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Sketch 58 Crack !NEW!

Early-age behavior may impact the long-term performance ofconcrete pavements leading to a number of distresses. These guidelines discuss the impact that early-age behavior mayhave on JPCP faulting and cracking, CRCP punchouts, and the impact on spallingdistress, which is common to both JPCP and CRCP. While JPCP faulting and cracking are modeled in HIPERPAV II, aspalling model was not included in the long-term prediction module because ofthe complexity required to account for all possible factors that affectspalling distress. Likewise, since CRCPlong-term performance was out of the scope of the HIPERPAV II project, nopunchout models were included in HIPERPAV II. It is important, however, that users of these guidelines understand thefactors influencing spalling and punchout distresses and the design andconstruction factors that should be considered to minimize their occurrence.

Sketch 58 Crack

Faulting of doweledpavements can also lead to other distresses, such as spalling and transversecracking. If the dowel bearing stressesare too severe, concrete above the dowels can break off to form a spall. This is discussed in more detail in sections 4.5 and 4.6. Transversecracking is also possible if the doweled joints are improperlyconstructed. Dowel misalignment cancause a transverse crack to form near the joint. For more details on this distress, see section 4.3.

Transverse crackingof JPCP can develop at early ages immediately after construction, or it canform years later due to fatigue. However, the mechanisms that cause these cracks to form are different. At early ages, transverse cracks formbecause of restrained volume changes. Concrete tends to expand and contract due to changing climaticconditions. When these internaldeformations are restrained by external slab-subbase restraint and self-weight,early-age transverse cracking is possible.(2) Because young concrete has not yet reached its full mature strength, itis more susceptible to tensile damage. More detail on this type of cracking is provided in section.

When transversecracks form after years of pavement use, the most common cause of cracking isfatigue. Over time, the cumulativenumber of traffic loadings increase, as do the number of seasonal climaticcycles. Stresses are generated in theconcrete, and eventually a transverse crack can form. Transverse cracks can either propagate from the top of thepavement down (top-down cracking), or they can propagate from the bottom of thepavement up (bottom-up cracking). Thedifference between these two cracking mechanisms is discussed in the followingsections.

This sectionexamines how early-age inputs influence transverse cracking in JPCP over thelong term. A flowchart connectingearly-age input to the long-term performance of JPCP is shown in figure 33. The six fields presented in that figure arediscussed in greater detail in the following sections.

Early-age inputsthat have a significant influence on transverse crack formation are listed instep 1 of figure 33. The concrete mixdesign will determine the strength and fracture properties of the concrete atearly ages and in the long term. Thecoarse and fine aggregate percentages also can influence the amount oftransverse cracking to the level aggregate gradation affects drying shrinkage.(35) Pavement design also controls the stresses generated in thepavement. Widened pavements cansignificantly decrease the pavement edge stresses that cause transversecracking. Joints alleviate tensilestresses that occur in the pavement at early ages. Shorter joints decrease frictional stresses at the slab-subbaseinterface and decrease the severity of transverse cracking in the longterm. Slab thickness also has asignificant influence on transverse cracking.(38) Thicker slabs are better able to resist deformation, and they are betterable to carry mechanical and environmental loading.(26)

Climatic conditionsat the time of pavement construction have a marked influence on long-termpavement performance. The temperaturegradient at the time of set determines the amount the pavement curls up ordown, and the moisture gradient controls the amount of warping. This built-in curling is discussed insection 2.4.2. Climaticconditions influence the set time and thermal gradient at set, and thereforesignificantly influence the magnitude of stresses generated over the longterm. Finally, construction methods arecritical in controlling transverse cracking. Time of joint sawing is important to control, since the joints aredesigned to alleviate internal pavement stresses and control where the cracksform. Both of these constructionmethods can improve the pavement's long-term performance.

The strength andfracture properties of concrete have a significant impact on transversecracking. The stiffness of the concretedetermines the magnitude of induced stresses and deflections. The strength and fracture properties controlwhen the concrete cracks.

Fatigue of concretehas been shown to be a function of the stress-to-strength ratio.(39) Ahigh-strength concrete will commonly have a greater fatigue life than alow-strength concrete. However,high-strength concrete also can have higher stress concentrations at flaws ormicrocracks, making it brittle. Pavements constructed of concrete with high flexural strength typicallyfracture more easily.(35)Forthis reason, it is recommended that concrete strength be kept within a midrange from 4.5 to 4.8 megapascals (MPa).

The pavement's critical stresses can either occur at the bottom of the pavement (bottom-upcracking) or at the top of the pavement (top-down cracking). The mode of failure depends, in part, on thepavement's "built-in" temperature gradient. The term "built-in" is in quotes because it is not really a fixedproperty. However, for simplicity, itis commonly assumed to be the temperature gradient at final set. To better explain some of the morefundamental concepts, it will be defined in this manner for the discussion tofollow. However, this gradient truly isnot fixed, but will instead change with time as the stresses in the slab relax.

Upward curling is less restrained than downward curling because only the corners of the slab lift. Downward curling requires that the slab joints bear down on the subbase (see figure 36). This downward curling requires more forcethan the upward curling.(22) Drying shrinkage most commonly increases the tensile stresses at the pavement surfacedue to an increase in the degree of upward slab curling.(39) For the first 72 hours after construction, the pavement's expansion and contractionstresses are partially restrained by the slab-subbase friction. If the internal stresses are greater thanthe strength of the concrete, transverse cracks may form. If the joints are sawed at the optimal time,early-age uncontrolled transverse cracking can be prevented. If dowels are used at the pavement joint andare significantly misaligned, transverse cracks can form at the joint. Care must be taken to construct and maintainthe pavement properly to prevent dowel bar corrosion. Dowel bar corrosion may lead to a locked transverse joint, whichmay cause additional transverse cracks to form.(40) Evenif no transverse cracks form in the pavement at early ages, they may still formin the long term due to fatigue. The upward and downward built-in set conditions increase the likelihood of midslabcrack formation due to subsequent mechanical and environmental loading. This is discussed in section 4.3.5.

Repeated tensilestresses in the pavements initially can damage the concrete at the microscopiclevel. Eventually, microcracks formthat act as stress concentrators. Afterthese microcracks have lengthened and connected, a macroscopic crack willform.

Curling and warpingstresses subject the pavement to tensile damage, and help generate transversecracks. As mentioned in section 2.4.2, built-in downward curling at set due to negativethermal temperature gradients increases the tensile stresses at midslab. The combination of thermal gradients andtraffic loading assist in transverse crack formation.

Even though thepavement may resist transverse crack formation at early ages, cracks may stillform in the long term due to fatigue. Fatigue loading causes any flaws in the concrete to lengthen. Repeated loads cause the microscopic flawsto blunt and resharpen until they form into a visible crack.(41) Fatigue depends primarily on the ratio of induced stresses to theconcrete's strength. Miner's damage law typically is used to calculate theamount of damage a pavement sustains over its lifetime.

The two mostsignificant factors affecting long-term fatigue are the combined effects oftraffic and environmental loading.(26) Traffic loads increase the stresses to which the pavements aresubjected. They contribute considerablyto bottom-up and top-down transverse crack formation. As figure 37 demonstrates, for bottom-up cracking, the criticalstresses that cause transverse cracking are at the pavement's midslab edges.(15)

Figure 38 demonstrates howwheel loading during the service life causes the pavement to deflect downward,putting the bottom of the pavement in tension. Traditionally, transverse cracking is assumed to initiate at the bottomof the pavement.

Similarly, wheelloading can put the bottom of the slab in tension when it is curled down. The interior of the slab may not havecontact with the subbase when it is curled down.(15) Thisloss of support increases the likelihood of midslab transverse cracking.

Environmentalloading changes subbase support. Thesubbase sometimes erodes, settles, pumps, or heaves, all of which negativelyimpact pavement performance.(38) Thedevelopment of these subbase phenomena is often directly related toprecipitation amounts. Figure 38 demonstrates that,for a curled-up pavement, the water softens the subbase, therefore increasingmidslab deflection and, consequently, the tensile stresses at the bottom. For this scenario, the potential ofbottom-up cracking is increased. For acurled-down pavement, loss of subbase support, coupled with traffic loadingclose to the joint, put the surface of the curled-down pavement in tension,increasing the potential for top-down cracking. This can be seen in figure 39.


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