NATIONAL BUREAU OF STANDARDS REPORT

8039

Strength of Concrete Collars Cast

Around Existing Columns

by

L* F« Skoda

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-12-10411 July 16, 1963 8039

Strength of Concrete Collars Cast

Around Existing Columns

by

L* F.* Skoda

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, th whole or in part, is not autho

Bureau of Standards, Washingl

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ss accounting documents intended

subjected to additional evaluation

i listing of this Report, either in e Office of the Director, National

' the Government agency for which

copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Strength of Concrete Collars Cast

Around Existing Columns

L. F. Skoda

1. INTRODUCTION

A test program was initiated by the Bureau of Yards and Docks to

determine a practical and economic method of attaching new concrete slabs

to existing columns* The specific problem was to transfer the weight of

a new concrete slab installed as fallout protection between the roof and

floor of an existing high ceiling building to the existing reinforced

concrete columns* An engineering study by the Bureau of Yards and Docks

indicated that the additional load could be safely absorbed by the

existing concrete columns and column footings* This additional load

was calculated to be 60,000 lbs per column. It was decided that the

simplest solution to the problem was to cast reinforced collars around

the columns that could support this load and transfer it to the column.

The primary objective of this test series was to determine the strengths

and stiffness of such a column collar assembly* The various factors

investigated were the depth of the collar, reinforcement , surface

preparation, and load slip relationship,

2* DESCRIPTION OF TEST SPECIMENS

2,1 Columns

The column stub was 14- by 14-in. in cross section and 30-in. long.

The longitudinal reinforcement was four 3/4-in. diameter bars. Lateral

ties were spaced at 12-in. intervals and were made of 3/8-in. diameter

bars* The same column stub was used in all five tests.

The concrete used for the column was made of Type III cement, siliceous

sand and a well rounded gravel of 3/4-in. maximum size. The proportions

were 1:3. 8:4. 9 by weight and a compressive strength of 3700 psi was

realized at time of test of the first collar.

Prior to casting of the first test collar the peripheral surface

of the column stub which was to bond with the concrete collar was sand

blasted so that the coarse aggregate was well exposed. The column was_

similarly cleaned prior to casting each of the subsequent collars tested.

Two different sizes of collars were tested. The first two specimens

were 4- in. thick and 8-in. deep* The final three specimens were 4- in. thick

and 12-in. deep. Views of typical collars are shown in figure 1.
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.... ' 9

=- ' v. * .
2

The main reinforcement was the same in all specimens and consisted of

a lap welded continuous 3/4-in. diameter bar located near the top of the

collar. Specimen No. 1 had two additional horizontal stirrups spaced 3- in.

on center parallel to the main vertical stirrup. The reinforement in

specimen No. 2 consisted of a continuous stirrup fashioned of No. 2 bar

that provided vertical reinforcement spaced every 2- in. around the periphery

of the collar at its midplane. The additional reinforcement used in

specimens 3, 4, and 5 consisted of varying amounts of 2- by 2- in. welded

wire fabric of different gages that extended throughout the depth of

the collar. Specimen No. 3 had a double layer of 6-gage fabric, No. 4

had a single layer of 6-gage fabric, and No. 5 was reinforced with a single layer of 12-gage fabric.

The mortar used in the collars had a sand to cement ratio of 3:1, by

weight, with the cement being of Type III. A water-cement ratio of 0.49

was used. The workability of the mortar was improved by adding lime in

an amount equal to 0.1 of that of cement, by weight. The specimens were

cured under wet burlap for 24 hr and then allowed to dry in air until

tested. This method of curing was selected to simulate field curing

conditions. The 7-day compressive strength of 2- by 2-in. control cubes

averaged 8000 psi. The collars were all 7 days old at time of test.

3. TEST PROCEDURE

The specimens were tested in an inverted position. The assembly was

supported by the collar above the platen of a 600,000 lb capacity hydraulic

testing machine by two 3- by 8-in. bearing blocks that rested on top of

two steel pedestals. The load was applied to the column stub in increments

of 10,000 lb until failure. Specimens in the testing machine can be seen

in figures 3 through 7.

Slip measurements were made on each side of the column directly over

the supports with 0.0001-in. micrometer dial gages. The gages were attached

to the column stub so as to measure the displacement of the collar with

respect to the column as the load was applied. (Figures 3, 5, and 7).

Readings were taken before and after each increment of load. A

time lapse of 2 min was observed between successive increments of load

so as to determine any slip that might occur at any given applied load.
wt« ... ' -

•

'

.

.

'
3

4. RESULTS

Load-slip relationships for the five specimens are shown in figure

8. Specimens numbered 1 and 2 had collars 8 in. deep, while 3, 4, and 5

were 12 ift. deep. The loads corresponding to first visible cracks appear

as enlarged points on each load- slip curve. The maximum plotted values

of slip shown on the curves were the last observed values before failure.

Table I summarizes the pertinent results shown in figure 8.

The mode of failure in all of the specimens was punching shear. There

was no evidence of bond failure at the interface of fc\ie column and collar.

The initial cracking in all cases occured at the top of the collar just

over the 3- by 8-in. reaction plates. Figures 2 through 7 are photographs

of the specimens after test.

The slip measurements made of the column collar assemblies at the

beginning and at the end of each period of sustained increment of load

indicated that no appreciable movement occured under sustained load

except at loads near failure.

5. DISCUSSION

An examination of the load-slip curves indicates that the significant

difference in load carrying capacity of the collars is almost wholly due

to the difference in the depth of the collar. The maximum loads attained

by the 8-in. collars were approximately half of those attained by the 12-in»

collars. While the primary horizontal stirrups was a 3/4-in. bar in all

specimens, the auxiliary reinforcing varied considerably in the 8-in.

and 12-in. collars; however, the shape of the load-slip curves as well

as the maximum loads attained were essentially dependent on the depth of

the collars.

The photographs show the comparative crack patterns in the 8- in.

and 12-in. collars. Considerably larger cracks and greater displacements

are visible in the 8-in. collars. The close-up of specimen No. 1 (figure 2) shows that no bond failure occured. This was typical of all

specimens. This supports the assumption that if the surfaces are

properly prepared and reasonable care is taken in casting of collars, bond

is no problem in spite of the minimal amount of curing.

COLLAR

No. I COLLAR

No. 2

COLLARS

Nos. 3,4 8« 5

22 "

A-A

FIG. I. DETAILS OF COLUMN AND COLLAR REINFORCEMENT.

No. 3 - 2 LAYERS 22-66 WELDED WIRE FABRIC

No. 4- I LAYER 22-66 WELDED WIRE FABRIC

No. 5- I LAYER 22-1212 WELDED WIRE FABRIC
31646 g
FIG. 2. CLOSE-UP OF COLLAR No. I AFTER FAILURE.

FIG. 3. COLLAR No. I AT FAILURE.





FIG. 6. COLLAR No. 4 AT FAILURE.


81646 7
200

180

ISO

140

120

100

80

60

40

20

C

Table

1.

Test

Results

I

I

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A.'V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C.,and

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front cover. WASHINGTON, D.t.

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Joint Institute for Laboratory Astrophysics- NBS Group (Univ. of Colo.).

HSr.OUM-NR.-'-— 3C