workability . Higher-strength uses smaller aggregate , and the aggregate itself is a factor . Fibers have become popular , but can produce a 'fuzzy' surface . Less water is always better , but it depends on the flow required . Tamping is good - gets rid of voids , but not to the point where the mix is vibrated apart ... Rebar and mesh are important to overall strength , too ... Curing is a chemical reaction , and even happens underwater . Slower is better for strength , though . Plastic can be put over a fresh pour , and water can be sprayed on it if in direct sunlight on a hot day , rapid drying can cause surface cracks ... Plastic underneath can also slow down the cure , and is recommended for future moisture-control ... Oddly , a pure carbon-dioxide atmosphere can achieve the same cure in a few hours that would require 2-weeks , if I'm remembering a Popular Mechanics article correctly .