Contributions to Industrial-Minerals Research

U.S. Department of the Interior

U.S. Geological Survey

Bulletin 2209–K

Progress in the Evaluation of Alkali-Aggregate 

Reaction in Concrete Construction in the Pacific 

Northwest, United States and Canada

Chapter K of
Contributions to Industrial-Minerals Research

James D. Bliss, Phillip R. Moyle, and Keith R. Long, Editors

Bulletin 2209–K

U.S. Department of the Interior

U.S. Geological Survey 

Progress in the Evaluation of Alkali-Aggregate 

Reaction in Concrete Construction in the Pacific 

Northwest, United States and Canada

By Fred H. Shrimer

Chapter K of
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

This report and any updates to it are available online at:

http://pubs.usgs.gov/bul/b2209-k/

For additional information write to:

U.S. Geological Survey

Box 25046, Mail Stop 421, Denver Federal Center

Denver, CO 80225-0046

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, it may contain copyrighted materials that are noted in the text. Permission

to reproduce those items must be secured from the individual copyright owners.

Cataloging-in-Publication data are on file with the Library of Congress

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, March 2, 2005

Text edited by George A. Havach

Layout and design by Stephen L. Scott 
Contents

Abstract---------------------------------------------------------------------------------------------------		 	 1

Introduction-----------------------------------------------------------------------------------------------		 	 1

Background-----------------------------------------------------------------------------------------------		 	 4

Control Specifications and Testing for Alkali-Aggregate Reaction--------------------------------		 	 4

Geology and Reactivity of Aggregates----------------------------------------------------------------		 	 5

Alkali-Aggregate Reaction in Concrete in the Pacific Northwest---------------------------------	 	 	 7

Testing the Alkali-Aggregate-Reaction Potential of Pacific Northwest Aggregates------------		 	 8

Recent Alkali-Aggregate-Reaction Test Data--------------------------------------------------------		 	 9

Summary--------------------------------------------------------------------------------------------------		 10

Recommendations---------------------------------------------------------------------------------------		 10

References Cited-----------------------------------------------------------------------------------------		 11

Figures

	 	 1. Scanning electron photomicrograph of an air void in concrete filled with

	 	 	 	 	 alkali-silica gel--------------------------------------------------------------------------------	 	 2

	 2–6. 		 Photographs of:

	 	 	 	 	 2. Concrete column in Stanley Park, Vancouver, British Columbia, showing

	 	 	 	 	 	 	 typical “map” cracking throughout entire column, induced by expansion

	 	 	 	 	 	 	 of concrete due to swelling of alkali-silica gel---------------------------------------		 	 2

	 	 	 	 	 3. Pier in Seattle harbor, showing well-defined “map” cracking induced by

	 	 	 	 	 	 	 alkali-aggregate reaction---------------------------------------------------------------		 	 3

	 	 	 	 	 4. Wall in Stanley Park, Vancouver, British Columbia, showing white deposits

	 	 	 	 	 	 	 of leachate emanating from cracks in concrete-------------------------------------		 	 3

	 	 	 	 	 5. Old Alexandra Bridge in Fraser Canyon, near Boston Bar, British Columbia

	 	 	 	 	 	 	 Fraser Canyon, B.C., with both alkali-aggregate-reaction-induced and

	 	 	 	 	 	 	 freeze-thaw damage in its piers and abutments-------------------------------------		 	 4

	 	 	 	 	 6. Part of a dock structure in Vancouver, British Columbia, showing “map”

	 	 	 	 	 	 	 cracking-----------------------------------------------------------------------------------	 	 4

	 7–9. 		 Photomicrographs of:

	 	 	 	 	 7. Polished section of a core drilled from alkali-aggregate-reaction-affected

	 	 	 	 	 	 	 concrete in a dam in southern British Columbia, showing “reaction rims”------		 	 5

	 	 	 	 	 8. Fracture surface in a sample of core drilled from alkali-aggregate-reaction-

	 	 	 	 	 	 	 affected concrete, showing dried, white gel patches on several

	 	 	 	 	 	 	 aggregate particles----------------------------------------------------------------------	 	 5

	 	 	 	 	 9. Polished specimen of concrete, showing cracking induced by pressure

	 	 	 	 	 	 	 from swelling alkali-silica gel----------------------------------------------------------		 	 5

10–13. 	Photographs of:

	 	 	 	 10. Section of a wall in a dam in southern British Columbia, showing well-

	 	 	 	 	 	 	 developed map cracking and efflorescence, both indicators of alkali-silica

	 	 	 	 	 	 	 reaction-----------------------------------------------------------------------------------		 	 8

	 	 	 	 11. Manette Bridge in Kitsap County, Wash., which was diagnosed with

	 	 	 	 	 	 	 alkali-silica reaction in its piers and abutments-------------------------------------		 	 8

	 	 	 	 12. Monroe Street Bridge in Spokane, Wash., undergoing rehabilitation work

	 	 	 	 	 	 	 in fall 2003---------------------------------------------------------------------------------	 	 8

	 	 	 	 13. Closeup of the Monroe Street Bridge in Spokane, Wash., showing section

	 	 	 	 	 	 	 of the main arch--------------------------------------------------------------------------	 	 8

	 	 	 	 14. Photomicrograph of polished sample of concrete from the Monroe Street

	 	 	 	 	 	 	 Bridge in Spokane, Wash., showing reaction rims on aggregate particles------		 	 8

Table

1. Summary of alkali-aggregate reaction test data for selected aggregates in the

	 	 	 	 	 Pacific Northwest, United States and Canada-------------------------------------------		 10 III


Abstract

The supply of aggregates suitable for use in construction 

and maintenance of infrastructure in western North America 

is a continuing concern to the engineering and resources-management community. Steady population growth throughout 

the region has fueled demand for high-quality aggregates, in 

the face of rapid depletion of existing aggregate resources 

and slow and difficult permitting of new sources of traditional 

aggregate types. In addition to these challenges, the requirement for aggregates to meet various engineering standards 

continues to increase.

In addition to their physical-mechanical properties, other 

performance characteristics of construction aggregates specifically depend on their mineralogy and texture. These properties 

can result in deleterious chemical reactions when aggregate is 

used in concrete mixes. When this chemical reaction—termed 

“alkali-aggregate reaction” (AAR)—occurs, it can pose a 

major problem for concrete structures, reducing their service 

life and requiring expensive repair or even replacement of 

the concrete. AAR is thus to be avoided in order to promote 

the longevity of concrete structures and to ensure that public 

moneys invested in infrastructure are well spent.

Because the AAR phenomenon is directly related to the 

mineral composition, texture, and petrogenesis of the rock particles that make up aggregates, an understanding of the relation between the geology and the performance of aggregates 

in concrete is important. In the Pacific Northwest, some aggregates have a moderate to high AAR potential, but many others 

have no or only a low AAR potential. Overall, AAR is not as 

widespread or serious a problem in the Pacific Northwest as in 

other regions of North America.

The identification of reactive aggregates in the Pacific 

Northwest and the accurate prediction of their behavior in 

concrete continue to present challenges for the assessment and 

management of geologic resources to the owners and operators of pits and quarries and to the users of the concrete aggregates mined from these deposits. This situation is complicated 

by the length of time typically required for AAR to become 

noticeable in concrete construction in the Pacific Northwest, 

commonly on such a scale that other deterioration mechanisms 

may have masked the effects of AAR. Distinguishing between 

the effects of AAR and those related to other problems in 

concrete is important for understanding the nature and severity of AAR throughout the Pacific Northwest. Furthermore, 

developing an understanding of the extent of the problem will 

assist efforts to maximize the intelligent and stewardly use of 

aggregate resources in the Pacific Northwest.

This chapter illustrates the current “‘state of the art” 

of AAR studies in the Pacific Northwest, a region with a 

common geologic heritage as well as many distinct geologic 

elements. The optimal use of aggregates in the construction of 

concrete structures that will achieve their design life is possible through an understanding of the engineering and geologic 

properties of these aggregates and of their geologic setting.

Introduction

This chapter has a twofold focus: (1) to review current 

research on the evaluation of alkali-aggregate reaction (AAR) 

in concrete structures from Alaska to Oregon (the “Pacific 

Northwest”), and (2) to assess AAR test data for aggregates 

throughout this region. I briefly appraise the problems and 

trends in AAR research in the Pacific Northwest region, 

together with a discussion of test methods, the specification 

limits applied throughout the region, and the various solutions 

that are available for managing AAR in concrete.

AAR is a chemical reaction that occurs in portlandcement concrete, involving alkalis that are present in the 

pore solutions of concrete and certain mineral phases in the 

aggregates with which the concrete is made. The typical pH 

of pore solutions in concrete is about 12 to 13. Several minerals are more soluble at a pH of more than 9 to 10 and thus are 

susceptible to attack by alkalis; most of these minerals are 

forms of quartz, or silica-rich minerals, although some carbonate species, zeolites, and certain clay minerals have also been 

reported to be alkali reactive. Depending on which minerals 

are involved, AAR is subdivided into alkali-silica reaction 

(ASR) or alkali-carbonate reaction (ACR). Worldwide, ASR 

is the more common form of AAR. No ACR occurrences have 

been reported in the Pacific Northwest.

The most AAR susceptible forms of silica are generally 

those with a more disordered crystalline structure, including 

Progress in the Evaluation of Alkali-Aggregate 

Reaction in Concrete Construction in the Pacific 

Northwest, United States and Canada

By Fred H. Shrimer1

_____________________

1

Golder Associates, Vancouver, British Columbia, Canada.

Introduction
 Contributions to Industrial-Minerals Research

chalcedony, flint, chert, opal, tridymite, cristobalite, strained 

quartz, fragmented/sheared/shredded quartz, quartz overgrowths, and quartz cement. In addition, other silicate minerals, 

such as micas and clays, and some hydrous silicates, such as 

zeolites, have also been reported as AAR susceptible. Feldspar, 

the most abundant rock-forming silicate mineral, has not been 

reported as alkali reactive in concrete. Quartz-deficient rocks 

generally are not alkali reactive, except some carbonate rocks.

Analysis by stereobinocular microscope or thin section alone, however, may be inadequate to characterize the 

AAR potential of some rocks. For example, Shrimer (1996) 

described a mafic rock, considered to be quartz deficient on 

the basis of thin-section petrographic analysis, that was determined to be alkali reactive. Further analyses using scanning 

electron microscopy (SEM) and X-ray analysis determined 

that an extremely fine grained quartz phase was present in the 

rock (a metadiabase), as a result of metamorphism. This finegrained, nearly undetectable quartz proved to be significantly 

reactive in the diabase aggregate.

Typically, AAR occurs in concrete between a reactive 

silica (or silicate) mineral and alkali compounds derived from 

the cement, producing an alkali-silica gel (fig. 1) that absorbs 

water from the concrete pores and subsequently swells. Swelling of the alkali-silica gel generates tensile stresses in the concrete that, if sufficiently great, may cause cracking. In other 

words, if enough reactive aggregate, alkalis, and moisture are 

present in concrete, expansion and cracking of the concrete 

may occur, affecting its serviceability or structural capacity. 

Features associated with AAR include cracking of concrete 

(figs. 2, 3) and leaching or “efflorescence” of carbonate and 

alkali-silica gel out of concrete through cracks (fig. 4).

The typical timeframe for AAR is measured in years, 

rather than days or months. Some strong reactions can cause 

noticeable, even serious, damage in concrete structures 

Figure 1. Scanning electron photomicrograph of air void in concrete 

filled with alkali-silica gel. Note microcracks leading from gel-filled 

void, resulting from extrusion of gel into concrete-paste matrix. 

Magnification, 100×.

Figure 2. Concrete column in Stanley Park, Vancouver, British Columbia, showing typical “map” cracking throughout entire column, 

induced by expansion of concrete due to swelling alkali-silica gel. 

Reactive aggregates include andesite and dacite from Mount Garibaldi near Whistler, British Columbia, as well as quartzite, chert, 

and granodiorite. Concrete was cast in the 1920s; specific aggregate source (that is, location of pit) is unknown.

within a short timeframe, for example, within 5 years of 

construction (Rogers, 1986). The typical length of time for 

AAR to become noticeable in structures in the Northwestern 

United States and western Canada, however, is 25 years or 

more. Other natural processes (freeze-thaw cycles, wind) 

and anthropogenic processes (traffic load, salt applications, 

impact) can contribute to the deterioration of concrete elements throughout the lifetime of concrete structures. AAR 

may rank as more severe than other causes if its effects are 

significant and are manifested early, and as less severe if the 

effects are not evident or serious before other deterioration 

mechanisms become evident. The prediction of whether various cement-aggregate combinations will produce no, slight, 

moderate, or severe reaction of this type is of greatest concern 

to owners of concrete construction, to concrete design professionals, to concrete producers, and to aggregate producers.


Depending on the nature of the concrete structure or element, and its intended service life, the effects of AAR may 

range from merely cosmetic (no significant structural effects) 

to serious (requiring extensive repair). In some cases, these 

effects may be limited to unsightly cracks and some efflorescence in concrete construction such as a sidewalk or a retaining 

wall (fig. 4), and the degree of damage may not be significant 

enough to warrant any particular repairs if the structure remains 

serviceable and safety is not an issue. In other cases, however, 

the effects of AAR can weaken the concrete altogether and 

reduce its structural capacity to such an extent that the concrete 

may require either significant and costly repairs or outright 

replacement. For such major civil structures as bridges (fig. 5), 

dams, and tunnels, this degree of damage can be particularly 

problematic, affecting not only serviceability and operation 

but also safety. At the costly end of the scale of repairs is the 

technique of cutting vertical slots in dams that exhibit expansion due to AAR, to relieve the pressures generated by swelling 

alkali-silica gel. These repair programs are expensive, sometimes costing millions of dollars. For example, Mactaquac Dam 

in New Brunswick, Canada, is a well-documented case of such 

a remediation scheme (Curtis, 2000).

For smaller concrete structures, load capacity may be 

compromised, requiring expensive repair or replacement. For 

example, Shrimer (2000b) reported on a dock facility in which 

relatively small concrete elements were affected by AAR (fig. 

6), reducing their structural capacity and requiring the replacement of more than 150 precast-concrete beams at a cost of 

more than $2 million.

Three “ingredients” (or conditions) are required for AAR 

to occur: (1) reactive aggregate, (2) alkalis content sufficient 

for the reaction to be significant, and (3) moisture. Should all 

three of these conditions not be present in the concrete, AAR 

will not occur. These conditions are normally met only when 

the concrete contains sufficient cement to elevate alkali levels 

to some threshold value (such as when the cement content is 

high or where high-alkali cement is used), and only when the 

Figure 3. Pier in Seattle harbor, showing well-defined “map” 

cracking induced by alkali-aggregate reaction. Reactive aggregates 

includes andesite from the Mount Rainier-Mount St. Helen’s area, 

Wash., as well as sandstone, chert, and quartzite of unknown provenance. Field of view, 1 by 1.3 m.

Figure 4. Wall in Stanley Park, Vancouver, British Columbia, showing white deposits of leachate emanating from cracks in concrete. 

These “efflorescence” deposits are composed of a mixture of 

alkali-silica gel and calcium carbonate.

Introduction
Contributions to Industrial-Minerals Research

concrete structure is located in an environment that exposes it 

to moisture. Typically, these moist conditions would be present outdoors and, if indoors, only in a setting where moisture 

was common (for example, a carwash, sewage plant, industrial facilities). Thus, AAR damage is more likely to occur 

in such structures as dams, bridges, reservoirs, dock facilities, and walls located at or near water than in those that are 

protected from the elements or located away from sources of 

moisture. In short, most residential and commercial construction, as well as some types of industrial facilities, generally is 

less AAR susceptible.

When AAR begins, and as it progresses over a period of 

time, either the alkalis in the concrete are consumed thereby 

ending the reaction (even though reactive aggregate remains), 

or the reactive aggregate is consumed, and the reaction ceases 

(while alkalis are still left in the concrete). Furthermore, if the 

supply of moisture is interrupted, swelling of the alkali-silica 

gel will cease, even if the reaction between the alkalis and 

reactive aggregate continues. AAR is specifically a chemical 

reaction that occurs between the minerals and the alkali solution, while water is the agent that enables or produces expansion of the concrete.

Evidence of AAR is determined through various means, 

such as macroscopic examination of the concrete structure, 

and microscopic examination and chemical analyses of concrete samples obtained from the structure. Visual evidence 

includes reaction rims (figs. 7, 8), cracks induced by the pressures exerted by swelling alkali-silica gel (fig. 9), and the 

presence of alkali-silica gel (figs. 1, 8).

Background

In the Pacific Northwest, the level of effort directed 

toward the identification, remediation, and prevention of AAR 

in concrete has been inconsistent. Some of the earliest studies of AAR in North America were conducted in this region. 

During the late 1930s and early 1940s, for example, Tremper 

(1941) and Coombs (1942) identified reactive aggregates 

in the Mount Rainier area and Puget Lowlands of western 

Washington. Their studies of cracked concrete in bridges and 

concrete pavements in central and eastern Washington and 

Oregon identified reactions that were attributed to the presence of opaline and chalcedonic silica in aggregate derived 

from Columbia Plateau basalt flows.

Holland and Cook (1953) described reactive aggregate in 

Oregon derived from the Clarno Formation, which consists of 

andesite-rhyolite flows and tuffs of Oligocene through Eocene 

age. More recently, Shrimer and Jones (2000) described 

examples of AAR in Washington and British Columbia. 

Overall, however, the level of effort directed towards the 

identification of AAR in concrete structures or toward the 

characterization of AAR-susceptible aggregates in the Pacific 

Northwest has been lower than in eastern North America, 

where AAR is recognized as more of a problem.

Control Specifications and Testing for 

Alkali-Aggregate Reaction

The testing and research efforts during the 1930s and 

1940s led to the development of a widely adopted specification geared to the control or prevention of AAR in new 

concrete structures in Washington and Oregon. The primary 

requirement was the use of low-alkali-content cement in 

concrete, because a link had been found between the use of 

cements with an alkali content greater than 0.6 weight percent Na2O equivalent and a higher or more severe incidence 

of AAR. This specification, which was adopted throughout 

Figure 6. Part of a dock structure in Vancouver, British Columbia, showing “map” 

cracking. Reactive aggregates include Mount 

Garibaldi volcanic rocks, opal (in volcanic 

vugs), granitic rocks, and quartzite. Aggregate 

was derived from the now-closed Britannia 

Pit, located near Squamish, British Columbia.

Figure 5. Old Alexandra Bridge in Fraser Canyon, near Boston Bar, 

British Columbia, showing piers and abutments damaged by both 

alkali-aggregate reaction and freeze-thaw cycles.
the Northwestern United States, is widely used as a general 

rule-of-thumb in concrete construction. To some extent, 

application of this specification has been successful because 

relatively few cases of serious AAR have been reported in 

the Northwestern United States for many years. This success 

rate may be largely coincidental because cement supplies in 

western North America generally have low alkali contents. 

With lower alkali contents, AAR has not been as frequent or 

as pronounced in the Pacific Northwest as in other regions, 

where cements with higher alkali contents are more commonly used.

Besides the general requirement for the use of low-alkali 

cement, other standard practices specific for the control of 

AAR were not adopted within the Pacific Northwest during 

approximately the 1940s to well into the 1990s, because no 

serious problems due to AAR damage in concrete had been 

diagnosed or, if so, they were poorly understood within the 

region. Generally, the use of low-alkali-content cement with 

the required limit of 0.6 weight percent Na2O equivalent was 

effective in preventing widespread AAR damage in concrete 

throughout the Pacific Northwest.

During much of the 1950s to1990s, few cases of AAR 

were reported in the Pacific Northwest. Some examples 

surfaced, but the incidence was low enough that the general 

concrete-construction community did not give AAR much 

consideration. This situation began to change in the 1980s 

and 1990s as cases of AAR became more apparent throughout the Pacific Northwest (British Columbia Ministry of 

Transportation and Highways, 1995). In some cases, AAR 

damage was merely cosmetic, with cracks appearing in the 

surfaces of concrete elements but without serious structural 

weakening of the concrete. In other cases, however, AARdamaged concrete structures were removed from service or 

demolished, or needed costly repairs.

In practice, many of the aggregates used for making concrete were, in fact, determined to be quite reactive, but owing 

to the lower alkali contents in Pacific Northwest cements, 

AAR did not set in and become noticeable until considerable 

time had elapsed. By the time damage was noted and investigated, frequently the deterioration had been attributed to 

the effects of freeze-thaw cycles, which can closely resemble 

those of AAR.

Currently, the primary methods used for evaluating 

AAR potential are the accelerated mortar-bar test (AMBT), 

referenced in American Society for Testing and Materials 

(2002) as ASTM C 1260 and in Canadian Standards Association (2004) as CSA A23.2–25A; and the concrete-prism test 

(CPT), referenced as ASTM C 1293 and CSA A23.2–14A. 

Other methods that have been used in the past include the 

“Potential Alkali-Silica Reactivity of Aggregates” (“Quick 

Chemical”) test (ASTM C 289) and the “Mortar-Bar Test for 

Potential Reactivity of Cement-Aggregate Combinations” 

(ASTM C 227), both of which have been shown to be largely 

unreliable indicators of AAR potential, and so they are no 

longer recommended for this purpose. Thus, the AMBT and 

the CPT are the currently preferred methods for evaluating 

AAR potential in the United States and Canada.

Geology and Reactivity of Aggregates

The low incidence of AAR in the Pacific Northwest is 

probably due more to the historical use of low-alkali cement 

than to the use of nonreactive Pacific Northwest aggregates. 

Recent research has shown that, in many areas of the Pacific 

Northwest, aggregates actually have an AAR potential which 

(in some areas) ranges from “moderate” to “high” as defined 

in CSA A23.1/2–04 and by ASTM. This observation is based 

on testing of aggregates from throughout the region, and on 

comparison of the geology of tested aggregates with that of 

aggregates from areas where little or no testing has been conducted (for example, Alaska, Oregon).

Aggregates used in the Pacific Northwest for the manufacture of ready-mix and precast concrete generally are 

derived from sand and gravel deposits of glaciofluvial or 

fluvial origin. Of the quarried supplies of aggregates in the 

Pacific Northwest, few are regularly used primarily as concrete aggregate. Some amount of crushed quarry rock or overFigure 7. Polished section of a 

concrete core drilled from an 

alkali-aggregate-reaction-damaged dam in southern British 

Columbia. Darkened reaction 

rims on aggregate particles 

indicate that reaction with 

alkalis has taken/is taking place. 

Field of view, approximately 30 

by 50 mm.

Figure 8. Fracture surface on 

a sample of core drilled from 

alkali-aggregate-reaction-damaged concrete, showing dried 

white gel patches on several 

aggregate particles. Darker 

“reaction rims” also are clearly 

visible on many aggregate 

particles. Field of view, approximately 60 by 100 mm.

Figure 9. Polished sample 

of concrete, showing cracking induced by pressure from 

swelling alkali-silica gel. Crack 

extends through sand/cement 

matrix and passes by several 

coarse aggregate particles. 

Field of view, 20 by 55 mm.

Geology and Reactivity of Aggregates
Contributions to Industrial-Minerals Research

size gravel may be blended in with natural, rounded gravel 

to increase concrete strength and durability, but as a rule, the 

use of concrete coarse aggregates composed of 100 percent 

crushed quarried stone is rare.

For aggregates produced from sand and gravel deposits, the geologic composition of the aggregates reflects the 

regional (or “upstream”) geology of the area in which the 

deposit is located. Because the AAR potential of an aggregate 

is a direct function of its mineralogy and lithology, an understanding of the geology of the aggregates, their provenance, 

and their mode of deposition is important. This information 

also is required if an overall appraisal of the engineering geology is desired.

Generally, the following rock types in Pacific Northwest 

aggregates are considered to be “potentially reactive,” that is, 

susceptible to or associated with AAR in concrete:

• Volcanic rocks of Cascadian origin, including intermedi- 

 ate-composition to acidic rocks (rhyolite-andesite). One 

 of the most reactive aggregates discovered to date in the 

Pacific Northwest is in the former gravel pit at Britannia, 

British Columbia, which contains a significant amount of 

 volcanic rocks derived from Mount Garibaldi, situated 

at the north end of the Cascade Range (Hickson, 1994).

• Basaltic volcanic rocks, such as Columbia River flows, 

which contain chalcedonic quartz as amygdule fillings 

 or vug linings, basaltic pebbles with opaline coatings, 

 and rocks from the Chilcotin Plateau, Anahim Belt, and 

Wells Grey-Clearwater volcanic fields in British Colum-

 bia, particularly those that contain chalcedony, opal, or 

 zeolites.

• Quartzite pebbles of widespread and varied provenance. 

 In British Columbia and western Washington, some 

 quartzites may originate from the Canadian Rocky 

 Mountains and the Purcell Mountains. Other quartzite 

 formations in Washington and British Columbia likely 

 also have contributed.

• Sandstone pebbles of widespread provenance, some from 

 the Rocky Mountains, and many from southwestern Brit- 

 ish Columbia and western Washington, including chert- 

 rich sandstone, greywacke, and lithic sandstone.

• Chert pebbles in Fraser River gravels, which may have 

 been derived from radiolarian chert beds in the vicinity 

 of Cache Creek, British Columbia, as well as numerous 

other sources. Fraser River gravel cherts have been deter-

 mined to be reactive in concrete. Chert, which is gener- 

 ally a reactive rock type, occurs in many formations of 

the Pacific Northwest.

• Some plutonic rocks, such as granitic rocks of the Coast 

 Plutonic Complex in British Columbia, that have been 

associated with AAR in concretes 40 or more years old. 

 Therefore, though rarely, other plutonic rocks within the 

Cascades or other areas in the Pacific Northwest may 

 exhibit some AAR potential as well.

• Metamorphic rocks in which cataclastic fabrics are present 

(Wigum,1996) or that contain strained quartz, owing to 

 their disturbed crystalline lattice.

Rock types that generally are considered to be nonreactive 

include:

• Most plutonic rocks, particularly if little or no disturbed 

or strained quartz is present. Mafic plutonic rocks generally 

 exhibit little or no AAR potential. Coastal gravel deposits 

 that contain a high proportion of Coast Plutonic Complex 

 rocks generally exhibit a low (that is, “safe” level of) AAR 

 potential, such as in the large gravel pits near Sechelt, Brit- 

 ish Columbia, where the deposits are primarily derived 

 from bedrock sources within the Coast Plutonic Complex.

• Carbonate rocks, provided they are not dolomitic, sili-

 ceous, or argillaceous. Special tests have been devised to 

 identify reactive carbonate rocks. Numerous limestone 

deposits are mined throughout the Pacific Northwest for 

 cement or agricultural use: a few of these deposits have 

 been evaluated for their AAR potential, and thus far, have 

 been found to be nonreactive.

• As noted above, mafic, quartz (chalcedony, tridymite, 

cristobalite, opal)-deficient volcanic rocks have generally 

 exhibited low or no AAR potential. However, careful 

 diagnosis is required to verify that these potentially reactive 

 forms of silica are absent.

On a finer scale, even within deposits, AAR potential 

may vary significantly as production penetrates different 

strata. Individual strata exposed within one section of a pit 

may differ lithologically from other strata above or below, 

with corresponding shifts in the proportion of reactive rock 

types. Typically, hydraulic sorting and a change in flow direction (with a corresponding change in upstream provenance) 

are responsible for these variations. Aggregate suppliers 

working these types of gravel deposits commonly consider 

their deposits to be nearly homogeneous, but, in fact, this is 

not always so. A more thorough evaluation of the lithology 

and AAR potential within individual pits or deposits may be 

appropriate in these cases.

In one pit in southern British Columbia, evaluation over a 

5-year period showed reaction levels ranging from low (“safe 

for use”) for one year’s production to high (“very reactive”) in 

tests conducted for another year’s production. To explain such 

seemingly inconsistent test results requires detailed petrographic analysis of the aggregates produced over the various 

time periods. In this pit, various rock types, provenances, and 

AAR potentials are represented, including Cascade volcanic rocks (reactive), quartzite and sandstone (reactive), chert 

(reactive), Coast Plutonic Complex and Cascadian plutonic 

rocks (nonreactive), metamorphic rocks (some reactive, some 

nonreactive), greenstone (nonreactive), and Interior Plateau 

basalt (mostly nonreactive). As the proportion of reactive rock 

types changed from one years’ production to another, the AAR 

potential also changed. For example, chert content within the 

pit ranged from ~7 to ~20 percent, and AAR potential ranged 

from low to high, respectively.

Therefore, even within a single aggregate deposit, seemingly small changes in lithology may be associated with 

significant changes in some engineering characteristics of the 

aggregate produced, including AAR potential. Thus, aggregate 

producers may need to regularly evaluate and test deposits 

in order to monitor such characteristics as AAR potential. 

Without such test data available, management of the aggregate 

resources to its optimal and best use becomes difficult.
In Washington, virtually all the aggregate sources tested 

in accordance with ASTM C 1260 have been identified as 

“potentially reactive” (Shrimer and Jones, 2000). Very few 

of these aggregate supplies have been evaluated by using the 

long-term, more definitive ASTM C 1293 test, and so no clear 

indication of the actual AAR potential of these aggregates is 

known. This question requires further assessment in Washington.

In British Columbia, approximately 80 percent of the 

aggregates tested in accordance with ASTM C 1260 have 

been identified as “potentially reactive” (Shrimer, 2000b). 

When tested in accordance with ASTM C 1293, however, the 

proportion of aggregates identified as “potentially reactive” 

is only approximately 40 percent of the total. Some pits have 

been identified that contain aggregates with an AAR potential 

low enough to be considered “nonreactive,” generally those 

pits in deposits with a high content of Coast Mountain Pluton 

(intrusive) rocks and low contents of volcanic rocks, chert, 

quartzite, and sandstone (Shrimer, 2001). For information 

on individual pits, it is best to contact the pit operator and 

request current and historical AAR test data, before assuming 

anything about the AAR potential of a particular supply of 

aggregate.

Alkali-Aggregate Reaction in Concrete in 

the Pacific Northwest

In the Pacific Northwest, varying levels of effort have 

been applied to the discovery of AAR in concrete structures, 

depending largely on previous experience. In areas where 

a serious problem stemming from the occurrence of AAR 

in an important structure was uncovered, the owners and 

managers of such structures were or are more vigilant in the 

recognition of other cases of AAR. In general, more effort 

in identifying AAR in existing structures has been applied in 

Washington and British Columbia than in Alaska, Oregon, 

or northern California. Interest in the discovery of new sites 

of AAR in the Pacific Northwest has risen since the early 

1990s, as more and more old structures have been reported to 

be affected by AAR.

In some cases, AAR has made the structure all but useless, 

requiring its demolition or replacement. For example, in the 

central Fraser Valley of southern British Columbia, two water 

reservoirs were demolished because of serious leaking resulting 

from AAR-induced cracks. In Fraser Canyon, the Alexandra 

Bridge (fig. 5), built in 1926, has been affected by both freezethaw cycles and AAR; the heavily cracked and deteriorated 

concrete is essentially unsalvageable. Management of the structure has become a source of concern for the British Columbia 

Ministry of Transportation and Heritage Canada, who have 

joint custody of and responsibility for the bridge.

In a recent study of a dam in southern British Columbia, Shrimer (2003) reported that its safe operation was 

being compromised by the expansion of concrete elements, 

resulting in poor operation of the radial gates. The typical 

condition of some AAR-affected concrete elements—effloAlkali-Aggregate Reaction in Concrete in the Pacific Northwest

rescence, discoloration, and “map” cracking (fig. 10)—illustrates the potentially serious effects of AAR. At the time of 

the study, the dam was 47 years old.

In other structures, extensive repairs have been required 

to reestablish the serviceability of the concrete. For example, 

repair of 40-year-old concrete support beams at a Vancouver, 

British Columbia, grain terminal was considered to be an 

unworkable solution, owing to the extent of AAR-induced 

cracking and the reduction of structural capacity. In these 

beams, andesite, quartzite, and chert were the primary reactive aggregates. Reactive opal was also identified as vug linings in basaltic aggregate in the concrete. Replacement of the 

beams cost more than $2 million.

In many structures, the effects of freeze-thaw cycles 

commonly accompany the damage incurred by AAR in concrete, such that the AAR damage is “masked” by freeze-thaw 

damage. Rarely, in other structures, the observed damage 

was first attributed to AAR, but upon conducting a detailed 

investigation, the primary cause was identified as freezethaw action. In some cases, AAR is diagnosed in the concrete 

but may be only incidental.

The Washington State Department of Transportation 

(WSDOT) has identified AAR in some structures as part 

of their routine inspection of their infrastructure. Affected 

structures include bridges and pavements in various locations throughout the State, such as the Manette Bridge (fig. 

11) in Kitsap County, the Woodward Creek Bridge near 

Walla Walla in southeastern Washington, and the historic 

Monroe Street Bridge in Spokane (figs. 12–14). In total, as 

many as 150 sites of suspected occurrence of AAR have been 

identified in WSDOT-owned structures. The confirmation of 

whether AAR is present, and to what extent, remains to be 

determined at these sites in Washington. At the time of this 

writing, additional studies to make such determinations have 

not yet been undertaken.

In British Columbia, more than 125 structures have 

been reported to be affected by AAR in concrete, including 

bridges and tunnels (owned by the British Columbia Ministry 

of Transportation in all parts of the Province), tunnels (south 

coast and Fraser Canyon), dams (south, southwestern, and 

north-central British Columbia) (Shrimer, 2003), port facilities 

(Vancouver, Prince Rupert), reservoirs (south and north coast 

regions), and privately owned structures in various locations.

Correct evaluation of AAR in a concrete structure is best 

achieved through a multicomponent investigation, carried 

out by an individual experienced in this type of work. Typically, such an investigation will involve a detailed inspection 

of the concrete and include a review of construction records, 

plans, and repair-and-maintenance records, as well as a 

laboratory evaluation of concrete samples from the structure. AAR can be evaluated by specialized methods, such as 

petrographic analysis of hardened concrete, (ASTM C 856), 

the American Association of State Highway Transportation 

Officials’ uranyl acetate test (AASHTO T–299), and other 

methods that have been developed more recently, such as the 

damage-rating-index test (Dunbar and Grattan-Bellew, 1995; 

Shrimer, 2000a).
Contributions to Industrial-Minerals Research

Testing the Alkali-Aggregate-Reaction 

Potential of Pacific Northwest 

Aggregates

Despite the awareness of AAR since the 1930s and 

1940s, Pacific Northwest aggregates were generally not 

tested for AAR potential on a routine basis, except for 

Figure 10. Section of a wall in a dam in southern British 

Columbia, showing well-developed “map” cracking and 

efflorescence, both indicators of alkali-silica reaction. 

Concrete was about 47 years old at time of photograph.

Figure 11. Manette Bridge in Kitsap County, Washington, which 

was diagnosed with alkali-silica reaction in its piers and abutments.

Figure 12. Monroe Street Bridge in Spokane, Wash., undergoing 

rehabilitation work in fall 2003.

Figure 13. Closeup of Monroe Street Bridge, Spokane, Washington, 

showing section of main arch. Concrete removed from demolished 

sections was determined to have been affected by alkali-silica 

reaction.

Figure 14. Polished sample of concrete from 

the Monroe Street Bridge in Spokane, Wash., 

showing reaction rims on andesitic aggregate 

particle and on smaller aggregate particles, 

indicating reaction with alkalis in concrete.
specific projects, in contrast with other regions of North 

America, where a clear and more widespread AAR problem 

was recognized. In those regions (for example, Ontario, 

Quebec, central/eastern Canada, New Mexico, Virginia, and 

other Eastern and Southern States), testing of aggregates for 

AAR potential was more common, and aggregate producers 

were not only accustomed to having the testing done but also 

familiar with the AAR potential of their aggregate supplies.

As more evidence of the occurrence of AAR was identified in some areas, a desire to understand the problem and 

avoid it in new or rehabilitated concrete structures arose, 

leading to a renewal of interest in testing both existing and 

proposed aggregate resources. This level of interest has generally not characterized aggregate engineering in the Pacific 

Northwest since the 1950s. The recent renewal of interest, 

in turn, has resulted in the revision of specifications for concrete construction materials by transportation and highways 

departments, by municipal, district, or county governments, 

and by hydro, rail, port and other infrastructure authorities.

Another factor that has generated further interest in 

AAR evaluation has been the importance and use of aggregates in areas where specification requirements differ and 

are more stringent than at the source, for example, the sale 

of aggregates from British Columbia to the Washington 

and California markets. The sale and transport of British 

Columbia aggregates to the United States has occurred since 

the late 1990s, partly owing to the favorable exchange rate 

between Canadian and U.S. currencies, as well as the growing demand in California for high-quality construction aggregates. Generally, the U.S. market requires a higher level of 

testing and prequalification than in British Columbia, and so 

Canadian aggregate supplies needed to be evaluated for AAR 

potential in advance of their acceptance for use in the United 

States. Additionally, the limit for AAR-induced expansion 

in ASTM C 1260 is lower than that specified in the equivalent Canadian CSA A23.2–25A. As a result, the chances of 

exceeding the U.S. expansion criteria are higher in the American test than in the Canadian test.

Recent Alkali-Aggregate-Reaction 

Test Data

Since about 1990, testing of Pacific Northwest aggregates 

for AAR potential has been done mostly in Washington and 

British Columbia. Because the Alaska and Oregon Departments of Transportation currently have no prequalification 

requirements specifically for the AAR potential of aggregates 

intended for use in new concrete construction, few aggregate 

supplies have been tested for AAR potential in those States, 

although some testing of aggregates from neighboring areas 

has been carried out. For example, WSDOT has conducted 

testing of four aggregate supplies from Oregon and four 

aggregate supplies from British Columbia, all of which were 

proposed for use in WSDOT projects. All of the Oregon 

aggregate supplies and three of four British Columbia aggregates tested by WSDOT were rated as “potentially reactive” 

when tested in ASTM C 1260.

AAR test data (Shrimer, 2000b; Shrimer and Jones, 

2000) generally indicate that many Pacific Northwest aggregates have a moderate to high AAR potential. Because of the 

length of time (1 year) required to run a CPT, many aggregates have been evaluated with the AMBT, which requires 

only 2 to 3 weeks to complete. Although the AMBT is faster, 

it also can be quite severe, and so it is generally considered 

by the aggregate-testing industry to be better used as a screening test. (The CPT, as discussed below, is considered to be a 

more accurate predictor of AAR potential in aggregates.) As 

a result, many aggregates exceed the limits in the AMBT. In 

Washington, for example, current data indicate that 98 percent 

of the aggregates tested exceeded the maximum-expansion 

limit of 0.10 percent specified for this test. In British Columbia, where the higher CSA limit of 0.15 percent is used, 82 

percent of the aggregates tested exceeded this limit.

The AMBT data listed in table 1 are based on the number 

of samples tested, which varies for the three States and one 

Canadian Province involved. The Alaska and Oregon datasets are based on very few tests, whereas the Washington 

dataset currently represents about 100 AMBTs, and the British Columbia dataset more than 250 tests. Recent testing of 

aggregates in neighboring Montana has also identified several 

potentially reactive aggregates, a result consistent with what 

has been determined in the adjacent, geologically similar 

regions of Alberta, British Columbia, and Washington.

The sparsity of available CPT data reflects the low incidence of use this longer-term test in Pacific Northwest States. 

The British Columbia dataset currently consists of more than 

100 CPT results. Of the two current methods, the CPT is 

considered to be a more accurate and reliable predictor of the 

AAR potential of aggregates. The 1-year time requirement for 

running a CPT, however, exceeds the typically short timeline 

for preparation for many construction projects, such that the 

project might be well underway or even finished before the 

test is completed. Thus, many aggregates are evaluated by 

using the AMBT instead.

To overcome this dilemma of “quick/harsh versus slow/

better test,” it is recommended that aggregate supplies be 

evaluated initially by using both methods, so that a “calibration” for the short-term versus the long-term test is established. Some specifications require a CPT to be run at least 

annually. After a reasonable track record of data has been 

achieved for a specific aggregate source, the 1-year requirement might appropriately be relaxed to every 2 years (or 

longer, for smaller-output supplies) provided that no obvious 

changes are observed in the aggregate’s geology, as determined by petrographic analysis.

To assist aggregate and concrete producers, specifying 

agencies, such as State departments of transportation, could 

issue a “qualification requirement” to suppliers of concrete, 

concrete products and concrete aggregates. Any aggregate 

and concrete suppliers who wished to provide their products 

to State or Provincial transportation projects would be given 

a 1-year-long period in which to conduct CPTs on their supRecommendations
10 Contributions to Industrial-Minerals Research

plies of aggregates. In this way, the necessary information 

could be obtained in a timely fashion, and the high “failure 

rate” observed in the AMBT could be accounted for. The 

specifying agencies could then review the submitted data and 

assess the test results to determine the acceptability of various 

aggregate sources for use in concrete construction.

After an initial set of AAR tests has been compiled, and 

once a CPT has been begun, an AMBT can be conducted to 

determine the AAR potential of current production aggregates 

by comparing the new data with historical calibration data. This 

approach has been used with success by aggregate quality-control staff in central Canada (De Grosbois and Fontaine, 2000). 

Such an approach generally requires a concurrent petrographic 

analysis to provide data on the consistency of the mineralogic 

and geologic composition of the aggregates under study.

WSDOT currently has a framework in place requiring 

that aggregate suppliers submit samples of their aggregates 

for testing, in the quick, 2-week ASTM C 1260. Aggregates 

that pass this test may be included on the State’s “Aggregate Source Approval” listing, whereas those that fail are 

excluded. This approach is somewhat similar to that used in 

Ontario, where the Provincial Ministry of Transport requires 

that suppliers of concrete and aggregates provide certified 

test data to demonstrate that their aggregate supplies will not 

cause deleterious AAR-induced expansion in concrete.

The Federal Highways Administration, which has extensively studied the AAR problem in publicly owned transportation infrastructure, provides helpful information and 

detection advice in their Handbook for the Identification of 

Alkali-Silica Reactivity in Highway Structures (Federal Highways Administration, 1991).

Summary

AAR has been found to vary in occurrence, location, 

and severity throughout the Pacific Northwest. Although the 

geology of Pacific Northwest aggregate supplies commonly 

supports the occurrence of AAR because of their moderate 

to high AAR potential, the geology of much of the carbonate source rock used to manufacture local Pacific Northwest 

cements has resulted in the production of cements with low 

alkali contents, helping the region avoid a far more serious 

AAR problem.

Recommendations

More concrete structures that have been affected by AAR 

are anticipated to be detected in the Pacific Northwest. Repair 

or replacement strategies should be developed on a case-bycase basis. An assessment of each occurrence can be facilitated 

through detailed field and laboratory methods, including petrographic analysis (ASTM C 856), damage-rating-index test 

(Shrimer, 2000a) and determination of the “potential for future 

expansion” (Berube and others, 1995).

Continuing evaluation of the AAR potential of aggregates 

throughout the Pacific Northwest will be required to more completely understand the reactivity of available aggregate sources. 

Although reactive aggregates may be the only choice for use 

on some AAR-sensitive projects, effective methods to suppress 

AAR in new concrete construction are currently available, 

holding promise for many reactive aggregates to be declared 

“safe for use” with respect to AAR. These methods include 

the use of such mitigative materials as fly ash, silica fume, 

and certain lithium products. Nonetheless, it is recommended 

that individual combinations of aggregate be evaluated with 

the proposed mitigative material(s) before acceptance of the 

aggregate for use. These mitigative materials have generally 

been found to be effective in making many potentially reactive 

aggregates safe for use; however, authorities typically require 

that reactive aggregates be carefully evaluated before being 

their approval for use. Caution is the safest approach when 

dealing with potentially reactive aggregates in the concrete that 

is required to build safe, durable, and economic structures or 

constructed works.

Table 1. Summary of alkali-aggregate-reaction test data for selected aggregates in the Pacific Northwest, 

United States and Canada.

[All values in percent. n/a, not available]
11

Implementation of these types of mitigative material 

would extend the service life of existing aggregate resources 

that may otherwise be reactive in concrete. If potentially reactive aggregate supplies throughout the Pacific Northwest were 

abandoned in favor of nonreactive aggregate supplies only, 

some areas would be left with few or no alternative aggregate 

resources and would need to transport aggregates into the 

market area, at significantly greater cost.

Therefore, the use of local, potentially reactive aggregates, in properly designed, AAR-mitigated concrete mixes, 

would alleviate the need to transport nonreactive supplies from 

other, more distant areas. The use of local versus nonlocal 

aggregates would reduce costs to projects and communities, 

would mitigate the potentially greater negative effects on livability and the environment within the region, and is consistent 

with a more stewardly use of aggregate resources.

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