These small-scale tests reproduce the macrocell type corrosion found in large reinforced concrete structures, particularly bridge decks. They are used to determine the rate of corrosion of reinforcing steel in both uncracked and cracked concrete.

Southern Exposure Specimen

(larger image)
The SE specimen consists of a small slab containing two mats of reinforcing steel. The slab is 180 mm (7 in.) thick by 300 mm (12 in.) square. The top mat of steel consists of two bars; the bottom mat consists of four bars. The bars are electrically connected across a 10 ohm resistor. A dam is cast around the top surface at the same time when the specimen is cast. The specimen is wet cured for 3 days and then air cured until the specimen is 28 days old.

Cracked Beam Specimen

This specimen is half the width of the SE specimen and has one bar on top and two bars on the bottom. A dam is also cast around the top surface of the specimen. "Cracks" are formed in the specimen using a 0.30 mm stainless steel shim, cast into the concrete and removed 24 hours after casting. The cracks are placed in the concrete either perpendicular or parallel to the reinforcing steel.

Test Procedure
For both of these specimens, a wetting and drying cycle is used to accelerate the corrosion process of the steel in concrete. The cycle consists of:
  • 4 days of ponding with a 15 percent NaCl solution at 22° C (72° F). 3 days of drying at 38° C (100° F). This cycle is repeated over a period of 12 weeks.

  • 12 weeks of continuous ponding with a 15 percent NaCl solution at 22° C (72° F).

This cycle is repeated 4 times for a total rest period of 96 weeks.

The corrosion potential of both mats of steel with respect to a copper-copper sulfate electrode (CSE), voltage drop across the resistor, and matt-to-matt resistance are measured every week.

According to ASTM C 876, "Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete", if the potential is more positive than -0.200 V versus CSE, there is a high probability that the reinforcing steel is not corroding. If the potential is more negative than -.35 V versus CSE, there is a probability that reinforcing steel corrosion is occuring.

As in rapid evaluation tests, the voltage drop across the resistor is transformed into a corrosion rate in µm/year. Higher mat-to-mat resistances may contribute to a higher corrosion rate.

ASTM G 109

The G 109 test was developed to study the effect of chemical admixtures on the corrosion of reinforcing steel. The specimen is 280 mm (11 in.) long, 115 mm (4.5 in.) wide and 155 mm (6 in.) thick. It has one reinforcing bar on top and two reinforcing bars on the bottom. Instead of the 15 percent NaCl ponding solution used for the Southern Exposure and Cracked Beam tests, a 3 percent NaCl solution is used for the G 109 test. The 10 ohm resistor is replaced by a 100 ohm resistor in the G 109 test.

The specimens are ponded for two weeks and then dried two weeks. The voltage drop across the resistor, and the corrosion potential of the bars against a reference electrode are measured starting in the second week of ponding. The current as a function of time is monitored until the macrocell current is 10 µA or greater. The tests are continued for three more cycles.

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Last updated June 17, 2005 by R. Solwa