A va va c u u m d e wa te r i n g demonstration Floor slab durability enhanced while construction time is reduced
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After initial consolidation of concrete in the warehouse slab, this double-beam vibrating screed levels the concrete to an elevation carefully set a little above the desired floor level to allow for compaction of concrete during dewatering. Camber can be introduced at this stage if desired.
Pliable plastic base filter pads are placed carefully on top of the freshly screeded concrete surface. The filter sheet minimizes loss of cement fines as water is removed.
he construction press was recently invited to a demonstration of vacuum vac uum dewater dewa tering ing in connec con nec-tion with floor slab construction for a warehouse in Bridgeport, Connectic Conn ecticut. ut. This Thi s journa j ourna l h as kept k ept its readers in close touch with the subject ever since 19 63, when there was an upsurge of Ame rican interest inte rest because of Eu ropean exploita tion of the pro c e s s. Although the Bridgeport demonstrat demon stration ion invo lved no new technical techn ical developments, it was of o f interest i nterest because the work w as supersu pervised by one of the major manufacturers of vacuum dewatering equipment. The vacuum dewat ering pro c es s, paten ted in Un i te d States nearly half a ce ntury ago, has attracted increased int e rest in recent years after seve ral Scandinavian firms simplified the equipment enough to make it practical for almost any builder. Vacuum dewatering is used widely t h roughout Eu rope today, and in Sweden the method is used for 40 4 0 to t o 50 percent perc ent of o f all al l concre co ncrete te floors. f loors. Basically, the proces roc esss impro i mproves ves streng str ength th,, dura bility, and an d other oth er pro pertie s of concrete by reducing the water-cement ratio immediately after the mix is placed, usually in floors and other flat work.
Demonstration At the Bri dgeport dem onstration, onstration , concrete concr ete was p laced in the th e slab s lab form s, consolidate consol idated d with immersion immers ion vibra tors, then leveled and vibrated again with a double-beam vib rating screed. This vibrating screed, which can be powe red by either gasoline or electric engine was pulled ove r the concrete surface by two men. After screeding, the conc rete was immediately cove red with a filter pad and a suction mat connected to a vacuum pump. Within a few seconds the vacuum created under the mat m at began to compre ss the concrete and cause water to be drawn through hoses to the suction pump. Vacuum Vacuum was applied applied for about about 3 to 5 minutes per inch of slab thickness. Typically this lowers the water content of the concrete by 20 to 25 percent, while the filter pad minimize mi nimize s loss of cement fines with the water which is being collected. Less than t han half hal f of o ne percent percen t of the cem ent is ordinarily rem oved during vacuum dewateri n g . After a given area of floor was de watered, watere d, the pad an d mat we re moved on to another freshly p laced section of the slab. The dewatered surfaces we re firm enough to be walk ed on, and we re immedia imm ediately tely floated float ed with a powe r trowel fitted with a planing disc d isc to remove any high spots or or i rregu l ariti ri ties es intro in troduc duc ed duri du ring ng the th e vacuu va cuum m pro pr oce ssing. Where skid-resistant skid-resista nt surfaces are desired, this completes compl etes the floor-finishing operation, operation, and the slab is ready ready for curing about abou t on e-hal f to o ne hour after vacuum tre atm e nt. Where a smoother surface is specified, power troweling is done, usual u sually ly 30 to 90 9 0 minutes minu tes a fter the planin pla ning g opera op era tion.
Productivity
Flexible suction mats are unrolled on top of the filter pads, enabling vacuum to be created over the surface of the concrete.
The entire process from concrete placement to completion 1 of surface finishing can be accomplished in as little as 2 ⁄2 hours, hours, with some time time variations riations depending depending on the surface finish fini sh desired desi red as well as the weather. Past experi expe rience has shown shown that daily capacities of 6000 to 8000 square feet
New vacuum pump, less than half the size of earlier models, is capable of providing suction for two mats simultaneously.
When the filter mat and pad are removed after 15 or 20 minutes of vacuum treatment, the concrete surface is hard enough to support foot traffic without damage to either polished street shoes or the concrete surface.
Manifold attached at the center of vacuum mat is connected to the vacuum pump by a hose through which water is withdrawn from the concrete. of 6-inch-thick floor can be finished per working day, using a single vacuum pump unit, organizing work to avoid ove rtime. Use of vacuum dewatering helps the contractor maintain better control of his slab construction schedules in both hot and cold we a t h e r. The vacuum processing method is said to be practical for both small and large contra ctors, with an investment of as little as $10,000 to $12,000 providing all of the necessary startup equipment. For this cost, the contractor can obtain the vacuum pump, two 20-foot-wide plastic base pads, two
This power trowel fitted with a planing disc completes the slab finishing operation where a nonskid surface is desired. c ove ring suction mats, one manifold with channel extending across mats to extract the collected water, and a powe r t rowel with planing disc to re m ove the surface irre g u l a ri t i e s i n troduced by the vacuum pro c e ss. A crew with basic knowledge of concrete placing p ractices can become familiar with dewatering techniques in one or two days.
ADVANTAGES OF VACUUM DEWATERING
Properties of concrete
J ob operations
Compressive strength improvements of about 30 perc ent over conventionally finished concrete are reported. The gain in strength is greatest at the upper we aring surface of the slab, where it is most needed. Young’s modulus of elasticity can be increased 10 to 15 percent. According to test results reported in Ge rm a n y, abrasion damage to vacuum treated concrete ave raged 24 percent less than for conventional concre t e. Hi g her resistance to freezing and thawing da mage has been demonstrated in lab ora tory test s. Quality variations evidenced by slump differences from load to load of ready mixed concrete can be eliminated because the vacuum pro cess evens out va riations in water content. Permeability of concrete is greatly reduced by vacuum dewatering; tests show water absorption reduced by 10 to 20 percent. The water-cement ratio at the surface of a concrete slab can readily be reduced from as much as 0.7 to 0.45. Consequently zero-slump concrete, or at least its practical equivalent, can be attained near the va cuum-treated surf aces. The vacuum cannot remove water needed for hyd ration of cement because capillary diameters in the cement paste decre a s e, limiting extraction of water when the water-cement ratio approaches 0.3.
Slab depths up to 12 inches can be effectively dewatere d by the vacuum process. Joint spacing can be increased because vacuum dewatering reduces shrinkage of floor slab c o n c re t e. Better use of construction manpower is possible with vacuum dewatering because positive control of floor finishing time can be established and ove rtime for floor finishers can be reduced or eliminated. For suspended slabs earlier form removal is often possible because of more rapid early s t rength development. Tilt-up construction may be speeded by the improved early strength development achieved with vacuum dewatering. The process can be used with light weight aggregate concrete. Pumps now available for establishing the vacuum are little l a rger than a cocktail table, some weigh no more than 215 p o u n d s. Mats shown in the photos are flexible, but rigid pads or plates can also be used to create the vacuum. The seal bet ween the vacuum mat or plate and the fresh concrete is important; the tighter the seal the larger the area that can be de watered with a single pump unit.
P UB LI CATI ON #C810503, Copyright © 1981, The Aberdeen Group, All rights reserved
