Proper planning and intelligent equipment selection are the first and fundamental steps before construction begins.
By Mel Marshall, P.Eng.
Regardless of whether you are considering entering the precast concrete industry, entering a new marketing area or expanding an existing facility, a number of factors will be a part of your decision-making process. The amount of money that you want to spend, and the amount that you can afford to spend, are not always the same.
One of your first decisions needs to be about the products that you intend to manufacture. You will want to conduct a market survey of the proposed location for your new facility to determine how many other precast concrete manufacturers, if any, are active in the market that you wish to enter. Keep in mind that even though there may be no producers in the area, that region may be serviced by precasters from remote locations. Several horror stories exist about folks who invested a great deal of money without doing a proper market survey to determine whether or not there was a need for the products they planned to produce.
Another early decision that must be made is the size and configuration of the property, as well as the size and shape of the building (if one is required). The building should be sized to provide suitable clearances around machinery and production areas to provide a safe environment and efficient operation. The location of the building on the property is important to minimize product handling, maximize yard storage, and ensure the efficient flow of trucks hauling raw materials onto the property and finished product away to the job site.
Having determined a market requirement for specific products, one of the initial points you will want to consider is how you are going to precast those products. Will you wet-cast or dry-cast? These two basic methods of manufacturing precast products involve different types of manufacturing equipment.
Wet-cast generally refers to concrete products that have a water/cement (w/c) ratio greater than 0.45, or a lower w/c ratio that uses a superplasticizer to make it flow. Because wet-cast concrete is very fluid and flows easily, one form set per piece of precast product is required to contain the concrete. After the concrete has set and sufficiently cured, the forms are stripped from the product and reused.
Although the investment in form equipment can be substantial, the cost is directly proportional to the number of pieces that you want to produce each day. Many concrete manufacturers start out with just a few forms, and then add to their form inventory as market demands increase.
One advantage of wet-casting is that it is not necessary for the precaster to invest in a concrete batch plant. Quality ready-mix concrete is readily available from a large number of suppliers, so utilizing this concrete source can be very effective, particularly during your formative years. As sales increase and your concrete requirement increases, there may be financial justification to construct your own concrete batch plant. With regards to ready-mix concrete, you will want to consider constructing your production facility as close to a ready-mix operation as possible. In fact, some wet-cast precasters have set up their production facility on the property of the ready-mix supplier, when space is available. In these situations, the ready-mix supplier will frequently offer free rent to secure the supply of ready-mix to the precaster.
Depending on the climatic conditions, it may or may not be necessary to construct a building to house the production process. Many wet-casters pour their products outdoors, while others erect a structure with a roof but no walls to offer some protection from the elements. If you are going to produce outside, however, it is important to protect the freshly poured product from the elements such as rain, direct sunshine and wind.
Another major consideration is the method of pouring concrete into the forms, as well as handling the forms and finished product. Concrete that is poured directly from a ready-mix truck or concrete batch plant into a concrete bucket can be distributed to the forms by forklifts or an overhead crane. Larger production facilities often use flying buckets to carry the concrete from the mixer to concrete-holding hoppers located at the respective production stations.
You will want to consider how you are going to vibrate the product. Vibrators are powered with electricity, pneumatics or hydraulics, and are available in a number of different sizes and shapes. A common method of internal vibration is to utilize stick vibrators (stinger vibrators) that are inserted into the concrete between the inner and outer forms. This can be an effective method, but it requires a significant labor cost in order to achieve consolidation.
Form vibration is less labor intensive than stick vibration, but it requires the mounting of vibrators on the forms. Although there is an investment in vibrators, labor is reduced and productivity is increased. It is important to mount the vibrators on brackets welded to the form stiffeners, rather than mounting them directly onto the skin of the form. Precasters using this vibration method need to work closely with their form and vibrator suppliers to ensure that the forms are of sufficient strength, and that the vibrators are located in the right positions on the form.
Table vibration is very effective for flat products, and it is increasing in popularity for smaller products as well. This system involves vibrators mounted below the table to impart vibratory forces to the product.
Self-consolidating concrete (SCC) has become very popular, as it has proven to be very effective for wet-cast production. This material utilizes fourth-generation superplasticizers (known as polycarboxylate admixes) and Viscosity Modifying Agents (VMAs) to produce a concrete that is almost as fluid as water but does not segregate. SCC is so fluid that it is measured by slump flow (spread) rather than slump. The spread can vary from about 20 to 32 in., depending on the product. Because SCC is self-leveling and consolidates under its own weight, it pours easily and quickly without the need for vibration. The finished product will have virtually no bug holes.
Dry-cast concrete is a zero-slump concrete with a w/c ratio below 0.4, but commonly in the range of 0.30 to 0.36. Because this concrete is very stiff, it is possible to strip it immediately from the form. Dry-cast production equipment is used to consolidate these products sufficiently so that the concrete can support its own weight when placed in the curing area.
A number of different companies manufacture dry-cast systems for the production of concrete pipe, manholes, box culverts, grade rings and a number of other products. Because it is not possible to pour dry-cast concrete from a ready-mix truck, a concrete batch plant is a necessity for this type of production. Also, a building structure is required for dry-cast production, although there are a few exceptions in warm areas.
The major advantage with dry-casting is that, although investment is required in a dry-cast machine, only one form set is required for the whole day’s production of a specific product. This is, as mentioned above, because of the concrete stiffness and effective consolidation. The labor cost of dry-cast products can be significantly lower than that of comparable wet-cast production but with a higher rate of output.
Dry-cast systems are available with different degrees of labor input, varying from manual to fully automated systems. Generally, investment is made in a dry-cast facility only when there is a large demand for the items being produced.
Depending on the type of machinery, off-bearing and stripping of product will be handled with overhead cranes, forklifts or automated handling equipment.
If you are going to be batching your own concrete, you will need to consider the size and types of aggregates you will be using, and suitably size the aggregate bins to match the requirements of your mixer. Also think about the number of cement silos you will need to hold cement, fly ash, slag and blended cements. Be sure to size the mixer properly. The size of the mixer can be dependent upon the quantity of concrete you pour per hour, or the size of the pouring bucket (for wet-cast products) that you plan to use. The larger the pouring bucket, the more concrete you can pour at one time. For example, if you are going to pour large utility boxes, you would likely consider a larger pouring bucket than if you are going to pour concrete into smaller forms such as small meter boxes.
Curing is an important part of concrete production that is all too frequently overlooked. Keep in mind that curing (the hydration reaction between the cementitious materials and water) is essential to the formation of concrete. It is, therefore, imperative that the moisture not escape from the concrete product during the curing process so that sufficient moisture will hydrate virtually every particle of cement. If adequate curing does not occur, it will not be possible to achieve the mix design strengths. Concrete products must be protected from wind and direct sunlight during the curing process.
Although not necessary, we strongly recommend the inclusion of a QC testing facility at your new plant. It is possible to arrange for testing at independent laboratories, but this can be costly and not nearly as convenient as conducting your own tests. By doing your own testing, you will find it much easier to modify your mix designs in order to achieve the most efficient and effective mix designs that will enable you to produce the strongest, highest-quality products.
If financial resources are limited, you may want to consider purchasing used equipment. This can be a very cost-effective way to start out, but you will want to ensure that the equipment you are purchasing is in good working order. There are frequently good buys on used equipment, but there is also a lot of dysfunctional used equipment.
Investing in a new facility is a major undertaking, so take the time to carefully study your requirements. Always keep in mind that it is far less costly to make mistakes on paper than at the building site. Use the assistance of experienced suppliers, who are very knowledgeable in their respective fields. With proper planning and intelligent equipment selection, there is every reason to expect your new plant to be very efficient, safe and productive.
Best wishes for a successful installation.
Mel Marshall, P.Eng., is owner of Mel C. Marshall Industrial Consultants Inc. based in Delta, British Columbia. He has been actively involved in the design, manufacture and construction of concrete products and structures for more than 40 years. Visit precastconcretebc.com.
Carl Carlson says
How are you? I hope all is well. I have a question regarding dry cast manholes versus wet cast manholes. Are there any scientific studies that show dry cast manhole allow more water in than wet cast manholes? I ask this because we are now wet casting manholes as you know. I have been told by a pre-caster in Mississippi and Tennessee that dry cast manholes are not allowed in either of those states because wet cast is less permeable than dry cast. Do you know of any study that supports this claim?
Thank you very much,
Carl V. Carlson