Concrete plant moisture probe effectiveness.
But seriously, do I need a moisture probe in my concrete plant? Do I need one for only the sand or also the rock or pea gravel? Ever wondered yourself about the pros and cons? Had trouble getting clear, honest answers? Let’s change that here and now!
Moisture probes were obviously not always part of the concrete industry. So, why do some of your customers (especially state agencies oftentimes) require the use of them today if concrete that was poured 100 years ago without them is still perfectly sound? The true bottom line answer is usually an incomplete understanding of the effectiveness of a moisture probe in the process of most plants. Does that mean we are against moisture probes? Not at all! Instead, we are for education.
The obvious idea of a moisture probe is to detect the moisture in a given material. You typically see them in sand bins because sand generally holds not only the most moisture of all other materials in a mix design of concrete (excluding water of course) but also enough moisture to radically impact the target slump of the concrete. Anyone who has batched concrete for more than a couple of days with or without a batching system can tell you that fluctuations in sand moisture can wreck your slump.
Before moisture probes, you had to take a manual moisture reading by weighing the wet sand, baking it dry, weighing the dry sand and subtracting the difference to get your percentage moisture to adjust your water for the day or period of time until you did another bake off. Early moisture probes did this part for us but were not always accurate and required recalibration very often. Modern moisture probes are far more accurate and require calibration less often (but still you should do it regularly).
The next logical step was for the batch computer to use the moisture probe reading to automatically deduct water based on the amount of moisture in the sand and then compensate by adding back in sand to compensate for the amount that was moisture. (Yes you could do this infinitely but beyond one major adjustment, the returns on effort are generally insignificant). Virtually all batch computers do this today.
So, this should have been the end of moisture fluctuations in concrete then right? If only! While we could debate about moisture in rock and gravel (depending on your area, the material and whether or not you run a sprinkler on your aggregates), the amount and type of admixtures you are using and other factors (including water retention on the loading belt) these all have generally a far less significant impact on the load of concrete. The real culprit in remaining fluctuations in concrete slump is the ready mix truck and driver practices.
If you do not insist your drivers spin out any excess water from their drums before every load, then you have just negated your moisture probe in large part. Simply put, your moisture probe can be accurate and your batching system can compensate diligently, but if your trucks are retaining water then you have just reset your process to worse than pre-moisture probe times because you cannot know the accurate amount of water in the truck regardless of how often you measure your aggregate moisture and compensate.
To make it a math problem, taking 1200 LB per yard of sand for a 10 Cubic Yard load of concrete you have 12000 LB of sand. At 6% moisture, 720 LB of that is actually water. Divide that by 8.34 (water weight varies with temperature but again we are going for impact not splitting hairs) and you have ~86.3 gallons of water trapped in the sand. Considering a total water design in the mix of 32 gallons per yard or 320 gallons for the load, that would be in increase of 27% over designed water. A lot of factors are involved in determining slump but for rough rule of thumb, that’s a change in slump of 8 inches! Now, if we compensate for all of that water, but the driver has 40 gallons riding in his drum, that’s a potential change in slump for a 10 yard load of 4 inches!
Therefore, moisture probe or not, it’s always a good idea to set yourself up for success by starting each batch with known variables and spinning out your drums greatly helps this.