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Shrinkage Of Concrete: Minimizing/Eliminating The Potential For Cracking

Español | Translation Sponsored by Nox-Crete Products Group

By: Charles Nmai, Mark Bury, Joseph Daczko

The Fall 2017 issue of Tilt-Up Today offered an article describing a recurring cracking pattern in tilt-up wall panels. The article by Robinson, Hooks and Lawson entitled “Verifying the Cause of Panel Cracking-A Case Study” concluded by highlighting the two standard and primary determinants of non-load-induced cracking, namely the shrinkage potential of the concrete mixture and the degree of restraint the element is experiencing. It is also worth noting that at the Tilt-Up Concrete Convention in Miami this past September, one of the final presentations of the convention discussed extending the joint spacing of concrete floors. Both require awareness of concrete shrinkage and restraint. In this article we offer a focus on concrete shrinkage, provide a general overview of the various types of concrete shrinkage, and discuss the effects of concrete ingredients and their proportions on shrinkage. We believe it makes the most sense to develop a broad, holistic perspective on this concept first. At a later date, a follow-up article in Tilt-Up Today will focus on restraint and the fundamentals on these two very important and timely subjects. The majority of the following information is taken directly from “Concrete Technology in Focus – Shrinkage of Concrete” published by BASF Corporation.

The need for adequate workability to facilitate placement and consolidation of concrete often results in the decision to use a greater amount of mixing water than is needed for the hydration process (reaction with portland cement). The loss of some of this excess “water of convenience” from a concrete matrix as it hardens results in a volume reduction that is known as shrinkage. If the volume reduction occurs before the concrete hardens, it is called plastic shrinkage. The volume reduction that occurs primarily due to moisture loss after the concrete has hardened is known as drying shrinkage.

In addition to drying shrinkage, hardened concrete can also experience volume reductions such as thermal contraction, autogenous shrinkage and carbonation shrinkage.

Due to the hydration process, the temperature of fresh concrete in the hours after batching is often higher than the ambient temperature. The magnitude of the temperature rise is dependent on, among other things, the type and amount of cement used, the use of pozzolans or slag cements, the size of the concrete member, and the ambient temperature. As the hot concrete cools to the ambient temperature, it contracts and it is this volume reduction that is referred to as thermal contraction.

Autogenous shrinkage occurs as a result of the chemical reactions that take place during cement hydration. It can be significant in concrete with a very low water-cementitious materials ratio. It is possible for such concrete to shrink without the loss of any water to the environment. Fortunately, the magnitude of autogenous shrinkage is not significant in the majority of concrete placed where shrinkage is a concern.

As implied by the name, carbonation shrinkage occurs when concrete becomes carbonated, that is, when the calcium hydroxide in the hardened matrix reacts chemically with carbon dioxide present in the atmosphere. This leads to the formation of calcium carbonate and water and, consequently, a reduction in volume.

The major concern with regard to the shrinkage of concrete is the potential for cracking either in the plastic or the hardened state. In most situations, the likelihood of plastic and drying shrinkage is often greater than that of the other types of shrinkage mentioned above. Therefore, further details on the mechanisms by which these two types of shrinkage occur and the influences of concrete mixture ingredients, ambient conditions, design, and construction practices are presented in the sections that follow.

Plastic Shrinkage
Loss of water from fresh concrete, which leads to plastic shrinkage, can occur in a couple of ways: evaporation and absorption. The predominant mode is through evaporation from an exposed surface. The rate of water evaporation is usually aggravated by a combination of high wind speed, low relative humidity, and high ambient and concrete temperatures. Though these conditions are most likely to be present during the summer months, they can occur at any time. The rate at which bleed water is transported to the concrete surface will impact the potential for the phenomenon or form of cracking commonly referred to as plastic shrinkage cracking. It has been reported that, if the rate of surface evaporation exceeds about 0.1 Ib/ft2/h (0.5 kg/m2/h), the loss of moisture may exceed the rate at which bleed water reaches the surface, thereby setting into motion the mechanisms that cause plastic shrinkage [1].

Concrete can also lose water through absorption by the subbase and in some applications the formwork. Such loss of water can aggravate the effects of surface evaporation. It is generally accepted that the loss of water from the paste fraction of concrete due to external factors generates negative capillary pressures that cause the volume of the paste to contract, hence the shrinkage.

FIGURE 1. Effect of Concrete and Air Temperatures, Relative Humidity, and Wind Velocity on the Rate of Evaporation of Surface Moisture from Concrete [1].

To use this chart:
1. Start with the air temperature, move up to relative humidity.
2. Move right to the concrete temperature.
3. Move down to wind velocity.
4. Move left and read approximate rate of evaporation.

In ACI 305R [2], it is recommended that precautions against plastic shrinkage cracking should be taken if the evaporation rate from the exposed concrete surface is expected to approach 0.2 Ib/ft2/h (1.0 kg/m2/h). The evaporation rate for a prevailing ambient condition can be estimated by using the nomograph shown in Figure 1.

Precautionary measures to control plastic shrinkage include adjustments to the concrete mixture and the use of proven construction techniques. Reducing the temperature of a concrete mixture, particularly in hot weather, or increasing its rate of setting can be beneficial. The latter is one of the primary reasons why accelerating admixtures are increasingly being used in the arid Southwest regions, where conditions for plastic shrinkage are prevalent. The use of micro-synthetic fibers has also been reported to be beneficial in controlling plastic shrinkage cracking.

Effective construction practices to control plastic shrinkage include the use of temporary windbreaks to reduce wind velocity and the use of sunshades to reduce concrete surface temperatures, and placing concrete at the coolest time of the day. But the most effective control method is to prevent the concrete surface from drying out until finishing operations have been completed and curing initiated. The use of an evaporation reducer, temporary wet coverings, waterproof sheeting or a fog spray can be beneficial in this regard.

Drying Shrinkage
The loss of moisture from concrete after it hardens (hence drying shrinkage) is inevitable, unless the concrete is completely submerged in water or is in an environment with 100 percent relative humidity. Thus, drying shrinkage is a phenomenon that routinely occurs and merits careful consideration in the design and construction of concrete structures.

The actual mechanisms by which drying shrinkage occurs are complex, but it is generally agreed upon that they involve the loss of adsorbed water from the hydrated cement paste [3-5]. When concrete is initially exposed to a drying condition—one in which there is a difference between the relative humidity of the environment and that of the concrete—it first loses free water. In the larger capillary pores this results in little or no shrinkage. In the finer water-filled capillary pores (2.5 to 50 nm size) due to loss of moisture, curved menisci are formed, and the surface tension of water pulls the walls of the pores. Thus, internal negative pressure develops when the meniscus forms in the capillary pores. This pressure results in a compressive force that leads to concrete shrinkage. Continued drying also leads to the loss of adsorbed water, a change in the volume of unrestrained cement paste, and an increase in the attraction forces between the C-S-H hydration products that leads to shrinkage [5]. The thickness of the adsorbed water layer has been reported to increase with increasing humidity [5]. Therefore, it is conceivable that a higher water content would lead to a thicker layer of adsorbed water, and hence, more drying shrinkage.

Physically, concrete that experiences a drying shrinkage of about 0.05 percent (500 millionths or 500 x 10-6) will shrink approximately 0.6 inches per 100 feet (50 mm for every 100 m). In more visual terms, that is about 2 inches for the length of a football field. There are several factors that affect drying shrinkage. These include the characteristics of the concrete mixture ingredients and their proportions, design and construction practices, and environmental influences.

Effects of Concrete Mixture Ingredients
There is conflicting data in the literature of the effects of concrete mixture ingredients on its drying shrinkage. However, without question the constituents of a concrete mixture that influence drying shrinkage the most are water and coarse aggregate. Both have a profound effect on minimizing the paste content.

Figure 2 illustrates the effect of total water content on drying shrinkage. The data [7] shows that the total water content of a concrete mixture has a significant effect on its drying shrinkage. For example, assume that a concrete mixture has a cement factor of 708 Ib./yd.3 (420 kg/m3) and a water content of about 320 Ib./yd.3 (190 kg/m3) for a water-cementitious material (w/cm) ratio of 0.45. The figure shows that, on average, this concrete will have a drying shrinkage of about 0.06 percent and that this shrinkage value can be reduced by 50 percent by reducing the water content to 244 Ib./yd.3 (145 kg/m3), which translates into a w/cm of 0.35. Therefore, to minimize the drying shrinkage of concrete the total water content must be kept as low as is practicable.

Contrary to common belief that shrinkage increases with cement content, data [7] for concretes with cement contents ranging from 470 to 750 lb./yd.3 (280 to 445 kg/m3) showed that cement content had little effect on concrete shrinkage. The total water contents for these mixtures range from 338 to 355 lb./yd.3 (200 to 210 kg/m3) and slumps were between 3 and 4 inches (75 and 100 mm). For practical purposes, the type, composition and fineness of cement have also been found to have relatively little effect on drying shrinkage.

FIGURE 2. Effect of Total Water Content on Drying Shrinkage [7].
(Shaded area represents data from a large number of mixtures of
various proportions.)

The effect of coarse aggregate on drying shrinkage is twofold. First, the use of a high coarse aggregate content will minimize the total water and paste contents of the concrete mixture and, therefore, will minimize the drying shrinkage. The effects of aggregate-cement ratio and water-cement ratio on drying shrinkage are illustrated in Figure 3. The figure clearly shows that, at a given water-cement ratio, drying shrinkage is reduced as the aggregate-cement ratio is increased. For example, at a water-cement ratio of 0.40, a 50 percent reduction in drying shrinkage was obtained when the aggregate-cement ratio was increased from 3 to 5 (and also from 5 to 7).

FIGURE 3. Effect of Aggregate-Cement Ratio and Water-Cement Ratio on Drying Shrinkage [8]. (Data from 5 inch [125 mm] square mortar and concrete specimens exposed to a 70 °F (21 °C), 50 percent relative humidity environment for six months.)

Second, drying shrinkage of the cement paste is reduced by coarse aggregate because of its restraining influence. As to be expected, the amount of restraint provided by the coarse aggregate is dependent on the type of aggregate and its stiffness, the total amount of the aggregate used, and the top-size. Hard, rigid aggregates, such as dolomite, feldspar, granite, limestone and quartz, are difficult to compress and will provide more restraint to the shrinkage of the cement paste. These aggregates should therefore be used to produce concrete with low drying shrinkage.

The use of sandstone and slate should be avoided if low drying shrinkage is desired. Aggregates with clay coatings should also be avoided. This is because, in addition to its inherent shrinkage and effect on water demand, clay will reduce the restraining effect of aggregate on shrinkage.

Effects of Admixtures
Admixtures form an integral part of concrete mixtures produced today. Their addition to concrete typically increases the volume of fine pores in the cement hydration product. As a result, studies have shown increased drying shrinkage when admixtures such as calcium chloride, slag cement and some pozzolans are used. With regard to water-reducing admixtures, ACI 212 reports that information on their effects is conflicting [9], but there may be less long-term shrinkage, depending on the degree to which the water content of the concrete is reduced. Reductions in drying shrinkage have been obtained in instances where significant reductions in total water content were realized through the use of high-range water-reducing admixtures [10, 11]. Similar results may be obtained with mid-range water-reducing admixtures.

A specific example of reduced drying shrinkage with a high-range water-reducing admixture (HRWRA) is shown in Table 1 for concrete mixtures with a nominal cement factor of 600 Ib./yd.3 (356 kg/m3) and slump of 9 inches (225 mm). The data shows that at 84 days a decrease in drying shrinkage of about 30 percent was obtained with an 18 fl. oz./cwt (1170 mL/100 kg) dose of HRWRA. The water reduction at this dose was approximately 30 percent. Therefore, mid-range and high-range water-reducing admixtures can be beneficial if they are used to obtain significant reductions in total water content.Air-entraining admixtures have been shown to have little or no effect on drying shrinkage.

The magnitude of drying shrinkage can be reduced significantly through the use of a shrinkage-reducing admixture. Shrinkage-reducing admixtures function by reducing the surface tension of water within the pores of concrete. This leads to a reduction in the capillary tension and the pull on the walls of the pores and, consequently, a reduction in drying shrinkage. These admixtures have been successfully used in the Far East and North American construction markets since their introduction in 1985 [12].

FIGURE 4. Drying Shrinkage of Concrete with and without Shrinkage-Reducing Admixture.

In addition to shrinkage-reducing admixtures, a first-of-its-kind crack-reducing admixture provides better performance under restrained shrinkage, resulting in smaller initial crack widths [13] in addition to reduced drying shrinkage of concrete.

Recent research indicates that shrinkage-reducing admixtures may be used beneficially to reduce evaporative water loss from fresh concrete, to reduce autogenous shrinkage, and thus to reduce early-age cracking, whether due to plastic shrinkage or autogenous deformation [14].

Effects of Design and Construction Practices
Design parameters that most influence drying shrinkage are the amount of reinforcement provided and the size, shape, and surface area-to-volume ratio of the concrete member.

Steel reinforcement will reduce the drying shrinkage of concrete because of the restraint provided by the steel.

In the same ambient environment, a small concrete member will, because of its higher surface area-to-volume ratio, shrink more than a larger member. The greater the exposed surface area, the greater the rate of moisture loss becomes, hence the potential for drying shrinkage. Therefore, it should be recognized that the drying shrinkage that will be experienced in actual concrete structures will only be a fraction of that obtained in the laboratory with the ASTM C 157/C 157M test method.

Improper concreting practices, such as retempering at the job site, will increase drying shrinkage because of the increase in the water content of the concrete. Prolonged moist curing will delay the onset of drying shrinkage, but in general the length of curing is reported to have little effect on drying shrinkage [3]. Steam curing will, however, reduce drying shrinkage.

Effects of Environmental Factors and Time
As mentioned earlier, the loss of moisture from hardened concrete leading to drying shrinkage is inevitable unless the concrete is in an environment with 100 percent relative humidity. This scenario, of course, is rarely the case unless the concrete is completely submerged in water. The magnitude of drying shrinkage is greatly affected by the relative humidity of the surrounding environment. The lower the relative humidity, the higher the magnitude of drying shrinkage. The magnitude of drying shrinkage is, however, not influenced by the rate of drying. The rate of drying is in turn not affected by wind or forced convection, except during the early stages of exposure. This is because of the very low moisture conductivity of concrete that allows for only a very small rate of evaporation.

The magnitude of drying shrinkage is also time-dependent. Though the bulk of drying shrinkage occurs within the first few months of drying, the process continues for years. Data from a comprehensive study spanning a period of nearly 30 years showed that, on average, nearly 50 percent of the drying shrinkage obtained at 20 years occurred within the first two months of drying, and nearly 80 percent within the first year [15].

Effects of Shrinkage
As stated earlier, the major concern with regard to the shrinkage of concrete is the potential for cracking. Other potential issues are curling of slabs and dimensional stability of concrete members. Dimensional stability is typically taken into consideration during design, and unless the actual shrinkage far exceeds the design value, there should be no problems. Cracking due to shrinkage occurs mainly because of restraint. Concrete that is unrestrained, for example a cylinder that is 4-by-8 inches (100-by-200 mm), will not crack due to shrinkage. As stated in the introduction, another article will discuss the issues for restraint and the mechanism for cracking.

Shrinkage of concrete, drying shrinkage in particular, is inevitable; and because of restraint, cracking can occur. However, with good concreting and construction practices, shrinkage and subsequent cracking can be minimized.

Control plastic shrinkage – Prevent the surface of fresh concrete from drying out until finishing operations have been completed and curing has been initiated. The use of ice or chilled water to reduce the batched concrete temperature and polypropylene fibers can be beneficial. Temporary windbreaks should be erected on windy days, if possible, to reduce wind velocity. To reduce concrete surface temperatures, temporary sunshades may be used. In arid regions where conditions for plastic shrinkage are prevalent, the use of accelerating admixtures and evaporation reducer should be considered.

Minimize drying shrinkage – Keep the total water content of the concrete mixture as low as is practicable for the intended application. This can be achieved by using a high content of hard, rigid aggregates that are free of clay coatings, and by using mid-range or high-range water-reducing admixtures. In addition, the concrete should not be retempered at the job site.

Consider admixtures – A shrinkage-reducing admixture or a crack-reducing admixture will reduce the drying shrinkage and the rate of drying shrinkage of concrete. In addition, their use will improve cracking resistance, reduce curling heights and the rate of curling, and reduce joint opening and the rate of joint opening. As stated earlier, crack-reducing admixtures also provide better performance under restrained shrinkage.

As has been discussed, concrete undergoes different types of shrinkage starting from the time of placement. These are loosely related to the age of the concrete, for example plastic shrinkage occurs while the concrete is still plastic, early-age thermal contraction can occur within the first 24 hours, while drying shrinkage occurs over days and weeks. In tilt-up panel construction, and with respect to the cracking under restraint, we are primarily concerned with early-age thermal contraction and drying shrinkage (and autogenous shrinkage, depending on the mixture proportions) that are likely to play the biggest role. Development of a low-shrinkage concrete mixture can help to alleviate panel cracking, in addition to extending joints in slabs-on-ground. As noted earlier, the water content of a mixture is the primary driver of long-term drying shrinkage. However, using a low water mixture for a wall versus a floor with floor flatness (FF) or floor levelness (FL) requirements are two very different propositions. The use of a shrinkage-reducing admixture or a crack-reducing admixture allows for the reduction of drying shrinkage and control of crack widths without driving water contents to such low levels that the concrete becomes overly sticky. Successfully developing low-shrinkage concrete mixtures requires good communication between the engineer, contractor and the concrete producer so that all of the performance requirements can be achieved.


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