To show that short time tests do not always indicate the relative values of several samples of cement, even when all of the samples are of the same brand, Tables 42 and 43 are given. All of the results in these tables are from samples of the one brand of natural cement.

Table 42. Comparison Of Short And Long Time Tests Of Samples Of One Brand Of Natural Cement

1 Mean one-to-one and one-to-two mortars.

2 Briquets immersed six days in water maintained at 60° C.

In Table 42 the results are selected from a large number of tests of this brand, and are arranged in groups according to the strength shown at a certain age. For instance, in Series A the results of twenty samples are given, arranged according to the strength at twenty-eight days. Three of the samples gave less than 100 pounds per square inch, neat, at twenty-eight days; the same three samples gave a mean strength of 121 pounds per square inch, neat, six to seven months. Seven samples, the strength of which fell between 100 and 150 pounds at twenty-eight days, gave a mean strength of 186 pounds at six to seven months. The results of this series show the harmony between short and long time tests when it is a question of comparing neat cement mortars.

In Series D of this table the samples are arranged in order according to the strength developed by one-to-two mortars one year old. Thirteen samples had a strength at this age of between 450 and 500 pounds, average 473 pounds. The same samples gave but 66 pounds, neat, seven days. Forty-eight samples, giving between 400 and 450 pounds, average 422 pounds, gave but 80 pounds, neat, seven days, while eighteen samples that developed only 300 to 350 pounds mean, 325 pounds at one year, showed a mean strength of 147 pounds, neat, seven days.

A little study of this table will show that the samples which were comparatively weak in seven and twenty-eight day tests, either neat or with sand, gave the best results in the long time tests of sand mortars. Series A shows that the neat tests at seven days and at six months are consistent, but in all cases where sand mortars are tested at six months to one year, the highest results are given by the samples showing the lowest strength in the short time tests in cool water. It is very seldom that this conclusion has not been indicated by the author's tests of this brand. It is not invariably true, however, for some samples which were selected as being defective in burn, gave low results both in short and long time tests. The conclusion stated above must therefore be understood to have limits even for this brand, and may not apply at all to many brands.

As to the results of short time tests of briquets stored in hot water, Series C and D indicate that such results are more nearly consistent with the long time tests, yet it is evident that even with hot tests one could not readily and accurately differentiate the best from the mediocre samples.

Table 43. Natural Cement. Rate Of Increase In Strength, Hardening In Water And Dry Air

Sample....... 84 U' O'

Fineness : Per cent, passing Sieve No. 120, Holes .0046 inch square.......... 80.5 87.8 89.7

Time Setting—to bear 1/12" 1/4 lb. Wire, min. . . . 54 23 97

Specific Gravity............ 3.012 2.950 3.145

U', underburned, O', overburned. All samples same brand, Gn.

229. The results in Table 43 will serve to illustrate the same point by showing the very different rates of increase in strength of three samples when the briquets are stored in water and in dry air. One of these samples, 84, was taken at random from a shipment, while U' and O' were supposed to be defective in burn. Of the water-hardened specimens, No. 84 gained in strength up to six months or one year and then suffered only a slight falling off. The underburned sample showed a continuous gain, but the overburned cement showed a marked decrease in strength after six months or one year. The air-hardened specimens were very irregular in strength, but the underburned sample gave very low results throughout.

Table 44 gives similar results obtained with several samples, the briquets being hardened in water as usual. 16 R is a fair sample of the best cement of this brand, and its rate of increase in strength with one to three parts sand is shown. Samples M and L were tested together, as were CC and DD. M and CC are of the class giving comparatively high results at seven days, while L and DD give high results at seven days, but develop only a moderate ultimate strength.

Table 44. Natural Cement. Difference In Rates Of Increase In Strength Of Several Samples Of The Same Brand

230. Conclusions

From the above tables one should not draw the conclusion that all strength tests are valueless because likely to be misleading. Some lessons, however, seem to be plain; conclusions drawn from the results of short time tests of strength alone are likely to be far from infallible. This is especially true of natural cements. The correctness of one's conclusions concerning the value of a sample is likely to depend very much upon his knowledge of the behavior of that particular brand, and the beginner in cement testing should not have too great confidence in his early conclusions. Samples under inspection should be tested in comparison with other samples of known quality, and the results of the strength tests studied in connection with all the information obtainable from the other tests of quality already outlined.