Backer rod is used to fill in gaps thereby reducing the amount of caulking used.
Backer rod is cylindrical soft foam tubing that comes in various sizes from 1/4 inch to 2 inches in diameter and it can be easily cut to fit tightly in the joint.
Push the backer rod in to allow half the width in depth of caulking.
A 1/2 inch joint means push the backer rod in to all 1/4 inch of caulking.
Aluminum grey is a close colour match to regular concrete and it is recommended that the caulking be purchased at a building supply store as opposed to a big box store. Caulking manufacturers such as DyMonic, Sikaflex or Tremco sell a product with a 25 year life expectancy while big box stores sell a product with a five year life expectancy and there is only a small difference in price.
After installing backer rod and caulking use a spoon or piece of wood dunked in water to tool the caulking to provide a uniform application of caulking. The water helps reduce the amount of caulking sticking to the tool.
Immediately upon installation of caulking a light dusting of sand will dull the shiny appearance of the caulking and replicate the surface texture of concrete.
Place caulking in any joint that might allow water through it and only apply it evenly between concrete slabs or between concrete and wall. Do not allow any to go over concrete edge as any thin area's of caulking will split away from concrete. If this happens in the future simply apply more caulking on top. There is no need to remove it to re-apply.