Equivalent thickness of concrete masonry units

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Equivalent thickness of concrete masonry units

C A N A D A

44203

EQUIVALENT THICKNESS

OF

CONCRETE MASONRY UNITS

by

T.

2. H a m a t h y and E . W , Oracheski

27.L $

= .

9nT

EQUIVALENT THICKNESS

OF

CONCRETE MASONRY UNITS by

T.

Z. Harmathy and E. W. Orache ski

In analyzing f i r e t e s t data it has been customary to a s s u m e t h a t f o r c o n c r e t e masonry units of different g e o m e t r i e s but made f r o m the

same mix a f i r e endurance versus equivalent thickness plot on log-log

Faper is approximately a straight line,

It

has since been proved ( 1 )

that this assumption is not necessarily correct, yet one can expect that

this practice of correlating f i r e t e s t data will ga on f a r some time t o

come. It i s of s m e practical importance, t h e r e f o r e , to have a simple

t e s t method available for the reasonably accurate assessment of the

equivalent thickness of hollow masonry units

.

The 'LStandard)m Method

as

w h e r e

The equivalent thickness of a concrete m a s o n r y unit i s defined

1 = equivalent thickness of masonry unit, f t

V

= "nett' volume of the unit, f t 3

F = surface area t o be exposed to fire, ft2

.

The "net" volume of the unit is interpreted as the volume that

is formed by the visible, surfaces of the concrete, and which, therefore,

includes not only the concrete itself, but also the a i r and w a t e r in the pores of the concrete.

Owing t o the usually complex geometry of the cavities in hollow

masonry units, the direct calculation of the net volurne is often v e r y

l a b o r i o u s . It b e c a m e customary, therefore, to determine the net

volume with the aid of an experimental technique described i n ASTM

D e s i g n a t i o n C140 (2). The u s e of this technique has also been advocated

B y

now it is well known, however, that the experimental method described in ASTM

C

140 ( r e f e r r e d to h e r e a f t e r a s the "standard" method)

is inaccurate and often g r o s s l y m i s l e a d i n g , especially if it is applied t o

lightweight c o n c r e t e units. T o understand the difficulties, it may be

advisable to r e - e x a m i n e the essence of the method.

ASTM C 140 offers the following formula f o r the calculation of

t h e bulk density of the concrete:

where

4

= ltstandard" bulk d e n s i t y of concrete in a i r - d r y condition,

determined according te the standard method, lb/f t3 = d e n s i t y of water = 62.4 lb/ft3

W

AD = weight of m a s o n r y unit in a i r , in air -dry condition, l b

wws

= weight of masonry _Ib unit in water, in "saturated" condition,

W

ASD

=

weight of masonry unit in a i r , i n " s a t u r a t e d - t h e n - d r a i n e d t 1

condition, lb.

According t o the standard method the ltsakur ated" condition is obtained by having the m a s o n r y unit completely s u b m e r g e d in w a t e r at

room temperature f o r 24 hr.

If

the unit i s then lifted out from the w a t e r and is allowed t o d r a i n f r e e l y for 1 min, while the surface water is re-

moved with a cloth, the ttsaturated-then-drainedt* condition i s obtained.

T o t e s t the validity of Eq. (Z), the following expressions of W's w i l l be utilized:

w h e r e

f? = actual bulk d e n s i t y of concrete in a i r d r y condition, lh/ft3

D

'SD = average bulk d e n s i t y of concrete in "saturated-then-dl-ainedt'

condition, lb/ft3

= average bulk density of concrete in "saturated" c o n d i t i o n , 'S _lb/ft3.

B y substituting expressions of W w s f rorn Eqs. (33, (45, and

(5) into Eq. ( Z ) , one obtains

F r o m this equation it is obvious that P *

=

Pg only in that particular D

c a s e when p = p S

SD

i. e . when the drainage fallowing the saturatiorl

takes place f r o m the surfaces of the concrete only.

Experience has shown, however, that in the case of lightweight

concrete (or any other materials of sufficiently rough pore structure)

in addition to the !'surface watera* a certain portion of the "pore watertt

(i. e. the watex that has penetrated into the pores during the 24-hr

saturation period) is also given off as the masonry unit i s removed from

the water, especially f r o m within the pores in the surface regions. It

is obvious, therefore, that f o r lightweight m a s o n r y u n i t s P

>

PSD and,

S

owing to the larger surface to volume ratios, the difference between

pS and p is g r e a t e r for hollow units than for solid units.

SD

One can now define a "standardrP net volume,

V*,

which is

expressed with the aid of P * instead of P as

D

D

Also, analogously'to Eq. ( I ) , a "standard" equivalent thickness,

I * ,

can b e defined as

B y substituting the expression of P* from Eq, (21, Eq.

(8)

can be D rewritten as B W A * =

-

.

ASD

-

w~~

.

F

P W

F u r t h e r m o r e , by combining Eqs. ( I ) , (41, (5), and 19) one f i n a l l y

From this equation it is clear that for lightweight concrete

masonry units, since p

SD

<

's'

k*

is always l e s a than h , in other

words, the equivalent thickness determined according to the

etandard

method

is always

smaller than the actual equivalent

thickness

that can

be obtained by calculations based on the geometry of the units.

The obviously absurd finding that, for solid lightweight units,

the "standards' equivalent thickness invariably turns out to be smaller

than the actual thickness has long puzzled the manufacturers of masonry

units. Anather, even m o r e disturbing fact, however, has not yet been

clearly realised, namely that at identical values of V/F the

x*/X

ratio

is less f o r hollow units than that f o r s o l i d units (because of the larger

surface to volume ratios) and it generally varies with the geometry,

T o illustrate the inadequacy of the standard method in determin-

ing the equivalent thickness, s m e results

of

a simple t e s t are reproduced

irl this Note. This t e s t was p e r f o r m e d on

a

solid concrete unit m a d e with

expanded slag aggregates. The following dimensions w e r e noted:

thickness: 5.625 in. = 0.4687 f t

breadth: 7.561in. =0.6301ft

length: 15.625in. = 1.3021 f t

Hence

and, naturally, the true equivalent thickne s s is equal to the thickne s s

,

The weight of the u n i t in air, in air -dry condition, was measured

as

= 3 6 . 9 6 lb;

WAD -

therefore the actual bulk density of the concrete is

A f t e x measuring

W

the unit was lifted out from the water bath and

WS

its weight in air was noted at 1 0 - s e c intervals as the drainage went

on, T h i s information is denoted by and is reproduced in Table I. ASD

(According t o the definition of the "saturated-then-drainedvt condition obviously

3

at 6 0 s e c is equal to W

ASD ASDm

1

T o

point out the absurdity of the s t a n d a r d method, calculated

values of

p*

and

I*

(obtained with the aid of Eqs. ( 2 ) and (91, respectively,

D

using value s of for

W

]arealsoincludedinthetable. T h e d a t a

ASD

clearly indicate th%??hihe drainage occurs mainly from within the pores and

that this kind of drainage is significant enough even during the first 1 0 sec

to completely falsify the values of P and

X

.

(The correct values of the s e

D

were calculated earlier. ) As the time lapsed, the values of

c*

and D

yielded b y t h i s t e s t deteriorated further and in the standard "saturated-

then-drained1* condition (i. e. 60 s e e . after the removal of the specimen

from the w a t e r ) the "standard" bulk density of the concrete was found to

be 10.7 per cent h i g h e r and the "standardtt equivalent thickness 9.7 per

cent lower than the correct values.

The Recommended Method

The difficulties presented by the standard method c a n clearly

b e avoided if steps a r e taken to prevent the measuring liquid from

penetrating into the pores of the concrete m a s o n r y unit, This can be

achieved either by (a) covering the surface of the unit with s o m e inn-

p e r m e a b l e coating or by (b) using, instead of w a t e r , some other liquid that does not enter the p o r e s .

The f i r s t of these techniques is not very convenient. It is

difficult to apply the coating mate rial t o form a reliably continuous layer.

Even small invisible imperfections in the continuity of the coat m a y markedly falsify the t e s t result.

The u s e of a non-penetrating liquid seems to be more p r o m i s i n g .

M e r c u r y would be suitable f o r this purpose because, owing to its h i g h

contact angle with concrete, it enters only t h e l a r g e s t surface pores.

Unfortunately, its toxicity and high c o s t make its u s e prohibitive.

Instead of real liquids if: would t h e r e f o r e seem to be more

convenient to u s e some bulk granular solid consisting of particles of

high spherity. N a t u r a l l y , any such a "pseudo-1iq~id'~mus't have a

In

the authors1 laboratory,

No.

10 lead shot is used f o r measur-

ing the volume of cavities in concrete maaonry units.

In

filling the

cavities with lead shot, three prdcedures w e r e investigated: tmscooping,

"

"pouring" and "tamping.

''

As

one might expect, scooping resulted in

the least dense and tamping in the densest packing of the shot. N e v e r -

theless, the djfferences in the bulk densities with these three kinds of application were not large, Typically, the b d k density with scooping

was about 1 per cent lower and that with tamping 2 to 4 per cent higher

than with pouring.

It

is somewhat more disturbing, however, that the bulk density

also depends on the shape of the containing walls.

k

t

the vicinity af the

walls the particles are l e s s densely packed, so that the average bulk

density d e c r e a s e s with increasing surface-to-volume ratio for the con-

tainer (or cavity masonry unit s). Typically, as the surface -to-volume

ratio decreased from 30 f t v f t 3 to 15 ft2/ftJ, the bulk density of the lead

s h o t increased from 400 to 4 3 5 ib/ft3 when using the pouring technique.

F o r the cavities of concrete masonry units 20 ft2/ft3 can be

regarded as a characteristic value. The bulk density corresponding to

t h i s value is about 415 lb/ft3 (when the shot is poured).

The way of measuring the volume of cavities in a m a s o n r y unit

is shown in Figure 1.

If

there are s o m e end cavities, the gross volume

of the unit is adjusted i n t o a r e c t a n g u l a r parallelepiped by the u s e of

two pieces of plywood. T h i s specimen assembly i s then placed in a

plastic pan an a scale. A l l cavities within the parallelepiped a r e filled

with lead shot. The weight of

the

filling material,

Wf

(lb)

is recorded.

The t o t a l volume of the cavities,

V

(ft3) is obtained as

C

w h e r e pf is the appropriate bulk density of the filling material in

lb/ft3

.

The equivalent thickness i s

v - v

P

C

1

=

F

where

V

is the volume of the rectangular parallelepiped in ft3.

Ip

It was

found that cavity volume measurements p e r f o m e d with the

References

1 . Harrnathy,

T.

2. Thermal Performance of Concrete Masonry

W a l l s in Fire. ASTM Spec. T e c h . Publ, 464, 1970, p. 209. 2. Tentative Methods of Sampling and Testing Concrete Masonry

Units. ASTM Designation C140-65T,

ASTM

Standards,

Vol. 12, 1966.

3. Fire Performance Ratings, l965. Supplement Na, 2 to the

ADDENDUM

11; has been suggested recently that the techniques described

in ASTM Designation

D

1556-64 and D 2167

-66

_(ASTM Standards,

Vol.

II.

1970) concerning the in-situ measurement of soil density

might also be successfully u s e d

in

measuring t h e volume of

TABLE

I

SOME RESULTS

OF

A TEST CONCERNING

THE

ACCURACY

OF THE

"STANDARD" METHOD

T i m e lapse after

removal of specimen

from water, sec.

15 20 30 40 50 Ic. 60 120 180 W e i g h t of wet spe c h e n in air,

WASD,

lb

-41.80 41.36 41.14 4 0 . 9 2 40,88 40.81 40.70 40.70

-

Pg

a lb/f t 101.78 1 0 3 . 7 9 104.83 105.89

106.09

I

106.43

106.97

106.97

x*,

f t 0,4425 0,4340 0.4297 0.4253 0.4246 0,4233

..

0.4211 0 . 4 2 1 1

F i g u r e 1 . The u s e of lead shot in measuring the

volume of cavitie s in concrete masonry