Humboldt H-2795 Roll-a-Meter - Manual (Page 6)

be invaluable. No change of mix can be made without a resulting change in the
percentage of air entrained with the same amount of air entraining agent being
used.
A prompt air meter measurement whenever a change occurs in temperature,
slump, time of mixing, richness of mix, or proportion of sand to gravel, will
usually reveal a corresponding change in
Use of roll-a-meter for specific gravity tests
To correctly Design mixes on technically controlled concrete work, accurate
specific gravity determinations are required. Using the Roll-a-Meter as a
pycnometer is one of the most convenient, accurate, and rapid methods of
testing for specific gravity (hereinafter referred to as S.G.).
Its use is based on the principle that any material immersed in a vessel full of
water will displace exactly its absolute volume of water (solid volume). S.G. is the
ratio of the weight of a unit volume of water to the same volume weight of the
material being tested. Displacing the water by a known weight of any material
and finding the loss in weight of the water displaced, compared to the weight of
the material immersed, gives the S.G.
In practice, three accurate gross weights are needed: the weight of the
Roll-a-Meter full of water (P), a known weight of gravel (B), and the weight
of the Roll-a-Meter refilled with water after gravel is added (Ps). The water
displaced is shown by the difference in the combined weight (Ps) from the sum
of the weights (P) and (B). Thus, if tile weight of water displaced is one-half of
the weight of gravel added, then the gravel must be twice as heavy for the same
volume as water. As the S.G. of water is 1.00, then the S.G. of the gravel must be
2.00. As the weight (P) is constant, the only weights necessary to be obtained for
the S.G. determination are weights (B) and (Ps). As the weight (P) is constant, the
only weights necessary to be obtained for the S.G. determination are weights (B)
and (Ps). The mathematics involved are very simple:
= Specific Gravity (S.G.)
Use a scale or balance with sensitivity of 114 ounce or less for weighing sand or
gravel. On fine materials such as cement, sensitivity should be .01 lb. or less.
Use about 10 Ibs. of sand or gravel and about 5 Ibs. of cement for a test. The
usual rolling of the Roll-a-Meter, to get all air released before final refilling and
weighing, will give very reliable accuracy. (The use of water instead of kerosene
for the determination of the S.G. of cement is more accurate for this purpose.)
The following example will illustrate the method:
(P) Weight of meter full of water (only) 30 1bs.
(B) Weight of sample of sand, gravel, or other material 10 Ibs .
(Ps) Weight of meter refilled with water (all air out) with sample of gravel. 37 Ibs.
(10 Ibs. immersed)
= Specific Gravity (S.G.)
(B)
(P) + (B) - (Ps)
10
30+ 10 - 37