Maintaining quality of your raw materials affects the health of your products.
By Dean Frank, P.E.
In a sense, raw materials used in concrete are just like the food we eat: They should be of good quality, they must be stored and handled properly, and they must not be contaminated. The old adage “You are what you eat” comes to mind, because the quality of raw materials going into a concrete mix has a direct effect on how the final product will turn out – in both fresh and hardened states. With that in mind, the storage and handling of raw materials will play an important role in determining a plant’s layout and equipment needs.
Let’s take a look at storage and handling requirements of concrete raw materials one at a time.
When designing a plant’s layout, the first thing to consider is the actual amount of storage space required. Determine the number, sizes and kinds of aggregates that will be used, taking possible future needs into consideration. Most plants carry at least a one-day supply of aggregate, depending on a number of factors, including availability and haul distance. Only one size aggregate should be stored in each storage bin or stockpile. Allow enough space to store each of the various types of aggregates you’ll need in sufficient quantity so that production schedules are not interrupted if you experience a delay in aggregate delivery. The last thing you want is to have your production personnel waiting around for concrete because you ran out of raw materials.
Aggregates are generally stored in open stockpiles on the ground or in bins above or below the ground. Regardless of the storage method, it is very important to prevent contamination and segregation of aggregate particles.
Possible sources of contamination include:
- Allowing adjacent aggregate stockpiles to overlap, causing
- Scooping up underlying soil when using a front-end loader to move aggregate from a stockpile
- Dumping the wrong size aggregate in a bin or pile
- Aggregate leakage through or around bulkheads in storage bins
- Allowing leaves and other contaminants to fall into the aggregate stockpile
- Allowing vegetation to grow in the aggregate stockpile
- Failure to let the receiving hopper and conveyor belt or elevator run until empty before adding a different size aggregate
- The supplier may have contaminated the aggregate or delivered the wrong aggregate
In order to prevent in-plant contamination, locate your ground-level aggregate stockpiles on a hard base with good drainage, preferably on slabs or planking. Keep them far enough apart or separate them with dividing walls of sufficient height to prevent cross-contamination. Do not drive trucks and loaders on aggregate stockpiles because their weight can break the aggregate particles and track mud and dirt into the piles. It may be necessary to cover the storage area to prevent contamination.
Segregation of aggregate is the separation of the course particles from the fine particles. Aggregates with a wide range of particle sizes tend to be most susceptible to segregation. This being the case, segregation of sand – especially damp sand – is generally not a problem. Aggregates should not be stored in large conical piles, since there is a potential for the course aggregate to roll out and separate from the smaller particles. It is better to store aggregate in small piles or in horizontal layers as there will be less tendency to segregate. In addition, the type of aggregate has an effect on the probability of segregation. Crushed aggregate is less likely to segregate than rounded aggregate. Finally, when discharging aggregate into bins, do it from directly above and onto the center of the pile. Discharging aggregate against the side of a bin or a baffle will tend to cause segregation.
Storage bins can be square or circular. However, it is very important that their bottoms slope not less than 50 degrees from horizontal on all directions toward a center outlet. In square storage bins, avoid flat bottoms and angular corners. Use rounded corners instead, ensuring that all material moves toward the outlet. Plan to keep the storage bins as full as practical in order to minimize possible changes in the gradation caused by withdrawal of the material and by breakage of the aggregate particles.
In cold climates, the preferred method of storage is in covered overhead or underground bins. In many cases, plants must use heaters to maintain a reasonable aggregate temperature and prevent freezing.
Controlling moisture in aggregate stock is important, since the surface moisture on aggregate particles can add a significant amount of water to the mix, which must be accounted for in the batching process in order to maintain a constant water-cementitious ratio. The easiest way to maintain the moisture content is to sprinkle the aggregates with water, keeping them in a constantly saturated condition. Moisture probes can be placed in the aggregate bins to automatically measure the amount of free surface moisture in the aggregates being batched. These probes are generally limited to the sand bins, since sand has more surface area than coarse aggregate and consequently contributes the majority of the extra water to the mix. In addition, the coarse aggregate has a tendency to be very rough on the probes, limiting their usefulness. Be sure to coordinate with your equipment supplier to best determine your needs.
Aggregate bins are generally filled using front-end loaders, conveyor belts, elevators or traveling buckets. All of these methods are acceptable. Your choice will depend on your needs. Front-end loaders are an option because they have multiple uses both inside and outside the plant; however, they must be manned and cannot fill bins that are higher than their reach. As such, overhead bins are usually filled by conveyor belts, elevators or traveling buckets. All of the components of these systems must be properly sized, dependable and able to restart when fully loaded.
Be sure to adequately shield dry, fine aggregate from the wind during handling in order to prevent the wind from separating the fine from the coarse material or blowing it away altogether.
Finally, since aggregate handling tends to increase the possibility of undesirable segregation, design your plant layout and equipment to keep handling operations to a minimum.
The majority of precast concrete plants today employ bulk cement in their operations. Bulk cement is stored in circular or square silos made of concrete or steel. Given that cement is sensitive to moisture and must remain dry until used, the silos must be watertight. If kept dry, cement will retain its quality for a very long time. However, if it is stored in the presence of damp air or moisture, it will lose some of its quality and set more slowly and with less strength than dry cement.
Be sure to talk to the silo manufacturer about the capacity of the silo. Because the cement tends to fluff during transfer into the silo, the actual capacity of the silo may be only 80 percent of the rated capacity. Most plants carry at least a two-day’s supply of cement. Given the unpredictable nature of cement supply and delivery lately, it would be wise to carry as much as possible.
Use vibration or dry, low-pressure aeration to keep the cement free-flowing and to prevent arching or bridging inside the silo, which can get clogged. In addition, silos should be drawn down every few months in order to prevent caking of older cement around the perimeter of the silo.
Bulk cement is often delivered by truck in trailers designed to be pressurized, using air to fluidize the cement and move it through a pipe or hose into the silo. Although cement is a fairly dense material, the particles are so small that it is possible to aerate and suspend the particles in a moving stream of air. Cement may also be delivered in bottom-dump trailers that use gravity to move the cement. Other options include screw conveyors and bucket elevators. Cement is generally moved from the truck and into the storage silo using one or more of these transport systems.
Finally, be sure to clearly mark the delivery location to ensure the correct material is delivered to the silo.
Supplementary cementitious materials
Equipment required for handling and storing of bulk supplementary cementitious materials (SCMs) such as fly ash, slag and silica fume is similar to that required for cement. However, it may be necessary to modify handling and storage equipment used with silica fume because it does not have the same flowing characteristics as other SCMs.
Although moisture will not affect the performance of some SCMs, they should be protected from moisture nonetheless to reduce problems with handling.
As with cement delivery locations, all SCM delivery locations should be clearly marked to ensure that contamination with another material does not occur. This is sometimes compounded by the fact that SCMs and cement sometimes have a similar appearance.
Most concrete admixtures come either in powder or liquid form. Both types must be protected from contamination.
Powdered admixtures can be stored in bulk bins or may be delivered in bags. Powdered admixtures must be protected from moisture, but may not be as sensitive to temperature fluxuations.
Liquid admixtures are generally stored in bulk containers or tanks. Clearly mark the tanks to avoid contamination and ensure appropriate use. In addition, do not allow liquid admixtures to freeze; store them in a heated enclosure or as required by the admixture supplier. Some admixtures will tend to separate over time, so you should stir them with agitators or circulating pumps prior to use.
The admixture supplier will generally supply dispensing equipment that can be used when batching. Keep this equipment near the batching location where it is easily visible and accessible for cleaning and adjustment.
Handling and storing reinforcing steel is fairly simple and does not require much in terms of equipment. If you receive reinforcement in long bundles, use a spreader bar to lift the bundles and prevent unintentional bending of the bars.
Like all of the other raw materials, store reinforcement in a manner that prevents contamination. The steel should be reasonably clean when it is placed into concrete. A light amount of rust is acceptable, but not to the extent that it is pitted or the cross-sectional area is reduced.
Reinforcement should be stored on some sort of blocking or racks, not directly on the ground. For extended storage periods, protect the steel from the weather.
You are what you eat
You should be just as concerned about how your concrete raw materials are handled and stored as you are about how your food is handled and stored.
You can expect to produce quality products only when using quality raw materials. The way in which you handle and store those materials has a direct effect on product quality.
Be sure to contact multiple equipment manufacturers in order to evaluate your options and determine the best equipment solution for your needs. They care about the quality of your products as much as anybody.
An easy way to visit with quality equipment manufacturers is by attending industry trade shows and expositions, where you can do all of your comparison shopping in one location while taking advantage of the numerous educational and networking opportunities. With technology constantly changing, it is hard to stay up to speed any other way.
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