Quality concrete that meets certain criteria is important in the manufacture and performance of the box culvert. Since the concrete in a box culvert is normally exposed to the weather and running water in it's typical application, it is important that a high strength, well proportioned mix design be used. Concrete is a mixture of portland cement, sand (fine aggregate), crushed stone (coarse aggregate), water, and air. Flyash is being used more often to help reduce the cost of the mix design, recycle material using less space in landfills, and increasing the durability and strength of the mix design. Hydration, a chemical process, causes the mix to harden and increase in strength from a plastic state to a hardened state.
Libraries of books have been written regarding mix designs and what makes good concrete. There are several factors that should be discussed that everyone should know about concrete. Not all concrete is the same. A large contributor to reduced concrete quality is the water to cement ratio (by weight). The lower the water-to-cement ratio several advantages start become apparent.
- Increased compressive strength.
- Decreases concrete permeability and promoting low absorption. This decreases the chance water will reach the steel reinforcing rebar and cause rusting that leads to spalling and structure weakening.
- Reduces the volume change from wetting and drying.
- Reduces the shrinkage and crackage.
Important benchmarks that help achieve concrete quality suitable for box culvert:
- Minimum 28 day ultimate compressive strength of the concrete used should be 5,000 psi.
- Water to cement ratio should be less than 0.44.
- Minimum of 700 pounds of cementitious material should be used.
- Air content should generally be no less than 4% and no more than 10% depending on the climate the product will be installed.
- An ACI Field Technician Grade 1 in good standing should test the concrete during production to document concrete properties for density (unit weight), air content, temperature, and to prepare strength cylinders to be tested at 28 days.