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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

AIR VOID ANALYZER (AVA) FOR FRESH CONCRETE,

LATEST ADVANCES

by

Claus Germann Petersen 1

SYNOPSIS

The background and correlation of the Air Void Analyzer (AVA) for measurement of the specific surface and the spacing factor of fresh concrete’s air entrained bubbles is reviewed. The benefits of the AVA are outlined. The different AVA models are described and the latest designed models, the AVA-2000 and the AVA- 3000 are compared from testing on 44 air-entrained concrete mixes. The testing ranges covered were 14 mm-1

to 44 mm-1 for the specific surface and for the spacing factor 0.11

mm to 0.64 mm. The slump was for all mixes 120 mm ± 30 mm. The AVA-3000 gives the same high degree of correlation with hardened air-void characteristics as does the AVA-2000. For both models the standard error is 1.3 mm-1

on the specific surface and 0.02 mm on the spacing factor under normal on-site conditions. The general conclusion is that the two AVA models provide the same information on the air-void structure of air-entrained concrete for the slump indicated. Keywords: Air Void Analyzer, air-void characteristics, air-void characteristics, AVA, durability, freeze-

thaw, fresh concrete, frost resistant concrete, spacing factor, specific surface.

_________________________ 1).Germann Instruments A/S, Emdrupvej 102, DK-2400 Copenhagen, Denmark

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

Mr. Claus Germann Petersen holds diplomas from the Technical University of Denmark and the Danish University of Business Administration and Economy. He founded the Germann Instruments companies in 1976, and has since been active in the development, marketing and implementation of GI's test systems for reinforced concrete structures. Mr. Petersen has extensive experience in the application of the test systems, and frequently lectures on NDT methods at universities and government or private institutions. He has authored a number of books and papers on the test systems and their applications, and has during the past 20 years received a number of awards for his work in the NDT field. He is a member of ACI committee 228 on Non-Destructive Testing and ASTM C09.64 Non-Destructive and In-Place Testing, as well as the European RILEM Technical Committee on Non-Destructive Testing

________________________________________________________________________

SPACING FACTOR AND SPECIFIC SURFACE

While concrete specifications have traditionally targeted total air content, it is generally recognized that it is the spacing of the voids, rather than total volume that is the relevant factor in providing a durable protection from freezing and thawing. The air-void system protects the paste portion of the concrete by providing relief from hydraulic pressures generated as a result of freezing of water in the capillary pores in the paste. The air voids must be spaced close enough to relieve these pressures. Two derived characteristics have commonly been used to describe air-void systems: spacing factor – the average maximum distance from a point in the paste to the edge of the nearest air void, and specific surface – the ratio of surface area of the air voids to their volume. The specific surface gives an indication of the size of voids, with smaller bubbles yielding higher specific surface values and is useful since a given volume of air distributed among small bubbles will have close spacing. Because it more directly addresses the spacing issue, spacing factor more often is cited as the characteristic to judge frost resistance. In general, for a durable frost resistant concrete, the specific surface has to be greater than 25 mm-1

and the

spacing factor less than 0.20 mm, 1)

, as illustrated in fig. 1.

Fig.1. Durability factor by ASTM C666 / C666M - 03(2008) Standard Test Method for Resistance of Concrete

to Rapid Freezing and Thawing related to the spacing factor of air-entrained concrete 2)

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

SIGNIFICANCE

The required values for the in-place concrete are not necessarily easy to obtain as the performance of any given air-entraining agent is affected by a host of factors during the production, transportation and placing of concrete. Concrete without a proper air-void system will prematurely fail in a freeze-thaw environment provided moist is present in the paste as the ice formed in the capillary pores during freezing will expand and crack the paste. Examples of freeze-thaw deterioration are shown in fig. 2 and 3.

Fig. 2. Freeze-thaw deterioration of longitudinal and transverse joints in a 15 year old concrete road pavement (left)

and of a 20 year bridge edge beam (right). Note the aggregates are not damaged, only the paste.

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

Fig. 3. Centerline joint spalling of a highway after 10 years in service in a freeze-thaw environment due to

improper air void structure in the paste, presence of moist and freeze-thaw cycles 2) .

Note the centerline joint under the bridge is not spalled. Under winter freezing and thawing conditions the

sun does not reach this area to thaw the concrete, hence no freeze-thaw cycles from sunlight and no

deterioration of the joint.

MEASUREMENT

For testing the air content of a fresh mix the commonly used procedures (the pressure meter or the roll-a-meter) only gives information of the total air content in concrete, that is the amount of entrapped air as well as the entrained air. Furthermore, with these procedures no information is given on the factors that directly

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

affect the durability of hardened concrete relative to freeze-thaw cycles, that is the specific surface and the spacing factor of the entrained air bubbles. The approach of only measuring air content (volume of air) has worked well in the past where a fixed relationship between the total air content and the air-void structure was deduced from experience, but with higher requirements for durability, and with changing materials used in concrete, this approach is not adequate for today’s transportation structures. The ASTM C 457-98 “Standard Test Method for Microscopical Determination of Parameters of the Air- Void System in Hardened Concrete” requires cores be removed from the hardened concrete, saw-cut longitudinal in half, polished correctly and analyzed under a microscope by the linear-traverse or the modified point-count method. This procedure leaves coring holes in the structure; it is cumbersome, allows only limited testing to be done and requires minimum two weeks before the test result is available. Furthermore, highly skilled, trained technicians are required for the preparation of the sample and for the ASTM C 457 microscopic analysis. Typically, therefore the ASTM C 457 test is only performed on field samples if durability problem have occurred. To improve and insure concrete quality, better and more rapid test methods are needed for measuring concrete’s in- place properties. These improved tests need to provide relevant information in a timely manner, so that decisions about the materials quality can be made at the job site. Timely test information will allow the contractor to make real time adjustments to the production process, and minimize the risk of placing concrete that does not have adequate durability. One new tool to provide this proactive information about fresh concrete is the Air Void Analyzer (AVA) described in this paper. The AVA (Air Void Analyzer) measures the specific surface and the spacing factors of the entrained air in freshly-placed concrete within half an hour from sampling, providing early assurance of the durability of the cement paste, rapid assessment of the effects of changes in the mixture or equipment or variations on the air-void system from placement conditions such as temperature, slump and pumping. A qualitative assessment can be made immediately concerning the efficiency on the freeze-thaw durability from e.g. types of air-entraining and water-reducing admixtures, dosages, water, sand and aggregate size and grading, mixing time and plant configurations. The AVA sampling does not leave any damage on the surface tested and the AVA technician can do the testing properly after one day of training. The procedure for testing with the AVA is illustrated in fig. 4 and test results are shown in fig. 5 and fig. 6.

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

Fig.4. The basic steps in the Air Void Analyzer (AVA) procedure:

1. Sampling of a 20 cm3 mortar sample in a syringe in a wire cage excluding aggregates >6 mm.

2. Inserting the syringe with the sampled mortar in a riser column with release liquid at the bottom and

de-aerated water above. The mix design parameters are entered in the software and the test is started

after the mortar sample is injected into the release liquid.

3. Stirring of the release liquid containing the sample for 30 seconds causing the bubbles in the mortar

sample to be released and rise through the water in the column.

4. Collecting the bubbles under an inverted Petri disc and recording the buoyancy over a maximum

time of 25 minutes.

5. The software records the buoyancy and calculate the specific surface and the spacing factor of the

entrained air bubble system as well as other air parameters as shown in fig. 5 and fig. 6 for AVA-

2000 and AVA-3000. In addition the bubble classes are available.

.

1 1 2

3 4

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

Fig.5. AVA-2000 test record

Fig.6. AVA-3000 test record

The air content measured by the AVA for cord sizes of bubbles < 2 mm is usually lower than the total air content by the pressure-meter as the AVA only measures the air-entrained bubbles. In an attempt to ensure that concrete used in the construction of pavements and bridges will resist freeze-thaw related damage, the inspecting agency, concrete supplier and contractor must test the concrete being used in the project and make necessary changes in the production process to create the proper air-void system in the concrete structure.

Added in the AVA-3000 software

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

HISTORY OF THE AVA In the late 1980s Mr. Bent Jensen of DBT in Denmark invented and developed a method for obtaining immediate information of the air-void system (the spacing factor and the specific surface) of fresh, still plastic concrete. He released the air bubbles from an extracted mortar sample into a viscous liquid by gently stirring, transferring the bubbles into a water column above the viscous liquid and registering the arrival of the bubbles by buoyancy as a function of time, larger bubbles arriving through the water column earlier than the smaller ones, following Stoke´s Law. In a series of tests he experimented with the sampling procedure, stirrer type and stirring time, the type of release agent, temperature influences, buoyancy recorder operation / sensitivity and the height of the water column to ensure sufficient bubble separation in time to classify the sizes of the released bubbles. The new method labeled the “AVA” was empirically calibrated to ASTM C 457 “Standard Test Method for

Microscopical Determination of Air-Void System in Hardened Concrete” from 1991 to 1994 in Europe 3) 4).

The general conclusion was that the AVA provides air-void characteristics within ±10% of the ASTM C 457’s. Similar calibrations were made in the US in 1996 5)

and 2001 6). The conclusions were that the AVA can provide information characterizing the air-void system and the system is applicable in quality control and assessment of concrete mixes. In 2002 the AVA became a focus technology in the US by the AASHTO Technology Implementation Group (TIG) after Kansas DOT (Kansas Department of Transportation) had demonstrated the benefits of the AVA on a number of highway projects in 2001 and 2002 7), and after it was realized that nearly half of the all highway concrete placed in the US does not have suitable air-void characteristics 8). As a result of that effort a test method was written and is now published by the American Association of State Transportation Officials (AASHTO) as a provisional test method. It is TP 75-08, Standard Method of

Test for Air-Void Characteristics of Freshly Mixed Concrete by Buoyancy Change 9)

A precision statement has been developed in a round-robin study based on testing results from 19 different operators and AVA-1000 / AVA-2000 machines. A paper detailing the testing was published by the Transportation Research Board in 2007 as “Development of Precision Statement for Determining Air Void Characteristics of Fresh Concrete with Use of Air Void Analyzer” 10). In this study the single operator coefficient of variation (COV) was determined to be 15%. The multiple operator COV was determined to be 19%.

BENEFITS OF THE AVA The study by the Kansas Department of Transportation (KDOT) 7) comparing the AVA to the ASTM C 457-98 analysis for monitoring three paving projects over two construction seasons conclude: 1. The AVA equipment is 4-5 times less in initial costs compared to the ASTM C 457 equipment needed (core drill, saw, polishing/lapping equipment and microscope) 2. Daily output:

2.1 AVA: Eight samples for two technicians 2.2 ASTM C 457 analysis: One sample

3. Return time of results: 3.1 AVA: One hour after placement of the concrete 3.2 ASTM C 457 analysis: Two weeks minimum.

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

4. Benefits of improving spacing factor were $ 1,136.000 for future reduced longitudinal joint repair costs up to a 20 year period and an additional 74% saving for transverse joint sealing and partial depth patching in pavement areas with harsh environment.

AVA MODELS FROM 1990s AND ONWARDS

The first developed AVA (AVA-1000) used in the 1990s was sensitive to external influences like temperature, vibrations and wind. Special precautions had to be taken to eliminate these influences when testing was made in the field. Modification in the design of the balance suspension and the electronics was made to overcome this drawback in the newer AVA-2000 model. This was done shortly after Germann Instruments purchased all rights to the AVA system from the developer DBT in Denmark. Further developments made by Germann Instruments between 2005 and 2007 in the design, the electronics and the data acquisition system resulted in the most recent model, the AVA-3000 11). This model also accommodate the latest requirements in the ASTM C 457-06, defining air entrained bubbles as less than 1 mm in cord size, and it is specially designed for testing very stiff mixes as for highway pavement. However, the basic principles, the software and the algorithm used have been left untouched from model to model.

COMPARISON BETWEEN THE AVA-2000 AND THE AVA-3000

Forty concrete mixes were produced over a period of 3 weeks. The mixes had varying types of sand, aggregates, w/c-ratio, air entrainment and dosages. Mixing time was varied as well. The slump for all mixes was 120 mm ± 30 mm. The concrete was poured in wooden moulds with a surface area of 30 cm x 60 cm, 20 cm deep and vibrated uniformly. Two AVA samples were taken 25 cm apart from the top surface and tested by one AVA-2000 and one AVA-3000, outdoor in wind, sun and misty weather conditions on a non-rigid wooden table. The ambient temperature varied between +10 C and +140 C. The sampling and testing was done by two trained technicians.

Fig.7. The AVA-2000 (left) and the AVA-3000 (right) used for the comparative

testing on forty different mixes

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

The air content of the mixes for air bubbles < 2 mm cord size varied between 1.4% and 10.0%. The measured specific surface and the spacing factor for the AVA-2000 and the AVA-3000 of the 40 mixes are compared in fig. 8 and in fig. 9. In addition to the 40 measurements made by Germann Instruments ( ) data from a similar comparison made by Aalborg Portland 12) are included ( ), giving a total of 44 comparisons.

Fig. 8. Specific surface by AVA-2000 and AVA-3000, 44 comparisons

Line of equality

R = 0.97 SAVA-2000 = 1.29 mm

-1

SAVA-3000 = 1.28 mm-1

Specific Surface (mm

-1)

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

CONCLUSION The AVA-2000 and the AVA-3000 were compared over a testing range of 14 mm-1

to 44 mm-1 for the

specific surface and 0.11 mm to 0.64 mm for the spacing factor in 44 tests. The slump was 120 mm ± 30 mm. Measurements taken on samples from the same concrete show that the AVA-3000 gives the same high degree of correlation with the air-void characteristics (the spacing factor and the specific surface) as does the AVA-2000. For both systems the standard error is 1.3 mm-1

on the specific surface and 0.02 mm on the spacing factor under normal on-site conditions. The general conclusion is that the two AVA models provide the same information of the entrained air-void structure of fresh concrete for normal concrete mixes.

REFERENCES

1) AASHTO AVA TIG: “Hardened Concrete Spacing Factor”, internal paper presented 7-22-2003 on TIG meeting, Topeka, Kansas, USA

2) Wojakowski, J.: “Real Air Testing in Real Time”, Kansas DOT, Topeka, Kansas, USA, 2001 3) Brite Euram Project No.: BE-3376-89, Task 2: “Quantitative and Qualitative Determination of the

Air Void Structure in Fresh Concrete”, March 1991-February 1994

Spacing Factor (mm)

Line of equality

R = 0.99 SAVA-2000 = 0.02 mm SAVA-3000 = 0.02 mm

Fig. 9. Spacing factor by AVA-2000 and AVA-3000, 44 comparisons

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Ninth ACI International Conference on Superplasticizers and Other Chemical Admixtures in

Concrete – Sevilla, Spain, October 13-17, 2009

4) Henrichsen A. & Vyncke: “Quality Assurance of Air Void Structures in Concrete”, International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin, Germany, 1995

5) Magura, D.D.: “Air Void Analyzer Evaluation”, FHWA-SA-96-062; Federal Highway Administration, Washington DC, USA, 1996

6) McReynolds, R.L.: “Air Void Analyzer, Real Air Testing in Real Time”, Kansas DOT, Topeka, Kansas, USA, 2002

7) Wojakowski, J. & Distlehorst, J.A.: “Air Void Analyzer Benefits: An example from KDOT”, 2003, Kansas Department of Transportation, Topeka, Kansas, USA

8) AASHTO: “Air Void Analyzer (AVA) – An apparatus that measures the air-void characteristics of fresh concrete – A 2002 Focus Technology”, Washington DC, USA, 2002

9) AASHTO: “Standard Method of Test for Air-Void Characteristics of Freshly Mixed Concrete by Buoyancy Change”, AASHTO Designation: TP 75-08, Washington DC, USA, 2008

10) Distlehorst. J.A. & Kurgan, G.J.: “Development of Precision Statement for Determining Air Void Characteristics of Fresh Concrete with Use of Air Void Analyzer” , Transportation Recearch Record: Journal of the Transportation Research Board, ISSN 0361-1981, Volume 2020/2007, Transportation Research Board of the National Academies, USA

11) Germann Instruments: “AVA Models between year 1986 and 2007”, Copenhagen, Denmark, 2007 12) Kristensen, L.F.: “Sammenligning af Unicons og PTEs Air Void Analyzer”, Aalborg Portland

A/S, Product Technology (PTE), Aalborg, Denmark, 2009.