NBSIR 85-3186

Development of Durcon, An

Expert System for Durable

Concrete: Part I

James R. Clifton

Bhaichandra C. Oltikar

Steven K. Johnson

U.S. DEPARTMENT OF COMMERCE - National Bureau of Standards

^ Gaithersburg, MD 20899

July 1985

J.S. DEPARTMENT OF COMMERCE

nr

ilATIONAL BUREAU OF STANDARDS

jlOO i ju56 I 185-5186 I 11985

NATIC''--.r. OF STAK’DARDS UERAHY

NBSIR 85-3186

DEVELOPMENT OF DURCON, AN

EXPERT SYSTEM FOR DURABLE

CONCRETE: PARTI

James R. Clifton

Bhalchandra C. Oltikar

Steven K. Johnson

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MO 20899

July 1985

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldriga. Sacratary

NATIONAL BUREAU OF STANOAROS, Enwrt AmMar. DInctet
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ABSTRACT

This Is a progress report on the development of DURCON an expert system to give

recommendations on the selection of constituents for durable concrete. Four

major concrete deterioration problems will be covered when the DURCON system is

completed; freeze-thaw, sulfate attack, corrosion of reinforcing steel, and

cement-aggregate reactions. The factual knowledge base for DURCON is based on

the American Concrete Institute Guide to Durable Concrete. Heuristic knowledge

is being obtained from experts on the durability of concrete. The approach

being taken in developing DURCON is discussed. Then a model expert system for

concrete exposed to freeze-thaw conditions is described.

Keywords; aggregate, artificial intelligence, concrete, corrosion, durability,

expert system, freeze-thaw, knowledge-based system, sulfate attack

ill
Table of Contents

Page

1. INTRODUCTION 1

2. PRINCIPLES OF EXPERT SYTEMS 2

2.1 Features of Expert Systems 2

2.2 Architecture of an Expert System 2 2.3 Knowledge Base 3

2.4 Control System 3

3. APPROACH FOR DEVELOPING DURCON 5

4. MODEL SYSTEM USING FREEZE-THAW KNOWLEDGE BASE 11

4.1 Features of System 11

4.2 Example of a Session 11

5. SUMMARY 16

6. ACKNOWLEDGEMENT 17

7. REFERENCES 18

List of Tables

Table 1. Contents of American Concrete Institute Guide to Durable

Concrete 9

Table 2. User Session with Model Expert System 12

Table 3. Example of an Explanation of a Question In DURCON 15

List of Figures

Figure 1. Major Components of an Expert System 3

Figure 2. The Structure for Knowledge of Freeze-Thaw Durability

of Concrete 6

Figure 3. Sequence of the Development of DURCON 8

Iv
1 . INTRODUCTION

Among the most active research areas in artificial intelligence during the past

20 years has been the development of expert systems, which also are known as knowledge-based systems. At first, expert systems were primarily of academic

Interest, with little attention being given to their practical applications.

The usefulness of expert systems in solving difficult practical problems, however,

has become recognized and their development is being pursued in many fields.

For example,, expert systems have been developed to give recommendations in such

areas as medical diagnosis [1], molecular structure identification [2], mineral exploration [3], computer configuration [4], and structural analysis [5].

An expert system should be able to give recommendations at a level comparable

to, or above, an expert. Expert systems are most useful when the knowledge is

based on heuristics and when mathematical relationships are not available,

which is often the case in the construction field.

The knowledge required to solve difficult problems encountered in the construction industry is not often readily available to the field engineer or designer.

Further, when information is accessible it is often too imprecise to be of

substantial assistance in solving specific problems. Until now the construction

industry had to depend on a relatively few well-qualified experts to give

recommendations on problem resolution. Expert systems appear to be an effective

tool for preserving and maintaining the knowledge of experts and for using it

in solving difficult problems. Research on the applications of expert system

to construction problems prlmarly has been carried out by Carnegie-Mellon

University [6] and the U.S. -Corps of Engineers [7]. Undoubtedly, the con- struction Industry will soon begin to more fully exploit the benefits of

expert systems.

This report discusses the early stages in the development of DURCON (DURable

CONcrete), a prototype expert system which is Intended to give recommendations

on the selection of constituents for durable concrete. The purpose of developing

DURCON is to demonstrate the application of expert systems to improving the

process of selecting construction materials. Concrete was selected as the

subject of this prototype system as it is the most widely-used man-made con- struction material. In the United States alone, over 700 million tons of

concrete are placed each year [8]. It is well-known that major problems

encountered in the durability of concrete can be significantly reduced by

improved selection of concrete constituents and concrete design. Development

of DURCON is feasible because of the large amount of specialized factual and

heuristic knowledge which exists on relations between the design of concrete

mixtures, including the constituents, and the durability of concrete.

1
2. PRINCIPLES OF EXPERT SYSTEMS

An overview of expert systems is presented in this section to introduce the

concepts involved in the development of DURCON. More comprehensive views of

expert systems are given in references 9, 10, and 11.

2.1 Features of Expert Systems

In the several definitions given for the term "expert system," the essential

concept expressed is the use of the knowledge of experts to solve difficult

problems. Feigenbaum [12], a pioneer in expert systems, gave the following

definitions in which the main concepts are presented:

"An Expert System is an intelligent computer program that uses knowledge

and Inference procedures to solve problems that are difficult enough to

require significant human expertise for their solution. The knowledge

necessary to perform at such a level, plus the inference procedure used, can be thought of as a model of the expertise of the best practitioners

in the field."

The knowledge of an expert system consists of facts and heuristics. Facts

constitute Information that is widely accepted to be valid and readily available

(e.g., in textbooks). Heuristics are rules of good judgement or rules-of-thumb

[13]. Often an expert will rely upon his own judgement (heuristics) to solve

problems because of the lack of well-understood theories or facts.

2.2 Architecture of an Expert System

An expert system is an interactive system consisting of three major components

(fig. 1):

(1) A knowledge base of facts and heuristics which can be applied to a specific case.

(2) A global data base (working memory) which contains, in temporary

storage, observations or evidence provided by the user about a specific

case, and all derived information about the case,

(3) A control system or inference engine which selects the appropriate

knowledge rules and recommendations for the solution of the problem.

Other features usually present in expert systems are:

• explanatory interface which permits the user to understand why certain

questions are asked and gives a justification for specific conclusions

or recommendations

• transparency of the computer program to the user (i.e., the user does

not become involved in programming).

2
CONTROL

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• modification of the computer program can be accomplished without a comprehensive re-programming, e.g., portions of the knowledge

base can be modified without the need for other restructuring,

2.3 Knowledge Base

The ability of an expert system to solve a problem (referred to as its "power”)

has been observed to increase with the extent of its domain (subject specific)

knowledge. Undoubtedly the most demanding phase of developing an expert system

is obtaining and representing relevant knowledge.

The domain knowledge of an expert system is often represented by production

rules which have the general form:

If: Logical conditions are satisfied (antecedent) (1)

Then: Take the indicated action (consequence)

These rules are further illustrated by the following rule taken from DURCON

If: (1) Severe freeze-thaw conditions are anticipated and

(2) The nominal size of aggregate is 3/8 inch (9.5 mm.)

Then: The percent of entrained air should be 7.5 (2)

An expert system usually has many judgmental or empirical rules (heuristics)

which lead to recommendations or conclusions which contain a certain level of

uncertainty. A numerical value (certainty factor) can be given to each rule to

indicate the degree of certainty. These certainty factors can be combined with

each other to obtain an overall certainty factor for the final recommendation

or conclusion. Certainty factors are measures of the confidence that can be

placed in any given conclusion or recommendation.

2.4 Control System

The control system decides which production rules should be invoked and it must

resolve any conflicts which may occur if several rules are satisfied. There

are two main approaches for evaluating production rules: backward chaining

and forward chaining [9, 11].

In backward chaining the system has a set of initial goals, such as solutions

to a problem. The rules are Invoked in reverse order, with the right-hand

sides of the rules stating possible goals, and the left-hand side of the rules

being examined to determine which goals are satisfied i.e,, consistent with

the evidence. The goals are proven or disproven until the "most-correct"

solution is obtained. Backward chaining is often used in diagnostic systems

e.g., MYCIN [1]

.

In forward chaining the system does not start with any defined goals. Instead

the system starts with a set of evidence and proceeds to invoke the rules in a forward direction (antecedent first), continuing until no further production

rules can be invoked. Forward chaining is often used in systems providing

recommendations on design related problems.

4
3. APPROACH FOR DEVELOPING DURCON

The acceptance of an expert system undoutedly depends on the credibility of

its knowledge base. In the conceptual design of DURCON, three criteria were

established for selecting the main source for the factual portion of the

knowledge bases. The facts should have been: (1) agreed upon as the consensus

of a team or commi ttee of experts on the durability of concrete; (2) published

so that the concrete community could judge their reliability; and (3) accepted

by concrete practitioners. The "Guide to Durable Concrete," developed by the

American Concrete Institute Committee (ACI) 201 [14], met the three criteria

and was chosen to be the factual framework of DURCON. The Guide consists of seven chapters and the subjects are given in Table 1. In the conceptual design

of DURCON, four major durability problems covered by the guide were considered,

freezing and thawing, sulfate attack, corrosion of reinforcing steel, and

reactions between the constituents of cements and aggregates. These four

problems were selected because they comprise the most persistent causes of the

premature deterioration of concrete (excluding inferior concrete attributed to

Inadequate quality management). The known major factors controlling the

response of concrete to these deterioration processes and recommendations of

measures to minimize their deleterious effects are given in the Guide.

The first step in representing this knowledge was the creation of tree

(hierarchy) structures. The tree structure for the knowledge of the freeze-thaw

durability of concrete is illustrated in fig. 2. Then the facts were expressed

in the form of production rules. The planned sequence of the stages in developing DURCON is indicated in fig. 3. After the factual structure is developed

experts in concrete durability will be requested to contribute to the heuristic

component of the knowledge base. Also further improvements in the knowledge

base and the recommendations undoubtedly will result from workshops of experts

and from users' evaluation.

It is conceivable that if DURCON proves useful, responsibility for its

maintenance could be assumed by a voluntary consensus standards committee such

as ACI 201, and it could be issued as a new type of guideline.

During the development of DURCON its knowledge is being partitioned into four

components, each dealing with a major durability problem (freezing and thawing,

sulfate attack, corrosion of reinforcing steel, and aggregate-cement reactions).

Undoubtedly, some knowledge overlap will occur and, where appropriate, the

individual knowledge bases will be linked or integrated. The first component

of the knowledge base being developed addresses the freeze-thaw durability of

concrete. A model expert system using this knowledge base is described in

Section 4.

5
6
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Table 1 . Contents of American Concrete Institute Guide to Durable

Concrete [14]

CONTENTS

Introduction

Chapter 1 —Freezing and thawing

1 .1

General

1.2

Mechanism of frost action

Chapter 2—Aggressive chemical exposure

2.1

General

2.2

Sulfate attack

Chapter 3—Abrasion

3 . 1

Introduction

3.2 Testing concrete for resistance

to abrasion

3.3

Factors affecting abrasion

resistance of concrete

3.4

Recommendations for obtaining

abrasion-resistant concrete

surfaces

1.3

Ice removal agents

1

.4—

Recommendations for durable

structures

2.3—Acid attack

3.5

Improving wear resistance

of existing floors

3.6

Studded tire wear on con- crete pavements

3.7

Skid resistance of pavements

Chapter 4—Corrosion of steel and other materials embedded in concrete

4 . 1

Introduction

4.2

Effect of concrete condition on corrosion of embedded steel

4.3

Causes of corrosion

4.4 Corrosion characteristics of

various materials in concrete

4.5

Recommendations where corrosion may be a problem

4.6

Corrective measures

4.7

General remarks

Chapter 5—Chemical reactions of aggregates

5.1

Types of reactions

5.2

Alkali-silica reaction

5.3 Cement-aggregate reaction

5 . 4

Expansive alkal i-carbonate

reactivity

5.5

Preservation of concrete

containing reactive aggregate

5.6

Recommendations for future

studies

9
Table 1 . Continued

Chapter 6—Repair of concrete

6.1

Evaluation of damage and selection

of repair method

6.2

Types of repairs

6.3“Preparations for repair

6.4—Bonding agents

6 .5

Appearance

6.6

Curing

6.7 Treatment of cracks

Chapter 7—Use of coatings to enhance concrete durability

7.1

Surface water repellents 7.3—Future of coatings

7.2

Plastics and elastomeric

coatings

10
4. MODEL SYSTEM USING FEIEEZE-THAW KNOWLEDGE BASE

4.1 Features of System

A model expert system was developed using the freeze-thaw knowledge base to

test the approach described in Section 3. In the present form, the system

contains 34 production rules and has the architecture shown in fig. 1. Its

control system supports a forward-chaining procedure. The computer program was originally written in BASIC and subsequently converted to PASCAL. It operates

on the IBM PC (personal computer) using the IBM PC disk operating system

(DOS)^.

4.2 Example of a Session

A session with the model expert system is presented in Table 2. In this session,

the user specified normal-weight concrete with a portland cement. It was anticipated by the user that the concrete would be exposed to a severe freezethaw environment. Note that an user can request an explanation of a question.

An example of a reply in response to a request for an explanation is given in

Table 3.

Another feature of the system is that the answers are self-consistent. For

example, if light-weight concrete is selected, then the program indicates that

light-weight aggregate must be chosen. Also, if either a moderate or severe exposure condition is selected, then the program Indicates that the use of

air-entraining agents must be specified.

^ The computer hardware and software are identified in this paper to adequately

describe the operation of DURCON; such identification does not imply endorsement by the National Bureau of Standards.

11
Table 2. User Session with Model Expert System

(The computer displays text which conveys information then requires user

input in response to instructions or questions).

THIS KNOWLEDGE BASED SYSTEM GIVEN RECOMMENDATIONS ON REQUIREMENTS FOR DURABLE

CONCRETE EXPOSED TO FREEZE-THAW CONDITIONS.

The Resistance of Concrete to Freeze-Thaw Cycles Is Largely Dependent on the

Following:

Water-to-Cement (w/c) Ratio

Type of Structure

Exposure Conditions

Aggregate Properties

Entrained Air

In the recommendations, ASTM Standard Nos. are used, and their titles can

be obtained by making the appropriate choice at the end of the session.

Please answer the questions the system will ask, as well as you can. Hit

RETURN when you are ready.

Which type of hydraulic cement is to be used for this application?

1) Portland Cement

2) Blended Cement

3) Please explain the question

4) Redo a previous answer

?1

Which admixtures will be mixed with concrete ingredients?

1) Pozzolan

2) Air Entrainment Agents

3) Other Chemical Admixtures

4) 1) and 2)

5) None

6) 2) and 3)

7) 1) and 3)

8) 1) and 2)

9) Please explain the question

10)

Redo a previous answer

?6

12
Table 2. Continued

Which of the following best describes this application?

1) Light weight Concrete

2) Normal weight Concrete with Normal Section

3) Normal weight Concrete with Thin Section

4) Please explain the question

5) Redo a previous answer

?2

Which of the following describes the anticipated exposure condition?

1) Dry

2) Moderate

3) Severe

4) Please explain the question

5) Redo a previous answer

?3

Aggregate selection will be made according to which of the following?

1) Natural aggregate meeting ASTM C33

2) Light weight aggregate meeting ASTM C330

3) Labtest ASTM C666

4) Past experience has shown the aggregate to be acceptable

5) Please explain the question

?1

There is 60 percent chance of satisfactory performance

If not sufficient then use ASTM C666; enter Y to use ASTM C666, any key otherwise

?y

Is the Durability Factor of 75 obtained?

1) YES

2) NO

3) Please explain the question

4) Redo a previous answer

?2

13
Table 2. Continued

Which nominal size of aggregate is to be used in this application?

1) 3/8 inch (9.5 mm)

2) 1/2 inch (12.5 mm)

3) 3/4 inch (19.0 mm)

4) 1.5 inches (37.5 mm)

5) 3.0 inches (75 mm)

6) 6 inches (150 mm)

7) Please explain the question

8) Redo a previous answer

?6

RECOMMENDATIONS

*Portland cement is required to conform to ASTM standard C150.

*Since the Durability Factor is less than 75, do test ASTM C671 (dilation test

of concrete subjected to freezing) with the specimens conditioned as stipulated

in ASTM C682. If it does not pass, reject the aggregate.

*With 6-inch (150-mm) aggregate selected for severe conditions, the percent of

entrained air should be 4.0 percent. The only exception to the above conclusion is the possibility that the aggregate is immune to freezing in sizes up

to 1 inch. (Consult the following paper for assistance in identifying such

aggregates: Verbeck, George, and Landgren, Robert, Influence of Physical

Characteristics of Aggregate on the Frost Resistance of Concrete,Proceedings

,

ASTM, Vol. 60, 1960.) ASTM test methods for air content determination require

that the concrete sample be wet-sieved over a 1.5-inch sieve before test.

The percent of entrained air in the sieved specimen should be 5.5 percent.

*Air void spacing must be 0.008 inch (.20 mm)*

*For applications using normal weight concrete for normal sections the recommended

water/cement ratio is 0.50.

*Concrete for severe exposure conditions should have a F^, of about 500 psi

(3.45 MPa) prior to one or two cycles of freezing and thawing while curing.

It should attain a F^ of about 4000 PSi (27.6 MPa) before exposure to severe conditions

.

14
Table 3. Example of an Explanation of a Question in DURCON

DURCON QUERY; Which of the following best describes the anticipated exposure

condition?

1) Dry

2) Moderate

3) Severe

4) Please explain the question

5) Redo a previous answer

USER RESPONSE: 4

EXPLANATION: The definition used for dry is indoor exposure or well protected

from moisture and freezing. Moderate is an outdoor exposure in

cold climate where the concrete will be only occasionally exposed

to moisture prior to freezing and where no deicing salts are used. Severe is an outdoor exposure in a cold climate where the

concrete may be in almost continuous contact with moisture prior

to freezing, or where deicing salts are used.

15
5 SUMMARY

This is a progress report on the development of DURCON, an expert system for

making recommendations on the selection of constituents for durable concrete.

Four major durability problems are being addressed, freeze-thaw damage, sulfate

attack, corrosion of reinforcing steel, and cement-aggregate reactions. The

factual knowledge base for DURCON is being based on the ACT Guide to Durable

Concrete. Heuristic knowledge is provided by experts on concrete durability.

The knowledge base dealing with the freeze-thaw resistance of concrete has been

developed. A model expert system giving recommendations for concrete exposed to

freeze-thaw conditions has been prepared and demonstrated. At present, it has

34 production rules and uses a forward-chaining control system.

16
6 ACKNOWLEDGEMENT

The authors wish to acknowledge the encouragement of and helpful suggestions by

Dr. Geoffrey Frohnsdorff, National Bureau of Standards, which contributed

significantly to the development of DURCON. The assistance of Mr. Paul Klelger,

Chairman of ACI Committee 201, Durability of Concrete, In providing a forum for

discussing DURCON Is greatly appreciated. The assistance of Mr. Robert Phllleo,

former president of the ACI and Private Consultant, In developing the freezethaw knowledge base also Is acknowledged.

17
7 REFERENCES

1. E. H. Shortliffe, Computer-Based Medical Consultation; MYCIN American

Elsevier (New York, 1976).

2. B, G. Buchanan and E. A. Feigenbaum, DENDRAL and Meta-DENDRAL

:

Their Applications Dimensions, Artificial Intelligence, Vol. 11,

pp. 5 to 24 (1978).

3. R. 0. Duda, J. Gasching, and P. E. Hart, Model Design in the Prospector

System for Mineral Explorations, in Expert Systems in the Micro Electronic

Age, pp. 153-167 (Univ. Edinburgh, Scotland, 1979).

4. J. Dermott, R1 ; An Expert Configurer, Technical Report CMU-CS-80-119

(Carnegie-Mellon Univ., 1980).

5. J. S. Bennett and R. S. Englemore, SACON: A Knowledge-Based Consultant

for Strucural Analysis. Proceedings Sixth International Joint Conference

on Artifical Intelligence, pp. 47-49 (Tokyo, 1979).

6. S. J. Fenves, M, L. Maher and D. Sriram, Expert Systems; C. E. Potential,

Civil Engineering (ASCE), pp. 44-47 (October, 1984).

7. F. Kearney, Expert Systems Program Manager, Construction Engineering

Research Laboratory, US Corps of Engineers, Champaign, Illinois.

8. Minerals Yearbook 1983, Dept, of Interior.

9. S. M. Weiss and C. A. Kulikowski; A Practical Guide to Designing Expert

Systems, Rowman and Allanheld Publishers (Totowa, New Jersey, 1984).

10. Building Expert Systems, F. Hayes-Roth, D. A. Waterman, and D. B. Lenat,

(eds), Addison-Wesley Publishing Co. Inc. (Reading, Massachusetts, 1983).

11. Expert Systems; Principles and Case Studies, R. Forsyth, (ed.). Chapman

and Hall (London, 1984).

12. E. Feigenbaum, Expert Systems in the 1980* s, in Machine Intelligence,

ed. A, Bond, Pergamon Press (1981).

13. J. Pearl, Heuristics, Addison-Wesley Publishing Company (Reading

Massachusetts, 1984).

14. Guide to Durable Concrete ACI 201.2R-77, Journal of the American Concrete

Institute (December, 1977).

18
NBS«114A (REV. 2 »eo) U.s. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR-85/3186

2. Performing Organ. Report No. 3. Publication Date

July 1985

4. TITLE AND SUBTITLE

Development of DURCON, an Expert System for Durable Concrete: Part 1

5. AUTHOR(S) James R. Clifton, Bhalchandra C. Oltikar, Steven K. Johnson

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

I. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

National Bureau of Standards

Gaithersburg, Maryland 20899

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, Is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant Information. If document Includes a significant

bibliography or literature survey, mention it here)

This is a progress report on the development of DURCON an expert system to give

recommendations on the selection of constituents for durable concrete. Four

major concrete deterioration problems will be covered when the DURCON system is

completed; freeze-thaw, sulfate attack, corrosion of reinforcing steel, and

cement- aggregate reactions. The factual knowledge base for DURCON is based on the American Concrete Institute Guide to Durable Concrete. Heuristic knowledge

is being obtained from experts on the durability of concrete. The approach

being taken in developing DURCON is discussed. Then a model expert system for

concrete exposed to freeze-thaw conditions is described.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by lem-colon i Aggregate, Artificial Intelligence, Concrete, Corrosion, Durability, Expert System,

Freeze-Thaw, Knowledge-Based System, Sulfate Attack

13. AVAILABILITY

XX I Unlimited

I I For Official Distribution. Do Not Release to NTIS

r I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

23

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