Plastic Shrinkage Cracking and Evaporation Formulae
Paul J. Uno
Appears on pages(s):
air temperature; concrete temperature; cracks in concrete; wind;
durability; evaporation; high-strength concrete; hot weather concreting;
humidity; plastic shrinkage; solar radiation; vapor pressure;
Freshly placed concrete exposed to hot, windy conditions often is prone to plastic shrinkage cracking (though other conditions can also promote this phenomenon). This type of cracking is normally noticed on slabs, pavements, beams, and other flat concrete surfaces. Many factors affect plastic shrinkage cracking, in particular the evaporation of water from the surface of freshly placed concrete. Other factors also influence the likelihood of plastic shrinkage cracking such as water-cement ratio, fines content, member size, admixtures, and on site building practices. Evaporation itself is a function of climatic variables such as relative humidity, air temperature, the temperature of the evaporating surface, and, very importantly, the wind velocity at the surface. This paper primarily explains the background to the evaporation nomograph found in ACI 305R-96 "Hot Weather Concreting," Manual of Concrete Practice, Part 2-1996, in which the graph provides a means of estimating the rate of evaporation of surface moisture from concrete. The paper offers an alternative nomograph and various formulae to predict an evaporation rate of surface water (primarily bleed water) from freshly placed concrete surfaces. Other factors related to evaporation and plastic shrinkage cracking also are addressed.