🪨 Determination of the water content of aggregates by drying — PN-EN 1097-5

Construction PN-EN 1097-5

Overview

THIS IS THE REFERENCE METHOD, NOT THE ONLY ACCEPTABLE ONE — AND THE STANDARD ITSELF SETTLES THIS. The scope, quoted from the text of the standard: "This standard describes the REFERENCE METHOD used for type testing and in cases of dispute for the determination of the water content of aggregates by drying in a ventilated oven. For other purposes, in particular FACTORY PRODUCTION CONTROL, other methods may be used provided that an appropriate working relationship with the reference method has been established." A reader with an accreditation scope gets two things from this at once. First: this standard settles disputes and is used for type testing. Second: in factory production control another, faster method may be used — but only if its relationship to the reference method has been established. "We measure differently because it is faster" is not enough on its own; an established relationship is needed.

WHAT THIS METHOD MEASURES AND WHAT IT DOES NOT. The standard says it plainly: "The oven-drying method provides a measure of the TOTAL FREE WATER present in a test portion of aggregate. The water can be from the surface of the aggregate and from water accessible pores within the aggregate particles." That is, the result covers surface water TOGETHER WITH water from water-accessible pores — it is neither surface moisture alone nor water absorption. Anyone who needs to separate these two things needs a different test.

CONSTANT MASS IS DEFINED NUMERICALLY, SO IT IS NOT A MATTER OF JUDGEMENT. Definition 3.3: "successive weighings after drying AT LEAST 1 H APART not differing by more than 0,1 %". Two conditions at once — the time interval and the mass difference — and both must be met. The standard adds a note that in many cases constant mass is reached after a predetermined drying time and that the laboratory may determine this time for its types and sizes of samples, depending on the capacity of the oven. This allows the routine to be shortened, but does not change the definition: in a dispute it is the difference between two weighings that decides, not the passage of time.

THE RESULT REFERS TO THE DRY MASS — THIS IS WHERE IT IS EASIEST TO MAKE A MISTAKE. The formula of the standard is w = (M1 − M3) / M3 × 100, where M1 is the mass of the wet sample and M3 the mass after drying to constant mass. The DRY mass stands in the denominator. The same mass difference referred to the wet mass would give a smaller number, yet it looks the same — "percent water". A report giving the percentage alone without indicating the basis of reference does not make it possible to decide which of these two numbers has been given.

PROTECTING THE SAMPLE AGAINST A CHANGE IN MOISTURE IS PART OF THE METHOD, NOT A HOUSEKEEPING RECOMMENDATION. The standard requires that at all stages of handling and preparation BEFORE the start of the test the laboratory sample and the test portion taken from it be protected against loss and against gain of water. That is why the container must be non-corroding and closed, and the standard separately warns against storing the sample for a long time in a plastic bag. THIS IS WHERE AN ERROR GIVES A SEEMINGLY CORRECT RESULT: water lost between sampling and weighing leaves no trace in the further procedure. Drying will proceed correctly, constant mass will be reached, the formula will compute without reservation, and the result will be UNDERSTATED by as much as evaporated on the way. No check in the laboratory will detect this, because at the moment the sample is received there is nothing to compare its mass with — that is why the conditions of transport and storage are part of the method, not an organisational matter.

THE MINIMUM SAMPLE MASS DEPENDS ON THE UPPER AGGREGATE SIZE. The standard gives the rule: if D ≥ 1.0 mm, the minimum mass in kilograms is 0.2 D; if D < 1.0 mm, the minimum mass is 0.2 kg. The test portion is obtained from the laboratory sample in accordance with EN 932-2.

Method principle

The weighed test portion of aggregate is dried in a ventilated oven at a temperature of (110 ± 5) °C to constant mass, cooled to room temperature and weighed again. The loss of mass corresponds to the total free water — surface water and water from water-accessible pores. The water content is expressed as a percentage of the mass of the dry sample.

Applications

• Type testing of aggregates and cases of dispute — the standard is the reference method for them
• Aggregates for concrete, for bituminous mixtures and for unbound mixtures
• Correcting the mix design of a concrete mix for the water brought in with the aggregate
• Settling the mass of aggregate delivered in a wet state
• Lightweight aggregates — the standard provides a variant of the method for them in Annex A
• Temperature-sensitive aggregates — with a reduced drying temperature
• Establishing the relationship between the reference method and a faster method used in factory production control

Key parameters

Standard

Standard number

PN-EN 1097-5:2008 (wersja angielska; wprowadza EN 1097-5:2008 [IDT])

Title (PL)

Badania mechanicznych i fizycznych właściwości kruszyw — Część 5: Oznaczanie zawartości wody przez suszenie w suszarce z wentylacją

Title (EN)

Tests for mechanical and physical properties of aggregates — Part 5: Determination of the water content by drying in a ventilated oven

Step-by-step procedure

1. Obtaining the test portion

   The laboratory sample is reduced in accordance with EN 932-2. The minimum mass is calculated from the upper sieve size D: for D ≥ 1.0 mm it is 0.2 D kilograms, for D < 1.0 mm — 0.2 kg.

2. Protecting the sample against a change in moisture

   Immediately after preparation the sample is placed in a clean and dry container, unless the determination is carried out at once. At all stages before the start of the test the sample is to be protected against loss and against gain of water.

3. Weighing the tray

   The trays are cleaned and dried, then weighed and their mass (M2) recorded.

4. Weighing the wet sample

   The sample is spread on the tray, the tray with the sample is weighed, and the mass of the wet sample (M1) is determined by subtracting the mass of the tray (M2).

5. Drying

   The trays are placed in an oven at a temperature of (110 ± 5) °C and dried until constant mass is reached. With fine aggregates evaporation may be assisted by occasional stirring — the stirrer is then left in the tray until drying is complete, so as not to remove solid material from it.

6. Checking constant mass

   The tray is cooled to room temperature (in a desiccator if necessary), the mass of the sample Md1 is determined, the tray is returned to the oven for at least one hour, cooled again and weighed (Mdi). If the difference from the previous result is less than 0.1 %, constant mass is taken as reached; if it is 0.1 % or more, the procedure is repeated.

7. Establishing the dry mass

   The last determined value Mdi is taken as M3. With several trays, M3 is the sum of the Mdi values from all trays.

8. Calculation

   The water content is calculated from the formula w = (M1 − M3) / M3 × 100 and reported to the nearest 0.1 %.

9. Report

   The report must contain: a reference to EN 1097-5:2008, identification of the sample, identification of the laboratory, the type and nominal size of the aggregate and the result. Optional information includes, among other things, the name and location of the source of the sample, the individual weighing results, a description of the sampling procedure, the mass of the dried sample and the date of the test.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Trays and the sample are above 100 °C when taken out of the oven — tongs and heat-resistant gloves, cooling in a desiccator
• Fine aggregates give off dust when spread and stirred — respiratory protection when working with sand fractions
• The oven remains hot for many hours, and the test by its nature requires repeated opening
• Calcium chloride as a drying agent is irritating to the skin and eyes; silica gel with an indicator is sometimes coloured with cobalt compounds — handle it as a reagent, not as a filler
• Samples weighing several kilograms lifted to the height of the oven — risk of overexertion and of dropping a hot tray

Frequently asked questions