Controlling the moisture loss in the early stages of hydration is vital to the longevity of your concrete. It is also extremely important to control temperature during this time. For example: on hot, windy days the moisture in the concrete can leave much too quickly. Also, in cold weather the concrete needs protection from the freezing temperatures. When water is allowed to enter and freezes the concrete, it will expand 9%. While the concrete is curing, protection of the concrete is more important than any other aspect. The following are some set-time guidelines based on the air temperature:
Temperature Set Time
90 °F 3 Hours
80 °F 4 Hours
70 °F 5 Hours
60 °F 9 Hours
50 °F 13 Hours
One major problem seen in the housing industry is the failure to properly understand concrete hydration. This results in concrete being put into service too soon, which greatly affects the concrete’s strength and durability properties. Another major problem seen in housing is early use of power washers to clean the concrete surface. The use of power washers forces water into the concrete’s surface at a high power. During the early stages of hydration, power washers should never be used. Power washers can be used after the concrete is at least 14 days old. Barricades are many times quickly removed by other contractors and the concrete is opened for service too soon. Foot traffic is usually fine within a day after casting, but vehicular traffic should wait a minimum of 7 days. During snow or ice cycles, the sub-contractors should never be allowed on the concrete until such time that it is turned over to the owner.
Note: Freshly placed concrete should never be allowed to freeze until it has achieved a minimum strength of 500 lbs per square foot. This normally takes between 36 and 52 hours. If the new concrete is exposed to moisture, such as rain, it should achieve strength of 3,500 lbs per square foot. This may take anywhere from 14 to 28 days.
The aggregate is a large volume of the concrete mix. Hard, clean, sound aggregates are the key to the concrete’s growth and life cycle. Unfortunately, many fail to upgrade aggregates when designing a concrete mix. The short-term price becomes the primary deciding factor; thus, long-term durability is neglected. Lower priced mixes might seem good at the bid in order to make budget, but one also needs to realize that choosing better aggregates will result in concrete that will satisfy the performance requirements, withstand exposed weather conditions and provide years of service. Using clean, hard aggregates with an optimized aggregate gradation in concrete mixes not only gives increased durability, but also will reduce labor and maintenance costs.
A generic mix design is as follows:
¨ Cement 11%
¨ Water 15%
¨ Fine aggregate 26 %
¨ Course aggregate 41 %
¨ Admixtures (air entraining) 6-8%
There are certain keywords in mix-designs that need to be understood. The first and most important for concrete exposed to freezing (especially repeated freezing cycles) is air entraining admixtures.
Air entrainment is an admixture added to all exterior concrete. It creates millions of microscopic bubbles in the concrete. These bubbles act as a relief valve to withstand the pressures of internal water freezing. Most concrete mixes allow for 8% of air and a place for the water to expand. Two types of air are found in concrete mixes: air that is created from normal mixing and air from air entrainment admixtures. The normal mixing action will create air spaces (usually 1-2% air). The admixture that is added to create an even void structure throughout the mix concrete containing should not be overworked. Over finishing air entrained concrete can work the air out of the concrete surface reducing the positive effects of air entrainment.
Voids are the area between the solid particles. Voids make up over 65% of the volume of the concrete.
Specific Gravity is the ratio of items mass to an equal volume of water.
Unit Weight is the mass of an item for a given volume (example concrete = 145.3 lbs/ft3).
Strength is the compressive resistance of the concrete in pounds per square inch after 28 days of cure time (the sample load divided by the sample square area).
Cubic Yard the most widely used term when ordering concrete. A cubic yard will fill a form 3ft x 3ft x 3ft (concrete is sold by volume but produced by weight).