Vibration techniques

Vibrators are easy to operate, although getting good consolidation requires a knowledgeable craftsman. Some European countries actually require vibrator operators to he licensed. Here are few tips to improve consolidation:

• Do not use vibrators to move Concrete horizontally—this can result in separation of the aggregate from the paste (segregation). Vibrators can, however, he used to “melt” a pile of concrete that was placed by bucket or buggy.
• Keep lifts in walls and columns less than 20 inches thick or about the same as the length of the vibrator head.
• The critical questions for the vibrator operator are how far apart to insert the vibrator and how deep to penetrate into the preceding lift.

• Vibrators should he sized and positioned to ensure that all concrete within the forms falls within the radius of influence (see Fig. 1). Use the largest vibrator that will fit between the reinforcement. The center-to- center distance between vibrator insertions should be 1 ½ times the radius of influence of the vibrator.
• Lifts should he placed while the preceding lift is still soft enough to penetrated by the vibrator. If that’s impossible, vibrate near the cold joint hut realize that a lift line will be visible.
• A graphical technique can help to determine how deep to penetrate into the preceding lift. Knowing that the vibration waves slope up at about a 30 degree angle from the tip, place a 30-60-90 triangle at the midpoint between  the insertion points and measure the depth of revibration into the preceding lift and then the total insertion depth (see Fig. 2). Making sure that the boundary between lifts is vibrated will knit together the lifts and eliminate lift lines in the concrete.
• Insert the vibrator as quickly as possible to the proper depth into the previous lift. Typically it will sink under its own weight.
• Hold the vibrator at its maximum depth for S to 1S seconds. Withdraw it at 3 in./second for structural concrete or 2 in./second for architectural concrete (these are the withdrawal rates recommended by AC! 309). Slow withdrawal allows the vibrator to stay below the escaping air forcing it up and out of the concrete.
• Moving the vibrator up and down slightly closes the hole behind the vibrator.
• Make sure the vibrator head remains vertical—don’t force it in at an angle.
• Don’t force the vibrator into congested reinforced concrete because you can end up with a vibrator that is stuck in the reinforcement.

• Proper consolidation is often a judgment call—experienced operators can tell if they are getting good consolidation by the sound of the vibrator.
• Despite all this, if you still have hug holes in the concrete surfaces (after stripping the forms), reduce the distance between insertion points by 20% to 30% below the standard 1½ times the radius of influence. If there are still some hug holes try spading near form surfaces
• Do not let the vibrator tip contact the form surfaces—this can damage the form panels. Rubber vibrator head tips can help reduce this potential damage.
• After the concrete surface has
stopped bleeding, revibration can he used to remove air pockets in the top lift. Don’t revibrate deeper than about 3 feet and also not with very stiff or harsh mixes.
• For exposed architectural concrete, experiment with vibration techniques to avoid color variations on the surface. For more information, read Guide to Cast-in-Place Architectural Concrete Practice, ACI 303 R-04. 

 

FIGURE 1 Use the proper size of vibrator and keep vibrator insertions

at 1 ½ time the radius of influence

 

FIGURE 2 A graphical technique can help in determining howdeep the vibrator shoul penetrate into the preceding lift.

Vibrating Self-Consolidating Concrete

By definition, self-consolidating concrete does not require mechanical consolidation. But the problems with SCC are that it is susceptible to segregation, requires more expertise and quality control, can be expensive, and is not always robust (meaning small changes in water content can cause big changes in workability). Even with true SCC (slump flow greater than 18 inches), contractors will sometimes use a little vibration just to make sure the mix is consolidated, especially with congested reinforcing steel. In a properly proportioned mix, this vibration will not lead to separation of the aggregate from the paste (segregation) although there can be some grout leakage at form joints. ‘With slump flows below about 23 inches,” said SCC researcher David Lange, professor at the University of Illinois Champaign, “there may be the need to vibrate just a little bit to help the material move, usually just at the end of the pour when finishing off the top surface.”

The other type of concrete that should be addressed is “high-flow” concrete—this is not quite SCC but is concrete that still flows easily (slump flows between about 14 and 18 inches or even up to 20 inches). This class of concrete is lower priced than 5CC and more robust but still has most of the advantages (rapid placement, fewer workers needed, good finished surfaces). This is a very practical material but it does require some mechanical consolidation (vibration). “If there is access for vibrators then don’t use SCC, but consider “high-flow” concrete,” said Jack Gibbons, technical director for the Concrete Reinforcing Steel Institute, Schaumburg, Ill. “Trump Tower and the Aqua in Chicago are great examples of using 21st century methods without the expense and risk of SCC.”

 

Proper consolidation is often a judgment call-experienced operators can tell 

if they are getting good consolidation by the sound of the vibrator

Vibration equipment

The critical questions for the vibrator operator  

 are how far apart to insert the vibrator and 

how deep to penetrate into the preceding lift.

The first decision to make in consolidating concrete is whether to use internal vibration or external vibrators mounted on the outside of the forms.
For this article, we will cover only internal vibration, because that is the most common technique used in the field. External or form vibration is nearly always used in precast plants and sometimes in field applications where the forms arc getting multiple uses and being moved as an assembly (gang forms) from place to place.
Internal vibration equipment is available from a variety of manufacturers although it all works in similar ways:
•Concrete vibrators develop vibration by spinning an eccentric weight within a housing (the vibrator head) typically at a very high speed. This action creates vibrations in the range of 10,000 to 17,000 vibrations per minute.
• Flexible shaft vibrators are the most common type used, where a flexible drive shaft in a housing spins the eccentric weight inside the vibrator head. The core of the flexible shaft is made from high-strength braided steel wire and the casing is steel reinforced rubber. Shafts come in various standard lengths up to about 20 feet long, although shafts can be coupled up to 65 feet.
• The shaft may be driven by an electric motor or by a gasoline or diesel engine. Some power units turn at speeds high enough to develop the required and others must be geared

to achieve the needed speed. There are also pneumatic motors available to drive flexible shaft vibrators.
• Some manufacturers have developed quick connect systems between power units and shafts and between shafts and heads.
• Gas power units often come rigged to he carried like a backpack, freeing a worker from having to hold the power unit in one hand.
• Electric motor-in-head vibrators are also available; in these vibrators a small three-phase induction motor is powered by a heavy electrical cable that also serves as the vibrator handle.
• Pneumatic vibrators are used when compressed air is readily available and when vibrators must run for extended continuous periods, such as in mass concrete. The pneumatic motor is in the vibrator head and tends not to heat up like other vibrator s.
• Radius of influence (or radius of action) is the critical parameter in vibrator selection—this is the distance from the center of the vibrator to the farthest distance where complete consolidation will occur in the concrete. Vibrator heads are rated for radius of influence, although this distance varies with the concrete slump— with high slump mixes it can he double the listed value.
• Vibrator heads come in various shapes, diameters, and lengths.
There is no definitive evidence that one shape works better than another, although some dimpled geometries seem to be more efficient. The head size should be matched to the desired radius of influence and the power unit should he matched to the head size.
• With large headed vibrators and larger power units (as big as about 5.5 hp), the radius of action can he as high as 18 inches. Vibrator heads come with regular steel heads or with urethane (rubber) heads. Rubber heads are required with epoxy- coated rebar, where a standard steel head can chip the epoxy and allow corrosion of the steel. Most DOTs require non-metal vibrator heads with epoxy-coated bars.
• Rubber tips are available for steel heads to protect form panels from damage by vibrator tips.
• Undersized extension cords for electric drive motors can lower a vibrator’s performance and even burn out the motor. For example, a 15 0-ft extension cord powering a 2.25 hp motor should have 8 gage wires.

• Manufacturers have developed rebar vibrators that have proven to effectively consolidate concrete and also grout in reinforced masonry. A study at the University of Tennessee on reinforced masonry showed that rebar vibrators were “an acceptable alternative to conventional pencil vibrators.”

Tips and Techniques for proper vibration

Concrete Placed Into forms always includes some entrapped air (as much as 30% of the volume of the concrete), especially stiffer mixes, and this air can get stuck in the concrete along form surface, near the corners, and beneath reinforcing steel. Consolidation is getting as much of this air as possible out of the concrete. What we arc trying to remove is entrapped air not entrained air. Entrained air is there for a purpose—to prevent freeze-thaw damage. Entrapped air serves no beneficial purpose, rather it results in honeycombed surfaces, poor bond development between the concrete and reinforcement, lower strength in the concrete, and higher concrete permeability that can result in rebar corrosion. Good consolidation overcomes these problems and also eliminates rock pockets and lift lines resulting in beautiful smooth concrete surfaces.

The science of how a material like concrete flows is called rheology. Traditionally, workability has been defined by slump, hut mixes with the same slump can behave very differently (have different rheology), especially when the mix includes water-reducing admixtures. The theological properties of fresh concrete include its yield stress and viscosity, which together define workability and how easy a concrete will consolidate. Rheology is something every concrete person should understand.

Stiff, low-water concrete mixes are intended to achieve high strength and low permeability, but without good consolidation these objectives can’t be achieved. Added water will allow easier consolidation but lower the strength of the mix. Fluidity can be achieved with admixtures, most notably superplasticizers (or high-range water reducing adrnixtures), but these admixtures are expensive and not always necessary with good consolidation technique.
With more fluid concrete and when placing flatwork, manually pushing and tamping the concrete or banging on the outside of the forms can usually remove most of the entrapped air. Stiff, low water content mixes, though, require more effort. With most formed concrete, mechanical consolidation is the best way to improve the quality of the work. Consolidation isn’t difficult, but can be ineffective without the proper equipment and technique. Here are some tips to improve your concrete through consolidation. 

Fluidity can be achieved using superplasticizers, 

but they are expensive and not always necessary with good consolidation technique

Planning, preparation Off in Indianapolis

At 1.2 million square feet, there is nothing small about the Indianapolis International Airport’s Midfield Terminal. The massive construction project was so complex that planning for the original design began more than 30 years ago. It wasn’t until 1999 that Wessler Engineering was given the green light to move forward on an ambitious effort to cost-effectively  manage water-quality treatment and aircraft deicing for the facility.

Its roles were largely twofold:

As the project’s stormwater master planner, Wessler was charged with developing drainage guidelines and a stormwatemanagement plan. The firm also was responsible for the evaluation, design, constructionadministration and part-time inspection of deicing controls and water-quality treatment for airside drainage.

By 2008, the facility was fully operational and accommodating clean stormwater, as well as stormwater runoff contaminated with aircraft andpavement deicing fluids. The firm employed a dual-collection system that segregates gate area runoff—where primary aircraft deicing often occurs—from the remainder of the ramp. Stormwater runoff is treated by two massive water-quality systems; it then flows to a diversion mechanism that directs the run ff to either high-concentrate storage, low-concentrate storage or the receiving waterway.

A 12,400-gallon-per-minute pump station transfers contaminated runoff from an underground storage facility to a lined, covered storage facility in the airfield. From this facility, runoff is pumped to another control facility before being discharged to the city of Indianapolis sanitary sewer system.

The underground facility is up to 40 feet deep at its base and is strong enough to supporthe weight of aircraft activity overhead. To accommodate thenecessary depth and ground-water conditions, engineers designed thicker concrete walls and placed shear keys along its length to increase the structure’dead load.

Designing the stormwater retention and diversion facility and necessary infrastructure was no small feat. “The under-ground facility is the size of a football field,” says Wessler

President Brent Siebenthal.

The facility was sized using

30 years of hourly precipitation data. The stormwater and runoff facilities also are equipped with a supervisory control and data acquisition system for remote control and monitoring. “Wessler led a team of 15 consultants during design and construction and coordinated stormwater design efforts for the entire project,” says Bill Leber,

Wessler project manager.

New ACEC/APWA/ASCE Sustainable Infrastructure Rating System in Development

A new web-based sustainable infrastructure project rating system, a joint effort of acec, the American Public Works association (aPWa) and the american society of civil engineers (asce), is set to launch next spring.the goal of the new system and its companion training program will be to enhance the sustainability of the nation’s civil infrastructure, including transportation, water and environmental  projects of all types.

Michael r. cline, chairman of acec’s energy and environment committee and council liaison for the collaboration, says when finalized, the new rating system will significantly enhance the sustainability rating process for Member Firms and public works clients.

“First, it will provide a process that could help our clients evaluate their pro-posed projects through many areas of sustainable concepts and design on an easy-to-use web-based rating tool that includes links to reference materials and best practices,” he said. “it also will provide a mechanism for proposed projects to be pushed further along the sustainability scale, while checked against a project budget. Finally, it will provide an opportunity 

to help grow and brand the firm in using sustainability design practices through the certification and training process that will come out of  this effort.”

Under the program, sustainable infrastructure project ratings will be recognized only after an independently verified performance assessment, which will give the program a higher validity than self-verified programs that are frequently the standard in today’s market. the ultimate vision for the rating system is to produce a tool that will promote a more dynamic project delivery approach, where the practitioner, agency and owner can consider alternative approaches to achieving higher degrees of sustainability, and use those approaches to facilitate the regulatory approval process. acec, aPWa and asce will work with other organizations and government agencies to review and comment on the rating system. The consortium also will offer sustainability training and develop a certification program in an effort to further promote sustainable design and practice. The first phase of this program is expected to go into testing in fall 2010, in preparation for a planned public release in spring 2011.

Stimulus Grant Targets ‘Holistic’ Solutions For Infrastructure Problems

Clemson University’s civil engineering department is offering a new master’s program in “sustainable and resilient infrastructure” that will train graduates to confront the nation’s infrastructure problems.

Funded by a $700,000 National Science Foundation (NSF) grant made available by the American Recovery and Reinvestment Act, the program will cover tuition for 14 master’s degree students over three years.

The first seven students will begin this fall. “We are preparing a generation of engineers ho will examine the nation’s infrastructure throughout its life, from the planning stagesthrough design, construction, operation, maintenance and rehabilitation,” said Ron Andrus, associate professor of engineering at Clemson and principal investigator on the project. Andrus also wrote in his abstract about the need for a “paradigm shift to look holistically at the nation’s infrastructure throughout its life.” 

Unlike traditional civil engineering programs, Clemson officials said, the new program will involve interdisciplinary coursework and internships with external partners to help students focus on broader issues involving the nation’s infrastructure problems, from aging roads and bridges to water supplies and power grids.

“The department of civil engineering recently has adopted resilient and sustainable infrastructure as a research and educational focus, and I believe this was instrumental in securing the grant from NSF,” said Nadim M. Aziz, department chairman.

The students will work in interdisciplinary teams, gain experience with businesses in the field and learn to communicate their ideas to technical and nontechnical audiences. External partners will include national laboratories, international engineering firms, state and local government agencies, and nonprofit and professional organizations.

“By working in project teams and through extensive interaction with our external partners, these students will gain expertise that will allow them to contribute immediately to the workforce in a critical area of the nation’s economy,”

A shot of the Springfield Interchange, also

as the Mixing Bowl, in Northern Virginia.