Why do we need moisture sensors in the sand bins? What is the effect on the concrete? Why do we still get variations in slump or moisture? What is a mixer water dosing system? What is a slump meter? Why do we need them? Why are some more expensive than others? What do I need for my plant? This page answers these questions and others.
Why do we need moisture sensors 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 and will 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 ensures that the batch is proportioned according to the DRY WEIGHT MIX DESIGN.
Why do we still get variations in slump or moisture? When the aggregates and cement are weighed out, small errors occur on each ingredient. The water, if added independently, is based on the weights being correct. Any error on the dry materials, therefore, will result in an error on the mix design and produce a variation in slump or moisture of the product. This can be reduced by improving the accuracy of batching but 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, based on the mix design. This method neglects the error in batched weights, adding 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 monitor the moisture of the mix and add the quantity of water required to produce the correct moisture, slump or water/cement ratio. For critical products we need this 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.
A water dosing system ensures consistent moisture, slump and product quality.
What is a slump meter? The water content of slump concrete can be controlled by monitoring the effort required to turn the mixer. As water is added to the dry ingredients, the effort increases until a "doughy" consistency is attained. 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. The effort can easily be measured 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. A Watt Transducer is easily connected via a current transformer and voltage transformer in the motor control cabinet. Indication can be by 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 MixTron type of system.
Slump metering can give good results when the same mix design and batch size are used 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 semi-dry material is analyzed first, then the water is added. In direct addition, the moisture of the mix is monitored while the water is added slowly; the water is cut off when the moisture is correct. There are two drawbacks to this method; first, it is slow because the water must be mixed in before it can be measured, which limits the speed at which it can be added. Second, 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. It is useless on slump concrete. MicroMix is a direct addition system which uses the better microwave sensing method to give accurate results.
In predictive addition, the moisture of the dry ingredients is measured as soon as they are added to the mixer. When a suitably accurate reading is acquired, the amount of water in the batch is calculated and, from the mix design data, the amount still required is calculated. The water can then be added quickly, through a volumetric meter or from a weigh scale. Because the measurement is done on the dry material, sensitivity is at its highest and accuracy is maximized. In addition, any type of concrete can be made in this way, from dry cast to high slump values, with no loss in precision. And, because the actual batched weights of aggregates and cement can be factored into the calculation, all the errors can be eliminated. Speed is higher than the first type, above, but slower than metering the water in without feedback. AquaControl and MasterMix are predictive addition systems 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. 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 sensors and only a manual adjustment on metered or weighed water. Others, even wet cast producers, insist on both aggregate moisture sensors and mixer water dosing systems. Typically, dry cast, block and paver producers need both aggregate moisture sensors and mixer water dosing, usually of the MicroMix, MasterMix or AquaControl types. 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 the complete system is installed, the water dosing system can be bypassed for less critical products, using 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.