Table 29 shows similar results for mortars made with one, two and three parts sand. With one part sand the wet mortar made from Gn, 21 R, which gave but 22 pounds per square inch at seven days, gave 429 pounds, or nearly the highest strength, at six months. A similar result is shown for sample 15 R of the same brand when mixed with two parts sand, the highest strength at one year and two years being given by the mortar containing the greatest per cent, of water. That mortars containing three parts sand to one cement may be more easily damaged by an excess of water, is indicated by the results on Brand Ln in this table.

167. The effect on the strength of Portland cement mortars, of variations in consistency, has been investigated by Mr. Eliot C. Clarke,1 M. Am. Soc. C. E., and byM. Paul Alexandre,2 Chief Engineer, Ponts et Chaussees. The results of one series of experiments made by M. Alexandre are given in Table 30. The mortars were mixed with fresh water and the samples immersed in sea water.

Table 29. Variations In Consistency Of Mortar. Effect On Tensile Strength, Natural Cement Mortar; Sand, Crushed Quartz 20—30

Cement.

Parts Sand to One Cement by

Weight.

Age of Briquets.

Tensile Strength, Pounds per Square Inch.

Brand.

Sample.

water Used Expressed as Per Cent, of Dry Ingredients by Weight.

8.5

10 to

11.1

12.5

13 to 14

15 to 15.3

16.6

18

20 to 20.6

22

24.5

25

26

30

Gn

21 R

1

7 days.

58b

71e

38h

22i

"

"

1

28 "

102b

170e

129h

76i

"

"

1

3 mos.

210b

379e

354h

278i

"

"

1

6 "

234b

416e

438h

429i

Ln

31 S

1

7 days.

76b

151c

120f

58h

29i

"

"

1

28 "

109b

225c

232f

125h

79i

"

"

1

3 mos.

138b

322c

389f

322h

221i

"

"

1

6 "

190b

367c

458f

378h

245i

Gn

16 R

2

28 days.

100b

154d

120e

89g

63h

"

"

2

6 mos.

230b

316d

363e

351g

283h

"

"

2

1 year.

234b

347d

366e

354g

308h

"

"

2

2 years.

249b

332d

399e

360g

317h

Gn

15 R

2

28 days.

184c

186d

166e

139g

109h

"

"

2

6 mos.

226c

260d

287e

313g

303h

"

"

2

1 year.

190c

232d

288e

291g

323h

"

"

2

2 years.

186c

220d

237e

265g

281h

Ln

31 S

3

7 days.

62b

42e

22f

13g

3h

"

"

3

28 "

108b

104e

84f

54g

36h

"

"

3

3 mos.

148b

224e

176f

102g

78h

"

"

3

6 "

160b

240e

218f

142g

75h

Note: — For significance of letters following each result, see Table 28.

Table 30. Variations In Consistency Of Mortar

Effect on Tensile Strength, Portland Cement Mortar. 25 pounds cement to 1 cu. ft. sand (about 1 to 4 by weight).

Consistency.

water

Per Cent, of Sand.

Resistance, Lbs. per Sq. In. at Age of

3 Days.

7 Days.

28 Days.

3 Mos.

1 Year.

2 Years.

3 Years.

4 Years.

Dry . .

14

31

56

73

77

69

67

88

Disintegrated

Ordinary .

22

25

46

74

116

153

170

162

190

Wet .

30

16

35

55

89

126

136

180

189

From " Recherches Experimental sur Les Mortiers Hydrauliques," par M. Paul Alexandre, Annates des Ponts et Chaussees, Sept., 1890

It is seen that the highest strength at three days and seven days is given by the dryest mortar, at twenty-eight days to two years by that of the ordinary consistency, and at three years by that containing the highest per cent, of water. All of the samples exhibited white spots in the broken section at three years, and at four years the dry mortar briquets had lost their coherence on account of their porosity permitting the sea-water to permeate them.

168. Conclusions

It may be concluded, then, that the consistency of the mortar has a very marked effect on the tensile strength obtained; that different samples of cement are not affected in the same degree by given variations in consistency ; that the effect of consistency is usually shown most plainly in short time tests; and that while the dryer or stiffer mortars give the highest results on short time tests, the moist mortars attain a greater strength after a certain time.

1 "Records of Tests of Cement made for Boston Main Drainage Works." Trans. A. S. C. E., Vol. xiv.

2 Annates des Ponts et Chaussees, Sept., 1890.