As t rikeoff tool. Vi b ra t o r s. At a m p e r. Finishing equip-ment. All these standardtools for placing tra d i-

tional, formed concrete flatworka re also vital components of a slip-f o rm paving machine. Althoughthey may appear quite complex,m o d e rn slipform concrete pave r sa re made up of a series of stri k e o f f ,consolidation, and finishing toolsf i rmly mounted onto a self-pro-pelled machine. The machine au-tomates traditional concre t i n gp ro c e d u res in a continuousp rocess that still relies on the fun-damentals of placing, consolidat-ing, and finishing concre t e.

Slipforming Is Extrusion

All modern slipform pavers con-solidate and shape concrete usingan extrusion process that pulls or“s l i p s” the forms continuously ove rand through the plastic concre t e.The most common example of ex-t rusion is squeezing a tube oft o o t h p a s t e. The squeeze prov i d e sthe energy to move and consoli-date the toothpaste; the top end ofthe tube is the mold. The tooth-paste emerges from the tube con-solidated and in the exact shape ofthe mold. The slipform paver pro-duces similar results: Co n c rete iss q u e ezed through the mold tof o rm the pavement. One major dif-f e rence is that in slipform pavingthe mold move s, not the concre t e.

The mold on the slipform pave ris the extrusion enve l o p e. Thecomponents of the extrusion enve-lope consist of the bottom of thep rofile pan and the interior sur-faces of the sideforms (Fi g u re 1).The profile pan is also known as ane x t rusion meter, or forming plate.

This system confines the concre t e(just like the toothpaste tube) andp rovides the mold for the desire ds h a p e.

A slipform paving machine pro-vides the energ y, or squeeze, to thec o n c rete to force it through themold. The concrete is squeezed ve r-tically by the weight of the pavingmachine on the profile pan, andh o ri zontally by the taper adjust-ment of the sideforms confining thec o n c re t e. The energy of the squeezecomes from two sourc e s :1) The continuous movement of

the machine’s mold throughthe confined concrete

2) The energy applied by continu-ous vibration, which fluidizesand consolidates the concrete

Uniformity Is Key to Quality Paving

One important chara c t e ristic off resh concrete that affects slipformpaving is its hyd raulic nature. Pl a s-tic concrete acts like a fluid, whichmeans it cannot be compre s s e d .

When confined inside the pavingm a c h i n e, the energy applied to thec o n c rete for consolidation or fin-ishing is transmitted through thec o n c rete in all directions back tothe pave r. As a result, any changesin the energy the paver applies tothe concrete or changes in thec o n c rete pro p e rties will alter theresults of the paving pro c e s s.

This is why uniformity is a cri t i-cal issue in slipform paving. Di s t ri-bution of concrete to the pave rmust be continuous to meet thedemands of high production, andthe squeezing of the concre t emust be constant to produce ac o n t i n u o u s, uniform shape andfinish. Un i f o rmity in design (mixand geometrics), logistics (con-c rete supply and delive ry), and en-e rgy levels (vibration, squeeze,and movement) are all vital for sat-i s f a c t o ry machine perf o rm a n c eand paving re s u l t s. Any changes inthese va riables will cause changesin the concrete—some undesir-a b l e. Pa vement smoothness,

Despite the complexities of today’s models, slipform pavers still follow the basicprocedures for placing concrete flatwork: strikeoff, vibration, and finishing.

Understanding Slipform Pavers

BY BOB RISSER

Designed to automate various concreting procedures, a modern slipform paverstill relies on basic practices and standard tools of the concrete trade

s t rength, dura b i l i t y, and ove ra l lp e rf o rmance may suffer as a re s u l t .If the concrete mixture and appliede n e rgy (squeeze and vibration) re-main constant, the result is a uni-f o rm, geometric shape and sur-face—a smooth, durable concre t ep a ve m e n t .

As with concrete flatwork, thec h a ra c t e ristics of the concrete mixaffect how the tools will perf o rm .Harsh or sticky mixes, which do notconsolidate or fluidize easily whenv i b rated, will affect the ability of thes l i p f o rm paver to produce asmooth, durable pave m e n t .

Sl i p f o rm paving also re q u i res abalancing act with the concre t eslump: The slump must be lowenough to produce sharp pave-ment edges, yet the mix must re-spond well to the extrusion pro c e s sand consolidate pro p e r l y. Ty p i c a ls l i p f o rm paving mixes re q u i re a 1-to 2-inch slump to meet these re-q u i re m e n t s.

Tools of the Slipforming Trade

Despite its modern complexi-t i e s, the slipform paving pro c e s sstill consists of using a series ofs t a n d a rd concrete tools to per-f o rm the functions of consolida-tion and finishing: an auger orpaddle for uniform concrete dis-t ribution in front of the pave r, as t rikeoff, a tamper bar (also usedfor consolidation), and a finish-ing tool (the profile pan). Often,

additional equipment for sec-o n d a ry finishing of the concre t es u rface behind the paver is usedto re m ove slight irre g u l a ri t i e sand provide a tight surface mi-c ro t e x t u re.

Fi g u re 1 shows each part of a typ-ical slipform pave r. To understandthe crucial elements that affectp a ver perf o rm a n c e, it is import a n tto begin at the extrusion enve l o p e,w h e re the slipforming process ac-tually takes place. Each machine el-ement affects the concrete at thef ront of the extrusion enve l o p e. Atthis crucial point, the machine per-f o rmance and quality of the finalp roduct are determ i n e d .

The most crucial area of con-c e rn in the slipform pavingp rocess is the front, or nose, ofthe profile pan. At this point, vi-b rated, fluidized concrete iss h e a red off at the proper eleva-tion to form the concrete slab. It isalso where uplifting forces thatthe concrete transmits to the ma-chine concentra t e. The key to asmooth paving job is to maintainu n i f o rm pre s s u re at the nose ofthe profile pan. Changes in pre s-s u re at this point will cause thes l i p f o rm machine to compensateby lifting or diving. Since the pro-file pan cannot compensate ade-quately for large changes in pre s-s u re at the nose, a nonuniforms u rface and nonuniform consoli-dation can re s u l t .

The angle at which thenose of the profile pan shearsthe concre t e, also known asthe angle of attack, is cri t i c a l .Since the mix chara c t e ri s t i c swill determine how much en-e rgy the machine needs toput into the concrete to suc-cessfully complete the extru-sion pro c e s s, the best angle ofattack will be different for dif-f e rent concrete mixes.Achieving the proper angle ofattack of the profile panmeans applying enough pre s-s u re to properly slipform thec o n c re t e, but not too muchp re s s u re, which—thro u g hh yd rostatic forces—will lift

the machine (Figure 2).Just in front of the nose of the pan

is the tamper bar. It perf o rms sec-o n d a ry consolidation on the con-c rete mix and helps the perf o rm a n c eof the profile pan by moving larg ea g g regate to just below the pave-ment surface at the critical shearpoint at the nose of the profile pan.The thin layer of mortar created bythe tamper bar at the surface be-t ween the concrete and the pro f i l epan lowers friction between the ma-chine and the concre t e, helping thes l i p f o rm pan slide along the newp a vement surf a c e.

Internal Vibration

Consolidation is a vital part of fin-ishing concrete flatwork. In slipformpaving, the role of the internal vibra-tor is even more vital to success—the paver simply will not perf o rmwell without proper vibration of thec o n c re t e. Im p roper concrete vibra-tion can also lead to the concre t ep a vement not perf o rming as in-t e n d e d .

T h e re are two purposes for inter-nal vibrators in the paving pro c e s s :to consolidate the concrete mass( re m ove undesirable voids), and tof l u i d i ze the concre t e, helping thep a ver move over and through thec o n c re t e. Fluidization reduces thef riction of the particles within thec o n c re t e, permitting the “s l i p p i n g”re q u i red for the slipform i n gp ro c e s s.

Figure 1. Typical slipform paver cross section. Each element of the paver can affect themachine’s reaction in the crucial extrusion envelope.

Each vibrator on a slipformp a ver is adjustable for both posi-tion and energy level. Position isadjusted mechanically prior topaving. En e rgy level is adjustedh yd raulically to a uniform level ofv i b rations per minute (vpm) to be-gin paving, and then it’s re a d j u s t e dto appro p riate levels dependingupon the reaction of the concre t e.

In t e rnal vibrators have a coni-cal-shaped zone of influence. Thezone of influence changes as thee n e rgy leve l — v p m — c h a n g e s. In-c reased vpm widens the zo n e, de-c reased vpm narrows the zo n e.The vibration energy level re-q u i red for a particular mix designand depth of placement may re-q u i re changing the number of vi-b rators and operating them athigher or lower energy leve l s.

Vi b rators are mounted in ap a ver in a line across the width ofthe slab. Spacings are set for aslight overlap of each zone of in-f l u e n c e. Typical vibrator spacingson slipform paving machinesrange from 12 to 24 inches. Ou t-side vibrators are usually set ap-p roximately 6 inches from eachs i d e f o rm of the machine (Fi g u re3). The ve rtical position of the vi-b rator should be kept as near hor-i zontal as possible.

The vibration energy level re-q u i red to consolidate and flu-i d i ze the concrete in a successfuls l i p f o rm operation va ries withmix design and depth of con-

c re t e. Ge n e ra l l y, vibrators are setto produce a uniform level of vi-b ration energy ranging fro m6,000 to 9,000 vpm.

The hyd rostatic head, oramount of concrete above thev i b ra t o r, also affects the effi-ciency of an internal vibra t o r.The greater the hyd ro s t a t i chead, the greater the effect thev i b rator has at the bottom ofthe slab. Pa vers today are de-signed with a “g rout box” (Fi g-u re 1) to confine the concre t eand take advantage of this ef-fect. Another benefit of con-fined vibration in the grout boxis the reflection of vibration en-e rgy back into the mass of con-c re t e. This lowers the energy re-q u i red to fluidize the mix andp re vents or limits segregation ofthe particles within the con-c re t e.

The grout box also provides aconstant head of concrete inf ront of the nose of the pro f i l epan, further reducing adjust-ments the machine has to makein the extrusion pro c e s s. A con-c rete level in the grout box of ap-p roximately 11⁄2 times the pavingdepth is re c o m m e n d e d .

Pa ver speed can also change thezone of influence of the vibra t o r.The forw a rd motion of mostp a vers should be maintained at 3to 7 feet per minute, with uniformc o n c rete delive ry in front of thep a ver to maintain constant speed.

Pa ver speedshould be ad-justed to matchchanges in con-c rete delive ryra t e s. Sp e e d sg reater than 10feet per minutemay re q u i rechanges in thenumber andspacing of the vi-b ra t o r s.

While neces-s a ry for thepaving pro c e s s,v i b rators will not

c u re other problems in the pavingo p e ration, such as poor paver ad-justment or improper paving tech-n i q u e. Vi b rators may identify andaccentuate a poor concrete mixdesign, but they do not cause thep roblem dire c t l y. Im p roperly ad-justed vibrators can create unde-s i rable paving re s u l t s.

Concrete Delivery to the Paver

At the front of the grout box is as t rikeoff plate (Fi g u re 1), whichp e rf o rms the same function as thes t raightedge used to strikeoff con-c rete for flatwork. The stri k e o f fplate is the first component toc o n t rol the amount of concrete inthe grout box and, there f o re, inf ront of the nose of the profile pan.

In front of the strikeoff plate isan auger or some device used toc o n t rol the distribution andamount of concrete in front of thepaving machine. The height ofc o n c rete at this point influencesthe amount of concre t e, or hyd ro-static head, in the grout box. Asdiscussed earlier, changes in con-c rete pre s s u re will affect vibra t i o nand machine perf o rm a n c e. There-f o re, it is important that distri b u-tion of the concrete in front of themachine be as uniform as possi-b l e.

As when placing concrete flat-w o rk in form s, concrete that willbe slipformed should be placed asclose as possible to its final posi-tion. Di s t ribution should be uni-f o rm in height. Since part of therole of the slipform machine is tov i b rate and consolidate concre t e,apply the same rule used for theo p e ration of hand-held intern a lv i b rators: Do not use the vibra t o r sto move the concrete.

Remember that the slipform ma-chine is designed pri m a rily to con-solidate and extrude concre t e, notto move the material. Co n t rol ofc o n c rete distribution at this pointis why many contractors use plac-e r / s p reader equipment in front ofthe pave r. A placer/spreader deliv-ers the concrete ve ry close to its fi-nal position, allowing the slipform

Figure 2. The angle the profile pan makes with theconcrete surface is referred to as the angle of attack. Ifthe angle of attack is too high, it must be overcome by acombination of machine weight and vibration.

machine to perf o rm its function ofconsolidation and extrusion, ra t h e rthan act as a bulldoze r.

Despite the complexities of mod-e rn slipform pave r s, the basics ofplacing and consolidating concre t estill apply. And although many ad-vances have been made since them a c h i n e’s invention in 1954, thereis always more to learn about thea rt of slipform paving. When a del-icate balance of concrete mix de-sign, delive ry, vibration, and ma-chine adjustments is achieved, theequipment can produce smooth,d u ra b l e, and economical concre t ep a ve m e n t s. Ha p py paving.

AcknowledgmentInformation for this article was basedon a presentation made by ChapinSipherd, national accounts manager ofconcrete paving products for CMICorp., Oklahoma City, at the Trans-portation Research Board annualmeeting in January 1995.

F i g u re 3. I n t e rnal vibration is anessential element of quality slipformpaving. Vibrators are typically sethorizontally at the paving surface at12- to 24-inch spacings.

PUBLICATION # C950521

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