method used to locate delaminations (areas where concrete has cracked internally due to internal stress). This device applies an impact force to the concrete and gives a reading. The test should be performed first on sound concrete, then applied to various portions of the concrete surface. When the hammer is used on a delamination, the reading will be significantly lower than the reading from sound concrete. This method can also be used with a standard hammer, while listening for a change in sound. This is similar to using a hammer to find studs in a wall.
Fundamental Frequency – This non-destructive method uses an apparatus capable of measuring the frequency or pulse velocity of a wave as it travels through concrete. The wave is delivered to one part of the concrete by a driving circuit, and is and received by a pickup circuit. Since it takes a wave longer to move around cracks, the readings from this test will be significantly different for cracked and sound concrete. The apparatus used in this test is often expensive. This method is also referred to as ultrasound.
Visual Observation – Visual identification is often simple, but usually if you can see the effects, the structure may be beyond repair. Cracks due to rebar corrosion are long, extending in the direction of the corroded rebar. Prior to cracking, one may observe staining at the concrete surface. Reddish-brown stains at the concrete surface indicates that the rebar below the surface is corroding. Once one observes the effects of corroding rebar, the environment can be used to determine whether chloride penetration is actually the cause of this corrosion.