808 on 5th is a five-story multi-family/mixed use building located in the heart of Coralville. Precast beams, columns, and hollowcore were used to form the underground parking structure.
Precast concrete contributes to green building practices in significant ways.
The primary ingredients of concrete – sand, gravel, and cement – are mineral based. When mixed with water, the cement chemically reacts to create a crystalline matrix with a high compressive strength. The matrix binds the sand and gravel together, creating concrete. The low water-cement ratios possible with precast concrete (in the range of 0.36 to 0.38) mean it can be extremely durable. Unlike other construction materials that can rust, rot, or otherwise degrade when in the presence of moisture, concrete can actually get stronger if there are unhydrated cement particles available to react with the water.
Because precast concrete is factory-made, there is little waste created in the plant, and it reduces construction waste and debris on site, reducing construction indoor air quality concerns. Components are made by highly experienced personnel who apply stringent quality-control measures. In the factory environment, precast producers are able to achieve consistency in temperature and moisture and low water-cement ratios that are not possible in field-fabricated concrete. Precast concrete can easily attain strengths of 5000 psi to 7000 psi or more, with densities that minimize permeability.
Precast concrete’s fresh and in-place performance can improve when several common industrial by-products are added. Fly ash, slag, and silica fume, which would otherwise go to landfills, can be incorporated into concrete as supplementary materials. These by-products can also reduce the amount of cement that is used in concrete. Reinforcement is typically made from recycled steel. Insulation and connections within the precast concrete also contain recycled content.
The thermal mass of precast concrete absorbs and releases heat slowly, shifting air conditioning and heating loads to allow smaller, more efficient heating, ventilating, and air conditioning systems. Insulation is often used in architectural panels to increase the thermal efficiency, with continuous insulation in walls being possible. The resulting savings are significant – up to 25% on heating and cooling costs.
The load-carrying capacities, optimized cross sections, and long spans possible with precast concrete components help eliminate redundant components. Precast concrete is also friendly to downcycling, in which building materials are broken down, because it comes apart with a minimum amount of energy and retains its original qualities. An example of downcycling would be the use of crushed precast concrete as aggregate in new concrete or as base materials for roads, sidewalks, or concrete slabs.