We are very conscious about handling of concrete; in most cases don’t pay head to handling of aggregate. Handling of aggregate is also important to have designed gradation, to avoid foreign materials in aggregate and also to have ease of batching and economy of the same too.
Stockpiling and handling of coarser aggregate can lead to segregation. While tippling and discharging aggregate usually rolls down along slope. The large size aggregates have more weight and smaller size aggregates are relatively lighter. Thus large size particles remain in the bottom, the lighter particles remains at the top.
These results uniform grading at all layer; following size fraction should be spitted
3/16” to 3/8”
3/8” to ¾”
3/4” to 1½” etc.
Splitting means stockpiling and handling separately; can be remixed during feeding into concrete mixer according to desired proportions. Even after this precaution segregation can be occurred but would be in narrow range within each fraction and even careful handling can reduce this segregation.
When aggregate particles to be handled are greater than 1½ in; there have every possibility to breakdown large aggregate particles. The precautions should be taken in this case are:
Using rock ladders to lower aggregate into bins
Not to drop from height.
In important and large projects, extra precaution to avoid breakage and segregation of aggregate is taken by providing rescreening just before feeding into bins. Thus excess of undesired undersize particles are eliminated.
Rescreening controls proportions of various sizes of aggregate effectively but with greater control more cost and complexity of batching operation are added.
Again a denser pack of aggregate leaves less space to take place finer cement particles; thus requirement of cement or cementitious materials are reduced and eventually economy is added against more cost of screening.
The uniformity of concrete also produces more workable mix facilitating placing. Again improper handling can cause contamination by deleterious substances and also by aggregate of low strength, less durable and undesired sizes. Sometimes aggregates are transported by contaminated container or sacks like sugar or salt etc. The affect of presence of salt in concrete has already been described in previous posts and in upcoming post we will learn about effect of adding sugar to concrete.