364. The Effect Of Thorough Gaging

The value of thorough gaging is a point frequently overlooked in the preparation of mortars and concretes. Table 78 gives a few of the results obtained in experiments to determine the effect of thorough work in mixing. The tests are made with two brands of natural and one of Portland, with two parts sand to one cement by weight. The two minutes' mixing with hoe and box method gave a more thorough gaging than could have been accomplished in the same time with a trowel, and represented about the amount of work put on mortars for testing. We are not, therefore, comparing well mixed and poorly mixed mortars, but: rather well gaged and better gaged. The effect of the additional work is shown in all cases; to double the time spent in gaging, increases the strength of the resulting mortar about five per cent., while to quadruple the time adds twenty-six per cent, to the strength.

Table 78. Effect Of Thorough Gaging

365. Regaging

When more mortar is mixed at one time than is required for immediate use, there is always a temptation to retemper the mass and use it, even though it may have been standing for some time. The practice is usually prohibited by specifications and strenuously opposed by engineers. The tests recorded in Tables 79 to 83 were made to determine the effect of regaging on the resulting strength of the mortar.

366. The results obtained with two brands of Portland cement are given in Table 79. The first result in each line of the table is the strength attained by the mortar when treated as usual. The severity of the treatment of the mortar as regards regaging is shown by the letters heading the columns and the corresponding foot notes. The first general statement to be made concerning the results in this table is that in no case is the effect of regaging Portland mortars containing sand shown to be seriously deleterious to the tensile strength. Neat cement mortar is not improved by regaging, and if allowed to stand more than one hour, and then made into briquets without any further addition of water, the strength is considerably decreased. If water is added and the mortar frequently regaged, however, even neat cement mortar does not suffer a great decrease in strength by three to six hours standing. Rich mortars, containing one part sand, are not seriously affected by standing three hours if regaged frequently. Poorer mortars, with two to four parts sand, show an actual increase in strength as the effect of such severe treatment as standing five hours, if re-tempered with more water once an hour. These two brands were slow setting Portlands, beginning to set in forty minutes to two hours. The increase in strength of the regaged mortars is doubtless due, at least in part, to the more thorough gaging which they received.

Table 79. Regaging Portland Cement Mortar

Treatment

a — Treated as usual.

b—Mortar allowed to stand 1 hour, and briquets molded without regaging. c — Mortar allowed to stand 1 hour, regaged and briquets made. d — Mortar allowed to stand 3 hours, regaged every hour. e — Mortar allowed to stand 5 hours, regaged every hour.

f — Same as c, except that water was added in regaging to restore original consistency. g—.Mortar allowed to stand 3 hours, then regaged and briquets made, water added. h — Same as d, except that water was added in regaging to restore original consistency. i — Same as e, except that water was added in regaging to restore original consistency, j — Mortar allowed to stand 6 hours, then regaged and briquets made ; water added. k — Mortar allowed to stand 6 hours, regaged every hour; water added to restore consistency.

Table 80 gives similar results of briquets one year old made at another time with two parts river sand. The fact that during the delay between the making and use of the mortar it should be frequently re tempered with water to make up for the loss by evaporation, is plainly shown.

Table 80. Regaging Portland Cement Mortar

Cement: Portland, Brand R, Sample 42 M. Sand: 2 parts "Point aux Pins," passing No. 10 sieve. Age of briquets, 1 year. Treatment::— a — Molded as soon as gaged.

c — Mortar let stand 1 hour, regaged and briquets made.

d — Mortar let stand 3 hours, regaged each hour.

h — Mortar let stand 3 hours, regaged each hour and water added to restore original consistency. e — Mortar let stand 5 hours, regaged each hour. i — Mortar let stand 5 hours, regaged each hour and water added to restore original consistency. f — Mortar let stand 5 hours, regaged and briquets made. j — Mortar let stand 5 hours, regaged and briquets made; water added to restore original consistency.

367. Similar tests with natural cements are shown in Table 81, and it appears that cements of this class, especially if mixed neat, will not stand the same severe treatments without injury.

Table 81. Regaging Natural Cement Mortar

Treatment

a — Molded as soon as gaged.

b — Mortar let stand 20 minutes after first gaging, then regaged and briquets molded, c — Mortar let stand 40 minutes after first gaging, then regaged and briquets molded. d — Mortar let stand 60 minutes after first gaging, then regaged and briquets molded. e — Mortar let stand 60 minutes after first gaging, then regaged and let stand 30 minutes; then again regaged and briquets molded. f — Mortar let stand 1 hour after first gaging, then regaged with enough water added to bring to about same consistency as before, and briquets molded. g — Mortar let stand total time of 3 hours, being regaged every hour with enough water added to bring to first consistency. h — Mortar let stand total time of 5 hours, being regaged every hour with enough water added to bring to first consistency. i— Mortar let stand 3 hours, then regaged once with additional water and briquets molded. All cements rather slow setting, Brand An very slow setting.