Compressive strength values for type N mortars from construction sites

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Compressive strength values for type N mortars from construction

sites

C A N A D A S er T H l 392 no. 82 c . 2

COMPRESSIVE

STRENGTH

VALUES

FOR

TYPE

N MORTARS

FROM

CONSTRUCTION SITES

J.I. Davison

COMPRESSIVE STRENGTH VALUES

FOR

T Y P E

N

MORTARS FROM CONSTRUCTION SITES

J.

1. Davison

During the period mid-

19

66 to mid-1968, field studies were conducted

at the Atlantic Regional Station, Division of Building Research, o n three ma-

sonry construction projects involving tbree different Type N mortars. The

studies were designed, 1) to obtain d a t a on mortars mixed on the job site, and 2) to experiment with certain new f i e l d t e s t s .

Compressive strength t e s t s on 2-in. cubes and 3

-

x 6-in. c y l i n d e r s

w e r e also included in the study. Procedures and requirements were the

same for both t e s t s . Recent demand f o r field control t e s t s for mortars, a result of increasing acceptance of the load-bearing m a s o n r y wall con- cept, has resulted in t h e inclusion of a field control t e s t for compressive s txength in the recently published

CSA

Standard A224, M a s o n r y Code of

Practice, and the _{Canadian Structural Design Manual (Supplement #4 to}

the National Building Code of Canada 197 0). During preparation of t h e s e

documents there was discussion about meaningful requir ernlent values and it may, therefore, be helpful to compare d a t a f r o m the field studies

with the requirements f o r Type

N

_{mortar in the new standards.}

N E W F l E L D

CONTROL

TESTS

F o r every s t o r e y t e s t s a r e required on five 2 - i n cubes of mortar random sampled from mortar boards currently in use. Cubes are molded,

cured and tested in accordance with CSA

A179-1967,

M o r t a r f o r Unit

Masonry. At 2 8 days the average compressive strength obtained from

any five consecutive t e s t s is r e q u i r e d to exceed 0. 80 of the value f o r the

appropriate type of mortar as shown _{in the} following table; no individual

t e s t shall be less than 0 . 6 7 of that value.

Type of M o r t a r Compressive Strength

2500 p s i

1800 psi

750 psi

3 5 0 psi 75 p s i

These values, taken f r o m

CSA

A179, are identical t o those in

ASTM

(7270.

For a type N mortar then, the average value for a 2 -in. cube m u s t ex-

c e e d 600 (. 80 x 750) psi, with none below 500 (. 67 x 7 5 0 ) psi.

FIELD STUDY

The mortars tested in the field study included, 1 ) a conventional 1: 1:

6

_{cement : lime putty : sand,} 2 ) a packaged 1: 1:

6

_{masonry mortar mix}

(said to be a blend of portland cement and hydrated 3jme plus additives],

and 3 ) a 1:3 masonry c e m e n t : sand (interground portland cement clinker

and limestone). Cylinders w e r e included because of their general a c c e p - tance on construction sites f o r concrete t e s t i n g . It was considered t h a t the u s e of a familiar t e s t such as

this

would help facilitate the ultimate

acceptance of mortar field testing, The s t u d i e s w e r e designed t o provide data on the correlation between the 2-in. cube and the 3 - x 6-in. cylinder

values.

Control values w e r e obtained from t e s t s on specimens molded

with mortar containing materials from the job site and mixed in the labora-

t o r y by the s a m e procedures as those contained in ASTM C270 (The

Canadian M o r t a r Specification was n o t issued until 1967, about mid-point in t h e study). Sand f o r all three jobs came from one source and met the

grading requirements of CSA A82.56, Aggregate f o r Masonry M o r t a r .

M o r t a r from t h e job site was sampled either from the box at the

mixer discharge or from the masongs board at the wall face. Specimens

w e r e molded by conventional methods. All cubes containing f i e l d mortar

were molded in the lab. The proximity to the laboratory of all t h r e e jobs

made it possible for mortar t o b e brought: in for t e s t s wi& a minimal

amount of delay. Some cylinders containing f i e l d mortar were also m o l d e d in t h e l a b o r a t o r y ; t h e remainder w e r e molded on the job s i t e . A f e w

f r o m the latter group were maved directly i n t o

the:

lab (before the m o r t a x

had received its initial s e t ) but most w e r e left in _{a s h e l t e r e d spot on site} o v e r n i g h t and maved into the lab the followbig day. During this p e r i o d (approximately 24 hours) the only concession to the curing p r o c e s s w a s

to place covers on the cylinders to r e d u c e water evaporation f r o m the

mortar. Except for the initial curing period of these s i t e - m o l d e d cylinders,

all specimens received normal l a b o r a t o r y curing. Compressive strength

t e s t s w e r e conducted at 28 days. A total of 243 cubes and 1 6 1 c y l i n d e r s w e r e t e s t e d d u r i n g the t h r e e studies.

It must b e emphasized that t h e studies were not d e s i g n e d f o r a

comparison of field strength values with a particular strength requirement.

h

fact, s t r e n g t h was n o t of real c o n c e r n on any of the projects. Perhaps

this enhances t h e value

of

t h e data. Because t h e r e w a s no pressure an the operator mixing the mortar to achieve a critical strength value, the

data truly reflects a cross-section s f the condition of the end p r o d u c t r e -

s ulting from the masonry construction pr actic e s on three small projects

.

SUMMARY

OF

RESULTS

1. Cylinder -Cube Relationship

Type of M o r t a r 1 :

l : 6 C :

L : S 1 : 1: 6 M a s o n r y mortar mix 1: 3M.C.: 5 Cylinder S t r e n g t h as a

%

of Cube Value C o n t r o l F i e l d 9 4 88

The cylinder -cube relationship is considered f i r s t so t h a t t h e correlation

established can be used in later discussions, There was good correlation

for the conventional 1 : 1 :

6

cement-lime and the 1 : 3 masonry cement

mortars. The lower c o r r e l a t i o n f o r the packaged 1 : 1 : 6 _{masonry mortar}

mix is inexplicable. Observations indicated much better control d u r i n g mortar preparation on this job and better correlation had been anticipated.

I t is also interesting to note the better correlation for site mortars than f o r Lab mortars in two of three instances. A lower limit of 80 per c e n t f o r

the cylinder -cube relationship would appear adequate for the 1 : 1 : 6 C : L: S and the 1 : 3 M,

C.

: S mortars.

2. Average Compxessive Strength Values vs Requirements

T y p e of Control Field

M o r t a r Cube

C

ylirrder Cube C y l i n d e r

I : 1 : 6 C : L : S 1824 psi 1 7 1 4 p s i 1269 p s i 1 1 2 1 p s i

1 : 1: 6rnassnrymortar mix 1415 p s i 940 p s i 1049 p s i 7 6 2 p s i

1 : 3 M . C . : S 1628 p s i 1341 p s i 1614 p s i 1364 p s i

All c o n t r o l values a r e average for six cubes and three cylinders.

1: 1: 41

C :

L: S Mortar. - Values for field rnoxtars are average f o r

1 0 5 cubes and 70 cylinders (the 3 : 2 ratio was majntained throughout the study, three cubes and t w o cylinders being molded from each sample of

field r n o x t a r ) . Average values w e r e far in e x c e s s of the required 7 5 0 p s i for Type

N

mortar. T h e r e was wide variation in individual values, f r o m

the low 672 p s i (cylinder) to the high 1850 p s i (cube), All individual cube values exceeded 750 p s i with

6

_{of 7 0 c y l i n d e r s below,} U s i n g the 88 p e r

cent relationship between cylinder and cube values (above] f o r f i e l d mortar,

all six values, including the l a w 672 psi, exceed the r e q u i r e d 660

C.

88 x 7 5 0 )

p s i .

1 : 1 :

6

_{Masonry M o r t a r Mix. - V a l u e s f o r field mortars are averagc} f o r 72 cubes and 47 cylinders. While average values are l o w e r than f o r the

1 : 1 : 6

C:

L:

S mortar t h e y all exceed t h e 7 5 0 p s i r e q u i r e m e n t . Individual

values ranged f r o m t h e low 523 p s i (cylindex) to the high 1400 p s i [ c u b e ) ,

T h r e e cube valves were under 750 p s i the lowest being 725 p s i , but all w e r e w e l l above the permissible 5 0 0 (. 57 x 7 5 0 ) p s i for individual t e s t s .

The

cylinder-cube correlation f o r this mortar was 73 per c e n t and cylinder

strength requirement would t h e r e f o r e b e 548 1 - 7 3 x 750) p s i . Thus, while 21 of the 47 cylinders were below 750 psi, only o n e was under 548 psi, and at 523 p s i i t was well above the minimum 3 65 (. 67 x 54 8) p s i for indi-

vidual t e s t s .

1: 3

M.C.:S

Mortar.

-

The job specification f o r this m o r t a r r e -

quired an 850 p s i compressive strength. Values f o r field mortar a r e a v e r a g e

mortar w e r e all molded nn the job, stored overnight and then moved i n t o t h e lab. All values, cube and cylinder, average and individual, w e r e

above the required 850 psi. There was considerable variation

in

_individual values between the low 8 5 6 p s i (cylinder) and the high 2400 p s i (cube).

3. Field M o r t a r v s Control M o r t a r

Type of

M o r t a r

Cornp. Strength of Field M o r t a r

as a

%

of Control M o r t a r Cube

1:

1 : 6

C : L : S _{70 (105)*}

l : 1 :

6

_{masonry mortar mix}

74 (72) P : 3

M.C.:S

₉₉

_{( 6 6 )} Cylinder 6 6 _{( T O )} 81 (47) 102 (44) *Figures in brackets represent the n u m b e r of specimens of f i e l d mortar tested.

It will b e noted that this comparison is on t h e b a s i s of a relatively

l a r g e n u m b e r of field specimens v s a small n u m b e r (six cubes and three cylinders) of control specimens. T h e r e a r e na obvious explanations for

t h e variation in results. The b e s t correlation occurred for the m a s o n r y cement mortar w h e r e observations of the mortar m i x i n g operation indi- cated inferior site control. Many factors are t h u s concluded to have in- fluenced t h e results and, as no particular effort w a s m a d e to control

t h e m , there is n o point in attempting to rationalize their effect.

It

is interesting

to

note that t h e lowest correlation was 66 p e r cent ( f o r c y l - k d e r s of 1: 1: 6

_{C :}

_{L:S mortar).}

CONCLUSIONS

1. Compressive strength values for 243 cubes and 1 6 1 cylinders of

three Type N f i e l d mortars all met t h e requirements in recently published

Canadian field t e s t s f o r mortars.

2 . 3 - x

6-in,

Cylinders or 2-in. cubes specimens can b e used f o r

c o m p r e s s i v e strength t e s t s on field mortars.

3 . Field mrsr tar values are lower than control mortar values, rang -

i n g 7 0 to

99

per cent for cubes and 6 6 _{to 102 per cent for c y l i n d e r s ,} Largest single factor in the strength reduction is the higher water content of field mortars. Manyvarisbles, including site control of mortar mixing, con-