Introduction

A s we make the shift toward performance-based specifications, it becomes necessary toidentify durability tests and criteria that can be used to define performance. Strength-based criteria for mixture proportioning, jobsite acceptance and referee testing for low

strength situations are well established. These concepts are statistically based and balance therisk between the producer and purchaser. Similar concepts are also appropriate for other per-formance tests and criteria.

Current durability provisions in industrystandards rely on the water to cementitiousmaterials ratio (w/cm) and a concomitantspecified compressive strength level thatserves for acceptance purposes. For almostall cases, the specified w/cm is intended tocontrol the permeability of concrete to fluidsand chemicals in solution that cause durabil-ity problems. Strictly speaking, w/cm is aprescriptive limitation intended to control aperformance property, permeability. Howev-er, with current mix design technology,which includes the use of a variety of supple-mentary materials and admixtures, there is awide variation in permeability properties atthe same w/cm based on how the mixture isproportioned. One commonly used alterna-tive to specifying maximum w/cm for con-crete designed to withstand aggressiveenvironments (chlorides, sulfates etc.) is tospecify performance criteria based on ASTMC 1202, Test Method for Electrical Indicationof Concrete’s Ability to Resist Chloride Ion Pen-etration, commonly referred to as the RapidChloride Permeability (RCP) test. Figure 1illustrates a typical setup of the RCP test.While the method has its detractors, it is oneof the only standardized tests that provides areasonably good indicator of the permeabili-ty of concrete. One of the problems is thatthe developers of the test intended it for

CONCRETE in focus ı 41

feature

Acceptance Criteria for Durability TestsBy Karthik H. Obla, NRMCA Senior Director of Research & Materials Engineering and Colin L. Lobo, NRMCA Vice President of Engineering

more controlled use in laboratories ratherthan for acceptance of concrete from sam-ples obtained at the jobsite.

Many state transportation agencies andother engineers are using ASTM C 1202 intheir specifications primarily for mixturequalification and some even include it forconcrete acceptance. There seems to be agreater desire among agencies to use this testfor concrete acceptance purposes. Beforethis is widely practiced, we believe that cer-tain knowledge and understanding isrequired in order to use this test in anappropriate manner. This article discussesacceptance crite-ria based onASTM C 1202as an indicator ofpermeability. Theconcepts will alsobe applicable toother perfor-mance tests thatare not currentlyin common use.

TestingVariation

The first andforemost aspectthat needs to be

kept in mind when selecting a test forqualification or acceptance of concrete is aconsideration of the precision of the testmethod and the associated risk to the pro-ducer and purchaser of rejecting or accept-ing acceptable or defect ive product ,respectively. Compared to the strength test(ASTM C 39), the RCP test (ASTM C1202) tends to have a larger testing vari-ability. In ASTM C 39, the precisionstatement indicates that the within testcoefficient of variation (V) of companion6 x 12-inch cylinders prepared in fieldconditions is 2.9%. It states that theacceptable range of individual cylinderstrengths prepared from the same sampleof concrete and tested by one laboratoryshould not differ by more than 8.0%. Incomparison, ASTM C 1202 reports a singleoperator coefficient of variation of testresults (measured in triplicate) to be 12.3%

V

+ -

Concrete

0.3NNaOH

3%NaCI

Chloride Ion PenetrationChloride Penetrability

HighModerate

LowVery LowNegligible

Charge Passed> 4.000 coulomb2000-40001000-2000100-1000<100

Figure 1 – ASTM C 1202(RCP) test setup.

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them, they serve to illustrate that the vari-ability of C 1202 test results can be on theorder of six times higher than that of the

strength test results. Let us try to put these numbers in some

perspective. An acceptable range of 42% forthe RCP test suggests that if we get two resultsfrom the same sample, one of 800 coulombsand the other of 1200 coulombs, they are stillwithin the acceptable range of testing error.However, if we get two strength test results of4000 psi and 6000 psi, clearly that is notwithin the range of testing error. Let us con-sider a project scenario with a specifiedstrength of 5000 psi and a specified RCP testvalue of 1000 coulombs. Clearly the 4000 psistrength test result is a low break and is a causefor inquiry. Does a RCP test result of 1200coulombs deserve an inquiry as well? Giventhat the acceptable testing range is 42%, it isquite possible that the 1200 coulomb result isdue to normal testing variability. A retest ofanother set of specimens made from the samebatch of concrete may very well yield a valuelower than 1000 coulombs. In this case thetest result of 1200 coulombs should not bea cause for inquiry. To avoid these prob-lems, one must develop a rat ional , statistically-based acceptance criteria for the

with an acceptable range of 42%. While theprecision estimates are not directly compara-ble due to the method used in developing

CONCRETE in focus ı 43

Case Study Project - Bridge 2003

0

2000

4000

6000

8000

100004/

19 5/3

5/17

5/31

6/14

6/28

7/12

7/26 8/9

8/23 9/6

9/20

10/4

10/1

8

11/1

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reng

th,

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0

2000

4000

6000

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02,

coul

ombs

28 day strength, Spec 6500 psi56-day RCP, Spec 2000 Coulombs

ASTM C 1202 ResultsAverage = 1800 coulombsStandard Deviation = 540 coulombsC. of Variation = 30%

ASTM C 39 (Strength) ResultsAverage = 7480 psiStandard Deviation = 632 psiC. of Variation = 8.4%

Figure 2: Strength and RCP test data for a bridge project illustrating differences in testresult variability.

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states that the strength of concrete shallbe considered sat is factory i f the testresults, defined as the average of at leasttwo cylinder strengths, meet both the fol-lowing criteria:1. Arithmetic average of any three consecu-

tive cylinder test results equals or exceedsspecified strength (f c)

2. No individual test result should be lessthan 0.90f cFor simplicity in this article, let us define

terminology for specified “permeability”based on ASTM C 1202 test results as p´cwhere the test result is the average of at leasttwo specimens. Based on strength provi-sions, it is proposed that the RCP test resultsof concrete shall be considered satisfactory ifboth the following criteria are met: 1. Arithmetic average of any five consecutive

test results is equal to or lower than speci-fied permeability, p c

2. No individual test result should be above1.30 p cWhen the permeability acceptance crite-

ria are compared to the strength acceptancecriteria, two things become obvious:

1. The concrete producer has to designmixtures for a required average RCP testvalue, p´cr, to be lower than the speci-fied permeability, p´c, unlike the case ofstrength where required averagestrength, f ´cr, is designed to be higherthan specified strength, f ´c. This isbecause with permeability measures, alower value is more resistant to fluidingress (and therefore more durable)than a higher value.

2. The proposed permeability criteria usingthe RCP test are slightly more lenient.This is a reasonable approach to accom-modate the five to six times higher test-ing variabil ity of the RCP test ascompared to the strength test. Taking thearithmetic average of five consecutivetests instead of three is proposed toreduce the influence of the higher testingvariability and reduce the risk of reject-ing acceptable concrete. The higherallowance proposed for an individual testresult compared to the specified perme-ability is to accommodate the largeracceptable range of single operator

RCP test. We propose to use the same widelyaccepted concepts as the current provisions forsubmittals and acceptance established forcompressive strength. This discussion assumesthat RCP test results during the course of aproject follow a normal probability distribu-tion, like strength test results.

Establishing Statistically-BasedAcceptance Criteria

If the specified strength, f´c, is greaterthan 5000 psi, ACI 318 Section 5.6.3.3

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Figure 3. Test set up for ASTM C 1556 test to measure apparent diffusion coefficientof concrete.

feature

strength test results as “good” control ingeneral construction testing.

This case is evaluated by three approaches.

Current ApproachCurrently, there are no industry-recom-

mended statistically-based acceptance crite-ria for the RCP test. Consequently, the testresult above the specified permeability isconsidered a “failure” and a cause forinquiry. Using fundamental statistical con-cepts that assume a normal distribution ofRCP tests, a concrete producer will have tochoose a level of risk and design mixturesaccordingly. If a 99% confidence level (or a1 in 100 failure rate) is chosen for an indi-vidual test (this is the same confidence levelused for calculating required averagestrengths by ACI 318), the required averagepermeability for the mixture can be calculat-ed as follows:

p´cr = p´c -2.33 (V x p´cr)

Where V is an estimate of the coeffi-cient of variation (standard deviation

expressed as a percentage of the average) oftest results obtained during the course of aproject.

This can be expressed as follows:

p´cr =

Assuming a coefficient of variation, V, of30%, and p´c = 1500 Coulombs, one gets p´cr= 882 Coulombs.

This approach is obviously overly conserva-tive and is even more stringent than the currentACI 318 approach for acceptance of concretefor strength. Clearly this approach requires aconcrete producer to design a mix for anextremely low average permeability, p´cr.

Approach Identical to ACI 318In this case, equations identical to Equa-

tion 5.1 and Equation 5.3 in ACI 318 Codeare considered

p´cr = p´c – 1.34 (V x p´cr) results in p´cr = 1070 coulombs

p´c(1 + 2.33V)

results. It is these authors opinions thatthe allowance of 30% higher RCP testresult compared to the specified value isa conservative approach to address thetesting variability. Other more robust cri-teria can be established when a bettermeasure of jobsite sample test variabilitycan be determined.

Case StudyWhat target average permeability is

required for a specified permeability, p c,of 1500 coulombs?

Assume an overall coefficient of varia-tion of RCP test results (includes testingand batch to batch variation) to be in therange of 30% for general construction test-ing. This is a reasonable number for goodquality control and is based on actual datafor a particular project communicated tothe authors. The strength and RCP testresults for an actual bridge project for agiven class of concrete are illustrated inFigure 2. ACI 214R, Evaluation of StrengthTest Results for Concrete, reports an overallcoefficient of variation of 9% to 11% for

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Proposed ApproachThe approach that was proposed ear-

lier in this article can be mathematicallyexpressed as the following two equa-tions:

p´cr = p c – (2.33/√5) x (V x p cr) results in p´cr = 1143 coulombs

p´cr = 1.3p c – 2.33 (V x p cr) results in p cr = 1147 coulombs

This approach will allow the producer todesign the mixture for an average permeabil-ity, p´cr of 1143 coulombs.

This is the most reasonable approachthat takes into account the higher testingvariability of the RCP test as compared tothe strength test. The Table below summa-rizes the p´cr determined by the three differ-

ent approaches: For a specified RCPT=1500 coulombsClearly the 1143 coulombs is a lot eas-

ier to design for than the 882 coulombs.The most conservative approach wil lresult in concrete mixtures with a muchlower w/cm and much higher cementi-tious paste content thus resulting in agreater tendency for cracking. Crackedconcrete will lead to poorer durabilityeven though the RCP test results in thelab are very low.

Acceptance CriteriaA comparison of the acceptance crite-

r ia for tests performed from samplesobtained at the jobsite can be summarizedas follows:

(continued on page 49)

p´cr = 1.1 x p´c – 2.33 (V x p´cr) results in p´cr = 971 coulombs

This will cause the producer to design foran average permeability, p´cr = 971coulombs.

Clearly this approach, while better than thecurrent approach, is still conservative since itdoes not account for the higher testing variabil-ity of ASTM C 1202 compared to ASTM C39. Note that the value of 1.34 in the firstequation in this approach derives from(2.33/√3) and is tied to the acceptance criteriathat the average of three consecutive testsshould equal or exceed the specified value.

It is not appropriate to subject two testmethods with widely different testingvariability to the same level for concreteacceptance.

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Approach p cr Coulombs

Current (no single test result below specified) 882Approach similar to Current ACI 318 Strength Provisions 970Proposed approach 1143

feature

Comparing Target Values of P'cr When Proportioning Concrete

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permeability, p c. These criteria need to beestablished to be acceptable at a higher per-meability level and this relationship proba-bly needs to be developed by a researchprogram similar to that used to establish thecriteria for strength of cores.

Specimen CuringThe type of curing and testing age for

the RCP test specimens should be clearlyspecified. This test being very sensitive, it isimperative that initial curing in the field, ifit is proposed to be used, should be in strictaccordance to those required for strengthtests (ASTM C 31 Section 10.1.2). Subse-quently, it is desirable to allow at least 56 to

90 days of moist curing of the test speci-mens. This allows the slower reacting poz-zolans such as fly ash and to some extent slagto be used to attain low RCP test results. Ifthe curing time is limited to 28 days ofmoist curing this might force the use ofmore expensive supplementary cementitiousmaterials and preclude the use of fly ash andslag. This is not a desirable approach forspecifying concrete. The design professionalshould evaluate whether using results after a90-day curing period provides a better indi-cator of the potential performance of astructure that is intended to have a 50+ yearservice life. In cases where waiting for 90days before accepting concrete test results isnot feasible, an accelerated specimen curingapproach that has been widely tested by theVirginia Transportation Research Consor-tium (VTRC) can be adopted. This curingmethod specifies 7 days of moist curing at73°F, followed by 21 days of immersion inhot water maintained at 100°F. The resultsobtained by using this accelerated curingprocedure have been found to correlate well

Referee TestingACI 318 includes provisions in the event

of strength test results that fail to meet thecriteria. The criteria for cores obtained fromthe structure are that the average of threecores should be equal to or greater than0.85f ´c and each individual core should notbe less than 0.75f ´c. These criteria allow fornon-standard curing conditions of concretein the structure and the process of removingcore specimens. The criteria are more lenientthan standard cured strength tests.

The same concepts for referee testing areappropriate for RCP tests on cores removedfrom the structure. Tests on these cores can-not be expected to comply with the specified

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Approach Acceptance criteria

Current Individual tests ≤ p´cSimilar to ACI 318 Strength Provisions Individual test ≤ 1.1p´c

Avg. of 3 tests ≤ p´cProposed approach Individual test ≤ 1.3p´c

Avg. of 5 tests ≤ p´c

featureComparison of Acceptance Criteria

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CONCRETE in focus ı 51

The design professional might be con-cerned with relying on concrete acceptancetest results based on later age testing if it isanticipated that the concrete structure willbe exposed to a salt environment at an ear-lier age, such as for marine structures. Inorder to better answer this, ASTM C 1556chloride diffusion test was performed on flyash concrete specimens, which tend toreduce permeability slowly. Fly ash concretespecimens that were subjected to chlorideimmersion at 28 days showed high chloridediffusion coefficients after 60 days ofimmersion in solution; but very low diffu-sion coefficients after one year of chlorideimmersion. At early ages, fly ash concretetends to be more permeable and allows thechlorides to penetrate easily. However,beyond 56 days, the permeability of fly ashconcrete reduces rapidly and the chloridefront cannot easily progress into the con-crete. This results in a reduction in chloridediffusion coefficient. Therefore it can beseen that even for the slower acting poz-zolans such as fly ash, it is appropriate to

use a 90-day or an accelerated 28-day RCPtest requirement for concrete acceptancebecause these mixtures have been found toattain low long-term chloride diffusioncoefficients in spite of the early age chlorideexposure.

Readers are cautioned that ASTM C1202 is not a perfect test and only provides areasonable indicator of concrete permeabili-ty. It does not work well when concrete con-tains high concentrations of ionic solutions,such as when inorganic corrosion inhibitorsare used. In some regions, the aggregatecharacteristics are such that it is very difficultto obtain low coulomb values even with verydurable concrete mixtures. Finally, in regionswhere this test is not currently performed, itwill take a while for laboratories to achievethe necessary proficiency to perform this testfor acceptance of concrete. Current laborato-ry inspection and personnel certificationprograms do not qualify a facility to performASTM C 1202. In such cases, the alternativeis to use the current specification and accep-tance criteria with w/cm and strength.

with RCP test results obtained from moistcured specimens at 73°F to later ages of 6 to12 months.

Another method for evaluating perme-ability of concrete is ASTM C 1556, Deter-mining the Apparent Bulk DiffusionCoefficient of Cementitious Mixtures by BulkDiffusion. In this test, the specimens aresubjected to chloride solutions after 28days of moist curing. After different periodsof immersion, the specimens are removedfrom the salt solution and the penetrationof chlorides is measured as a function ofdepth from a surface. Figure 3 illustratesthe test specimens and set up. From themeasured chloride contents at differentdepths, a bulk chloride diffusion coefficientis calculated. Note that ASTM C 1556 is avery involved test that cannot be performedby most commercial laboratories and is notintended for testing samples obtained at thejobsite. However, it can be used to demon-strate the validity of specifying RCP test at90-day curing period or the 28-day acceler-ated curing procedure.

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CONCRETE in focus ı 53

they must be cured for a longer period than28 days. It is suggested that 90 days of moistcuring or an accelerated 28-day curing envi-ronment is most desirable. Even thoughmuch of the discussion in this article is cen-tered around ASTM C 1202, a similarapproach is recommended for other testssuch as the shrinkage test (ASTM C 157) orthe Air Void Analyzer test, all of which tendto have a higher testing variability than theASTM C 39 compressive strength test.

Reference1. Ozyildirim, C., “Effects of Temperature

Development of Low Permeability in Con-cretes,” VTRC 98–R14, Virginia Transporta-tion Research Council, Charlottesville, 1998.

2. Obla, Karthik, and Lobo, Colin, “Experi-mental Case Study Demonstrating theAdvantages of Performance Specifications,”Report to the RMC Research Foundation,Project 04-02, January 2006, 34 pgs.

3. ACI 318-05, “Building Code Require-ments for Structural Concrete,” ACIManual of Concrete Practice, American

Concrete Institute, www.aci-int.org.4. ASTM C 39, “Standard Test Method for

Compressive Strength of Cylindrical Con-crete Specimens,” Annual Book of ASTMStandards, Vol. 4.02, www.astm.org.

5. ASTM C 1202, “Standard Test Methodfor Electrical Indication of Concrete’sAbility to Resist Chloride Ion Penetra-tion,” Annual Book of ASTM Standards,Vol. 4.02, www.astm.org.

6. ASTM C 1556, “Standard Test Methodfor Determining the Apparent ChlorideDiffusion Coefficient of CementitiousMixtures by Bulk Diffusion,” AnnualBook of ASTM Standards, Vol. 4.02,www.astm.org.

7. ASTM C 157, “Standard Test Method forLength Change of Hardened Hydraulic-Cement Mortar and Concrete,” AnnualBook of ASTM Standards, Vol. 4.02,www.astm.org.

8. ACI 214R-02, “Evaluation of StrengthTest Results of Concrete, Part 1,” ACIManual of Concrete Practice, AmericanConcrete Institute, www.aci-int.org. ■

SummaryThis article presents an approach for using

the rapid chloride permeability test, ASTM C1202, for concrete acceptance for permeability.It illustrates that the current approach of reject-ing concrete based on test results that exceedspecification limits is not based on sound sta-tistical principles An approach based on theACI 318 strength test acceptance provisions,modified to take into account the higher test-ing variability of ASTM C 1202, is proposed.The advantages of this approach are: 1) It pro-vides a performance measure of the intendedperformance property for which current w/cmlimits are not that reliable; 2) Good qualityconcrete is not rejected due to the inherentlyhigher testing variability of ASTM C 1202;and 3) The value of the required average per-meability, p c,. is more realistically achievableand avoids high cementitious factors that willbe more prone to cracking.

Also it is noted that a single RCP testresult should be the average of two speci-mens. More care is necessary for initial cur-ing of RCP test specimens in the field and

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