field experience in using resistivity tests for concretedocs.trb.org/prp/15-3357.pdf · fresh...

1 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: jagan.m.gudimettla.ctr@dot.gov 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: gary.crawford@dot.gov 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

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.

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

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 95-11(10).

Table 1: Chloride Ion Penetration Classification

Chloride Ion Penetration

RCP Test Surface Resistivity Test

Charges Passed (Coulombs)

4"x8" Cylinder (KOhm-cm)

6"x12" Cylinder (KOhm-cm)

High > 4,000 < 12 < 9.5 Moderate 2000-4000 12 – 21 9.5 – 16.5

Low 1000-2000 21 – 37 16.5 – 29 Very Low 100-1000 37 – 254 29 – 199 Negligible <100 > 254 > 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.

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

Rapid Chloride Permeability Surface Resistivity Bulk Resistivity

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 State Year Project Cement,

lbs/yd3 Flyash, lbs/yd3

Slag, lbs/yd3

w/cm ratio Aggregate Max.

Agg. 1 North Carolina 2011 I-540 465 140 --- 0.41 Granite 1"

2 California 2011 I-80 452 54 169 0.37 Gravel, Quartz 1.5"

3 Nevada 2012 I-80 564 141 -- 0.38 Granite 1.5" 4 Virginia 2012 RTE 58 447 149 --- 0.43 Limestone 3/4" 5 Iowa 2012 US 71 449 112 0.40 Quartzite 1" 6 Pennsylvania 2013 RTE 202 540 95 -- 0.40 Limestone 1" 7 Alaska 2013 Petersburg 658 -- -- 0.41 Gravel 1.5 8 Arizona 2013 L303 451 113 --- 0.44 Gravel 1.5" 9 Illinois 2013 Tollway 455 175 70 0.37 Gravel 1" 10 Michigan 2013 US 10 375 -- 125 0.42 Limestone 1.5" 11 Florida 2014 I-4 350 150 -- 0.45 Limestone 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.

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 56 Day Fresh Concrete Properties 28 Day Compressive

Strength*

State SR

Air Conten

Unit Weight Slump MCW*

NC X --- X --- --- X X X --- X CA X --- X X --- X X X X X NV X X X X X X X X --- X VA X X X X X X X X --- X VA X X X X X X X X X PA X X X X X X X X X X AK --- --- X X X X X X --- --- AZ --- --- X X X X X X --- X IL X X X X X X X X X X MI X X X X X X X X X X FL 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

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

there was such a strong correlation between the SR and BR readings, data from SR measurements are presented for the rest of this paper.

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.

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

0 10 20 30 40 50

ity, K

Surface Resistivity, KOhm-cm

Bulk vs. Surface @ 28 Days

Nevada VirginiaIowa PennsylvaniaIllinois MichiganFlorida

y = 0.4337x + 0.5481R² = 0.98

1:1 Line

2:1 Line

0 10 20 30 40 50

ity, K

Surface Resistivity, KOhm-cm

Bulk vs. Surface @ 56 Days

Nevada VirginiaIowa PennsylvaniaAlaska ArizonaIllinois MichiganFlorida

y = 0.4553x + 0.1435R² = 0.97

1:1 Line

2:1 Line

(56 DAY)y = 2840.3x-0.625

R² = 0.89

0 1000 2000 3000 4000 5000 6000 7000

RCPT, Columbs

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

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<0.15 for Michigan, North Carolina, Nevada) there was no relationship between the two testing ages. Figure 5b shows a general relationship between SR at 28 days and RCPT at 56 days as well as SR and RCPT at 56 days. There was a moderate (R2<0.5) relationship between the SR at 28 days and RCPT at 56 days and a much stronger correlation at 56 days (R2>0.85). Some researchers believe that the SR at 28 days may be a good indicator of 56 day RCPT, however, from the data presented in Figures 5a and 5b, it can be observed that this may not be the case for all the mixtures. This could be due to two reasons; 1) with the use of binary mixtures the permeability will tend to be lower at later ages due to the continued hydration 2) for the same mixture and from the sample plant, there could be production variability associated with cementitious contents and potential change in sources.

Figure 5a: Relationship between SR at 28 Day and 56 Day

Figure 5b: Relationship between SR at 28 Day and RCPT at 28 and 56 Days

Variability of Resistivity measurements in Production Table 4 shows the COV associated with RCPT, SR, and BR measurements from 10 field projects (Alaska project not included due to limited data points). The COV associated with fresh concrete properties and compressive strength is also presented in Table 4. The numbers next to the COV percentages in parenthesis in each cell represent the number of data points used to arrive at the COV. As mentioned before, compressive strengths and microwave water contents were measured on a subset of the total samples taken from each project so there were fewer data points for these test results. The variability associated with test methods on cement-based materials is based on many factors. In general, the total variation can be attributed due to the sum of the individual variabilities associated with testing equipment, operator, material, production and curing (9). Interpreting total variations as in Table 4 presents a big challenge. There is no way to determine if the variation seen in these tests are purely a function of the variability of the tests or of mixture consistency.

Table 4: Coefficient of Variation of Various Properties from Field Projects

State 56 Days

Slump Air Unit Weight

Microwave WC

Comp Strength @

28 Day RCPT SR BR NC NA 14% (13) NA 16% (13) 12% (13) 1% (13) NA 8% (5) CA NA 14% (15) NA 46% (15) 35% (15) 3% (15) 6% (9) 13% (5) NV 31% (12) 25% (12) 24% (12) 16% (12) 12% (12) 1% (12) NA 5% (4) VA 14% (6) 11% (10) 9% (10) 25% (10) 16% (10) 1% (10) NA 8% (3) IA 7% (10) 7% (10) 7% (10) 16.5% (10) 5% (10) 0.2 (10) NA 8% (8) PA 13% (10) 12% (10) 7% (4) 19% (10) 9% (10) 1% (10) 2% (3) 6% (4) AZ 23% (9) 14% (9) 14% (9) 41% (9) 15% (9) 1% (8) NA 5% (3) IL 13% (7) 14% (7) 19% (5) NA 8% (7) 1% (7) 1% (3) NA MI 18% (9) 9% (9) 10% (9) 18% (9) 10% (9) 1% (9) 5% (7) 4% (4) FL 6% (5) 5% (5) 5% (5) 20% (5) 15% (5) 1% (5) 3% (4) 8% (3) Average COV 16% 13% 12% 24% 14% 1% 3% 7%

5.0 15.0 25.0 35.0Surf

Surface Resistivity @ 28 Day, KOhm-cm

SR @ 28 and 56 Day

California North Carolina NevadaVirginia Iowa PennsylvaniaIllinois Michigan Florida

(56 DAY)y = 2840.3x-0.625

R² = 0.89

(28 DAY)y =926x-0.549

R² = 0.47

0 1000 2000 3000 4000 5000 6000 7000

RCPT, Columbs

56 Day28 Day

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

HighModerateLowHigh

Moderate

From Table 4 it can be seen that the COV of RCPT is higher than COV of the SR and BR for almost all the projects. The COV of the SR and BR for all the projects were very close with the exception of Illinois. The COVs for RCPT, SR and BR for the Nevada project were quite high even though this was not reflected in the fresh concrete properties. The reason for the high COV for this project is due to variability in the cementitious contents. This was confirmed by heat signature of the samples from which the resistivity and RCPT tests were conducted. Interestingly, the California project has a high variability in its fresh concrete properties. Nearly half the samples from this project were taken at the plant site and the other half were taken on the grade. Due to this, there were substantial differences in the fresh concrete properties for this project. However, this high variability was not noticed in the SR data collected from this project. This is one of the benefits of the resistivity tests. The results do not depend on where during the construction process the measurements are conducted, but only on the quality of the concrete produced. Even though a one to one comparison of COVs cannot be made between the SR and compressive strength data due to the smaller number of compressive strength data points from each project, it could be still be observed that the overall COV of compressive strength is quite low. SR COV is better than RCPT; however, it is still much higher than compressive strength. Relationship between SR and Concrete Properties Relationship between SR and Fresh Concrete Properties The permeability of the concrete plays a significant role on the service life of a structure exposed to deicing salts or to salt water. Permeability of concrete depends on the water cementitious materials (w/cm) ratio, total cementitious content, aggregate gradation, and the amount of supplementary cementitious materials in the mixture. According to Henkenseifken (16), other parameters remaining constant, the resistivity measurements depend directly on the water-cementitious ratio. In this section, the 56 day SR readings were correlated to fresh concrete properties (slump, unit weight, air content and water cementitious ratio). Since this was not a controlled experiment, and there could be numerous interactions between several properties, good correlations between the various parameters and SR readings were not expected. Nevertheless, Figure 6a, 6b, 6c, and 6d plots slump, microwave water content, air content and unit weight with the corresponding 56 day SR readings. The plots were made on samples within each project i.e. samples from the same mixture. As can be seen from the four figures, there was little to no correlation between the 56 day SR and the various fresh concrete properties and in many cases the trend lines were opposite for different projects. Relationship between SR and 28 Day compressive strength Figure 6e shows the relationship between 56 day SR and 28 day compressive strength. Only projects with more than 4 compressive strength data points are shown in Figure 6a. Figure 6a indicated that similar to fresh properties, there was no direct correlation between SR and compressive strength.

Figure 6(a): Slump Figure 6(b): Microwave Water Content

Figure 6(c): Air Content Figure 6(d): Unit Weight

Figure 6(e): 28 day Compressive Strength

Figure 6: Relationships between 56 day SR vs Fresh and Hardened Concrete Properties

0.0 10.0 20.0 30.0 40.0

Surface Resistivity @ 56 Days, KOhm-cm

SR @ 56 Day vs. Slump

California North Carolina Nevada Virginia Pennsylvania Arizona Michigan Florida

10.0 15.0 20.0 25.0 30.0 35.0 40.0

SR @ 56 Day vs. Microwave W/C

California Michigan Florida

5.0 15.0 25.0 35.0

SR @ 56 Day vs. Air Content

California North Carolina Nevada Virginia Pennsylvania

Arizona Illinois Michigan Florida

10.0 15.0 20.0 25.0 30.0 35.0 40.0

SR @ 56 Day vs. Unit Weight

California North Carolina Nevada Virginia Pennsylvania

Arizona Illinois Michigan Florida

10.0 15.0 20.0 25.0 30.0 35.0 40.0

SR @ 56 Day vs. 28 Day Strength

California North Carolina Nevada Pennsylvania Michigan

Discussion on the use of Resistivity Tests for Quality Control/Acceptance As seen from the data presented in this paper, both the SR and BR tests are much better than RCPT not only in terms of testing time and effort but also in terms of the variability associated with the test method. Based on the mobile lab’s experience with the resistivity tests, there is definitely a potential for either of these tests to be routinely used for quality control and acceptance purposes. However, the most importance aspect is with handling the specimens in the field since testing temperature, curing temperature, and curing solution all have significant impact on the resistivity readings (9). In the case of the mobile lab, due to the access to the grade or the plant, specimens cast from various days of production were brought to the lab within a day of the specimens being cast and all specimens from each project were subjected to the same conditions. However, these issues (curing method, curing temperature and testing temperature) need to be clearly defined before using these tests routinely for QA/QC purposes. All the previous studies including this one had clearly shown that there is a general relationship between SR and RCPT, but there seems to be slight differences in the curves from the various studies. This suggests that this relationship depends on local materials and testing conditions. So before embarking upon using the SR results in specifications basing on the relationship between SR and RCPT from previous studies, these relationships have to be established for local conditions and mixtures.

CONCLUSIONS Based on the work performed in the study, the following conclusions can be drawn:

1. The 56 day SR, 56 day BR and 56 day RCPT measurements correlate very well for a

wide variety of mixtures produced in the field. This work confirms the relationship between 56 day SR and RCPT correlations from past research which was primarily performed on laboratory cast specimens. Based on this work, it can be said that either the SR or BR methods could be used in lieu of the RCPT test method for measuring concrete permeability from specimens cast in the field.

2. SR and BR results on 4"x8" specimens correlated extremely well. SR results are typically1.9 times larger than the BR results at 28 and 56 days.

3. Based on the results from this paper, it appears that the relationship between SR at 28 and 56 days is mixture specific. There was a moderate relationship between the 28 day SR and 56 day RCPT data which suggests that 28 day SR may not be a good indicator for 56 day RCPT for all mixtures.

4. The 56 day SR and BR variability associated with production samples were similar and ranged from 5 to 25%. As expected, this variability was lower than that of RCPT.

5. For the mixtures used in this study, SR results did not correlate with any fresh concrete properties. The relationship between 56 day SR and compressive strength varies from project to project.

ACKNOWLEDGEMENTS The contributions from the following individuals is gratefully acknowledged; Mr. Nicolai Morari with the FHWA MCL who performed majority of the testing associated with this study and Mr. Mario Parades and Mr. Robert Spragg for sharing FDOTs and Purdue University’s experiences with the SR and BR tests. REFERENCES

1. Stanish, K.D., R. D. Hooton, M.D.A. Thomas. Testing the Chloride Penetration Resistance of Concrete: A Literature Review, University of Toronto.

2. ASTM C1202-12. Standard Test Method for Electrical Indication of Concrete’s Ability to resist Chloride Ion Penetration. Annual Book of ASTM Standards, Vol.04.02, ASTM, Philadelphia, PA, 2013.

3. AASHTO T277-07. Standard Test Methods for Preparing Precision and Bias Statements for Test Methods for Construction Materials. Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Part 2B: Tests, 26th Edition, AASHTO, Washington D.C. 2006.

4. Understanding AASHTO T277 and ASTM C1202 Rapid Chloride Permeability Test. Technical Bulletin TB-0100.Grace Construction Products.

5. Chini,A., L. Muszynski, and J. Hicks. Determination of Acceptance Permeability Charcateristics for Performance Related Specifications for Portland Cement Concrete. Publication BC 354-41. University of Florida, Gainesville, Florida, 2003, p.162.

6. Kessler, R., R. Powers, E. Vivas, M. Paredes, Y. Virmani. Surface Resistivity as an Indicator of Concrete Chloride Penetration Resistance, 2008 Concrete Bridge Conference, St. Louis, Missouri, May 4-7, 2008.

7. Rupnow, T., and P.J. Icenogle, Evaluation of Surface Resistivity Measurements as an Alternative to the Rapid Chloride Permeability Test for Quality Assurance and Acceptance, Louisiana Transportation Research Center, Project Report, July 2011.

8. Ryan, E.W., Master’s thesis, Comparison of Two Methods for the Assessment of Chloride Ion Penetration in Concrete: A Field Study, University of Tennessee, Knoxville, 2011.

9. Spragg, R., Bu, Y., K. Snyder, D. Bentz, and J. Weiss. Electrical Testing of Cement-Based Materials: Role of Testing Techniques, Sample Conditioning, and Accelerated Curing. Publication FHWA/IN/JTRP-2013/28. Joint Transportation Research Program, Indian Department of Transportation and Purdue University, West Lafayette, Indiana, 2013.

10. AASHTO TP-95. Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration. Washington, D.C., July 2011.

11. Spragg, R., J. Castro, T. Nantung, M. Parades, and W. Weiss. Variability Analysis of the Bulk Resistivity Measured Using Concrete Cylinders. SPR-3509, FHWA/IN/JTRP-2011,Joint Transportation Research Program.

12. ASTM C1760-12. Standard Test Method for Bulk Electrical Conductivity of Hardened Concrete. Annual Book of ASTM Standards, Vol.04.02, ASTM, Philadelphia, PA, 2013.

13. Spragg, R., C. Villani, K. Snyder, D. Bentz. J. Bullard, and J.Weiss. Electrical Resistivity Measurements in Cementitious Systems: Observations of Factors that Influence the Measurements.; TRB 2013.

14. Tanesi, J., and A. Ardani., Surface Resistivity Test Evaluation as an Indicator for the Chloride Permeability of Concrete, FHWA Publication No: FHWA-HRT-13-024. Turner-Fairbank Highway Research Center, 2013.

15. Presuel-Moreno, F., Suarez,A., and Liu, Y. “Characterization of New and Old Concrete Structures Using Surface Resistivity Measurements.” Final Report, August, 2010

16. Henkenseifken, R., J.Castro, D. Bentz, T.Nantung, and J. Weiss. Water Absorption in Internally Cured Mortar Made with Water-filled Lighweight Aggregate. Cement and Concrete Research. Vol 39, No. 10,2009, pp.883-892.

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