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The aim of this work is to determine the compressive strength of concrete at 28 days age by using concrete samples collected on some uncontrolled construction sites. The later are randomly selected at some regions in Lebanon. The concrete for whole structure elements like columns, slabs, foundations, retaining walls, stairs...is tested. Results are shown according to some parameters such as mixing and aggregate origin. In general, the obtained results show weak values the compressive strength and in some cases an unexpected low values. An interpretation of these values is shown in this paper. The information obtained on the concrete compressive strength allows design engineers to take into consideration in their conception a high probably weak concrete when the construction is performed. So, the safety of construction is improved and we can avoid building collapses as happened recently in our countries. The paper also gives some simple recommendations which could be easily realized directly on site in order to improve the compressive strength of the concrete. The recommendations were essentially based on real conditions of concrete preparation observed directly on uncontrolled sites construction where the samples are taken.