Most decorative concrete contractors know that consolidating concrete is as essential to a quality finished product as mix ratio or proper finishing and curing. Consolidating forces air out of the concrete and encourages aggregate to rotate and fit together tighter, creating stronger concrete. Shake a bucket full of loose gravel and notice how the level of the gravel goes down - that's what consolidation does.
Against forms, proper vibrating (consolidating) pulls paste to the forms and eliminates honeycombing. It also pulls the fines to rebar, creating a tighter and stronger bond.
Concrete is stronger and more durable when the aggregate is evenly distributed and tight. Air pockets weaken concrete, allowing moisture to linger within the finished product, rendering it more vulnerable to the freeze-thaw cycle. The introduction of self-consolidating concrete in the late 1980s may have raised hopes that vibrating concrete would become a thing of the past. That has not been the case, as the relatively high cost of material used in such concrete continues to hinder its widespread use.
Consolidation tools include various types of hand-held head vibrators, form vibrators, vibrating screeds, and even vibrating tables for use in countertop manufacturing. The rule of "the right tool for the right job" applies as much with consolidation tools as it does with any other aspect of construction.
Hand-held or "stinger" vibrators are the most commonly used ones throughout the concrete industry. These are the vibrators with the heads or shafts (either round or rectangular) that are inserted into freshly placed concrete. Round heads are prevalent, though some manufacturers continue to offer rectangular attachments. There seem to be no advantages to round versus rectangular heads - the choice is said to be mainly a function of operator preference. There are some simple rules of thumb for optimum results with a stinger vibrator:
- Vibrating naturally brings moisture to the surface of concrete (particularly flatwork) in circular patterns. Make these circles overlap throughout the pour - this bonds batches and lifts of concrete into a monolithic pour.
- Completely immerse the stinger into the concrete.
- Insert the stinger vertically and quickly, but withdraw slowly.
- When concrete is placed in multiple pours or lifts, insert the stinger about 6 inches into the previous layer, intermingling the lifts.
- Start vibration when the stinger is completely submerged into the concrete.
- Stop vibration when the surface becomes shiny and there are no more air bubbles surfacing.
Form vibrating is generally faster than stinger vibration - however, your forms need to be stronger. As a result, form vibrators are the vibrators of choice for some contractors who are reusing forms (within the precast industry, for instance). Some tips for using form vibrators include:
- The vibrator(s) should not be fastened directly to the form. Mounting brackets should be welded onto the form. The vibrator is then attached to the mounting bracket.
- The vibrator should be located where it's potential will be maximized. The supplier should offer specifications.
- Start vibrating when the concrete is a few inches above the location of the vibrator.
- Stop vibration when the concrete has a glossy surface and there are no more air bubbles surfacing.
Vibration tables are popular in the concrete countertop industry. They offer more uniform control and results, allowing countertop artisans to create consistent products. Frequency (vibrations per minute) is the key to achieving the best results with a table vibrator. Manufacturer's guidelines should be followed and experimentation with test pours is encouraged.
Regardless of the type of vibrator being used, surface imperfections may be caused by either too little or too much vibrating. Undervibrating can result in honeycombing and excessive entrapped air. Overvibrating may cause segregation of aggregate, form damage or defl ection, and sand streaks. It's a good idea to perform a small test pour to determine the optimum vibration time and frequency.
Allen Engineering Corp. - Power Vibe and Power Vibe Pro
The Allen Power Vibe and Power Vibe Pro, from Allen Engineering Corp., are hand-held concrete vibrators powered by four-cycle 35cc gasoline motors. They feature flexible shaft lengths and a variety of head sizes. These lightweight vibrators are user-friendly for work on slabs, stem walls and foundations. The low-friction vibrator heads allow extended run time outside of concrete without burning up.
Northrock Industries Inc. - Pro Series
Offered by Northrock Industries Inc., the Northrock Pro Series of electric vibrators includes three models ranging from the Pro 1.5, for small jobs requiring a 1 1/2-inch head or less, to the Pro 2+, which can power up to a 2 1/2-inch head, and the Pro 3 for the most demanding applications. Northrock's electric motors are protected by high-strength cast aluminum housing. The shock-absorbing frame protects the housing and acts as a carrying handle. The vibrator heads are factory-sealed, require no maintenance and have lifetime warranties. Optional soft tips and coated heads are also available.
Stone Construction Equipment Inc. - Vibrators
Stone Construction Equipment Inc. offers a full line of concrete vibrators. Its electric and gas-powered vibrators come with a wide range of interchangeable flex-shafts and heads ranging in diameter from 3/4 inch to 2 1/2 inches, including a 1 7/8-inch rubber head for epoxy-coated rebar applications. Stone concrete vibrators feature a unique mold-injected nylon quick-disconnect system to allow quick and easy flex-shaft changes. The American-made Stone concrete vibrator line is backed by a 90-day warranty. Plus, it delivers quality in a super-tough design encased in a chrome-plated tubular steel frame. Oversized rubber mounts absorb vibration to reduce operator fatigue. Lightweight for easy operation, the electric motors also feature a viewing port for quick inspection of brushes.
Wacker Neuson Corp. - Vibrator
Wacker Neuson's versatile high-frequency vibrator can be adapted to every type of application. Heads and shafts can be easily combined and rapidly exchanged to match the right equipment to the job. Unique hybrid heads promise optimum concrete consolidation and movement.
Quick and easy flex shaft replacement with quick disconnect coupling.
Grounded motor for improved safety.
Dual air filters extend motor life and allow for easier maintenance.
Suited for on-site vibration of concrete for foundations, walls columns and slab work, as well as for in-plant vibration.
Vibco - US-900
Vibco offers a wide range of electric, pneumatic and hydraulic vibrators. The US-900 electric form vibrator is one of their hardest-working and most popular. Featuring 900 pounds of force and a maximum 10,000 vibrations per minute, the US-900 is good for either precast or pour-in-place applications.
Contractors who have used the US-900 in concrete wall applications reportedly say they have achieved a smoother finish with no air bubbles, significantly fewer labor hours spent on hand-finishing, and an increased rate of production.
Wacker Neuson Corp. - AR3, AR5 and AR7
Wacker Neuson offers a range of external vibrators (the AR3, AR5, and AR7), each of which provides maximum performance under extreme conditions with continuous operation. The vibration-proof and heat-resistant coils in the electric motors as well as the special heavy-duty bearings provide for durability and reduced maintenance.
Three different fastening systems are offered: on a base plate with a taper pin, with a quick-connect clamp device (for model size 5) or with a flanged attachment (different bore patterns available).
Units feature Class H insulation for reliable continuous operation.
All models are available in special versions for all international standard frequencies and voltages.
The Concrete Countertop Institute - Vibrating-Table Plans
The Concrete Countertop Institute recently began selling vibrating-table plans for countertop manufacturers who use the wet-cast method of making precast concrete countertops. Their plans allow countertop artisans to:
- Learn about the best and most cost-effective vibrators for a concrete countertop casting table.
- Learn exactly where to place the vibrators for even and optimum vibration.
- Use detailed, step-by-step instructions for building the table and mounting the vibrators properly.
- Take the guesswork and mistakes out of constructing a vibrating casting table.
Vibco - Vibrators and Accessories
Vibco supplies a variety of vibration options to the concrete countertop artisan: UMC-1, -2, or -3 adapter brackets that make it possible to attach external vibrators to a table, the model PJT-100 portable vibratory table, and the Vibra-Beam, which mounts to a sawhorse and is a low-cost alternative to a standard vibration table. Each enable the contractor to control the speed and vibration needed, and each plugs into a standard outlet or generator.
Allen Engineering Corp. - E-Screed by Magic Screed
Allen Engineering Corp. has expanded its wet screed equipment line with the introduction of the E-Screed by Magic Screed. The E-Screed is a precision-engineered lightweight wet screed for single operator strike-off of concrete. This high-frequency screed produces uniform vibration distribution over the entire blade length.
The E-Screed utilizes an environmentally friendly lithium iron phosphate 36-volt battery. The battery can be charged at any time and is maintenance-free.
The E-Screed can be used with existing Magic Screed blades. Allen Engineering is the exclusive distributor of Magic Screed products in the United States.