Gudimettla, Crawford
Field experience in using Resistivity Tests for Concrete
Jagan M. Gudimettla, P.E.* Project Engineer/Manager Federal Highway Administration/ATI,Inc Room E73-105C, HIAP-10 1200 New Jersey Ave, SE Washington, DC 20590 Email:
[email protected] PH: 202 366 1335 FX: 202 403 2070 Gary L. Crawford Concrete Quality Engineer Federal Highway Administration Room E73-438, HIAP-10 1200 New Jersey Ave, SE Washington, DC 20590 Email:
[email protected] PH: 202 366 1286 FX: 202 403 2070 *Corresponding Author Submission date: November 14th, 2014 Word Count: Text = 4767, Abstract = 233, Tables = 4, Figures = 6; Total = 7,500 Number of tables: (4 x 250 = 1000); Number of figures: (6 x 250 = 1500) = 2,500 words Paper Submitted for Presentation and Publication to the 94th Annual Meeting of the Transportation Research Board
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ABSTRACT Surface Resistivity (SR) and Bulk Resistivity (BR) are gaining popularity for testing concrete’s ability to resist chloride ingress in lieu of the Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration Test (American Association of State Highway and Testing Officials (AASHTO) T277 / American Society of Testing and Materials (ASTM) C1202) which is commonly referred to as the Rapid Chloride Permeability Test (RCPT). Previous comparison studies among the three tests were mostly focused on laboratory cast samples. This paper analyzed SR, BR, and RCPT data collected by the Federal Highway Administration (FHWA) Mobile Concrete Laboratory (MCL) from 11 concrete paving field projects from across the country. The data showed that the results from the three tests correlate well and confirms the work performed by previous research on laboratory samples. However, the data indicates that care should be taken when using 28 day SR data in lieu of 56 day RCPT data since they may not correlate well for all mixtures. The variability associated with production for SR, BR, and RCPT tests was also analyzed from the field projects. Depending on the project, the Coefficient Of Variation (COV) for SR and BR for production samples ranged from 5% to 25% with an average of 13% and 12% respectively. The SR and BR COVs for all the projects were lower than the COV of RCPT. Data also showed that little to no correlation existed between 56 day SR test results, fresh concrete properties and 28 day compressive strength.
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INTRODUCTION For concrete highways and bridges, one of the major forms of environmental attack is chloride ingress due to salt water or deicing chemicals and freeze thaw damage for northern states. Chloride ingress leads to corrosion of the reinforcement and subsequent reduction in strength, serviceability and aesthetics of the structure (1). To mitigate this issue, checking concrete for its permeability to chloride ingress is a very important agency activity both during the mixture design phase (qualification) as well as during construction (acceptance). Electrical Indication of concrete’s ability to resist chloride ion penetration test (AASHTO T 277 /ASTM C 1202) (2,3) more commonly referred to as the Rapid Chloride Permeability test (RCPT) is the most widely used test method by state agencies for this purpose. This test was originally developed by the Portland Cement Association under a research program paid for by the FHWA (4). The RCPT method was developed to replace ponding tests such as the AASHTO T259-80 “Resistance of Concrete to Chloride Ion Penetration”, which usually take 90 days or longer (4). One of the draw backs with the RCPT test method is the large variability in test results and the time required to run the test. Per ASTM, on companion samples tested by different laboratories, the results may differ as much as 51%. In recent years, the use of resistivity testing to assess the ability of concrete to resist chloride ingress has gained enormous popularity in lieu of the traditional RCPT testing. Previous studies by states such as Florida (5,6), Louisiana (7), Tennessee (8) and Indiana (9) have shown good correlations between measurements from surface resistivity and RCPT tests. Compared to RCPT, electrical resistivity testing offers several advantages such as 1) rapid testing (less than a minute) 2) no sample preparation requirement (standard size 4"x8" or 6"x12"specimens can be tested) 3) ease of use 4) better precision and 5) directly related to fluid transport (9). All of these advantages could lead to significant cost savings for both agencies as well as contractors. Realizing the significant cost savings associated with the resistivity tests, states such as Florida and Louisiana have started implementing resistivity tests in their specifications, while states such as Indiana and Michigan are currently in the process of doing the same. In addition to using it for mixture qualification, quality control and acceptance (9), there is also growing interest in evaluating the resistivity tests for assessing long term durability of concrete for potential use in performance related specifications. BACKGROUND and LITERATURE REVIEW Resistivity Testing Resistivity is a material property that is independent of specimen geometry and electrode configuration. However, the resistivity is determined based on a test of electrical resistance of a material. This resistance must be corrected for specimen size and electrode configuration (9). There are two common methods to measure electrical resistivity of concrete: 1) Surface Resistivity and 2) Bulk Resistivity. Surface Resistivity (SR) The Surface Resistivity test method requires four probes to be directly placed on the surface of the test specimen to measure its electrical resistance. The concrete electrical resistivity is calculated from the measured electrical resistance, the test cylinder dimensions and the spacing between the probes. This method has a distinct advantage in that it is rapid and easy to perform
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on the surface of a cylinder. However, according to Spragg et.al (9), the probe spacing, geometry of the sample, aggregate size and surface moisture conditions can all influence the measurements in this method. Since moisture conditions affect the readings, care should be taken to protect the concrete specimens against drying prior to testing. The additional advantage of this method is the ability to make in-situ measurements. However, care should be taken in interpreting the in-situ results since the readings depend on geometry, degree of saturation, leaching of alkalis and ingress of deicing salts, temperature, and location of rebar (9). There is currently an AASHTO provisional test method (AASHTO TP 95) which addresses several of the concerns regarding probe spacing, sample geometry etc (10). This provisional test method has been accepted for use by Louisiana and Florida DOTs. Table 1 show the RCPT and SR classification for permeability of concrete from AASHTO TP 9511(10). Table 1: Chloride Ion Penetration Classification Chloride Ion Penetration High Moderate Low Very Low Negligible
RCP Test Charges Passed (Coulombs) > 4,000 2000-4000 1000-2000 100-1000 254
6"x12" Cylinder (KOhm-cm) < 9.5 9.5 – 16.5 16.5 – 29 29 – 199 > 199
Bulk Resistivity (BR) or Uniaxial Resistivity The resistance of a concrete cylinder can also be evaluated by using plate electrodes that can be placed on the end of the sample. The resistance value obtained can be normalized by specimen geometry, to obtain the sample resistivity, termed as the bulk resistivity (11). The term Resistivity (units in k ohm-cm) is universally adopted in the concrete industry. However, there is another way of measuring/describing this property called conductivity (units are Siemens per meter). The parameters resistivity and conductivity are simply the inverse of each other (9). Bulk resistivity is also a rapid test and has a simple geometry factor. The primary advantage of bulk resistivity is the more uniform distribution of current throughout the sample, unlike the SR test where measurements are taken on the surface of the specimen (9). The ASTM designation for this test method is ASTM C1760-12 Standard Test Method for Bulk Electrical Conductivity of Hardened Concrete (12). The ASTM C1760 test method uses the RCPT setup and sample preparation but uses a single measurement after voltage is applied for a minute. Even though the ASTM C1760 test method measures the conductivity of the sample, for the purposes of this paper, only bulk resistivity measurements were analyzed. The BR measurements were made using the same apparatus used to measure SR with some minor “add-ons”. Figure 1 shows the photos of the RCPT, SR, and BR equipment.
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Rapid Chloride Permeability
Surface Resistivity
Bulk Resistivity
Figure 1: Permeability Measuring Test Equipment Comparisons between Resistivity and RCPT Since 2003, work done by several researchers (5-9, 11, 13-14) from various states has found a strong relationship between RCPT and resistivity tests (mostly SR). Chini et.al (5) was the first one to establish this relationship. Louisiana (7) tested more than 30 laboratory mixes with several combinations of cementitious materials and water cementitious ratios. They also tested some field samples and found that the relationship between the two methods at 56 days was very good with a correlation coefficient of 0.89. In the same study, the authors found that, for the mixtures used in this study, suitable correlations were found to exist between both the 14-day and 28-day SR values and the 56-day RCPT. Researchers at FHWA (14) have shown similar relationships between 56 day SR and RCPT for high volume Fly Ash mixtures prepared in a laboratory setting. Ryan (8) tested samples at 28 and 56 day for SR and RCPT from concrete used in bridge decks all over Tennessee and found similar relationship shown by Chini. From this research, it was seen that there was a moderate correlation between SR at 28 and 56 days (R2=0.592). Spragg et.al shows that there was an excellent relationship between SR and BR methods (13). Presuel-Moreno et al. showed that a correlation exists on SR testing on samples tested in field conditions (non-saturated) and samples tested in a laboratory (wet condition) (15). Except for a few studies (7,15), most of the work performed to date on developing relationships between RCPT and resistivity tests focused on laboratory specimens i.e. two or three specimens were cast and tested per mixture using SR and RCPT. Similarly, when field samples were used, only a few specimens were cast from each project and tested for SR and RCPT. None of the published literature reported evaluating this relationship exclusively from actual field projects where samples were collected on a daily basis to see if 1) the resistivity and RCPT relationship matched the laboratory experience reported by other researchers and 2) how resistivity results change with typical changes in daily production. This paper attempted to address these two issues using data from actual pavement projects. DATA COLLECTION For the past twenty five years, the FHWA Mobile Concrete Laboratory (MCL) has been instrumental in taking promising new technologies from research and assisting states with the implementation of these innovative technologies through a variety of technology transfer activities. One such activity is by showcasing these promising new technologies during active construction projects, testing local materials and showing the benefits to the highway agencies and contractors. This helps the agency and contractor staff increase their confidence
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and understanding of the equipment and see results first hand which significantly increases their likelihood of adopting the new technology that was showcased. Since 2011, the SR and BR tests were two of the many new technologies that were showcased by the MCL. Table 2 shows the various states in which the resistivity tests were performed by the MCL in the last few years. This paper presents data generated by the MCL during visits to 11 field pavement projects across the country. TABLE 2: Pavement Projects and Mixture Designs S. No 1
Cement,
Flyash,
Slag,
lbs/yd3
lbs/yd3
lbs/yd3
I-540
465
140
---
w/cm ratio 0.41
2011
I-80
452
54
169
0.37
2012 2012 2012 2013 2013 2013 2013 2013 2014
I-80 RTE 58 US 71 RTE 202 Petersburg L303 Tollway US 10 I-4
564 447 449 540 658 451 455 375 350
141 149 112 95 -113 175 -150
----
0.38 0.43 0.40 0.40 0.41 0.44 0.37 0.42 0.45
State
Year
Project
North Carolina
2011
2
California
3 4 5 6 7 8 9 10 11
Nevada Virginia Iowa Pennsylvania Alaska Arizona Illinois Michigan Florida
----70 125 --
Aggregate Granite Gravel, Quartz Granite Limestone Quartzite Limestone Gravel Gravel Gravel Limestone Limestone
Max. Agg. 1" 1.5" 1.5" 3/4" 1" 1" 1.5 1.5" 1" 1.5" 1"
OBJECTIVES Based on the data collected by the MCL from the various field projects, this study has three objectives: • • •
Evaluate / validate the relationships between SR, BR, and RCPT test measurements from specimens cast during actual concrete production. Assess the variability in resistivity measurements from production samples and Evaluate relationships with various fresh and hardened concrete properties.
APPROACH and TEST MATRIX Approach As mentioned previously, most of the studies evaluating the relationships between SR, BR and RCPT were from laboratory cast specimens and most of the studies compared either SR data with RCPT or BR data with RCPT, but not all three of them together. In the few cases where comparison studies were performed using field mixtures, mixture design was the only variable i.e. samples from each mixture would be the same from a fresh concrete property standpoint since the replicate samples for the mixture were cast from the same batch. In this work, mixture design was not the only variable i.e. for the same mixture, other variables such as air, slump, compressive strength also varied.
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Test Matrix Table 3 shows the overall test matrix for this study. Due to the nature of this type of work, the test matrix is not balanced i.e. the number of samples from each project change from state to state. However, based on information from Table 2, it can be seen that data was collected from a wide variety of mixtures (one straight cement mixture, eight binary mixtures, and two ternary mixtures), various maximum aggregate sizes and different geographical regions. In most of the projects listed in Table 3, 10 to 15 samples were collected from each field project (some projects had fewer than 10 samples) ranging over a two week period ranging from one to three samples per day. From each sample, fresh concrete properties were measured and one 4"x8" specimen was cast for SR testing. On a subset of the samples from each project, Microwave Water Content (MWC), and 28-day compressive strength were also measured. In the MCW test, the water content of fresh concrete is determined by evaporating the water in a small sample using a microwave oven. The test is fast and allows the water content to be determined in less than 15 minutes. The water to cementitious ratio (w/cm) can then be determined by dividing the amount of water from the microwave test by the amount of cementitious materials indicated on the batch ticket. Since w/cm ratio affects the permeability of concrete, it is expected that an increase in w/cm would lead to a decrease in resistivity indicating a more permeable concrete. The testing process evolved with time. During the first couple of projects (NC and CA in Table 2 and 3), only SR testing was performed on all the samples collected. From 2012 onwards, on most projects, 28 and 56 day SR and BR were performed followed by RCPT testing after 56 days. Table 3: Test Matrix of Tests Performed 28 Day State NC CA NV VA VA PA AK AZ IL MI FL
SR X X X X X X ----X X X
BR ----X X X X ----X X X
56 Day SR
BR
X X X X X X X X X X X
--X X X X X X X X X X
Fresh Concrete Properties Air Unit Slump MCW* RCPT Conten Weight --X X X ----X X X X X X X X --X X X X --X X X X X X X X X X X X X --X X X X --X X X X X X X X X X X X X X X
28 Day Compressive Strength* X X X X X X --X X X X
*A subset of all samples taken from each project were tested for Microwave Water Content and 28 Day Compressive Strength
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SPECIMEN FABRICATION and TESTING Specimen Preparation According to Spragg et.al (9,13) electrical resistivity tests can be influenced by 1) specimen geometry 2) degree of saturation, 2) testing temperature 3) curing temperature 4) leaching of alkali’s from pore solution (curing conditions) and 5) age of the specimen. In order to minimize these effects on the results, all of the parameters (conditioning method, curing temperature, test temperature etc.) except age were kept constant in the study. Irrespective of the maximum aggregate size, all the specimens cast in the field for SR, BR, and RCPT were 4"x8" size. The specimens were cast either at the concrete plant site or on the grade. Curing All the specimens for this study were cured in lime water baths inside the MCL. The temperature inside the MCL was set to be maintained at 72⁰F. There were some instances when the MCL was in transit from one location to the other, when the temperature inside the MCL was not maintained exactly at 72⁰F, however, all the specimens from each project experienced the same temperature profile and were tested under the same laboratory conditions. Since all the specimens were cured in lime water baths, the SR readings were increased by 10% per AASHTO TP 95 requirement. Testing Equipment A CNS Farnell SR meter was used to perform all the testing (Figure 1a). For BR testing, the SR meter was used along with some additional platens to measure resistivity from the top and bottom of the specimen (Figure 1c). At the end of this study, the CNS Farnell meters used in this work were compared with newly acquired Proceeq meters (which are more common) with the MCL. Both the meters produced similar results. Testing Only one specimen was cast from each production sample and this specimen was first tested for SR and BR at 28 and 56 days respectively. After the 56 day SR and BR testing, the same specimen was cut, epoxied, vacuum saturated and tested for RCPT on the 58th day. In a few cases, depending on the MCL schedule, some specimens were tested for RCPT beyond 58 days. TEST RESULTS Relationship between SR, BR, and RCPT. Relationship between SR and BR Figures 2 (a) and 2 (b) show the relationship between SR and BR measurements at 28 and 56 days. Each data point in the figure represents test results from a single specimen. Based on the figures, it can be seen that the measurements correlate extremely well with each other at both ages. Only 9 of the 11 projects from Table 2 are shown in Figure 2, since BR measurements were not taken in earlier projects. Both the figures show that when testing 4"x8" cylinders, the SR results are typically 1.9 times higher than the BR results. Spragg et.al conducted a round robin study of SR and BR testing with INDOT laboratories and reported the same ratio between the SR and BR results from laboratory mixtures (9). The strong correlation between the two tests for 4"x8" cylinders indicates that there was practically no difference of one test over the other. Since
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there was such a strong correlation between the SR and BR readings, data from SR measurements are presented for the rest of this paper. Bulk vs. Surface @ 28 Days Nevada Iowa Illinois Florida
40
Virginia Pennsylvania Michigan
1:1 Line
30 20
2:1 Line
10
Bulk vs. Surface @ 56 Days
50
y = 0.4337x + 0.5481 R² = 0.98
0
Bulk Resistivity, KOhm-cm
Bulk Resistivity, KOhm-cm
50
Nevada Iowa Alaska Illinois Florida
40 30
Virginia Pennsylvania Arizona Michigan
2:1 Line
20 10
y = 0.4553x + 0.1435 R² = 0.97
0 0
10
20 30 Surface Resistivity, KOhm-cm
40
1:1 Line
50
0
10
20 30 Surface Resistivity, KOhm-cm
40
50
Figure 2: Relationship between BR and SR measurements at 28 and 56 days Relationship between SR and RCPT Figure 3 shows the overall relationship between the SR and RCPT measurements from all the projects listed in Table 2. As mentioned previously, the SR measurements were made at 56 days and RCPT tests were conducted on the same specimens on the 58th day (56th and 57th day were used to fabricate the specimens for RCPT). Each data point on the figure represents test results from a single specimen. There was a very good relationship between the two measurements. It is interesting to note that of the 69 data points from 9 states represented in Figure 3, the majority of the specimens were classified in the moderate and low permeability category by both SR and RCPT tests. 50
Surface Resistivity,KOhm-cm
45 40 35 (56 DAY) y = 2840.3x-0.625 R² = 0.89
30 Low
25 20
Moderate
15 10
High Low
5
High
Moderate
0 0
1000
2000
3000 4000 RCPT, Columbs
5000
6000
7000
Figure 3: Relationship between RCPT and Surface Resistivity for the MCL Mixtures at 56 days age Figure 4 shows the relationship between SR and RCPT from all the previous studies and this one (the solid blue line represents the trend line from this study). The overall trend from all five
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studies shown is the same. However, the figure shows that at lower permeability the trend lines from all the studies were close to each other and as the permeability of concrete increases, the trend lines tend to drift apart. As mentioned before, all the data from this study was obtained from specimens cast in the field during concrete pavement production and shows the production variability within each mixture. Overall, Figure 4 shows that there is a definite trend between SR and RCPT for field produced samples, too. However, based on the slight differences in the curves, it appears that this relationship is specific to local conditions and mixtures so the corresponding limits to classifying the concrete for its permeability based on SR should be based on RCPT-SR curves prepared based on local conditions until such time that more experience is gained.
Figure 4: Comparison of relationships between RCPT and SR at 56 Days Relationship between SR measurements at 28 versus 56 Days. Figure 5a show the relationship between SR at 28 and 56 days from the various field projects. The correlation between SR at 28 days and 56 days appear to be project specific i.e. in some cases there was a very good linear correlation (R2>0.80 for Iowa, Illinois, Virginia) and in other cases (R2
