Table 122. Adhesion Of Cement Mortar To Brick. Effect Of Lime Paste In Portland Cement

Notes: — Bricks cemented together in pairs in cruciform shape.

Age of all mortars when tested, three months.

Cement, Portland, Brand R, Sample 14 R. Sand, " Point aux Pins".

Lime paste slaked about six months before use.

Each result in cohesion, mean of five to ten briquets.

Each result in adhesion, mean of eight to sixteen pairs of bricks.

Half of pairs were hard burned brick and half soft burned.

Composition of mortars: A B C D E

Grams P. P. (river) sand, 960 960 960 960 960

Grams cement, 240 240 200 180 120

Grams lime paste, 0 80 120 180 360

Grams lime contained in paste, 0 27 40 60 120

Lime as per cent, lime plus cement, 0 10 16.7 25 50

413. The results of a more complete set of tests on this subject are given in Table 123. The mortars used included one made with four parts sand to one of cement by weight; one in which about ten per cent, of lime by weight, which had previously been made into lime paste, was added to the mortar; a third in which lime, in the form of paste, was substituted for one-sixth of the weight of the cement used in the first mortar; a fourth, in which lime was substituted for one-fourth of the cement; and finally, a mortar composed of lime paste and sand only.

The adhesive strengths of the mortars are given in the table. The difference in the adhesion of Portland cement mortar to hard brick and to soft brick is not clearly brought out. Neither is the strength of air-hardened specimens much different from that of the mortars stored in damp sand. The use of lime paste with Portland cement in the amounts tried here more than doubles the adhesive strength of the mortar.

The first point to notice in the case of natural cement is that the adhesive strength of this mortar without lime is nearly double the adhesive strength of Portland mortar without lime. The adhesive strength of mortars hardened in damp sand is somewhat greater than the strength of similar mortars hardened in dry air. The addition of a small amount of lime paste increases the adhesive strength somewhat, and when as much as twenty-five per cent, of the cement is replaced by lime in the form of paste, the adhesive strength of the natural cement mortar is not usualfy diminished. The effect of lime paste, however, on the adhesive strength is not nearly so great as it is in the case of Portland mortars.

Table 123. Adhesion Of Cement Mortar To Brick. Lime Paste With Cement

Notes : — All mortars three months old.

" Fresh" paste had been slaked three days to one week before used. " Old " paste had been slaked five months before used.

The following conclusions may be briefly stated: The ratio of adhesive to cohesive strength is much greater with natural cement than with Portland. If a high adhesive strength is desired, Portland cement should not be mixed with more than two parts sand unless lime paste is added to the mortar, as the use of lime paste materially increases the adhesive strength of lean mortars. Tests of cohesion of similar mortars containing lime paste are given in Art. 48.

414. The Adhesion Of Cement To Rods Of Steel And Iron

The tests recorded in Tables 124 and 125 were made to determine the adhesion of cement mortar to iron rods, or the strength of a bolt anchorage secured with cement mortar, and the style of rod and kind of mortar which would give the best results. The bars were made in an ordinary concrete mold, ten inches by ten inches by four and one-half feet. The rods or bolts were placed in a row along the center of the box, being spaced about nine inches apart, and the mortar was rammed about them. After being allowed to set in a warm room for twenty-eight days, the rods were pulled by means of two hydraulic jacks, a special grip being used to grasp the free end of the rod, and an hydraulic weighing machine serving to measure the pull required to start it. The supports against which the hydraulic jacks were braced bore at points on the concrete bar about three or four inches from the center of the rod which was being tested.

415. The rods given in Table 124 were imbedded in mortar composed of one part of Portland cement to two parts limestone screenings. The rods were cut from bar iron and were perfectly plain, without nuts or fox wedges. The results indicate that the force required is proportional to the area of contact. Comparing the different styles and sizes of plain rods, no difference in favor of one style or size can be determined; the apparent higher resistance per square inch offered by one-inch rods would probably disappear in a large number of tests.

Table 124. Resistance To Pulling Of Iron Rods Of Various Forms Imbedded In Mortar

Notes: — Cement, Portland, Brand R.

Sand, limestone screenings passing 3/8 inch slits, two parts by weight to one cement. Mortar one month old when tension was applied to rods.

The rods given in lines seven to nine were made by twisting a piece of one inch square bar iron. The twisted portion was eight inches in length. Comparing the plain one inch square bolts with the twisted bolts, it appears that the former offered a resistance of 2,245 pounds per inch depth while the latter gave 2,405 pounds, an increase of less than eight per cent.

416. In the tests recorded in Table 125, the ordinary river sand used in construction was employed. The mortar was made neat and with two and four parts sand to one of cement. The depth the rods were imbedded varied from two inches to ten inches. The one-to-two mortar gave nearly as good results as neat cement, but the one-to-four mortar gave much lower results. The resistance seems to vary directly as the area of contact without reference to the depth imbedded, except as this enters in obtaining the said area. The results obtained in this table do not compare favorably with those obtained in Table 124, where limestone screenings were used.