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The Water, Moisture, Cement, Concrete relationship
Why do we need moisture measurement in the sand bins?
Because your mix design assumes that aggregates, cement and water are present in the correct proportions, according to the dry material weights. If your sand contains 10% moisture, when you weigh out 1000 lb, only 900 lb is sand; the rest is water. You can allow for this by estimating or measuring the moisture of the sand and increasing the amount that you weigh in proportion.
If your sand moisture decreases by 2% without being noticed, however, the batching system will weigh out 2% more sand than you require. As a consequence it will also add appreciably less water than needed, making a dry batch. If you correct for this by adding more water, the water/cement ratio will increase, reducing the strength of the product. If the moisture had been measured accurately, the proportions would all have been correct and there would have been no need to add more water.
The aggregate moisture sensor works with your controller to proportion the batch according to the DRY WEIGHT MIX DESIGN.
Why do we still get variations in slump or moisture?
When the controller weighs out the aggregates and cement, small errors occur on each ingredient. If you add the water independently, based on the weights being correct, any error in the weights will give a variation in slump or moisture of the product. You can reduce this by improving the accuracy of batching but it will never be entirely eliminated, especially if batching speed is important. Accurate moisture or slump requires correction of the water added to the mixer.
What is a mixer water dosing system? Why do we need it?
There are two ways to add water to concrete. The first is to meter the water volumetrically or by weight, based on the mix design. This method adds the same amount of water regardless of the actual weight of cement or aggregates in the batch. Variations in slump are the result. If this is not adequate, the second way is to make a moisture measurement on the mix and add the quantity of water required to produce the correct moisture, slump or water/cement ratio.
For critical products we need to eliminate the variations and produce consistent product. We have already seen that measurement of aggregate moisture will not fully correct the final product quality. The conclusion: a water dosing system based on moisture is necessary.
A water dosing system ensures consistent moisture, slump and product quality.
What is a slump meter? Why do we need it?
You can control the water content of slump concrete by monitoring the effort required to turn the mixer. As you add water to the dry ingredients, the effort increases until it gives a “doughy” consistency. Any further increase in water results in a drop in the effort as the mix starts to liquify. This drop is very rapid and is a sensitive measure of the slump. You can measure the effort with a wattmeter in the motor’s electrical circuit. Ammeters have been used with some success, but the current is affected by the power voltage, introducing errors as a result. Read more on Slump Meters
You can easily connect a Watt Transducer via a current transformer and voltage transformer in the motor control cabinet. You can display the resulting power value with a meter or, in the case of BatchTron, by a bar-graph on the touch screen. The reading is repeatable when making the same mix design and batch size but can change if the mix design or batch size changes, making it more difficult to apply than the AquaControl type of system.
Slump metering can give good results when you use the same mix design and batch size for long periods.
Why are some water dosing systems more expensive than others?
There are two methods of water dosing. Direct addition of water while measuring the moisture, and predictive addition, where the system analyzes the semi-dry material first, then adds the water. In direct addition, the system monitors the moisture of the mix while adding the water slowly. The water cuts off when the moisture is correct.
There are two drawbacks to this method. Firstly, it is slow because the water must distribute throughout the material before it can be measured. As a result, it limits the dispensing rate. Secondly, the sensitivity of most moisture sensors, especially the older resistive probes, reduces as the moisture increases, and is very poor at normal dry cast moistures. The result is that it is useless on slump concrete. MicroMix and Fast MicroMix are direct addition systems which use the better microwave sensing method to give accurate results.
In predictive addition, the system makes a moisture measurement on the dry ingredients soon after adding them to the mixer. When it detects a suitably accurate reading, it calculates the amount of water in the batch. Then it calculates the amount still required from the mix design data. As a result, it can add the water quickly, through a volumetric meter or from a weigh scale. Because it measures the moisture in the dry material, sensitivity and accuracy are high.
In addition, you can make any type of concrete in this way. From dry cast to high slump values, with no loss in precision. And, because it can factor-in the actual batched weights of aggregates and cement, it can eliminate all the errors. Speed is higher than the first type, above, but slower than metering the water without feedback. AquaControl using MasterMix is a predictive addition system with all the refinements mentioned here.
Of course, this method requires more equipment than the first and is more expensive as a result. Also, since speed in reading the initial moisture is critical, the software for averaging the wide variations in sensor reading is complex and expensive to develop. If high quality, consistent product is necessary, however, this method is the best. In comparison, slump metering is effective in a limited number of applications and is low in cost.
There is no free lunch; the better systems cost more – but the payback could be impressive!
What do I need for my plant?
It depends on your end product and factors like strength, workability, texture and colour. Some precast producers making dry cast products can manage with no aggregate moisture measurement and only a manual adjustment on metered or weighed water. Others, even wet cast producers, insist on both aggregate moisture measurement and mixer water dosing systems.
Typically, dry cast, block and paver producers need both aggregate moisture measurement and mixer water dosing, with moisture sensor in the mixer. Wet cast and ready mix producers can usually get by with only aggregate moisture sensors but for high quality, high strength products, especially for SCC, a mixer water dosing system is still necessary.
Note that once you install the complete system, you can bypass the water dosing system for less critical products. Consequently you can use metered water with no moisture feedback to gain faster mix times and more production per hour. The BatchTron controller allows this by saving your choice right in the formula.