Over-Vibration in Concrete

Do these photos look familiar? Are you finding water stains and “honeycombing” on the concrete when forms are removed? See Figures 1 and 2. Over-vibration could be the cause. Let’s take a closer look.

 

Pumpable concrete has become the norm on commercial jobsites, especially for vertical pours. Highly flowable concrete is delivered into the forms, and a vibrator is used to consolidate it. When the forms are removed, the surface of the concrete may show water stains or even honeycombing. These issues typically occur near the bottom of the wall or at formwork corners.

It’s a common misconception that honeycombing indicates an area missed by the vibrator operator. This can be true when dealing with low-slump, stiff concrete. However, with flowable concrete, honeycombing and water staining often result from material separation caused by vibration energy waves—not operator error.

A vibrator emits two types of energy waves: S-waves (shear waves), which are slower-moving and travel through solids but not liquids. Their intensity increases as the vibrator speed increases and can cause separation. The other type, P-waves (pressure waves), are faster-moving and can travel through both solids and liquids. The strength of this wave increases with the size of the vibrator head, which helps prevent material separation. (See Figure 3)

To avoid material separation, we need to control the energy output of the vibrator by reducing speed (S-waves) and increasing weight (P-waves). Slowing the speed of the S-wave allows water to remain in place, while the heavier vibrator head (P-wave) delivers energy that effectively removes air pockets and consolidates the concrete.

Testing has shown that simply reducing vibrator speed decreases material separation, as illustrated in Figures 4–6 (bucket test). Notice the amount of water driven through the holes in the bucket at each speed. On a larger scale, water separation becomes even more apparent when vibrator speeds are too high. You may observe water being driven out of the bottom of the form and pooling.

Material separation weakens the integrity of the concrete and leads to costly surface repairs. Vibrator manufacturers now offer controlled-speed vibrators that allow you to manage the energy being introduced into the concrete. These motors improve the integrity of the finished product and reduce the carbon footprint by minimizing surface rework.

If you’d like to learn more about controlled-speed vibrators, contact us. We’re always here to help!

Article from Minnich Manufacturing