Research report sponsored by the Re· inforced Concrete Research Council

Title No. 62-3

Technique for Investigation of Internal Cracks in

Reinforced Concrete Members

By BENGT B. BROMS

A method is described by which the interna I crack pattern, crack width, and crack spacing in reinforced members can be investigated.

The method consists of injecting a resin into loaded tension or flexural reinforced concrete members. The applied load is maintained for 6 to 8 hr while the resin is allowed to set. The members are then cut open by a diamond saw and the internal cracks (maintained in their original posi­tions) are studied with a microscope.

Key words: beam; cracking; crack pattern; crack spacing; crack width; microscopic study; reinforced concrete; research; resin injection; technique .

• THE INTERNAL CRACK PATTERN OF reinforced concrete members is fre­quently different from that observed on the surface of the members.* For example, the crack width measured at the surface of flexural mem­bers may be two to three times the crack width close to the reinforce­ment. Internal secondary and longitudinal cracks often develop at rela­tively high load levels. These internal cracks originate at the level of the reinforcement, but are frequently confined to the immediate vicinity of the reinforcement.

This paper describes a method by which the spacing, width, and pat­tern of internal cracks has been investigated in tension members.

DESCRIPTION OF TEST METHOD

Description of test members

A tension member reinforced concentrically with a single reinforcing bar is shown in Fig. 1. It is loaded by a tensile force applied to the two protruding ends of the reinforcing bar. The tensile load transferred from the reinforce­ment to the surrounding concrete causes local tensile cracks tD form in the concrete around the reinforcement. Such local secondary tensile cracks which

•Broms, Bengt B., "Mechanism of Tension Cracking in Reinforced Concrete Members" (Publication pending).

35

6.0"

T-C5-I

JOURNAl OF THE AMERICAN CONCRETE INSTITUTE

..

...

. . . .

~APPLIED LOAD

. .

REINFORCING BAR

SECONDARY TENSILE CRACKS

~APPLIED LOAD

Fig. I -Tension member

8.10" 11·.25". ,. . I I . I

®!· 35" .. .. -· . I . 0

January 1965

.I .··.:~[]2.5'' ~ SAW-CU SAW-CUT ~SA~-CUT.

T- RC3-I

# 8 BAR

I I 1· I · •

0 : I I I

.. I I • . • ·I I 1·1.

:·:

T-RC4-I

Fig. 2 - Dimensions of test members·

8.0"

INVESTIGATION OF INTERNAL CRACKS

ACI member Bengt B. Broms has been director of the Swedish Geotechnical Institute, Stockholm, since 1964. Prior to ioining SGI, Dr. Brems was attached to Shell Development Company, Houston, Texas, until 1959 when he ioined Cornell University as associate pro­fessor of civil engineering. He is the author of technical papers concerned with buckling of reinforced concrete columns, failure criteria for concrete, lateral resistance of piles, shear strength of clays, bearing capacity of flexible pavements, and frost action in soils. He is a member of ACI-ASCE Committee 441, Reinforced Concrete Columns, and ACI Committee 543, Concrete Piles.

37

do not reach the surface of the tension member are shown in Fig. 1. The distri­bution of internal cracks was investigated for three tension members. The dimensions of the test members (T-C5-1, T-RC3-1, and T-RC4-1) are shown in Fig. 2. The average compressive strength, as measured from at least six compression tests on 6.0 x 12.0 in. concrete cylinders capped with Hydrostone, was 3940, 5140, and 5140 psi for Members T-C5-l, T-RC3-l, and T-RC4-l, respectively. The members were reinforced with a single #8 bar with a yield strength of 93,800 psi (determined by the 0.2 percent -offset method). The in­ternal crack pattern was investigated at an applied load of 70 kips (correspond­ing to a nominal steel stress of 88,700 psi).

Preparation of test specimen

To investigate the internal crack pattern, each test specimen was provided with a small hole parallel with and close to the reinforcement as shown in Fig 3. The holes were formed during casting by a small neoprene hose with an outside diameter of ¥s in. The hose was held in place by two standard brass compression unions placed in two holes bored in the end sections of the forms. Each compression uni-on was provided with 1fs in. standard pipe threads at one end and with a compression fitting, for a tubing with an outside diameter of Ys in., at the other end.

The neoprene hose was threaded through the two compression unions (the diameter of the hole thr-ough the unions was enlarged to ¥s in. to provide room for the neoprene hose). The hose was stretched slightly and locked in its stretched position by two screw clamps. The surface of the hose was greased to decrease the bond between the hose and the c-oncrete during setting. During casting of the tension members, the concrete was placed carefully around both the hose and the reinforcement.

One or two hours after casting, the tension members were placed in a moist room. A few days after casting, the neoprene hose was removed. The pull applied to the hose during removal decreased the diameter of the hose slightly and freed it from the surrounding concrete. The specimens were wet cured for 28 days.

After completion of the curing, the test specimens were coated with a fast drying gum to provide an airtight seal on the surface of the test members. Nylon tubing with an outside diameter of ¥s in. was attached to the compres­sion unions at both ends of each test member. The nylon tubing attached to the lower end of the test member was connected to a reservoir containing a resin (Fig. 4). The tubing attached to the upper end of the test specimen was connected to a vacuum source. Two screw clamps denoted A and B in Fig. 4 were clamped to the nylon tubing close to the compression unions.

38 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE January 1965

Testing procedure

The test members were loaded by applying axial tension to the two pro­truding ends of the reinforcing bar. At the maximum load level (after internal tensile cracks had formed), a vacuum was applied. The upper screw clamp (Clamp A) was opened and the lower clamp (Clamp B) was closed during application of the vacuum. The vacuum removed part of the air trapped inside the test specimen (within the secondary and longitudinal tensile cracks). The gum applied to the surface ·of the test members provided an airtight seal and prevented air from intruding into the test members (the trapped air would have prevented the resin from entering into the very fine narrow cracks).

After 5 to 10 min, Clamp B was opened slowly while maintaining the vacuum. The vacuum drew the resin* into the test specimen. When resin was observed to flow out through the upper nylon tubing, Clamp A was closed.

*Six parts of EOPCAST 530 to one part of Hardener 9816, both manufactured by the Furane Plastics, Inc. The resin was colored black for ease of observation,

SCREW CLAMP

1/8-IN UNION

1/16-IN NEOPRENE---..,_

HOSE "

1/8-IN UNION

SCREW CLAMP

'.

.,

..

FORM

~REINFORCING / ~BAR

Fig. 3 -Casting of test member

INVESTIGATION OF INTERNAl CRACKS 39

CONTAINER

AIR TIGHT SEAL

..

0 ' -----

Fig. 4- Injection of resin

----

SCREW CLAMP

lt13-IN BRASS COMPRESSION UNION

...

1/8-1 N NYLON TUBING

40 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE January 1965

An air pressure (up to 100 psi) was then applied to the reservoir system as shown in Fig. 4, which forced the resin into the very fine internal cracks. The air pressure in the reservoir was regulated by a standard air regulator. (It is possible that the applied pressure caused some of the internal cracks to open up since the axial force resulting from this pressure is appreciable. At, for ex­ample, a pressure of 100 psi, the axial force at a secondary crack with a radius of 2.0 in. is 1250 lb. A better procedure might have been to limit the applied air pressure to 10 psi at which pressure, the resulting axial force is small, and probably can be neglected.)

The air pressure was applied for about 10 min, whereafter Clamp B was closed.

The axial load was maintained for approximately 6 to 8 hr until the resin had hardened. The axial load was then released and the resin maintained the internal cracks in their original positions. The test member was cut open by a high-speed diamond saw close to the reinforcement to permit study of the internal secondary tensile cracks as a function of the distance from the rein­forcement.

The internal crack pattern was investigated with the aid of a 40-power microscope. The width of the internal cracks was measured to the nearest 1/1000th in.

CRACK WIDTH I/1000 IN

b

A

v

Fig. 5- Crack formation (Member T-C5-I)

INVESTIGATION OF INTERNAL CRACKS 41

TEST DATA

Member T-CS-1

The cylindrical tension member T-C5-1 was cut approximately across its diameter close to the reinforcement. Secondary tensile cracks can be seen in Fig. 5. The upper two secondary cracks were confined to the vicinity of the reinforcement and could not be observed on the surface of the test member. The maximum secondary crack width was measured close to the le.-el of the reinforcement. The lower two tensile cracks originated close to the reinforcement and reached a maximum at the surface of the test member. The two lower tensile cracks were visible on the surface of the test member along part of the perimeter.

Member T -RCJ-1

The rectangular tension member T-RC3-1 was cut parallel with its wide side. The secondary tension crack visible at the center of the cut member was also visible on the surface of the member. This secondary tensile crack did not traverse the total width of the member. The maxi­mum crack width was measured close to the level of the reinforcement .

......,:»<..C--.!!W.!..!.I Dw.T.!:!.H.....,:I/1000 IN

4 5 2

Fig. 6- Crack formation (Member T-RC3-I)

42 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE

CRACK WIDTH

1/1000 IN

Fig. 7-Crack formation (Member T-RC4-1 J

Member T-RC4-1

January 1965

The rectangular tension member T-RC4-1 shown in Fig. 7 was cut across its wide side parallel with the reinforcement. The cut traversed two secondary tensile cracks which extended over only a part of the wide face of the member. Fig. 7 thus shows the variation of the secondary crack width over the thickness of the member. The crack width close to the level of the reinforcement was only one-fifth to one-third the crack width at the surface of the member. This variation indicates that the widths of cracks which penetrate to the surface is larger at the surface of the member than at the level of the reinforcement. Thus the widths of these cracks increase with increasing distance from the rein­forcement. Cracks which are confined to the vicinity of the reinforce­ment (and do not penetrate to the surface) reach their maximum widths at or close to the level of the reinforcement.

The minimum measured crack width was 0.001 in., indicating that the :resin could not penetrate into cracks with a width less than approxi­mately 0.001 in. Probably additional cracks were present at the maxi­mum applied load but the widths of these cracks were probably less than 0.001 in. These cracks closed when the members were unloaded and could not be detected when the test specimens were cut open and examined with a microscope.

INVESTIGATION OF INTERNAL CRACKS 43

CONCLUDING REMARKS

The method described in this paper can also be used to study the in­ternal crack formation in other types of reinforced concrete members, such as flexural members.

ACKNOWLEDGMENT

The investigation reported in this paper was a part of a test program spon­sored by the Reinforced Concrete Research Council with the financial support of the Bureau of Public Roads, Portland Cement Association, and the C{m­crete Reinforcing Steel Institute. This investigation was conducted under the administrative guidance of George Winter, head of the Department of Struc­tural Engineering at Cornell University. The experimental part of the investi­gation was carried out by M. Khartabil, formerly research assistant in the Department of Structural Engineering.

Received by the Institute June 30, 1964. Title No. 62-3 is a part of copyrighted Journal of the American Concrete Institute, Proceedings V. 62, No. 1, Jan. 1965. Separate prints are available

at 50 cents each, cash with order.

American Concr~te lnsrittJte, P.O. Box 4754, R~dford Std!ion, Detroit, Mich. 48219

Discussion of this paper should reach ACI headquarters in triplicate by Apr. 1, 1965, for publication in the September 1965 JOURNAL.

Sinopsis- Resumes- Zusammenfassung

Tecnica para Ia lnvestigaci6n de Grietas lnternas en Miembros de Concreto Reforzado

Se describe un metodo por el cual se pueden investigar la configuracion de grietas internas, asi como su ancho y espaciamiento en miembros reforzados.

El metodo consiste en inyectar una resina en miembr·os de concreto reforzado sujetos a tension o a flexion. La carga aplicada se mantiene por 6 u 8 horas para dar tiempo a que endurezca la resina. Los miembros son cortados por despues medio de una sierra de diamante y las grietas internas (que se mantienen en su posicion original) son estudiadas por medio de un microscopio.

Procede pour Relever les Fissures Internes a l'lnterieur des Elements en Beton Arme

On decrit un procede qui permet d'etablir la configuration, la largeur et l'espacement des fissures qui se produisent a l'interieur des elements de betQn arme. Ce procede consiste a injecter une substance resineuse dans les elements de beton tandis que ceux-ci sont soumis a une charge de flexion ou de torsion.

On maintient l'element sous charge pendant une periode de 6 a 8 heures ce qui permet a la substance resineuse de se coaguler a l'interieur des fissures. A !'aide d'une scie a diamant, on sectionne ensuite l'element puis Dn etudie au microscope les fissures internes qui ainsi n'ont pu se refermer apres le retrait de la charge.

44 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE Janu8ry 1963

Verfahren z:ur Erforschung von inneren Bri.ichen in verstarkten Betonteilen

Eine Methode ist beschrieben, nach der die innere Rissbildung, Rissweite und der Rissabstand in verstarkten Betonteilen ermittelt werden konnen.

Die Methode besteht darin, Harz-Bindemittel in spannungs- oder biegungs­belastete verstarkte Betonteile zu injizieren. Die angewendete Belastung ist fiir 6 bis 8 Stunden unterhalten, wahrend das Bindemittel erhartet. Die Teile werden dann mit einer Diamantsage aufgeschnitten und die inneren Brliche (in ihrer Original-Position erhalten) werden dann unter dem Mikroskop studiert.