By Luke M. Snell
One of the big challenges on construction projects is to get concrete adequately vibrated. On many projects, the person vibrating the concrete is poorly trained and does not realize that the job is one of the most important to achieving a well consolidated concrete.
This has become more critical; now we can place concrete much faster than we can consolidate it. Thus, it is management’s responsibility to adequately train employees on how to consolidate concrete. When we place concrete it contains an excessive amount of entrapped air. Without removal of this air, concrete will have voids (called honeycombing), low strength, be porous and be unattractive. Consolidation of concrete is the process of removing this excessive air and making the concrete into a uniformly solid mass with the needed properties. Consolidation also levels the fresh concrete and improves the bonding with the reinforcement.
How is consolidation achieved on the construction project? The simplest way consolidation is achieved is by hand rodding. Although it can be successful on the higher slump concretes, it is slow process. Thus most companies use the internal vibrator called the poker or spud vibrator. This vibrator has a spinning eccentric weight encased in a metal housing that will shake or vibrate the concrete in close vicinity to it. The vibrator causes the nearby concrete to settle and forces the entrapped air out of the concrete.
What are the effects of poor vibration techniques? Poor vibrations will result in voids at the surface of the concrete – normally called honeycombing. This is caused because the entrapped air was not removed, the concrete did not settle, and the voids were not filled with the paste from the concrete mixture. Since this concrete has voids, the strength of the concrete can be low. This concrete can also deteriorate quickly since water and other materials can enter the concrete through these honeycombed areas resulting in corrosion of reinforcements and further deterioration to the concrete as it goes through freeze thaw cycles.
Examples of honeycombed concrete.
What are the good techniques to prevent honeycombing problems? There are simple “rules” for how to vibrate concrete and preventing honeycombing issues:
1. The area affected by the vibrator is about six times the diameter of the vibrator. A one-inch diameter vibrator consolidates a circle with a diameter of about six inches. This means that the vibrator must be inserted in several locations to properly consolidate the concrete.
2. Vibrators must be inserted into fresh concrete vertically and not used to move the concrete. When moving concrete with the vibrator, the concrete will segregate; the lighter materials move (basically water) while the heavier materials (cement and aggregates) stay where the concrete was deposited.
3. The vibrator needs to be inserted into the previous layer about six inches. This knits together (combines) the recently placed layer and the previous placed layer.
4. The vibrator should remain stationary in the concrete from 5 to 15 seconds then removed slowly. This timing will adequately vibrate most concrete.
How do I know if the consolidation is completed? There are four things that the user must observe that indicates that the consolidation is complete.
1. The concrete levels: One of the first results of vibration is that the concrete levels or is at a uniform height in the forms. Some refer to this as “melting down the concrete”.
2. The large aggregate will disappear: The vibration is causing the paste to fill in voids, thus the large aggregates will not be visible.
3. The air bubbles will cease to be rise: One reason we vibrate the concrete is to remove the entrapped air. As you vibrate concrete, the air bubbles will rise and exit the concrete surface. When these air bubbles stop, you are near the end of vibrating the concrete in that area.
4. A thin film of water appears at the surface: Water is lighter than cement and aggregates so when vibrating concrete, water is expected to rise to the surface.
Note: The above means that you have vibrated concrete in that one area and are ready to move the vibrator to a new area to begin the consolidation process again. You need to reinsert the vibrator so that the all the concrete in vibrated.
Can I over vibrate the concrete? Yes, concrete can be over vibrated. If the vibrator is left in the concrete for too long, a thick film of water will be on the concrete layer and the cement will tend to sink. This results in a lower strength at the surface of the concrete. I have seen few problems with over vibrating concrete. I have seen many cases of concrete that was not properly vibrated. I would recommend that it is better to over vibrate concrete than to under vibrate concrete.
Does the person vibrating determine how fast I can place concrete? The person vibrating the concrete is the key person that determines the placement rate of concrete. We can place concrete with a pump or from the chute much quicker than we can vibrate concrete. Thus, the consolidation rate determines how quickly we can place concrete. ACI Committee 309 (Consolidation of Concrete) provides a table that can help determine the speed of placement. For example a single one-inch diameter vibrator can limit the placement rate to 1 to 5 cubic yards per hour. A six-inch diameter vibrator allows a placement rate of 25 to 50 cubic yards per hour. Thus the number and size of vibrators on the project determines the rate of placement that can be achieved.
Can vibrators break down? Yes! For those who use them regularly they will tell you to expect them to break down. The vibrators are used in an environment that is dusty, wet and in all weather types. The vibrator will have cement splatter on it. All of these conditions can interfere with the motor’s operation. Breakdowns are probable and should be expected. A contractor should always have spare vibrators available and easily accessible so they can maintain the desired placement rate and the placement is not delayed while trying to find the spare vibrators.
What is the best way to make sure I get adequate vibration? The best way to get adequate vibration on the job is to make sure the people doing the vibration are well trained. This procedure is not obvious and management must take time to explain the correct procedure. Management must also explain how to know if the vibrator is working properly, and when the concrete vibration is complete. These extra steps will eliminate many of the problems and greatly cut down on repair problems.