354. The Proportion Of Sand

The rate of change in the strength of mortars as the proportion of sand is increased varies greatly for different cements. The fineness and chemical composition of the cement, and the quality of the sand, are the most important factors influencing this rate of change upon which the question of the relative economies of different mortars is so largely dependent.

Table 67 gives the results of tests with two brands of Portland cement mixed with from two to ten parts of river sand, the age of briquets being six months and two years. It is of interest to notice that the strengths of the mixtures are approximately in the inverse ratio of the number of parts of sand used. Thus the strength with six parts sand is approximately two-sixths of the strength with two parts, while with ten parts sand, the strength is nearly two-tenths of that with mortar containing two parts.

Table 67. Rate Of Decrease In Strength With Addition Of Sand

Portland Cement; River Sand, " Point aux Pins ".

Parts Sand to 1 Cement by Weight.

Tensile Strength, lbs. per Sq. In.

Proportionate

Strength, Two Years, if 1 to 2=100.

6 Months.

2 Years.

h

r

h

r

Mean.

2

512

504

534

548

541

100

3

390

335

363

355

359

66

4.09

295

261

296

288

292

54

6

175

144

191

174

182

35

8

113

96

132

132

132

24

. 10

64

74

104

116

110

20

355. In Table 68 similar results are given for two samples of Portland cement and two kinds of sand, neat cement specimens being included in the comparison. The one-to-one mortars give a higher strength than neat cement, and even the mortar containing two parts of the limestone screenings is stronger than the neat specimens. From the one-to-one mortars the strengths decrease rapidly as more sand is added, until five parts sand are used, but the strengths then decrease less rapidly as larger additions of sand are made.

Table 68. Rate Of Decrease In Strength With Addition Of Sand

Portland Cement, Brand R; Sand, Crushed Quartz and Limestone Screenings.

Parts Sand to 1

Cement by

Weight.

Tensile Strength, Pounds per Sq. In.

Proportionate

Strength if Strength

1 to 1 Mortar = 100.

Sample

Cement H H,

Crushed Quartz

Sand, 20-30.

Age Briquets,

6 1/2 Months.

Sample

Cement II,

Limestone

Screenings, 20-30.

Age Briquets,

6 Months.

Crushed

Quartz.

Limestone

Screenings.

0

689

686

82

78

1

840

881

100

100

2

521

703

62

80

3

368

508

44

58

4

236

335

28

38

5

203

267

24

30

6

156

178

19

20

8

104

138

12

15

10

78

98

9

11

356. In Table 69 two samples of natural cement are treated in a similar manner, from one to eight parts river sand being used in the mortars. With Sample II the strength is diminished rapidly until five parts sand have been added, but with further additions of sand, the strength is decreased more slowly. Sample 18 S gives quite a different curve, as the one-to-two mortar is stronger, and the one-to-three mortar is but little weaker than the one-to-one. With four parts sand the mortar shows a marked falling off in strength, but further additions of sand diminish the strength more slowly.

357. Increase In Tensile Strength With Time

In Table 70 are given the results obtained in tests of tensile strength with twelve samples of Portland cement, illustrating the rates of increase in strength from seven days to three years. It is seen that rich mortars gain strength rapidly, neat and one-to-one mortars showing usually eighty to ninety per cent, of their ultimate strength in twenty-eight days. Mortars containing not more than four parts sand to one cement give practically their ultimate strength at six months. It is also of interest to notice that the variations in strength among the several samples are not very great. The lowest strength at the end of two to three years is seventy-five to eighty per cent, of the highest.

Table 69. Rate Of Decrease In Strength With Addition Of Sand. Natural Cement, Brand Gn; River Sand, "Point Aux Pins"

Parts Sand to 1

Cement by Weight.

Age.

Tensile Strength, Pounds per Square Inch.

G Months.

2 Years.

Proportionate Strength, Two Years if 1 to 2 = 100.

Sample Cement

II.

18 S.

18 S.

0

380

1

297

308

280

86

2

260

314

324

100

3

183

280

294

91

4

128

193

187

58

5

81

161

165

51

6

69

142

172

53

7

56

119

156

48

8

53

101

114

35

358. In the case of natural cements, results for ten brands of which are given in Table 71, only fifty to seventy per cent, of the ultimate strength is gained in the first twenty-eight days; with mortars containing three parts sand to one cement the average result at twenty-eight days is less than forty per cent, of the strength at two years. Most of the samples gain some strength after six months, but two samples fail at two years which had given a fair result at six months. The variations in strength among the several samples are very much greater than with Portland cements; even omitting the two samples that failed, the strength of the highest is two or three times the strength of the weakest sample at two years.

Table 70. Portland Cement, Twelve Samples. Strength Of Mortar And Fineness

Cement.

Fineness: Per Cent.

Passing.

Tensile Strength, in Pounds per Square Inch.

Neat Cement.

One Part Sand to One Cement by Weight.

Three of Sand to One Cement by Weight.

4 Sand to 1

Cement.

5 Sand to 1

Cement.

Brand.

Sample.

Sieve No.

100.

Holes .0065

In. Square.

Sieve No.

120.

Holes .0046

In. Square.

7 days.

28

Days.

6 mos.

2 yrs.

3 yrs.

7 days.

28

Days.

6 mos.

2 yrs.

3 yrs.

7 days.

28 days.

6 mos.

1 yrs.

2 yrs.

2 yrs.

2 yrs.

H H R I

P

K

F

C

N

J

A

G

5S 21 S 300 S 8S

2

3

4 12 S 18 S 17 S 20 S

1

...

90.7

...

89.5

...

81.3 86.7 86.6 83.0 89.0 84.5 92.0

...

82.6

...

76.4

...

67.7 79.0 76.0 70.4 79.1 73.8 80.0

497

567 523 469 516 518 387 532 403 574 556 542

570 710 619 562 626 628 575 711 479 743 637 616

619 736 717 669 721 646 685 797 574 737 691 732

633 703 676 748 638 643 728 790 637 767 681 730

668 804 768 719 741 745 723 751 604 745 698 762

501 566 525 486 467 477 404 502 399 503 498 469

650 669 591 563 653 606 614 621 546 665 616 560

740

722 756 727 752 722 761 756 650 743 675 661

768 840 813 796 814 860 855 687 649 764 795 704

749 802 746 725 728 778 793 620 719 722 737 672

176 197 202 177 148 164 101 144 133 181 150 175

272 266 255 250 251 237 212 219 185 261 227 240

456 434 384 387 381 367 385 362 311 392 374 338

398 395 394 388 388 383 379 376 351 350 350 333

384 350 370 390 365 354 382 332 318 343 327 311

...

283 280

...

268

...

233 ...

249

...

...

...

228 224 215

...

...

...

...

...

...

198 184

Sum

...

...

6084

7476

8324

8374

8728

5798

7354

8665

9345

8791

1948

2874

4571

4485

4226

...

...

Mean

...

...

507

623

694

698

727

484

613

722

779

733

162

240

381

374

352

...

...

Table 71. Comparative Tests On Ten Brands Of Natural Cement

Ref.

Cement.

Tensile Strength, Pounds per Square Inch.

Brand.

Sample.

Neat Cement.

One Part Sand to One Cement.

Two Parts Sand to One Cement.

Three Parts Sand to One Cement.

1 day.

7 days.

28 days.

6 mons.

7 days.

28 days.

3 mons.

6 mons.

2 yrs.

7 days.

28 days.

6 mons.

2 yrs.

28 days.

3 mons.

6 mons.

2 yrs.

a

b

c

d

e

f

g

h

i

j

k

l

m

n

o

p

q

r

s

1 2 3 4 5 6 7 8 9 10

Mean

"

" "

Bn In Jn Kn Gn Hn Ln Mn Nn Dn of 10 r of 4, 8 of 2,5, of 1, 3

27s 28s 29s 30s 844 26s 31s 39s 40s 9s esults and 9 7 and 10 and 6

46 63 93 100 74 117 148 162 179 54

104 147 85 85

130 103 174 189 140 203 229 189 210 258

182 196 182 169

293 180 208 289 178 344 296 263 316 226

259 289 220 282

376 329 510 326 326 468 468 473 493 335

410 431 364 451

103 80 125 164 104 198 196 164 169 341

164 166 180 142

247 179 237 385 242 342 275 247 252 413

282 295 277 275

324 366 457 452 410

..

..

..

..

..

..

..

..

298 395 445 505 358 346 436 644 670 386

428 540 394 363

358 369 177 532 337 271 395 631 665 411

415 609 378 265

72 39 69 95 46

..

..

..

..

..

..

..

..

148 106 132 217 129 223 183 158 184 279

176 186 174 168

202 332 330 483 384

..

..

..

..

..

..

..

..

195 401 106 661 386 157 339 515 550 322

353 542 362 153

85 26 64 101 63 113 114 93 104 175

94 99 94 87

102 148 128 204 191 207 217 157 171 240

182 197 199 146

122 189 213 321 256 200 253 259 260 204

228 280 225 178

117

265 99 372 285 130 270 340 328 327

253 347 287 115

In general, each result is the mean of five briquets.

359. Table 72 shows in some detail the rate of increase in strength of a sample of natural cement when the specimens are maintained in air and in water. This table has several points of interest. When hardened in water, the cement gained steadily in strength up to six months, when it began to fall off, and at two years this cement failed, as is shown in Table 71. The neat cement specimens hardened in air are very irregular, as usual. These specimens showed high strength at three months, suffered a marked falling off at six months and one year, but showed a remarkable strength, equal to neat Portland cement, at two years. The strength developed at one year by specimens of this sample containing two parts sand to one cement and hardened in air, is also equal to that shown by similar mortars of Portland cement.

Table 72. Rate Of Increase In Strength In Water And Air

Tensile Strength, Pounds per Square Inch.

Age of Briquets.

Mortar Two Parts Sand

Neat Cement.

by Weight to One

Cement.

water.

Air.

water.

Air.

1 day......

81

152

....

....

7 days.....

192

254

135

142

14 days.....

232

315

....

....

28 days.....

305

473

232

271

3 mos......

390

551

367

459

6 mos......

437

372

409

475

1 year.....

432

314

249

537

2 years.....

395

731

....

....

All cement, Brand Hn, Sample 26 S, which fails in water after two years, see Table 71.