Concrete Pixels

Concreting in one go of high walls, big columns (one meter or more in height) or other similar structures is often done in layers of less than half a meter. In pouring such structures it is very important to ensure that before pouring the next layer the elder layer should still be in its vibratable state or has not yet set so that they can be virbated together otherwise the connection between these two layers is weak and cold joints are formed. It is therefore very important to know beforehand the revibration limits. As the concrete setting time behaves differently for every mix influenced by many factors such as the amount & type of binders, additives and admixture, the starting temperatures, the environmental condition, etc, there is no specification to fix when two layers of concrete can still be vibrated. Standard tests done at the laboratory will only give an indication but not necessarily true with the actual condition. Manytimes in most construction sites the time is a wild guess based on the textbooks recommendation or on the experience of the engineer but this is very risky. But there is actually a very simple non-standard method to determine such parameter. You just need a bucket and one poker vibrator. Put an 8 liters of concrete in a bucket and note the time when it is batched. Every hour since the time of batching the concrete in the bucket is vibrated and visually see if the vibrator hole closes or not after taking out the vibrator. A thermometer to measure the concrete temperature everytime the vibration is done would also be good inorder to have an idea of the temperature development of the concrete as this will also give hint on the setting time. As long as the hole closes after lifting the vibrator, then you are confident that the concrete in use is still in its revibration limit. Note though that the bucket should be in the same environmental condition as the concrete section being poured. So next time you are not sure of the revibration time of your concrete, do this simple experiment on site.