The characterization of hardened concrete was analyzed by testing the weight change of water absorption. A simple testing method is submitted and discussed in this paper. As one of the test results, it was reported that the relationship between weight change (at) and time (t) of water absorption test would be well fitted with at=(α×t)/(β+t). Based on the result, laboratory studies of a water absorption test were carried out. Test results were discussed for estimation of mix proportions, compressive strength, and carbonation of different kinds of concrete.