NATIONAL BUREAU OF STANDARDS REPORT

6396

Test of Precast Concrete Shower Receptors

byJames 0. Bryson

Report to

Federal Housing Administration

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4810 April 28 , 1959 6396

Test of Precast Concrete Shower Receptors

by

James 0. Bryson

To

Federal Housing Administration

NATIONAL BUREAU Of" intended for use within

to additional evaluation

listing of this Report, ei the Office of the Directo however, by the Goverm

to reproduce additional

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

or progress accounting documents

is formally published it is subjected

ting, reproduction, or open-literature rmission is obtained in writing from

C. Such permission is not needed,

-.ally prepared if that agency wishes

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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Test of Precast Concrete Shower Receptors

by

James 0. Bryson

Precast concrete shower receptors were tested to

determine the flexural strength under a concentrated

load, watertightness while under load, and the quality

of the concrete. The tests were made with five different types of receptors, two of which were reinforced

and the other three were plain. One of the three plain

receptors contained a lightweight concrete.

The receptors were tested in duplicate and had

average flexural strengths for the five different types

ranging from 913 lb to 12^5 lb. The reinforced receptors failed to show evidence that the reinforcement

served to increase their strength. All receptors were

watertight up to their maximum loads.

1, Introduction

At present there are no standard specifications for precast concrete shower receptors. In view of this, the Federal

Housing Administration in a letter dated December 3» 1958* requested the National Bureau of Standards to perform tests to

aid in determining the structural characteristics needed in a precast concrete shower receptor to allow its use without the

added protection of a metal shower pan.

1.1 Scope

Data on the properties of the receptors were obtained by

performing tests which fall into three categories. They are:

a strength test of the receptors under a concentrated load, a water permeability determination during the load test, and miscellaneous tests to determine the quality of the concrete.

The concentrated load test was to represent the most severe

load condition that could possibly be found in service, such as

a heavy man standing in the center of a receptor supported at

diagonally opposite corners. The water permeability test was

to determine the point at which the receptor would cease being

watertight under the applied load. Tests to determine the

water absorption and the compressive strength of the concrete

2 .

fall in the miscellaneous tests category. An examination of

the receptors in "as received" condition and determination of

the location and type of reinforcement were also included

among the miscellaneous tests.

2.

Test Specimens

The precast concrete shower receptors were supplied by

the Fiat Metal Manufacturing Company, Inc., of Long Island

City, New York. They were 36 in. square receptors with a shoulder on the border approximately 1 l/4-in. wide and were

bound with a sheet metal strip standing 4-in. high. All of

the receptors received for testing were of the same model

which was called "Sierra" by the manufacturer.

Five types of receptors were included in this test. The

types were classified by material composition as follows:

1. Dense concrete, plain,

2. Dense concrete, reinforced.

3. Terrazzo surfacing on dense concrete base,

plain,

4. Terrazzo surfacing on dense concrete base,

reinforced

5. Terrazzo surfacing on lightweight concrete

base, plain.

The receptors were to be load tested at the age of 28

days and 2-in. cubes cast from the same batch of materials

were to be tested to determine the compressive strength of

the concrete. However, the shipping schedule followed by

the manufacturer was such that the age of the receptors at

the time of test varied from 27 to 31 days. Since the manufacturer did not supply cubes representing all the receptors

tested, -it became- necessary to drill_cores from the receptors

to determine the compressive strength of the concrete.
.

i*

.
3.

4. Test Procedures

4.1 Concentrated load test

Several factors were given consideration in establishing

the concentrated load test procedure. The specimens had to

be supported above the lower platen of the testing machine

high enough to allow observation of the underside throughout

the test. The testing machine which had sufficient clearance

to admit a 36- by 36 -in. receptor was a 600,000 lb capacity

machine which had a minimum load range of 0 - 30,000 lb. For

this reason, a 5,000 lb capacity load cell was used to measure

the applied load with a greater precision than could be attained with the indicator of the testing machine.

The receptors were supported on diagonally opposite corners, with the point of support at a distance 3 in. in from each

corner. This provided a bearing length of 6 in. at each support, The flexural span, from center to center of the supports,

was 44 7/8 in. Heavy steel frames supported the- specimens

approximately 30 in. above the platen of the testing machine

with 3/4 in. square aluminum bearing bars set in plaster on

the steel frames to serve as line reactions. Strips of leather

were placed between the test specimens and the bearing bars to

compensate for any uneveness along the bearing surface. The

load was applied through a vertical steel pipe 5 3/4 in. in

outside diameter and l/4 in. in wall thickness. This pipe was

set in plaster in the center of the specimen. The ends of the

loading pipe were machined parallel to each other and perpendicular to the longitudinal axis. A l/2 in. thick steel plate

was placed on top of the loading pipe to support the load cell

which made contact with the head of the testing machine.

The deflection of the specimen under load was measured

with two 0.001 in. dial gages symmetrically located about the

center of the span at a distance of 1 3/4 in. from the surface

of the loading pipe. Figure 1 shows the over-all view of the

test setup.

The load was applied to the specimens at a rate slow

enough to allow dial gage reading to be made without interrupting the application of the load. The loading rate, in

terms of the deflection of the specimen was 0.003 in. per min

(or about 100 lb per min). Dial gage readings were made at

loads of 250 lb, 500 lb and at 100 lb increments thereafter.

4 - An exception to this was the first specimen tested (plain,

dense concrete base with terrazzo surfacing) where the readings were made at 100 lb increments from zero load.

During the loading test the water permeability of the

specimen was determined by filling it with water after applying a sealer at the base of the loading pipe which covered

the drain opening. Th. e load observed at the instant when

the water penetrated the specimen and became visible on the

underside was recorded, as was the load at first crack and

the maximum load. Water did not become visible on the underside of the specimen at first crack in all cases. In all

specimens but two, it became necessary to continue deflecting the specimen to permit leakage to occur, and during this

process the load, having passed the maximum value, began to

drop off. For this reason, the loads recorded in Table 1 as loads corresponding to the first leak are in most cases less

than the loads at first crack.

4.2 Compressive strength test

Three small cores, approximately 1 in. in diameter, were

drilled from each receptor after the completion of the load

test to be used for determining the compressive strength of

the concrete. The cores were drilled from the receptors near

the shoulder where the thickness of the conrete was about

2 in,, giving a height to diameter ratio of about 2. Thp ends

of the^ cores were capped with a high strength gypsum plaster

which was allowed to harden over night. The compressive .load

was applied to the cores at a rate that would cause failure

within 1 to 2 minutes.

4.3 Water absorption test

Segments taken from the receptors subjected to the concentrated load test were used to determine the water absorption of the concrete. The specimens were approximately 25

sq in. in surface area with thicknesses equal to the depths of

the respective receptors.

The specimens were dried in a ventilated oven at a temperature of 110° C for 72 hr after which they were cooled to

room temperature and weighed. They were then submerged in a tank filled with clean tap water at room temperature and

allowed to soak for 24 hr. At the end of the soaking period

the specimens were taken from the tank, blotted with a damp

towel and immediately weighed. The absorption is reported as

a percentage of the dry weight.

5.

4.4 Miscellaneous examinations

Upon receipt of the receptors, one of each type was

visually examined for over-all general appearance and shrinkage cracks. In addition, one receptor with a terrazzo surface and one of plain concrete were stored in the laboratory

and filled with water for one week. The two receptors were

observed daily to detect any damp spots showing on the undersides .Following the concentrated load test each receptor was

broken up to determine: (1) the type and location of reinforcement (if any); (2) the depth of the receptor base at

the shoulder and at the center; (3) the thicknesses of the

materials in the composite units at the shoulder and at the

center.

5. Results

The receptors were square and conformed to the specified dimensions within a very small tolerance. There were

no surface defects or shrinkage cracks visible on any of the

five types of receptors ”as received.” This was also indicated by the examination of those receptors which were permitted to hold water for one week. After one week’s test,

neither the terrazzo surfaced receptor nor the plain concrete

receptor showed any dampness on their bottom surfaces.

The test results for the Individual receptors are given

in Table 1. In the concentrated load tests, the maximum

loads varied from a low of 913 lb (average for the two plain

dense concrete receptors) to a high of 1270 lb (average for

the two plain terrazzo-dense concrete receptors). All of the

receptors failed in the same manner which was characterized

by a sudden drop-off of the load without warning with visible

cracks appearing simultaneously. Two exceptions to this were

the plain terrazzo-dense concrete receptor No. 2 and the reinforced terrazzo-dense concrete receptor No. 2 . In these cases

the cracks were detected just before the maximum loads were

reached. In all cases the cracks first appeared in the center

of the receptors at the edge of the drain and proceeded in the

directions of the free corners of the specimens. The crack

patterns on all the receptors were essentially the same and

are Illustrated by figures 2 and 3 which show the cracks in a plain (non reinforced) receptor and a reinforced receptor,

respectively.
.

‘

'
The relationship between the center deflection and the applied load for each receptor is illustrated by the curves in

figure 4* Although the term center deflection is used, the

points of measurements of the deflections were 4 5/8 in. from

the center of the specimens. However, because of the proximity

of the points at which the measurements were made to the center

of the receptor, this approximation is believed to cause no

serious error.

During the load tests the receptors developed leaks either

at the maximum load or at some stage of loading beyond that

producing the maximum load. That is, most of the receptors did

not develop leaks immediately when the maximum load was reached,

but only upon developing considerably larger deflections than

those observed at the maximum loads. Thus, only the plain

terrazzo-dens e concrete receptors Nos. 1 and 2 allowed water

penetration at the maximum loads, while all other receptors

maintained their watertightness up to some stage of loading

beyond the maximum load.

6. Discussion

An examination of the data indicates that there was no

clearly defined relationship between the compressive strength

of the cores removed from the receptors and the strengths of

the receptors as tested. It is noted that while the compressive strengths of the cores ranged from 1990 to 6140 psi , the

maximum loads supported by the receptors varied from 825 to

1375 lb.

It is believed that the sheet metal strip located around

the edges of the receptors contributed greatly to the bending

strength of the units. This belief is supported by the fact

that the two plain receptors composed of lightweight concrete

which showed relatively low compressive strengths, supported

practically the same loads as the plain dense concrete receptors and the reinforced dense concrete receptors.

The failure of the reinforped receptors to support greater

loads than the plain receptors might be due to the fact that

the wires in the welded fabric made an angle of 45° with the

direction of the flexural span, thereby reducing considerably

the effectiveness of the fabric as structural reinforcement.

If we assume that the design load on the receptor is produced by an individual weighing 300 lb, then the ratios of the

maximum load to the design load will be found to range from

2.8 to 4» 6.
:

. '

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•
7 .

The receptors can be rated as watertight up to the maximum load. Only two receptors, the plain terrazzo-dense concrete receptors, developed leaks as soon as the maximum loads

were reached. All other receptors developed leaks only upon

reaching considerably larger deflections than those observed

at the maximum loads

USCOMM-NBS-DC
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Table

1.

Properties

of

Shower

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Value

observed

after

the

maximum

load

had

been

passed

and

the

load

started

to

drop

off

while

the

specimen

continued

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deflect.

The

design

load

is

assumed

to

be

an

individual

weighing

300

lb.

tj. S. btel’AftTMENT OK COMMERCE

Lfcwln !j, 3tntl)p«, Secretory

NATIONAL MJMAU ()P STANDAHDH

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