Drying shrinkage dilemma

The Drying Shrinkage Dilemma Some observations and questions about drying shrinkage and its consequences

BY WILLIAM F. PERENCHIO

lthough many things can go wrong with plastic c o n c re t e, even more can go wrong with hardened c o n c re t e. A case in point: drying shrinkage problems, including failures at filled joints, slab curling, and excessive cracking.

Many designers of concrete structural elements know that concrete shrinks when it dries, but they often fail to consider this in their designs. And even if they do, there are conflicting opinions about the best ways to counter the harmful effects of drying shrinkage. Unfortunately, test data aren’t always available to confirm or disprove these conflicting opinions, as the following examples illustrate.

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Joint Failures The U.S. Bureau of Reclamation ran shrinkage tests on 4x4x40-inch beams that were dried for 38 months (Ref. 1). Their tests showed that 34% of the 38-month shrinkage occurred in the first month, and 90% occurred after 11 months of drying. However, unlike floors, these beams dried from four surfaces.

A different picture emerges for slabs on grade. Based on formulas developed in Reference 2, Figure 1 shows how much time is re q u i re d for slabs on grade of different thicknesses to reach various percentages of ultimate drying shrinkage. A 6-inch-thick slab drying from one side reaches only 60% of its ultimate shrinkage after 12 months of drying in laboratory air at 50% relative humidity. This can have some

undesirable consequences.

Anyone who reviews design specifications for slabs on grade often sees the following requirement: Joint fillers are not to be applied sooner than 90 days after concrete placement. But consider the same 6-inchthick slab in Figure Figure 1. Time required for slabs on grade of different 1. After being ex- thicknesses to reach various percentages of ultimate posed to air for drying shrinkage. Specimens were dried in laboratory air at three months, the 50% relative humidity. slab has undergone only 30% of its ultimate drying Probably the best approach is to shrinkage. require the caulking contractor to If filler is applied to the joints of fill the joints as late as possible, this slab at three months, failure then later refill areas where the will occur due to additional widenfiller has torn or separated fro m ing of the joint. Depending on the the concrete. nature of the filler, this failure can occur within the filler or the concrete, or at the filler-concrete interface. If the contractor waits, howe ve r, and applies the filler after the floor is placed in service, slab edges may spall due to traffic exposure.

Figure 2. Top surface deflection of a 20x20-foot, 6-inchNo practical so- thick warped slab with free edges. Curling occurs in slabs lution to this on grade that are exposed to the atmosphere on the top p roblem has ye t surface. The top of the slab shrinks due to drying while the been pro p o s e d . bottom of the slab does not dry.

Slab Curling Curling is one consequence of drying shrinkage. Curling occurs because the top of the slab shrinks due to drying while the bottom of the slab does not dry. This causes the top of the slab to be shorter than the bottom, which makes the slab curl upward, as shown in Figure 2.

The corners curl more than the sides, because at a corner the curling is a function of the shrinkage along both of the sides adjacent to it. Because the slab edges no longer touch the subbase, the weight of the concrete near the edges causes an uplifting force at the slab center.

Figure 3 represents the contact area between the slab and subbase.

The open area at the perimeter is the portion of the slab not in contact with the subbase while the cross-hatched section re p re s e n t s the area that is in contact. The shaded area at the slab center shows the portion that is in contact with the subbase but exerts little pressure due to the uplifting action of the cantilevered slab edges.

The thickness of a slab significantly affects the amount of curl that will occur. But while some authorities suggest using thicker slabs to reduce curling (Ref. 3), a para-

Figure 3. Because the edges of a curled slab are no longer in contact with the subbase, the weight of the concrete near the edges causes an uplifting force at the slab center. The cross-hatched section is the area in contact with the subbase while the shaded area at the slab center is in contact with the subbase but exerts little pressure due to the uplifting action of the cantilevered slab edges.

HOW DOES INITIAL CURING AFFECT ULTIMATE SHRINKAGE? Early drying shrinkage of concrete is believed to be largely due to surface tension in water that’s left in capillary pores in the hydrated paste. Later shrinkage is believed to be caused by loss of water adsorbed on the surfaces of hydrated cement paste. If this is the case, at a fixed water content, high-water-cement-ratio concrete should shrink more initially, but low-water-cement-ratio concrete should exhibit higher later shrinkage. Which mechanism contributes most to ultimate shrinkage?

Consider the figure below, which presents the drying shrinkage of small laboratory prisms after one year of drying in air vs. their initial moist-curing periods. The unpublished data on which the figure is based came from a study I conducted in 1963. The figure clearly shows that concrete with the lowest water-cement ratio (0.30) has the highest one-year shrinkage, regardless of the initial moist-curing time. Concrete with the highest water-cement ratio (0.70) has the lowest shrinkage when it’s cured less than seven days. These concretes also have the highest and lowest potential hydrated paste contents, respectively. At seven days of curing or less, the data indicate that the concrete with the

least curing and the highest water-cement ratio—and therefore the lowest cement content—has the lowest shrinkage. Of course, this should be true because this concrete contains the least amount of hydrated cement— which is responsible for drying shrinkage—and the greatest amount of aggregate, which resists the shrinkage.

The relationships in the figure also imply that ultimate dr ying shrinkage of 0.70-water-cement-ratio concrete can be minimized by curing either for a very short time (one day) or a very long time (90 days). Seven days of curing produced the highest ultimate shrinkage for this concrete. Is it possible that there’s a pessimum initial curing time with respect to drying shrinkage, and that the curing time that produces the greatest shrinkage is a function of watercement ratio? Curves for other water-cement ratios in the figure show a similar shape, but the differences in shrinkage aren’t as pronounced. I’m not suggesting that all properties of high-watercement-ratio concrete can be improved by reducing the curing period. But it is curious that the standard seven-day curing period produced the most shrinkage.

Drying shrinkage of concrete after one year vs. length of initial moist-curing period. (The data for the concrete with a water-cement ratio of 0.70 cured for six hours are missing because the concrete specimens were too weak at that age to be handled.)

metric study using finite element methods (Ref. 4) indicates that curling deflections increase with increasing slab thickness. We need deflection measurements from floors of differing thickness to confirm or refute the results from theoretical calculations.

Excessive Cracking Restraint to drying shrinkage.

Curling stresses cause cracking due to loss of subgrade support under the curled portion. But another cracking cause is restraint to drying shrinkage. Every slab on grade is in contact with a subbase or subgrade (at least those portions of the slab that haven’t lifted due to curling). When the slab tries to shorten, the subbase or subgrade opposes the movement. The degree to which it is successful is directly p ro p o rtional to the subgrade friction, or the amount of force the subgrade can exert.

Figure 4 shows typical values for the coefficient of friction for most materials typically in contact with the bottom of a slab (Ref. 5). These values, multiplied by the weight of the slab, equal the amount of force the subgrade is able to exert to resist slab movement. The very low value for sand—nearly as low as for polyethylene sheeting—explains the tendency for good designers to choke off a crushed stone or gra ve l subbase with sand. This allows for a wider joint spacing with no increase in cracking tendency.

Although polyethylene sheeting also produces a low coefficient of friction, concrete placed in direct contact with such sheeting bleeds longer, delaying finishing. The practice may actually promote greater curling because concrete at the bottom of the slab doesn’t dry as fast, and thus doesn’t shrink as fast as concrete at the surface.

Again, howe ve r, there doesn’t appear to be any test data that show slabs placed directly on impermeable surfaces curl more than those placed on absorptive surfaces.

Figure 4. Coefficient of friction values for various subgrade materials for a 5-inchthick slab.

Joint spacing. Probably the most basic consideration when dealing with drying shrinkage in slabs on grade is contraction joint spacing.

Different agencies give different suggestions, most of them based on slab thickness. Usually control joints are specified (in feet) to be two to three times the slab thickness (in inches), or 12 to 18 feet for a 6-inch slab. This rule-of-thumb may be too lenient.

My experience indicates that if the joints are spaced no farther apart than 15 feet, regardless of slab depth, cracking will be minimal. If the concrete is expected to shrink more than normal concrete, the joints should be even closer together. If the concrete is expected to shrink less, the joints can be farther apart. When in doubt, I use a 15-foot spacing.

I know of no studies that relate cracking frequency to floor-joint spacing. If more data were available, rational rules could perhaps be formulated.

William F. Perenchio is a senior consultant with Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.

References 1. S.H. Kosmatka and W.C. Panarese, Design and Control of Concrete Mixtures, 13th ed., Portland Cement Association, Skokie, Ill., 1994, p. 155.

2. T.C. Hansen and A.H. Mattock, “Influence of Size and Shape of Member on the Shrinkage and Creep of Concrete,” Journal of the American Concrete Institute, Feb. 1966, pp. 267290. (Reprinted as Bulletin D103 by the Portland Cement Association.)

3. R.F. Ytterberg, “Shrinkage and Curling of Slabs on Grade,” Concrete International, June 1987, p. 78.

4. M. Al-Nasra and L.R.L. Wang, “Parametric Study of Slab-on-Grade Problems Due to Initial Warping and Point Loads,” ACI Structural Journal, March-April 1994, pp. 198-210.

5. A.G. Timms, “Evaluating Subgrade Friction Reducing Mediums for Rigid Pavements,” Highway Research Board Record No. 60, 1964, pp. 48-59.

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