An Investigation of Internal Stresses in Concrete

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An Investigation of Internal Stresses in Concrete

Nielsen, K.; National Research Council of Canada, Division of Building Research

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Title:

NATIONAL RESEARCH COUNCIL OF CANADA Technical Translation TT-621

A n _{investigation of internal stresses i n concrete,}

( ~ ~ e n s ~ ~ n n i ~ s u n d e r s ~ k n i n ~ )

Author: Rnud Nielsen,

Reference: Swedish Institute for Cement and Concrete Research,

Translator: H.A. G. Nathan, Translations Section, N. R. C. Library,

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PREFACE

In beginning a long-term study of the problem of creep in concrete the Division of Building Research is following its usual practice of making a careful study of all available material in this field,

In the course of this investigation, 1,Tr. W.G. Plewes, who is conducting the study within the Building Design Section, was much impressed by a paper which had appeared in Sweden as a contribution from the Swedish Institute for Cement and Concrete Research, an organization with which the Division of Building Research is privileged to have close liaison,

With the permission of the Director of this Institute this paper has been translated for the Division by M r , H,A.G.

Nathan; it is now published in this form in view of its general interest beyond the immediate needs of the Division,

Ottawa,

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AN INVESTIGATION OF INTERNAL STRESSES IN CONCRETE

INTRODUCTION

Since Mr. Nielsen is taking up duties outside the Swedish Institute for Cement and Concrete Research (c,B.I,) on August 1 of this year, the g e n e r a l p m g ~ ~ m m e fo~theinvestigation of internal stresses has been revised. This rearrangement means in effect that a part of the very extensive project will now con- stitute an independent programme planned in such a way that it may be carried out under Mr, Nielsen during his part-time service at

this institute, According to an agreement between the C.B.1, and Mr, Nielsents new employer, the Dansk Cement Central (The Danish Central Cement Agency) he will spend one-quarter of his usual working time at the C.B,Im as from August 1, 1956 to July 31,

1958,

The present report comprises a description of said part of the investigation of internal stresses, a test programme, a summary of the present situation and a time-table as well as a plan for publishing the results.

DESCRIPTION OF PROBLEMS TO BE ITTVESTIGATED

According to an earlier description of the investigation of internal stresses in concrete the programme includes the study of the f'ollowdng problems:

A, The local stress concentration in the large particles of the coarse aggrega*e in non-reinforced concrete during setting and as a result of conversion of water,

B, Stresses and deformations in nonarei,nfo~ced concrete due to moisture and temperature gradients.

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C, Stress and deformation patterns in specimens of non- reinforced concrete under constant load and constant elongation, respectively, Le,, stress and deformation patterns due to contraction and relaxation, respectively,

Do The effect of reinforcement on the states of stress and deformations,

E, As a final stage in the investigation, plans are being made for a study of the effect of internal stresses on fracture.

The effects mentioned above should be studied as a function of a large number of variables, namely, the composition of the concrete, the hygrometric and thermal conditions during setting, the storing and testing, size of test specimen, mould and method of reinforcing.

Nielsen9s stress investigations are conf'ined to those listed under A and C and thus include only a limited number of variableso

The experiment is set up on the basis of the following theory and with a view to verifying it and determining the

material constants,

Concrete is considered to be a two-phase system, where one phase is composed of particles of broken stone and the other of cement mortar,

ma his

constitutes a very considerable simpli- fication of the actual conditions, but it is claimed that this assumption produces results in the

treatment

of internal stresses of concrete. Properly speaking, concrete should be considered a four-ph~lse system, because the cement mortar itself may be re- garded as a system consisting of at least two phases, i.e., sand glue and cement glue,)

In order to comply experimentally with the assumption of a two-phase system as long as possible, the concrete used for the

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experiment is decidedly a "gap-gradedf' one, composed of only a fraction of broken stones of the same size as the embedded stress gauges (32 m ) and a cement mortar whose m a x i m grain size

should not exceed

4

to 8 mm,

It is assumed that in this two-phase system each phase has certain rheological properties and that by combining these two systems the interaction between the systems can be treated theoretically,

It may also be assumed that at the beginning the rheological properties of each phase may be cornposed chiefly of three different types of rheological curves:

A. Elastic deformation (recoverable

-

reversible) of instan- taneous character according to Hooke's law, represented by the modulus of elasticity, E+

B. Viscous deformation (irrecoverable

-

reversible) expressed by the equation,

where E = elongation,

t = time,

o = stress

k = viscosity constant.

C, Flow (irrecoverable

-

irreversible), whereby is meant deformation as a result of adjustment of "dislocationstq. As yet it has not been possible to form a satisfactory expression for e-a-tb the interrelationship for flow, Various attempts have been made without resulting in a definite opinion regarding the appearance of this type of deformation,

The conversion of water may be interpreted in quite general terms as an external force, acting on the test specimen

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exactly like temperature and load in the usual sense, The

initiation of the conversion of water is brought about according to the theories suggested by Powers, Brownyard and Picket.

By studying the rheological curve for each phase separately and also for the combined system (concrete), and at the same time measuring the stresses in the large particles of the coarse aggregate with the aid of the embedded stress gauges, and also measuring separately the water-vapour pressure within the test specimen for both the concrete and the cement mortar, it is possible, under otherwise similar external conditions, to verify

the theoretical curve for these three phases and to determine the constants.

TEST PROCEDURE

Firstly, the rheological curve for concrete and the individual phases of the system will be studied in the unloaded state (experimental stages I and 111, and secondly with constant external load and constant elongation (experimental stage III),

Experimental stage I

-

sealed test specimens

-

was

added in order to make it possible to study the internal stresses during setting without conversibn of water with the surrounding medium and also the internal conversion of water under the same conditions (self-dessication), This experimental stage lasts

l-4

-

28 days, its end being determined by the results,

Constant load

3

Constant elongation -

1,293 1,293 Wdthout "external force"

I I 1 I 21°c, sealed in moulds

U+

-

28 appr, 200 (time in days)

I1 21°C,, 6% rel. hum, of the air, I11 21°C,, 60% rel. hum, of the air.

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Experimental stage I1

-

a i r storage i n otherwise unloaded s t a t e

-

was included i n order t o study the i n t e r n a l s t r e s s e s when conversion of water with the surrounding medium can take place f r e e l y (drying-shrinkage s t r e s s e s ) . T h i s experimental stage l a s t s a t l e a s t h a l f a year and i t s termination i s d e f i n i t e l y determined by t h e r e s u l t .

Experimental stage 111, which according t o t h e above t a b l e begins not l e s s than 200 days a f t e r pouring, includes s u i t - a b l e creep and r e l a x a t i o n t e s t s and l a s t s a t l e a s t a year and a h a l f . Af'ter reaching the age of 200 days the r a t e of hydration has become so slow t h a t the continued hydration can no longer appreciably i n t e r f e r e with creep and r e l a x a t i o n , However, t o be on the s a f e s i d e during t h i s experimental s t a g e , c o n t r o l specimens a r e n o t exposed t o any e x t e r n a l f o r c e other than the thermal

and hygrome tri c conditions,

I n order t o keep within reasonable l i m i t s the experimental work t o be accomplished i n the s p e c i f l e d two-year period during which the experiment should be concluded, the v a r i a b l e s a r e t o be r e s t r i c t e d t o t h e following:

1, Composition of the Concrete

Only f o u r d i f f e r e n t concrete compositions a r e studied, namely, the f o u r combinations obtained by combining two d i f f e r - e n t percentages of broken stone ( A and B ) with two d i f f e r e n t cement mortars ( C and D ) , The s e l e c t e d percentages of broken stone a r e made a s d i f f e r e n t from one another a s p o s s i b l e with a view t o the p o u r a b i l i t y of t h e f r e s h concrete. The two types of cement mortar d i f f e r from each o t h e r with r e s p e c t t o cement con- t e n t and water-cement r a t i o , p o s s i b l y i n such a way t h a t the cement content o f concrete type AC becomes equal t o t h a t of con- c r e t e type BD, I n other r e s p e c t s a l l the components of the con- c r e t e a r e i d e n t i c a l ,

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2, IJyarometric and Thermal Conditions, Stora~e

During the entire experiment the temperature of the surrounding medium is kept constant at 21°C, The hygrometric conditions are identical for all the test specimens. As

mentioned above, these are stored

14

to 28 days in sealed moulds followed by storage at 6- _{relative humidity of the air for the}

remainder of the experiment,

3 0 All the test specimens are identical in size and form except for the test specimens made from large particles of coarse aggregate, where size and form may be chosen freely in a manner most suitable for the study of the specific properties of the

large aggregate,

For a fair evaluation of the result obtained from the embedded stress gauges a knowledge of the interaction between these gauges and the surrounding material is needed as well as a knowledge of the distribution of the stresses in the large part-

icles in the coarse aggregate.

During the past year these problems were the subject of intense study, first in the form of a continuous experiment in which the stresses arising in concrete specimens during setting

(some stored in water and some in air) were recorded with the aid of ten embedded gauges, and second an optical investigation of the stress distribution in the material surrounding the gauge in the case of pure contraction.

Further light is thrown on the distributions of the stresses in the coarse aggregate (the elongation at the stress gauges) according to the measurements carried out during experimental stages I and 11, where all the test specimens of each concrete type (three test specimens, each with two embedded gauges) are absolutely identical in every respect,

The numbers of both the variables and the test specimens that may be studied is also limited in advance by the supply of'

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PRESEM: SITUATION

1, Measuring Technique

The work of evolving the inductive system of measure- ment was concluded during the past year.

A set of

24

conductive stress gauges is being manufac- tured. The work of these is in an advanced stage; all the de- tails have been taken care of, leaving only the work of assemb- ling, checking and calibrating,

A sample inductive elongation meter was produced and tested with entirely satisfactory results. As a matter of fact, the final design is clear, and its production may begin within a

very short time,

The inductive load meters are still in the design stage, But it is estimated that this will be concluded within a very short time and production may then begin.

For both the inductive elongation meter and the inductive load meter the material that is difficult to obtain has been pro- cured and the coil formers of steatite are now ready. It is

reasonable to assume that the production of these two meter ty-pes tirill not encounter any difficulties corrrparable with the

serious obstacles v~hich had previously slowed down the manufacture of the incluctive measuring equipment.

7 L e r 5Xrs

-s are used for measuring the temperature within the concrete. 'A" are quite familiar with this measuring

technique, since the preparatory work was concluded during the

past year, Therefore, no difficulties will be encountered in the manufacture of thermometers either, This may then be considered a very simple matter with respect to both the quantity of work and the costs,

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The preliminary hygrometer work may on the whole be considered finished, although the continuous routine measurenlents with the test meters are taking a long time, However, line pro-

duction of hygrometers m y be started shortly and no difficul-

ties are anticipated here either, However, preparatory develop- ment work and calibration are consuming quite a lot of time,

2, Concrete Technolorn

As -mentioned above, an experiment which was begun last

summer with ten stress gauges embedded in the concrete is still continuing, This experiment was intended primarily for testing the method of embedding the gauges and for studying their oper- ating capacity, The results may be sumnarized briefly, The technique used in embedding the gauges was found to be correct, and, up to the present, all the gauges, with the exception of one, were found to operate satisfactorily.

The embedding test involves two beams with five gauges each. This experiment will be continued for the time b e h g , but one of the beams will be broken open in the near future in order to remove the five gauges and to check the zero point,

(~espite the fact that these ten test gauges might be

able to operate after their removal from the concrete they cannot be employed for actual research in the future because of various

differences in design and operation from the set of 24 gauges

mentioned above, )

The optical investigations of the stress distribution about an embedded gauge was carried on long enough so that all the tests for determining the material constants and establishing the testing technique have been concluded, Fwthermore, all the

equipment for the actual investigation is ready, The main ex-

periment involving this part of the internal-stress investigation is to begin immediately and it should be concluded within approxi- mately two months,

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TIhIE SCHEDULE

Up u n t i l August 1 o f t h i s year, a f t e r which M r , Nielsen w i l l only be working part-time a t the C , B . I , , t h e time i s being devoted c h i e f l y t o the design, manuf'acture and c a l i b r a t i o n of the measuring equipment, t h e completion of the o p t i c a l i n v e s t i - g a t i o n of the s t r e s s e s , the mnuf'acture of t h e moulds and the

t e s t i n g of the colnposition o f the concrete,

I n a d d i t i o n t o the c u r r e n t program of designing and producing the measuring equipment, the work of designing and possibly even producing the apparatus f o r keeping load and elon- g a t i o n constant i s about t o begin, However, the l a t t e r equipment

w i l l not be needed u n t i l 200 days a f t e r the pouring begins, a t the very e a r l i e s t ,

In the optimum case the pouring m a y begin j u s t before August 1, b u t more l i k e l y not before the f a l l of t h i s year,

The t h e o r e t i c a l s t u d i e s w i l l be resumed a f t e r August 1

a t the very e a r l i e s t and will be c a r r i e d out c h i e f l y i n Denmark, p a r a l l e l with t h e r o u t i n e measurements c a r r i e d out by the COB. I,

according t o the experimental procedure o u t l i n e d above, PUBLICATION OF RESULTS

On March 1, 195!ja a preliminary p l a n was made f o r the p u b l i c a t i o n of the r e s u l t s obtained from the i n t e r n a l - s t r e s s in- v e s t i g a t i o n i n the broadest sense, This p l a n involves t h r e e each of the following r e p o r t s , namely:

Report no, 35

-

The use o f heat accumulators f o r ternper- a t u r e measurements a s well a s f o r temperature c o n t r o l and temper- a t u r e recording ;

Report no, 36

-

Hygrometers f o r recording the water-

vapour pressure i n s i d e concrete;

Report no, 37

-

Inductive s t r e s s , elongation and load meters,

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Howevbr, the time-table laid down in the plan could

not be maintained and it is considered unwise to allow additional experiments to crowd the publf cation progpamne, Instead the

following time-table is suggested for the publications: Report no,

35 -

about October,

1956;

Report no,

36 -

about February,

$957;

Report no,

37

-

during fall

$957,

These dates are those on which the Swedish manuscripts

will be available,

The results of M r , Nielsenvs pa~ticipation in the concrete-technological aspect of the internal-stress investigation are expected to be ready for publication within a year from August 1, 1958, However, a small, summarized internal report

intended for the C,B,I,, the Dansk Cement Central and the re- s e a ~ c h council might be available immediately after August 1,