Aggregates influence the properties of concrete/mortar such as water requirement, cohesiveness and workability of the concrete in plastic stage, while they influence strength, density, durability, permeability, surface finish and colour in hardened stage.
Field Tests for Aggregate
1. Surface Moisture Content & Absorption
The surface moisture in the aggregate, influences the water cement ratio, strength and durability of the mix. To determine the surface moisture of moist or wet aggregate, method is as follows-
Frying Pan Method
The following apparatus are required:
A frying pan or metal tray,
Gas stove or an electric hair dryer,
A metal or glass stirring rod and
Scales to measures.
The following procedure is followed during the test
For coarse aggregate 2 kg sample is adequate and for fine aggregate 0.5 kg sample is adequate.
The wet/moist sample is then weighed wet weight (W)
Heated very gently on the frying pan or metal plate and stirred with a glass or metallic rod to maintain uniform distribution of heat, until the sheen disappears from the surface. The fine aggregates become surface dry when it just starts showing free flowing characteristics.
The aggregates is then cooled and reweighed. The surface dry aggregate weight (Wsd) is noted. The surface moisture is then calculated as follows-
Surface moisture = [(W – Wsd) / Wsd] x 100%
Overheating must be avoided, as it will not give the correct surface moisture.
If heating is continued either on the fry pan or in the oven till the aggregate is bone dry aggregate weight is then noted (Wbd) after continuous heating. The absorption (absorbed water content) is then calculated as follows-
Absorption = [(Wsd – Wbd) / Wbd] x 100%
Similarly, if the dry aggregates are received on site and absorption capacity is to be determined then the aggregates are first soaked in water and then the above methods are deployed to determine the absorption capacity of aggregates.
2. Silt Content Test for Sand
The permissible silt content in sand (fine aggregate) must not exceed the values as specified in the standards. However, this method can only be used for natural sand, it should not be used for crushed rock sand.
The apparatus required for this test is only 250 ml glass measuring cylinder.
The silt content determination by volume is done in the following manner:
The glass cylinder is filled with salt-water solution (concentration of the solution will teaspoon full of common salt for every 570 ml) upto 50 ml mark.
Add sand until the level of the sand is upto 100 ml mark.
Add further salt-water solution till 150 ml mark is reached.
Place the palm on the mouth of the glass cylinder and shake it vigorously.
Place the cylinder on hard levelled surface and tap it all round so that sand is leveled.
Wait for three hours for silt to settle on top of sand.
Measure the thickness of the silt layer and the height of the sand. The silt content can be calculated as follows:
Silt (%) by volume = [(Thickness of silt layer/ Height of sand + Silt) x 100 %]
If silt content by weight exceeds 3% then washing of sand is necessary. After conducting few tests, a co-relation can be developed for silt layer thicknesses at various intervals of time. The silt content at 10 minutes can be fixed as inspection criteria.
3. Bulking of Sand
When sand is damp, the water coating on the surface of each sand particle causes separation of particles from one another due to surface tension. This causes sand to bulk. Bulked sand occupies more volume and hence if volumetric measuring is done while proportioning it, bulking correction is necessary.
The bulking test is done as follows:
The sand is filled, in loose condition in a box of measured height (H cm).
The box is then flooded with water and rodding is done to make the sand settle and consolidate. Care should be taken that sand does not overflow during the flooding and compaction.
The sand is then leveled in the box and the drop in height is measured (h cm).
Bulking is calculated as: Bulking % = h/H x 100%
Dry sand occupies the same volume as fully saturated sand. The bulking will vary from load to load and day to day depending on the fineness of sand and its surface moisture content. It is there-fore, very essential to make bulking corrections by checking the actual bulking of sand proposed to be used by volumetric batching for mortar or concrete.
Moisture contents %age by wt.
Bulking % by volume
4. Sieve Analysis
Sieve analysis is done to check the gradation of aggregate. The test is done as follow.
Take required amount of aggregate sample (for coarse aggregate take apprx. 2.5 kg and for fine aggregate take 0.5 kg)
Arrange the required no of sieves as per the contract or job requirement in an descending manner. (i.e. keep the sieve having largest size opening at the top and the smallest size opening at the bottom)
Shake vigorously the sieve set for at least 2 minute.
Then measure the weight of aggregate on each sieve and express it as the percentage of passing.
Now compare these values with the recommended values to know whether it falls within the range or not. If not falling within the desired gradation then take necessary action.
Grading limit of coarse aggregate and fine aggregate is given below for reference.
This test is done initially for concrete mix design and later conducted periodically for mix proportion adjustments if it is suspected that the grading of aggregates has changed considerably.
5. Fineness Modulus
Fineness modulus is generally used to get an idea of how coarse or fine the aggregate is. More fineness modulus value indicates that the aggregate is coarser and small value of fineness modulus indicates that the aggregate is finer.
Sieve the aggregate using the appropriate sieves (80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron & 150 micron)
Record the weight of aggregate retained on each sieve.
Calculate the cumulative weight of aggregate retained on each sieve.
Calculate the cumulative percentage of aggregate retained.
Add the cumulative weight of aggregate retained and divide the sum by 100. This value is termed as fineness modulus
Compare the test value with the values given in the following table and you can get an idea about how coarse or fine the sand is.
Only sand between FM 2.6 to 2.9 is considered suitable for nominal mix proportion.