Concrete mixes balance, strength and workability during construction to produce a strong, high-quality finished product. Portland cement is available in different types for special purposes; but recently engineering specifications and concrete technology has evolved so that modifications during mixing, placing and curing can replace production of specialty cement for high strength requirements.
Concrete is a mixture of Portland cement, water and aggregate. The water/cement ratio is the determining factor of the strength of concrete. Concrete is stronger with less water in the mix. But water makes the concrete workable; the typical water/cement ratio balances strength and workability. Aggregate is also a factor in the strength of concrete; a strong concrete mix has a low water/cement ratio and strong aggregate. Concrete reaches its full strength through the chemical reaction between cement and water, called hydration.
The temperature of hydrating (setting) concrete has to be kept within a certain range to achieve its final strength. Masses of concrete generate high temperatures with no release, which is detrimental to concrete quality. Portland cement Types II and IV are intended for massive concrete structures, such as heavy retaining walls and gravity dams, respectively. These types hydrate slowly at lower heat than other cements. But usually concrete mixes and practices are modified to address temperature conditions and produce high strength concrete.
Concrete Mix Modifications
Cementitious materials can be modified for concrete mixes to meet low hydration and strength requirements by replacing part of the Portland cement with Class F fly ash or slag cement. To keep the temperature below the maximum of 70 degrees Fahrenheit, the water in the mix is chilled, or ice is substituted for part of the water, or liquid nitrogen is injected into fresh concrete. Additional measures are taken during curing to prevent temperature extremes.
General Purpose Concrete
More than 92 percent of the Portland cement produced in the U.S. are Types I and II. Type I is for general purpose construction, which encompasses a wide range of construction, including buildings and bridges. In addition to the mixture, curing properly is critical so that temperature and moisture conditions allow concrete to hydrate to full strength. The essential curing period is three days; concrete continues to gain in strength for a long time after that.