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When combining raw materials into the mix, you’ll want the right equipment.
By Brian Miller
For a precast plant to perform at its best, it should have the proper batching and mixing equipment. Using outdated and/or improperly maintained equipment typically wastes money and time, and may even place employees at risk. Selecting the right equipment will help optimize your operation.
This article provides a general overview of batching systems, moisture sensors, mixers and proper maintenance of these systems. However, the technology that goes into these systems is much more intricate and constantly evolving. For detailed information on available options and new developments, contact the manufacturers, suppliers and/or consultants in the batching and mixing industry.
Once you establish your mix design, the first task in preparing a concrete mixture is to batch raw materials, which generally include cementitious materials (cement and supplementary cementing materials, or SCMs), water, admixtures, aggregates and sometimes fiber reinforcement. The goal is to collect raw materials in defined measured quantities (referred to as the mix design) and mix them together to a given consistency. The collection process is referred to as batching, which may be done either by weight, volume or a combination of the two. Regardless of which batching method you use, the equipment should perform within the permissible tolerances. These tolerances are typically a part of project specifications and/or noted in the “NPCA Quality Control Manual for Precast Concrete Plants.”
Cementitious materials are usually weighed in a separate hopper. Cement is commonly moved by means of a screw drive or auger, and sometimes pneumatically. Cement silos may also be located above a weigh hopper using gravity to load the hopper. Typically, cement is weighed first along with any supplementary cementitious materials in the same hopper. It is important when installing or replacing a cement auger that the auger is designed for use with cement. Retrofitting old grain augers, etc., is not advisable. This can lead to excessive wear and less than optimum performance. Also, silos should be inspected routinely to ensure there are no holes or water leaks. Moisture will activate the cement, which will waste material and possibly lead to system shutdown.
Water may be batched by volume or weight. Volumetric batching or metering of water is common, usually using a flow meter. The flow meter measures the volume of water that passes through the valve. One concern is that a flow meter does not distinguish the difference between air and water, and therefore some error may be introduced. Error may also result from leaky valves that allow water to dribble through when they are closed.
Considering the importance of water-cementitious ratio (w/c ratio), make every effort to ensure the accuracy of the water content. The most accurate method for measuring water is by weight as an alternative to a flow-meter.
Admixtures are typically batched by volume through a dispenser. Usually an admixture supplier will provide the dispensers at no cost to the producer, so you should investigate this option. This should reduce the likelihood of improper dosing. Be sure to introduce admixtures into the concrete mixture in accordance with the manufacturers’ recommendations.
Some producers batch admixtures by hand. Use caution when batching admixtures by hand in full production, since doing so increases the risks of contamination, improper dosage and use of the wrong admixture.
Most precasters batch aggregates by weight. Aggregates can be weighed in hoppers, on weigh belts, or in or directly from aggregate storage bins. Hoppers have load cells attached to them to measure the weight of the aggregates. Typically aggregates are moved to the weigh hopper by conveyor belts and weighed cumulatively.
Weigh belts are being used more often in the precast industry. However, the term “weigh belt” is a misnomer. A true “weigh belt” weighs while it is moving, but many batch plants use live bottom-weigh hoppers (LBWHs), whose belt is stationary when weighing aggregates. This type of belt may be started under load to move the weighed aggregates to a transfer belt, hopper or skip hoist. (A skip hoist is a type of hopper that collects all weighed materials and then disburses them into the mixer. The advantage is that another batch can be weighed and waiting in the skip hoist to go into the mixer immediately after the previous batch is discharged. This can save time and increase output.)
Some systems may also incorporate adjustable gates. These are knife-like gates that help blend aggregates on the belt before getting to the mixer, thereby reducing mixing time and improving homogeneity of the mixture.
Storage bins may also have load cells attached to them that weigh the amount of aggregate remaining in the bin. The difference between the starting and final weights is assumed to be the aggregate weight going into the mixer. This is called a decumulative weighing system and is an indirect method of measuring the aggregates to be batched. This type of system is usually done with smaller batching systems and reduces costs by eliminating additional gates, weigh hoppers and LBWHs. The theory of this mechanism assumes that all the aggregates leaving the bin go into the mixer. This may not be accurate, since aggregates can deflect or ricochet elsewhere from freefall.
Volumetric measuring is typically used in continuous mixing systems. There is also volumetric batching, which is another means of measuring raw materials for a single batch. Concrete produced by volumetric batching should meet ASTM C 685, “Standard Specification for Concrete Made by Volumetric Batching and Continuous Mixing.” This type of batching is used in some precast operations. It should be noted that while volumetric batching can produce good quality concrete, it is not yet universally accepted.
Batching equipment should be maintained and operated in accordance with ASTM C 94 or ASTM C 685. Hoppers and scales should be calibrated annually or whenever there is reason to question their accuracy. Refer to the “NPCA Quality Control Manual for Precast Concrete Plants” for more details.
It is very important to make adjustments for the moisture in the aggregates, especially the fine aggregate or sand. Surface moisture in sand typically can contribute large amounts of water, which dramatically increases the w/c ratio and weakens the concrete. Usually a moisture probe is located in the sand bin(s) to determine the sand’s moisture content. The weight of moisture-laden sand by volume will be heavier than normal, so you must add more sand to attain the required volume and then reduce the amount of mix water as necessary.
Another probe is typically located in the mixer. This sensor corrects for the additional moisture from both fine and coarse aggregates in the mixer. Another concern is that when aggregates are less than Saturated Surface Dry (SSD), more water should be added to compensate for the aggregates’ absorption and achieve SSD. SSD is the condition when aggregates will neither absorb nor give off moisture. This condition is not generally found outside the laboratory and is used for calculation purposes. It is important to note that moisture probes measure the total amount of water in aggregates or concrete. As such, adjustments should also be made for absorption of the aggregate.
There are two common types of probes or sensors:
Electrical resistant probes
This type of system uses a set of two probes working together. Additional sets of probes may be used as well. These can be installed in aggregate bins and/or mixers. They work by sending a small electrical charge into the aggregates and measuring the strength of the charge at the other probe. These devices do not measure the amount of water directly, since water does not conduct electricity. The charge is conducted through minerals or contaminates in the water. Therefore, a change in mineral content will change the conductivity for the same amount of water, thereby changing the measurement. This is a potential problem when using city water, since the mineral content may vary or change without notice.
The most common type of moisture sensor today is a microwave probe. These can be installed in aggregate bins and/or mixers. Originally developed by NASA, microwaves emit frequencies that spin the hydrogen atoms in water molecules and thus generate heat. Moisture sensors emit a higher-frequency microwave into the aggregate. These waves begin to spin the hydrogen atoms just slightly, and the sensors measure the transmission of energy required to do this. As moisture increases, the sensor absorbs more waves and transmit greater energy. Essentially, these measure the true number of hydrogen atoms and therefore the water content.
Microwave probes require complete calibration upon initial installation. This calibration requires several measurements from dry to wet aggregates and correlated to probe measurements. Follow-up verification testing should be performed every couple of days to confirm that the correlation of the measurement to moisture content is still accurate. See the “NPCA Quality Control Manual for Precast Concrete Plants” for more details on calibration frequency requirements.
Mixing and mixers
All mixing should be performed in accordance with ASTM C 94 or ASTM C 685.
When discharging materials into the mixer, take care that all of the weighed raw materials go into the mixer, especially the cementitious materials. The mixer should be in motion while discharging raw materials. This reduces wear and tear on the mixer. If startup under full load is required, verify that your mixer is designed for it.
The feeding sequence is also important. Typically you will add aggregates and some water, followed by cementitious materials. Admixtures are added to the batch at varying times, depending on the type. For example, water reducers are usually added with the mix water (up front), while a superplasticizer is added at the end of the mix cycle. As always, follow the manufacturer’s recommendations.
Selecting a mixer
When replacing a mixer, you may not need a larger one. The newer mixers generally are faster than their predecessors. A new 1-yard mixer will most likely have a greater hourly output compared to an older mixer of the same size. This means that its discharge-to-discharge time is less (the total time between discharges of the concrete mixture into the delivery system). This includes batching, mixing and cleaning. Some mixer manufacturers’ published cycle times are based on the mixing cycle only and do not take batching into account. It is important to clarify this when selecting a mixer and determining which one is right for you. Keep in mind that discharge-to-discharge time includes the raw material feed time as well – updating only the mixer will not improve discharge times if the batching system is too slow.
There are many things to consider when determining mixer size, such as the total concrete required each day, labor capacity, labor costs, future capacity and growth, and total volume of concrete needed at peak production, the latter of which may be the most important. For example, if two plants require 200 cubic yards of concrete per day, but plant A requires 200 cubic yards of concrete in four hours and plant B requires 200 cubic yards in eight hours, their peak-hour demands are different. Plant A has the peak-hour demand of 50 cubic yards per hour, whereas plant B has a peak-hour demand of 25 cubic yards per hour. If plant A based its mixer size on peak-hour demand, a 1-yard that mixer’s maximum discharge-to-discharge time would need to be one minute and 12 seconds. However, if the plant used a 2-yard mixer, the mixer’s maximum discharge-to-discharge time would need to be two minutes and 24 seconds. In this example, plant A would most likely need the 2-yard mixer.
Here is another important note: Most mixers’ batch capacities are actually about 67 percent (or two-thirds) their total volume. Therefore, if a plant needs to batch 1 yard of concrete, the mixer’s total volume would need to be 1.5 cubic yards. It is important to know how a mixer is rated, either by batch volume or total volume.
It is also important to evaluate the entire production process when identifying limiting factors that will likely control the production rate. Usually more than one combination of components or mixing systems will meet a plant’s requirements. You may consider talking with batch plant design consultants and manufacturers to optimize your plant’s performance and maximize your return on investment.
Types of mixers
There are two categories of mixers: continuous and batch. Continuous mixers provide a steady stream of concrete. These systems commonly incorporate the use of a screw drive that continuously mixes raw materials together as they progress through the screw. Volumetric measuring does not directly account for aggregate moisture content, nor does it measure aggregate density changes. Adjustments must be made for these, so frequent calibrations are usually required. Volumetric continuous mixing is not used for dry-cast operations.
Batch mixers are more common for most precast operations, and there are several types. These include rotating drum, horizontal shaft (paddle, spiral or ribbon blade), pan and twin shaft mixers. A description of each type of mixer follows, and includes information about the suitability for the type of concrete mix used.
The term “wet-cast” generally refers to concrete that has an initial w/c ratio of 0.40 or higher. Concrete with lower w/c ratios behave as a “dry-cast” or no-slump mixes. This usually includes superplasticized mixtures. It is important to note that when a superplasticizer is used, generating slumps in excess of 9 inches, you must first mix the batch thoroughly before adding the superplasticizer. Since these concrete mixtures typically have w/c ratios of 0.40 or less, they are initially a low-slump or dry-cast type of mix.
This type of mixer is one of the oldest and consists of a large rotating drum with fixed blades on the drum and/or down the center. The mixing action actually segregates the materials and then drops them back on top of each other. Rotating drum mixers typically are mounted on trucks for the ready-mixed concrete industry and should be used for wet-cast concrete only. With this type of mixing action, dry-cast concrete will usually clump together. Also, this type of mixer cannot have a moisture sensor attached to it due to the rotating drum. Some precasters use ready-mixed trucks to mix and move concrete to the forms, or they may purchase concrete from a ready-mixed supplier. If you are purchasing ready-mixed concrete, make sure that the producer is certified by the National Ready Mixed Concrete Association (NRMCA) or state DOT and has all required documentation. See the “NPCA Quality Control Manual for Precast Plants” for more details.
With this type of mixer, the shaft and blades (or paddles) rotate through the concrete mix, and the container or drum remains fixed. This was the second innovation of mixers during the early 1900s. While these mixers have double the mixing intensity of a drum mixer, they may require the same amount of time to thoroughly produce a mix. Horizontal shaft mixers include:
Pan mixers consist of a large pan with a flat bottom. The gearing system may be located either in the center of the pan or out of the mixing area, usually overhead or to the side. These are the most common type of mixers used in the precast industry and are used for both wet-cast and dry-cast concrete mixes. Pan mixers may have a stationary (fixed) or rotating pan. Stationary pans can have multiple discharge gates and moisture control devices placed in the bottom of the pan since they are fixed. Rotating pans have centrically fixed discharge gates and use fixed stick-type moisture sensors.
There are two primary types of mixing action in pan mixers: counter-current and compulsory. The paddles in a counter-current mixer rotate in opposite directions, creating a counter-current mixing flow. This may also be achieved with a rotating pan. In a compulsory mixer, materials are essentially thrown into each other at high rates of speed.
Pan mixers come in two varieties:
These mixers utilize two horizontal shafts with paddles on them. These mixers are very good for wet-cast, but are not commonly used for dry-cast. However, some producers have had success with dry-cast applications. Although twin shaft mixers mix very quickly, they require cleaning after every mix. Twin shaft mixers create a “zone” where material is suspended and compulsory-mixed into each other. This allows for very well-mixed concrete.
Overall, money and time invested into a well-engineered and maintained batching and mixing system are good investments. This is important to optimize the performance of plant operations and prepare for future growth. For more information on batch plant specifications, see ACI 304, “Guide for Measuring, Mixing, Transporting, and Placing Concrete.” You can also attend NPCA’s Production and Quality School or contact NPCA members who consult, engineer, manufacture and sell batching and mixing systems.