A concrete problem at Kingston, Ontario

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A concrete problem at Kingston, Ontario

NATIONAL RESEARCH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

A CONCRETE PROBLEM AT KINGSTON, ONTARIO by

R.F. Legget and E.G. Swenson

I..NAlYZED

Report No.

115

or the Division of Building Research

Ottawa May,

1951

PREFACE

The preparation and issue of this internal report by the Division of Building Research of the National Researoh Council is an unusual step for a research organization, even allowing for the fact that the report is essentially a private document. The report is being issued, however, in view of the importance of the problem discussed, especially in the Kingston area, and with the full agreement of all the

authorities concerned with this problem with whom the Division has been privileged to maintain full and cordial liaison.

The problem with which the report deals concerns the serious disintegration of some concrete in the Kingston area. The problem has been recognized locally and also by agencies which have construction work in the Kingston area. Something

is known of the causes but research work is still actively in progress, not only in Canada but also in the United States.

The final solution to the problem is, however, not yet in sight. If the Division of Building Research takes no action to publicize what is already known of the problem more concrete will be placed in the Kingston area without the benefit of even

this partial knowledge. Even though the final solution is not yet known, enough information is available to assist those who are responsible for concrete work in the Kingston area to

minimize the possibility of trouble with concrete yet to be placed.

This report has therefore been prepared to show clearly what is being done regarding the problem and to advise those in

the Kingston area of the present state of knowledge regarding the problem SO that they may be guided accordingly in their design

and construction work.

Eventually complete papers will be published on the

scientific aspects of the problem which are at present baffling, and on practical solutions. This report represents a private interim progress report only.

The Division of Building Research accepts full

responsibility for preparing and iSSUing to a limited circulation, this private report at this time. The Division is pleased to

do this with the full agreement of those concerned with the problem. One of the most satisfactory aspects of this research

projeot has been the oordial good-will shown by all those concerned, despite the real interests which are so definitely affected by

Finally, it must be stressed that although discussion of the problem suggests very serious trouble, and although

there are a few isolated cases of concrete work in the Kingston area where the trouble has been serious, concrete has been

used in this area for many years with much success and

acceptability. The work now in progress is designed to provide a solution to the problem of excessive expansion of some concrete,

8 problem that has been recognized for a long time, and thus to

ensure the continuation of the use of concrete in the Kingston area as a major material of construction. With the continued co-operation of those who have already assisted the Division with its studies and those who can assist with local knowledge, it is hoped that a solution will be found.

Ottawa May

1957

Robert F. Legget Director

TABLE OF CONTENTS

Page

Field Observations • • • • • • • • • • • • • • • • • • 2

Laboratory Studies • • • • • • • • • • • • • • • • • •

4

Appendix - Tentative Method Suggested by the Division

or Building Research for Testing Expansion or Concrete made with Kingston Limestone Coarse Aggregate.

A CONCRETE PROBLEM AT KINGSTON, ONTARIO

by

R.F. Legget and E.G. Swenson

In the fall of

1955

the Division of Building Researoh

was asked by a large government construction agency to investigate some unusual cases of concrete deterioration at Barriefield near Kingston, Ontario. A large bUilding program was involved and considerable trouble had been experienced with structures such as sidewalks, curbs, and floors.

The deterioration in these cases was characterized by an abnormally high expansion of the concrete, usually accompanied by a more or less uniform pattern cracking depending on ･ｸｰｯｳＧｾ･

conditions. This cracking defined roughly hexagonal areas of apparently intact concrete about four inches across. Sections cut out of sidewalk slabs showed that these cracks extended

deeply into the concrete and were to be distinguished, therefore, from shrinkage craoking or "crazing". This trouble occurred

principally under conditions of dampness or wetting and drying, and was found in elements protected from the weather as well as in exposed sections.

In preliminary laboratory tests, it was possible readily to reproduce the abnormal expansion of concrete as observed in the field, using materials from the same sources as those used in a sidewalk which had shown distress. The wide use of these particular materials and the occurrence of the problem in an

unusually large number of structures at Barriefield was considered sufficient justification for the rather extensive investigation SUbsequently undertaken by the Division of Building Research.

During the course of more than a year of research,

including field stUdies, it has become evident that the problem is more serious and widespread than was first thought to be the case. It has been established that the limestone coarse aggregate used in the cases investigated is involved in what is apparently an ususual type of cement-aggregate reaction whioh produces

abnormal expansion of the concrete. Present indications are that the alkali content of the cement is a factor and that the causative mechanism is an alkali-aggregate reaction of a kind not previously reported in the literature. Although much additional research and testing will have to be done to clarify the situation completely

2

-and to provide a practical field solution, the evidence accumulated to date as to the general nature of the problem is considered sufficient to warrant an interim report at this time.

Field Observations

The first field examinations were made in the Barriefield area in connection wit!! the original cases invostigated in the laboratory. The coarse aggregate used in these instances was crushed limestone from a quarry which will be hereinafter

designated as quarry A, and which has been in operation for not more than

9

years at the time of this writing. Subsequently, a rather extensive examination of concrete was carried out in and near the city of Kingston. It was generally limited to sidewalks, curbs, walks, steps, and floors. Accurate information as to the time of placing and the sources of aggregates and cements could not, in most cases, be obtained. The cases where reliable

information was available indicated quite conclusively that the problem existed before the opening of quarry A. It is understood that the only other major source of limestone coarse aggregate is from a second quarry, hereinafter designated as quarry B, which has been in operation for at least 20 years, according to reliable sources of information.

The above observations are based on cumulative evidenoe obtained from many field examinations. The following oases are cited as examples:

(i) Sidewalk in Barriefield, 3 years old; excessive expansion and pattern oracking; coarse aggregate definitely from quarry A;

(ii) A house about

3

years old several miles outside

the oity of Kingston; positive evidence of exoessive

･ｸｰｾｮｳｩｯｮ and map cracking in walks, steps, retaining wall, basement walls, and partition walls; ooarse aggregate definitely from quarry Aj

(iii) A concrete structure in Kingston, about

17

years old; shows much evidenoe of map oracking and distress in concrete; coarse aggregate definitely from quarry B;

(Iv) An army building in Barriefield, about

4

years old; clear evidence of abnormal expansion and map cracking in concrete steps, platform, gutters and inside floors; in this instance special attention had been given to good concreting practice because of previous experience with the problem; source of aggregate not certain but probably quarry A;

3

-(v) Army building in Barriefield, about 20 years old; severe case of pattern cracking; source of aggregate not known, but definitely not quarry A; and

(vi) Outside walk at a home in Kingston about 12 years old; shown ,to writer by local contractor; typical case of pattern cracking; source of aggregate uncertain, but definitely not quarry A.

In addition to the six cases ｣ｩｾＬ much additional evidence of the occurrence of this characteristic concrete deterioration in Kingston prior to the opening of quarry A was found in many sidewalks and other structures examined. In most instances the observations were made in company with interested and informed local residents whose knowledge and opinions could be trusted. In many cases, the affected concrete was said to have been placed from 10 to 30 years ago.

The occurrence of abnormal expansion and pattern cracking appeared to be largely, but not always, confined to elements on or near the ground, where the concrete was subject to contact

with moist soil or to high humidity conditions. The characteristio cracking pattern occurs apparently where one side expands at

some more or less uniform rate as a result of a constant supply of moisture, whereas expansion on the other side is interrupted by periods of drying. In the case of a sidewalk also, for example, this differential movement appears to have initiated cracking on the top surface. The first cracks may have been shallow but these would permit drying at a greater depth, producing deeper cracks. In a sidewalk about two years old, the cracks had penetrated about two thirds of the way through a six-inoh slab.

Abnormal expansion was detected in sidewalks through

the closing up of expansion joints and the extrusion of the joint filler. In some curbs the expansion had produced a bending or breaking at the joints. In two notable cases, expansion of the concrete foundations had caused wide cracks to occur in the

masonry superstructure. There appeared to be no evidence, however, of structural failure of any building in Kingston as a result of this problem.

In addition to map cracking, there was considerable

evidence of spalling, crumbling and "gross" cracking of concrete in such elements as sidewalks and curbs. These cases can

probably be attributed to other destruotive agencies such as the action of frost or salt on poor quality concrete. There is

evidence, in some of these cases, that initial deterioration had occurred through the abnormal expansion and map cracking phenomenon.

4

-In some of the caRes examined, where no visible evidence of map cracking could be detected, the concrete had evidently been placed in two courses, ｴｨｾ top course containing little or no coarse aggregate. Gross cracking had occurred in many of these cases, probably as a result of excessive expansion of the bottom course. TYio-course sidewalk construction was reported to have been practisen to a considerable extent in Kingston.

The problem was discussed with a number of those who

have been associated with local concrete construction for periods varying up to

3S

years. There was general agreement that the problem has existed for many years. Opinions differed as to the probable cause of the trouble. Some held the limestone

responsible but many considered the sand or cement to be at fault. In view of the possibility of alkali-aggregate reaction being the cause of the problem, the alkali contents of the cements used in this area are of interest. The main source of cement for the Kingston area is a cement plant in southern Ontario which produces a normal portland cement with an alkali content of 1.2 per cent or higher. Where alkali-aggregate reaction is involved, a high alkali cement is usually considered to be one which contains more than

0.6

per cent total alkali, calculated as sodium hydroxide. As far as is known, all cements manufactured in Eastern and Central Canada are high alkali cements. Certain European cements have

been imported into this area during recent years due to shortages of domestic cements. Analyses of some of these imported cements are available and they were all high alkali cements.

ｾ｡｢ｯｲ｡ｴｯｲｹ Studies

The research program carried out by the Division of Building Research in connection with the problem was designed

(a) to isolate the material or materials responsible for the trouble; (b) to determine the conditions which promote abnormal expansion and map cracking as observed in the field cases; (c) to determine the basic nature of the problem; and (d) to test the effectiveness of certain possible remedial measures. The methods used have inclUded chemical tests, petrographic

evalua-tion by recognized authorities, and extensive physical tests.

. It was early established that a limestone coarse aggregate was involved in the reaction. This stone was obtained from quarry A and was the source of coarse aggregate used in the case which was first investigated. This rock can be generally ､･ｳ｣ｲｾ｢･､ as a fine-grained calcareous dolomite or dolomitic limestone containing about

13

per cent of material insoluble in hydrochloric acid. On the basis of conventional acceptance tests this rock has been rated satisfactory for use as coarse aggregate in concrete.

5

-Present indications are that the sand component does not contribute to the primary phenomenon of rapid and abnormal expansion of the concrete. From the limited laboratory

experiments to date, it does appear, however, that certain characteristics of the sand may play at least a minor role in the problem. The particular sand studied in connection with the original problem investigated shows some reactivity of a nature not yet understood. Another local sand, when used with a, limestone coarse aggregate from an outside source, did not produce excessive expansion of the concrete. Physically, the sands were considered

satisfactory, although one was rated petrographically as not of the highest quality.

It was found tbat certa in agenc ie s which are known to produce physical symptoms of expansion and cracking, such as

frost action, salting, drying shrinkage, and sulphate attack, were not responsible for the trouble in this case. In one important phase of the investigation it was shown that, after about six months of conditioning, rapid expansion followed by cracking was most pronounced in cases where concrete test beams were being exposed merely to a relative humidity of close to 100 per cent at 73°R, or to wetting and drying cycling. Outside exposure during spring, summer and fall only, produced somewhat less expansion. The least expansion at this age was observed in specimens SUbjected to two cycles of freezing and thawing per day.

Provision was made at the start of the research program for investigating thoroughly the possibility of cement-aggregate reaction and, more particularly, alkali-aggregate reaction.

Several but not all of the symptoms of such a reaction were present in this case. Recognized ASTM test methods were used.

The standard mortar bar test, (ASTM designation C227-S2T), gave negative results although more than the normal amount of

expansion was obtained in samples made from limestone from quarry A and a high alkali cement. In this test the aggregates are reduced to sand sizes and blended to give a specific grading. Mortar bars are made which are then conditioned at 100°F. and 100 per cent relative humidity and measured for length change.

The chemical test employed for determining alkali

reactivity (ASTM designation C289-S4T) also gave negative results but these results were so unusual that they could only be

interpreted as resulting from extraordinary properties of the limestone in question. In this test the potential reactivity of the aggregate is determined by digesting a prepared grading of the material in a sodium hydroxide solution, then determining the

6

-The Conrow test (ASTM designntion c342-55T) gave

negative results also. In this test samDles similar to those in the mortar bar test are made but the conditions of test are different. It is designed to reveal excessive expansion in aggregates susceptible to a cement-ageregate reaction peculiar to certain materials in the central parts of the United States. This reaction is not an alkali-aggregate reaction and the

mechanism of the expansion is not kno\vn.

Two independent petrographic evaluations, by two recognized authorities in the field, agreed that an unusual type of cement-aggregate reaction is present in this case, and that there is a definite possibility of some type of alkali-aggregate reaction being involved. Doubt remains however, as to the identity of the deleterious component of the limestone.

Studies on actual concrete beams, using both high and low alkali cements, indicated, after more than eight months of test, that the degree of expansion is affected by the alkali content of the cement. High alkali cements, such as those used

in the field, have produced abnormally high expansion in concrete prisms, whereas low alklai cements have produced much lower

expansions. These results suggest that some kind of alkali-aggregate reaction is the cause. Further support for this view is found in the effective reduction in expansion obtained when part of the high alkali cement is replaced by an active pozzolanic material called California calcined shale. Two commercial fly ashes retarded expansion, but not sufficiently to be considered acceptable. Other possible correctives are being studied.

The only test method which is considered reliable in the light of these experiences, to determine whether concretes have expansive properties or not, when made with the reactive limestone, is one in which concrete prisms are continously

conditioned at near 100 per cent relative humidity and a constant temperature of about 73°F. The beam size used is 3 by 4 by

16

inches. A measuring stud is inset at each end of the beam at the time of fabrication. The test beams are kept continuously in a curing room at 100 per cent relative humidity and 73.4°F., and removed for short periods only during which measurements of

length are made. Comparison specimens made with innocuous materials of known performance are ｡ｬｷ｡ｾｳ used for comparison.

An advantage of this test is that it provides for the testing of actual mixes that may be used on the job, with a ooe-inch maximum placed on the stone size. A serious disadvantage is that six to twelve months may be required to determine, with any degree of assurance, whether or not the concrete will expand abnormally. This method may be used, for the time being, as a rejection test. The limits of expansion beyond which a concrete may be considered unsatisfactory have been tentatively calculated

7

-from laboratory da ta which, a 1 t hough limited, are cons idered rellable at this time. It follows that both the method of test and the expansion limits mAy be subject to modification when more work has been carried out. The details of the test method, inclUding the method of me asur-Lng Lengt h change, are given in the Appendix to this report.

It must be emphasized again that the basic cause of the problem in the Kingston case is still not known with certainty and may not be discovered for a long time. Nevertheless, remedial measures generally employed to control expansion due to alkali-aggregate reaction appear to offer promise of an ultimate solution in this case.

Discussion

It would be premature, on the basis of the limited work done to date, definitely to classify the present phenomenon as an alkali-aggregate reaction. In samples of both field and

laboratory concretes which have expanded abnormally and cracked. petrographic examination has revealed only meagre quantities of alkalic-silica gel whlch is characteristic of alkali-aggregate reaction, and reaction rims around the coarse aggregate particles are very narrow or ill-defj.ned. The deleterious mineral or rock constituent, whatever it is, does not appear to be anyone of those associated with other alkali-aggregate reactions. Never-theless, many of the usual symptoms of alkali-aggregate reaction are present in this case. These have already been discussed.

The apparently anomalous situation in the Kingston area, where some concretes have shown distress and others have not,

the materials used in these concretes having presumably come from the same sources, might appear to be baffling. The same situation, however, can and does occur where a Lkal.Lvaggr-egate reaction is

involved, and this phenomenon has been demonstrated by experiment in other laboratories. An optimum proportion of deleterious

material to innocuous material for the production of excessive expansion in concrete has been found to occur, for example, where reactive opal is involved. In the case of the Kingston limestone

studied, the rock from one shallow stratum has not produced excessive expansion in concrete prisms after one year of test. The rate and degree of expansion of concretes in Kingston are reported to have varied greatly. This also is in keeping with what is known about cement-aggregate reactions generally.

Despite the present uncertainty as to the real nature of the reaction, solutions to the field problem in Kingston appear, from laboratory results to date, to be given by the use of the conventional measures for controlling alkali-aggregate reaction. Classically, these are: (8) selection of aggregate from another source known to be unreactive; (b) use of a cement with a suf-ficiently low alkali content; or (c) use of an effective admixture such as a proven pozzolanic material.

8

-The importation of coarse aggregate from an outside source will probably add considerably to the cost of concrete. The availability of satisfactory materials within a reasonable distance from Kingston is a matter for investigation. It has been reported that suitable ｬｯ｣ｾｬ gravel deposits are few and inadequate. The use of crushed granite in this vicinity is a

ｰｯｳｾｩ｢ｩｬｩｴｹ but the practicability of such a step, and the quality of the material available, must first be assessed.

The importation of a low alkali cement into the area appears at the moment to be even less practicable from the economic standpoint. The nearest Canadian source of suitable cement may be found to be a plant in Alberta. Possible American sources have not yet been examf.ned , The possibility of modlfications in cement manufacturing in nearby plants, presently producing high alakli cements, in order to reduce the alkali content, is a matter which cannot be considered in this report. Nevertheless, the

development of a sufficient supply of an effective low alkali cement for this area may not be an impossibility, and will no doubt be carefully considered by companies presently supplying cement.

Portland-pozzolana cements have been used successfully

in the field to control excessive expansion due to alkali-aggregate reaction. The effectiveness of a pozzolan in controlling the

reaction in the case under study is presently under investigation in the D.B.R. laboratory. A California calcined shale, already

useQ successfully in controlling cases of alkali-aggregate reaction, has shown fair promise after seven to eight months of test when

used as a

25

per cent repl'lcement of cement by weight. Two American fly ashes, both probably available in quantity and at reasonable prices, do not appear to be sufficiently effective to provide a solution. A Canadian fly ash is under test but is not

expected to be usable in its present form because of its very high carbon content.

It will be recognized that a oartial replacement of cement by a pozzolan may reduce the early strengths of the

resulting concrete very materially. On many jobs it is important that the concrete develop a certain flexural strength at some early age. Such cases must be considered carefully. It is clear that the effect of such materials on the various properties of concrete must be thoroughly investigated before they are used in the field.

A variety of other pozzolanic materials are know, including ､ｩ｡ｴｯｭｾ｣･ｯｵｳ earths, volcanic ashes, and finely ground siliceous materials. The effectiveness of these materials in controlling excessive expansion due to alkali-aggregate reaction is dependent primarily on the reactivity of the siliceous component. Certain chemical compounds also act as

9

-inhibitors of alkali-aggregate reaction.. None of these materials should be used until they are proven, by proper testing, to be effective in nontrolling expansion and, at the same time, not detrimental to other desirable properties of concrete. It I'oL'low s that the numer-ous admixtures for concrete available on the market can not be expected to

correct the present problem.. Calcination of nqtive shales or clays may provide mgterial which will give an answer. In,the event that none of the thpse major corrective measures suggested is found to be economically practical, thot'e is still the

possibility that a combination of a cement of moderate alkali content and a fly ash may prove effective.

It has already been sug3ested that ｾｯｯｲ concreting practices may have aggravated many of the cases of expanding concrete in Kingston. It is pos:->ible that the risk of abnormal expansion and cracking can be minimized by producing dense,

impermeable concI'etes thI'ough the use of low water-cement ratios, proper grading and proportioning, and good compaction techniques. In view of the apparent contributing action of the cement, it would appear that the recognized good practice of using a minimum cer.lent content is desirable, providing it is consistent with job requirements.

Since Vlarmth and moisture have been sho1l'm to promote abnormal expansion, the risk may be minimized if, after a

reasonable curine period, the concrete is permitted to dry out reasonably well. Consideration should therefore be given in the design of concrete structures to features which will reduce the extent of exposure to dampness or wetting and drying

conditions, and which will facilitate drying. An important consideration, will naturally be the provision for expansion

joints in slabs and walls (for example) to acconmodate more than normal movement in the concrete.

It has become obvious, from an assessment of the whole situation, that a final solution of the Kingston problem will not come from laboratory research alone, but this research work must be backed up by satisfactory field performance. For this reason, careful records should be kept on all possible jobs of details such as sources of materials, mix data, and conditions of placing and curing. Whenever possible, test specimens should be made from field m:i.xes and these should be conditioned and

measured periodically for length change in accot'dance with methods similar to the one used in the D.B.R. laboratory. Preliminary consideration has been given to a field test exposure pr-ogr-am in Kingston. This would be a major undertaking and would depend also on the further development of laboratory resef:lrch.

10

-The inference that may be drawn from this report that all limestones in the vicinity of Kingston are to be held suspect may be entirely unjustified. It is evident that this aspect of the situation can be clarified only by a detailed study of all pmential sources of limestone. Such a large scale investigation would require the co-operation of several laboratories and would entail detailed petrographic evaluation of all local aggregate sources and the various strata or

formations in each source.

The problem in Kingston is a serious one. Although much has been learned about the nature of the problem and

possible methods for controlling it, much additional work munt be done before a practical and economical solution can be worked out. This report, therefore, is an interim statement of the situation, based on studies to date, and is intended to provide some gUidance for concreting work in the immediate future. Any conclusions drawn from it must be weighed carefully in the light of the materials actually studied and of the cases already observed.

APPENDIX

Tentative Method Suggested by ｴｾ･ Division of Building Research for Testins EXEansion of Concrete Made

With Kingston Limestone Coarse Aggregate

Scope

1. (a) This method of test is intended to determine whether concrete made with Kingston limestone as coarse

aggregate is to be rejected on the basis of degree and rate of expansion according to prescribed

specifications.

(b) This method involves the measurement of length change in laboratory specimens made with materials intended for job work, the mix to be the same as the job mix except that the maximum-size coarse aggregate is to be limited to one inch. The

conditions of test are to be those herein prescribed. (c) This method is regarded as particularly applicable

to cases where Kingston limestone as coarse aggregate is used with or without pozzolans, with high or low alkali cements. It is intended to be a tentative test only, and may be modified when further field and laboratory data become available.

Preparation of Materials

2. Materials shall be prepared in accordance with the procedure set out in A.S.T.M. Designation C192-55, using the materials needed on the job, but limiting the size of crushed limestone coarse aggregate to a one-inch maximum.

Weighing Materials

3.

As in A.S.T.M. Designation C192-55. Mixing Concrete

4.

As in A.S.T.M. Designation C192-55. Consistency and Yield of ｃｯｮ｣ｲ･ｴｾ

2

Number of Specimens

6.

At least two, but preferably three, specimens shall be made from each test batch. General guidance is given in A.S.T.M. Designation C192-55.

Size of Specimens

7. _{The test specimens shall be prisms measuring}

₃

_by

4

_by

16

inches. Apparatus

8.

(a) Moulds - Moulds shall be used which are fitted wfth"end plugs into which standard measuring

studs are placed prior to placing the concrete. Moulds similar in design to those used by D.B.R. are recommended.

(b) Preparation of moulds - The clean mould parts shall be coated with·a layer of vaseline on all inside surfaces. For moulds of the type used by D.B.R., a piece of 2-mil polythene sheeting shall be cut to cover the base and a hole punched at each screw hole •. The sheeting shall be then

smoothed on the greased surface with a dry cloth. The side and end plates shall then be assembled

and tightened snugly. A piece of polythene sheeting shall be then cut to cover the inside of each side plate and to fold over top, leaving at least one inch hanging over the outside wall. The sheeting shall be smoothed on the greased surface with a dry cloth. The same procedure shall be followed for each end, leaving a hole somewhat larger than the end plug. The purpose of this procedure is to avoid greased or oiled surfaces on the concrete samples. The end plugs shall be amply greased and put in place. Mea suring studs shall be deg rea sed in acetone for a few minutes, allowed to dry, and

screwed snugly into the end plugs so that no grease comes in contact with the portion to be bonded by the concrete.

(c) (d)

Tamping; rod - as in A.S.T.M. Designation c192-5.5. Comnarator - an apparatus for ｮｾ｡ｳｵｲｩｮｧ accurately length changes in'the beams. A simple but ｡｣｣ｵｾ｡ｴ･

3

Moulding Specimens

9. The procedure to be followed shall be that outlined for Flexural Test Specimens in A.S.T.M. Designation C192-55, with the add:itional provision that the end plug shall be held tightly against the end plgte during the filling and tamping operation by grasping the outside end of the plug and pulling outward. This prenaution shall ｢ｾ taken whenever necessary until the mould and specimen are placed in the curing chamber. Extra

tamping ar-ound the inserts is very important to ensure good bond.

Curing the ｓｅｾ｣ｩｭｾｾ

10. The moulds containing the placed specimens shall be carefully placed in a curing chamber conditioned at near 100 per cent relative humidity and at a temperature of 70 to 75°F.

Demoulding the Specimens

11. _{The moulds contalning the placed specimens shall be removed}

after 24

±

2 hours, and the specimens removed from the mould. If the moulds used are similar to those used by D.B.R., this operation is to be carried out as follows:

(a) The screws on the end plates shall be removed by means of a suitable wrench.

(b) The end plates shall be gently tapped away from the mould in a uniform manner so as not to exert pressure on the insert plug. Pressure on these plugs may

loosen the inserts since the bond developed at this early age is very small.

(c) A small wrench shall be used to remove the insert plugs, leaving the free insert exposed in the end cavity.

(d) The side plates are unscrewed and removed by

gentle tapping, and the specimen may be lifted

free from the bottom plate. The polythene sheeting is removed from the snecimen.

(e) The whole operation of demoulding shall be completed quickly to prevent drying out of the samples.

Storage and Measurement of Test Specimens

12. (a) Identlf:ication - Immediately after demoulding, the specimens should ba clearly identified by appropriate code letters.

(b) Initi8I Mea S"llre..!!1ent - Following the demoulding and identification of the specimens, they shall be measured immediately for length to the

near-os t 0·.0001 inches. If the comparator used is similar to that used by D.B.R., the specimen shall be placed on end on the sample support. The counterbalanced system shall be then adjusted so that the bottom contact face is snugly in

contact with the measuring stud of the specimen. The micrometer at the top shall be then adjusted against the top measuring stud by means of the ratchet attachment, and a reading taken. The reference bar shall then be measured in the same manner, and the readings recorded in a suitable record book.

(c) After the initial measurement, the specimens shall be placed in the conditioning chamber, which is maintained at close to 100 per cent R.R. and at a

temperature of 70 to 75°F., for the duration of the test.

(d) Subsequent Measurements - At 7 days of age the specimens sball be measured again for length in the same manner, with a minimum time of exposure to room conditions, and protection against drying provided by covering with a moist cloth.

Measurements then shall be made at 28-day age intervals to 168-day age, following which the measurements may be made at intervals judged appropriate in the light of previous behaviour of the sample.

Calculation and Report

13. _{Calculate the difference in length of the specimen after}

the initial 24-hour measurement (making the necessary correction with reference bar readings) and any sUbsequent measurement to

the nearest 0.001 per cent of the effective gauge length which in this case, is ｉｌｾＭＳＯＸ inches. This is simply calculated by multiplying the length difference in incbes by 7. _{Report as per}

cent expansion, using negative signs for contraction values, taking the average of duplicate or triplicate samples.

Reproducibility

14.. Reproduclbili ty shall be considered satisfactory if the expansion of any specimen does not differ by more than 0.005 per-centage point from the average of the duplicate or triplicate

samples from the same batch, except when the expansion has reached a value exceeding 0.020%. After this, reproducibility shelJ. be considered satisfactory if the expansion of any test specimen moulded from the same batch does not differ from the average by more than 15 per cent of the average.