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21st Century Dam Design —

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the region’s

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

The information contained in this publication regarding commercial projects or firms may not be used for

advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

Copyright © 2011 U.S. Society on Dams

Printed in the United States of America

Library of Congress Control Number: 2011924673

ISBN 978-1-884575-52-5

U.S. Society on Dams

1616 Seventeenth Street, #483

Denver, CO 80202

Telephone: 303-628-5430

Fax: 303-628-5431

E-mail: [email protected]

Internet: www.ussdams.org

U.S. Society on Dams

Vision

To be the nation's leading organization of professionals dedicated to advancing the role of dams

for the benefit of society.

Mission — USSD is dedicated to:

• Advancing the knowledge of dam engineering, construction, planning, operation,

performance, rehabilitation, decommissioning, maintenance, security and safety;

• Fostering dam technology for socially, environmentally and financially sustainable water

resources systems;

• Providing public awareness of the role of dams in the management of the nation's water

resources;

• Enhancing practices to meet current and future challenges on dams; and

• Representing the United States as an active member of the International Commission on

Large Dams (ICOLD).

Strength Input Parameters 227

SELECTING STRENGTH INPUT PARAMETERS FOR STRUCTURAL ANALYSIS OF AGING CONCRETE DAMS

Timothy P. Dolen1

ABSTRACT

The Bureau of Reclamation Dam Safety Program performs periodic examinations and risk analysis of all of their dams. Structural analysis results are typically used during a risk analysis to estimate the failure probabilities of concrete dams under static, hydrologic, and seismic loadings. The probability of failure is estimated, often considering average, high, and low strength properties of mass concrete. Not all dams have a complete history of construction and the strength properties must be assumed. A database of mass concrete core tests from the past 50 years was analyzed to determine bond strength input parameters. The average direct tension and shear properties of lift lines were compared for different state-of-the-practice construction methods in the 20th Century. The properties were estimated for three primary construction eras and separately for dams suffering from alkali-aggregate reaction. In addition to bond strength, the percent of bonded lift lines greatly influenced the input parameters for these construction eras.

INTRODUCTION The Bureau of Reclamation (Reclamation) is responsible for about 370 storage dams and dikes that form a significant part of the water resources infrastructure of the western United States. As owner of these facilities and in accordance with the mission of the Dam Safety Program, Reclamation is committed “to ensure that Reclamation facilities do not present unreasonable risks to the public, public safety, and/or the environment.” Reclamation has established a risk-based framework to assess the safety of their structures, to aid in making decisions to protect the public from the consequences of dam failure, to assist in prioritizing the allocation of resources, and to support justification for risk reduction actions where needed. Structural analysis results are typically considered during the risk analysis to better assess the performance of concrete dams under static, hydrologic, and seismic loads and the likelihood of a structural failure resulting in the uncontrolled release of water and loss of life. Concrete materials properties are needed for structural analysis programs. In some cases, the materials properties for a given concrete dam are unknown. Concrete materials properties are estimated from the aging concrete information system (ACIS) database of historical mass concrete core test results (Dolen 2005). Since the properties of mass concrete dams evolved throughout the 20th Century, the estimates must reflect the state-of-the-practice at the time of construction. Therefore, the average properties are estimated from test results of dams with comparable construction methodology and quality. In addition, the range in materials properties can be used to better estimate the range of levels of risk of structural failure. Of particular interest is the range in tensile and shear properties of lift lines. Estimating the materials 1 Bureau of Reclamation, P.O. Box 25007, 86-68180, Denver, Colorado, 80225, 303-445-2380, Fax: 303-445-6341, [email protected].

228 21st Century Dam Design — Advances and Adaptations

properties of mass concrete is only a part of the overall risk analysis and risk assessment program. Risk-based Framework for Dam Safety Decision Making Risk is defined as the probability of adverse consequences, and is estimated as the product of the loading probability, the probability of failure given the load and the consequences of that event. The annualized loss of life is estimated from the product of the annual loading probability and the probability of failure given the load. Reclamation identifies potential failure modes for each dam in our inventory and estimates annual probability of failure and risk for plausible failure modes that could result in an uncontrolled release of water from the reservoir. Risk analysis is the quantative calculation of risk. Risk assessment is performed at a given dam once a risk analysis is completed and is the process of deciding whether risk reduction actions are needed. Annual probability of failure estimates and risk estimates are compared to Reclamation’s Public Protection Guidelines as part of risk assessments (Reclamation 2003). For a given dam, Reclamation considers potential failure modes in three different loading categories - static, hydrologic, and seismic, and risk are estimated over a full range of loading conditions for each potential failure mode. Reclamation may take measures to reduce risk and/or to better quantify the uncertainties associated with the risk before taking action. Reclamation considers risks greater than 0.01 lives per year to require expedited action to reduce risk. For risk estimates between 0.01 and 0.001 lives per year and for annual probabilities greater than 0.0001, there is justification for taking action to reduce risk, normally within the scheduled dam safety program budget or evaluation process. For risk estimates to be less than 0.001 lives per year and for annual failure probability estimates less than 0.0001 there is diminishing justification to implement risk reduction measures, or actions considered reasonable and prudent are implemented. Risk Analysis and Structural Analysis For many potential failure modes related to concrete dams, a structural analysis provides valuable information on stresses and stability of the structure. During risk analysis, the probability of failure for each failure mode is estimated, based on the risk team’s evaluation of structural analysis results using assigned materials properties of mass concrete. Strength Input Parameters for Risk and Structural Analysis Reclamation became concerned that variations in the estimates of materials properties and the range of these estimates were significant factors when estimating the risk of dam failure. In some cases, the range of estimates for mass concrete strength was enough to produce a wide range of potential risk. Reclamation began a research program to better identify the materials properties of mass concrete and in particular, the bond strength of concrete at horizontal construction joints, or “lift lines.” Lift lines are planned interruptions in the placement, the concrete has chemically “set” and gained sufficient strength before the next placement. Practically defined, the lift lines are nearly horizontal concrete surfaces resulting from planned work stoppages, usually corresponding to the end of a work day or shift, or topping off formed placements. Lift lines usually must be


cleaned to remove top surface “laitance” and debris just before placing the subsequent lift to assure impermeability, adequate bonding, and monolithic behavior. Typical lift lines are the regularly spaced construction joints resulting from topping off a formed placement. The bond strength of lift lines normally represents the weakest concrete from a strength perspective. ACIS Database The ACIS database is a relational database of historical mass concrete tests. The database was originally created to investigate the properties of aging concrete compared to other like concretes without aging (Dolen 2005). Concrete data can be categorized by date, structure, mixture properties, specimen size, and various aging mechanisms for later sorting. For this investigation, the database was modified to add individual direct tension and direct shear tests, including failure mechanisms. Figure 1 shows an example histogram of direct tension strength tests showing the distribution of lift line tests that failed at the lift line bond, average of 155 lb/in2, and those that failed away from the lift line, average of 195 lb/in2. Figure 2 shows the distribution of direct tension tests of parent (no lift line) concrete that failed near the center of the specimen, average of 215 lb/in2, and those that failed near end plates, average of 220 lb/in2. Neither the center, nor near-platen failure histograms follow a typical normal distribution. This is attributed to combined data from different dams with both low strength and high strength concrete in the same sample populations. Based on the failure mode analysis, lift line versus no lift line test failures were considered separate data populations, whereas the parent center and end plate failure mechanisms were combined into a single population. From these overall sample populations, generational subgroups were identified based on the year of construction and lift line surface preparation methods. When sorted by specimen diameter, there was little difference between the average direct tensile strength of 6 in diameter tests compared to larger diameter test specimens, 9 to 18 in diameter (for each characteristic population). The individual shear, break-bond tests were added to the database to produce subgroups based on the year of construction and lift line surface preparation methods. The data includes the test normal stress and shear failure strength for both lift line and parent concrete test specimens.

TIMELINE FOR CONCRETE DAM CONSTRUCTION AND LIFT LINE SURFACE PREPARATION

A historical timeline was developed to identify bond strength input parameters for dams constructed in different eras by different lift surface cleaning and placement methods. Reclamation analyzed construction records, drilled core logs, and test results to determine the average bond strength properties for different construction methods. Lift Surface Cleaning and Preparation Methods Reclamation concrete dam evolution followed the rapidly developing state-of-the-practice in mass concrete construction throughout the 20th Century. Initially, dams were constructed using relatively rudimentary techniques, equipment, and materials. As dams became larger and more complex, the construction techniques also evolved. The


construction of Hoover Dam from 1933 to 1935 led to a vast advancement in mass concrete technology. Figure 3 shows a historical timeline of the evolution of lift line surface cleaning methods.

0

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Freq

uenc

y

Direct Tensile Strength - lb/in2

Direct Tension No LL Failures Direct Tension Lift Line Failures

Avg. 195 lb/in2 Avg. 155 lb/in2

Figure 1. Histogram of direct tensile strength of mass concrete lift lines and failure modes.

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uenc

y

Direct Tensile Strength - lb/in2

DT Parent - Center Break Failures DT Parent - Near Platten Failures

Avg. 210 lb/in2 Avg. 230 lb/in2

Figure 2. Histogram of direct tensile strength of parent mass concrete and failure modes.


1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

BroomingWashing

BroomingAir-Water Jetting

Green-cuttingPrimary

Sand BlastingSecondary

Wet SandblastingPrimary

High-pressure WaterJetting

Hydro-demolition(Old Concrete)

Uncontrolled W/C Mortar Bonding Mortar No Mortar

Figure 3. Timeline of Reclamation concrete dam lift line surface cleaning methods. The properties of lift lines were affected by the available equipment, the speed of construction, ambient temperature and humidity, the strength of the underlying concrete, and quality control and inspection practices. There is a narrow lift line cleaning “window” from about 8 to 96 hours where the concrete strength is low enough to be effectively cleaned by the available equipment and methods before placing subsequent lifts of concrete. As the strength of concrete increased, the window of opportunity for effective lift surface cleaning decreased. New equipment and methods were therefore needed to compensate for the ever-increasing strength and performance requirements. About five types of lift line surface cleaning methods were identified:

• wire brooming and washing, • brooming and air-water jetting, • early age “green-cutting,” • delayed, wet sand blasting, and • high-pressure water blasting.

There is some overlap between when one method was abandoned in favor of a more cost efficient and effective method. Aging Concrete Dams The properties of aging concrete dams form a subset of the overall properties of mass concrete. Aging dams include those affected by freezing and thawing deterioration


and/or alkali aggregate reaction (AAR). The strength input parameters of AAR affected dams were separated from the properties of unaffected dams.

CONCRETE CORING AND TESTING Reclamation has drilled hundreds of feet of concrete cores from dams up to 100 years age. These cores are tested for a variety of strength parameters. Core drilling and test methods have evolved over the years. Test specimens themselves can be subject to sample bias, depending on the specific purpose of the core drilling program. In some cases, the drilled cores are biased to investigate specific areas of poor quality in a dam, perhaps in areas of low strength or deterioration. The test data was carefully examined before summarizing strength input parameters. The test averages represent primarily vertically drilled cores tested in the saturated moisture condition. Estimating the Percent Bonded Lift Lines Without incorporating the percent bonded lift lines, bond strength test results represent a somewhat biased sample record. An un-bonded lift line cannot be tested for strength, only friction parameters. Thus, the results must be evaluated to determine if they are truly representative of the overall dam quality. A drill core log represents the best available information on the integrity of lift lines. The percent of bonded lift lines was estimated based on the available records from the number of lift lines obtained intact compared to the total number of lift lines intercepted. Figure 4 shows the change in percent of bonded lift lines over time. The moving average was used to smooth trends. One factor affecting lift line bond strength was placing overly wet mixtures with excessive bleeding and laitance. Additional water was added to transport concrete in long chutes and to consolidate the mass concrete in higher lifts. Introduction of the highline bucket method of placing mass concrete about 1930 and the internal mass concrete vibrator in 1934 allowed placing lower slump concrete, with a subsequent improvement of lift line properties. In addition, Reclamation required placement of about a ½ to ¾ inch thick layer of mortar broomed into the cleaned lift line surface immediately before placing mass concrete up to about 1965. The average properties with and without mortar were compared where appropriate. Broken lift lines must be examined carefully to determine if they are un-bonded or caused by mechanical breaks. The percent of bonded lift lines was considered a significant factor in overall mass concrete dam performance, and can be correlated to better lift line cleaning methods. The average and range of percent bonded lift lines are summarized in Table 1. There is more uncertainty if only a single core is drilled from a dam than from multiple spaced drill holes which provide a more global estimate of the percent of bonded lift lines. If multiple core holes indicate there is a continuous, un-bonded lift line, additional analysis should evaluate this condition. From these estimates, the designer may choose a more conservative estimate of the direct tensile strength or cohesion of bonded lift lines by reducing the average values by the estimated percent of bonded lift lines. Or, appropriate bond strength values could be applied to bonded lift lines and apparent cohesion applied to un-bonded lift “contact


surfaces,” based on the estimated percent bonded lift lines. The most conservative approach is to perform analysis assuming the lift lines are un-bonded with only apparent cohesion and sliding friction resistance. The estimated percent bonded lift lines are added with the average values for shear and tension tests in the ACIS database. As shown in Table 1, early Reclamation dams averaged about 50 percent bonded lift lines until about 1930. After Hoover Dam, the average percent bonded lift lines increased to about 80 percent. The increase in percent bonded lift lines after Hoover Dam may be the result of several factors including:

• introduction of controlled, green-cutting and/or wet-sand blasting, • increased overall concrete strength, • improved quality control methods and inspection, and • introduction of internal vibrators for consolidating concrete.

A few dams had noticeably lower intact lift line recovery including several AAR affected dams, dams with high early strength concrete, and the first dam to use high-pressure water blasting as the primary lift line surface cleaning method.

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1/0/1900 9/8/1913 5/18/1927 1/24/1941 10/3/1954 6/11/1968 2/18/1982 10/28/1995

Perc

ent b

onde

d lif

t lin

es

Date introduced

Shear DT Combined Moving …

(combined shear and direct tension data sets)

brush / wash green-cut / wet sandblast high pressure water jet

with mortar without mortar

early high-pressure water jetting

high strength arch dams

Hoover & Grand Coulee Dams

AAR dams

Figure 4. Moving average of estimated percent bonded lift lines for mass concrete dams constructed from 1905 to 1993.


Table 1. Estimate of percent bonded lift lines observed from drilled cores of Bureau of Reclamation concrete dams.

Year Constructed Average bond Range low Range high percent percent percent

1905 - 1933 50 9 83 1933 - 1964 85 66 100 1965 - 1993 1965 - 1993 *

74 83

23 66

96 96

AAR** 1925 - 1938 40 12 89 * Average for 1965 to 1993 not including early development of high-pressure water blasting, lift surface cleaning technology. ** Alkali aggregate reaction affected dams constructed between 1925 and 1938.

Sample Selection and Bias This study analyzed the direct tensile and biaxial shear tests of “parent concrete” (no lift lines) and of lift lines. Laboratory tests of parent concrete represent the in-situ properties of the mass concrete. Lift lines are normally centered in the test specimen. The shear and tensile properties of good quality concrete and construction methods of lift lines approach the properties of the parent mass concrete. Poor quality concrete and construction practices significantly lower lift line properties compared to parent concrete. The ratio of strength between lift lines and parent concrete gives an indication of the quality of lift line treatment and construction. Direct Tension Test The direct tension test is performed according to USBR No. 4914, “Procedure for Direct Tensile Strength, Static Modulus of Elasticity, and Poisson’s Ratio of Cylindrical Concrete Specimens in Tension” (Reclamation 1992). The current test method, shown in Figure 5, uses double–bolted, steel platens bonded to each end of the specimen with epoxy resin to distribute the tensile stress evenly at the platen - concrete interface. The results of parent concrete and lift line tests should be used cautiously since they normally represent the better quality concrete of testable size, and/or the bonded lift lines. During a test program, the direct tension lift line specimens are visually examined to determine if the concrete is in fact failed or broken at the lift line surface, or at another plane of weakness. In some cases, the strength of testable lift lines may be quite adequate, but a low percentage of bonded lift lines will significantly bias the analysis by reducing the bond strength for the entire dam (monolith) section, which could conclude the dam is potentially unsafe to operate. Bi-axial Direct Shear Test McLean and Pierce described the current practices of Reclamation bi-axial shear strength testing according to USBR No. 4915, “Procedure for Direct Shear of Cylindrical Concrete Specimens” (McLean and Pierce 1988, Reclamation 1992). Four data sets may be obtained from these tests:

• break-bond test cohesion and coefficient of internal friction of “parent” mass concrete,


• break-bond test cohesion and coefficient of internal friction of bonded lift lines, • apparent cohesion and sliding friction resistance of the broken surfaces of the

parent concrete or, of lift lines after initial break bond failure, and • apparent cohesion and sliding friction resistance of “un-bonded” lift lines.

Figure 5. Direct tensile test performed on 18 - inch diameter drilled concrete core using double-bolted, steel end plates.

Several specimens are required for each sample set to determine the cohesion and internal friction angle of parent concrete and bonded lift lines. Three or more “break-bond” specimens are tested with different (constant) normal loads until there is a failure, or large horizontal displacements of the specimen are recorded. A graph of each normal versus shear strength test specimen is plotted and a linear, “best fit” curve is used to determine the cohesion, C (intercept at zero normal stress) and the internal friction angle, φ as shown in Figure 6.


Break-bond(Intact specimens)

y = 2.53x + 95R² = 0.87

Average Sliding Friction(broken or un-bonded lift lines)

y = 0.95x + 67R² = 0.99

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Shea

r Str

ess -

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Normal Stress - lb/in2

Break - bond Tests (bonded lift lines)

Sliding Friction (broken and/or un-bonded lift lines)

Linear (Break - bond Tests (bonded lift lines))

Linear (Sliding Friction (broken and/or un-bonded lift lines))

Figure 6. Example combined plots of shear break-bond (BB) and sliding friction data sets.

The coefficient of internal friction, tan φ, is obtained from the slope of the linear regression equation. For each broken or un-bonded lift line test specimen, three to six sliding tests are performed on each specimen with the normal stress increased after large displacements are recorded. Apparent cohesion, Ca is the projected intercept of the linear best fit equation at zero normal stress; obtained from sliding tests of broken surfaces, or of un-bonded lift lines. Phia is the sliding friction angle obtained from the slope of the linear regression equation of sliding tests. A second sliding equation is also projected from the origin of the graph (0, 0) up to the first recorded sliding test. This would be used assuming no apparent cohesion, but has a steeper friction angle. The broken shear specimen surfaces, Figure 7 are examined visually and with a depth profile gauge to investigate surface asperities and/or possible encapsulating cement interference. Individual Versus Weighted Average Properties The individual direct tension and shear test results were added to the database for each dam. The bond strength averages are based on all of the individual tests representing each subgroup. In a few of the smaller sub-groups, the overall average of these tests may show bias due to one dam test program with a larger number of tests. Weighted averages were calculated for compressive strength and elastic properties, and splitting tensile strength based on the average strength weighted by the number of tests. Thus, the average strength for many tests from one dam outweighs those where only a few specimens were tested.


Figure 7. Lift line surface after direct shear, break bond test.

BOND STRENGTH INPUT PARAMETERS The following tables summarize the input parameters for direct tensile and shear strength of mass concrete for use in risk analysis. The average properties and the range are suggested for estimating risk for most projects. The range is shown as plus or minus one standard deviation from the average strength. The lowest values are used for “worst case” conditions. However, the estimated percent of bonded lift lines, Table 1, is critical to using strength input parameters estimates correctly for risk analysis. Dams constructed before about 1933 have a higher probability for many un-bonded lift lines and the potential for a completely un-bonded lift line across the entire dam (monolith) section. The lift line cleaning methods and effectiveness is the most prominent factor influencing the bond strength of mass concrete. The tests indicate that parent concrete strength is not necessarily a good indicator of lift line strength. Dams suffering from alkali-aggregate reaction have differing strength input parameters. In addition, these dams have the potential for a significant percentage of un-bonded lift lines. Even AAR dams with high lift line bond, where the concrete has little reaction, show degrading lift line strength where the reaction is more severe (Joy 2010). In these instances, the dam should be analyzed assuming no tensile strength across lift lines. Direct Tensile Properties of Parent Mass Concrete and Bonded Lift Lines The direct tensile properties of parent mass concrete and bonded lift lines are summarized in Table 2. This table includes the average properties for all direct tension tests, followed by the average properties for three construction eras, 1905 to 1933, 1934 to 1965, and 1965 to 1993. The lift line averages of sub-groups represent those tests that were identified as having failed at the lift line. The average lift line strength is about 80 percent of the parent concrete strength. The average strength of lift line specimens that


did not fail at the lift line interface is about 90 percent of the parent mass concrete strength. The average direct tensile strength of bonded lift lines of dams from the pre-Hoover Dam construction era is about half the parent concrete strength. Table 2. Direct tensile strength of parent mass concrete and bonded lift lines (no AAR).

Average strength

Standard deviation Minimum Number of tests

lb/in2 lb/in2 lb/in2 All parent mass concrete 220 80 40 230 1905 – 1933 (1905 – 1933)*

225 (195)* 85 60 53

1934 - 1964 220 80 40 53 1965 - 1993 220 75 70 112 All bonded lift lines 175 80 10 154 1905 - 1933 [50]** 100 75 10 28 1934 - 1964 [85] 165 70 70 16 1965 - 1993 [74] 195 85 70 27 Lift line failures 155 90 10 65 Not lift line failures 195 70 50 81 * (Parent direct tensile strength average of six dams; not weighted) ** [Estimated percent bonded lift lines for each subgroup]. Direct Shear Properties of Parent Mass Concrete and Bonded Lift Lines The average direct shear properties of parent mass concrete and bonded lift lines are summarized in Tables 3 and 4, respectively. Example data sets for tests of parent concrete and bonded lift lines are shown in Figures 8 and 9, respectively. The average cohesion and internal friction angles are derived from a linear regression of all tests for specific subgroups. This may include both stronger and weaker concrete in the same data set, resulting in very low correlation coefficients. The “90 percent exceeding” cohesion values were obtained by projecting the average linear regression line with the average phi angle to the point where 90 percent of test values were above it. The average cohesion of all bonded lift lines is similar to the direct tension results, about 80 percent of the average parent mass concrete strength. Table 3. Direct shear, “break-bond” properties of parent mass concrete, 1905 to 1993. Average

cohesion 90 percent exceeding

Internal friction angle, φ Number of tests

lb/in2 lb/in2 degrees All parent mass concrete 575 160 48 25 1905 - 1933 [estimated]*

515 [435]

210 [145]

12 [45] 8

1934 - 1964 495 350 61 11 1965 - 1993 595 500 75 6 1934 - 1993 635 440 53 17 * [1905 to 1933 data extrapolated with φ equal to 45 degrees].


Table 4. Direct shear “break-bond” and sliding friction properties of bonded mass concrete lift lines, 1905 to 1993.

Average cohesion

90 percent exceeding

Internal friction angle, φ

Number of tests

lb/in2 lb/in2 degrees All bonded lift lines 470 160 49 107 1905 - 1933 [50]* 415 250 32 46 1934 - 1964 [85] 505 250 54 35 1965 - 1993 [74] 575 380 53 26 1934 - 1993[80] 540 340 53 61 Sliding friction 1905 - 1993

(Ca) 70 (φa)

46

130 * [Estimated percent bonded lift lines]. The data representing parent concrete from dams constructed between 1905 and 1933 show an unusually low phi angle. Another estimate for cohesion was found by projecting a dashed line with a 45 degree slope through the 1905 to 1933 data in Figure 8.

Parent Concrete 1934 - 1993y = 1.31x + 636

R2 = 0.48 Phi = 53 deg90 % exceed C = 440 psi

Parent Concrete 1905 - 1924y = 0.21x + 516

R2 = 0.005 Phi = 12 deg90 % exceed C = 225 psi

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reak

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/in2


All Parent 1934 - 1993

All Parent 1905 - 1924

Linear (All Parent 1934 - 1993)

Linear (All Parent 1905 - 1924)

90 % Exceedance

90 % Exceedance

Figure 8. Shear break-bond properties of parent mass concrete from 1905 to 1933 and from 1934 to 1993.

The internal friction angle of bonded lift lines for dams constructed between 1905 and 1933 is also low. This could have been caused by relatively low strength concrete at lift lines and related to early 20th Century placing techniques using the “chuting” transport method. Overly wet mixtures with slumps of 6 in or more were used to facilitate transporting and consolidating concrete in the absence of suitable mechanical equipment. These lift surfaces had excessive bleed water, and little aggregate interlock due to


settlement. An example of this condition was found at Gerber Dam, Oregon, which had good parent concrete strength properties but only 5 percent bonded lift lines (Joy 2010).

Sand Blasting, Green Cutting, HPWJwith / without Mortar (1934 - 1993)

y = 1.33x + 540 R² = 0.48 Phi = 53 deg

90 % exceed C = 340 psi

Broom - Washwith Mortar (1905 - 1924)

y = 0.62x + 414R2 = 0.10 Phi = 32 degrees

90 % exceed C = 250 psi

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All BB LL 1934-1993 GC-WSB -HPWJrAll BB LL 1905-1924 Br/Wash w/MortarLinear (All BB LL 1934-1993 GC-WSB - HPWJr)Linear (All BB LL 1905-1924 Br/Wash w/Mortar)

90 % Exceedance

90 % Exceedance

Figure 9. Shear, break-bond properties of bonded mass concrete lift lines; effects of different lift surface cleaning methods.

The shear strength of early 20th Century dams is about 70 to 80 percent of the strength of the Hoover and later dams. There were two notable changes in lift placing methods after about 1930. Mass concrete mixtures were placed using the overhead highline and bucket method after about 1930 and the first internal vibrators were introduced for mass concrete consolidation about 1934. These two changes led to noticeably superior concrete and improved bond strength at lift lines. Mass concrete could now be placed at a slump of about 2 in; the reduced water content decreased the paste volume, resulting in less surface laitance from bleeding and less aggregate settlement. Coupled with the introduction of more efficient wet sand-blasting, lift lines now had a more undulating profile with better aggregate interlock. The 1934 to 1964 and 1965 to 1993 data represent lift lines with and without a ¾ inch layer of bonding mortar spread over lift surfaces. For convenience, Figures 8 and 9 compare the 1905 to 1933 data to the combined 1933 to 1993 mass concrete test data for parent concrete and for bonded lift lines, respectively. Lift Line Properties of Alkali Aggregate Reaction Affected Dams, 1925 to 1938 Lift line properties for dams suffering from AAR were noticeably lower than comparable dams of that generation. Table 5 summarizes the average direct tensile properties of mass concrete suffering from AAR. The tests are compared to other dams constructed during the same time frame not suffering from AAR. The average direct tensile strength of AAR-affected parent concrete is about 40 percent, of the non-affected concrete. Lift line strength of AAR-affected dams is about 30 percent of comparable non-affected


dams. It should be noted the few tests performed on bonded lift lines is indicative of the low percent of testable, bonded lift lines recovered from AAR affected dams. This may be the result of both AAR and/or the placing conditions during construction (in arid climate). A few dams suffering from AAR with comparatively good lift line bond also seemed to be de-bonding in specific areas with the more severe reaction and expansion. Table 6 shows the shear and sliding friction properties of lift lines for AAR affected dams compared to un-affected dams built between 1925 and 1938. The cohesion is about 70 percent of comparable concrete and the internal friction angle is reduced by about 10 percent.

Table 5. Direct tensile strength of parent mass concrete and bonded lift lines with and without alkali aggregate reaction.

Average strength

Standard deviation Minimum Number of tests

lb/in2 lb/in2 lb/in2 AAR parent mass concrete 1925 - 1938 100 50 10 81 AAR bonded lift lines 1925 - 1938 [40] *

70 60 30 9

No AAR** parent mass concrete 1925 - 1938

225 90 90 9

No AAR** bonded lift lines [80] 235 85 100 17

* [Estimated percent bonded lift lines]. ** Results of tests from comparable dams with no AAR.

Table 6. Direct shear, “break-bond” and sliding friction properties of AAR affected lift lines and comparable unaffected mass concrete, 1925 to 1938.

Lift lines Average cohesion

90 percent exceeding

Internal friction (phi)

Number of tests

lb/in2 lb/in2 degrees AAR 1925-1938 [40]* 365 200 49 28 No AAR** 1925-1938 [80] 505 275 56 23 AAR sliding friction 1925 - 1938

(Ca) 65 (phia)

47

88 * [Estimated percent bonded lift lines]. ** Results of tests from comparable dams with no AAR. Weighted Average Strength Properties Table 7 summarizes the weighted average compressive strength and elastic properties of mass concrete dams for different construction eras. The weighted average takes into account the average strength for various test programs weighted by the number of tests for each program. The average compressive strength and elastic properties of early 20th Century dams is about two-thirds of those completed after Hoover Dam, indicating better quality concrete and improved construction methods. The properties of AAR affected dams are compared to un-affected dams constructed from about 1925 to 1938. The average compressive strength and modulus of elasticity of AAR affected concrete is about 30 percent lower than un-affected mass concrete. In many cases, the AAR data are


biased with better quality concrete due to the inability to test severely deteriorated concrete. Table 7. Weighted average compressive strength and elastic properties of mass concrete,

1905 to 1993, and with or without alkali aggregate reaction, 1925 to 1938. Average

Strength Modulus of Elasticity

Poisson’s ratio

Number of tests

lb/in2 106 lb/in2 All concrete 4550 4.14 0.19 565 1905 - 1933 3680 3.16 0.20 256 1934 - 1964 5040 5.19 0.19 185 1965 - 1972 5940 4.73 0.20 63 1973 - 1993 5300 4.51 0.16 63 AAR dams 1925 - 1938 4230 3.21 0.11 399 No AAR ** dams 1925 - 1938 5800 4.62 0.23 23

* Results of tests from comparable dams with no AAR. The weighted average splitting tensile strength of parent mass concrete is about 415 lb/in2, and ranges from about 375 lb/in2 for dams constructed before 1933 to about 515 lb/in2 for dams constructed after about 1933. The weighted average splitting tensile strength of AAR affected dams was about 340 lb/in2, about 25 percent lower than un-affected dams built between 1925 and 1938, about 450 lb/in2. Roller Compacted Concrete Dams Roller-compacted concrete (RCC) dams form another subgroup of dams constructed after about 1981. There is less data available on cores obtained from RCC dams. Generally, the bond strength of RCC dams, particularly early RCC dams is significantly affected by compaction at the bottom of lifts (Drahushak-Crow 1988). Lift line strength is also affected by the age after placing the subsequent lift and the lift surface cleaning methods (Dolen 2004). Although there is less data available on modern RCC dams, the lift surface data is currently being compiled for RCC dams, where available.

CONCLUSIONS Risk evaluation is an accepted practice for dam safety decision making. Structural analysis is used during a risk analysis to estimate the probability of failure for a given event. The range of probability of failure was found to depend on the range of concrete materials properties assumed in the analysis. A database of concrete materials properties was examined to better estimate the average bond strength and the range of strength for dams constructed between 1905 and 1993. The data analysis shows there is sufficient justification to apply separate strength input parameters in risk analysis for dams constructed with different state-of-the-art lift line preparation methods. Both the percent of bonded lift lines and the average and range of strength are considered significant. Of particular interest is the difference in bond strength properties of early 20th Century dam construction methods compared to improved methods developed for Hoover Dam. The introduction of the internal vibrator for consolidating mass concrete is under appreciated


with regards to improved lift line performance. The average and range of properties of AAR affected dams are also noticeably lower than comparable dams with similar construction.

REFERENCES Bureau of Reclamation, Concrete Manual, 9th Edition, “Direct tensile strength, static modulus of elasticity, and Poisson’s ratio of cylindrical concrete specimens in tension,” US Department of the Interior, Denver, Colorado, USA, 1992, pp. 726-731. Bureau of Reclamation, Concrete Manual, 9th Edition, “Direct shear of cylindrical concrete specimens,” US Department of the Interior, Denver, Colorado, USA, 1992, pp. 732-737. Bureau of Reclamation, “Guidelines for achieving public protection in dam safety decision making,” U.S. Department of the Interior, Denver, Colorado, USA, 2003. Dolen, T.P., Materials properties model for aging concrete, Bureau of Reclamation Dam Safety Program Report No. DSO-05-05, Denver, Colorado, USA, 2005. Dolen, Timothy P., “Long-term performance of roller compacted concrete at Upper Stillwater Dam, Utah, USA,” Berga and Buil, (ed.), Roller Compacted Concrete Dams, Proceedings of the 4th International Symposium on Roller Compacted Concrete Dams, 17-19, Nov. 2003, Madrid, Spain, Taylor and Francis, London, UK, 2006, pp. 1117 - 1126. Drahushak-Crow, R. & Dolen, T.P., “Evaluation of cores from two RCC gravity dams,” K. Hansen & F. McLean (ed.), Roller Compacted Concrete II, ASCE Proceedings of the 2nd Conference on RCC Dams, San Diego, California 1988, New York, NY, USA, pp. 203-219. Joy, W.T. “Seminoe Dam – 2009 concrete coring - laboratory testing program,” Bureau of Reclamation Report No. MERL-2010-07, Denver, Colorado, USA, 2010. Joy, W.T., “Gerber Dam – 2009 and 2010 concrete coring and laboratory testing program,” Bureau of Reclamation, Report No. MERL-2010-22, Denver, Colorado, USA, 2010. Mc Lean, F. G. & Pierce, J.P. “Comparison of joint shear strengths of conventional and roller compacted concrete,” K. Hansen & F. McLean (ed.), Roller Compacted Concrete II, ASCE Proceedings of the 2nd Conference on RCC Dams, San Diego, California 1988, New York, NY, USA, pp.151 – 169.

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