Concrete mixture is made of one or more of sand, aggregate, cement, pozzolans, and other materials and is transported on a conveyor assembly from bins or silos to an area where water and other additives are incorporated into the concrete mixture to homogenize the mixture in preparation for installation and curing. The sand and other aggregates naturally contain a varying amount of water and determining the amount of water already existing in the concrete mixture is important so that the proper amounts of water can be added later during homogenization. It is well known that the water to cement ratio is directly related to the concrete product strength. Improper amounts of water added during the mixing process can impact the curing time, strength, durability, and appearance of the cured concrete. In some instances, entire batches of concrete must be destroyed or sold as cull product if the water content is not appropriately controlled.
Under current methods, water content of the mixture moving on the conveyor is estimated by random sampling on the run and by direct measurement of water in sand and aggregate stockpiles. Usually, such measurements are made once or twice a day and cannot estimate the variability of the water in the stock pile from surface drying during the day and/or rainfall that may occur. Such measurements also take a longer time, usually requiring about half an hour or more. Typically, the operator takes a select amount from the stock pile, weighs the amount, dries it on an electric or gas stove top and finds the mass lost to evaporation. The mass lost is determined as a gravimetrical percent moisture based on dry or wet mass of initial total aggregate.
In another method widely used in industry for determining water content, a probe is directly buried in sand or sand and aggregate within bins and/or hoppers. The material may be held stationary or may flow out of the bin past the probe or hopper to a conveyor belt. The method relies on the correlation of the dielectric properties of the water content of the material. The probe measures the dielectric properties of the material, and, based on a calibration, the water content of the material is determined. This method has limited accuracy in estimating the bulk water content of the material due to following draw backs: 1) when the material is stationary, the measurement volume of the probe is small compared to the majority of the material volume, resulting in an inaccurate detection of water in the bulk volume of the sample and 2) when material is flowing past the probe, the flow characteristics such as random air pockets, density variations, and turbulent material flow significantly increase the variance, reducing the average values, and leading to inaccurate water content data.
Nuclear and non-nuclear methods have been used to measure water content of construction materials for more than five decades. One such method described in U.S. Pat. No. 3,213,280 incorporates a neutron source and a slow neutron detector that utilized the fast neutron thermalizing effect or slowing down effect of hydrogen to measure water content in sand. The method described there was to measure the water content of sand used for molds and cores. The neutron source and the detector were placed inside a cylindrical probe. In calibration, the probe was buried in a container of carefully measured dimensions filled with sand so that the measurement volume of the probe covers most of the volume of sand in the container. Although this method is good for that particular application, the measurement volume is still a small fraction of the volume of bins and hoppers used in concrete plants. Chemical composition errors remained in the systems as well as density errors associated with moisture values.
Methods that determine water content of a concrete mixture when the mixture is moving on a conveyor belt have the advantage of estimating water content of a large integral fraction of the mixture. When the mixture has water distributed non-uniformly, water content estimate for the bulk volume has better accuracy with “on the run” averaging. Furthermore, due to the nature of concrete plant operation, at a given measurement position or location, height, mass, and density of the mixture moving on the belt varies with time. When determining the water content, methods should be used to compensate for such variations.
To compensate moisture measurements for the variation in height and mass of the mixture moving on the belt with time, one practice may be to use two or more independent methods for determining the height or mass thickness, and a quantity related to the water content of the mixture. Thereafter, physical relationships between the height or mass thickness, and a quantity related to water content are used to estimate or obtain direct measurements of the water content of mixture.
In nuclear techniques based on this proposed method, gamma-ray techniques are used to measure height or mass thickness and neutron techniques are used to measure a quantity related to water content of the mixture. Such methods are described in a report by Muller, R. H. (1963), Anal. Chem., Vol. 35(1), pp 99A-101A, and U.S. Pat. Nos. 3,255,975, 3,431,415, 3,748,473, 3,955,087, 4,362,939, and 4,884,288. Problems with the previous such methods is that they use nuclear radiation sources of large strength or activity, may have mechanical constraints to keep the mixture passing near the gauge at a constant height, and use specialized nuclear radiation detectors and sources, and complex electronic circuitry that were problematic for plant maintenance. Furthermore, error corrections on the fly associated with chemical composition, real-time corrections to flow discontinuities associated with random material height, thickness, density, and mass thickness, linked to belt speed and separation of detectors, are not fully described in previous art.
A need therefore exists for a method or solution that addresses these disadvantages and provides a real-time assessment of the concrete mixture. This solution can produce real time or near real time data, averaged, integral, and filtered results instantaneously.