Basics of Concrete

Subfloors & Underlayments


  • Contains about 60% lime, 25% silica, 5% alumina, and the rest is iron oxide and gypsum.
  • Lime comes from limestone, oyster shells, chalk, and marl (type of clay)
  • Silica and alumina come from shale, clay, silica sand, slate, and blast furnace slag.
  • Iron oxide comes from iron ore, pyrite, and other materials.


  • Crushing: quarried limestone dumped into primary crushers which smashes rock into softball size pieces. Secondary crushers or hammer mills then break rock into 3/4″ pieces.
  • Grinding: crushed rock and the other raw materials are placed into ball mills and tube mills which contain thousands of steel balls which grind the mixture into fine particles; can be done wet(slurry) or dry.
  • Burning: ground material fed into higher end cement kiln which rotates about one turn per minute; travels slowly to lower end as kiln rotates; takes about four hours; heats materials to 2600 to 3000° F; all moisture and carbon dioxide are removed allowing chemical reactions to occur; changes materials into marble size pieces of “clinker” which, chemically, is a new compound.
  • Finish Grinding: after cooling by large fans, reground with small amount of gypsum in ball or tube mills; gypsum regulates setting time.


  • Water
  • Portland Cement
  • Fine Aggregate (sand)
  • Intermediate Aggregate (3/8″ – 1/16″ Pea gravel)
  • Coarse Aggregate (gravel)

Water and Portland cement form a paste which coats the aggregates. When the paste hardens, the aggregates are bonded together in a solid mass of hardened concrete. The quality of the paste has a major effect on the quality of the concrete, and is affected by the amount of water in the mix.

  • Water/ cement ratio determines the quality of cement paste.
  • w/c ratio = (amount of water / amount of cement)
  • Minimum w/c ratio needed for complete hydration is 0.22 to 0.25
  • Lower w/c = higher compressive strength of finished concrete.
  • When concrete sets, its volume remains almost unchanged, so too much water in the paste causes empty water pores which in turn lessen the strength of the concrete.

HARDENED CONCRETE: Consists primarily of relatively insoluble compounds including simple and complex calcium silicates, silica (sand and aggregate), aluminum silicates, unhydrated and partially hydrates dicalcium and tricalcium aluminates, and small amounts of ettringite as well as other compounds. Relatively soluble compounds are hydroxides of calcium, sodium, and potassium which are available for the transport of moisture and potentially harmful chemicals.

Healthy new concrete is alkaline – pH of 12.5 or higher. Once cured and dried, alkalinity on surface, through carbination, drops to the normal range.


  • Chemical reaction between Portland cement and water.
  • Concrete hardens as a result of hydration – not water evaporation
  • Hydration continues as long as water is present in concrete.
  • Produces crystals – the closer together the crystals, the stronger, more durable, more watertight the concrete becomes
  • Curing, or retaining water in concrete, is essential for complete hydration.

Ideally, w/c ratio would be 0.50 giving a slump of 1-3/8″ – but this would not be very workable concrete; slump test is an indicator of plasticity in concrete.

  • ASTM C143
    • Test is run using a cone 12″ high with a 4″ diameter hole on top and an 8″ diameter hole in the base
    • Cone is filled with concrete at job site and concrete is tamped
    • Cone is removed and placed next to concrete it is removed from
    • Slump is measurement in inches of the distance from the top of the cone to where the concrete has slumped.
    • The higher the slump, the weaker the concrete; the lower the slump, the stronger the concrete
    • Slump for a floor should be no more than 5″
    • It is important to remember that while slump is the accepted test for workability, in reality, it only measures how much the concrete mix slumps. A low slump with good gradation of aggregate and more sand may be more workable than a mix of higher slump with poor gradation and low sand content.
    • The slump is increased if a water reducing agent is used.


  • Curing is essential for hydration
  • If water evaporates before concrete is completely cured, causes surface to crack and become weak and friable.
  • For complete hydration to occur, concrete must be kept wet, ideally for 28 days;
  • Maintaining presence of mix water in concrete during the early hardening period – includes ponding and immersion, spraying or fogging, or wet coverings such as cotton mats, rugs, burlap or concrete curing blankets..
  • Prevent loss of mix water by sealing the surface – includes impervious paper or plastic sheets as liquid membrane forming curing compounds. If used, they must be removed prior to the installation of any adhered material.
  • Accelerate strength gain by supplying heat and additional moisture to concrete – includes live steam, heating coils, or electrically heated forms or pads
  • Concrete should be moist cured for at least seven days; failure to do so may increase permeability four times or more
  • While curing, slab continues to give off moisture, ideally in both directions
  • Curing has strong influence on properties of hardened concrete
  • Surface is especially sensitive to curing as strength development can be reduced significantly if curing is defective.
Updated on October 26, 2022