by William M. Kelly
Chapter 7: Ready Mix Concrete
The term “ready mix concrete” refers to a type of concrete that is manufactured in a batching plant or factory according to a specific formula and is then delivered to a work site by truck-mounted transit mixers. Although the first load of ready mix concrete was delivered to a building site in Baltimore, Maryland, in 1913, the early ready mix plants appeared in the 1930s when the standard of practice was for contractors to mix concrete at the work site from bagged cement and aggregates delivered separately. The ready mix concrete industry expanded significantly in the 1960s. Modern ready mix concrete is made under computer-controlled conditions. This results in specialty mixtures designed for very specific purposes. The use of ready mix concrete allows a precise recipe to be delivered to the work site. This eliminates the need for an on-site concrete mixing plant and the need for storage space for the materials to make concrete at the site. Ultimately, the use of ready mix concrete reduces noise and labor costs and can improve air quality at the work site.
The concrete is delivered freshly mixed and in a plastic state from a centrally located batching plant that can serve a wide area. This is an advantage as the plant can be located, for instance, in an industrially zoned area and make deliveries to residential districts or into congested city work sites. However, when the materials are combined at a batch plant, the mixing begins at the plant so travel time from the plant to the work site is critical. Batch plants cannot be too far away from the site. The concrete should be placed within ninety minutes of being mixed. The weight of concrete is 1,776 kg/cubic meter (3,915 lbs/cubic yard) for general purpose concrete, and means that the transit mix truck is 33 tons for a normal 7.6-cubic-meter (10-cubic-yard) load. Therefore, concrete manufacturers strive to keep transportation distances to a minimum to avoid hauling heavy loads great distances.
Ready mix concrete is commonly manufactured in batches of 1.5 to 9 cubic meters (2 to 12 cubic yards). The manufacturing plants are relatively simple. Facilities for handling bulk raw materials are present, including silos for cement storage, wheeled loaders, and perhaps conveyors to move aggregates (sand, gravel, crushed stone) from on-site storage piles to a mixer. A source of clean water is required. The batch plant weighs the various ingredients and feeds them into a weigh hopper. Aggregates comprise about 60 to 75 percent of the mix by volume. Ten to 15 percent is cement and 15 to 20 percent is water. Entrained air bubbles may be up to 5 to 8 percent. The batch plant and weigh hoppers are atop an elevated structure that allows the mixer trucks to drive underneath the plant to be loaded (Syverson 2008).
Additives are solid or liquid substances that improve workability, reduce shrinkage, or modify setting times. Air entraining substances provide resistance to freeze–thaw cycles. Additives can reduce the amount of water required in the mix, to increase slump (flowability) and improve workability. They provide increased strength and they reduce cracking due to shrinkage. Some additives protect reinforced concrete from corrosion caused by exposure to de-icing salt or a marine environment. Plastic or cellulose fibers increase the strength of the concrete and reduce shrinkage and cracking. Coal fly ash and slag are added as a replacement for portland cement. These materials make concrete stronger and less permeable and extend the set time.
Ready mix concrete plants in New York are of two varieties, batch and central mix facilities (Figure 28). At batch plants, the components of the concrete are weighed and loaded directly into the transit mixer truck. Then the requisite water is added, and the final mix is made in the truck. In central mix plants the concrete is made in batches and loaded into the truck as a wet mix (Figure 29). In both cases the stone, gravel, sand, cement, water, and additives are delivered to the mixer, either truck-mounted or stationary, from hoppers that weigh the requisite components. These in turn are controlled by computer systems.
Batch (left) and central ready mix concrete plants.
(Courtesy Northern Ready Mix, Inc.)
Central ready mix concrete plant in the process of lifting the batch mixer to load the wet mix into a transit mixer truck. (Courtesy Northern Ready Mix, Inc.)
Probably the most widely used form of ready mix concrete is as a cast-in-place material. Cast-in-place concrete is ready mix that is transported to the work site and placed in forms. The concrete is mixed according to specifications at an off-site location. It is used for most building foundations and slabs as well as walls, columns and beams, and floors and roofs. Cast-in-place concrete is used for large sections of bridges as well as for pavement. This material is used because of its long-term durability and structural strength. A variation of pre-cast concrete that uses ready mix concrete is “tilt-up” construction. In this process, reinforced concrete products are cast in forms at the work site and then “tilted-up” or lifted into final position. Structural elements, such as wall panels and bridge girders, can be made in this way. This technique has the advantage of ease of construction. Furthermore, large structural members or panels do not have to be transported by truck to the work site.
A related product is self-consolidating concrete, also known as self-compacting concrete. This material was new to the market in the 1980s. It is a highly flowable concrete that fills forms and is capable of encapsulating very dense arrangements of reinforcing steel without leaving voids and without the necessity of mechanical vibration. It settles into place entirely due to its own weight. This is accomplished by the addition of “superplasticizers” and viscosity modifiers, resulting in a product that is easily pumped and will flow into complex shapes and into hard-to-reach areas of the forms. The product commonly has more cement volume, less coarse aggregate, and more sand than typical concrete mixtures. Use of this material can reduce costs, labor requirements, and noise levels on the work site (National Ready Mixed Concrete Association 2010d).
Flowable fill, also called “controlled low-strength material,” is self-compacted, cementitious material used primarily as backfill or structural fill. It is an economical alternative to compacted granular fill. Flowable fill is a self-leveling material that does not require vibration or tamping. It hardens with minimal subsidence. Compressive strength of flowable fill is much lower than that of normal concrete. It must be less than 8.3 MPa (1,200 psi), is commonly less than 2.1 Mpa (300 psi), and may be as low as 1.4 MPa (200 psi). This allows for future excavation by hand if necessary. Mixtures with more than 20 percent entrained air by volume are used to reduce the strength of the material. Compressive strength in the range of 0.3 to 0.7 Mpa (50 to 100 psi) have a load-bearing capacity similar to well-compacted soil. Density when placed is between 171 kg/m3 and 216 kg/m3 (115 and 145 lb/ft3). Density can be further reduced by the addition of light-weight aggregate or fillers. Because the material flows into place, neither compaction nor leveling is necessary. Flowable fill can be made with very large amounts of such nonstandard materials as fly ash or aggregates in amounts not suitable for concrete. Flowable fill is used for slab support in unsuitable soil conditions, in closing roadway cuts and utility trenches, and as pavement base. It is used to fill cavities in abandoned mines and tunnel shafts, underground structures and tanks, under pavement, and with rip-rap in river bank and ocean soil erosion control. Flowable fill is not designed to replace concrete. It will not resist freeze-thaw cycles, abrasive environments, or aggressive chemicals. However, if flowable fill degrades in place, it will continue to act as granular fill (National Ready Mixed Concrete Association 2010a).
Pervious concrete allows storm water to pass directly through it because it contains little, if any, fine aggregate and the size range of the coarse aggregate is restricted to allow for little packing. Both rounded and angular aggregate can be used, leaving voids in the final product that constitute 15 to 35 percent by volume. Allowing water to pass through reduces the requirements for drainage infrastructure associated with pavements such as roads, driveways, and parking lots. Pervious concrete reduces storm water runoff and increases groundwater recharge. The exposed coarse aggregate enhances vehicular traction and reduces hydroplaning. This type of concrete can achieve compressive strength up to 20 MPa (3,000 psi), which is strong enough to support such heavy vehicles as fire trucks. In addition to parking lots and low traffic-volume streets, pervious concrete is used for sidewalks, paths, retaining walls, and slope protection (National Ready Mixed Concrete Association 2010b). Due to its ability to reduce or eliminate surface runoff from a building site, pervious concrete is becoming very popular in New York.
Ready mix concrete is used in insulating concrete form construction. This technique involves the use of interlocking rigid foam blocks or panels that are assembled on the work site in place of the traditional wood and steel concrete forms. Reinforcing is added and concrete is poured into the cavities in the forms. The foam forms are left in place. This type of construction offers significant thermal advantages. Walls so made have an insulation value of approximately R-20. In addition, they allow little to no air infiltration because they form an unbroken envelope on the building. They also deaden external noise. Finally, the thermal mass of the concrete moderates external temperature fluctuations (National Ready Mixed Concrete Association 2010c). The insulating value, air-tight construction, and thermal mass combine to an equivalent insulation value of R-40.
A market is developing in New York for roller-compacted concrete. The difference between this material and typical ready mix concrete is the method of placement. Roller-compacted concrete contains the same ingredients as normal concrete (i.e., fine and coarse aggregate, cement and additives) but it is a drier mix. Large capacity and mixing efficiency are critical to economical construction by this method. When spread, typically with an asphalt paver, roller-compacted concrete is sufficiently stiff to bear the weight of and be compacted by vibratory rollers. It is a high-strength concrete that can be constructed without expansion joints and without forms or reinforcing steel. It requires no finishing. If appearance is important, joints can be sawn into the surface after curing. If not, the material is allowed to crack naturally (Portland Cement Association 2010). It is commonly used for heavy-duty pavements, although the use of this technique began in the construction of gravity dams (American Concrete Institute 1999). Roller-compacted concrete is used for ports, military facilities, parking, storage and staging areas, intersections, and low-speed roads. In New York, this material has recently been used in large quantity for runways at the U. S. Army Military Post at Ft. Drum and in the harbor at Oswego (G. Novitzki, pers. comm., 2010).
In New York there are approximately 350 ready mix plants of which 274 are New York State Department of Transportation-approved facilities. Ready mix plants are not as likely to seek Department of Transportation certification as, for example, crushed stone quarries, as some of the ready mix facilities will never produce concrete for New York projects. The plants are located across New York with concentrations in the populated areas (Figure 30). Ready mix concrete, as construction aggregate, is a high-volume, low-value commodity. This, combined with the relatively short life span between mixing and placement, means that the plants must be relatively close to their markets.
Location of ready mix concrete plants in New York. Note that this map only shows facilities with NYS Department of Transportation approval. Some plants are located outside New York boundaries but feed into the New York market.
The greatest concentration of ready mix concrete facilities is in the New York City Metropolitan Region. According to the New York City Concrete Promotional Council, forty-four plants located in Queens (19), Brooklyn (15), the Bronx (6), and on Staten Island (4) produce 2.7 million cubic yards of concrete annually. This is a sufficient volume of concrete each year to build Yankee Stadium forty-seven times, the Lincoln Tunnel twenty times, or lay a two-lane road from New York City to Detroit.