Introduction
The 1990's have seen an increase in concrete design strengths in pavements, pre-cast products, and throughout the general construction market. This phenomenon has posed many on-site placement problems, but one concept that is often misunderstood is vibration. Higher design strength usually means a lower water/cement ratio. Even the substitution of a water-reducing admixture may not eliminate the gradation or placement problems that may occur. The end results of placement may not be satisfactory, and the contractor is often left wondering how it happened and how it can be avoided the next time. The answer is clear and simple - you can't. The same material amounts, mixed identical to the day before, may have some placement variability. High-strength mix design may have the contractor and the materials and equipment suppliers scratching their heads. There are no universal answers. Although there is much variability in design and placement, the problems associated with vibration are not hopeless. An understanding of the tool and the medium is helpful in achieving the desired effects.
The Mix
If the aggregates in the concrete mixture are well-graded, and there is enough fluidity present in the paste matrix, placement should go well. If there is barely enough water for the hydration process and the coarse aggregate does not have an even-size differential from largest to smallest particle, it may present undesired effects. Also, the air entrainment, if not checked and controlled, can affect the level of consolidation that is achieved. These instances are the basic considerations, and additional placement variables can also affect results.
The Tool
Concrete pavements are placed with a variable speed hydraulic vibrator. The vibrator head operates by spinning an off-centered weight about 10,000 times a minute. In the process of the vibrator trying to center itself, a head housing with a certain weight slams into particles of concrete. Those particles transfer the energy to surrounding particles much like the principles of another vibration product - sound. The parameters of this action can be expressed as: speed x weight = force. An amplitude measurement is a linear measurement of the displacement of the vibrator's center point. Amplitude is a consideration in "radius of action."
The Dynamics
In pavement placement, the properties of the mix and dynamics of the vibrator are now being investigated in the United States on the State level. Although more testing needs to be done, the initial tests reflect the studies that are available in the Portland Cement Association library. The vibrator speed can have a bad affect on air entrainment. It can also affect how the coarse aggregate is distributed throughout the pavement depth. As in sound waves, higher pitched (higher frequency) waves affect smaller particles. The lower pitched (lower frequency) waves affect larger particles. Since force facilitates the compaction process, the relationships of speed and weight and the development of force is key in the vibration process.
Air Arrangement
From past studies we realize what the adverse affects of speeds (frequencies) can be. The vibrator needs to be designed to extract entrapped air (macro-air) from the placed mix while not disrupting the air entrainment (micro-air). If the frequency is too high in order to arrive at the necessary force to consolidate, the micro-air system may be damaged. The amount of design slump, amount of air entraining agents and temperature, type of sand, types of aggregates, and the duration of time spent with a vibrator in a cubic foot of concrete all play an important role in concrete compaction and ultimate durability.
Particle Arrangement
Concrete consists of many different sized and shaped particles with a variety of specific gravities. These particles are held to their location by the consistency of the paste matrix. High-slump concrete can have some separation (larger rock moving toward the bottom) if the vibrator is left in a cubic foot of concrete too long. In reality, most problems occur from under-vibration (expressed in time per cubic foot) given a reasonable consistency of the mix design. With low slump, high-strength concrete mixtures, it is possible to move aggregate away from the vibrator head, and the void is then back-filled by paste as the vibrator exits. These variances are referred to as post holes or vibrator trails. This condition is believe to be caused by the vibrator frequency not matching the natural frequency of the mix. The frequency may not be in a range to properly energize the larger particles to snap back into place after the vibrator exits. Again, speed is the culprit.
