DETERMINING SPECIFIC GRAVITY OF AGGREGATES

 

 

SCOPE

 

This method describes two procedures used for determining the bulk specific gravity of aggregates proposed for use in Portland Cement Concrete. This method is also described in Laboratory Test Method 201.

 

 

PROCEDURE A – SPECIFIC GRAVITY OF AGGREGATES USING A PYNCNOMETER

 

A.     Apparatus

 

1.     Balance having a capacity of at least 5,000 grams, accurate to 0.5 grams

 

2.   Pycnometer – a fruit jar supplied with a gasket and conical pycnometer top. A two-quart pycnometer is used for coarse aggregates, and a one-quart pycnometer is used for fine aggregate. If a two-quart pycnometer cannot be obtained, a one-quart jar may be substituted (The engineer may require 2 samples be obtained and tested in separate 1-quart pycometers for some aggregates). The quantity of aggregate would be approximated 1,100 grams for the one-quart pycnomter.

 

3.   Thermometer – a thermometer with a range of at least 50°F to 100°F

 

4.   Sieve – a No. 4 sieve

 

B.     Field Sample

 

1.     Obtain a field sample as prescribed in IM 301.

 

C.    Preparation of Test Sample

 

1.     Fine Aggregate

 

a.     Obtain a test sample of approximately 1,100 grams from the material to be tested by one of the following methods:

 

(1)  Use of a sample splitter

 

(2)  Method of quartering after being thoroughly mixed and in a damp condition

 

(3)  By taking small scoops of material from various places over the field sample, after it has been dampened and thoroughly mixed. In order to avoid segregation, the material must be damp enough to stand in a vertical face when cut with a trowel. This method of sample reduction is applicable to sands only.

 

b.     If the material has been continuously wet before being received on the job, it may be assumed to be saturated. Otherwise, the sample must be saturated by immersing it in water for period of not less than 15 hours.

 

c.      After soaking, pour off the free water, spread the wet sample on a flat, non-absorbent surface, and allow it to come to a surface-dry condition by natural evaporation of free moisture. Circulation of air by means of a fan may also be used to attain the surface-dry condition. The sample should be stirred frequently to secure uniform drying.

 

2.     Coarse Aggregate

 

a.     Obtain the test sample as prescribed in IM 336, Methods of Reducing Aggregate Field Samples To Test Samples (See Sections on Quartering or Splitting).

 

b.     Sieve the test sample over the No. 4 sieve. The sample should be of sufficient size to produce approximately 2,100 grams of material retained on the No. 4 sieve. Discard the material that passes this sieve.

 

c.      Immerse the sample (plus No. 4 sieve size) in water for a period of not less than 15 hours.

 

d.     After soaking, pour off the free water and allow the sample to come to a saturated-surface-dry condition by spreading the sample on a flat, non-absorbent surface. The forced circulation of air by means of a fan, if available, may hasten this process. The sample should be stirred frequently to secure uniform drying. The predominance of free moisture may be removed initially by rolling the sample back and forth in a clean, dry, absorbent cloth.

 

e.     The sample may be considered to be saturated-surface-dry when the particles look comparatively dull as the free moisture is removed from their surfaces. For highly absorptive aggregates, the saturated-surface-dry condition is reached when there is an absence of free moisture.

 

D.    Calibration of Pycnometers

 

1.     Fill the pycnometer jar nearly full of water at the temperature to be used in the actual test, plus or minus 3°F. This may be done either before or after the actual test.

 

2.     Screw the pycnometer top down tightly on the jar and mark the position of the top on the jar by a scratch or mark on the threaded rim and a scratch in a corresponding position on the jar, which will establish a constant volume.

 

3.     Fill the pycnometer completely by pouring water into the hole of the pycnometer top until a bead forms above the opening. Immediately wipe the bead of water level with the pycnometer opening. Wipe all other excess moisture from the outside surfaces of the pycnometer. If a bead of water forms at the opening during the final wiping, it should remain for weighing. Weigh the pycnometer to the nearest 0.5-gram.

  

E.     Test Procedure

 

1.     Weigh the saturated-surface-dry sample to the nearest 0.5-gram. For ease in calculations, the fine aggregate sample may be brought to exactly 1,000 grams weight, and the coarse aggregate sample may be brought to exactly 2,000 grams weight.

 

2.     Place the sample in the appropriate pycnometer containing approximately two inches of water.

 

3.     Nearly fill the pycnometer jar with water at the same temperature plus or minus 3°F as used in the calibration.

 

4.     Screw the cap down into the proper position by lining up the mark on the pycnometer top and the jar.

 

5.     Entirely fill the pycnometer by adding additional water through the hole in the pycnometer top.

 

6.     Hold one finger over the hole in the top and gently roll and shake the pycnometer to remove any trapped air in the sample.

 

7.     When further rolling and shaking brings no more air bubbles to the top, fill, dry and weigh as in step C3.

 

F.     Calculations

 

1.     Calculate the saturated-surface-dry (SSD) specific gravity to the nearest 0.01 by the following formula:

 

Bulk Specific Gravity (SSD) =

 

            Where:

 

            S    =    Weight in grams of aggregate in a saturated-surface-dry condition.

 

            P    =    Weight in grams of the pycnometer filled with water.

 

      W   =    Weight in grams of the pycnometer containing the sample and sufficient water to fill the remaining space in the pycnometer.

 

 

 

Pycnometers for Coarse and Fine Aggregates

 

 

PROCEDURE B – SPECIFIC GRAVITY OF COARSE AGGREGATE (AASHTO T85)

 

A.     Apparatus

 

1.     Balance having a capacity of at least 5,000 grams, accurate to 0.5 grams

 

2.     Sample Container – A wire basket of No. 6 (0.132 in.) or finer mesh, or a bucket of approximately equal breadth and height, with a capacity of 4 to 7 L. The container shall be constructed so as to prevent trapping air when the container is submerged.

 

3.     Water Tank – A watertight tank, into which the sample and container are placed for complete immersion while suspended below the balance, equipped with an overflow outlet for maintaining a constant water level.

 

4.     Suspended Apparatus – Wire suspending the container shall be of the smallest practical size to minimize any possible effects of a variable immersed length.

 

5.     Sieve - A No. 4 sieve

 

6.     Thermometer – a thermometer with a range of 50°F to 100°F

 

B.     Field Sample

 

1.     Obtain a field sample as prescribed in IM 301.

C.    Preparation of Test Sample

 

1.     Prepare the test sample identical to that described in Procedure A.

 

D.    Test Procedure

 

1.     Weigh the saturated-surface-dry sample to the nearest 0.5-gram. For ease in calculations, the fine aggregate sample may be brought to exactly 1,000 grams weight, and the coarse aggregate sample may be brought to exactly 2,000 grams weight.

 

2.     After weighing, immediately place the saturated-surface-dry sample in the sample container, remove all entrapped air by shaking the immersed container, and determine its weight in water at 73.4°F ± 3°F. Make sure the water is at a depth sufficient enough to cover the container and sample.

 

E.     Calculations

 

1.   Calculate the saturated-surface-dry (SSD) specific gravity to the nearest 0.01 by the following formula:

 

Bulk Specific Gravity (SSD) =

      Where:

 

S    =    Weight in grams of aggregate in a saturated-surface-dry condition.

W   =    Weight in grams of the saturated-surface-dry sample in water