TECHNICAL INFORMATION ON BUILDING MATERIALS

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FOR USE IN THE DESIGN OF LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

November 4, 193b.

MORTAR AND BRICE PROPERTIES AND THEIR RELATION TO BONE

This is a brief digest of Research Paper RP683 , "A Study of the

Properties cf Mortars and Bricks and Their Relation to Bond, "(May

1934) ,1 by L, A. Palmer and D. A. Parsons, issued by the National

Bureau of Standards.

Investigations were conducted at the National Bureau of Standards

with cooperation of producers of brick and masonry materials. Ti .e studies extended over a twelve month period in an effort to determine

the relation between some of the pronerties of mortars and bricks,

their tensile and transverse strength, and the durability of joints in

masonry construction. The physical properties of fifty different mortar compositions were determined, fifteen of these being used with six

types of bricks in suecimens of masonry construction which were tested

for strength and durability of bond.

Materials

Bricks : Six representative types (numbered from 1 to 6 as a means of identification) showed absorption rates in their order from

high to low, and surfaces in parenthesis as follows: No. 1 very high

(mechanically smooth); No. 4 high (smooth but uneven); Nc. 6 high

(very smooth for side cut bricks); No. 2 intermediate (very rough);

No. 3 low (moderately rough); and No. 5 extremely low (relatively

smooth for side cut bricks, also glossy). All suecimens had a tendency

to expand slowly but slightly, by prolonged immersion (one month) in

water; t-ha^e data agreeing with previous research. Average eorpressive

strengths varied from a low of 4gzc pounds to a high of 16,025 pounds

for No. 6 ana 5 respectively, while transverse strengths were from b09

pounds to 2665 pounds respectively for the same bricks.

'"Available

(Price 5 cents)

from Superintendent of Documents, Washington

,

D. C.

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Mortar : Fifty varied -proportions , the ingredients of which were Potomac Fiver sand (passing through a tTo. 8 sieve) and twelve masonrycements, two nortland cements and four limes, these cements "being ren- resentative products of manufacturers in various parts of the United

States. Descriptions of these mortars—their mix, materials and waterretaining capacity—given in full in Research Paper RPb83 include their

proportions "by volume and "by weight.

Tests and Results

Water Retention Properties of Mortars : The stiffening rate of

each mortar caused by the loss of water when in contact with an absorptive brick was estimated. The mortars were mixed with water in amounts

somewhat greater than normally required by the brick mason; and were

then subjected to a suction test for one minute, drawing water from the

mortar in a manner resembling the effect -produced by contact with an absorptive brick, at which time measurements were made to determine the

water retaining capacities. Of the fifty mortars tested those containing slaked lime, putty, or natural cements with metallic stearates showed

highest water retaining capacities. The effect produced by partially

substituting lime for cement on the water-retention property cf cementlime mortars depended upon the qualities of limes. Two of the four limes

used. showed marked effects by increasing in water-retaining capacity.

Shrinkage of Mortars Dur i ng Early Hardening : Four hundred and

fifty specimens (l" x 1" x 10") of fifty mortars at three consistencies

(dry, intermediate, and wet), were measured! during the first fortyeight hours showing on an average highest shrinkages for straight limesand and 1PC:3L:12S mortars and lowest for straight portland-cementsand mortars.

Volume Changes of Mortars Subsequent to Hardening : Strain-gage

readings were takenl for the expansion and contraction of mortar

specimens (l" x H" x 12"). Initial readings were made when the specimens were one week old; readings thereafter over a one year period. In

these tests, wetting and drying cycles were produced to roughly

parallel average climatic conditions; shrinkage being greater than ex- pansion over the course of a year with greatest shrinkage taking place

during the first drying period. The cement-lime mortars rich in lime

showed the least and those rich in portlend cement the most changes in

volume subsequent to hardening. The changes of the mortars with some

of the masonry cements were greater than for the cement-lime mortars;

with the rest the changes were about the same.

!"8y methods described in national Bureau of Standards Research

Paper RP321, "Volume Changes in Brick Masonry Materials". Available

from Superintendent of Documents, Washington, D. C. (Price 30 cents).

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Transverse an d Comnressive Strengths of Mortars : Transverse

strength tests were made at the end of one year with specimens (l" x

U" x 12" dry,, intermediate and wet) that had previously been used for

testing volume changes. Compressive tests were made with 2" cubes.

•Higher transverse and compressive strengths were associated with

mortars of high: cement-water ratios, .portland-cement-lime mortars being

highest and those richer in po.rtland-cement than the 1PC:1L;6S mix

being greatest. The average strength, either transverse or compressive,

of the above mortar was equal to or greater than the strongest of the

twelve masonry cement mortars and that of 1PC:2L:9S mortar which com- pared favorably with six of the twelve masonry cement mortars. Compressive strength of lime (dol.omitic hydrated) mortar, 1L:3S(1L;10»52S)

at periods of three months and one year, compared favorably with four

of the masonry cement mortars.

Discussions contained in Research Paper RPo83 concerning "Modulus

of elasticity and extensibility" and "Sorption"! are omitted from this

digest.

Brick "suction" was determined by immersing a dry brick in water

to a depth of l/S" and measuring the amount of water (in grains )2 absorbed in one minute. 'When this suction exceeded sixty grams, poor bonding

resulted by too rapid stiffening of the mortar, the area of bonding being less than with brick which were wetted, or those of lower suction.

The strongest joints resulted with all mortars when the brick suction

was between ten and forty grams (approximately 0.05 to 0.2 grams per

square centimeter ), 3 the bond strength of the joints increasing with the

increase in compressive strength of the mortars. However, with bricks

having a suction in excess of sixty grams ( approximately 0.3 grams per

square centimeter), the bonded area and the ratio of strength of the

joint with dry absorptive bricks to that with the same bricks set wet was greatest with mortars of medium of high water retaining capacities.

Alternate wetting and drying did not cause any appreciable weaken-,

ing of bond nor was there evidence to indicate that volume changes,

subsequent to hardening, destroyed or weakened the bond of specimens

when their initial bond "'as good. The effect of properties of mortars

on the durability of joints exposed to cycles of wetting, freezing,

thawing, and drying was determined with bricks having a suction of twelve

grams or less (less than 0.06 grams uer square centimeter) or which had

to be wetted to produce that suction. Most of the joints were highly

resistant to this treatment, the exceptions being mortars having unusually low strengths or high sorptions.

^Sorption refers to moisture added to a body by any or all of the

three processes: abs:rption (physical), adsorption (physico-chemical),

and hydration (chemical).

p _ One gram - 0.0353 ounces. 3one square centimeter = 0.155 square inches.

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