University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

UNIT WEIGHT, YIELD, AND AIR CONTENT (GRAVIMETRIC) OF CONCRETE

ASTM C138

Terminology

Yield - volume of concrete produced per batch, cubic yard, or cubic meter

Air content - percentage of air voids by volume in concrete.

Apparatus

Measure - a cylindrical metal watertight measure, a yield bucket (nominal 1/3 ft3 )

Tamping Rod (5/8” diameter)

Scale (0.01 lb accuracy)

Mallet - rubber, weighing approximately 1.25 lb

Flat trowel

Materials

0.4 ft 3 of mixed plastic concrete

Procedure

The test procedure for the air content by pressure method test is part of ASTM 192.

Download the procedures for Unit Weight, Yield and Air Content here

1) Weigh the empty measure.

2) Fill the measure with freshly mixed concrete in three layers of approximately equal volume. Rod each layer with 25 strokes of the tamping rod.

3) After each layer is rodded, tap the sides of the measure smartly 15 times with the mallet (this procedure is required to release any large trapped air bubbles). After consolidation, the measure must not contain any excess of concrete protruding above (approximately 1/8”) the top of the yield bucket.

4) Strike off the top surface with a sawing motion of the flat trowel (using little vertical pressure).

5) Clean all excess concrete from the exterior of the measure (use a dampened towel if necessary, and then dry).

6) Weigh the measure with concrete.

7) Calculate the unit weight of concrete as the ratio between net weight of concrete and measure volume:

γconcrete = Wconcrete/ Vmeasure

where: γconcrete = unit weight of concrete (lb/ ft3)

Wconcrete = net weight of concrete (lb)

Vmeasure = volume of measure (ft3)

8) Calculate the total weight of all materials batched, i.e., the sum of the weights of the cement, fine aggregate (sand), coarse aggregate (gravel), the mixing water, and any admixture:

W1 = Wc + Ws * + Wg * + Ww * + Wa

where: Wc , Ws * , Wg * , Ww * and Wa are the weights of cement, sand, gravel, water, and admixture, respectively. (Note: Ws * , Wg * , and Ww * are adjusted weights for absorption and moisture content).

9) Calculate the yield, in yd3 /batch as:

Yield = W1 /(27 γconcrete )

10) Calculate the theoretical unit weight of concrete (on an air-free basis):

γtheoretica = W1/V

where: V = the total absolute volume of the component ingredients in the batch (ft3);

V = [(Wc /Gc ) + (Ws /Gs ) + (Wg /Gg ) + (Ww /Gw ) + (Wa /Ga )]/ γw

where: Gc is the specific gravity of cement (=3.15)

Gs , and Gg , are the bulk specific gravities (dry) of sand, and gravel, respectively,

Ga is the specific gravity of the admixture,

Gw is the specific gravity of water (=1.00),

γw is the unit weight of water, 62.4 lb/ ft3, and

Ws , Wg , and Ww , are the weights of sand, gravel, and water (unadjusted for absorption and moisture).

11) Calculate the air content as

% Air = 100( γtheoretical - γconcrete )/ γtheoretical

Report

1) Report the value of unit weight to the nearest 0.1 lb/ft3.

2) Report the value of theoretical unit weight to the nearest 0.1 lb/ft3.

3) Report the air content to the nearest 0.1%.

4) Report the yield to the nearest 0.01 yd3.