Pumping concrete successfully
Successfully pumping concrete means transporting the concrete from the ready-mix truck to the desired project location quickly without any segregation. Compliance must also be maintained with applicable concrete building code criteria such as ACI 318, ACI 301, and ACI 332 (“Residential Code Requirements for Structural Concrete”). ACI 318, the concrete building code, and ACI 301, “Specifications for Concrete Construction,” contain minimal narrative about pumping concrete other than to advise that the discharge of pumped concrete should not result in segregation.
It is essential to remember that the pumped concrete in place must comply with all existing concrete compliance criteria, such as minimum compressive strengths, maximum water-cement ratios, air contents, and maximum slumps for exposure categories and classes. ACI 301, Table 18.104.22.168, stipulates minimum cementitious materials content for floors. For a nominal ¾-inch maximum size aggregate – which is the favored pump mix aggregate top size – the minimum cement content is 540 pounds per cubic yard, equivalent to a 5.75 bag concrete mix.
Anyone involved in placing concrete using pumping methods should review ACI 304.2R, “Guide to Placing Concrete by Pumping Methods,” because this standard discusses equipment use, proper mixtures for good pumpability, and field practices. While ACI 304.2R is not code, it is an excellent guide, and the project team should discuss it before the commencement of construction.
One of the first things to discuss is the concrete mix design proportions of the proposed pumped mix. The project team should always use a concrete mixture with a proven track record of success related to pump mixes. There is nothing wrong with selecting concrete mix proportions that enable the mix to be a pro-pump mixture, provided the mix proportions comply with the applicable building codes. Sixty percent of concrete is pumped on commercial projects, while the remaining is pumped on projects designated residential. The commercial pump mixes have to comply with the compliance criteria of ACI 318 and ACI 301, and the residential pump mixes must adhere to the compliance criteria of ACI 332 and ACI 301.
Determining the cementitious materials content for a pumpable mixture follows the same basic principles
used for any concrete. The popular idea that you should use extra quantities of cementitious materials to make the concrete mix pumpable is not well-founded nor economical. The substitution of fly ash for a portion of the portland cement improves the concrete mixture pumpability. The use of a water-reducing admixture will increase the workability and pumpability of the concrete.
The coarse aggregate in the pump mix must comply with code (ASTM C33), and the amount of coarse
aggregate per cubic yard will be defined by ACI 211.1, Table 6.3.6, including the tolerances allowed by Table
6.3.6. Most concrete pump mixes use coarse aggregate ranging in sizes from ¾-inch (top size) down to #7, #8,
and #89. There is a consensus that gravel mixtures pump a little better than crushed stone because stone tends to be angular, while gravels tend to be more rounded. In concrete pump mixes, the maximum coarse aggregate size is limited to one-third of the smallest inside diameter of the pump or pump line.
The fine aggregate (sand) properties play a more prominent role in proportioning pumpable concrete mixtures versus coarse aggregate. Together with cement and water, the fine aggregate provides the mortar that conveys the coarse aggregate in suspension and limits its motion to adjust to delivery line configuration changes. ASTM C33 requires that any sand used to produce concrete have a fineness modulus (FM) between 2.3 and 3.1. The FM is obtained by adding the percentages of material retained on selected sieves and dividing the sum by 100. For a concrete pump mix, a perfect FM would probably be about 2.5.
If the concrete mixture utilizes lightweight aggregate and the mix is pumped, the lightweight aggregate must
be pre-soaked before batching at the concrete plant. Let’s assume the lightweight aggregate has a total
absorption rate of 6%, and the concrete is batched with the lightweight aggregate registering an absorption of 3% when measured before batching. Under the pressures exerted by pumping, not only may that 3% be forced into the lightweight aggregate in the pump line, but line pressures can force even more moisture into the aggregate during the pumping process. This loss of water from the mortar in the pump line reduces fluid properties and the pumpability of the concrete. Pump line blockages occur, which is the last thing you want to happen when placing lightweight concrete.
Nothing is more critical to the success of pumping concrete than the proper selection and use of the
entire pumping system, including what size and type of pump to use. You must choose the most efficient
configuration of the pumping system, the pumping rate, line diameter and length, horizontal and vertical
distances, number of elbows, and bends. Resistance in the line will increase if there is a reduction in pipe
diameter along the path the concrete travels. The further and higher the concrete needs to go, the more
pressure it will take to get the concrete there. If significant height and horizontal distance is involved, a good
option is to use two lines and two pumps. When the pump logistics are correct, there is little doubt that
pumping is an efficient, reliable, and economical means of placing concrete. Sometimes it’s the only way to get the concrete to specific locations.
Loss of Air Content in Pumped Concrete
Specifiers often request that the air content of the concrete be tested and reported at the discharge end of
the concrete pump hose at the point of placement. While understandable, this request conflicts with the code (ASTM C 31), which requires the air content test associated with the QC sample be taken from the ready-mix truck chute. In some cases, the air content is much lower when tested at the point of placement than the concrete QC samples tested at discharge from the truck chute. It’s not unusual to experience a slight
air loss of 0.5 to 1.0 percent as concrete is conveyed through a pump system. However, when long boom
pumps have the boom oriented with a long, near vertical downward section of pipe, the air content
at the point of placement (from the discharge hose) may be less than one-half of the concrete going into
the pump hopper. When the boom is upward or horizontal, or if there is a 12-foot section of rubber
hose placed horizontally at the discharge end, there generally is no significant loss of air due to pumping.
Why the Air Loss
Air loss will occur if the weight of concrete in a vertical downward section of pipe is sufficient to overcome
frictional resistance, which allows a slug of concrete to slide down the pipe. As this occurs, it develops a
vacuum on the upper end that significantly expands the air bubbles. When the concrete hits a pipe elbow in
the boom or a horizontal surface, the bubbles collapse. The transition from high pressure in the pump to a
near-vacuum condition in the pump line can further exacerbate air loss.
How to Prevent Air Loss
When possible, avoid vertical or steep downward boom sections. Lower the boom as close as possible to
the surface where the concrete will land. Utilize a 12-foot section of rubber hose at the discharge end of the
pipeline with at least 5 feet of the rubber hose horizontal at the end of the pump line and lined up to properly discharge into fresh concrete. Lastly, ensure a continuous stream of concrete within the pump and the pump line.
In general, the influence of pumping on air-entrained concrete is minimized by maintaining the lowest possible pumping pressure, minimizing freefall within a vertically descending pipeline, and reducing impact by directing discharge from the hose into previously placed concrete. The discharge height from the end hose should be minimized to a maximum of 3 feet (ACI 304.2R).
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