By Alex Morales, M.Ed.
Editor’s Note: This is the fifth article in a year-long series that focuses on the details and more technical aspects of one common thing precast concrete producers do on a daily basis.
Every precast product design assumes a dense, homogeneous, consistent mass of concrete surrounding the reinforcement embedded within it. Concrete’s density, bond to reinforcement and adherence to design are critical for any structure to perform as anticipated.
As a composite material of cement paste and aggregate, however, conventional slump concrete does not flow like a liquid to completely take up the space of its container. As fresh concrete is placed in formwork, pockets of air will form. These air voids, called entrapped air, are of irregular size and shape and can compose up to 20% of the placed volume. Entrapped air will prevent proper bond to reinforcement, decrease the strength and increase the porosity of hardened concrete, all while creating formed surface defects like rock pockets, bug holes and honeycombing. These pockets of entrapped air must be remediated as completely as possible to ensure the concrete will have the density and uniformity required by design.
Consolidation is the process of densifying a concrete mix by reducing the volume and proportion of voids. A consolidated mass could easily be achieved if concrete could behave like a liquid and freely flow into all corners of the form and prevent entrapped air pockets created during placement. Fortunately, we can rely on a phenomenon known as liquefaction to consolidate the mix. Liquefaction is the temporary change in behavior of a solid to a liquid due to an external shaking or rapid loading.
In order to create liquefaction of a fresh concrete mix, we need to introduce an independent force that will liquify the mixture so that it fills entrapped air pockets. The most commonly used consolidation method is vibration. Temporary liquefaction of the concrete mix by means of vibration will coax the entrapped air pockets to the surface of fresh concrete and ensure the concrete sufficiently consolidates to completely fill the formwork.
The use of vibrating equipment has long been associated with consolidating a concrete mixture. The most commonly specified procedure for consolidation by vibration is ACI 309, “Guide for Consolidation of Concrete.” Much like the chemical reaction between cement and water (called hydration), the temporary liquefaction of a mix in order to expel entrapped air voids is a bit of a science.
Vibrators typically are powered by electric or gas motors, compressed air (pneumatic) or hydraulic gear motors. Electric and pneumatic are the most common in the precast industry. Regardless of the power source, there are three main factors to consider during the vibration process:
- Type of vibrator
- Vibrator settings
- Vibration time
The specific mix design, type of form and amount of reinforcement will impact these three consideration factors.
Concrete vibrators are divided into two major categories: internal and external. External vibrators are attached directly to formwork and vibrate the concrete through the form, while internal vibrators feature a vibrating head that is inserted directly into the fresh concrete mix.
Internal vibrators are popular in the precast industry and consist of a power source, a shaft and a vibrating head. Internal vibration is a manual procedure requiring hard work and careful attention, but don’t give in to the temptation to give the task to the newest worker. Training is crucial to the proper operation of the equipment. The operator must interpret what it means when air bubbles burst at the surface and know when to stop vibrating. An experienced operator will know when the concrete has been properly consolidated.
An internal stick vibrator should be inserted vertically into concrete under its own weight at a rate of one foot per one second. It should be withdrawn at a slightly slower rate, about one foot every three seconds. For the first concrete lift, the vibrator head should be inserted to almost touch the bottom of the form. Vibration forces extend laterally from the head, not below the tip of the head, so the operator should watch for the zone of influence, which is the circumferential area consolidated when the vibrator is inserted into the concrete. When the vibrator is inserted each subsequent time, it should be done close enough to the previous location so that the zones of influence overlap. This process should be repeated over the entire product so the whole concrete mass is affected by the radiuses of influence of all the vibrator insertions. When vibrating subsequent lifts, the vibrator head should always penetrate the lift beneath it to prevent a cold joint. The operator should also know that concrete should be vibrated in lifts of 12-to-18 inches.
Your internal vibrator’s head diameter will play a role in the effectiveness of the consolidation process. The size of the head will affect the vibrator’s zone of influence. The larger the head, the greater the impact on zone of influence. If you’re accustomed to an insertion pattern with one vibrator, you may need to make adjustments if you must use a different vibrator with a different size head. Operators should always pay attention to the effect of their equipment on the concrete during vibration.
External vibrators are often called form vibrators, but it’s important to understand that while they are attached to formwork, they are designed to vibrate concrete. Their location must consider the thickness or mass of the concrete being vibrated. As a rule of thumb, external, form-mounted vibrators should not be turned on until the concrete level is at least 6 inches above the location of the vibrator. If the supplier suggests a higher concrete level above the vibrator, that recommendation of the vibrator supplier should be followed. External vibrators should never be turned on when the forms are empty. Following this recommendation will prolong the life of both the vibrator and the formwork.
External vibrators will usually consolidate a concrete radius of about 18-inches but the exact impact will vary from case to case. This type of vibrator will impart vibratory forces on both the formwork and the concrete and will therefore consume more power than internal vibrators in order to obtain the same level of consolidation. Some precasters prefer them, however, because they require less time for consolidation compared with internal vibrators. Form vibrators are the vibrator of choice for zero-slump concrete and are often attached to the core and jacket of dry-cast forms. Dry-cast equipment may be designed to work as a system (a specific vibrator with a particular form system), so use caution when attaching a different type of vibrator to a dry-cast form. Always consult the equipment manufacturer and the vibrator supplier when making such a change.
Form vibrators should never be attached directly to the form skin. This may lead to fatigue stresses on the form in the area near the vibrator mount. Attach form vibrators to a structural stiffener in locations recommended by the form manufacturer.
Whether you’re using an internal or external vibrator, there is usually a weight within it called the eccentric. The movement of an unbalanced eccentric is what causes the vibration. The size of the eccentric affects two important settings of a vibrator: its frequency and amplitude.
The frequency and amplitude of the unit will impact liquefaction and consolidation. The frequency setting refers to the number of cycles the vibrator makes per unit of time, typically referred to in units of vibrations per minute. The greater the frequency, the more vibratory impacts the concrete receives. High frequency settings on vibrators primarily affect fine particles, moving sand and slurry around coarse aggregate and ultimately liquefying the mixture.
Amplitude is the unit’s deviation from the point of rest. The higher the amplitude, the greater the distance the vibrator head will move and the stronger the vibratory energy imparted to the mix. Higher amplitude settings are preferred with large aggregate mixes as they move coarse aggregate and determine radiuses of influence.
The frequency of pneumatic vibrators can be adjusted easily by changing the air pressure, while some electric vibrators have adjustable frequency settings. The amplitude typically can be changed by changing the mass of the eccentric weight or head diameter of internal vibrators. The mass of the eccentric weight can also be changed in certain types of external vibrators.
Optimum frequencies and amplitudes will vary by product, mix design and form size. A consultation with plant engineering staff or the vibrator supplier will assist in determining the ideal setting for your mix design, type of form and amount of reinforcement. This is true whether you are using an internal or external vibrator.
The ideal vibration time is not always a set number of minutes and seconds. It is the amount of time a vibrator needs to completely consolidate the specific mix design, in its particular formwork, with its unique reinforcement configuration.
Experience with a vibrator, the mix design and formwork is the best way to determine ideal vibration duration times at particular amplitude and frequency settings.
The length of time you vibrate the concrete should ensure the release of all entrapped air pockets but prevent the segregation of the heavier coarse aggregate particles from the paste matrix. At the start of the vibration procedure a large quantity of entrapped air bubbles can be seen escaping the concrete at the surface. As vibration continues, the rising of air pockets slows and a sheen will appear over the concrete surface. The sound of the vibrator will deepen as it densifies the fresh concrete.
The amount of time you should vibrate concrete is also affected by the vibrator type, size and settings. Consult your vibrator supplier for this information and let experience be your guide.
No two uses of concrete vibrators are exactly alike. With factors such as mix design, reinforcement configurations, vibrator type and size all coming into play, the best way to ensure a quality product is training, experience and following manufacturer guidelines.
Alex Morales, M. Ed., is NPCA’s director of workforce development.
For step-by-step guidelines throughout the entire vibration process using internal or external vibrators, check out NPCA’s Precast Learning Lab video on proper consolidation at precast.org/learning-lab