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Publication Number: FHWA-RD-01-164
Date: March 2002

Section 2

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Summary on Laboratory Testing

The diagnosis of MRD in portland cement concrete (PCC) often requires the use of various laboratory procedures for identifying the extent and mechanism of distress. These laboratory tests do not always lead to absolute characterization of the distress for a variety of reasons. First, most cases of concrete distress occur as the result of multiple distress mechanisms. As a result, it is often difficult to isolate the specific cause of failure or even determine the principal cause of failure. Second, the laboratory methods commonly used often provide results that may be interpreted differently, depending upon the motivation or objectivity of the analyst. To minimize the latter case of misinterpretation, it is advised that a thorough, complete examination of the concrete be performed using the data collection forms and approach described.

Clearly the most useful tools for examining concrete are the stereo OM and the petrographic OM and/or the SEM. There is a significant body of technical information available discussing OM observations of concrete and concrete distress. This information is useful as a benchmark when evaluating a specific concrete specimen. The SEM is quickly becoming an equally valuable tool for evaluating concrete. It is advised that anyone charged with the examination of MRD in concrete become familiar with this equipment and the capabilities it offers. Most notable of these is the ability to perform a chemical analysis of phases within the concrete. This allows for the absolute identification of reaction and hydration products, greatly facilitating MRD identification

Finally, the general approach of “asking the materials questions” must be followed. As is discussed in the next section, it is often the case that a process of elimination is required to determine what distress is not present, thereby leading to the short list of possible distress mechanisms. Also, it is important to remember that the concrete as observed may have undergone a significant metamorphosis over its service life and the degradation seen may be the final product of years of exposure. It is only through the careful and methodical application of the described laboratory methods that the true cause of distress may be identified.

DATA ANALYSIS AND INTERPRETATION

The interpretation and diagnosis of MRD relies primarily on information collected during laboratory investigation, supplemented with information collected during the review of the records and visual assessment of the pavement surface. When diagnosing a concrete distress, often there is no clear answer as to which distress mechanism caused the failure as multiple mechanisms are observed. This makes it difficult to determine which mechanism(s) might be responsible for the initial deterioration versus those that occurred after the fact as opportunistic distress mechanisms. Various types of distress mechanisms can occur simultaneously in concrete and each can incrementally contribute to the ultimate failure of the material. This fact must be taken into account when evaluating MRD in concrete pavements.

In approaching laboratory diagnosis of MRD, the analyst must put aside preconceived notions as to what the MRD might be. Instead, diagnosis should be approached through systematic data collection, linked to a process of elimination. A general philosophy of "asking the material questions" must be adopted where the analyst determines which diagnostic features are identifiable within the concrete. For example; “Are there microstructure features indicating AAR?” or “Is the air-void system adequate for the concrete service conditions?” After examining the concrete and noting all available information, the analyst can only make an educated judgment as to why the material failed. In some cases, there will be a clear cause while in other cases there may be multiple mechanisms at work making it difficult to determine precisely which factor is primarily responsible.

Description of the Data Interpretation Flowcharts

The purpose of this section is to outline the procedure for gathering and interpreting data for identifying MRD in concrete pavements. The proposed method uses the flowcharts and diagnostic features to guide the analyst toward diagnosis. The flowcharts present a systematic method for diagnosing MRD in concrete pavements. The analyst inspects the concrete using the methods described previously, being guided by the hierarchy of questions presented in the flowcharts. The responses to the questions presented in the flowcharts determines what analytical procedures will be performed. As the analyst moves through the flowcharts, there is the potential for more than one MRD being identified. The analyst keeps track of all possibilities identified by checking the appropriate response in the table at the top of the flowchart. Tables presenting the diagnostic features are then consulted to help isolate the most likely MRD(s).

Flowchart for Assessing the Likelihood of MRD

As discussed, figure I-14 is used to evaluate the likelihood of an MRD causing the observed distress. This flowchart, which should already have been completed, uses information gathered during field observations to determine if a laboratory evaluation is warranted. However, it is worthwhile for the analyst to ask the same questions and/or review the relevant data to understand the nature and scope of the distress. The questions asked are based upon visual inspection, with a positive answer to any question resulting in the diagnosis of a possible MRD necessitating further study.

Flowchart for Visual Inspection

Next, the analyst needs to perform a visual inspection of the concrete to assess its general properties. This may be done on the as-received cores, on broken pieces, and/or on polished slabs. The questions to be asked are presented in figure II-4. In most cases, visual inspection helps build a hypothesis of failure, but does not itself directly lead to the cause of distress. Most of the observations will be confirmed at a later stage using OM or SEM. However, there are some general conditions, with visual diagnostic features, known to contribute to, or directly cause, distress. The process to identify these conditions is illustrated in figure II-15 and is described as follows:

Flowchart for Analysis of the Paste and Air

After completing the visual inspection, the concrete is examined with the stereo OM and/or staining techniques are performed. In this process, data on the general condition of the concrete are collected using figures II-5 and II-8. Having completed this analysis, the analyst can begin the process of assessing the condition of the paste and air system. Quantitative measurements of the air system parameters should be obtained using methods consistent with ASTM C 457 and the results entered in figure II-7. In figure II-16, the process for analyzing the paste and air fraction of the concrete is shown. To complete this flowchart it may be necessary to use the petrographic OM, SEM, chemical analysis, or XRD. The results of these analyses are recorded in figures II-7 through II-14, as appropriate. The process to evaluate the paste and air is illustrated in figure II-16 and is described as follows:

Possible Distress

Present

Additional Information

Error in mix proportioning

Yes

No

See recommended literature

Poor placement

Yes

No

See recommended literature

Poor finishing/curing

Yes

No

See recommended literature

Poor steel placement

Yes

No

See recommended literature

Carbonation at depths > 5-10 mm

Yes

No

See recommended literature


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Alternative Text for Figure II-15

Figure II-15. Flowchart for assessing general concrete properties based on visual examination.


Possible Distress

Present

Additional Information

Shrinkage cracks or sample preparation cracks

Yes

No

See recommended literature

Corrosion of embedded steel

Yes

No

Table II-1

Paste freeze-thaw

Yes

No

Table II-2

Aggregate freeze-thaw

Yes

No

Table II-3

Sulfate attack

Yes

No

Table II-4

Deicer attack

Yes

No

Table II-5

Infilling material

Yes

No

Figure II-17


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Alternative Text for Figure II-16

Figure II-16. Flowchart for assessing the condition of the concrete paste and air.


Flowchart for Analysis of the Aggregate Structure

After completing the visual inspection, the concrete is examined with the stereo OM. In this process, general data on the condition of the aggregates are recorded using tables II-5 and II-8.


Possible Distress

Present

Additional Information

Corrosion of embedded steel

Yes

No

Table II-1

Sulfate attack

Yes

No

Table II-4

Deicer attack

Yes

No

Table II-5

Alkali–silica reaction

Yes

No

Table II-6

Alkali–carbonate reaction

Yes

No

Table II-7

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Alternative Text for Figure II-17

Figure II-17. Flowchart for identifying infilling materials in cracks and voids.


Having completed these analyses, the analyst can begin the process of assessing the condition of the aggregate structure. Aggregate quality is fundamental to concrete performance and any degradation of the aggregates should be closely scrutinized. To complete this process it may be necessary to use the petrographic OM, SEM, chemical analysis, and/or XRD. Record the results of these analyses in figures II-7 through II-14, as appropriate. In figure II-18, the process for examining the aggregate fraction of the concrete is presented.

Any cracks resulting from distress may only be apparent in selected cores (e.g., cores A and B from near the joint/crack)


Possible Distress

Present

Additional Information

Natural cracking of aggregate

Yes

No

See recommended literature

Sample preparation cracks

Yes

No

See recommended literature

Aggregate freeze-thaw

Yes

No

Table II-3

Natural weathering of aggregates

Yes

No

See recommended literature

Alkali–silica reaction

Yes

No

Table II-6

Alkali–carbonate reaction

Yes

No

Table II-7

Infilling material

Yes

No

Figure II-17


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Alternative Text for Figure II-18

Figure II-18. Flowchart for assessing the condition of the concrete aggregates.

Flowchart for Analysis of the Secondary Deposits

In figure II-17, the procedure for identifying infilling material is presented. The identification is accomplished by using staining techniques, petrography, and/or x-ray microanalysis using the scanning electron microscope. The identification is fairly straightforward with staining techniques yielding a yes/no answer and x-ray microanalysis yielding a full chemical analysis that can be used to identify the specific material. Optical petrography can also be used, but it is more difficult than the other methods as the optical properties are more difficult to discern. ASTM C 856 includes a complete table of common minerals found in PCC and their optical properties. Regardless of the composition of the infilling material, the analyst must do two things to confirm the MRD type based upon identification of the infilling material.

Use of the Diagnostic Feature Tables

Tables II-1 through II-7 summarize the principal, common, diagnostic features for each MRD, as characterized by laboratory methods and as seen during field evaluation. Included in these tables are the diagnostic features, methods of characterization, and specific comments relative to either the observed characteristic or the test method. The tables are intended to serve as a quick reference during the diagnosis of MRD by assisting the analyst in identifying other diagnostic features to help confirm the presence of a particular MRD. The tables also serve to tie together the results of visual inspection performed in the field, with laboratory inspection and diagnosis.

While performing the laboratory analysis, the analyst will be guided by the flowcharts presented in figures II-15 through II-18. The flowcharts will be asking yes/no type questions about laboratory observations that are not always definitive. Usually, each question will focus on one diagnostic feature, the answer of which cannot be used alone to positively identify a specific MRD as the cause of distress. To confirm the identification, other diagnostic features need to be investigated and confirmed. At this point, the analyst should use the diagnostic tables presented in tables II-1 through II-7 to see what he/she should look for to help confirm the suspected MRD. It may be possible to perform the confirmation immediately, but in many cases, different samples must be made or techniques employed (e.g., thin sections, petrographic OM, etc.). In the latter case, it is recommended that the analyst proceed completely through the flowcharts once before going back to confirm specific MRD types. As other possible MRD types are identified, consult the diagnostic tables to determine other possible confirming diagnostic features. Note on the flowcharts which MRD types have been indicated as possible by circling yes or no in the table included at the top of each flowchart.

Table II-1. Diagnostic features of corrosion of embedded steel.

Diagnostic Feature

Method of Characterization

Comments

Spalling and delamination of concrete over reinforcing steel

Field Evaluation
Visual Inspection

Visual inspections can be used to readily identify areas affected by corrosion of embedded steel. It is characterized by rusting steel at the bottom of the spalled out area and rust stains on the loose pieces.

Visible corrosion products

Field Evaluation
Visual Inspection
Stereo OM
Petrographic OM

The “rust” seen may contain crystalline magnetite but is primarily amorphous.

Continuing through the flowcharts before going back to confirm the first MRD identified accomplishes two things. First, it helps remove the tunnel vision of looking at only one possible MRD. Second, moving completely through the flowcharts will identify questions that require additional techniques to answer. Overall, this process helps to identify 1) what questions need to be answered, 2) what techniques will be required, and 3) what samples will be needed. After completing the required analyses, the analysts should go through the flowcharts one more time, isolating the most probable MRD(s).

In the end, it is not always possible to identify a single MRD as the cause of the observed distress. This conclusion should not be viewed negatively, but instead as a recognition that on many occasions more than one MRD may be active in a distressed concrete pavement, making absolute identification of the primary distress mechanism difficult or impossible. In such cases, the analyst should list all possible MRD mechanisms and assign a relative rating as to the likeliness of each being responsible for the observed distress. The rating scheme should be simple and subjective, possibly along the lines of a scale ranging from highly unlikely, unlikely, possible, probable, highly probable. In this way, the analyst can present to the engineers and other interested personnel what they think is the most likely cause(s) of distress while still presenting all possibilities. In the end, this will help focus the repair/rehabilitation efforts and preventative strategies for future construction without turning a blind eye to other possible causes.

SUMMARY OF DATA ANALYSIS AND INTERPRETATION

This second guideline presents laboratory procedures and an approach to diagnosing materials-related distress in concrete pavements. It is emphasized that trained, skilled personnel are needed to execute the recommended laboratory procedures, carry out the data collection function, and draw conclusions from the data collected. It is also emphasized that these guidelines provide only a framework for analysis and interpretation, and that additional sources of information need to be consulted in the course of an evaluation. And finally, it is hoped that the use of these guidelines will enhance communication between the analyst (typically a petrographer) and the field/design engineer, so that each can develop a better understanding of the other’s needs and expectations regarding the strengths and limitations of such an investigation.

Table II-2. Diagnostic features of paste freeze-thaw damage.

Diagnostic
Feature

Method of Characterization

Comments

Surface scaling or subparallel cracking

Field evaluation
Visual

Look for loss of paste at road surface, exposed coarse aggregate, and/or scaling. Can be isolated to surface or through slab depth.

Stereo OM
Petrographic OM

Look for delamination/ subparallel cracking at surface or evidence of an overworked surface such as decreased air content at the surface.

Inadequate air- void system

Stereo OM

Measure air-void parameters consistent with ASTM Method C 457. Typical parameters for good concrete are as follows:

Spacing Factor (symbol ) = 0.1 - 0.2 mm
Maximum Spacing ( symbol ) = 0.2 mm
Typical Specific Surface (a) = 23.6 - 43.3 mm-1
Paste/Air Ratio - 4 - 10

The air-void size distribution should also be noted as different size distributions can yield similar values of symbol and a.

Secondary deposits filling air voids

Staining

Deposits stained cannot be analyzed by any other method to determine their composition.

Stereo OM

Deposits result as water freely moves through the distressed paste. In extreme cases, the air-void system may be further compromised when significant numbers of voids are filled with secondary deposits. Common deposits include calcium hydroxide, calcium carbonate, ASR reaction products, and various sulfates including ettringite.

Petrographic OM

Secondary deposits are commonly identified using the petrographic OM.

SEM

The SEM operated at high vacuum is very useful for determining the composition of secondary deposits. Direct output of phase composition allows for absolute identification.

Microcracking around aggregates

Stereo OM
Petrographic OM

Cracking will be in the paste. If cracks pass through aggregates, check table II-3 for coarse aggregate freeze-thaw, table II-6 for ASR, and table II-7 for ACR. Cracks will fill with secondary deposits.

LVSEM

Severe cracking in the paste occurs if PCC is observed in a CSEM. Hydration products and ASR reaction products dehydrate in the high vacuum in the CSEM.

 

Table II-3. Diagnostic features of aggregate freeze-thaw deterioration.

Diagnostic
Feature

Method of Characterization

Comments

Cracking near joints/cracks

Staining/Spalling

Field evaluation

Has a very characteristic cracking pattern concentrated at corners, joints, and cracks (SHRP 1993).

Increased permeability results from the cracking. Calcium hydroxide is leached and re-deposits on surface where it carbonates.

Cracks through
nonreactive coarse aggregates

Visual inspection
Stereo OM
Petrographic OM
SEM

Cracks through nonreactive coarse aggregates are very typical of D-cracking. Be very careful to completely rule out alkali–aggregate reaction. See tables II-6 and II-7 for diagnostic features of ASR or ACR.

Nonuniform gaps around coarse aggregates

Visual inspection
Stereo OM

Gaps between the aggregate and paste form. These gaps may result from the dissolution of calcium hydroxide at the aggregate/paste interface or coarse aggregate dilation due to freezing. Subsequent redeposition of calcium hydroxide or calcite may occur in the cracks. Check for sulfate or ASR reaction products in cracks surrounding aggregates. A negative result helps to confirm aggregate freeze-thaw.

Known freeze-thaw susceptible aggregate

Large top size aggregate

Records review
Visual inspection
Stereo OM
SEM

Check aggregate sources for known freeze-thaw performance

Aggregate freeze-thaw is more common in large aggregates (> 38 mm) and rare in aggregates smaller than 12.5 mm.

Poor void structure in the aggregate

Petrographic OM
CSEM
LVSEM

As a percentage of the total aggregate void space, excessive amounts of voids in the aggregate with diameters less than 5 microns is thought to be detrimental to aggregate freeze-thaw resistance.

 


Table II-4. Diagnostic features of sulfate attack.

Diagnostic Feature

Method of Characterization

Comments

Map cracking

Field evaluation

Paste expansion commonly results in map cracking over entire surface. In some cases, it is isolated to joints/cracks.

Deteriorated paste

Sulfate attack may result in paste “crumbling,” commonly at joints. Loose aggregate observed in resulting void.

External source

of sulfur only

Soil analysis
Deicer analysis

Identify a sole source of sulfate that is external to the concrete to confirm external sulfate attack. Having both external and internal sources confounds the diagnosis.

Internal source

of sulfur only

Records review SEM
XRD

Identify a sole source of sulfate that is internal to the concrete to confirm internal sulfate attack. Having both external and internal sources confounds the diagnosis.

Paste expansion

Stereo OM
Petrographic OM
SEM

Expansion occurs, usually over a large area. Gaps form around aggregates with the gap width proportional to the aggregate diameter.

Significant sulfate deposits in cracks and voids

Staining
Stereo OM

Stained deposits or stained paste cannot be accurately analyzed by SEM to determine their composition. A common sulfate deposit is ettringite. This is commonly recognized by acicular needle-like crystals infilling voids and cracks.

Petrographic OM

Common sulfate deposits can be identified. Mixtures with other phases may be more difficult to identify.

SEM

All deposits are readily identified using the SEM in high vacuum mode. Co-deposition with other phases may be more closely studied.

Significant sulfate deposits in the cement paste

Petrographic OM

Fluorescent dye epoxy impregnation assists in identifying microcracks in the paste. Cracks unfilled with epoxy should be assumed were created during sample polishing.

LVSEM/ESEM

Cracks resulting from sulfate expansion can be viewed in an LVSEM/ESEM but caution should be exercised in identification of micron scale microcracks. Even in an ESEM, some dehydration does occur, leading to possible cracking. Cracks unfilled with epoxy should be assumed were created in the SEM or during sample polishing. If viewing an unimpregnated specimen, cracks unfilled with secondary deposits should be assumed were created in the SEM or during sample preparation.

Microcracking

Stereo OM
Petrographic OM

For filled cracks, the cracks may have been present from other distress and secondary deposits formed in the cracks. Fluorescent dye epoxy impregnation greatly improves the identification of microcracks in the paste. Cracks not filled with epoxy are probably artifacts of sample preparation.

SEM

A characteristic spectrum for dehydrated ettringite has approximate element ratios of 1:2:4 (Al:S:Ca) by weight.

Table II-5. Diagnostic features for deicer scaling/deterioration.

Diagnostic Feature

Method of Characterization

Comments

Staining at joints or cracks

Field evaluation

Staining results from calcium hydroxide depletion and subsequent carbonation at surface.

Scaling or crazing of slab surface

Field evaluation
Visual inspection
Stereo OM

Common visual diagnostic feature. Similar and possibly related to paste freeze-thaw damage. See table II-2 for more on paste freeze-thaw damage.

Calcium hydroxide depletion near joints

Stereo OM
Petrographic OM
SEM

Calcium hydroxide (CH) is most soluble near the freezing point of water. Cyclic freezing and thawing from repeated deicer applications can accelerate the dissolution of CH near joints/cracks.

Secondary deposits of chloroaluminates

Petrographic OM
SEM

Chloride ions released from dissolved salts can form these phases with aluminate phases in the paste.

 

The procedures presented are intended to lead the analyst through identification of common concrete pavement MRD types based upon the consideration of typical diagnostic features. Although one MRD may be present, the flowcharts have the analyst examine diagnostic features for all MRD types, thereby minimizing the probability of prematurely "zeroing in" on a single MRD without considering all possibilities. The flowcharts do not address every possible combination of MRD nor do they address every set of possible diagnostic features that may be seen in distressed concrete. It should be understood that the mechanisms responsible for MRD are complex and may manifest themselves differently under different conditions and the exact nature of a given distress may vary.

It is very common to observe multiple distress mechanisms at work when examining distressed concrete. When multiple distresses are present, it is clearly more difficult to diagnose an absolute cause of failure and the analyst should be cautious when trying to do so. Instead all possibilities should be presented with the analyst using a subjective ranking as to the likelihood of each. Such an attempt will almost invariably require some assumptions of the order, magnitude, or cause of the earliest failures. Likewise, analysts and engineers are cautioned from trying to diagnose MRD based on laboratory results alone. However, a complete laboratory analysis, when combined with field evaluation data and construction and service records, will help accurately describe the condition, environment, and performance of the concrete pavement in question. From this type of broad-based evaluation of a MRD problem, conclusions about the source of distress can often be reached.


Table II-6. Diagnostic features of ASR.

Diagnostic Feature

Method of Characterization

Comments

Map cracking with or without exudate

Evidence of pavement expansion

Visual inspection

ASR is characterized by widespread map cracking. Can be more severe at joints and may be preferentially oriented perpendicular to the direction of least restraint (e.g., in pavement slabs, longitudinal cracks often predominate). Exudate common but not always observed.

Evidence of joint closing or shoving of shoulder or fixed structures are possible indicators of expansion.

ASR reaction product in cracks and voids

Stereo OM
Petrographic OM

A glassy clear to white amorphous reaction product resulting from an alkali–silica reaction. ASR reaction product is found within reacted particles, cracks, and air voids. The presence of ASR reaction product alone does not indicate ASR distress, as it must be of sufficient volume and composition to cause deleterious expansion.

SEM

ASR reaction product can be chemically characterized with the SEM operating at a high vacuum. Primarily high alkali (low calcium) ASR reaction products are expansive.

Reaction rims on aggregates

Visual inspection
Stereo OM
Petrographic OM
SEM

Reaction rims are often seen on most reactive aggregate. Reaction rims are common on aggregates that are undergoing ASR. Good gravel aggregates can exhibit rims that appear similar to ASR reaction rims. These are typically the result of weathering. Reaction products present help confirm ASR.

Aggregate known to be reactive

Records review

Check to see if the aggregates used were from a source that is known to be reactive.

Microcracking radiating from reacted cracked aggregates

Softening of the aggregate

Visual inspection
Stereo OM
Petrographic OM
LVSEM/ESEM

Reacted aggregates may break down internally and often partially dissolve. As the aggregate degrades, the ASR reaction product produced may be expansive and cause cracking to occur. The cracks are within the periphery of the aggregate but around the center. Often the cracks will narrow from the center of the aggregate out. Coarse and fine aggregates can both cause ASR distress. Common reactive aggregates are composed of or include chert, flint, siliceous shale, strained quartz, and porous volcanic glasses.


Table II-7. Diagnostic features of ACR.

Diagnostic Feature

Method of Characterization

Comments

Map Cracking with or without exudate
Evidence of expansion

Visual inspection

ACR is characterized by widespread map cracking. Can be more severe at joints. Exudate common but not always observed.

Evidence of joint closing or shoving are possible indicators of expansion.

Cracks radiating from the coarse carbonate aggregate into the paste

Visual inspection
Stereo OM
Petrographic OM
LVSEM

Deterioration from ACR results from the expansion of the aggregate that causes cracks in the aggregate, which propagate into the paste. The expansion is the result of reaction products produced in the dedolomization reaction.

Aggregate known to be reactive

Records review

Check to see if the aggregates used were from a source that is known to be reactive.

Characteristic texture of ACR reactive aggregates

Petrographic OM

Most ACR reactive aggregates have a characteristic texture. ASTM C 856 states the basic texture as being relatively larger rhombic dolomite crystals in a fine-grained calcite matrix with clay and silt-sized quartz. Substantial amounts of both dolomite and calcite are present. Other textures have been reported as reacting, with a common thread being soluble magnesium phases that react to form expansive products.

CSEM
LVSEM

Calcium and magnesium silicate hydrates are common reaction products.

 

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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