The formation and role of ettringite in iowa highway concrete deterioration

The role of magnesium in concrete deterioration

Despite over 25 years of experience with the problem of bridge reinforcement corrosion due to chlorides from deicers or sea water exposure, the solution to the problem is still unknown. Many different corrosion protection systems have shown promise in laboratory and preliminary field studies, but their performance on the basis of long-term field tests either has not been evaluated or has been questionable. In the portion of the work discussed in this report, an extensive literature search was conducted to identify corrosion protection systems and field test methods. In addition, a survey of TXDOT districts was conducted to identify corrosion protection systems used in Texas and to determine the districts' experiences concerning the different systems. Most importantly, a field testing program was developed to evaluate and characterize the corrosion performance of bridges and to increase the information learned from corrosion protection system test installations in Texas. Preliminary questionnaire surveys distributed to all the TXDOT districts revealed that many different types of corrosion protection systems have been installed in Texas, but that little information on their performance has been collected from the structures since their completion. The field testing program developed in this study was used to investigate eight preexisting bridges representing different protection systems and various overall exposures and service conditions. The test program, which included determination of half-cell potentials, concrete permeability, chloride content, cracking patterns, and delaminations, was found to accurately reflect the corrosion condition of bridges protected by a variety of corrosion protection measures. The tests selected for use in the field surveys were generally successful and accurate, with different tests confirming each other's results.

Field evaluation of bridge corrosion protection measures

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Deterioration of Iowa Highway Concrete Pavements: A Petrographic Study

The effects of diethylenetriaminpenta(methylenephosphonic acid) (DTPMP), a phosphonate inhibitor, on the growth of delayed ettringite have been evaluated using concrete in highway US 20 near Williams, Iowa, and the cores of six highways subject to moderate (built in 1992) or minor (built in 1997) deterioration. Application of 0.01 and 0.1 vol. % DTPMP to cores was made on a weekly or monthly basis for one year under controlled laboratory-based freeze-thaw and wet-dry conditions over a temperature range of -15 degrees to 58 degrees C to mimic extremes in Iowa roadway conditions. The same concentrations of phosphonate were also applied to cores left outside (roof of Science I at Iowa State University) over the same period of time. Nineteen applications of 0.1 vol. % DTPMP with added deicing salt solution (about 23 weight % NACL) were made to US 20 during the winters of 2003 and 2004. In untreated samples, air voids, pores, and occasional cracks are lined with acicular ettringite crystals (up to 50 micrometers in length) whereas air voids, pores, and cracks in concrete from the westbound lane of US 20 are devoid of ettringite up to a depth of about 0.5 mm from the surface of the concrete. Ettringite is also absent in zones up to 6 mm from the surface of concrete slabs placed on the roof of Science I and cores subject to laboratory-based freeze-thaw experiments. In these zones, the relatively high concentration of DTPMP caused it to behave as a chelator. Stunted ettringite crystals 5 to 25 micrometers in length, occasionally coated with porlandite, form on the margins of these zones indicating that in these areas DTPMP behaved as an inhibitor due to a reduction in the concentration of phosphonate. Analyses of mixes of ettringite and DTPMP using electrospray mass spectrometry suggests that the stunting of ettringite growth is caused by the adsorption of a Ca2+ ion and a water molecule to deprotonated DTPMP on the surface of the {0001} face of ettringite. It is anticipated that by using a DTPMP concentration of between 0.001 and 0.01 vol. % for the extended life of a highway (i.e. >20 years), deterioration caused by the expansive growth of ettringite will be markedly reduced.

Reduction of Concrete Deterioration by Ettringite Using Crystal Growth Inhibition Techniques: Part II Field Evaluation of Inhibitor Effectiveness

Research on the topic of calcium sulfate impurities in deicing salts adversely affecting the durability of portland cement mortar is covered in this paper. Natural rock salt may contain as much as 4.0 percent calcium sulfate. When combined with chloride solutions the solubility of calcium sulfate increases by as much as 3.5 times; hence, the calcium sulfate impurities contained in rock salt brine become highly detrimental, causing attack on cement mortar samples. These impurities can collect in pavement joints and cracks, reaching high concentrations through evaporation and the repeated use of deicing salts. Such a destructive mechanism may account for premature field failure of pavements that passed durability tests in the laboratory. Durability studies using brines containing differing amounts of gypsum in proportion to the sulfates occurring in natural rock salts have been conducted. To model field conditions, samples were concurrently subjected to brines and rapid freeze-thaw testing. Deterioration proved so rapid that testing was stopped at 88 cycles. Tensile tests showed strength losses up to 40 percent for samples subjected to brine solutions containing gypsum impurities. Mercury intrusion porosimetry showed significant losses in pore volume for the treated samples as compared to samples frozen and thawed in water only. X-ray diffraction tests discovered increased amounts of ettringite and Friedel's salt, a tricalcium aluminate chloride hydrate.

D-Y Lee

Papers

Sulfate impurities from deicing salt and durability of portland cement mortar