Materials

a — Hammered bar iron.

b — Potsdam sandstone, cleavage surface.

c — Drummond Id. limestone, cleavage surface.

d — Ground plate glass.

e — Kelleys Id. limestone, sawn surface.

f — Soft terra cotta, filed surface.

g — Soft red building brick, sawn surface.

403. The Adhesion Of Neat And Sand Mortars

Table 115 shows the cohesive and adhesive strengths of different mortars, the adhesion blocks being all of the same material, Kelleys Island limestone. The Portland mortar giving the highest adhesive strength at six months is that containing one-half part sand to one part cement, though the greatest cohesive strength is given by the one-to-one mortar. With natural cement the one-to-one mortar gives the highest strength, both in adhesion and cohesion. The ratio of the adhesive strength to the cohesive strength is greater for natural than for Portland. It also appears that between twenty-eight days and six months the adhesive strength increases more than the cohesive strength.

404. Effect Of Consistency On Adhesion

Table 116 gives the results of tests to show the relative effects of the consis-. tency of the mortar on the adhesive and cohesive strength. It is seen that the effect of consistency on the adhesive strength is less than on the cohesive strength, but that the best results in adhesion are given by a mortar that is considerably more moist than that which gives the highest strength in cohesion. The practical bearing of this point on the use of mortars is evident.

Table 115. Adhesion Of Mortars Containing Different Amounts Of Sand

Notes : — Sand, crushed quartz, 20 to 30.

Adhesion blocks, 1 in. X 1 in. X 1/4 in., Kelleys Id. limestone, sawn surface, saturated before used.

Table 116. Adhesion Of Mortars. Varying Consistency

Notes: — Sand," Point aux Pins," pass No. 10 sieve, one part to one cement by weight.

Adhesion blocks, 1 in. X 1 in. X 1/4 in., Kelleys Id. limestone, surfaces filed smooth, saturated with water before used.

405. Effect Of Regaging On Adhesive Strength

The tests given in Table 117 were designed to show the effect of regaging on the adhesion of cement mortar to stone. A comparison is made between mortars used fresh and those that were allowed to stand three hours and gaged once an hour. There are but few tests from which to draw conclusions and the treatment is very severe, but it appears that while the regaging to which these mortars were subjected usually resulted in a slight increase in cohesive strength, the adhesive strength was considerably impaired. The decrease in adhesive strength was greater for natural cement than for Portland, and greater for rich than for poor mortars. The effect of regaging on the cohesive strength is treated in Art. 47.

Table 117. Effect Of Regaging On Adhesive Strength

Notes: — Sand, crushed quartz, 20/30. Each result, mean of two to five specimens, broken at age of six months.

In adhesive tests, pieces Kelleys Id. limestone, 1 in. X 1 in. X 1/4 in., placed in center mold and two ends mold filled with mortar.

Results in columns headed "Fresh" from mortar treated as usual.

Results in columns headed "Regaged" mortar allowed to stand three hours before use, mortar being regaged each hour.

406. Character Of Surface Of Stone

In the tests recorded in Table 118 all of the adhesion blocks were of Kelleys Island limestone, but part of them were finished with smooth filed surfaces, while the others were grooved with a coarse rasp. In the twenty-eight-day tests there is but little difference in the adhesion to the different surfaces, but at six months the adhesion to the smooth surfaces appears to be slightly greater, ex-cept in the case of one-to-two natural cement mortar.

Table 118. Adhesion Of Mortars. Effect Of Character Of Surface Of Stone

407. The Effect Of Plaster Of Paris On The Adhesion Of Mortar To Stone

The results in Table 119 show the effect on the adhesive strength of adding small percentages of plaster of Paris to cement mortars of Portland and natural cement. The Portland cement used was a quick setting sample, neat cement pats of which began to set in eighteen minutes. The effect of plaster of Paris on the cohesive strength of mortars from these samples hardened in dry air, is shown in Table 92, § 378. It is seen that the addition of from one to three per cent, plaster has no deleterious effect on the adhesive strength of these samples at one year. Six per cent, plaster, however, ruins the Portland and the neat natural cement.

Table 119. Effect Of Plaster Of Paris On The Adhesive Strength Of Cement Mortars

Notes : — Adhesion pieces between two halves of briquet were of Kelleys Id. limestone, sawn surfaces, saturated with water before used. Cement and plaster Paris passed through No. 50 sieve. All briquets stored in tank in laboratory. Each result, mean of four to ten briquets.

Found badly cracked and separated from limestone prisms after three days.

408. The Adhesion Of Cement Mortar To Brick

Tests of the adhesion of cement mortar to brick were made by cementing pairs of brick in a cruciform shape, with a one-fourth inch joint of mortar. The brick were placed together flatwise, with the bed down, so that in the case of stock brick, one stock mark, or depression in one side, was filled with mortar. The mortar was made more moist than was ordinarily used for briquets, but not so moist as would be used in brickwork. The top brick of each pair was slightly tapped to place with the handle of a pointing trowel, and the excess mortar cut away. About forty-eight hours after cemented, the pairs of brick were packed in damp sand in a large box prepared for the purpose, and the sand was kept in a moist condition by a thorough daily sprinkling. For pulling the bricks apart, a special clip was devised to equalize the pull on the two ends of each brick, and a simple lever machine was used to measure the force required.