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Factors affecting shrinkage Powers; Meininger; and Tremper and Spellman are the three references I have used most to explain the causes of drying shrinkage. Each of them point out that water demand of the separate materials used in concrete is the major determinant of the shrinkage of concrete. I maintain that variations in water demand caused by the separate concrete ingredients has a far greater effect on concrete shrinkage than does the common concern overthe variation in slump.
Powers, and Tremper and Spellman both emphasized the cumulative effect on shrinkage of making poor choices in the selection of material to be used. Powers’ shrinkage results, clarified by Mather in the form shown in Table 1, show the individual and cumulative effects of the most unfavorable versus the most favorable material choices with regard to six factors influencing the amount of shrinkage. Powers assumed a constant water-cement ratio, and concluded: “Wrong choices of alternatives (with respect to volume change) can result in about seven times as much shrinkage as would result from best choices.”
Table 1 below shows that concrete with ¾ in. (19 mm) maximum sized aggregate will shrink about 30 percent more than concrete with 1 ½ in. (38 mm) maximum sized aggregate. But, concrete placing costs may increase slightly when larger aggregate is used. Designers therefore should specify that the maximum sized coarse aggregate be slightly less than ⅓ the slab thickness, with the understanding that the small increase in cost will be offset through lower shrinkage and a more productive floor slab.
If you have questions about your industrial concrete floor contact one of our sales engineers today.
Kalman Floor Company, known for its innovative approach to creating solutions for industrial concrete floor customers, concluded its exhibition and sponsorship of educational and technical discussions at SWANA’s WASTECON®. Kalman installs highly abrasion resistant floors for transfer stations, materials recycling facilities and associated maintenance shops but also works to educate the industry and meet its ever changing needs.
Kalman is currently scheduling presentations about specialty floors for transfer stations and materials recycling facilities which will be beneficial to owners, operators, builders and designers. To arrange for a meeting at your location or to learn more about Kalman systems contact a sales engineer today.
Since 1916, Kalman Floor Company has been the worldwide leader for industrial concrete floor solutions. Kalman Absorption Process® Topping is a highly abrasion resistant concrete floor topping for use in waste management facilities and other industrial environments. In addition, Kalman Seamless Concrete® eliminates floor joints and reduces maintenance costs for its industrial users. A Kalman floor means lower costs and higher productivity.
Kalman Floor Company will be exhibiting at this year’s WASTECON show August 23-25 in Nashville, Tennessee.
Kalman sales engineers will be at the show to discuss long lasting floor solutions for solid waste facilities in booth 249 in the exhibit hall. Kalman will also be sponsoring the Transfer Station Design and Expansion technical education sessions.
WASTECON is conference hosted by the Solid Waste Association of North America (SWANA) for solid waste professionals. SWANA serves municipal solid waste professionals throughout North America with conferences, certifications, publications and technical training courses.
Kalman Floor Company installs long lasting, durable, abrasion resistant industrial concrete floors in waste facilities. Since 1916, Kalman has been working side-by-side with owners/municipalities, architects, design-builders and contractors to provide specialty concrete floors for waste facilities and other heavy duty industrial applications. Kalman has installations in nearly every major city in North America, including Monterey, California’s waste management district’s, MRF which lasted 10 years longer than originally expected. Find out how Kalman can help your facility, contact a sales engineer today.
Solid waste facility floors typically last five years before replacement is required at great expense and operational downtime. At the Monterey (CA) Regional Waste Management District MRF the floor lasted 10 years longer than expected saving time and money through reduced maintenance costs.
In 1996, the Monterey Regional Waste Management District in Marina, California selected Kalman Floor Company to install a 40,000 square foot materials recovery facility (MRF) floor anticipating replacement in five years. In 2001 the Monterey MRF called Kalman to reinstall.
Kalman inspected the floor there was no need for replacement. Over the next few years, Kalman and the owner monitored the floor’s performance. Eight years went by, then 10, 12 and finally after 14 years it was determined to resurface approximately one-third of the facility – there was no need to replace the entire floor or perform a full-depth removal and replacement. The same ¾ inch thick Kalman Absorption Process® concrete topping as utilized originally was used for the resurfacing.
Kalman’s Seamless Concrete® slab and Absorption Process® floor topping stood up to the abuse of tracked vehicles and was still in working order. It is well known in the industry that Kalman’s floors are much more abrasion resistant, more dense and have 80% fewer joints than a conventional concrete floor. These qualities make for a low maintenance, cost effective operation with higher productivity. Contact a sales engineer today to learn how Kalman can help you and your facility.
Shrinkage cracking and upward slab edge curling are common problems of enclosed industrial floor slabs on grade. The edges of these slabs curl upward because of differential shrinkage when the top of the slab dries to lower moisture content than the bottom of the slab. This can be caused by moist subgrades, low humidity air on the upper slab surface, and because in order to make it workable, concrete must be made with much more water than is needed to hydrate the cement. Evaporation of moisture from the upper surface of slabs is what causes drying shrinkage. Curling is caused by the difference in drying shrinkage between the top and bottom of the slab.
The effects of shrinkage and curling due to loss of moisture from the slab surface often are overlooked by designers because of the great emphasis placed on compressive strength and slump testing, and also because of the lack of information on curling. Owners expect floor slabs to be relatively free of shrinkage cracks and free of curled edges at control and construction joints. In my opinion, for enclosed slabs on grade made with Portland cement concrete, these problems are worse today than 25 years ago. This is true for a number of reasons:
- There are two basic references for floors on ground: PCA’s “Concrete Floors on Ground”, I and ACI’s “Guide for Concrete Floor and Slab Construction” (ACI 302).2 Neither of these emphasizes the need for low-shrinkage concrete for floor slabs on grade. The words “warping” and “curling” do not appear in the table of contents of the PCA book. That book implies that if slump is low, then almost everything has been done to minimize shrinkage. Neither publication suggests that the designer specify shrinkage testing of both cement and concrete and choose the cement and aggregates that will provide the lowest shrinkage concrete. With foreign cements and clinker flooding the U. S. market in the 1980s, designers must check cements. The PCA and ACI documents should specify that shrinkage testing is every bit as important as compressive strength testing for enclosed slabs on grade.
- The 1960 to 1980 market demand for high-early-strength cement to allow rapid form removal in multistory construction has resulted in cements that have high shrinkage. The use of these high-early cements is promoted for slabs on grade by ACI 302, which somewhat arbitrarily requires a minimum 3-day compressive strength of 1800 psi (12.4 MPa) for all floors. High-early- strength cements increase slab shrinkage. The ACI and PCA documents should specify that cements with limited shrinkage be used for slabs on grade.
- ACI 302 requires 4000 and 4500 psi (27.6 and 31.0 MPa) 28-day compressive strengths for enclosed single-course industrial floor slabs on grade, up from 3500 psi (24.1 MPa) specified 25 years ago. Despite a lower water cement ratio, these higher 28-day strength concretes usually contain either more high-early strength cement or admixtures that increase the total water content and thereby increase shrinkage.
- Clean, low shrinkage aggregates are less available today than 25 years ago because environmental considerations restrict quarry operations.
- Floor slabs are being built on higher moisture content subgrades as the cost of good industrial land has risen. Moist subgrades increase the moisture gradient through the slab, which increases upward curling at free edges.
- Excessively low slumps such as the 3 in. (75 mm) maximum slump currently allowed by ACI 302 for Class 4 and 5 floors has encouraged the use of high range water reducers (HRWR) in order to increase workability. There is some evidence that HRWRs actually may increase shrinkage even though the total water content is not increased. Fortunately ACI 302′s maximum allowable slump is proposed to be raised to 4 in. (l00 mm) in 1987. This change will provide workability without using HRWRs.
Drying shrinkage: definition and amount
The definition used in this paper for the phrase “drying shrinkage” is borrowed from Mather’s Highway Research Board Committee3 which defined drying shrinkage of concrete as “the reduction in concrete volume resulting from a loss of water from the concrete after hardening.” That committee quoted Washa4 as saying that “drying shrinkage of concrete is caused principally by the contraction of the calcium silicate gel . . . when the moisture content of the gel is decreased.”
All practical portland cement concrete shrinks about 400 to 800 millionths [0.0004 to 0.0008 in.lin. (0.004 to 0.0008 mm/mm)] due to drying, according to the PCA document, “Volume Changes of Concrete,” 5 but that document states “when drying shrinkage is restrained by reinforcing, shrinkage can be reduced by up to one-half.”
Aggregate and shrinkage
To provide the workability needed for placement, practical concrete mixes always contain more water than is needed to hydrate the cement. When this excess water evaporates, the cement paste shrinks. To fully restrain shrinkage of the cement paste, concrete would have to contain the maximum practical amount of an incompressible and clean aggregate.
If the dry-rodded volume of an incompressible and clean coarse aggregate were equal to the concrete volume, then the coarse aggregate would fully restrain cement paste shrinkage. That is never the case, though, for conventional floor slab concrete because such a stony concrete mix would be totally unworkable.
In actual practice, the dry-rodded volume of the coarse aggregate is only 50 to 60 percent of the concrete volume if Y2 in. (13 mm) maximum sized aggregate is used, but can be as high as 75 percent of the concrete volume if 1Y2 in. maximum sized aggregate is used, according to Table 5.3.6 of ACI 211, “Standard Practice For Selecting Proportions for Normal, Heavyweight, and Mass Concrete.”6 Therefore, a large maximum sized coarse aggregate, slightly less than \13 the thickness of the slab, should be specified in order to maximize the amount of aggregate in floor slab concrete. This aggregate must be low in shrinkage.
The floor is where all the action is in the Byrne Arena at the Meadowlands Sports Complex in East Rutherford, New Jersey. Elephants pound on the floor during circus performances, hockey players race down the frozen floor and musicians construct massive stages atop the 18,000 square foot Kalman Absorption Process® industrial concrete floor. Though, spectators will never see the jointless, super-hard, abrasion resistant concrete floor that literally floats on a structural surface base slab beneath.
Kalman Floor Company, specializes in building durable, non-dusting, abrasion resistant floors, using 80% fewer floor joints that its competitors. Kalman Floor Company ensures the industrial concrete floors they install are a smooth, hard, durable and perform seamlessly under the most extreme conditions.
The Byrne Arena, can handle any type of event without interruption or need for costly, time consuming repairs. Kalman has installed the same type of floor system in other arena facilities including Madison Square Garden, Pepsi Center, Atlantic City Convention Hall, and the Denver Coliseum, just to name a few.
Concrete floors are one of the most troublesome items in today’s industrial buildings. Surveys over the years indicate that about six of every ten buildings have floor problems, most often because the floor is manufactured on the construction site and subjected to variances of temperature and other weather conditions, inexperienced work crews, and poor quality control during and after its manufacture. In addition, it receives greater use than any other part of the building. Typical problems reported include:
- Base slab concrete or sub grade conditions that produce floor cracks and rocking slabs.
- Surface dusting, uneven or premature wear, or a floor that never looks clean.
- Joints which break down and are difficult, and sometimes impossible, to repair.
- Misapplication of floor surfacing materials-light duty concrete vs moderate or heavy duty; non-corrosion resistant surface vs corrosion resistant.
From The Bottom Up
To start at the bottom and work up; subsoil must be firm. The load of the slab itself is great, but the dead weight factors for machinery and stored goods, as well as the dynamic factors created by materials handling vehicles, are even greater. Therefore, the soil must be permanently firm or the slab must be designed as a structural slab.
If soil conditions are perfect, but a building is not enclosed, problems can occur. A lack of levelness caused by a rainstorm, for example, can result in cracking because as the slab dries it shrinks, with resistance created by the friction set up between the bottom of the concrete slab and the top of the sub grade. When the sub grade is uneven, the resistance increases because the slab is keyed into the sub grade at both high and low points. Moreover, if the high points in the sub grade are substantial in any one linear dimension, a plane of weakness is introduced, almost guaranteeing cracks. This can usually be avoided simply by waiting a few days and correcting the rain-caused soil condition. The slab should never be poured unless the roof and walls are tight.
The mix design also will have a substantial effect on slab shrinkage. Most slabs are poured too wet and with too much sand. These two factors make for good workability, but they cause concrete to shrink and crack more than necessary.
In theory, the solution to this problem is easy: make the mixes harsher. But, each mix has to be
looked at individually. The gradations of sand and stone used by ready mix plants- even site adjacent plants- often vary widely.
Surface Failures
Two major misunderstandings about industrial concrete floors seem to be at the root of surface failures. Many owners feel that specifying high compressive strength for a concrete floor is all that is required. However, there is a vast difference between compressive strength and surface wear resistance. A 3000 or 4000-psi concrete, for example, means only that it will withstand that compressive force in a 6 by 12-in. concrete cylinder. The compressive strength figure relates only to an internal breaking point, not wearability.
Another misunderstanding involves aggregates. Special aggregates are often used to help toughen floor surfaces. A variety of these materials are commercially available ranging from large stone aggregates to sand or metallic aggregates. These materials do help, but they are not a panacea. Their full contribution cannot be made if other elements are not properly controlled.
The major factor in producing good Portland cement concrete floors is the water-cement ratio. The strength of a concrete mixture depends on the quantity of mixing water used in the batch, so long as the concrete is workable and the aggregates are clean and structurally sound. The strength of the concrete decreases as the water ratio
increases.
Three stipulations are given equal importance: the water-cement ratio, workability, aggregate cleanliness and type. Starting out with the strongest material, we want to include as much of it as possible that is the coarse aggregate. The minimum amount of water for Portland cement is 3.1 to 3.2 gallons per sack. Anything in excess of that, according to the water-cement ratio law, will take away some qualities of the Portland cement. Finally, workability is important because without it all the cement particles will not be saturated with water, the cement paste coating of aggregates will not be complete, voids will be trapped in the mix when it is placed, and the mix so produced will be costly because of the long mixing times required and the extreme difficulty in handling it.
Unfortunately, when we try to mix a batch of cement with only a 3.1 to 3.2 ratio, including the water that is in the aggregate- especially the sand- we find that it is not workable if it has the maximum amount of coarse aggregate.
A plethora of concrete admixtures will produce workable concrete with low water content. However, while producing easily finished concrete, lower water contents permitted by the use of some admixtures does not result in lower shrinkage concrete, the ultimate goal. Overuse of admixtures can result in the coarse aggregate sinking to the bottom of the slab which aggravates slab edge curling. A shrinkage compensating concrete (not shrinkage reducing admixture concrete) is a very good choice to eliminate floor joints thereby eliminating curling (no joint = no curl) and avoiding the main location of surface distress in concrete.
The Deferred Topping
The best industrial concrete floor are constructed with a separate surface- a deferred topping. For this, the base slab concrete is cast about 3/4-in. below finished elevation and the separate surface is applied after the slab hardens. The thin topping allows water to be removed from the mix after workability for mixing and placing is no longer needed. The topping may be applied to the base slab the next day or later. Delayed installation is preferred because it allows most of the drying shrinkage to occur in the base slab.
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Immediately before the topping is placed, the base slab is saturated so that it will not suck water from the topping mix. Also, a cement grout is thoroughly scrubbed onto the slab surface as a bonding agent. (The slab must, of course, be prepared with a bondable surface.) The topping mix is then placed, usually containing a cubic yard of coarse aggregate for every cubic yard of topping mix. Enough fine aggregate is incorporated to fill all spaces between the particles of coarse aggregate. After the topping mix has been straight-edged to finish floor grade the workability water is extracted.
A burlap blanket is placed on the mix and an absorbing material spread over the blanket. This starts pulling water from the mix below the burlap almost immediately, and in from 10 to 20 minutes all possible water removal is completed.
Obviously, this material can be taken off before maximum water removal. But, if it is left until the maximum potential of the technique is accomplished, the resulting topping mix will be so hard and stony that special vibrating float equipment is needed to "work" the surface. Troweling does three things: it smoothes the surface, densifies it, and imparts a built-in gloss.
Smoothing can be done relatively quickly, and most floors get only this amount of troweling. The resulting surface is so "soft" that the floor will probably wear quickly and unevenly. The complete benefit of troweling may not be realized until six trowelings under the most favorable weather conditions in summer, ten trowelings in unfavorable weather in winter. Troweling squeezes moisture from the surface due to blade pressure.
Each troweling pass pulls out water particles which cling to the outside of the cement particles. As part of the same action the fibers of the concrete which were disrupted by the moisture movement are consolidated into the floor surface. Troweling must be continued until the concrete sets so hard that it has no plasticity.
The last step is water curing. The floor is flooded with water and covered with paper. This water must be available to the cement once the complicated chemical process of cement hardening proceeds.
For Average Use
Many facilities do not subject their floors to the sort of use that requires the brute strength of a topping.
For these the monolithic floor can be beefed up. After the concrete has been placed, straight-edged to finish grade, and the surface smoothed, aggregate mixed with cement is applied to the surface and embedded to a depth
of about 3/4-in. This densities the surface and helps absorb the shock and impacts which would weaken, and eventually destroy, an ordinary cement paste.
The cement paste at the surface also must be strong enough to withstand its share of punishment. Therefore, the full value of the aggregate densification can only be achieved by repeated trowelings just as in the topping installation procedure. A curing compound is then applied, and water curing is not needed because there is enough water sealed in the slab by the curing compound.
Stress relieving joints are put in the floor so that the tensile stresses developed during shrinkage will not become greater than the tensile strength of the concrete. If that point is reached, the concrete will crack. With proper mix design and sub grade conditions, 50-ft square panels are satisfactory. However, if freedom from cracks is a prime design criteria, the panels can be made smaller. Contraction joints may not be needed if small cracks are permissible.
Tooled construction joints spell trouble. Tooling produces a rounded, wide opening in the floor. Small diameter wheels, and even big ones, slam into this joint opening causing joint breakdowns. Any joint should be filled and refilled as needed to protect the shoulders against breakdown by traffic, and for sanitary reasons.
Joints are also used to isolate the floor from differential movement of other parts of the building. Columns, for example, are usually boxed because some downward movement can be expected. However, the traditional column box is not necessarily needed. In many instances all that is needed is some form of separation between the floor slab and the column.
Construction joints are to be avoided and a considerable amount of hand troweling is required to finish them properly. Construction joints must be joined in some fashion so that the weight impositions of moving loads can be transferred from one slab to the other without any movement of either. These days is accomplished by steel bars embedded into the edge of the concrete. Additionally dowels are placed in the center of the slab, with foam-sided clips to allow for horizontal and lateral shrinkage movement.
Kalman Floor Company received the 2006 Construction Communications’ Plaque of Honor for its work on the Safeway Distribution Center in Auburn, Washington.
Kalman Floor Company poured more than 1,051,802 square feet of Kalman Seamless Concrete® and Kalman Monorock® for this award-winning project. The 116-acre site consists of 8,913 square feet of administration space, a 20,352 square foot truck maintenance building with Tilt-up concrete skin, steel joist and beam roof structure metal roof decking with three ply insulated roof, a 31,054 square foot recycling center, a 524,636 square foot grocery warehouse with 50 x 40 foot bays, a 662,368 square foot refrigeration warehouse, and a fueling station with a reinforced concrete slab and holding area with a steel column and joist canopy, metal decking with a three ply insulated roof.
The award acknowledges Kalman Floor Company, Turner Construction and SLL Leo A. Daly for contributions in helping to build one of the region’s most important projects.
Kalman Seamless Concrete®, which eliminates 80 percent of floor joints, combined with Kalman Monorock®, a durable, self-polishing abrasion-resistant floor finish, is often used in industrial warehouses such as the Safeway Distribution Center to maximize material handling speeds under demanding conditions.
Since 1916, Kalman Floor Company has been eliminating miles of floor joints by installing seamlessly smooth, durable industrial concreted floors for cold storage, meat packing and grocery distribution facilities, beverage distribution warehouses, waste management facilities and large manufacturing plants.
Kalman Floor Company won the 2004 Excellence in Concrete Award presented by the Northern New England Concrete Promotion Association.
Working with PCI Architecture and ProCon Construction, Kalman Floor Company installed more than 310,500 square feet of Kalman Seamless Concrete® and Kalman Monorock® for F.W. Webb’s central distribution warehouse in Amherst, New Hampshire. The warehouse consists of 250,000 square feet of high bay pallet rack storage, 140,000 square feet of parts storage and 24 loading docks for shipping and receiving. Additionally, the facility houses 30,000 square feet of office space and a training center.
Kalman Seamless Concrete®, which eliminates 80 percent of floor joints, combined with Kalman Monorock®, a durable, self-polishing abrasion-resistant floor finish, is often used in industrial warehouses such as F.W. Webb’s distribution center to maximize material handling speeds under demanding conditions.
Since 1916, Kalman Floor Company has been eliminating miles of floor joints by installing seamlessly smooth, durable industrial concreted floors for cold storage, meat packing and grocery distribution facilities, beverage distribution warehouses, waste management facilities and large manufacturing plants.
