Concrete Construction Procedures
PRIOR TO CONSTRUCTION
Most concrete containment facilities should be designed or reviewed by an engineer prior to beginning construction. As mentioned previously, the lagoon location should be chosen only after reviewing all ISDA, county and local siting requirements. The exact lagoon location and size should then be marked for the contractor. The local utility companies should also be notified at this time so they can mark any utility line locations, including, electric, gas, water, sewer, phone and cable TV. The landowner should also mark any known utilities, including the barn waste line, water trough lines, drain lines and electric.
CLEARING THE SITE AND BASE PREPARATION
The area of the new lagoon should have the vegetation and top 6" of soil removed. This material should be stockpiled onsite for later use as backfill against the concrete walls. For below ground storage facilities, the site should be excavated with a backhoe or excavator. The hole should be excavated 3–4' larger on all sides of the facility to allow room for forming the walls and slab. The hole should be over excavated 6" to 12" below the bottom of the slab for placing base material (sand or gravel). The soil in the bottom of the hole, the subgrade, should then be compacted. Compaction has been discussed previously in the Earthen Lagoon section of these guidelines, however, the compaction equipment in this case may be different. Small jobs can be compacted with a plate compactor or small rubber tired equipment.
After the subgrade is properly compacted, the base material may be placed in 6" lifts. Base material usually consists of Pit Run Gravel, Roadmix Gravel, or Clean Sand. All of these materials can be purchased from a local gravel plant or may be available onsite This base material has two purposes, it provides a very stable and uniform foundation for the concrete slab and it keeps water from accumulating around the concrete. The base material should be compacted between lifts using the same compaction methods previously mentioned. The final lift of base should be leveled very carefully to provide a uniform surface for the concrete.
Forming will not be discussed in these guidelines, however, it is a very important step in providing a good finished product. Waste facilities will usually be large enough to require ready-mixed concrete to be delivered by a concrete truck. When ordering the concrete from the ready-mix plant the following criteria should be used for ordering (if criteria was not provided by an engineer): 4000 psi concrete (5.5 bag minimum mix), 1" maximum aggregate size, Water/cement ratio of 0.49 or lower, 6% air entrainment and a 3–4" slump. The 4000 psi concrete requirement is the standard structural grade of concrete for the slab and vertical walls of the facility. The Water/cement ratio should be low to improve the strength and durability of the concrete; some ready mix plants will try to use a slightly higher ratio which is less durable. The air entrainment increase the weather resistance of the concrete and is highly recommended for concrete subjected to freezing and thawing. The 3–4" slump criteria will produce a stronger final product than a 6" slump, but it will be more difficult to place and work.
Wire fabric is usually used as the reinforcement in reinforced slabs and should be placed on chairs (a small manufactured wire device) or masonry blocks to hold it above the bottom of the slab. Rebar is used as the reinforcement for walls and footings. The rebar should be spaced and tied per the design drawings and must be placed at least 1½" from the forms to insure adequate concrete cover over the rebar (See: Figure 11 for an example of what can happen without proper cover).
Figure 11. Exposed Rebar in a Concrete Wall
Figure 12. Wall Corner (photos by Michael Mitchell)
When placing the concrete for a slab, a shovel, rod or vibrator should be used to consolidate the concrete near the forms. The concrete surface should then be leveled off with a power screed or piece of lumber spanning across the forms. The concrete can then be floated (or smoothed), troweled, or broomed depending on the surface finish required. Walls should be vibrated as concrete is added to the forms. This will insure that the concrete is fully consolidated near the forms. (see Figure 12 for an example of what can happen when the corner of the forms are not properly compacted).
Concrete usually takes approximately 28 days to cure (or dry) to its full strength. At 7 days, the concrete has about 75% of its strength. Depending on the weather, the forms can usually be stripped from the concrete within several days, but backfilling should wait until the concrete has cured for several weeks. During cold weather, the concrete should be covered with insulated blankets and heated if necessary to keep the concrete above freezing for at least one week. During hot weather, the concrete should be kept damp for the first few days to slow the curing process.