INTRODUCTION
Although many people view aggregates as just a product that is taken from a quarry, transported and used in a structure, this is far from the truth.

Aggregates and sand is a low value product and therefore it is expensive to move the product.

The importance of aggregate gradation limits and the maximum aggregate sizes is well known.  The grading and maximum sizes of aggregates affect relative aggregate proportions, as well as cement and water requirements, workability, economy, porosity, and shrinkage of concrete.

In order to control the quality of the aggregates (and concrete), the trend of late has been to prescribe an end result specification.  Specifications generally require “all aggregates to be handled and stored in such a manner that will minimize segregation, avoid contamination, and secure a uniform grading of the material within the specified gradation band”.  This naturally places the responsibility for quality control in the hands of the contractor.  Since contractors generally rely on commercial producers for the source of their aggregates, the responsibility essentially has been transmitted through the contractor to the material supplier.

The guidelines on recommended practices for handling aggregates in this report should, if followed, result in maintaining quality of gradation and cleanliness characteristics.  The recommendations trace a path from the producer to the user in three broad areas:

1.  Hauling to stockpile
2. Stockpiling
3. Unloading stockpiles and charging mixer

Hauling to stockpile
The hauling of aggregates and sand from the point of production is usually by truck or rail; however, there are many circumstances where economics dictate that other modes or combinations be used.

Regardless of mode used, the possibilities of degradation, segregation and contamination arise each time the material is shipped.

1.  Economics is the most important factor to consider when selecting the route and method of transportation.
2. Care must be exercised through all stages of transportation to avoid segregation of particle sizes and in some cases degradation

Specific Suggestions

a. Transportation costs frequently are greater than aggregate costs except when nearby sources are available.

b. When designing a mix, consider alternates of transportation before selecting a source.

c. Tougher environmental rules and stricter regulations will cause sources to be located greater distances from built-up areas.

d. Designers should be aware of the difference in hauling costs when considering concrete and asphalt pavement designs.

Stockpiling
The method of stockpiling has a large effect on the variation in gradation and degradation characteristics of an aggregate.  The most economical and acceptable method of forming and reclaiming stockpiles from aggregates delivered in trucks is to discharge the loads in such a way that they are rightly joined and to reclaim the aggregate with a front-end loader.  When aggregates are not delivered in trucks, the least expensive acceptable results are obtained by forming the stockpiles in layers with a crane bucket and reclaiming the aggregates with a front-end loader.

Concrete can be proportioned to permit use of a fairly wide variety of gradations, but excessive variations in grading may noticeably affect such properties as slump and cement efficiency.  Deviation from the design, gradation, especially on the finer sieve, is particularly undesirable.

Smaller size aggregate pieces have greater surface area than larger ones.  Consequently, when undersized particles are concentrated, through segregation or degradation, water demand is significantly increased.

Specific Suggestions

a. Cone type stockpiles should be avoided under all conditions.  Many producers and contractors still tend to stockpile coarse aggregates by coning because of simplicity, even though the practice has demonstrated that segregation will invariably occur.

b. Segregation is minimised when aggregate stockpiles are formed by spreading in thin layers to minimise gravity flow.

c. If space is available, long flat stockpiles are considered desirable.

d. Backing truck loads of aggregates onto the pile and dumping in close packed piles is most desirable from an economical standpoint.  Unfortunately, some materials will break down under this action while some will not provide traction to allow the trucks to pull up on the pile.  In other cases, the stockpile terrain or size of the stockpile area prohibits the long gradual slopes essential for the success of this type of stockpiling; and other methods of stockpiling must be used.

e. Front end loaders used for stockpiling should have the widest tyres available and should avoid following the same path every load.

f. Economics should not be the final criteria for constructing stockpiles.  Segregation, degradation and contamination problems should be avoided even if more costly.  Contractors must do more toward seeing that their producers adhere to good stockpiling and handling practices.  It is too late to do much about segregation and degradation after you have received the material.

g. The average loss on a stockpile placed on an unpaved site is approximately 3 percent.  This would indicate that paving of a site would be desirable from an economic angle.  Usually local bituminous hot mix plants are available for this work.  Cement treated sub-base and lean concrete mix can be used to pave the plant site.  The great majority of contractors do not pave but use a clean, cheap, local stone as a mat under their pile to minimize contamination by muddy soil.

h. The effect of pollution requirements must be considered in the selection of stockpile areas.

i. Water drainage from stockpiles due to rainfall or pre-wetting activities must be controlled or treated to meet pollution requirements.

j. When the contractor designs, the mix, consideration should be directed toward using a single coarse aggregate stockpile.

k. A continuous sampling and testing program must be maintained to assure that material received meets gradation requirements and that stockpiling techniques are retaining these attributes.  A control chart system using averages and ranges is an effective visual aid technique that can easily be instituted.  This system will provide advance warning that the material being stockpiled may be out of specification and can provide a clue as to cause.

Unloading Stockpiles and Charging Mixer
Special attention should be given to unloading stockpiles and blending of materials on the charging belt in order to assure uniformity and homogeneity of the concrete produced.  Research has demonstrated that segregation of the mix can result if proper blending of the materials is not accomplished prior to entry into the mixer.

Specific Suggestions

a. Care must be taken in unloading a stockpile to avoid segregation.  One recommended technique is to have the front-end loader rake down the slope prior to filling the bucket.

b. Blending can be accomplished by coordinating the rate of flow from each bin or from the individual compartments within a single bin setup.  Where flow from separate bins is combined on the main belt leading to the mixer, blending techniques are relatively simple.  In this case, the speed of individual belts is adjusted so that beginning and ending flow on each coincides.  A belt scale system could be used to augment this blending method.  Where batch portions for each size aggregate flow from individual compartments onto the main belt located below the bin, blending is more difficult to achieve.  If it is possible to install a full width belt sampler installation on the main belt, information for making adjustments and for continuous monitoring of aggregate gradation is readily available.  However, on most setups, space for such installation is not available, and an incremental belt sampling technique must be used.

Sieve analysis made on five or more samples obtained in this matter should provide sufficient information that blending is adequate, or adjustments are needed.  Belt sampling requires that the loaded belt be stopped while the sample is taken, putting a severe strain on the system.  The following sampling sequence will help alleviate this situation:

1. Open bin gates with belt running
2. Close bin gates at predetermined time
3. Empty material on belt into the mixer
4. Open bin gates for predetermined time (usually 2 to 4 seconds)
5. Close bin gates
6. Stop belt with all the incremental sample available for sampling
7. Obtain full belt width sample
8. Repeat same procedure for additional samples from batch.

A plot of aggregate gradation for several of the sieve sizes will provide visual information where adjustments are needed.

CONCLUSION
It is recognised that many of the suggested techniques may not be feasible to implement on each project

The costs involved with implementing these suggestions are minimal in comparison with expected benefits.