Chances are that when a load of concrete shows up at the jobsite, somebody's going to add some water. And that's not necessarily a bad thing.
Both ASTM, the organization that sets test procedures, and ACI, which describes how to make and work with concrete, specifically permit the practice of adding water at the jobsite but with some very clear directions.
In its hardened state, concrete has to be sound and strong, and often its finish must conform to certain expectations. But in the plastic state, between the mixing and the placing, the contractor's main concern is with the concrete's workability.
At first that sounds like a vague term, but ACI offers this useful definition: Workability is “that property of freshly mixed concrete or mortar that determines the ease with which it can be mixed, placed, consolidated, and finished to a homogenous condition.” Obviously, that workability depends on how the concrete is being placed. For example, a stiff mix that's workable for slipform paving would likely not be suited for hand placement.
One standard tool for gauging concrete's workability is its slump. Mix designs that use viscosity-modifying and water-reducing admixtures can make concrete much more fluid than a standard mix without increasing water content, which is the other way to increase slump. To take an extreme example, self-consolidating concrete is so fluid that a slump measurement becomes meaningless—the material turns into a pancake-like puddle. (That has given rise to the slump flow test, ASTM C 1611, which measures the diameter of the puddle or its spread.)
Nonetheless, slump remains a good relative indicator of workability for standard concrete. It's useful in monitoring the consistency of multiple loads of concrete from the same source and using the same mix design. That, coincidentally, brings the test full circle back to its original purpose: “This test method is intended to provide the user with a procedure to determine slump of plastic hydraulic-cement concretes.” All the slump test tells us is the slump of the concrete, but field crews know that in a particular application they need concrete with a certain slump to ensure easy placement and good consolidation.
The specific procedure for testing concrete slump is spelled out in ASTM C 143, “Standard Test Method for Slump of Hydraulic-Cement Concrete.” It boils down to filling a 1-foot-tall truncated cone—8 inches in diameter at the bottom and 4 inches at the top—with freshly mixed concrete. The cone then is removed and the concrete generally slumps because it no longer is confined. The height of the concrete after the cone has been removed is subtracted from the original height to get its slump.
Although there are very stiff mixes that do not slump at all, a typical job specification calls for concrete to be delivered with a slump in the 2- to 6-inch range. Because many variables can affect concrete consistency between the batch plant and the jobsite, it is very helpful to have a system in place that allows for adjusting the slump in the field, says Ian Blair, CEO of Blair Concrete Services, Wilmington, N.C. “Mix design and workability are very important,” Blair says, “and the two are tied together.” That is one of the primary reasons the prepour conference is so important. Blair says the contractor, ready-mix producer, independent testing lab, owner's representative, general contractor, and engineer all should be there to discuss the placement procedure. That includes which testing procedures will be used, how they will be performed, and what options there will be available if, for example, a load of concrete arrives without enough slump.
It's at that meeting where, among other things, the project team agrees upon the procedure for adding water onsite. “They all have to buy into it,” Blair says. Additionally, it turns out that is exactly what the ACI and ASTM guidelines stipulate.
“ASTM C 94 permits the addition of water onsite where the specifications permit it and provided you don't exceed the water-cement ratio for the mix,” says Colin Lobo, senior vice president/engineering of the National Ready Mixed Concrete Association. To facilitate, producers batch the concrete to a certain slump at the plant and hold back on the water, he says. This allows the addition of water, within limits, at the jobsite. To be sure of when to stop, “The purchaser always can request to have the limit of water addition printed on the delivery ticket,” Lobo says.
The purpose behind allowing this water addition is outlined in both the ACI and ASTM documents. ASTM C 94 says if the project specifications allow it, a one-time addition of water is permitted in a truck mixer “when on arrival at the jobsite the slump of the concrete is less than that specified.” It goes on to say the drum should go for another 30 revolutions, or more if needed, at mixing speed to ensure the extra water is fully mixed in.
Section 126.96.36.199 (“slump adjustment”) in ACI 301, “Specifications for Structural Concrete,” says first that when water is added, it should be done as described in C 94, and then goes on to add a few cautions as well. They include: don't exceed the design w/c ratio or slump; don't add water unless the equipment can mix it properly; and don't add water after admixtures have been added onsite to achieve flowable concrete.
Blair says to make sure this item is addressed in the prepour meeting, noting one of the problems he has encountered are those engineers and owners who simply refuse to allow water to be added. When allowed, there are other details to attend to: How will the producer indicate how much water has been withheld or how much can be added at the jobsite? Who will be the one to add the water?
Producer practices vary, depending upon the automation of the batching and delivery systems. Blair says some of the producers he works with will print the amount of water withheld on the batch ticket, while others simply write it in by hand.
At the jobsite, the various parties look at the concrete, gauge its workability, and decide whether additional water is necessary. If so, it's only the producer's personnel who turn the valve. Typically both the producer's quality control person and the independent testing lab's representative read the gauge to verify the amount of water added, but the contractor personnel stay focused on the concrete.
One automated system that has emerged in the last few years uses high-tech drum rotation and hydraulic sensors to monitor the concrete as it is transported. Water or admixtures are added automatically to temper the material so the concrete arrives on the jobsite at the required slump. All additions are documented on the delivery ticket, as well as the standard information about the concrete. The ticket also includes the delivery time, location and amount of concrete, including its slump at that time, based on the sensor inputs. Although the C 94 specification does not yet include such automatic systems as alternatives to existing procedures, trucks have been using the system in the field for more than four years. Adopting such technology should eventually improve the consistency of concrete deliveries.
A note on pumping
Workability is a top concern when concrete is pumped. Although a pumpable concrete mix begins with a good mix design, it also requires good communication with the pump operator, whether it's one of the concrete contractor's own employees or a pumping subcontractor.
A good mix for pumping is dense, cohesive, and has sufficient mortar. Beyond that, though, the pipeline diameter should be 3½ to 4 times the maximum aggregate size, and that means the pumper needs to know about the concrete mix ahead of time in order to schedule the right equipment. It's more important if the mix will include fibers or larger aggregates.
“If the pumping company knows well in advance, they have opportunities to identify possible issues, such as site conditions, mix designs, and setup obstacles,” says Keith Bauer, operations manager, Coastal Carolina Pumping, Charlotte, N.C.
He also notes the importance of knowing ahead of time how much equipment will be needed. “With equipment weight restrictions on highways, pumpers don't always carry extra systems,” Bauer says.
Scheduling is another issue to keep in mind. Bauer says it's very important to allow the pump operator enough time to set up. “Overhead obstacles and power lines often are overlooked by the contractor and only discovered when the operator arrives onsite,” he says. “Also, soil conditions may cause the operator to have to make several changes to his machine's setup to ensure safety.”
The best precaution? Don't hurry.