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Bleeding described as the separation of water from a concrete mix due to gravitational settlement of the solid materials was recognized many years ago as a form of segregation, the excessive occurrence of which leads to laitance, green shrinkage, sand streaking, non-uniformity of strength, water gain voids under aggregate with consequent increased transverse permeability and cold weak joints between successive courses of placed concrete. Later it came to be recognized that reasonable amounts of bleeding could be desirable, particularly with regard to the placement of concrete slabs or pavements in hot, dry, and windy weather. Bleeding rate and total bleeding, the latter referred to as bleeding capacity in ASTM Standard Method of Test for Bleeding of Cement Pastes and Mortars (C 243), are affected by several factors (1)1 type, fineness and setting characteristics of the cement, (2) amount of grading of the sand, particularly the −100 mesh fraction, (3) grading of the coarse aggregate, (4) water/cement ratio (5) cement factor and (6) the atmospheric conditions of concrete placement. Fundamentally, established is the fact that the ability of any solids/water mixture to retain its mixing water is directly related to the total surface area of the solid matter and the viscosity of the liquid phase. This ASTM standard is an adaptation of the carbon tetrachloride method developed by Valore, Bowling, and Elaine of the National Bureau of Standards and was originally intended to serve as a cement performance test which would make possible the elimination of fineness limits on cement. However, the Workability Subcommittee eventually came to recognize that (1) correlation between cement bleeding characteristics was impossible of attainment because of the many factors, including job factors, which control bleeding of concrete in the field and (2) even if correlation were established, realistic limits on bleeding rate and capacity could not be set because variable job and mix conditions, particularly atmospheric, vary the requirements for bleeding or its limitation. For these reasons, C 243 has been retained as a research method without limits.
cement and concrete, cement bleeding, concrete construction, water-cement ratio
Mardulier, F. J.
General ManagerPersonal Member ASTM, W. R. Grace & Co., Cambridge, Mass.