KTC-03-02
"Investigation of Soluble Salts on Kentucky Bridges" S. Palle, R. Younce, T. Hopwood
ABSTRACT
Invisible soluble salts present on steel
highway structures can act to promote corrosion even after maintenance
painting. Soluble salts include chloride, nitrate and sulfate ions.
Soluble salts found on bridge decks and roadways are the result of usage
of deicing salts (chlorides), and the deposition of atmospheric
pollution (nitrates and sulfates) on exterior surfaces. Typical
pollution- related sources of soluble salts are coal-fired power plants,
refineries, farming operations, vehicle emissions, and chemical plants.
Sites for soluble salt damage on bridges include:
• Beam ends under open or leaking expansion joints,
• Flanges and lower portions of webs of overpass
structures,
• Lower chords and connecting members (end posts,
portals, vertical posts and diagonals), and guard rails of truss bridges
near roadway level, and
• Outside faces of fascia girders and other bridge
elements constantly exposed to the environment.
Maximum acceptable concentrations of soluble salts are very low (e.g. 10
µg/cm2 for chlorides).
Several state highway agencies are looking at ways to increase the durability of their bridge maintenance painting projects and are using/investigating various chemical neutralizers/soluble salt removers to preclude problems caused by soluble salts. KYTC (Kentucky Transportation Cabinet) officials initiated this study to investigate presence of soluble salts on Kentucky bridges and to determine if they posed a significant problem to bridge paint durability. This work was undertaken by Kentucky Transportation Center (KTC) under study KYP 02-0056 .Unforeseen Investigations.. The objectives of the study were to: 1) Determine the levels of soluble salts on KYTC bridges. 2) Study the effect of various cleaning methods (pressure washing, power and hand tool cleaning, etc.) on soluble salts on bridges. 3) Establish whether commercially available chemical neutralizing/soluble salt removing agents are capable of reducing salt levels to where they will not damage new paint nor cause early .rust back..
KTC researchers selected a field test method developed by CHLOR*RID International, Inc., a firm that supplies soluble salt testing materials and a chemical salt remover sold under the trade name CHLOR*RID™. The firm also markets a field test kit (i.e., CHLOR*TEST”CSN” SALTS™) that extracts soluble salts from bridge surfaces and analyzes the extract to provide a measure of salt concentration.
KYTC officials chose three bridges located on different routes that they anticipated to be subject to differing amounts of deicing-salt applications (and consequently differing amounts of chloride contamination for testing). Those were the I-471 Bridge over the Ohio River in Campbell Co. (tied arch and urban area), I-71 N Bridge over I-75 S in Boone Co. (deck girder and main line overpass in a rural area) and KY 355 Bridge in Owen Co (through truss in a rural area).
Tests were conducted at different areas on the bridge where corrosion or rusting was evident. Readings were taken in the existing condition, after hand tool/power tool cleaning, and, in some instances, after pressure washing at approximately 3500 psi with and without CHLOR*RID™.
The tests of the three bridges revealed low levels of soluble salts in most instances, even in the untreated or .existing conditions.. Pressure washing with potable water was effective in reducing the amounts of soluble salts as was pressure washing with CHLOR*RID™. The testing did not reveal a significant problem related to soluble salts, but the testing was too limited to make a determination about all Kentucky bridges.
Recommendations are provided for future testing of bridges for soluble salts prior to maintenance painting along with recommendation for performing washing tests on structures with high soluble salt levels (i.e., above 30 µg/cm2 for any types of soluble salt), and adding contract requirements for reducing the surface salt level to about 10 µg/cm2 prior to painting (with the contractor having the choice of method to treat the surfaces to that level).