DETECTION OF CHLORIDES IN CONCRETE USING Low RADIO FREQUENCIES By I. L. AI-Qadi; Member, ASCE, R. H. Haddad/ and S. M. Riad 3 ABSTRACT: An experimental program was conducted to study the effect of chloride contamination of portland cement concrete (PCC) on its complex permittivity over low radio frequencies (RF) range (0.1-40.1 MHz). Two types of aggregate (quartzite and limestone) were used to prepare the PCC mixes at a water to cement ratio (w/c) of 0.45. Two methods were used to induce chlorides into PCC: partial immersion in NaCI solution (6% by weight) and the addition of NaCI to the PCC mixing water. For the NaCI addition to the mixing water, three levels of chloride content were used. Dielectric measurements were taken for PCC specimens during the 28-day curing period and continued for six months. Powder PCC samples were collected periodically to estimate the chloride content in the PCC specimens. Results showed that complex permittivity of PCC specimens (im- mersed in NaCI solution) quantitatively increased as the chloride content increased, until quantitatively 75% of the chloride content estimated theoretical saturation volume. Aggregate type and frequency also show a signif- icant effect on the measured dielectric constant. INTRODUCTION Chloride intrusion into reinforced portland cement concrete (RPCC) has been the interest of many research projects over the years because of chloride's special properties, such as be- ing a catalyst and an accelerator of PCC reinforcing-steel cor- rosion. Corrosion of reinforcing steel in PCC is a major prob- lem in the civil infrastructure; bridge deterioration is an example, where deicing salts are being used extensively, es- pecially in the 25 snowbelt states where the annual amount of snowfall is high. This results in severe internal and external deterioration that cost billions of dollars (Bradford 1992). When RPCC is exposed to deicing salts, free Cl- ions, mov- ing through the PCC pore system, react with the positively charged Fe +2 and form FeCI 2 . In the presence of moisture, further reaction with FeCl 2 leads to the formation of iron hy- droxide and the release of more Cl- and H+ ions. The iron hydroxide reacts with oxygen and results in the formation of Fe203 (rust). The latter compound, after further stages of re- action, produces up to six times the original steel volume (Bradford 1992). This results in reduction of the steel effective area and creates tensile stresses in surrounding PCC, thus caus- ing cracks. The cracks allow more chlorides to intrude the RPCC pore system. This aggravates the problem and leads to the formation of delamination and spalls in the RPCC struc- ture. Repair and maintenance procedures of deteriorated RPCC structures, deteriorated as a result of reinforcing-steel corro- sion, have not yet contributed significantly to reducing the problem because the deterioration is usually discovered at late stages. Therefore, efforts were made toward finding new meth- ods to measure chloride contamination prior to the deteriora- tion process. As a result, different nondestructive testing tech- niques have been developed to detect both corrosion activity and/or RPCC deterioration, such as three electrode linear po- larization (3LP), Geocor, impact echo, and the utilization of ultrasonic and infrared waves (Heitz et al. 1992; Tasker et a1. 'Assoc. Prof., The Via Dept. of Civ. Engrg., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061-0105. 2Formerly, Grad. Res. Asst., The Via Dept. of Civ. Engrg., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA. 'Prof., Bradley Electr. Engrg. Dept., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA. Note. Discussion open until July 1, 1997. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on February 27,1995. This paper is part of the Jour- nal of Materials in Civil Engineering, Vol. 9, No. I, February, 1997. ©ASCE, ISSN 0899-1561/97/0001-0029-0034/$4.00 + $.50 per page. Paper No. 10227. 1990; AI-Qadi et al. 1993). However, limited success has been achieved using these methods because many of them detect the deterioration rather than determining the cause. Recently, the use of electromagnetic waves to characterize PCC at dif- ferent frequency bands was introduced. Ground penetrating radar (GPR) is one of the techniques that has been investigated over the past 25 years for civil en- gineering applications. Studies using GPR at different fre- quency bands were conducted on RPCC structures; rigid and flexible pavements; and bridge decks to detect deterioration, predict moisture content and/or measure pavement thickness (Clemena et al. 1987; Alongi et a1. 1982; Cantor and Kneeter 1982; AI-Qadi 1993). Because PCC is a heterogeneous ma- terial, and analyzing the GPR output was based only on signal recognition approach, using GPR to evaluate PCC perfor- mance did not always achieve satisfactory results. In addition, the imaginary part of the complex dielectric properties of PCC was overlooked. To improve the performance of GPR in field applications, an understanding of PCC dielectric properties should be es- tablished. Thus, the effect of PCC composition, maturity, moisture, and deterioration type on the PCC electrical prop- erties should be investigated. Electromagnetic waves at 1 kHz were used to study the cement paste setting-stages through the evaluation of the change in the electrical properties response (McCarter and Curran 1984). The result of the study demonstrated that the chl;U'acteristics of the electrical response of cement paste set- ting-stages can be used as an effective means for studying the progress of hydration and structural changes occurring within the paste. Electrical conductivity of cement paste was also studied during the first 24 h of curing (Whittington et a1. 1981; Perez-Pena and Roy 1989). The change in the cement con- ductivity over time was used as a measure of cement setting progress. Electrical resistivity was also used to study paste and PCC properties (Hansson and Hansson 1983; McCarter and Whittington 1981; McCarter et al. 1985). Based on the fact that a higher conductivity of the material correlates to a lower resistance, effects of different proportions and properties of PCC constituents, temperature, and curing time were evaluated by many researchers. The mechanical properties of a mature cement paste were studied using both the dielectric properties and conductivity of the cement matrix constituents to estimate the volume of each constituent (Taylor and Atulanandan 1974). This has been accomplished using experimental testing and theoretical ma- nipulation based on the mixture theory. Results indicated that electromagnetic waves can be used successfully to estimate JOURNAL OF MATERIALS IN CIVIL ENGINEERING / FEBRUARY 1997/29 J. Mater. Civ. Eng. 1997.9:29-34. Downloaded from ascelibrary.org by GRAINGER ENGINEERING LIB E on 07/24/13. Copyright ASCE. For personal use only; all rights reserved.