Concrete Floors

TIM FRANCE, P.E.

Wet CuringSingle-use curing blankets

provide uniformly colored slabs

The curing blanket lies down flat as it is unrolled. Below, left: Sheets stay together without taping when over-lapped only 2 to 3 inches. Below, right: The curing blankets are unrolled onto a wet slab surface.

here is no quick and easy wayto wet-cure concrete—at leastthat’s what I used to believe. After

18 years as a structural engineer, I haveworked on numerous slab-on-grade proj-ects and used traditional wet-cure meth-ods. Most of these techniques, particu-larly synthetic curing blankets, were time-intensive, expensive, and they sometimesleft discolored areas on the slab. Becauseof recent industry trends, our firm hasconsulted on numerous integrally col-ored concrete slabs, where discolorationfrom traditional wet-curing is a majorissue. Recently however, I had an op-portunity to test an innovative, wood-pulp fiber fabric that readily absorbs andmore evenly distributes water on bothintegrally colored and gray slabs.

A bit of historyTo cure concrete slabs cast on the

ground, contractors traditionally useACI-recommended methods of waterwet-curing. To achieve proper hydra-tion at the concrete surface, contrac-tors will mist or soak the concrete sur-face or cover the slab with soaked burlap,coated paper, plastic sheeting, or syn-thetic-fiber plastic-backed blankets.

Each of these water retention tech-niques has the same goal of hydratingthe surface, thereby improving abra-sion and wear resistance once cured.We have learned, however, that each ofthese practices has its own set of con-sequences.

Soaked burlap, for example, re-quires periodic rewetting, which maynot always be done, and even then itoften leaves spotty wet and dry surfaceareas. This can discolor concrete andleave a fabric pattern on the slab. Withthe trend toward decorative concretefloors, this discoloration can be a seri-ous problem. To prevent this discol-oration, burlap has to be sized andcleaned prior to use.

Plastic sheeting and coated paperboth keep a slab surface moist, but thesetechniques are labor-intensive since theedges of the sheeting and the paper needto be overlapped and taped. If not prop-erly taped down or weighted, windscan cause the sheeting and paper tocome up in areas, leaving dry spots anduneven curing of the slab surface. Coatedpaper and plastic sheets can also causediscoloration from the contact and non-

TBy Tim France, PE

contact areas. The non-contact areasoften are significantly lighter, and insome cases, efflorescence can appear.This contrast in coloration can be animportant issue since most owners wantaesthetically pleasing concrete floors,particularly for retail stores.

The most problematic hydrationtechnique is wetting by misting, sprin-kling, soaking, or flooding. Besides re-quiring round-the-clock observation,the wetting procedure is impractical formost construction projects and maycause problems from water runoff ifnot properly supervised. Excessive cur-ing water runoff could result in the slabsubbase becoming water soaked, whichcould weaken the base and increasecurling of the slab.

Problems with curing blanketsTrying to rectify the issues caused

by wet curing, several manufacturershave developed improved curing blan-kets made of synthetic fibers bonded toimpervious opaque plastic sheeting. Al-though these blankets are an improve-ment, they have some shortcomings.First, the blankets do not readily absorbwater, nor do they easily lie flat aftertheir initial use. Although the blanketscan be reused, they get dirty, are hardto store, and are difficult to clean. Un-fortunately, if these blankets are notcleaned, they may create the same slabdiscoloration problems as burlap. Car-rying project debris from job to job doesnot lend itself to quality, especially onan integrally colored slab. Also, thesesynthetic blankets, like paper and plas-tic sheeting, must be overlapped, taped,and weighted to stay in place underwindy conditions. And, just like withplastic sheeting, the blankets can causediscoloration and efflorescence.

A curing blanket that worksRecently, I had the opportunity to

test a single-use curing blanket and tocompare it with the alternatives. Theproduct we have started to use is awood pulp-based fiber fabric with animpervious clear plastic cap layer. WhatI liked about this blanket is that it ar-rives at the worksite clean, virtuallywrinkle-free, and sealed in rolls. Oncethe 57-pound rolls are opened, the cur-ing blanket is rolled out rather thanlaid on the wet slab. The fiber fabric

readily absorbs water, and because ofits wicking action, helps distribute thewater to other areas on the slab to cre-ate an even, wet-cured surface.

With this new curing blanket, Ul-traCure NCF, there is no guessing wherethe dry areas are. Because of its clearplastic backing, workers can easily seeareas that are dry, and wet those areasby moving water under the blanket. An-other advantage of the new blanket overolder synthetic products is that the Ul-traCure blanket edges do not have tobe taped if the pieces are properly over-lapped 2 to 3 inches. This overlap provesto be a significant cost saver when com-pared with synthetic products, whichare overlapped 8 to 12 inches. Thispulp-based blanket also helps preventdiscoloration because of the blanket’sabsorption capability. However, I be-lieve that despite all of these advances,the best feature of this new curing blan-ket is its price, which at this time isabout one-half to one-third the cost oftraditional synthetic curing covers.

Typical installationTypical installation of this curing

blanket is to flood the surface withwater until a thin film of water, about1⁄32 to 1⁄16 inch, stays on the surface.Remove the packaging, position on theslab with the roll edge parallel to theslab edge and unroll it, watching andcorrecting its position to eliminate wrin-kles. Monitor the absorption, and addwater if needed. If dry spots are noted,add water under the blanket with asmall diameter hose or by using a rollersqueegee to move the water arounduntil the entire blanket has becometranslucent. The time to remain on theslab is typically referenced in the proj-ect specifications. Most specificationsrequire at least seven days; however thisvaries from a minimum of three daysto 14 days for most projects; follow theproject specifications.

Comparison testingHow well does this new single-use

curing blanket perform compared toreused synthetic fiber blankets? To findout, we conducted a side-by-side fieldcomparison using two slabs, both about15,000 square feet, wetted over theirentire surfaces. We used the UltraCureblanket on one slab and previously used

ers folded the product over itself sev-eral times and left it on the edge ofthe slab where it was picked up anddiscarded. In comparison, the syn-thetic cover was folded, placed on aflatbed trailer, and hauled offsite forstorage and eventual reuse on anotherproject. The cover was not cleaned ordried before it was stored.

During the curing process, the fiberson several areas of the synthetic blan-ket had worn off, leaving only the plas-tic sheeting on the slab. Also, we ob-served that considerably more waterwas retained on the pulp blanket slabsurface than by the synthetic blanket.Finally and more importantly, when thetwo slabs dried, we clearly saw that thesurface covered by the pulp-based coverwas much cleaner and more uniformin appearance.

Test resultsAs a result of our testing, we found

the pulp-based cover to have numer-ous advantages. First is its deep hy-dration capability. When properly in-stalled, the UltraCure blanket providesa fully saturated surface, helping the

concrete to achieve more complete hy-dration than other products we haveused. Contractors will like the wood-pulp curing product because it is cost-effective in labor and material andeasier to handle than reused syntheticblankets. This technique provides amore uniformly colored concrete sur-face, especially important with inte-grally colored decorative slabs. n

—Tim K. France is president ofTKF Engineering Associates, BrokenArrow, Ok.

For additional information:UltraCure NCF, McDonald

Technology Group, 866-913-8363,www.ultracure.net, or circle 1 on thereader service card.

synthetic fiber blankets on the other.One thing we discovered was that

using the noticeably dirty synthetic blan-kets was time-consuming and labor-intensive since they had to be laid out,unfolded, and then repositioned. Wealso had problems keeping the blanketsin position because of winds.

In comparison, handling the pulp-based blanket was much easier. Be-cause the product comes in a roll, la-borers simply pushed the roll out onthe slab where it laid flat. Althoughthere were a few wrinkles, most werequickly eliminated with a roller. Repo-sitioning was minimal, and becauseof the nature of the product, the windwas not a problem in keeping the blan-kets on the slab.

As the slabs cured, the wind re-moved about a third to half of the syn-thetic blankets, even though those blan-kets had been weighted but not taped.The pulp blanket, not weighted downor taped, remained completely in place.We attribute this to the pulp blanket’sabsorbed water weight.

Removing the pulp-based coverwas as easy as putting it down. Work-