Slump can be tested for acceptance at the point of discharge or point of placement. This small detail can have large implications. So where should you test concrete slump?
The point of discharge is at the end of the truck chute. The point of placement is at the final location of the concrete, typically at the end of the pump. The slump, air, and temperature of the concrete can change during pumping. Therefore, concrete producers may need to deliver concrete that is out of specification (slump and air too high, for example) so that it will be in specification after pumping.
[End of truck chute]
The concrete producer would generally prefer to test at the point of discharge. After the concrete leaves the truck, it is usually out of the producer’s control. Each job has different placement conditions, so the need to custom adjust concrete for each job can create a significant workload. In addition, the point of placement can be difficult to access, making sampling and testing difficult.
The contractor is generally concerned with both the point of discharge and point of placement. The point of placement is where concrete will be finished, which can dramatically affect contractor productivity. However, the contractor should also be concerned with point of discharge so that concrete can be pumped quickly and without blockage. The contractor needs the concrete producer to take changes during pumping into consideration when proportioning a mixture, something that can be achieved by testing for acceptance at the point of placement. The American Society of Concrete Contractors (ASCC) Position Statement #20 recommends testing for acceptance at the point of discharge, with additional samples at the point of placement to establish the effect of placement on concrete properties.
Engineers are similarly more likely to be concerned with properties at point of placement. Slump at the jobsite is not an indicator of strength, water content, or segregation resistance and should be a concern primarily of the contractor. Properties like temperature and air content should be the consideration of the engineer.
In reality, both point of placement and point of discharge are important. The engineer, contractor, and producer should closely coordinate throughout construction to ensure properties are correct at both locations.
Testing at the point of placement can create significantly more work for the concrete producer. Most dispatch and batch software programs have only one field for one slump, and no way to indicate whether that slump is at point of placement or point of discharge. (It’s usually point of placement.) Therefore, adjusting the concrete to account for point of placement is often a manual process that occurs outside of these software applications. The producer must have an internal process in place to manage each project to account for point of placement testing. Dispatchers, sales, and QC must closely coordinate with contractors to determine the properties needed at both point of discharge and point of placement. Changes to target properties at discharge over the course of the project are not uncommon.
Verifi can help by ensuring the concrete is consistent and fully documented at the point of discharge. Contractors should closely watch the properties of concrete indicated by Verifi at discharge and immediately contact their concrete producer if they need changes.