Things you know you should do, but frequently don't: eat right, exercise regularly, brush after every meal, change the oil every 3,000 miles. In the concrete construction world: consolidate by vibration.
The average concrete contractor may understand the reasoning behind consolidating concrete by vibration, but still choose not to do it. Why? Largely because if he's not forced by a specification or diligent quality control, it's a corner that can easily be cut, usually without immediate consequences—or often any consequences, since the potential damage from the compromised concrete may not show up for years. And surface defects that do show up immediately may be attributed to a host of other factors. For those wishing to, it's not difficult to camouflage skipping the vibration step.
Nonetheless, consolidation by vibration is an important part of doing the job right. And, what's more, consolidation by vibration isn't particularly difficult, isn't particularly complicated, isn't particularly time-consuming, and the equipment, in terms of service life, isn't particularly expensive. This article will examine the basic hows and whys of this sometimes overlooked, yet necessary, process.
Freshly poured concrete, in addition to the standard ingredients, contains a sizeable degree of entrapped air—up to 30% in some cases. If the air is allowed to remain—if the concrete is unconsolidated—the concrete is vulnerable to early deterioration, honeycombing, high permeability, poor bonding, decreased strength, and an aesthetically unpleasing appearance. To be sure, it's still concrete, and the building isn't going to collapse, but it's imperfect, incomplete, not watertight concrete.
What consolidation by vibration does (with the able help of gravity) is compact all the solid particles together in an even mix, liquefying the mortar, releasing the air bubbles, eliminating voids, and resulting in structurally solid and sound concrete. Think of it in terms of a jigsaw puzzle, with all the components fitting together tightly in their proper places, instead of a loose collection of separate pieces arranged in the rough outline of a rectangle.
Or, perhaps, as a glass of ice water, with the ice bunched together tightly at the top or bottom. A little shake of the glass, and the ice more evenly distributes itself, often with a few air bubbles rising to the top. The same principle holds for concrete vibration: shake it up, let the air out, and the concrete—like the water and ice mix—will take the shape of its container, flowing around reinforcing steel and into all the form's corners and empty spaces.
There are two methods of consolidation, although one is now almost obsolete. Manual consolidation via tamping, rodding, or spading was standard some decades ago, even after mechanical means became available. Today, however, while you may see a worker with a shovel handle or piece of reinforcement moving some concrete into a corner, manual consolidation is rarely done. With the exception of extremely small jobs, consolidation today is done by mechanical vibrating equipment.
That equipment falls into two categories, internal and external. With internal vibration, a vibrating head on a shaft is inserted directly into the mix. With external vibration, also called form vibration, the equipment is attached to brackets or tracks on the forms themselves, which transmit the vibration energy to the concrete. Both types have their appropriate uses, and there are several choices within each category.