THE LAB TRIAL
• Test for compatibility and make adjustments
THE PLANT TRIAL
• Test in uncontrolled conditions, and adjust accordingly to wind and humidity variances
Even if you are able to use the same raw materials in your lab trial that will be used on-site there are still variables that cannot be duplicated from the field. These differences include:
• Air temperature in the lab is always controlled (23° +/- 2° C)
• The wind and humidity are controlled
• Materials are used when ready – there is no delay in driving the concrete to the site
• Timing is controlled – you can add materials when you want
• Time of day variance is not present – on the jobsite properties such as temperature and moisture content of materials can change from morning to night
Because of the above differences it is always good practice to perform a plant trial following a lab trial. Between lab and plant trials the types of testing are similar; most commonly slump, air content and compressive strength tests are performed on each batch of concrete over three days.
The sampling frequency for some tests in plant trials is different than lab trials due to the volume of concrete produced. For example, samples of concrete are taken at the beginning, middle, and end point of the discharge from the mixer for testing. At a plant where concrete is batched in a central mixer and mixed in a truck mixer, testing should be performed on several different truck mixers to evaluate consistency.