A vacuum dewatering demonstrationFloor slab durability enhanced whileconstruction time is reduced

After initial consolidation of concrete in the warehouseslab, this double-beam vibrating screed levels the concreteto an elevation carefully set a little above the desired floorlevel to allow for compaction of concrete duringdewatering. Camber can be introduced at this stage ifdesired.

Pliable plastic base filter pads are placed carefully on top ofthe freshly screeded concrete surface. The filter sheetminimizes loss of cement fines as water is removed.

Flexible suction mats are unrolled on top of the filter pads,enabling vacuum to be created over the surface of theconcrete.

The construction press was recently invited to ad e m o n s t ration of vacuum dewatering in connec-tion with floor slab construction for a warehouse inBri d g e p o rt, Connecticut. This journal has kept its

readers in close touch with the subject ever since 1963,when there was an upsurge of American interest becauseof Eu ropean exploitation of the pro c e s s. Although theBri d g e p o rt demonstration invo l ved no new technical de-ve l o p m e n t s, it was of interest because the work was super-vised by one of the major manufacturers of vacuum dewa-tering equipment.

The vacuum dewatering pro c e s s, patented in Un i t e dStates nearly half a century ago, has attracted increased in-t e rest in recent years after seve ral Scandinavian firms sim-plified the equipment enough to make it practical for al-most any builder. Vacuum dewatering is used widelyt h roughout Eu rope today, and in Sweden the method isused for 40 to 50 percent of all concrete floors. Ba s i c a l l y, thep rocess improves strength, dura b i l i t y, and other pro p e rt i e sof concrete by reducing the water-cement ratio immediate-ly after the mix is placed, usually in floors and other flat-w o rk .

Demonstration

At the Bri d g e p o rt demonstration, concrete was placedin the slab form s, consolidated with immersion vibra t o r s,then leveled and vibrated again with a double-beam vi-b rating screed. This vibrating screed, which can be pow-e red by either gasoline or electric engine was pulled ove rthe concrete surface by two men. After screeding, the con-c rete was immediately cove red with a filter pad and a suc-tion mat connected to a vacuum pump. Within a few sec-onds the vacuum created under the mat began to compre s sthe concrete and cause water to be drawn through hoses tothe suction pump.

Vacuum was applied for about 3 to 5 minutes per inch ofslab thickness. Typically this lowers the water content of thec o n c rete by 20 to 25 percent, while the filter pad minimize sloss of cement fines with the water which is being collected.Less than half of one percent of the cement is ord i n a rily re-m oved during vacuum dewateri n g .

After a given area of floor was dewatered, the pad andmat we re moved on to another freshly placed section ofthe slab. The dewatered surfaces we re firm enough to bewalked on, and we re immediately floated with a powe rt rowel fitted with a planing disc to re m ove any high spots ori r re g u l a rities introduced during the vacuum pro c e s s i n g .W h e re skid-resistant surfaces are desired, this completesthe floor-finishing operation, and the slab is ready for cur-ing about one-half to one hour after vacuum tre a t m e n t .W h e re a smoother surface is specified, power troweling isd o n e, usually 30 to 90 minutes after the planing opera t i o n .

Productivity

The entire process from concrete placement to comple-tion of surface finishing can be accomplished in as little as2

1⁄2 hours, with some time va riations depending on the sur-

face finish desired as well as the we a t h e r. Past experi e n c ehas shown that daily capacities of 6000 to 8000 square feet

of 6-inch-thick floor can be finished per working day, usinga single vacuum pump unit, organizing work to avoid ove r-t i m e. Use of vacuum dewatering helps the contractor main-tain better control of his slab construction schedules inboth hot and cold we a t h e r.

The vacuum processing method is said to be practical forboth small and large contra c t o r s, with an investment of aslittle as $10,000 to $12,000 providing all of the necessarys t a rtup equipment. For this cost, the contractor can obtainthe vacuum pump, two 20-foot-wide plastic base pads, two

c ove ring suction mats, one manifold with channel extend-ing across mats to extract the collected water, and a powe rt rowel with planing disc to re m ove the surface irre g u l a ri t i e si n t roduced by the vacuum pro c e s s. A crew with basick n owledge of concrete placing practices can become famil-iar with dewatering techniques in one or two days.

New vacuum pump, less than half the size of earlier models,is capable of providing suction for two mats simultaneously.

This power trowel fitted with a planing disc completes theslab finishing operation where a nonskid surface is desired.

When the filter mat and pad are removed after 15 or 20minutes of vacuum treatment, the concrete surface is hardenough to support foot traffic without damage to eitherpolished street shoes or the concrete surface.

Manifold attached at the center of vacuum mat isconnected to the vacuum pump by a hose through whichwater is withdrawn from the concrete.

Properties of concreteCo m p re s s i ve strength improvements of about 30 perc e n t

over conventionally finished concrete are re p o rted. The gainin strength is greatest at the upper we a ring surface of the slab,w h e re it is most needed. Yo u n g’s modulus of elasticity can bei n c reased 10 to 15 percent. Ac c o rding to test results re p o rt e din Ge rm a n y, abrasion damage to vacuum treated concrete av-e raged 24 percent less than for conventional concre t e. Hi g h-er resistance to fre ezing and thawing damage has beend e m o n s t rated in labora t o ry tests. Quality va riations evi-denced by slump differences from load to load of ready mixedc o n c rete can be eliminated because the vacuum pro c e s se vens out va riations in water content. Pe rmeability of con-c rete is greatly reduced by vacuum dewatering; tests show wa-ter absorption reduced by 10 to 20 percent. The water- c e m e n tratio at the surface of a concrete slab can readily be re d u c e df rom as much as 0.7 to 0.45. Consequently ze ro-slump con-c re t e, or at least its practical equivalent, can be attained nearthe va c u u m - t reated surf a c e s. The vacuum cannot re m ove wa-ter needed for hyd ration of cement because capillary diame-ters in the cement paste decre a s e, limiting extraction of wa-ter when the water-cement ratio approaches 0.3.

Job operationsSlab depths up to 12 inches can be effectively dewatere d

by the vacuum pro c e s s. Joint spacing can be increased be-cause vacuum dewatering reduces shrinkage of floor slabc o n c re t e. Better use of construction manpower is possiblewith vacuum dewatering because positive control of floor fin-ishing time can be established and ove rtime for floor finisherscan be reduced or eliminated. For suspended slabs earlierf o rm re m oval is often possible because of more rapid earlys t rength development. Tilt-up construction may be speededby the improved early strength development achieved withvacuum dewatering. The process can be used with light-weight aggregate concre t e.

Pumps now available for establishing the vacuum are littlel a rger than a cocktail table, some weigh no more than 215p o u n d s. Mats shown in the photos are flexible, but rigid padsor plates can also be used to create the vacuum. The seal be-t ween the vacuum mat or plate and the fresh concrete is im-p o rtant; the tighter the seal the larger the area that can be de-w a t e red with a single pump unit.

P U B L I C AT I O N #C810503, Copyright © 1981, The Aberdeen Group, All rights re s e r v e d

ADVANTAGES OF VACUUM DEWATERING