Sand topping concrete mixes are used for concrete overlays less than 2 inches thick and for leveling and patching floors, steps and walkways. High strength concrete supports heavy loads with less concrete and is most commonly used to build high-rise buildings.
Sand Topping Mixes
Sand topping mixes are a form of high strength mortar made from a mixture of sand and Portland cement. In addition to patching and leveling, sand topping concrete is used to repair cracks in sidewalks and chimneys, fill the cores of masonry block and provide a floor for ceramic tiles.
Mixing Sand Topping
Commercial sand topping concrete mixes composed of cement and sand are available at most building supply stores. Commercial mixes ordinarily contain three parts sand to one part cement. You add more sand to get a ratio of five parts sand to one part cement then add water to the mix, and it's ready to use. Sand topping concrete is also called “deck mud” or “dry pack mortar,” and you can make it yourself by mixing five parts of sand to one part Portland cement. Be careful not to get sand topping concrete too wet when you add water or it won’t dry properly.
High Strength Concrete
High strength concrete is made by adding fly ash and silica fume to create additional calcium-silicate-hydrate gel, commonly called C-S-H gel, that gives concrete its strength. The American Concrete Institute notes that high strength concrete should have a compressive strength greater than 6,000 pounds per square inch (psi). High strength concrete with a compressive strength up to 12,000 psi was used to construct the tallest concrete building in the U.S., a 969-foot-tall high-rise building in Chicago. Concrete able to bear 19,000 psi was used to build two high-rise buildings in Seattle.
C-S-H gel accounts for about 50 percent of the volume of the cement paste to which sand or gravel are added to form concrete. Fly ash and silica fume additives, called pollanzans, give high strength concrete the ability to bear heavy weight. Pollanzans are named for the Pozzuoli region of southern Italy where volcanic ash was traditionally added to cement. Thousands of tiny C-S-H pores grow within the cement to form a network. These pores harden as they fill with water, making stronger concrete.