Recently I was contacted by a friend who was performing a concrete slab pour. He needed advice on how to prepare for the pour.
The slab he is making will be used for maintaining a large rainwater tank for use in his home and garden. The tank will hold up to two thousand gallons of water with an approximate weight of 16,000 pounds. The tank is about eight feet high and eight feet in diameter.
When you first see a tank of this size you may not expected to weigh as much but you must take into account how much the water weighs when you are designing the support system it will sit on. Even the tank itself weighs about 250 pounds so when it is full it will be much heavier.
The most important thing when designing a slab for a heavy item is whether or not the entire base of the tank will contact the slab and distribute the weight evenly or if it will be necessary to install footers under contact points of legs or a stand.
In our case the entire base of the tank will rest on the slab and distribute the weight evenly. For this reason we do not have to build the slab to any specifications greater than a concrete driveway that can support a vehicle.
When building a concrete pad you must first remove all topsoil down to a gravel base. Topsoil will easily erode and compress under heavy weight this means it could compromise the structure of the concrete slab. Once you have removed all of the topsoil you should install a minimum of 3 inches of mixed crushed gravel. 6 inches or more of concrete should be poured and a half inch rebar should be used every 3 feet on center. Concrete walkways may use a wire mesh rebar however it is not meant to support heavy loads.
When rebar is installed it should be placed in the center of the slab thickness. Since our concrete slab will be 10 inches thick we will install concrete rebar in two layers.
First we will define the perimeter of the slab pour with 2x 12 inch boards. We will then come in 6 inches from the perimeter and drive 2 foot long rebar spikes into the ground to support our rebar system. The first layer of rebar will be attached to the spikes 3 inches up from the bottom and the second layer will be attached 3 inches down from the surface of the slab. Each of the rebar rods must be fastened to the rebar spikes with metal wire to hold it securely during the concrete pour.
Final note
It is always better to over build your concrete slabs when you are predicting heavyweight will be placed upon them. There are many books and guides that can help you define the correct use of rebar and concrete to build the right slab for most conditions. It is then in your best interest to build above these guidelines to extend the life of your install.
Removing topsoil is a very important step even if you must find clean fill to bring the grade up to the level where the slab can be poured it is important to dig deep enough to remove all topsoil.