EFFECT OF AGE DIFFERENCE OF ELEMENTS AND LONG TERM

STORAGE ON THE FLEXURAL BEHAVIOR OF SPLIT BEAMS

by

J. 0. Bryson and L. F. Skoda

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Department of the Navy

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

421.01-1-4212411 January 31, 1966 9033

EFFECT OF AGE DIFFERENCE OF ELEMENTS AND LONG TERM

STORAGE ON THE FLEXURAL BEHAVIOR OF SPLIT BEAMS

by

J. 0. Bryson and L. F. Skoda

to

Bureau of Yards and Docks

Department of the Navy

NATIONAL BUREAU OF S

for use within the Government, and review. For this reason, tl whole or in part, is not authc Bureau of Standards, Washing

the Report has been specificalt

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

>ss accounting documents intended

i subjected to additional evaluation

! listing of this Report, either in

le Office of the Director, National

1 the Government agency for which

copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Effect of Age Difference of Elements ana Long Term

Storage on the Flexural Behavior of Split Beams

by

J. 0, Bryson and L. F. Skoda

ABSTRACT

Two split beams were constructed with a one month delay between

prestressing the tensile element and casting-on the compressive element

to explore the effect of the difference in ages of the two halves of

the beams on their long term behavior. After casting, one beam was aged for 1 1/2 years and the second beam was aged for 1 year. At the

end of the storage periods the maximum strains developed from elastic,

plastic, and shrinkage deformations were about 2100 micro in. /in. for

the first beam and 1600 micro in. /in. for the second beam.

In the load tests the beams showed no effect from the delayed

casting and long storage period when compared with similar beams that were cast and tested without delay.

INTRODUCTION

In the study of the flexural behavior of split beams—^ conducted

at NBS, two principal variables were introduced to investigate the

effect of abrupt changes of strains in the composite member at the

interface on the crack pattern and resistance to bending moment of the

beams „ The variables were: The location of the interface between the

prestressed and non-prestressed portions with respect to the midplane

of the composite beam and the magnitude of the prestress at the interface. The effects of creep and shrinkage of the concrete were minimized

by continuous moist curing. The time from casting to testing was also

held to a minimum with the use of Type III cement. The concrete developed the prescribed strength for prestressing in 3 to 5 days and for

testing in 4 to 6 days. The time between the prestressing of the tensile

element and the placing of the concrete for the compressive element ranged

from 1 to 3 days. Therefore, the overall time from casting to testing

ranged from 10 to 14 days.

The effect of differential shrinkage was, for all practical purposes,

eliminated in the earlier tests. However, in actual construction practice

differential shrinkage stresses might occur quite often. This would re- sult from a time delay between the casting and prestressing of the tensile

element and the casting-on of the compressive element.

To explore the effect of a difference in age of the two halves of

split beams on their long-term behavior, two beams were cast with a 1 month delay between prestressing the tensile element and the casting-on

of the compressive element. These beams were stored indoors simply supported

—Flexural Behavior of Prestressed Split-Beam Composite Concrete Sections,

by J. 0. Bryson, L„ F. Skoda, and D. Watstein, Journal of the Prestressed

Concrete Institute, Vol. 10, No. 3, June 1965.

1

on a 9 ft span to age for 1 year in one case and i year and 5 months

in the other case, after which the beams were tested to failure in

flexure. The ages are reckoned from the time that the tensile elements

were prestressed.

TEST SPECIMENS

The test specimens included the two split beams, 3-by 4-by 16-in.

free shrinkage prisms, and 6-by 12-in. cylinders. The split beams were

4-by 12-in. in cross-section and 10 ft in length. The depth of the tensile element and the compressive element were each 6-in., therefore, the

interface of the two elements coincided with the midplane of the composite

unit. The beams were post-tensioned and the tendons were unbonded. A

single steel bar* fixed in a parabolic profile served as the prestressing

tendon. The tendons were located in the cross-section so that the calculated prestress in the top fiber of the tensile element at midspan was

zero. The force in the tendon was calculated to produce a prestress of

2250 psi in compression at the bottom fiber of the tensile element. This

prestress includes the effect of the weight of the beam on a 9 ft simply

supported span. In short, these beams were identical, in all respects, to

2 / the "A" beams— in the first series of tests.

A group of 6-by 12-in. control cylinders were cast from each batch of

concrete for each beam. The 3-by 4-by 16-in. shrinkage specimens were cast

from the concretes of the first beam only. There were three prisms cast

with the tensile element and three with the compressive element.

INSTRUMENTATION

Gage lines for a 10-in. Whitt emore strain gage were established on each side of the beams and the shrinkage prisms parallel to the longitudinal axis. The gage lines were established at the time of casting with

gage plug inserts fastened to the inside of the forms. There were four

gage lines on each side of the split beams and one gage line on each side

I^ibid

~

2

of the shrinkage prisms. The gage lines on the split beams were located

at midspan approximately 1-in. from the top and bottom surfaces and l-in„

above and below the interface. The gage lines on the shrinkage prisms

were centrally located on the sides.

The use of Whittemore strain gage instrumentation allowed observation of the strains in the concrete during the early stages of curing

and served to reveal the nature of the stress block developed over the

cross-sections. Initial readings on all gage lines were made one day

after casting each element of the split beam and the shrinkage specimens.

The tensioning force in the tendon was measured with a steel dynamometer attached to the tendon at the end of the beam opposite to the

jacking end.

Bonded wire electrical resistance strain gages were used to measure

longitudinal concrete strains in the beam during load tests. These gages were mounted on the specimens with a quick setting adhesive immediately

before testing. The gages were located on both sides of the beam at midspan and were spaced along the depth of the beam to show the distribution

of strains over the cross-section.

The deflection of the beams at midspan were measured with respect

to the platen of the testing machine with 0.001-in. dial indicators.

MEASUREMENTS AND TEST PROCEDURE

Whittemore measurements on the split beams and the shrinkage prisms

and readings of the prestress dynamometer were made at intervals of about

1 mounth over the storage period. At the end of the storage period, the

beams were tested in flexure simply supported with the load applied at the

third (1/3) points. The test procedure was the same as for the split beams

of the first series referred to earlier.

3

RESULTS AND DISCUSSION

The two beams were designated AC-1 and AC-2. The initial prestress

in both beams tapered from a maximum of 2250 psi in the bottom fiber of

the tensile element to zero at the top fiber of this element. The com- pressive element was cast-on stress free.

The results of measurements made over the storage period and during

the load test are presented in Table 1. Figures 1 through 4 show the

variation of strains developed over the storage period. The strains at

the bottom and top gage lines of beam AC-1 are presented in Figs. 1 and

2, respectively, and those at the bottom and top gage lines of beam AC-2

are presented in Figs. 3 and 4, respectively. The measurements from

shrinkage specimens cast with beam AC-1 were used to estimate the plastic

strains in the beam* The curves in Fig. 1 show the total strains,

elastic strain, shrinkage and plastic strains occuring at the bottom gage

line for beam AC-1. The total strain for the top gage line and the cor- responding shrinkage strain were nearly the same as can be seen from the

curves in Fig. 2. Also, it is noted that the elastic strain computed from

the loss in prestress is very small and therefore it is assumed that no plastic strains occurred at this level in the beam. Shrinkage specimens

were not cast with beam AC-2, therefore, only the total strain and elastic

strain curves are shown.

At the end of the storage period, the total strain measured at the

bottom gage line was approximately 2100 p in./in. for beam AC-1 and approximately 1600 |jl in./in. for beam AC-2. This difference in the total

strain is significant and is not due to the difference in the lengths of

the storage periods since the strains in both beams tended to level off

after about 200 days of storage. The difference in the strains is apparently caused by the difference in the concretes. It is noted that at

the end of the storage periods (time of test) the concrete strengths for

the top and bottom elements was 4710 psi and 4940 psi for beam AC-1, and

5640 psi and 5380 psi for beam AC-2, respectively.

4

In the load tests, the ultimate loads for the beams were

24,150 lb for beam AC-1, and 24,850 lb for beam AC- 2. Both beams

failed by flexural compression. These results show favorable agreement with those for split beams and reference beams of the first

series where the ultimate loads for all beams ranged from 22,200 lb

to 25,800 lb.

The load-deflection relationship for the beams are shown by the curves in Fig. 5. For comparison, curves of the load-deflection re- lationship for a reference beam (Rl-1) and a similarly constructed split

beam (A-2) from the first series of tests are shown in Fig. 6. The

values of deflection at the applied load of 12,000 lb for these beams

are 0.14 in,, 0.10 in., 0.12 in., and 0.12 in. for beams AC-1, AC-2,

Rl-1, and A-2, respectively. These values of deflection are presented

for comparison since they correspond to the value of applied load computed

for zero stress at the bottom fiber of the beams.

The crack patterns for both beams are shown in Fig. 7. These

beams developed several more cracks than did the beams tested in the

first series. However, the patterns are essentially the same. As was the case in the earlier beams, the first crack developed at mldspan and was shortly followed by additional cracks symmetrically located about the midspan. The crack patterns also indicate that complete composite action whs

carried out through the load tests.

CONCLUSIONS

The one month delay between prestressing the tensile element and

casting-on the compressive element and the long storage period for the

two split beams, had no measurable effect on the flexural characteristics

of these beams. The two beams when tested after more than a year of

storage, showed essentially the same response to load, ultimate load properties, and crack patterns as the split beams in the first series of tests

that were protected from drying shrinkage and tested shortly after casting.

5

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