Alkali-Silica Reaction Degradation of Nuclear Power 

Plant Concrete Structures: A Scoping Study

Prepared by:

K.A. Snyder

H.S. Lew

National Institute of Standards and Technology

Engineering Laboratory

100 Bureau Drive

Gaithersburg, MD 20899

Manuscript Completed: March 2013

Date Published: April 2013

Jacob Philip, NRC Program Manager

NRC Job Code: V 6367

Office of Nuclear Regulatory Research

NUREG/CR-####

NISTIR 7937
ii

NISTIR 7937 

Alkali-Silica Reaction Degradation of 

Nuclear Power Plant Concrete Structures: 

A Scoping Study

K.A. Snyder

H.S. Lew

Materials and Structural Systems Division

Engineering Laboratory

March 2013

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
iii

ABSTRACT

Alkali-silica reaction (ASR) is a concrete degradation phenomenon in which the alkalis that are typically 

found in portland cement react with certain amorphous or micro-crystalline siliceous phases in the 

aggregate and, in the presence of moisture, form an expansive gel that is capable of cracking the concrete 

and generating macroscopic expansion. As the cracking occurs and progresses, there is a change in the 

mechanical properties of the concrete. Unfortunately, there is no standard guide or industry best practice 

for quantifying the reduction in the mechanical properties and the commensurate reduction in the 

structural capacity of ASR-affected elements. Furthermore, ASR is a complex chemical phenomenon, 

the rate and extent of which depend upon a number of material and environmental parameters, and the 

interactions among parameters is not fully understood. Therefore, although there are standardized test

methods to characterize the susceptibility of a specific concrete mixture to future ASR degradation, there 

is no standardized procedure for assuring that a concrete mixture will be entirely immune to ASR 

throughout its intended service life. 

A scoping study of alkali-silica reaction in concrete is performed to support future activities that include 

evaluating the effects of ASR on the structural capacity. The study begins by summarizing the current 

knowledge of a number of technical topics related to the diagnosis, maintenance, and evaluation of 

concretes affected by ASR. The summary is then used to identify knowledge gaps for each technical 

topic. A technical plan is developed for filling in the knowledge gaps and for providing the tools that are 

needed to evaluate structural capacity of ASR-affected concrete and estimating the useful service life.
iv
v

TABLE OF CONTENTS

ABSTRACT............................................................................................................................................................................III

LIST	OF	FIGURES ...............................................................................................................................................................VII

EXECUTIVE	SUMMARY .....................................................................................................................................................IX

ABBREVIATIONS ................................................................................................................................................................ XI

1 INTRODUCTION.............................................................................................................................................................1

1.1 SCOPING	STUDY ................................................................................................................................................................................2

1.2 STATE-OF-THE-ART	REVIEW.........................................................................................................................................................3

1.3 GAP	ANALYSIS...................................................................................................................................................................................3

1.4 TECHNICAL	PLANS ...........................................................................................................................................................................3

2 STATE-OF-THE-ART	REVIEW ...................................................................................................................................4

2.1 STATE, SEVERITY, AND	RATE	OF	ASR..........................................................................................................................................4

2.2 IN-SITU MECHANICAL	PROPERTIES	OF	ASR-AFFECTED	CONCRETE......................................................................................7

2.3 IN-SITU STRENGTH	OF	ASR-AFFECTED	CONCRETE ..................................................................................................................8

2.4 APPLICABILITY	OF	ACI DOCUMENTS	TO	ASR-AFFECTED	CONCRETE ...................................................................................8

2.5 MANAGING	THE	EFFECTS	OF	ASR.................................................................................................................................................9

2.6 NUMERICAL	MODELING	OF	ASR ................................................................................................................................................10

2.7 ASR PREVENTION	FOR	NEW	CONSTRUCTION .........................................................................................................................11

2.8 EFFECT	OF	ASR ON	TRANSPORT	PROPERTIES ........................................................................................................................13

2.9 SUMMARY........................................................................................................................................................................................14

3 GAP	ANALYSIS............................................................................................................................................................. 15

3.1 STATE	AND	SEVERITY	OF	ASR; PREDICTING	RATE	OF	REACTION.......................................................................................15

3.2 IN-SITU MECHANICAL	PROPERTIES	OF	ASR-AFFECTED	CONCRETE...................................................................................15

3.3 EFFECTS	OF	ASR ON	ANCHORAGE	OF	REINFORCEMENT.......................................................................................................18

3.4 CAPACITY	OF	ASR-AFFECTED	CONCRETE	STRUCTURES .......................................................................................................18

3.5 MANAGING	THE	EFFECTS	OF	ASR..............................................................................................................................................19

3.6 NUMERICAL	MODELING	OF	ASR ................................................................................................................................................20

3.7 ASR PREVENTION	DURING	NEW	CONSTRUCTION..................................................................................................................20

3.8 EFFECTS	OF	ASR-INDUCED	CRACKING	ON	TRANSPORT	PROPERTIES ................................................................................20

3.9 SUMMARY........................................................................................................................................................................................21

4 TECHNICAL	PLAN....................................................................................................................................................... 22

4.1 ARTICULATED	PRIORITIES...........................................................................................................................................................22

4.2 STRUCTURAL	CAPACITY	OF	EXISTING	STRUCTURES...............................................................................................................22

4.2.1 Determine	Structural	Capacity	of	an	Element .....................................................................................................23

4.2.2 Recommended	Plan	for	Assessing	In-Situ Mechanical	Properties	of	ASR-Affected	Concrete: .......24

4.2.3 Estimating	Future	Mechanical	Properties .............................................................................................................25

4.2.4 Predicting	the	Effect	of	ASR	on	Bond	and	Anchorage ......................................................................................26

4.2.5 Methodology	for	Assessing	Response	Characteristics	of	ASR-Affected	Structures	to	Earthquake	

Loads 26

4.3 EFFECT	OF	CRACKING	ON	FLUID	TRANSPORT .........................................................................................................................26

5 SUMMARY..................................................................................................................................................................... 28

6 REFERENCES................................................................................................................................................................ 30
vi
vii

LIST OF FIGURES

Figure 1. Schematic representation of alkali-silica reaction and cracking in concrete (from Deschenes et 

al. 2009). The process is shown in three stages: hydroxyl and alkali species react with the silica to 

form a gel; the gel absorbs water, creating expansive forces; and the stresses can ultimately generate 

sufficient stress to crack both the aggregates and the surrounding paste.............................................. 1

Figure 2. Sequence of laboratory tests for evaluating aggregate reactivity (from Thomas et al. 2008).... 12

Figure 3. Relationships between ASR expansion and loss in compressive and tensile strengths (from 

Smaoui et al. 2006).............................................................................................................................. 17

Figure 4. Relationship between ASR expansion and loss in modulus of elasticity (from Smaoui et al. 

2006).................................................................................................................................................... 17
viii
ix

EXECUTIVE SUMMARY

Alkali-silica reaction (ASR) is a concrete degradation phenomenon in which the alkalis that are typically 

present in concreting materials react with certain amorphous or micro-crystalline siliceous phases that 

can be present in the aggregates and, in the presence of moisture, form an expansive gel that is capable of 

cracking the concrete and generating macroscopic expansion. As cracking occurs and the expansion 

progresses, there is a loss of the mechanical properties of the concrete, and a commensurate decrease in 

the structural capacity of the affected elements. Because this is a form of internal degradation, and 

because there may be sufficient moisture in the concrete itself for ASR to progress in the absence of 

external moisture, long-term solutions can be limited. 

From a safety perspective, an important consideration is the remaining capacity of a structure exhibiting 

distress due to ASR. Determining mechanical properties from cores taken from the structure can be 

problematic for a number of reasons. First, the extent of the degradation will vary throughout the 

element as a function of the moisture content, and as a function of the degree of restraint provided by the 

steel reinforcement. Also, it may be difficult to predict the properties of the concrete using cores taken 

from the structure because the size of the defects caused by the ASR may be large compared to that of 

the cylinder (resulting in anomalously low tested strength), but small compared to the structure 

(suggesting there may be sufficient capacity). Alternatively, one could relate the relative change in 

concrete strength to the relative extent of the reaction. The challenge with this approach is that there is 

no reliable means of estimating the degree of the reaction in an existing concrete structure. 

For safety-related concrete structures that require extended periods of safe operations, a monitoring 

strategy is needed that can predict the future evolution of the reaction to ensure that there is sufficient 

time to execute a safe cessation of operations. Because the evolution of mechanical properties as a 

function of the degree of reaction is not known, near linear extrapolations of past changes in behavior 

have not been validated.

A scoping study of alkali-silica reaction in concrete is used to identify knowledge gaps in the evaluation 

of the present capacity of concrete structures affected by ASR, and the prediction of the future loss of 

structural capacity. The gaps are identified through a literature search of journal articles, standards, and 

codes related to ASR. Based on the gaps identified, a technical plan is proposed for closing each 

knowledge gap.

For existing structures, the technical plan includes activities to support the evaluation of the current state 

of the structure and activities to support the estimation of the future performance of the structure. 

The plan focuses on modifying the ACI 318 relationships for estimating strength, modulus of rupture, 

and modulus of elasticity from the compressive strength of distressed concrete; these are critical 

parameters in a structural evaluation. The assumption is that these material properties decrease as a 

function of the degree of degradation, which is either the equivalent degree of free expansion, or the 

extent of the chemical reaction. The structural testing consists of making large-scale specimens with 

reactive aggregate and measuring the mechanical properties and the expansion of these elements over 

time. At the same time, samples are extracted and tested to determine the mechanical properties. 

In concert with the mechanical testing, extracted specimens will be evaluated to determine the degree of 

the degradation. A number of tests for characterizing the degree of degradation will be considered: 

residual expansion tests and petrographic tests. The purpose of this testing is to identify reliable 

observables in the field that can be used in existing structures to determine the extent of the reaction, so 

that one may estimate the amount of future expansion to be expected. It is expected that a relationship 
x

between core test values and the mechanical properties of a reinforced concrete element will depend, in 

part, on the extent of the reaction.

By using a number of different concrete mixture designs that are representative of NPP mixture designs, 

a relationship will be developed between the degree of the reaction and the modification needed to the 

ACI 318 relationships for estimating mechanical properties required to evaluate structural capacity. 

The remaining technical plan addresses predicting the future extent of ASR degradation as a function of 

time. Of the current approaches being used to estimate future properties of ASR-affected concrete, the 

one recommended here is a first-order kinetic approach that captures the contribution of the primary 

factors affecting the rate of ASR. Although the resulting model will not have the accuracy of a more 

sophisticated model, it should provide general guidance for estimating the progression of the reaction 

with time. From this prediction, one can use the relationships developed from the mechanical testing for 

the ACI code parameters to predict the future mechanical properties of the concrete, and thus evaluate the 

future structural capacity of a structure exhibiting distress due to ASR.
xi

ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials

ACI American Concrete Institute

ASR Alkali-Silica Reaction

ASTM (formerly) American Society for Testing and Materials, (presently) ASTM International

DRI Damage Rating Index

FHWA Federal Highway Administration

NIRAS Nonlinear Impact Resonance Acoustic Spectroscopy

NIST National Institute of Standards and Technology

NRC Nuclear Regulatory Commission

NPP Nuclear Power Plant

ORNL Oak Ridge National Laboratory

SDT Stiffness Damage Test
xii
1

1 INTRODUCTION

Alkali-silica reaction (ASR) was identified decades ago (Stanton 1940) as a degradation mechanism in 

concrete whereby a reaction between the alkalis in the cement and certain siliceous phases in the 

aggregate produces a gel that causes an expansion that leads to cracking and strength loss (Hansen 1944, 

Taylor 1990). The gel forms initially in the partially saturated pore space of the hardened cement paste 

that binds the aggregate together. Because the resulting gel is hydroscopic, which is how the pressure is 

generated, water is a key component to the reaction and subsequent damage by cracking. Generally, the 

reaction rate is relatively slow, so the onset of cracking can occur years or decades after construction, 

depending on the reactivity of the mineral phases and the alkalinity of the pore solution.

The conceptual model for ASR and the consequent damage is shown schematically in Figure 1, which is 

taken from Deschenes et al. (2009). Inside the concrete are reactive aggregates containing siliceous or 

microcrystalline silica phases. The hydroxyl (OH-) and alkali species (e.g., K+ and Na+) are in solution 

and react with the siliceous phases to form an expansive hydroscopic gel. As the gel absorbs water, it 

expands, generating stresses that can be in excess of 10 MPa (nominally 1500 psi) (Rigden et al. 1995, 

Ferraris et al. 1997). These stresses are sufficient to generate cracking, both in the aggregate and in the 

surrounding hardened cement paste, and can even cause macroscopic expansion of the structure.

Figure 1. Schematic representation of alkali-silica reaction and cracking in concrete (from Deschenes et 

al. 2009). The process is shown in three stages: hydroxyl and alkali species react with the silica to form a 

gel; the gel absorbs water, creating expansive forces; and the stresses can ultimately generate sufficient 

stress to crack both the aggregates and the surrounding paste.

At the microscopic length scale, the reaction occurs in two stages. The initial stage is dissolution of 

silica on the surface of the reactive phase within the aggregate. At the interface between the reactive 

phase and the alkaline pore solution, the hydroxyl ions (OH-) attack the silanol groups (Si-OH) and the 

siloxane bonds (Si-O-Si) of the reactive silica network. The siloxane bonds are broken and replaced with 

silanol bonds, which together with other silanol bonds in the reactive mineral, further react with 

hydroxyls (Dent Glasser and Kataoka 1981) producing SiOH-:

Si − O − Si + OH! → Si − OH + OH − Si (1)

Si − OH + OH! → SiO! + H!O (2)
2

Within the alkaline pore solution, the concentration of the hydroxyl (OH-) species is nearly equal to the 

combined concentration of the potassium (K+) and sodium (Na+) species. The reaction mechanisms 

occurring in Eq. 2 are complex, and supply the pore solution with silica species that diffuse through the 

pore system and react with OH-, Na+ and K+ to form a gel: this is referred to as the “through-solution”

mechanism of gel production (Dron and Brivot 1992, 1993). The gel incorporates water, either through 

absorption (Dent Glasser and Kataoka 1981) or through osmotic pressure (Diamond et al. 1981). This is 

a somewhat condensed representation of ASR, and a broader review of the ASR mechanism can be found 

in Helmuth et al. (1993). Although an understanding of the alkali-silica reaction helps to inform test 

method development, a thorough review of the fundamental studies of the reaction is not presented here 

because answering any remaining fundamental questions about the reaction does not directly support a 

timely solution to the principal objective of this scoping study.

One of the remaining relevant technical challenges, however, is predicting the reaction kinetics from 

information that can be obtained from an existing structure: e.g., the aggregate mineralogy, the pore 

solution composition, the temperature and moisture conditions, and the structure geometry and location. 

Part of the challenge is identifying and quantifying the rate-controlling processes. These may include the 

diffusion of pore solution through the aggregate and the dissolution rate at the reactive mineral phase 

surface. Depending on the mineral composition and the morphology of the aggregate, and the exposure 

conditions, ASR may not become evident until decades after construction.

One of the practical challenges in preventing ASR in new structures is the diminishing supply of 

aggregates that have a history of good performance. Regionally, there may be little, or no aggregate 

locally available that is known to be immune to ASR. For large construction projects that require on the 

order of 105 cubic meters of concrete, it may be impractical to ship in reliable aggregate from a long 

distance away. 

The other important challenge in mitigating ASR is the uncertainty of if or when ASR damage will 

occur. For many concrete structures, a 20-year service life may be sufficient to meet the required 

performance until the structure becomes functionally obsolete. For certain critical components of the 

nation’s infrastructure (e.g., dams, bridges, and nuclear power plants), assurances of long-term 

performance can be a financial and a safety concern. Premature replacement due to failure is costly to 

society, and if the structural capacity is lost at a rate that is faster than anticipated, there is a potential 

safety risk to the public. Moreover, these larger critical infrastructural elements are not conducive to 

large-scale testing to assure the overall structural capacity.

1.1 Scoping Study

The breadth of ASR research and field investigations spans a wide literature base: (1) laboratory studies 

of the reaction and the reaction products that are published in the technical literature; (2) standardized 

test methods for evaluating materials; (3) codes and industry best practice guides for material selection 

and concrete formulation; and (4) reports on the diagnosis and condition assessment of structures 

exhibiting distress due to ASR. 

Although the majority of these documents may not have been developed with the purpose of facilitating 

structural evaluation, they may nonetheless play an important role. Given the uncertainties in evaluating 

and predicting the capacity of a structure exhibiting distress due to ASR, a scoping study is needed to 

identify past work on the subject and to devise a plan for developing the engineering tools needed to 
3

support the evaluation of an existing structure exhibiting distress due to ASR. The components of the 

scoping study are outlined below.

1.2 State-of-the-Art Review

A state-of-the-art literature review is performed to assimilate all this knowledge in one place. 

Furthermore, the scope of this review is limited to assessing the capacity of a structural concrete element 

exhibiting stress due to ASR, and predicting the future capacity of the same element. This scope is 

further broken down into the specific technical needs that, when combined together, meet the overall 

objective. The literature search is summarized for each of these specific technical needs.

1.3 Gap Analysis

A gap analysis is performed for each of the specific technical needs by assessing the degree to which the 

existing body of knowledge meets the need. Because the majority of a body of knowledge was not 

developed to meet the critical needs covered in this report, identifying the gaps requires one to first distill

the knowledge into a coherent and comprehensive whole.

1.4 Technical Plans

Technical plans are developed to meet the gaps identified for each specific technical need. The overall 

objective of the technical plan is to have a process by which one can assess the capacity of a structural 

concrete element affected by ASR, and make an assessment of the future structural performance of that 

element.
4

2 STATE-OF-THE-ART REVIEW

A state-of-the-art review was conducted to address a number of engineering issues related to the 

evaluation of structural concrete elements exhibiting distress due to ASR. These issues are related, 

broadly, to understanding the current state of the structure and forecasting future performance. There are 

guides that provide strategies for confirming the presence of ASR, for mitigating the effects of ASR in 

new construction, and for providing repair strategies for existing structures. For the problem of 

quantifying the extent, rate, and future potential of the ASR reaction, however, the literature was taken 

largely from the research community, including archival journals and technical reports.

There are many general guides that a practicing engineer can use to find understanding and guidance in 

mitigating and addressing ASR. The British Cement Association (1992) has published guidelines for 

diagnosing the existence of ASR. Hobbs (1986) and Fournier and Bérubé (2000) provide practical 

strategies for mitigating the effects of ASR in new and existing structures. Farny and Kerkhoff (2007) 

provide broad guidance in identifying, controlling, and preventing ASR, primarily through materials 

testing and selection. The Federal Highway Administration published a comprehensive report on the 

diagnosis, prognosis, and mitigation of ASR affected concrete (Fournier et al. 2010), and a surveying and 

tracking guide (Thomas et al. 2012b). The Institution of Structural Engineers (UK) has produced a guide 

for the appraisal of existing structures (ISE 1992). RILEM is developing guidance on the appraisal and 

management of structures exhibiting damage due to expansive aggregate (Godart et al. 2012, Godart et 

al. 2013). Although each of these guides serves a purpose, none provides guidance for evaluating the 

structural capacity of a structure exhibiting distress due to ASR. 

2.1 State, Severity, and Rate of ASR

There are a number of reliable techniques for confirming the presence of ASR in existing structures, most 

notably petrographic analysis and staining tests. These tests can be an indirect indication of the severity 

of ASR by defining the spatial extent of the affected concrete. Unfortunately, these tests only indicate 

the presence of ASR on a surface that is accessible; the presence of ASR at greater depths can only be 

inferred unless cores are removed. Regardless, it is not possible to relate test results to the state/extent of 

the reaction (i.e., initial stages vs. later stages), or the past and current rates of the reaction.

Visual Inspection: The most common way of characterizing the state of (advanced) ASR reaction is 

through visual examination. Distress due to ASR can manifest itself in a number of indicators: 

unidirectional cracks, map (random pattern) cracking, closed joints, spalled concrete surfaces, and 

relative displacement of different portions of a structure (Farny and Kerkhoff 2007). Stark (1991) 

collected examples of ASR cracking in highway structures, along with descriptions for distinguishing 

among cracks due to different degradation mechanisms. A recent field identification handbook published 

by the FHWA (Thomas et al. 2012a) provides a description and some examples of a number of visual 

symptoms: cracking, expansion, localized crushing of concrete, extrusion of joint material, surface popouts, and surface discoloration.

Surface Staining Methods: Over time, the gel resulting from ASR is forced through the pores, and if 

sufficient pressure is generated in the hydrated cement paste, cracks will form that (may) eventually 

propagate to the surface. The gel will flow through these cracks and eventually show at the surface as a 

white, yellowish, or colorless fluid that appears waxy, rubbery, or hard (Farny and Krikhoff 2007). Two 

surface chemical tests exist for identifying the presence of ASR reaction gel (Powers 1999). Natesaiyer 

and Hover (1988) developed a surface test in which uranyl-acetate is applied to the surface, rinsed with 
5

water, and the surface is viewed under ultraviolet light; this test is described in the Annex to 

ASTM C856. Guthrie and Carey (1998) developed a similar method in which sodium cobaltinitrite is 

applied to the surface, which reacts with the potassium in the ASR gel and results in a yellow stain. A

second solution, rhodamine B, is applied to the surface, and if ASR gel is present, it produces a pink 

background and a darker pink stain where reaction gel is present. Neither staining test, however, can 

give an indication as to the severity or rate of the ASR reaction.

Petrographic Examination: Petrographic test methods (ASTM C295/C295M, ASTM C856 or AASHTO 

T 299) have been proven to be effective in identifying many reactive minerals in aggregates or reaction 

products in concrete (Hurcomb 2009). A prepared concrete specimen is examined with optical (Walker 

et al. 2006) and scanning electron microscopes (Clark et al. 1992). Under optical analysis of a cross 

section, the silica gel appears as a darkened area in the aggregate, or around its edges (Farny and

Kerkhoff 2007). FHWA recommends conducting a petrographic examination of the aggregate to be used 

in new construction in accordance with ASTM C295/C295M (Thomas et al. 2008). Rivard et al. (2002),

Rivard and Ballivy (2005), and Grattan-Bellew and Mitchell (2006) have assessed the effectiveness of 

the Damage Rating Index (DRI) that is based on a number of petrographic features observed on a thin 

section: e.g., cracked aggregate, cracked paste, or air voids lined with gel. Each observable is given a 

rating factor, and the overall rating was a sum of the individual factors. Although the method yields 

results that are correlated to the extent of expansion, the method yields comparative results, and the DRI

cannot be used to make an absolute statement about the extent or the rate of the reaction without a preexisting correlation. From a practical perspective, Thomas et al. (2012b) presented applications of the 

use of DRI to bridge structures for the diagnosis of ASR.

Quantification of Cracking: The Cracking Index (CI) is a type of crack mapping analysis that attempts to 

quantify the degree of cracking as an indication of the severity of the reaction. The CI method was 

introduced by the Laboratoire Central des Ponts et Chaussées in 1997 (LCPC 1997) (Fournier et al. 

(2010) provide an description in English) as a tool for assessing transportation structures affected by 

ASR. The method consists of drawing a square (approximately 0.5 m on a side), on the surface of the 

affected concrete and recording the total width of cracks at the intersection of the cracks and the sides of 

the square. The numerical result is the ratio of the total width (millimeters) of the intersected cracks to 

the total length (meters) of the sides of the square; the result is expressed in units of (mm/m). Optionally, 

one can report the “horizontal” and “vertical” components of the CI separately. Fournier et al. (2010) 

provide criteria for determining whether more detailed investigations are needed: a CI greater than

0.5 mm/m, and/or individual crack widths greater than 0.15 mm.

Ben Haha et al. (2007) developed an image analysis technique, applied to polished cross sections, to 

quantify the degree of reaction and correlated this to the amount of expansion. Dunant and Scrivener

(2010) extended this model to include a prediction of the loss of stiffness as a function of the extent of 

the reaction.

Properties of Silica Gel: Detecting ASR gel from bulk analysis is difficult because the chemical 

composition of the gel is similar to the calcium-silica-hydrate gel that forms from the reaction of water 

and portland cement. Therefore, new techniques for detecting ASR gel in concrete will need to exploit 

properties of the gel that make it distinguishable from the other phases present. Studies have been 

conducted to quantify the properties of the amorphous silica gel emanating from the reaction site. 

Hamoudi et al. (2008) combined X-ray diffraction, nuclear magnetic resonance spectroscopy, and infrared Fourier transform spectroscopy to characterize the gel formed from high purity silica reacting in an 

alkaline environment at elevated temperatures. Cong et al. (1993) used nuclear magnetic resonance 

spectroscopy to study polymerization of natural materials, reacted with potassium hydroxide in the 
6

presence of calcium hydroxide, and noted that the reaction products were similar to the calcium silicate 

hydrate gel produced by portland cement hydration. Presently, however, no single technique has 

emerged that can provide a reliable quantification of the ASR gel.

Stiffness Damage Test: The Stiffness Damage Test (SDT) was proposed by Chrisp et al. (1993) to 

characterize the extent of damage in ASR-affected concrete, and it has been adopted in the United 

Kingdom by the Institution of Structural Engineers (ISE 1992). The method uses cyclic (5 cycles) 

uniaxial compressive loading, typically between 0 MPa and 5.5 MPa (800 psi), of concrete core samples,

to quantify changes in certain mechanical properties:

1. The reduction in the Young's modulus of elasticity, and

2. The energy dissipated during the load-unload cycles, which corresponds to the area of the 

hysteresis loops in a stress-strain plot, and

3. The accumulated plastic strain after these cycles, which is related to the closure of the existing 

cracks and to a slip mechanism, and thus represent a measure of the damage in the specimen 

(microcracking) in the direction of the applied stress.

One of the drawbacks to this method is that correlations are needed to relate the results of the SDT 

method to the expansion measured from the concrete prism tests (ASTM C 1293) discussed in 

Section 2.7. This is a serious drawback for an existing structure, particularly one that is decades old and 

for which there are no samples of the original aggregate. In this case, the process would have to be 

modified, possibly by using the aggregate extracted from the affected structure; this approach would only 

provide an approximate measure of the subsequent expansion.

Smaoui et al. (Smaoui et al. 2004a, Smaoui et al. 2004b, Bérubé et al. 2005) performed an extensive 

study to compare the effectiveness of the surface cracking index (CI), the stiffness damage test (SDT)

(but cycling up to 10 MPa), and the damage rating index (DRI) in estimating the extent of ASR reaction. 

In the third report (Bérubé et al. 2005), the methods were compared when applied to existing structures. 

The authors found that surface crack widths provided good estimates of concrete expansion if the 

degradation was sufficiently severe. The DRI method was not able to differentiate some of the most 

affected concretes from the lesser affected ones. The SDT method proved to be the most promising 

method for estimating non-prestressed concrete expansion; the method underestimated expansion in 

prestressed concrete members. More recently, Sanchez et al. (2012) have shown that the best results for 

determining the internal condition of concrete affected by ASR can be obtained when the concrete 

specimens are loaded to about 40 % of their design strength.

Ultrasonic Assessment of ASR Reaction: A number of techniques have been proposed that use ultrasonic 

stress waves to assess the severity and rate of ASR. Saint-Pierre et al. (2007) used Fourier analysis of the 

compressive wave pulse through a cylinder specimen to relate the pulse velocity and the wave 

attenuation at 100 kHz to the expansion. With the growing number of acoustic techniques for detecting

cracking in concrete, Kodjo et al. (2011) have investigated ultrasonic techniques that may be able to 

distinguish between cracking due to ASR and cracking from mechanical damage. However, the test 

requires analyzing the creep response of specimens, and although comparative differentiation was 

possible, it is not clear whether an absolute determination can be made. Recently, nonlinear resonance 

spectroscopy techniques have shown promise in detecting defects in concretes having distributed thermal 

damage (Payan et al. 2007), and the same technique may be applicable to ASR damage. The concept 

was extended to a nonlinear impact resonance acoustic spectroscopy (NIRAS) principle for detecting

ASR damage in concrete (Leśnicki et al. 2011), whereby changes in the nonlinear acoustic response of a 
7

prism specimen undergoing ASTM C 1293 testing are compared to the expansion occurring during the 

test. The results suggest that the test might be used to evaluate the evolution of the ASR reaction, and it 

can be applied to cores taken in the field, suggesting the possibility of a future field test.

2.2 In-situ Mechanical Properties of ASR-Affected Concrete

During the past several decades, a number of studies aimed at assessing the effects of ASR on the 

mechanical properties of concrete have been carried out (Ahmed et al. 1999, 2002 and 2003, Bollinghaus 

1985, Chana 1988 and 1989, Chana and Koroboski 1991 and 1992, Clark 1989, McLeish 1990, 

Deschenes et al. 2009, Nixon and Hobbs 1988, Smaoui et al. 2006, Swamy and Al-Asali 1988). These 

studies focused on determining the extent of reduction of the compressive and tensile strengths and the 

modulus of elasticity of ASR-affected concrete. In general, the compressive strength was determined by 

testing cores (taken from either in-situ structural members or laboratory specimens), standard test 

cylinders (laboratory conditioned for accelerated expansion), or cubes (prepared from cores or laboratory 

conditioned cube specimens for accelerated expansion). The tensile strength, in all cases, was determined 

by splitting tensile tests of cores or standard cylinders or, to a lesser extent, direct tension tests of cores. 

The modulus of elasticity was determined either from axial compression or direct tension tests or simple 

beam tests. 

Provided water is available in the pore system, the compressive strength of non-ASR-reactive concrete 

increases as it ages due to additional hydration. On the other hand, the above-mentioned studies have 

shown that the compressive strength of ASR-affected concrete undergoes a progressive reduction in 

strength with increasing expansion (Chana 1988, Nixon and Hobbs 1988, and Swamy and Al-Asali 

1988). These studies presented the degradation of strength of concrete as a function of the expansion. For 

example, the reduction in strength reaches about 29 % at 0.39 % expansion, as reported by Smaoui et al. 

(2006). 

The tensile strength of in-situ concrete determined by testing of cores (either splitting tensile test or direct 

tension test) would vary substantially depending on the location of the cores with respect to the 

placement (upper half or lower half of a beam, for example) and the direction of loading with respect to 

the general direction of microcracks. It was noted by Smaoui et al. (2006), that the direct tensile strength 

is particularly sensitive to the cracks perpendicular to the direction of the applied load. In addition, 

because the development of microcracks due to ASR in a structural member is heterogeneous, it is 

reasonable to expect that the variation of the tensile strength of ASR affected concrete from core tests 

would be large. Both McLeish (1990) and Deschenes et al. (2009) have shown a large variation of 

tensile strength using cores. Therefore, it is reasonable to expect that the reduction of the in-situ tensile 

strength of ASR-affected concrete members would vary substantially depending on (1) the location 

where cores are taken, (2) the test method (splitting tensile test vs. direct tension test), and (3) the 

direction of applied tensile stress with respect to the general direction of cracks due to the ASR 

expansion.

The modulus of elasticity can be determined from the stress-strain relationship obtained from the 

compression and direct tension tests. Both tests yield about the same elastic modulus value (Smaoui et al. 

2006). In the case of nonreactive concrete, the stress-strain relationship remains relatively constant with 

time. However, because the stress-strain relationship of ASR-affected concrete changes over time, the 

modulus of elasticity also changes. The reduction in the modulus can be significant (up to 30 %) even at 

only 0.04 % expansion (Samoui et al. 2006). The reduction the elastic modulus increases as the concrete 

expands further due to ASR. 
8

The transfer of stress between reinforcement and concrete is a complex mechanism that is influenced by 

a number of factors. Bond or anchorage capacity between the reinforcement and the concrete is necessary 

to ensure composite action of the two materials in resisting applied load. In general, the anchorage 

capacity of a reinforcing bar in concrete, depends on the type of reinforcement, the depth of concrete 

cover and the tensile strength of the cover concrete. Depending on the type of reinforcing bar, anchorage 

capacity is developed by (a) chemical adhesion between the steel and the concrete, (b) frictional

resistance to sliding after the adhesion has been broken and (c) bearing of the concrete against the 

reinforcing bar surface deformations (or lugs). Research has shown that bond strength is proportional to 

the tensile strength of concrete (Ferguson 1966, Van de Meer 1975, Orangun et al. 1977). Limited 

research on the effect of ASR on the bond strength (Ahmed et al. 1999, Chana 1989) shows that ASR 

causes substantial reductions in the anchorage capacity, as compared with that of the sound concrete. 

Conventional pull-out tests also have shown (Hobbs et al. 1987) that as much as a 50 % reduction in the 

pull-out strength was observed in ASR-affected concrete if there was no transverse reinforcement to 

confine the concrete. 

2.3 In-situ Strength of ASR-Affected Concrete 

To assess the adequacy of the structure, the actual in-situ concrete strength must be known. At present, 

the transient properties of ASR-affected concrete are estimated by testing of cores taken from the actual 

structural members. Because ASR activity is not uniform throughout the member cross section, the 

concrete strength determined from cores can vary through the cross section. Consequently, the strength in 

the central and cover regions of a concrete member could vary significantly. In addition, the variability of 

concrete strength through the cross section is affected by the location within a placement of concrete and 

the direction of testing. This variation increases as a result of ASR expansion and is non-uniform (Chana 

and Koroboski 1991) because: (a) the structural member would be subjected to differing restraints due to 

loading, geometry and reinforcement, and (b) the extent of expansion and cracking can vary in different 

directions. As a result, the strength derived from tests of cores taken from an ASR-affected concrete 

structure may vary considerably. It should be pointed out that the non-uniformity also exists in sound 

concrete members. However, the range of the non-uniformity is known and reasonably well documented. 

Furthermore, the presence of transverse reinforcement can also influence actual in-situ strength of the 

ASR-affected concrete (Chana and Koroboski 1991). All these factors must be considered in determining 

the strength of the in-situ concrete. This will require testing more cores for ASR-affected concrete than 

are typically tested for sound concrete.

In addition, the long-term relationship between compressive strength and tensile strength of in-situ

concrete must be established because design is based on the code-provided relationships, which are based 

on sound concrete. As a result, the failure mode may change from that of the original design 

assumptions. Thus, the performance of structural members may change over their service life due to ASR 

degradation of concrete.

2.4 Applicability of ACI Documents to ASR-Affected Concrete

ACI standards related to design and construction of nuclear power plants, ACI 349 (Code Requirements 

for Nuclear Safety-Related Concrete Structures and Commentary), and ACI 359 (Code for Concrete 

Containments) are for sound concrete. These standards assume that the materials are selected so that 

ASR is prevented. Similarly, ACI 318 (Building Code Requirements for Structural Concrete and 
9

Commentary), to which both ACI 349 and ACI 359 refer, is for sound concrete. Thus, ACI 318, ACI 349 

and ACI 359 do not include any provisions for assessing the strength of ASR-affected concrete members.

There are several ACI documents (committee reports) that are relevant to ASR. These are:

a. ACI 349.3R (Evaluation of Existing Nuclear Safety-Related Concrete Structures) 

This document provides information on inspection of concrete nuclear structures subjected to 

age-related degradation including chemical attacks, thermal exposure, cement aggregate 

reactions (ASR) and others. It also suggests test methods that could be used to evaluate 

potential for ASR.

b. ACI 224R (Control of Cracking in Concrete Structures)

This document provides information on the principal causes of cracking and recommends 

crack-control procedures.

c. ACI 221.1R (Report on Alkali-Aggregate Reactivity)

This document summarizes currently available information on alkali-aggregate reactivity 

including alkali-silica reactivity (ASR) and alkali-carbonate reactivity (ACR). It provides 

information on design of concrete mixture (materials) and test methods to evaluate potential for 

ASR. The document also provides limited information on repair methods for cracks.

2.5 Managing the Effects of ASR

The hydroelectric dam community has been studying management strategies for ASR-affected structures

for many years. Fournier et al. (2004) discussed options for determining the structural integrity and 

evaluating the potential for future deterioration, and provided a process for taking appropriate 

management actions. Fournier et al. (2010) and Charlwood et al. (2012) discuss managing the long-term 

expansion, and options for maintaining serviceability.

In any structure, the ongoing chemical reaction can be difficult to stop using repair or rehabilitation. The 

three key contributors to ASR are reactive aggregate, alkalinity, and water, and one can slow the rate of 

reaction by eliminating one or more of these contributors. The most common approach is to reduce 

access to water, typically by making the structure impermeable to water. This approach works best for 

stand-alone elements like columns. For a buried structure, the concrete-soil interface may be

inaccessible. Furthermore, for a buried structure like a tunnel in contact with groundwater at the exterior 

surface, drying only the interior of the structure will have the effect of driving more water through the 

structure, and the evaporation front near the interior surface will concentrate alkalis, thus possibly 

accelerating the ASR near the interior surface. Flushing an element with excessive water to dilute the 

alkalis is problematic because it will difficult to ensure that the water will infiltrate the entire member; 

this would likely only be attempted if the structure exhibited extensive cracking, but the water would 

almost certainly only flow through the cracks and not through the affected hardened paste. Furthermore, 

the net effect may be to provide a reservoir of water for continued reaction in the uncracked portions of 

the concrete. As a result, there are no established methods or procedures at present to prevent progress of 

deterioration after the initiation state of the ASR.

On approach that may have promise in reducing future ASR-induced expansion is the use of sealers to 

prevent additional moisture ingress. Bérubé et al studied the effectiveness of different sealer 
10

technologies on laboratory specimens (Bérubé et al. 2002a) and highway median barriers (Bérubé et al. 

2002b). Some of the technologies were effective in reducing expansion to an acceptable level, if applied 

early on in the ASR reaction, and that subsequent expansion was strongly correlated with the change in 

concrete mass due to moisture transport. Drimalas et al. (2012) also reported on the effectiveness of 

sealers on highway structures. They concluded that topical silane treatments can be effective in slowing 

down expansion, but developing conclusive evidence may require many years. 

Another solution to managing the effects of concrete degradation is to catalog the performance of 

existing structures. The Oak Ridge National Laboratory (ORNL) has created a Nuclear Concrete 

Materials Database (Naus 2011, Ren 2012) using an existing ORNL materials database infrastructure. 

Some of the ultimate uses of the database would be to facilitate electronic and mathematical processing 

for analysis and modeling in support of facilities management. In addition, the FHWA has developed a 

guide for surveying and tracking alkali-silica reactivity in concrete (Thomas et al. 2012b). The guide 

was intended to help State Highway agencies survey and track transportation infrastructure that has been 

affected by ASR, primarily resulting in cracking and expansion.

2.6 Numerical Modeling of ASR

A model for the pore pressure has been developed as a function of aggregate size and paste transport 

properties (Bažant and Steffens 2000). The model was based on a spherical reactive glass particle that 

generates gel uniformly over its surface, and the predicted pressure generation was consistent with the 

observed pessimum aggregate size effect observed by Meyer and Baxter (1998) and Jin (1998). Further 

modeling is needed, however, to account for the role of aggregates in the early stages of reaction and the 

role of the hardened paste in the later stages of the reaction (Dunant and Scrivener 2012). 

Ulm et al. (2000) developed a thermo-chemo-mechanical model for ASR in concrete. Although the 

model yields realistic responses, it requires a large number of material parameters as inputs, and it is not 

clear to what extent that model is applicable to existing structures already exhibiting ASR damage. (One 

might use such a model as part of a monitoring program where future observations are used to improve 

the model parameters). Saouma and collaborators (Saouma and Perotti 2006, Saouma et al. 2007) have 

developed structural models using the functional form of the thermodynamic model to predict future 

properties; the thermodynamic model parameters are determined empirically from measured mechanical 

properties of the concrete.

Bangert et al. (2004) developed a chemo-hygro-mechanical model for ASR that incorporates the silica 

dissolution reaction, a mixture model for the volume change, a kinetics model, the viscosity of the gel, 

and the saturation dependent permeability. The model predicts the expansion, the expansion rate, the 

damage parameter (ratio of damaged cross section to the original cross section), and the reaction force as 

a function of the load factor. Although it is an interesting comprehensive model, the authors do not 

discuss the applicability of the model to structural concrete elements containing steel reinforcement. 

Pesavento et al. (2012) developed another model that accounts for non-isothermal and changing moisture 

conditions. Although this model is able to reproduce strain and mass variations measured in the 

laboratory, there are many model parameters. Recently, Puatatsananon and Saouma (2013) have 

developed a chemo-mechanical model for ASR that also incorporates the effects of internal moisture and 

ion concentration on the transport properties of the concrete. It is a two-stage model that couples a finite 

difference transport (pore solution ions and ASR gel) model to a finite element nonlinear analysis to 

estimate the resulting expansion.
11

2.7 ASR Prevention for New Construction

Mixture Proportioning Guides: Industry organizations provide guidance in developing concrete mixtures 

to mitigate the potential effects of ASR if aggregates that exhibit some measureable reactivity have to be 

used. The Portland Cement Association provides guidance for the specification of concrete (PCA 2007). 

The recommendations typically limit the total alkali to less than 0.5 % on a mass basis for ordinary 

portland cement mixtures, and increase the allowable alkali contents with increasing replacement of 

portland cement with pozzolans, or slag cement, or both (Lane and Ozyildirim 1995, Lane and 

Ozyildirim 1999, Thomas et al. 2008). AASHTO has adopted the Federal Highway Administration 

(FHWA) guides (Folliard et al. 2006, Thomas et al. 2008) for selection of concrete materials for 

preventing ASR, and has recently established a standard practice (AASHTO PP65) for determining the 

reactivity of aggregates and selecting appropriate for preventing deleterious expansion (Thomas et al. 

2012c, Thomas et al. 2012d). There are also recommendations for using Class N fly ashes to mitigate 

ASR (Ballard et al. 2008). Alternatively, one might accept that ASR is going to happen and use fibers to 

resist the expansion (Bektas et al. 2006), or starve the reaction by adding additional reactive aggregate as 

a powder (Carles-Gibergues et al. 2008). The use of air entrainment in concrete was reported to reduce 

expansion due to ASR (McCoy and Caldwell 1951; Jensen et al. 1984; Pleau et al. 1989), but generally 

not enough to adequately prevent deleterious expansion and cracking due to ASR. Mather (1999) 

recommended using satisfactory slag cement or pozzolan, and maintaining the concrete’s internal relative 

humidity below 80 % throughout its service life. 

Materials Chemical Test Methods: There are test methods to evaluate both the aggregate and the 

cementitious materials to be used for a specific concrete (Thomas et al. 2006) that report the unrestrained 

expansion of a test specimen as a function of time: 

• ASTM C227 (Mortar Bar Test) was the original mortar bar test. Leaching of alkalis from the 

mortar bars during the test, however, makes the test method unsuitable for determining critical 

alkali levels, and the test is not considered to provide an accurate assessment of the potential 

reactivity of an aggregate with a given cement.

• ASTM C441/C441M (Pyrex Mortar Bar Test) is similar to ASTM C227, but uses Pyrex 

(borosilicate) glass to test the effectiveness of pozzolans in mitigating ASR when used with a 

high alkali portland cement. 

• ASTM C1260 (Accelerated Mortar Bar Method) is an accelerated mortar bar test in which the 

proposed aggregate is crushed and sieved, added to a prescribed mortar mixture that includes 

additional alkali, formed into a mortar bar (25 mm x 25 mm x 250 mm) and submerged in a 

NaOH solution at a temperature of 80 °C. The test requires 16 days to complete.

• ASTM C1293 (Concrete Prism Test) can be used to test either the coarse or the fine aggregate, or 

the combination of an aggregate with a pozzolan or slag. The specimen is made with both coarse 

and fine aggregate, includes the addition of an sodium hydroxide (NaOH) solution to achieve a 

prescribed alkalinity, and the material is cast into a prism that is approximately 75 mm x 75 mm x 

275 mm. The test requires a year to complete.

• ASTM C1567 (Accelerated Mortar-Bar Method) is a rapid evaluation of the effectiveness of a 

proposed supplementary cementitious material (SCM) to mitigate the effects of ASR for a given 

fine or coarse aggregate. Test conditions are the same as in ASTM C1260, and the test requires 

16 d to complete.
12

• Autoclave Mortar Bar Tests (Tang et al. 1983, Fournier et al. 1991) accelerate the testing by using 

high temperature (150 °C) and pressure, typically on for a few days. The samples are often 

prepared with an alkaline mix solution and are steam cured, which accelerates the overall testing 

plan. These methods, however, have not yet been standardized.

FHWA has developed a guide for evaluating the reactivity of the aggregate (Thomas et al. 2008). The 

format is a flow chart, as shown in Figure 2, and the process involves two of the test methods described 

above. The resulting decisions are either that the aggregate is non-reactive and no precautionary 

measures are needed, or the aggregate is alkali-silica reactive and either preventive measures are needed 

or the aggregate should not be used.

Figure 2. Sequence of laboratory tests for evaluating aggregate reactivity (from Thomas et al. 2008)

14

This effect of cracks on transport is limited to the crack portion of the concrete. If there are surface 

cracks that only penetrate a (relatively) short distance to the reinforcement, the net effect is minimal

(Snyder 2000). Therefore, to gauge the effect of cracks on the bulk transport coefficient, one must 

characterize both the topology of the cracks and the depth of the cracks.

2.9 Summary

Significant research has been conducted in the mitigation and detection of ASR in concrete. 

Unfortunately, relatively little research has been applied to estimating the remaining capacity of ASRaffected structures over time by relating observable quantities in the field to the mechanical properties of 

the concrete. Furthermore, there is relatively little technical knowledge or existing data for establishing 

such relationships. This gap is discussed further in the following chapter.
15

3 GAP ANALYSIS

While research summarized in Chapter 2 has resulted in a better understanding of mechanisms of damage 

to concrete by alkali-silica reactivity, there are still knowledge gaps on how to control alkali-silica 

reactivity, how to predict future development of cracks, and how to predict the performance of concrete 

structures affected by alkali-silica reaction. In this chapter, gaps in the current state of knowledge of 

methods for detecting ASR activity before cracks develop, for preventing further progress of crack 

development, for assessing the mechanical properties of ASR-affected concrete, and for predicting the 

progress of deterioration due to ASR are discussed. 

3.1 State and Severity of ASR; Predicting Rate of Reaction

There are no reliable direct test methods for determining the degree or the rate of the ASR in existing 

structures. The severity of the reaction can be otherwise inferred from the degree of expansion and 

cracking. Unfortunately, expansion will not be detected until it becomes macroscopic (unless 

displacements have been monitored precisely over time), and the amount of expansion and cracking 

depend on the degree and the extent of the confinement provided by the steel reinforcement. In general, 

more cracking implies more reaction and expansion, but there is no simple relationship between the two. 

For an existing structure, for which samples of the original aggregate do not exist, there are no reliable 

test methods for quantifying the state or the rate of ASR. One could monitor the degree of in-place 

expansion or cracking, but this approach has limitations: the expansion must be monitored over a 

sufficiently long period to observe quantifiable displacements; the degree of cracking depends upon 

factors, like confinement by reinforcement, that are not related to the extent or rate of the reaction. 

Furthermore, although the expansion often exhibits a sigmoidal shape over time, one must have data over 

a sufficiently long time to ascertain conclusively the stage of the reaction. Moreover, expansion vs. time 

curves are most often developed for unrestrained specimens such as the mortar bars and prisms used in 

standardized test methods.

Bulk chemical detection and quantification of the gel is also hindered by a lack of uniqueness of the ASR 

gel properties. The alkalis react with the silica, which can incorporate calcium from the portlandite

produced during cement hydration. As a result, the chemical properties of the ASR gel are similar to the 

chemical properties of the strength-producing calcium-silicate-hydrate (C-S-H) gel that forms as a 

hydration reaction product of portland cement and water. Furthermore, the amorphous silica in the 

aggregate will be difficult to differentiate from the amorphous silica in the ASR gel using X-ray 

diffraction. There are some laboratory studies of the gel product formed under controlled conditions

(Hamoudi et al. 2008), but additional laboratory research is needed to assure that one can quantify the 

amount the gel uniquely and consistently in field specimens.

3.2 In-Situ Mechanical Properties of ASR-Affected Concrete

The main mechanical properties of concrete that need to be determined for the assessment of the 

structural adequacy are:

a) Compressive strength 

b) Tensile strength

17

Figure 3. Relationships between ASR expansion and loss in compressive and tensile strengths (from 

Smaoui et al. 2006)

Figure 4. Relationship between ASR expansion and loss in modulus of elasticity (from Smaoui et al. 

2006)

It should be pointed out that, the long-term relationship between the compressive and tensile strengths of 

in-situ concrete must be established because the original design was based on the code-provided 

-80	

-60	

-40	

-20	

0	

0	 0.1	 0.2	 0.3	 0.4	 0.5	

Loss	in	strengths	(%)	

Expansion	(%)	

Compression	

Splitting	tensile	

Direct	tension	

-80	

-60	

-40	

-20	

0	

0	 0.1	 0.2	 0.3	 0.4	 0.5	

Loss	in	modulus	of	elasticity	(%)	

Expansion	(%)	

Compression	

Direct	tension	

19

As indicated in Section 2.3, ASR reactivity is not uniform throughout the cross section of a concrete 

member. Thus, the concrete strength determined from cores can vary across the cross section. In order 

to determine the mechanical properties of the weakest parts of the structure, it would be desirable to 

detect, using a non-intrusive method (such as nondestructive test methods), the locations where ASR is 

most active. At present, no standardized nondestructive testing methodologies are available to map ASR 

activity in structural members. 

3.5 Managing the Effects of ASR

There is no definitive means of managing the effects of ASR to ensure long-term performance of ASRaffected structures. Measures to prevent additional water penetration will have an unpredictable efficacy 

because there is no way to know how much more damage will be caused by the water already inside the 

concrete. This is because it is not possible to determine the degree of the reaction, from which to 

estimate future expansion from existing water. Attempts to dry out one side of a structure will hasten 

precipitation near the drying surface, which will concentrate the alkalis, resulting in accelerated ASR 

near the surface.

Options for managing the ASR reaction in existing concrete structures are limited. One approach is to 

stop, or significantly slow, the reaction. Because the reactive silica is in the aggregates, the remaining 

options are to starve the reaction of water or alkalis. Assuming that water is available from the 

environment, the use of water barriers is limited to structures for which the external surface is accessible. 

Reducing alkalis requires changing the pore water (flushing the concrete), which would likely ensure 

complete saturation of the concrete. This, in turn, may provide sufficient moisture to maintain the 

reaction for a number of years. 

One repair alternative is to fill existing cracks with a sealant such as epoxy, but the effectiveness of such 

an approach is difficult to quantify. If successful, this could flush out water that was in the cracks, and 

fill the cracks with an impermeable material that would prevent additional water from entering the 

concrete. The overall success of this approach, however, may depend strongly on the moisture content of 

the concrete. If the concrete is saturated with water, the epoxy will not displace the large quantity of 

water in the cement paste, and the reaction will continue. If there is sufficient water that additional 

cracking occurs, these cracks will form new avenues for additional water to enter the concrete. 

It may also be difficult to quantify the efficacy of drying the concrete element at the internal surface

when the external surface is exposed to moisture, as this may also cause problems. Just below the 

internal surface of the concrete there will be a drying front at which the moisture in the concrete 

transitions to vapor. At this transition point, the alkalis in the pore solution will be a maximum, which 

could further drive the ASR reaction. With a constant source of moisture on the external surface, 

transport through the element will continue to supply additional alkalis to the drying front. Complex heat 

and moisture transport models are needed to determine whether such an approach is beneficial or not.

Any approach to managing the effects of ASR on structural concrete will require some means of 

assessing the structural properties of the concrete because of the evidence that cracking in general, and 

ASR specifically, can reduce the capacity of structural concrete elements. Therefore, a lack of methods 

for evaluating the structural properties would be a serious gap in the ability to develop a comprehensive 

aging management plan. Furthermore, if there are no tools or tests that one can use to predict the rate of 

degradation, one will have to incorporate a higher rate of inspection (with respect to the rate of inspection 

of concrete without degradation). 
20

3.6 Numerical Modeling of ASR

No reliable models exist for simulating alkali-silica reactions at the microscopic length scale. Moreover, 

there is still debate on whether the gel forms at the surface of the aggregate or whether silica species 

diffuse through the pore solution and react with other dissolved species within the pores. Furthermore, 

details of the reaction may depend on the chemical nature of the specific reactive phase; there are 

numerous possible reactive phases to consider. 

None of the numerical models for the alkali-silica reaction are directly applicable to assessing the 

condition of an existing structure. The models are typically “expanding sphere” models that require a

relatively large number of material properties of the undamaged concrete. 

3.7 ASR Prevention During New Construction

There are no reliable test methods or industry best practice guides for testing and qualifying materials 

that may be sufficiently immune to ASR for 75 years to 100 years. Current test methods, such as the 

accelerated mortar bar test and the concrete prism test, and the acceptance performance criteria that are 

often required from this testing, were developed to assure reliable performance in concrete structures that 

would be in service for only 50 years to 75 years. Presently, the most up-to-date guide is the AASHTO 

PP 65 standard practice (Thomas et al. 2012c, Thomas et al. 2012d).

3.8 Effects of ASR-Induced Cracking on Transport Properties

Much is still unknown about the specific effects of cracks on transport properties, for any degradation 

mechanism. The challenge with ASR is that expansion occurs wherever the reaction occurs, so the 

cracks tend to be distributed throughout the concrete. Furthermore, the cracking can be altered 

significantly as a result of the amount and the orientation of steel reinforcement. 

The cracking that occurs from extensive ASR can provide pathways for fluid transport through the 

concrete. If the concrete structure is located in a region with brackish water, infiltrating groundwater 

could accelerate the rate of corrosion in reinforced concrete. For degradation mechanisms that depend 

upon diffusion (e.g., corrosion of the reinforcement, sulfate attack), any water-filled cracks will 

accelerate the rate of diffusion.

The degree to which the cracking will accelerate degradation depends on the nature of the cracking and 

the degradation mechanism. If cracks are “finely” distributed, they contribute to the overall bulk 

transport through the concrete. If cracks are long and oriented, the cracks in proximity to individual 

reinforcement elements may play a far greater role in influencing the rate of corrosion than cracks farther 

away from the reinforcement. 

For “finely” distributed cracks, one can approximate the overall effect by characterizing the relative 

change in the bulk transport property. Because for saturated concrete relative changes in diffusivity are 

(nearly) proportional to relative changes in electrical conductivity, a doubling of the electrical 

conductivity would suggest there is a commensurate doubling of the diffusivity. Therefore, either buried 

or surface electrodes could be used to monitor changes in the transport properties.
21

For wide cracks oriented parallel to the transport direction, a slightly different strategy is needed, but one 

that still relies upon exploiting the relationship between conductivity and diffusivity. The change in the 

diffusivity of the cover concrete is equivalent to the change in conductivity. Therefore, measuring 

changes in the electrical conductivity between the outside surface and the reinforcement should give a 

meaningful indication of the effect of the large oriented cracks on the rate at which dissolved species like 

chlorides will diffuse to the reinforcement. Again, this is valid provided conductivity is measured when 

the concrete is saturated.

The challenge in using these electrical methods is accounting for the differences in the electrical 

conductivity of the pore solution in the hardened paste and the electrical conductivity of the solution in 

the cracks. As a first approximation, one could approximate the contents of the cracks by the 

groundwater composition.

3.9 Summary

This section addressed current gaps in our understanding of the performance of ASR-affected concrete. 

The primary gaps include (1) the inability to estimate the extent and rate of the chemical reaction without 

having historical data; (2) the inability to estimate the in-situ strength of the concrete; and (3) the 

inability to estimate quantitatively the effects of the ASR-induced cracking on transport (and other 

degradation mechanisms). Chapter 4 provides a recommended research plan to address the gaps 

identified in this chapter.
22

4 TECHNICAL PLAN

The strategies for evaluating existing concrete construction affected by ASR are based on the assumption 

that relatively little historical data exist for either the expansion exhibited by specific structural elements 

or the relevant material properties of the materials used during construction. Furthermore, separate plans 

are given for evaluating the existing properties of concrete and for estimating future properties of the 

concrete.

4.1 Articulated Priorities

The technical plan required to meet all the objectives addressed in Section 4.2 will be composed of a 

number of tasks, with each task contributing to one or more of the objectives. The technical plan in this 

chapter is developed to address these objectives assuming the following priority order as determined by 

NRC:

1. Identifying and evaluating method for managing the effects of ASR to maintain intended 

functionality

2. Identifying criteria to assess the state of the ASR with respect to structural performance

3. Identifying and evaluating methods for determining the state and severity of the ASR 

degradation, and for predicting the rate at which the reaction is progressing

4. Identifying the effects of ASR on the capacity of anchorages

5. Identifying and evaluating methods for determining the impact of ASR on the mechanical

properties of reinforced concrete as they relate to the static and dynamic performance

6. Identifying parameters to be measured and the criteria to be used to determine when the ACI 349 

code provisions and other related standards and codes may not be valid

7. Identifying data needed to develop reliable numerical models for the formation of ASR and for 

evaluating the performance of ASR-affected concrete.

8. Quantifying the change in transport processes through ASR-affected concrete and identifying 

how it may affect other degradation mechanisms such as corrosion of the steel reinforcement

9. Identifying standard test methods for new construction that may improve the effectiveness of 

reducing the likelihood of ASR.

As mentioned in Section 3.5, assessing the structural performance of a reinforced concrete element 

exhibiting distress due to ASR is an important component of a management plan. Therefore, the top two 

priorities are closely related. Furthermore, an aging management plan that includes a revised schedule 

for inspection and evaluation for affected concrete will require some means of understanding the rate of 

the ASR degradation. Therefore, a technical plan that focuses on developing tests for estimating the 

current and future structural properties will address the five highest priorities, and contributes to next two 

highest priorities.

4.2 Structural Capacity of Existing Structures

The Technical Plan for existing structures includes two objectives:

1. Determining the current structural capacity of an element

2. Estimating the future structural capacity of an element
23

The goal of the first objective is to develop a testing procedure that will yield reliable material property 

data for performing a structural evaluation of the current structure. The goal of the second objective is to 

develop a methodology for estimating the extent of the reaction over time, and building a relationship 

between the extent of the reaction and the changes in mechanical properties. 

The plan is described briefly as follows:

1. Estimating the degree of confinement in ASR-affected reinforced concrete element using a linear 

elastic expansion.

2. Demonstrate that mechanical properties measured under confining stresses provide reliable 

parameters for evaluating existing structural capacity of the affected reinforced element.

3. Relate the resulting expansive forces to the extent of the reaction, via materials analysis and the 

extent of surface cracking, or both.

4. Develop a relationship between changes in the mechanical properties and a combination of the 

ASR-induced cracking and the confinement stress.

5. Create a model to estimate the future extent of ASR (including expansive forces and the extent of 

cracking).

4.2.1 Determine Structural Capacity of an Element

Objective: Validate an approach for estimating the structural capacity of ASR-affected reinforced 

concrete elements.

Overview:

1. Cast large-scale ASR-reactive reinforced concrete elements to represent common structural 

components, and embed tri-axial strain gauges at selected locations.

2. Perform a computational expansion analysis of the reinforced concrete element. Determine the 

degree of expansion that yield a strain distribution that best agrees with the strain gauge data.

3. Develop a reliable protocol for determining the extent of the reaction in an existing structure. 

This may include a combination of, test methods such as measurement of: surface cracking (e.g., 

CCI), mechanical tests (e.g., Stiffness Damage Test (SDT)), acoustic tests (e.g., acoustic 

emission, NIRAS), petrographic tests (e.g., mineral analysis, gel analysis), or image analysis of 

polished sections. 

4. Validate the extent of the reaction to the expansive forces obtained from the expansion analysis.

5. Remove cores from the large-scale elements and perform compressive and tensile tests under triaxial restraint consistent with the restraint indicated by the strain gauges. The expansion analysis 

will also be used to determine the ideal core orientation.

6. Relate the expansive forces and the internal confinement to the changes in the mechanical 

properties.

25

Because the direction of coring with respect to the direction of concrete placement affects the test results 

of specimens cores should be taken from the large block specimens in three orthogonal directions with 

respect to the orientation of the block specimen (see Figure 5).

Figure 5. Direction of taking core samples

4.2.3 Estimating Future Mechanical Properties

Objective: Validate a relationship between changes in the extent of the reaction and changes in the 

mechanical properties of the concrete.

Overview: 

1. Identify a test method for estimating the ultimate extent of the reaction. This is an asymptotic 

value of the expansion sigmoidal curve. This would be a two-step process. Laboratory test would 

be used to relate accelerated residual expansion tests values to ultimate expansion values. The 

relationship would then be validated on large specimens.

2. Develop a first-order kinetic model for estimating the future expansion, given information about 

the mixture design and the ultimate expansion. This model will likely require information about 

temperature and moisture dependence, and may require coupling to physicochemical service life 

computer models for concrete that can simulate both heat and moisture transport. 

The first-order power-law approach may be a middle ground that balances simplicity and accuracy. This 

approach identifies the major contributing factors to ASR and uses physical or chemical knowledge to 

make first-order assumptions about the contribution of that factor to the overall rate of the reaction or 

degradation. Such an approach may be sufficiently accurate for determining the decade during which a 

structure may not have sufficient structural capacity. In such a case, one might then use a more rigorous 

monitoring strategy as the structure nears the predicted decade. 

Deliverables: 

4”x8”	or	6”x12”	core

Direction	of	internal	cracks

Test	configuration

Figure		4.2					Direction	of	taking	cores	for	tests
26

• Test method for estimating the ultimate expansion.

• Develop a monitoring strategy where estimated percentages of the change in structural capacity 

could be used to determine the periodicity of coring and testing for verification.

4.2.4 Predicting the Effect of ASR on Bond and Anchorage

The following two types of tests are required to assess the anchorage of reinforcing bars in concrete 

which will affect the required length of lap splices and the anchorage length required to develop the yield 

strength of reinforcing bars. 

1. Flexural tests with four-point loading using beams that have undergone various stages of ASR 

expansion. The test specimens are prepared with various lengths of lap splices in the constant 

moment region.

2. Pullout tests of single bars embedded in concrete specimens that have undergone various 

levels of ASR expansion. The bar diameter and embedment length are varied to determine 

the effect of ASR expansion on the anchorage strength.

4.2.5 Methodology for Assessing Response Characteristics of ASR-Affected Structures 

to Earthquake Loads

In earthquake prone regions, structures experience lateral movement due to ground shaking. To avoid 

sustaining serious damage, structures are designed such that the lateral displacement of vertical members 

(such as columns and walls) and the end rotation of horizontal members (such as beams and girders) 

would not exceed the allowable values. If the stiffness and energy absorbing capacity are reduced due to 

ASR, the response of structural members to earthquake excitation may exceed the design limits, thereby 

making the structure susceptible to serious damage or collapse during a design level earthquake. To 

assess the structural response of ASR-affected concrete members to the cyclic loading generated by an 

earthquake, the following properties are needed: 

1. Compressive and tensile strengths and the modulus of elasticity of the concrete,

2. Reinforcing bar anchorage under cyclic loading, and

Energy absorption properties of members and connections under cyclic loading.

4.3 Effect of Cracking on Fluid Transport

Although it is understood that cracks increase the average rate of fluid transport through concrete, the 

specific manner in which distributed cracks contribute to transport is still largely unknown. Test 

methods are needed to characterize the overall effect of cracks on transport properties. These tests must 

also address the degree of saturation within the cracks. 
27

1. Identify viable test methods for characterizing the effects of cracks on transport properties. These 

tests may include measurement of electrical properties on the surface or through the laboratory 

specimens or specimens extracted from the structure..

2. Validate the test method as a means of providing concrete service life estimation tools with 

information for modifying transport coefficients to account for the cracking present.

When ASR-affected concrete structures with surface cracks are exposed to aggressive conditions (such as 

chlorides and carbonation), their service lives are impaired due to corrosion of reinforcing steel. In 

addition, corrosion-induced stresses may further widen surface cracks. Thus it is important to determine 

the current rate of corrosion of reinforcing steel.

1. Detection of corrosion activity in reinforced concrete structures.

2. Measurement of corrosion rate in existing concrete structure using techniques such as the linear 

polarization resistance method (ASTM G59, ASTM G102).
28

5 SUMMARY

Evaluating the structural capacity of existing concrete structures exhibiting distress due to alkali-silica 

reaction (ASR) is a challenging problem. There are three primary reasons for this: the reaction rate is 

very slow; any observed expansion would be a function of the specific degree of restraint in the structure; 

and there is no test one can perform that can quantify the extent of the reaction. The first two reasons 

impact efforts to determine the present capacity, and the third reason impacts efforts to determine the 

future capacity.

The relatively slow rate of the reaction in concrete may require decades before the concrete exhibits 

distress at the surface. The slow rate of the reaction is an indication that there are many factors that 

contribute to ASR, and the variability in the rate of reaction reflects the variation in the properties of the 

concretes and their exposure conditions. When studying ASR in the laboratory, the challenge is to study 

representative concretes in a reasonable time frame. To achieve measureable expansion in a reason 

period of time, one must modify the mixture in a fundamental way (e.g., different aggregate sizing, 

additional highly reactive aggregate, alkaline mix water). Doing so, however, risks creating a material 

that does not represent the mechanical and chemical properties of the concrete being evaluated.

One can make correlations between laboratory expansion measurements and tests of mechanical 

properties, but these tests are typically performed on specimens that differ from the field concrete in one 

or more fundamental ways. The implicit assumption is that the visco-elastic properties of the laboratory 

concrete are sufficiently similar to the field concrete that the correlations are reliable for evaluating the 

field concrete over time. Laboratory degradation-strength correlations are typically performed using the 

free expansion of laboratory specimens. In the field, the concrete is almost always constrained by steel 

reinforcement and the continuity of the structure. Therefore, expansion measured at the surface of the 

structure will not be equal to the free expansion that would be observed in an un-reinforced concrete 

element for the same extent of ASR.

For existing structures, the challenge of determining the structural capacity is even greater. The 

degradation-strength correlations for the in-place concrete do not exist. Therefore, one must extract 

specimens and test them. The challenge is that the cracks that form due to ASR have a finite size, and 

they grow in size over time. The strength measured on a concrete core is a function of the mechanical 

properties of the concrete, the size of the specimen, and the ratio of the typical defect (i.e. crack) size to 

the specimen size. For intact (no defects) concrete, the measured core strength can be used to evaluate 

structural capacity in accordance with design provisions in existing codes. For concretes with distributed 

cracking, the effects of the cracks on the structural capacity are not known, and there are no industry 

guides for evaluating the structural capacity of ASR-affected reinforced concrete elements.

Estimating the future performance of an existing concrete exhibiting ASR distress is an even greater 

challenge. In addition to the challenge of estimating the current structural capacity of the distressed 

structure, the engineer must predict the rate and extent of future distress. This task is all the more 

challenging because the reacting phases may not have been accurately characterized, the details of the 

reaction for a particular mineral phase may not be known, and cracking that occurs in concretes exposed 

to the environment will likely accelerate ingress of ionic species that may also influence the rate of the 

reaction. In addition to this, the ingress of external species may accelerate other degradation mechanisms 

(e.g., leaching, corrosion of the steel reinforcement, sulfate attack) that would also influence the 

structural capacity of a reinforced concrete element.
29

A technical plan has been proposed for addressing these challenges. The plan focuses on modifying the 

ACI 318 relationships for estimating tensile strength, modulus of rupture, and modulus of elasticity from 

the compressive strength of distressed concrete; these are critical parameters in a structural evaluation. 

The assumption is that these material properties decrease as a function of the degree of degradation, 

which is either the equivalent degree of free expansion or the extent of the chemical reaction. The 

structural testing consists of making large-scale specimens with reactive aggregate and measuring the 

mechanical properties and the expansion of these elements over time.

In concert with the mechanical testing, extracted specimens will be evaluated to determine the degree of 

the degradation. A number of tests for characterizing the degree of degradation will be considered: 

residual expansion tests, mechanical tests, and petrographic tests. By using a number of different

concrete mixtures that are representative of those used in nuclear power plants, relationships will be 

developed between the degree of the reaction and the modification needed to the ACI 318 relationships 

for estimating mechanical properties required to evaluate structural capacity.

The remaining technical plan addresses numerical simulation of ASR degradation. Of the current 

approaches being used to estimate future properties of ASR-affected concrete, the one recommended 

here is a first-order kinetic approach that captures the contribution of the primary factors affecting the 

rate of ASR. Although the resulting model will not have the accuracy of more sophisticated models, it 

should provide general guidance for estimating the progression of the reaction with time. From this 

predicted extent rate of reaction, one can use the relationships developed from the mechanical testing for 

the ACI code parameters to evaluate the future structural capacity of a structure exhibiting distress due to 

ASR.
30

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