Though light-weight, air-infused concretes are available to reduce the heft of the furniture, a project as large as a picnic table will still be cumbersome. Be absolutely certain you are committed to the placement of the table, since moving it later will be difficult, if not impossible. Also, since concrete molds may have to sit for several days while curing, be aware that the grass will turn yellow and possibly die in the area you are working.
Just like a wooden picnic table you have many practical design choices: Round or square? Bench or chair? Seats four or eight? However, unlike wooden picnic tables that can only be painted or stained, the decorative choices with concrete are only limited by your imagination. Concrete no longer has to be the boring gray of garage floors and driveways. Dyes and stains are available to create various color schemes. Dyes come in water-soluble and solvent based. Water-soluble dyes work best if you want to create a variegated or marbleized effect. Solvent-based dyes provide a uniform color. Where dyes soak into the concrete leaving a vibrant color, stains react with the calcium hydroxide and do not penetrate the surface. If you are feeling artistic, create mosaics using rocks, sea shells or broken china pieces. Consider pressing large large sycamore or tulip poplar leaves into the wet cement and using their impressions a permanent place mats. First do-it-yourself project with the kids? Have them write their names in the pieces they helped pour.
Working with concrete is labor intensive. This is not a Saturday afternoon do-it-yourself project. To begin, you must create molds for the concrete pieces. Next, pour the concrete, and depending on the type of concrete you purchased, allow it to cure for several days at least to ensure that the pieces will not crack due to shrinkage. Then you apply the dye and allow it to soak into the concrete. With or without the application of a dye, the last step before assembling is applying a sealant to prevent dusting. This is an essential step to any concrete furniture. If you have ever sat down on a concrete park bench and stood up to find your backside covered in a fine white powder, this bench was not sealed.