Testing of industrial products - Aggregates for construction

The methods for the determination of the Particle Density and Water Absorption of Coarse Aggregate

The principle of the "saturated, surface-dry" (SSD) method.

The "SSD" particle density is determined from four masses: the mass of the saturated, surface-dry, test portion, the mass of the pyknometer containing the test portion and topped up with water, the mass of the pyknometer full of water, and the mass of the oven-dry test portion.

It is defined as:

pssd = (mass of aggregate + mass of water contained in the accessible voids)/(volume of aggregate including the inaccessible and accessible voids)

In the "SSD" method, the test portion is immersed in water for 24 ± 0.5 h, and then dried with absorbent cloths, spread out, and left exposed to the atmosphere until all visible films of water are removed but the aggregate still has a damp appearance. It is then weighed to give the mass of the test portion in the SSD condition. The other weighings are then carried out so that the mass of the oven-dry test portion is determined last.

The temperature of water used in the test should be within the range 22 ± 3 °C. All weighings are carried out to an accuracy of 0.1 % of the mass of the test portion. The calculation of the particle density assumes that the density of the water is 1.000 Mg/m3.

The method also allows the water absorption of the aggregate to be determined from the same four weighings.

The principle of the "pre-dried" method.

The "pre-dried" particle density is determined from three masses: the mass of the empty pyknometer, the mass of the pyknometer containing a test specimen of the coarse aggregate, and the mass of the pyknometer containing the test specimen and topped up with water. It is defined as:

pp = (mass of dry aggregate)/(volume of aggregate including inaccessible voids but excluding accessible voids)

Thus both the mass and the volume in the definition of density are higher in pssd than in pp, and the two definitions will lead to identical densities if the aggregate has no accessible voids.

In the "pre-dried" method, the test specimen is first oven-dried, and placed in a pyknometer. The pyknometer is then topped up with water, and placed in a water-bath for at least one hour before the third weighing is done.

The calculation requires the volume of the pyknometer (established by weighing the pyknometer empty and then full of water), and the density of water (a table is given in the Standard giving the density of water at a range of temperatures).

Pyknometers of nominal capacity 1000 mL are normally used, although other sizes are permitted. Before a pyknometer is to be weighed containing water it is placed in a water bath at 25.0 ± 1.0 °C for at least one hour.

All weighings are carried out to an accuracy of 0.1 % of the test specimen mass.

The proposed CEN methods.

The proposed CEN "SSD" method is one that has been used for many years in the UK and in one or two other countries to determine particle densities and water absorptions of coarse aggregates, and is mainly used for concrete mix design. The proposed CEN "pre-dried" method is one that has been used for many years in Germany and some other countries to determine the particle density of coarse aggregates, and is used in connection with bituminous mixtures. According to the draft Standard, the latter method is suitable for normal weight, non-porous, aggregates.

Participants in the cross-testing experiment were sent copies of the methods taken from the version of the Standard that was prepared in July, 1996, for submission to CEN members for CEN enquiry.

At the time the experiment was being planned, it was hoped to carry out both methods on the same test portions. The "SSD" method ends with the test portion being oven dried, so it should be possible, in principle, to do just one further weighing (the third weighing of the pre-dried method, with the test portion in the pyknometer, topped up with water) to obtain the pre-dried particle density. The minimum masses of test portions required by the "SSD" method are two or three times as large as the minimum masses of test specimens required by the "pre-dried" method. Hence it should be possible in practice to carry out the "SSD" and "pre-dried" methods on the same test portions, by splitting the test portion used for the "SSD" method into test specimens suitable for use in the "pre-dried" method. However, it turned out to be too complicated to organise the experiment in this way. The partners did not know the size of the pyknometers the participating laboratories would want to use, and how they would want to split the test portions used for the "SSD" method to produce test specimens for the "pre-dried" method.

The two methods are not consistent with respect to their treatment of the density of water. The "SSD" method allows the water used in the test to be within a tolerance of ± 3°C of the nominal test temperature. The "pre-dried" method requires a stricter tolerance of ± 1°C. The "SSD" method assumes that the density of water is 1.000 Mg/m3. The calculations of the "pre-dried" method require the density of water at 25°C to be used correct to four decimal places. Either one method is too strict or the other is too lax.