NATIONAL BUREAU OF STANDARDS REPORT

5814

A STUDY OP Z- CRETE AS UNDERGROUND

PIPE INSULATION

by

Selden D. Cole

Paul R. Achenbach

Prank J. Powell

Report to

Office of the Chief of Engineers

Bureau of Yards and Docks

Headquarters, U. S. Air Force

U. S. DEPARTMENT OF COMMERCE

NATIONAL BOREAO OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT MBS REPORT

1003-20-4881 March 18, 1958 58l4

A STUDY OF Z-CRETE AS UNDERGROUND

PIPE INSULATION

by

Selden D. Cole

Paul R. Achenbach

Frank J. Powell

to

Office of the Chief of Engineers

Bureau of Yards and Docks

Headquarters, U. S. Air Force

3MP08TAHT NOTICE

HA7I0AAI BUlEAli Of 8' intended for use within the to additional evaluation and

lilting of this Report, althet the Office of the Director, h however, by the Governmon

to reproduce additional sop

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

’ progren accounting document!

formally published It ii subjected

gi reproduction, or open-literature

lision ii obtained In writing from

> Such permission Is not needed,

ly prepared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FOR OFFICIAL USE ONLY

A Study of Z-Crete as Underground Pipe Insulation

by

Selden D 0 Cole, Paul R. Achenbach, and Frank J. Powell

Abstract

At the request of the Office of the Chief of

Engineers, laboratory and simulated field tests

were made of a Z-Crete system as a means for

insulating underground steam and hot water lines.

A typical Z-Crete system consists of an envelope

of insulating concrete six inches thick around

the pipe with the insulating concrete enclosed

in a waterproof membrane, all supported on a

dense concrete base pad. Z-Crete is the trade

name for a lightweight concrete made of Portland

cement, expanded vermiculite, an admix, and

water. It is poured in the field and allowed

to cure for a limited time before sealing up

the waterproof covering on the outside. Simulated field tests in an earth-filled box showed

that the Z-Crete did not crack under heat, that

it did not adhere to the pipe because of the

corrugated paper wrapping on the pipe, and that

there was no measurable leakage of water through

the waterproofing. The heat loss of the fourinch pipe ranged from 212 Btu/hr per linear foot

when the system was surrounded with air-dried

earth to 239 Btu/hr per linear foot with water

standing over the earth cover. The waterproof

covering prevented the complete drying out of

the Z-Crete. The moisture in the one inch of

Z-Crete nearest the pipe was transferred to the

material farther from the pipe and remained

there as long as the pipe was heated. The average

moisture content in this inner one inch was

about 7 percent whereas that in the remainder

ranged from 50 to 100 percent of the dry weight

of the material. Only the air space and about

one inch of Z-Crete nearest the pipe were very

effective as insulating material. The thermal

conductivity of the oven-dry Z-Crete was

0.84 Btu/hr (ft^) ( °F/in. ) at a mean temperature

of 128 °F. The average thermal conductivity of

the Z-Crete envelope as tested underground was

- 2 -

more than 2 1/2 times this value. About three

weeks were required to completely dry out a

Z-Crete annulus 5 3/8 in. thick without exterior

waterproofing when heated at the center.

1. Introduction

In response to the request of the Office of the Chief of

Engineers,, Department of the Amy, an investigation was made of

Z-Crete as an insulating material for underground heat distribution pipes operating at a steam pressure of about 120 psig

corresponding to a saturation steam temperature of 350F.

Z-Crete is the trade name of a lightweight concrete made

of Portland cement, expanded mica, an admixture, and water.

The expanded mica used in Z-Crete is known commercially as

Zonolite. Z-Crete is made with different admixes and with different sizes of Zonolite particles. The admix used for the

Z-Crete specimen tested in the earth-filled box was identified

as Zonolite Liquid Admix and the aggregate was Zonolite No. 3.

More recently, the Zonolite Company submitted two cylindrical specimens of a new Z-Crete mix for water absorption tests.

The admix used in these specimens is B-Crete admix and the

aggregate is "Zonolite No. 4 aggregate waterproofed,” as identified by the manufacturer.

The sieve analysis for the standard Zonolite grades is as

follows

Sieve Size Cumulative Percent Retained

Maximum Minimum

Grade No. 3 (Density 7 to 10 lb/ft3)

8

16

30

50

100 100

10

60

95

98

0

20

65

75

90

Grade No. 4 (Density 8 to 12 lb/ft^)

16

30

50

100 100

5

65

98

0

15

60

90

-3

A typical Z-Crete installation consists of a pipe or pipes

surrounded by an envelope of insulating concrete six inches

thick and enclosed in a waterproofing membrane,, all supported

on a concrete base pad. The proportions of the ingredients of

such lightweight heat insulating concrete are varied as circumstances indicate. The mixture used for this investigation

was that recommended for most ordinary conditions; namely,, one

bag of Portland cement (about 9^- lbs. or 1 cu.ft), eight cubic

feet of expanded mica (Zonolite), seven quarts of admix and

twenty-six gallons of water.

To determine the characteristics of Z-Crete, thermal conductivity measurements, moisture absorption tests, and full

scale trench tests under simulated use conditions were made.

The laboratory phase consisted of (l) determining the

thermal conductivity by the standard hot plate method, and

(2) determining the thermal conductivity of several concentric

annuli of Z-Crete cast around a cylindrical ceramic heating

tube as the material gradually dried from an initial condition

after setting to a dry condition.

The full scale simulated use tests consisted of measuring

the heat transfer of a Z-Crete envelope cast around a standard

four-inch pipe under steady state conditions with the pipe temperature at 350F when (l) the envelope had not yet been covered

with earth; ( 2 ) the envelope was covered with nominally dry

earth; (3) the envelope was covered with earth and submerged

under water; (4) the envelope was cooled to ambient temperature

while under the high water table, followed by lowering the

water table and reheating the pipe to 350F.

Moisture determinations were made of the earth surrounding

the Z-Crete and also of the Z-Crete at selected distances from

the heated pipe. Moisture absorption of the Z-Crete used in

the tr'ench tests and of a newer mix made with finer particles

of Zonolite and a different admix was studied.

2. Description of Equipment

The thermal conductivity of Z-Crete was measured in an

eight-inch guarded hot plate apparatus conforming with the

requirements of Federal Specification LLL-F-321b and ASTM

C177“^5- The specification requires two specimens 8x8x1

inch in size, oven-dried to constant weight at 215F.
'

’
-4

The Robinson ceramic core apparatus was used for determining the thermal conductivity of Z-Crete at various stages

of drying. For this test a three foot long cylinder of

Z-Crete, 12 inches in diameter, was cast on the 1.29-inch

ceramic core which was centered lengthwise in the specimen.

The ceramic core has numerous coaxial openings running lengthwise through it which contain electric resistance wires for

heating the core. Thermocouples were placed at selected

stations within the ceramic core to determine core surface

temperature and at each third of the radial distance from the

core to the surface of the specimen. The thermocouples used

for determining thermal conductivity were centered lengthwise

of the cylinder.

After casting, the cylinder was allowed to cure in the

metal casing for six days, and in air for an additional seven

days after removing the casing. The test was made without the

metal casing. The specimen was supported on a platform scale

so the loss of moisture could be observed as the test progressed.

The test of the Z-Crete system under conditions simulating an underground installation was conducted in a watertight box 4 x 4 x 12 ft in size. A piece of four-inch black

iron pipe 14 feet long was suspended along the longitudinal

center line of the box and pitched at a slope of 1/4 inch per

foot toward the outlet end. The 14-foot length was divided

into a four-foot measuring section at the center and a fivefoot guard section on either end. The condensate formed in

the measuring section was isolated from the guard sections by

two half-moon baffles in the bottom and two half-moon baffles

in the top of the pipe, each located slightly upstream of the

lower baffles. Separate internal condensate drain lines were

brought out from each of the three sections and connected to

high pressure steam traps. The condensate from the measuring

section could alternately be directed through a sight glass

instead of the steam trap. In each case, the condensate from

the traps, or the sight glass, was passed through a watercooled heat exchanger to reduce its temperature to near room

temperature so it could be collected and weighed in open vessels without loss by evaporation.

fORfllrrii 11 !> A

5-

An electrically-heated steam boiler whose input could be

varied in the range from 1.5 to 12 KW (5000 to 47., 500 Btu/hr)

produced steam at the desired pressure. The water level of

the boiler was maintained by a float which opened and closed

contacts in a solenoid valve circuit. The solenoid valve was

in a line from a supply tank held at a higher pressure than

that of the boiler. A check valve in the supply line before

the tank and one in the line before the solenoid valve prevented

loss of pressure in the system from back feed.

Steam was delivered from the boiler through a 3/4-in.

pipe extending about one foot into the inlet end of the fourinch pipe. The one-foot extension was capped off at the end

and was drilled with a series of l/4-in. holes in its side

walls. The aggregate area of the holes equalled 1.5 times the

area of the open end of the 3/4-in. pipe. This produced a

spray of steam on the side walls at the inlet end instead of

a jet down the longitudinal center of the pipe.

The box was filled with dirt, a typical Chester loam, to

something less than mid-height. A wooden form was then placed

on the dirt below the pipe to make a sand and gravel concrete

pad 22 inches wide and 4 inches thick, the upper surface to be

six inches below the under side of the pipe and one edge to

extend six inches from the side of the four-inch pipe. After

the concrete was placed and set the first form was removed and

a second wooden form constructed about the pipe and resting on

the concrete pad. This form was 16 1/2 x 22 1/2 inches in

size and was lined with a water resistant membrane as follows:

six-foot lengths of 15-pound roofing felt were placed lengthwise on the sides of the wooden forms such that there was

approximately an eight-inch gap between the two strips on the

concrete pad and about six inches projected over the upper

edge of the form. The gap on the concrete pad was covered

with a strip of felt to give an overlap of about six inches.

Hot pitch was mopped on the concrete pad and resoftened before

the side felts were placed. The bottom was again hot mopped

before the filler piece was laid inside. A second layer of

felt pieces was placed over hot pitch in the same manner as

the first layer but with the end joints being broken with

respect to those beneath by about eight inches. Wooden bulkheads comprised the ends of the wooden form and these were not

felted.

FOR OFM USE ONLY
**
6 “

An eight-inch wide strip of fabric was set in hot pitch

in the lower inside corners for reinforcement.

The four-inch pipe was now wrapped with a covering of

corrugated paper secured with cotton string. The paper was

used to prevent the bonding of the Z-Crete to the pipe and to

allow space for linear movement of the pipe in the Z-Crete due

to thermal expansion and contraction of the pipe. A two-inch

pipe representing a return condensate line was suspended in

the form such that its bottom edge was in a plane with the

bottom edge of the four-inch pipe and its closest distance was

four inches from the side of the four-inch pipe and six inches

from the side of the form. The two-inch pipe was covered with

corrugated paper in the same manner as the four-inch pipe.

See Pig. 1.

The form was then filled with Z-Crete. The Z-Crete was

mixed in a paddle type mixer and consisted of one bag or

one cubic foot of cement, eight cubic feet of Zonolite,

seven quarts of admix and 22 gallons of water per batch,

designated as a one to eight mix.

After allowing the Z-Crete to set for about one week, the

forms were removed, the top was covered with felt performing

the operations in reverse order to that used for the bottom

and the exposed portions of the envelope weremopped with hot

pitch. The envelope was flashed to the walls of the box

where it passed through the ends.

Before pouring the Z-Crete, five thermocouples were

peened into the top of the four-inch pipe at positions 23

inches apart starting at midlength and extending in both

directions. Five thermocouples were fastened to the surface

of the concrete pad at positions two feet apart starting at

the middle and extending in both directions. Thermocouples

were placed for encasement in the Z-Crete envelope at midlength in four strings in a plane perpendicular to the fourinch pipe. These four strings extended downward, laterally

in both directions and in an upward direction from the fourinch pipe. The thermo couples were spaced on each string as

follows: (l) on the pipe, (2) 1/2 inch from the pipe, (3) two
' . • r 7 k
Fig. 1

-7

-

at one=inch intervals from the preceding ones 5 (4) two at

1 1/2-inch intervals^ (5) three at two-inch intervals on the

long side only., (6) one on the inside surface of the waterproof membrane. The two-inch pipe had four thermocouples on

it: one on the pipe and one on the surface of the paper two

feet from the middle in each direction. Temperature stations

were located on the steam traps and on the boiler. All thermocouples were of 26-gage copper-constantan wire and were used

with a Rubicon portable precision potentiometer to indicate

temperature

.

In order that the water table in the box might be adjusted

six l/2-inch diameter pipes were spaced equally along each

side of the box and extending vertically from the top to bottom.

A 12-inch length of two -inch pipe was fitted to the top of

each l/2-inch pipe for ease in introducing water. There were

100 feet of cotton rope on the bottom of the box placed to

provide distributing channels for the water introduced through

these vertical pipes. Two 1-inch diameter vertical pipes

placed diagonally from each other one foot from either end of

the box and equally spaced between conduit and box side served

as stand pipes for measuring the actual water level.

The 4 x 4 x 12 ft box and its necessary auxiliary parts

were set up in a specially erected building., 12 feet wide.,

24 feet long and about 10 feet high., constructed of insulated

prefabricatedpanels . An air chiller with capacity to freeze

the contents of the box if desired was placed at one end in the

building. The air chiller was used to maintain a steady

ambient temperature about the box when a test was in progress.

The building was supplied with a 100-ampere., 3-phase, 208-

volt electric service and a 1/2-in. water line with a line

pressure of about JO pounds,

3. Test Procedure and Results

3.1 Thermal Conductivity

The thermal conductivity of the Z-Crete was measured by

casting two specimens 8x8x1 in. in size for placement in

the hot plate apparatus after oven drying at 21^>F

.

The results

of the thermal conductivity test are as follows:

.1 j/k
'

1
-8 -

Densit'y, as tested, lb/ft^ 27*9

Thickness,, as tested, inch 1.00

Mean temperature of specimens, °F 128.0

Temperature gradient in specimens, °F/in. 4l . 1

Thermal conductivity, Btu/hr ft 2 (deg F/inch) 0.842

Loss of weight on drying, percent 3-7

Gain in weight during test, percent 0.8

3.2 Tests with Ceramic Core Apparatus

The results obtained on the cylindrical Z-Crete specimen

with the Robinson ceramic core apparatus are shown graphically

in Fig. 2. The thermal conductivity and density of the material

are shown together with the rate of heat transmission and core

surface temperature for a 31 -day test period while the specimen

was losing moisture. The electric energy supplied to the

ceramic core was reduced in a stepwise manner to maintain a

core surface temperature between 350F and 400F until a steady

state was reached with a final core temperature of 350F.

Fig. 2 shows that a steady state condition was attained

after about 21 days of operation at which time the thermal conductivity of all three concentric annuli was essentially equal

at about O .87 Btu/hr(sq ft)(°F/in.), the density was about

26 lb/cu ft, the heat loss rate was 55 Btu/hr per foot of

length, and the core surface temperature was 350P- The upper

group of curves in Fig. 2 shows the progressive reduction in

thermal conductivity of the several annuli of Z-Crete as drying

progressed. About six or seven days wererequired to dry the

inner third of the cylinder, about 15 days were needed to dry

the middle third, and about three weeks were required to attain

steady state thermal conductivity in the entire specimen.

About 18 pounds of water were evaporated per cubic foot of

Z-Crete during the tests.

3„3 Controlled Underground Tests

For underground studies of the heat loss of the Z-Crete

system with controlled conditions, the manufacturer's recommended directions were followed in preparing the specimen, and

simulated field conditions were used in carrying out the investigation.



HEAT LOSS, THE RMALCONDUCTIVITY AND DENSITY OF Z-CRETE BY CERAMIC-CORE APPARATUS

-9-

With air surrounding the Z-Crete envelope, steam at a

temperature of 350? was supplied to the four-inch pipe for

a total of 44 days at which time the temperature observations

in the system and the condensate measurements indicated that

near steady state conditions of heat flow prevailed.

Dirt was then added to the box to cover the system while

the heat remained in it. This test condition was maintained

for 12 days. Pour days after introducing the dirt a core

sample was taken for moisture measurement. A vertical core

was taken from top to bottom, midlength of the box, and half

way between the Z-Crete envelope and the box on the narrow

side. The sample was placed in a closed container for weighing

before heating at a temperature of 215? until constant weight

was observed. The moisture content of the dirt at the time of

sampling was found to be 14.5 percent of the dry weight.

At the end of this 12-day heating period and with the

heat on, water was added to the box until the surface of the

dirt was under two inches of water. This water height was

maintained for 14 days at which time observed temperatures and

condensate rates indicated steady conditions prevailed.

At the end of the 14 days the heat was turned off and the

water height maintained for ten days after which the 350?

steam was again turned into the four-inch pipe. In a period

of 48 hours the temperatures and condensate rate were at

nearly the same values they were before the heat was turned

off. After heating a few more days, with the water table

below the Z-Crete envelope, the envelope was uncovered and

opened up while keeping the heat on.

A section was sawed completely out of the top half of the

conduit, crosswise of the length and at the middle of the

measuring section. An additional slice about 1 1/2 inches

thick was then sawed off the exposed end of the envelope and

one-inch cubes were removed from the slice to permit measurement of the moisture gradient in the envelope in three directions

from the heated pipe, as indicated by the cross section diagram in Pig. 3. The blocks were placed in weighed containers

as soon as possible and sealed airtight immediately. After

ir mm H h

lit

RATIO,

MOISTURE

TO

DRY

Z-CRETE,

PERCENT

BY

WEIGHT

MOISTURE CONTENT OF Z-CRETE VS

DISTANCE FROM PIPE

4

INCHES

5 6

FROM

7

PIPE

LONG

SHOR

TOP

oss section showing

moisture specimen locotion

FIf; 3

-10

oven drying at 215 F the percent moisture to oven dry weight

was computed. Although these ratios are portrayed graphically

in Fig. 3 as a smooth curve., each plotted point represents an

average moisture for a radial increment of one inch in most

cases

.

Fig. 3 shows that the average moisture content of the

inner annulus one-inch thick was about 6.8 percent whereas

that for the second annulus of the same thickness was about

49,5 percent. The moisture gradient was reasonably consistent

in all three directions from the heated pipe to a distance of

four inches from the pipe, but in the longer horizontal direction

was slightly lower in the range from four to six inches from

the pipe. It reached 95 to 100 percent in all directions just

inside the waterproof covering. It should be noted that the

Z-Crete specimen had been under heat a total of 84 days at

the time of these measurements with one period of ten days

intervening when the pipe was not heated.

The heat loss from the measuring section of the Z-Crete

system in the earth-filled box is summarized in Table 1 for

the three test conditions tried. There was only about ten

percent greater heat loss with the system submerged than when

surrounded with air-dried earth, and the heat loss with air

surrounding the system was about half way betwqen these two

values. These data indicate that a 13 to 15 inch layer of

air-dried earth surrounding the Z-Crete provided a little more

insulating value than the air flLlm that existed when there was

no dirt in the box, and the water-soaked earth provided a little

less insulating value than the air film.

The temperatures observed in the Z-Crete envelope during

the tests with air-dried and water-soaked earth around it are

shown in Fig. 4 and 5 * respectively. The graphs show the

temperatures observed in four directions from the heated pipe

after substantially steady state conditions were established.

The temperature gradient observed when the specimen was surrounded by air was not plotted because this is not a typical

operating condition.

Table 1

Heat Loss of Z-Crete System in Earth-Filled Box,

Ambient Air Temp, about 80F, Pipe Temp 350P

Surrounding

Media

Earth Cover

above C. L.,

ft

Water Table

above C. L.

ft

Duration

of Test,

days

Heat Loss,

Linear

ft

Btu/hr

Sq Ft

Pipe

Surf

Air None None 44 224 190

Air-Dried Eartn +2 None 12 212 180

Earth & Water +2 +2 14 239 203

*Moisture content,, 14.5 percent, based on sampling during the test.

TEMPERATURE OF Z-CRETE

VS DISTANCE FROM PIPE

INCHES FROM PIPE

TEMPERATURE,

DEG.

F

TEMPERATURE OF Z-CRETE

VS DISTANCE FROM PIPE

INCHES FROM PIPE

FIG. 5 M OFFICIAL USE ONLY

- 11 -

Figs. 4 and 5 show a rather consistent temperature pattern

in all directions from the pipe at distances from 1 1/2 to

4 1/2 inches from the pipe with some variation at distance^

less than 1 1/2 inches from the pipe. Since the Z-Crete

envelope was not symmetrical with respect to the heated pipe

the temperature pattern was observed at distances up to 9 1/2

inches from the pipe on the longer side. Figs. 4 and 5 show

that the temperature on the outer surface of the Z-Crete

envelope under the waterproofing (about 5 1/2 inches from the

pipe) would be between 150F and 160F when surrounded by airdried earth and between 130F and 140F when surrounded by

earth flooded with water.

The average temperatures in the Z-Crete around the pipe

were also plotted on semi-log graph paper in Fig. 6 for the

two test conditions. The temperature gradient in a cylindrical envelope of uniform thermal conductivity would normally be

a straight line when plotted on semi-log paper. The temperature gradient observed in the test specimen can be approximated by three straight lines of different slopes. The section

of the curve on the right with the flattest slope corresponds

to the air space formed adjacent to the pipe by the corrugated

paper. This space was only about l/8-inch thick. The center

section of the curve in Fig. 6 corresponds approximately to

the annulus of Z-Crete one inch thick which was essentially

dry by virtue of its temperature being 212F or higher, whereas

the left section of the curve with the steepest slope corresponds

to the portion of the Z-Crete envelope which contained 50 to

100 percent of moisture and, therefore, had higher thermal conductivity.

In Fig. 6 the slope of the curve for the air space

corresponds to a thermal conductivity of 0.25* the slope for

the first inch of Z-Crete nearest the pipe corresponds to a

thermal conductivity of 2.3* and the steepest part of the

curve for the wetter portion of the envelope corresponds to

a thermal conductivity of 7*6, all expressed in Btu/hr(ft2)

(

<F/in.). These results indicate that the l/8-inch air space

adjacent to the pipe added appreciably to the insulating effect

of the envelope, and that the insulating characteristics of

the wet portion of the Z-Crete were comparable to that

expected for a clay loam containing 10 to 20 percent moisture.

FOR OFFICIAL USE ONLY
(


- 12 -

These values for thermal conductivity were computed from

the following formula for a cylindrical insulating envelope

using the observed heat loss and temperature data.

6 Q In

r° ri

k = ,lat where

Q = latent heat of saturated steam at the pipe

temperature times the weight of condensate

collected per hour in pounds

L = length of test section, ft

r , r.- = radius of the outside and inside of

insulating envelope, inches

AT = temperature difference between positions r

Q and r^ in the insulation as measured from the

center of the pipe, °F

A small error in the values of the thermal conductivity

is probably involved in this computation because the envelope

of Z-Crete was rectangular and asymmetrical in cross section.

However, the error is small because the thermal conductivity

of the outer part of the Z-Crete was of the same order of magnitude as the surrounding earth.

Fig. 7 shows the average temperatures observed in the

Z-Crete envelope for the two tests with air-dried and watersoaked earth around it, and the corresponding average thermal

conductivity of several thicknesses of Z-Crete as measured

from the pipe surface. That is, the average thermal conductivity from the pipe to a distance one inch from the pipe was

1.07 and 1.17 Btu/hr(ft 2

)

( °F/in. ) for the two test conditions

whereas that from the pipe to a distance four inches from the

pipe was 2.23 and 2.30 Btu/hr ( f

t

2

)

( °F/in. ) for the corresponding

conditions. These average thermal conductivities include the

effect of the l/8 -inch air space formed adjacent to the pipe

by the corrugated paper wrapping. Fig. 7 shows that the average

thermal conductivity of the entire Z-Crete envelope was more

than 2 1/2 times as great as the hot plate value for the dry

material
,* .
TEMPERATURE,

DEG.

F

TEMPERATURE AND THERMALCONDUCTIVITY

OF Z-CRETE VS DISTANCE FROM PIPE

EMPERATURE

INCHES FROM PIPE

FIG. 7

-13-

There were no visible breaks in the waterproof covering

and no cracks in the Z-Crete envelope at the fend of the tests.

The coal tar pitch revealed some plastic flow when the pipe was

heated before covering the specimen with earth. There was no

evidence of significant change in the moisture content of the

Z-Crete during the course of the tests. Free water was visible

on the inside of the waterproof covering when the Z-Crete was

unwrapped. The appearance of the Z-Crete specimen tested in

the earth-filled box is shown in Fig. 8 at the time it was

uncovered after the tests.

3.4 Water Penetration Tests

Several blocks of Z-Crete were cut from the underground

specimen after completion of the tests in the earth-filled box.

Fig. 9 shows the rate of water absorption of three blocks of

Z-Crete when immersed in cold water. Two of the three specimens absorbed 100 percent of their dry weight, of water in 24

hours and the third specimen required about four days to

absorb the same amount. All three specimens had absorbed

about 130 percent of their dry weight of water after 30 days

soaking.

The two cylindrical specimens of Z-Crete containing BCrete admix and No. 4 aggregate were received wrapped in a

plastic film to retain the initial water used when mixing.

They were subsequently oven-dried at 215^ and then immersed

in water and periodically weighed to determine their absorption

rate. The results observed on these two specimens and a similar one taken from the original Z-Crete used for the other

tests are shown in Fig. 10.

Fig. 10 shows that the two new specimens, as received,

had a water content of 86 percent and 69 percent of their dry

weight, respectively. After oven-drying they absorbed 35 percent and 31 percent of water relative to their dry weight

during the first day of immersion whereas the Z-Crete specimen

cast here absorbed 132 percent of its dry weight during the

same period. After 18 days immersion the water absorption was

64 percent and 58 percent for the two new specimens and 156

percent for the specimen cast here, related to the dry weight

in each case. These results show a considerably lower water

absorption for the newer Z-Crete mix than for that used for

the simulated underground tests.



MOISTURE

ABSORPTION

OF

3

Z-CRETE

BLOCKS

SUBMERGED

IN

WATER

1H9I3M Nl NIV9 %

TIME

.DAYS

1H9I3M N I NIV9 %

TIME

,

DAYS

-14-

.4. Discussion and Conclusions

The laboratory and simulated field tests of Z-Crete indicated the following conclusions about the installation tested:

1. The Z-Crete did not crack during the course of the

tests^.

2. There was no evidence of leakage in the waterproofing

surrounding the Z-Cretet

3. The Z-Crete did not adhere to the pipe because of the

corrugated paper wrapping on the pipe:

4. The waterproofing prevented the complete drying out

of the Z-Crete and resisted the entry of water from the surrounding earth:

5. The simulated field installation of Z-Crete had a higher thermal conductivity than oven-dried specimens from the

same pour because the envelope was not dried out in the field,

6. The insulating concrete used for the simulated field

test was hygroscopic and could absorb in excess of 100 percent

of its dry weight in water. A newer Z-Crete mix was more

resistant to water absorption than that used for the underground tests;

7. In the underground installation the moisture in the

one-inch of Z-Crete nearest the pipe was transferred to the

material farther from the pipe and remained there as long as

the pipe was heated.

Only the air space and about one inch of Z-Crete nearest

to the pipe was very effective as an insulating material. In

this zone the Z-Crete had an average moisture content of about

seven percent. Beyond the one-inch annulus the moisture content ranged from 50 to 100 percent of the weight of the dry

Z-Crete. In this zone the thermal conductivity was comparable

to that expected for a d{£y loam with 10 to 20 percent moisture

-15-

8. The thermal conductivity of the oven dry Z-Crete

used for the underground tests was 0.84 Btu/hr ( ft ^ )

( °P/in.

)

at a mean temperature of 128F. The average thermal conductivity of the Z-Crete envelope as tested underground was

more than 2 1/2 times this value. Consequently, the dry

thermal conductivity of the material cannot be used to determine the heat loss from a pipe insulated in the manner of the

test specimen.

9. About three weeks were required to completely dry

out a Z-Crete annulus 5 3/8-inches thick cast dm a ceramic

core when the core was maintained at a temperature between

350F and 400F and there was no waterproof covering on the

exterior. These results indicate that it would be impractical in most cases to attain complete drying in the field

before application of the waterproofing.

FOR 0

:mh\

a bans.

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

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

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