At its essence, concrete comprises three ingredients: cement, water, and aggregate. But from these, architects can get an endless variety of concrete mixtures, tailored to a project’s site conditions, location, climate, and performance requirement. But what differentiates one mixture from another?
Cement Type
Cement—a term often incorrectly interchanged with concrete—acts as a binding agent for the aggregate and generally comprises about 10 to 15 percent of the concrete mixture by weight. Portland cement—the most widely used binding agent—is a mix of crushed calcium compounds, silica, alumina, and iron oxide, and comes in six common types.
Type I cement has a high calcium silicate content, which is good for early strength development in general construction projects. Type II has a low calcium silicate content, good for underground or submerged structures. Like Type I, Type III cement has a high calcium silicate content but is more finely ground, expediting cure times for rapid construction and projects in cold climates. Type IV has a low calcium silicate content and releases heat slowly during curing, limiting the concrete’s internal temperature and making it desirable for massive projects such as dams. Type V boasts high sulfate resistance and a very low content of calcium silicate, suiting it for structures exposed to high levels of corrosive sulfate ions, as in marine environments. Finally, White Portland cement performs similarly to Type I cement, but is more expensive due to its light coloring. It is often used in decorative applications.
Water-to-Cement Ratio
The water-to-cement (w/c) ratio of concrete is calculated by dividing the weight of the water in a mix by the weight of the cement. Ratios typically range from 0.45 to 0.60. Since the two values are inversely related, the strength and durability of the concrete will increase as the w/c ratio decreases—that is, the lower the water content, the better the choice for high load-bearing projects. More water in a mix also results in more capillary pores in the concrete, which increases its vulnerability to freeze–thaw cycles, and thus cracking.
However, mixtures with low w/c ratios are harder to place during construction. Air entrainment can also improve the concrete’s workability during placement, which decreases with the w/c ratio. Adding 5 to 8 percent (by volume) of entrained air can relieve the concrete’s internal pressure, improving its durability when water in the pores expands.
According to the American Concrete Institute (ACI) and the National Precast Concrete Association, the w/c ratio of concrete should not exceed 0.40 for projects exposed to large amounts of salt, such as marine applications . Concrete exposed to freezing temperatures should have a maximum w/c ratio of 0.45, and watertight concrete should have a maximum w/c ratio of 0.48.
Aggregates
Rocks, gravel, and sand make up 60 to 75 percent of the total volume of the concrete mix. Fine aggregates have a maximum diameter of 0.3 inch; coarse aggregates should have a maximum diameter of 1.5 inches to ensure proper binding and coverage. Mixtures utilizing high proportions of fine aggregates are suitable for road surfacing, decorative concrete, and paving paths. Coarser aggregates can reduce costs and increase durability and strength by reducing the amount of voids between particles as well as the amount of cement needed.
Admixtures
Contingent on project needs, admixtures can be introduced into a concrete mix to enhance performance or placement. Generally speaking, admixtures fall into five categories: water-reducing, retarding, accelerating, superplasticizers, and corrosion-inhibiting. Water-reducing admixtures improve the workability of mixes with low w/c ratios. Retarding admixtures prolong the curing of the concrete and are suitable for construction in warm climates where the ambient heat may hasten cure times. On the opposite end are accelerants, such as calcium chloride, which speed up curing when the weather is cold. Superplasticizers reduce the viscosity of concrete, which is good for pouring concrete in tight areas around rebar. Lastly, corrosion-inhibiting admixtures increase the durability of concrete in high-traffic applications.
Strength
The strength of concrete can be monitored during curing by testing concrete cylinders assembled on the jobsite in conjunction with its placement. According to ACI guidelines, samples should be tested at least on the 28th day of curing to assess its potential ultimate strength. At this time, a compressive strength between 3,000 psi and 5,000 psi can be expected. If the project requires high strength earlier, a three- to seven-day test can be conducted to confirm its performance matches expectations. High-strength concrete for tall buildings can top upwards of 19,000 psi.
Advances in Concrete
The production of concrete, and cement in particular, is energy intensive. Ready-mixes called Engineered Cementitious Composites include fiber-reinforced concrete that contain flyash (a byproduct from the coal industry) and polymer-based fibers for flexibility and enhancing the tensile strength of concrete—especially important in areas with high seismic activity. Self-compacting concrete that also self-levels into formwork without the need for mechanical vibration can save projects time and money. Recently, researchers at MIT tested a concrete mix containing pulverized, recycled plastic aggregate that was briefly exposed to gamma rays to stregthen their crystalline structure. According to the team, preliminary tests showed that this new mixture was 20 percent stronger than traditional concrete, carving the way for alternative aggregates.
Resources
The ACI’s Building Code Requirements for Structural Concrete offers minimum requirements for concrete structures while the ASTM International also provides informative and important suggestions, guidelines, and terminology for specifying concrete mixes.
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