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Publication Number: FHWA-04-122 Date: February 2005 |
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This section discusses input parameters that influence the early-age behavior of JPCP and CRCP. This section will also outline general guidelines for designing and constructing these pavement types. Inputs to these guidelines are divided into the following five basic categories; these are discussed in the following sections.
The following sections detail the specific recommendations and guidelines for input selection in each of the five categories. Because these guidelines provide only general guidance for selecting the input parameters, the user must exercise sound judgment when selecting the final input parameters. To make the final decision, other factors such as economics, material availability, and ease of construction also should be considered. These guidelines are not intended to optimize the decision process, but they do provide a means by which to predict the success or failure of a particular combination of inputs to the overall performance of a JPCP or CRCP.
Pavement design parameters typically are selected before construction based on the project site location. Factors such as type of subbase to be used can significantly impact both the early-age and long-term performance of the JPCP and CRCP. Other design parameters, such as thickness, drainage, and joint spacing (if applicable) also have an influence on the pavement's long-term behavior.
Four general types of PCCP are constructed today. JPCP is the most common. Its transverse joint spacings are typically 4.5 to 6 m, and it contains no slab reinforcement. If a crack occurs at midslab, only the aggregate interlock will transfer load across the joint. Jointed reinforced concrete pavement (JRCP) is another type of pavement. Its transverse joints are longer, with spacings typically between 12 to 30 m. This pavement is reinforced at midslab, allowing for the longer joint spacing. Cracks can occur midslab where the reinforcement holds the pavement together, providing load transfer efficiency. CRCP does not have transverse contraction joints. It is heavily reinforced, and this reinforcement holds the cracks closed that do form. The last type of pavement is prestressed concrete pavement (PCP). It is constructed using the prestressed concrete girder ideology—by applying a compressive stress via post-tensioned cables to the pavement, it can resist greater loads, and thus may be constructed with a smaller cross-sectional area and longer spans. FHWA and the Texas Department of Transportation (DOT) recently have sponsored some recent pilot sections in Georgetown, TX, for using precast PCP in rapid construction projects, and additional work is underway for implementation of this technology in other States.(45)
Of these four pavement types, JPCP comprise the greatest number of new and existing concrete pavements. CRCP is increasingly popular due to the minimum maintenance required, but only accounts for a small fraction of the lane-miles of concrete pavement. PCP and JRCP are not as commonly used as the other types. Only JPCP and CRCP will be discussed further in these guidelines.
Thickness design of JPCP or CRCP usually is based on long-term pavement performance requirements. Traffic-induced loading can lead to fatigue and other forms of cracking, spalling, and faulting. Each of these distress types can be controlled to different degrees by specifying a thicker cross section. In general, a thicker pavement will lead to a lower potential for early-age and long-term damage.
For CRCP, the thickness of the slab is believed to affect crack width. Thicker pavements tend to result in smaller crack widths. A possible explanation for this behavior may be that thicker pavements have less exposed surface in relation to their volume than thinner pavements. As a result, thicker pavements are less affected by drying shrinkage.(16, 19) With respect to crack spacing, thicker slabs provide a higher stiffness to the pavement, thus reducing the level of deflections at which the pavement is subjected due to wheel loads. Thinner slabs, on the contrary, are subjected to larger deflections, and therefore higher stresses develop, contributing to the development of additional transverse cracks, thus reducing the crack spacing.
In these guidelines, joint spacing is a design input only for JPCP. At early ages, joint spacing controls the amount of stresses that accumulate in the pavement. The joints relieve the thermal and frictional interface stresses by allowing controlled cracks to form. Joint spacing is also a significant factor that influences the joint opening size. At later ages, joint opening determines the load transfer efficiency of the aggregate interlock across the joint. Joint sawing (and dowels, if used) are costly, so a tradeoff between cost and allowable stress level usually occurs in the selection of the optimal joint spacing.
Reliability is a measure of design (or system) effectiveness based on the variability of each individual component. Each of the input parameters identified in this study has a different degree of variability associated with it. Reliability concepts are used to reduce the individual variabilities to a system reliability. By selecting a certain level of reliability, the user is, in essence, selecting the confidence level of the analysis and an acceptable level of risk. For example, if a reliability of 95 percent is selected, the user is willing to accept a 1 in 20 chance (5 percent) that the results obtained using these guidelines will be outside the bounds of what is predicted (unconservative). By selecting a higher level of reliability, the odds decrease. However, the higher the reliability, the more conservative the design may become, making it cost prohibitive.
The user should select the optimum reliability level based on the function and importance of the facility. For low-end facilities, such as streets or local roads, a lower degree of reliability may be chosen (50 to 85 percent). For primary or interstate highways, a higher degree of reliability should be selected (90 to 95 percent).
Water drainage from pavement layers is extremely important if the pavement is to provide suitable long-term performance. Excess water, in combination with heavy traffic loads, can lead to early distress of the pavement structure, such as faulting, transverse cracking, and corrosion of steel elements (dowels and steel reinforcement). Water can also reduce the strength of unbound granular subbases and subgrade soils. Water causes pumping and erosion of material from under the slab, and results in loss of pavement support. For CRCP, crack widths should be kept less than 0.5 mm to prevent water infiltration.(2) Pooling of the water between the slab and the subbase also can cause concrete deterioration that leads to spalling, especially in frost-susceptible regions. The effect of water on pavement properties, and its affect on the structural capacity of the pavement, must be considered when predicting the long-term behavior of JPCP and CRCP.
For pavements, three different drainage systems are commonly used: 1) surface drainage; 2) groundwater drainage; and 3) structural drainage. These drainage systems are designed to drain water as quickly as possible away from the pavement system. The design of all important highways should consider the use of a drainage layer in the pavement structure.
There is commonly an engineered subbase course beneath JPCP and CRCP pavements. A subbase is typically a layer of granular or stabilized material. The subbase layer imposes restraint to volume changes occurring in concrete pavement during the early age. The restraint is the result of friction, adhesion, and bearing stresses that develop as a function of the subbase materials and construction procedures. For CRCP during the early age, the bond strength of the concrete to the steel is still weak, and the frictional restraint plays a more significant role in the pavement. After the bond strength increases with time, the subbase restraint is more negligible compared to the restraint imposed by the steel.(46)
By selecting a subbase with minimum frictional resistance at the slab-subbase interface, restraint can be minimized. This reduces the stress development in the pavement at early ages. A bond breaker can minimize slab restraint as well as reduce the possibility for reflective cracking from high frictional subbases such as cement stabilized or hot-mix asphalt (HMA) subbases. Typical values of slab-subbase friction for various subbase types can be found in the HIPERPAV II software.
The type of subbase used has been found to influence the long-term performance of the pavement significantly. The subbase should be chosen to allow for optimal water drainage away from the slab. If water cannot drain, the concrete at the joint can deteriorate, and spalling damage is more likely in the long term. The amount of faulting is also related to the ability of the subbase to drain. In the presence of water, fines can pump underneath the slab, increasing the potential for faulting. On an impermeable subbase, such as cement-treated base, the pumping of fines underneath the slab on one end and the erosion of fines at the end of the other slab can lead to a loss of support, which can result in transverse cracking. These distresses can be prevented or delayed if the subbase provides good drainage. Good drainage is one of the most efficient ways to prevent pumping.(15)
The properties of the subgrade influence the long-term behavior of the pavement, especially in seasonal frost areas (see section 5.3.2). In these regions, the subgrade's resilient modulus varies significantly with changing seasons. Different subgrades are affected differently by effects such as thaw-weakening. Well-drained sandy soils tend to have a higher bearing strength, while clayey soils are generally weaker.(17)
Load transfer commonly is understood as the ability of a pavement to transfer loads across a joint or crack, from one slab to another. Load transfer at joints or cracks in a concrete pavement can be provided by aggregate interlock and mechanical devices such as dowels. When loads are applied near a joint or crack for a pavement with poor load transfer efficiency, significant deflections often develop. Good load transfer efficiency between joints or cracks greatly reduces the level of these deflections and stresses. The deflection and stress is reduced because the adjacent slab helps support the load. Primary factors known to affect load transfer efficiency include load magnitude, number of applications, slab thickness, joint opening, subgrade support, and aggregate characteristics.(47)
JPCP pavements are susceptible to cracking due to restraint to expansion and contraction movements with changes in temperature and moisture. Transverse and longitudinal joints commonly are sawed or formed at regular intervals to control the spacing of the cracks and provide a uniform shape that is easier to seal against infiltration of water or incompressibles. The joint depth must be designed to create a weakened plane that intensifies the stresses at the joint location and therefore induces the crack to occur at that point. However, the depth of the joint also affects the load transfer, because the area of aggregate interlock through the thickness of the slab is reduced.
Application of loads, either by temperature or wheel loading, can be transmitted from one slab to the other by using dowels with JPCP or steel rebars with CRCP. Dowels in JPCP allow slabs to expand and contract while restraining the vertical movement from one slab to the other to avoid faulting. With CRCP, the expansion and contraction of the slabs is restrained by the steel to keep the cracks tight. Although steel is not designed for load transfer, the steel restrains the vertical movement of CRCP, thus reducing stresses. Cyclic loadings commonly cause crushing of the concrete particles at the dowel interface due to the high compressive stresses that develop at those locations. Eventually the dowel becomes loose, because voids are created around the dowel as the crushed concrete particles are removed.
In the absence of dowels, aggregate interlock can provide load transfer at the joints and cracks. Aggregate interlock is primarily a function of the characteristics of the aggregate, including aggregate size, angularity, and abrasion. In a controlled laboratory study, Nowlen investigated the effect of aggregate characteristics on load transfer.(13) Major findings from that study included:
The width of the contraction joint or crack plays an important role in the load transfer efficiency provided by aggregate interlock. Nowlen developed a looseness factor, which is a function of aggregate size and joint opening.(13) In his study, he showed that wider joints correspond to less effective load transfers provided by the aggregate interlock. In the study performed by McCullough et al., a reduction in load transfer efficiency in CRCP was also found with larger crack widths.(2)
Primary steel properties known to influence CRCP transverse cracking include the percent of longitudinal steel, bar size, and vertical location of longitudinal steel with respect to slab thickness.
An increase in percent of longitudinal reinforcement will increase restraint. As the level of restraint increases, so does the number of cracks that develop, resulting in shorter crack spacings. In addition, the spacing of the transverse steel is commonly a function of the amount of steel used. It is believed that as the amount of steel increases, the average steel stresses are reduced, producing less elongation of the steel, which, in turn, leads to a decrease in crack width.(16) Previous investigations have shown that this effect is significant for steel percentages of 1 percent or less.
For a given amount of steel, larger bar sizes result in less steel surface area. The average crack spacing decreases with an increase in ratio of steel surface area to concrete volume. A possible explanation to this effect is that the high tensile stresses in the steel at crack locations are transferred to the concrete as a function of the steel surface area and deformation characteristics of the longitudinal steel.(46) On the other hand, the greater the bond area, the more the steel imposes restraint to movement of the concrete, and therefore, tighter cracks are expected to result. However, some studies have found that although this theory holds for limestone aggregates, the opposite trend is observed for the case of siliceous aggregates.(19)
During the early age, the concrete is subjected to differential thermal and moisture gradients through the depth of the slab, primarily as a function of climatic conditions and curing methods. The location of the steel with respect to pavement depth will affect the crack widths. For example, since drying shrinkage is more pronounced at the surface of the slab, placing the steel close to the surface maintains crack widths tight more effectively. However, adequate concrete cover also should be maintained around the steel. Two-mat placement has been implemented in the Texas DOT specifications for pavements thicker than 330 mm. This helps in maintaining optimum steel bond area to concrete volume ratio and also allows for steel closer to the CRCP surface, where shrinkage strains tend to produce larger crack widths. In HIPERPAV II, the number of steel mats is used to determine the change in percent steel only. As new models are available that consider the effects on shrinkage and other behaviors as a function of the steel depth and number of bar mats, they maybe included in HIPERPAV II.
PCC consists of three basic constituents: aggregate, cement, and water. To modify a particular property of the PCC, SCM and chemical admixtures can also be used. SCM include, but are not limited to, pozzolans, fly ash, and ground granulated blast furnace slag (GGBFS). Chemical admixtures that can be used include superplasticizers, water reducers, and retarders. Fiber reinforcement, synthetic or steel, is another additive. SCM and admixtures can often affect the early-age behavior of the PCC during the hydration period, especially in fast-track mixes. However, modifications of the three basic PCC components, both in type and content, can also affect the overall concrete performance.
The following sections will highlight the influence of material parameters on the behavior of PCC, with emphasis on the fast-track mixes.
Aggregates are, both by volume and by mass, the largest component of a PCC mix. By volume, they account for 60 to 80 percent of normal weight concrete. Because of this, aggregate properties can significantly affect the early-age and long-term performance of the concrete. The characteristics of the bond between the concrete paste and the aggregate is also a significant factor on concrete performance. At very early ages, the bond between the aggregate and the cement mortar is not yet at its maximum. Failure will most likely occur in this weak zone. At this very early age, aggregate strength does not strongly influence concrete behavior. However, once this bond has formed, then aggregate strength has a greater influence on concrete strength. PCC properties typically are modeled as a weighted combination of the individual component's properties.
Chemically, aggregates are composed of one or more minerals. Aggregates typically are formed naturally, but they can be manmade (such as steel slags). Many aggregates can be divided into two general categories of calcareous or siliceous. Calcareous aggregates include limestones and dolomites. Siliceous aggregates include granites and quartzes. Some aggregates, particularly sands and gravels, can be a blend of these two types.
All other factors being equal, concrete pavements constructed with aggregates that have a low CTE (e.g., many calcareous aggregates) perform better than those built with aggregates with a high CTE (e.g., many siliceous aggregates). Higher stresses can form in the pavement because the high coefficient aggregates experience greater expansion and contraction from temperature changes. These higher stresses may produce cracks that reduce long-term performance. If an aggregate with low CTE is used, there is less thermal shrinkage. In the final selection of aggregate type, factors other than CTE, such as availability and cost, should be included in the decisionmaking process. If siliceous aggregate with a high CTE is the only locally available material, precautions should be taken to minimize early-age damage. These precautions, such as cooling the coarse aggregate before use, will be discussed in more detail in the following sections.
The aggregate proportion of the mix design can also influence early-age behavior, although it is considered to be relatively insensitive when compared to others. However, by increasing the relative proportion of aggregate in the mix design as compared to the other constituents, the stiffness of the mix will be higher for both the plastic and solid states. Therefore, the stresses will be greater, and the potential for early-age damage will be greater.
Other aggregate properties, including shape and gradation, influence the behavior of the pavement in the long term, as well as at early ages. In JPCP, it may be only aggregate interlock that transfers the load across the joints. Aggregates with an angular shape provide better load transfer across the joint than do round aggregates. Likewise, large aggregates are better able to bridge a joint opening, and hard aggregates are able to resist the joint shear stresses. The bond between the angular aggregate and the cement matrix is generally higher as well, often resulting in a higher flexural strength. However, angular aggregate can reduce workability of the fresh mix, increasing the air content during placement.
Durability of aggregate also affects the long-term performance of pavements. Aggregate should resist deterioration caused by weathering, such as freezing-thawing and wetting-drying. Clay balls and shales should be avoided because of their susceptibility to moisture swelling. If freeze-thaw susceptible aggregates are near the surface of the pavement, they can expand and break out of the surface of the pavement. Low durability aggregates typically have a higher porosity, and are easily saturated. In pavements with poor drainage, saturated aggregates at the bottom of the slab can cause D-cracks to form, which can progress upward.
The absorptivity of aggregate directly correlates to concrete shrinkage. Granites and quartz have low absorption and low shrinkage, while sandstone and slate have higher absorption and therefore have higher shrinkage.
The type of coarse aggregate also has a marked effect on spalling and cracking distresses. In CRCP, it was found that pavement constructed with crushed limestone had less damage that those constructed with siliceous river gravel. Reasons for this may include the lower stiffness and better bond to the cement matrix in the limestone concrete.
Primary aggregate factors known to influence the cracking characteristics of CRCP include its angularity, bond strength properties, the CTE, and the drying shrinkage properties. Previous investigations have shown how the angularity and bond strength properties of the aggregate play an important role in the development of tensile strength in the concrete. As cracking in CRCP form whenever the tensile stresses exceed the tensile strength of the concrete, the characteristics of the coarse aggregate type used will have an effect on crack spacing. Particularly, aggregates with increased angularity provide higher tensile strengths than rounded aggregates.(46) Because the coarse aggregate type is one of the major components in the mix, the drying shrinkage and CTE of the aggregate commonly will govern the contraction/expansion of the concrete. As previously mentioned, differences in the CTE of the steel and concrete induce an internal restraint to volume changes that generates stresses in the concrete and therefore influences cracking. In general, coarse aggregates with a high CTE result in shorter crack spacings than aggregates with low CTE.(19) Some studies have also found that aggregates with low CTE result in smaller crack widths.(19) Drying shrinkage in the concrete is a result of several factors, including the w/c ratio, relative humidity, curing method, and coarse aggregate type. The resulting drying shrinkage will affect crack spacing, as drying shrinkage contributes to the volume changes in the concrete and therefore to stress development. Aggregates with higher drying shrinkage characteristics usually result in wider cracks and closer crack spacings.
Properties of concrete change as a function of time, primarily due to changes in the cement paste. This is particularly true at early ages. Because the hydration process is complex and mix specific, these general guidelines should be used with caution. Improper selection of the cement type can lead to a poor performing pavement. The four basic types of cement commonly used in paving concrete are:
These four types of cement are all hydraulic cements that react chemically with water. This reaction generates heat, and for each cement type, heat is released in different quantities at different rates. The strength of PCC is directly related to the amount of heat generated by this chemical reaction. In general, Type III cement releases its heat earlier than do the other types of cement. It is a high early strength cement. Type V is at the other extreme. It releases heat slowly and gains strength slowly as a result.
Type III cement often is specified in fast-track mixes. It allows pavements to open only a short time after construction. However, detrimental side effects can result. Because of the high heat generated, the early-age pavement stresses can be quite significant. Cracks may be noticeable as the concrete cools down from the maximum heat of hydration. The use of Type III cement for fast-track paving should be monitored carefully. In addition to Type III cements, Type I and even Type II cements have been used in fast-track mixes. These applications can be successful if measures are taken to maximize the concrete's strength gain during pavement construction for all but the shortest opening criteria.
In general, Type I cement is recommended in fast-track mixes, unless a very short opening criterion is demanded, or if Type I cement can not generate the required strength gain. When a Type III cement is needed, it is recommended that the time of the maximum heat of hydration be set to a different time as the maximum air temperature. This may require that construction takes place in the late afternoon or evening. Night paving is recommended in the southwest United States during the summer months, for example.
The amount of cement in a concrete mix influences its early-age behavior. Large cement contents usually cause shrinkage problems and also produce significant levels of heat in the slab. However, higher cement contents can improve workability and strength. However, the same two benefits can be achieved by adjusting the w/cm ratio, or by using admixtures. The later is recommended to improve concrete's workability and strength. The w/cm ratio is defined as the mass of mix water (including free moisture on the aggregates) divided by the combined mass of the cement and any additional cementitious materials, including fly ashes, silica fume, and GGBFS. This ratio is important, as it determines the overall strength of the mix as well as other mechanical properties, such as creep and shrinkage. Because increasing the w/cm ratio generally will decrease strength, it should be kept as low as possible while still maintaining a workable mix. Additional information on mix design proportioning can be found in the literature.
The different types of cement also have different long-term performance. Generally, the finer the cement is ground (as for Type III), the higher the strength gain at early ages, but the lower the strength gain at later ages. Concrete constructed using Type I cement continues to gain strength as it matures in the field. The finely ground cements (Type III) are less durable in the long term than are the coarser ground cements (Type I).(5) Likewise, higher strength concretes often have numerous cracks.
Several types of chemical admixtures can be added to concrete for pavement construction. Admixtures can be used to allow concreting in hot and cold weather conditions. The following is a list of the various types of chemical admixtures and how their use affects PCC mixes.
Accelerator admixtures increase concrete's strength gain at early ages. Calcium chloride commonly is used in accelerator admixtures. Accelerators can be used in place of Type III cement if the cement is not available or is too expensive. The same cautions for using Type III cements are given for using accelerators. Their use can also lead to long-term durability problems associated with early-age high heat generation, increased drying shrinkage, reduced creep capacity, potential reinforcement corrosion, and scaling.
Air entrainers are the most commonly used PCC admixture. They entrain microscopic air bubbles in the concrete. A minimum level of air entrainment commonly is mandated in construction specifications, since it has been shown to increase concrete's long-term durability as well as its workability. However, air-entrainment does reduce concrete strength, so care should be taken when using it, especially in hot-weather conditions.
Retarders extend the concrete's set time, reduce the rate of strength gain, and commonly act as water reducers. They can be useful with long haul times from the batch plant to the pour site. However, they can be detrimental to the concrete's structure. Retarders can offset the increased strength gain when constructing in hot weather, but they are rarely used in fast-track pavements.
Superplasticizers can reduce the amount of water required to achieve a workable concrete mix, if used properly. They are high range water reducers that can reduce water demand by 12 to 30 percent. They can be used to reduce the w/cm ratio, increase concrete strength, and expedite strength development. Superplasticizers are often used in fast-track mixes because of these beneficial effects under hot weather conditions. Typically, they are added at the construction site because the increase in workability is short-lived, from 30 to 60 minutes.
Water reducers act in a similar manner to superplasticizers, but to a lesser extent. They reduce water demand at least 5 percent and can produce the desired slump. They can extend the working time of concrete by a couple of hours and also accelerate the concrete's strength gain. However, they may significantly increase drying shrinkage, so caution must be exercised when using them. Water-reducing admixtures improve workability by changing the electric charge of the cement particles, which improves dispersion of the cement particles and makes more water available for hydration. Therefore, the hydration of the cement is improved, with a consequent slight increase in strength development.
Of these chemical admixtures, air entrainers are considered to be an essential element in PCC (where applicable). In general, many mix designs can be developed using an air entrainer, superplasticizer, and possibly accelerators. These materials should be used based on past experience, availability, and economics.
Fly ash, silica fume, and GGBFS have all been shown to increase later age compressive and tensile strengths and/or durability of concrete (reduced permeability) when used properly. It is difficult to generalize the effect of SCMs on concrete properties. Each one can influence the behavior of the concrete differently.
Pozzolans such as fly ash are the most common type of SCM. These minerals partially replace the cement, and some (particularly Class F fly ash) improve workability, reduce the heat of hydration of the concrete, and retard concrete's setting time. Benefits of replacing cement with fly ash include improved workability and lower cement demand.(48) The strength of concrete at early ages is usually reduced by incorporating fly ash.(49)However, satisfactory long-term strength gains are obtained. Common ranges of cement replacement with fly ash are between 10 and 50 percent. In general, fly ash retards cement hydration, consequently retarding the strength gain. This retarding effect is more considerable at low temperatures. Class C fly ash possesses more cementitious properties than Class F fly ash due to its higher free lime content.
GGBFS is used commonly in concrete operations for economy reasons at proportions of 25 to 50 percent by weight replacement of cement. Higher slag contents are used for durability or sulfate resistance. Incorporating GGBFS in the mix tends to prolong the hydration, but in the long term, strength increases to a satisfactory level depending on the percentage of cement replacement. The setting time of concrete increases with the use of GGBFS.(49) Final setting of PCC can be delayed as much as several hours, depending on the ambient temperature and mixture proportions.(49) Significant retardation in setting time of the PCC has been observed for curing temperatures lower than 23 °C.(49) At temperatures lower than 10 °C, the strength development of GGBFS is poor, and using it is not desirable.
Silica fume reduces the permeability of the mix and significantly increases its compressive strength. The use of silica fume, despite its benefits, is often limited due to reduction in workability, placeability, flowability, and finishability. Replacing cement with silica fume has been limited in some standards to 10 percent.(48)However, up to 15 percent of silica fume has been incorporated successfully in concrete.(49) Incorporating silica fume improves cement hydration. The development of heat of hydration in ordinary PCC with silica fume has been observed to be as high as that of rapid hardening cement alone for certain mix proportions.(9)
Concrete mix designs should be optimized for workability, strength, economy, and other concrete properties before using these guidelines. Several mix designs are appropriate for concrete pavements. The selected design should be tailored to the specific environment the pavement will be subjected to during its lifetime. For concrete pavements, high early strength, high ultimate strength, and low permeability are all desired criteria for high-performance pavements. A number of other factors should be considered in mix design.
One of the most important factors affecting the properties of hardened concrete is the w/cm ratio. This ratio controls the quality of the paste and therefore impacts the durability and strength of the concrete. The concrete is a mixture of the paste and the aggregate, so the aggregate properties and characteristics are very important to the concrete's quality. Aggregate gradation and particle size, shape, and surface texture are all important.
Another key factor is the concrete's 28-day strength. The 28-day strength should be selected to satisfy the long-term design criteria. Proper selection can delay, or even prevent, fatigue and traffic-induced cracking. It is recommended that the highest possible strength be selected in design, balancing the adverse effects of high shrinkage and rapid heat generation that can result.
Other factors, such as shrinkage, should also be accounted for during the mix design process. If the concrete's paste content is too high, shrinkage cracks can form. To achieve a lower paste content, the gradation of the aggregate can be optimized and, in some cases, also will result in an increase in the strength of the mix.
Because many of the PCC constituents can exert both positive and negative influences on the properties of the concrete, the engineered mix should first be verified against the numerical guidelines. If the mix fails this test, modifications can be made to the mix based on engineering judgment and the criteria outlined here.
The strength development in concrete begins after the concrete sets. Strength develops primarily as a function of the w/cm ratio; cement, admixtures, and aggregate characteristics and content; the curing temperature; and moisture state. The strength of the concrete depends on the strength of the cement paste, the strength of the aggregates, and the bond strength of the cement/aggregate interface.
The rate of strength development is a function of the cement properties such as the cement fineness and cement compounds, along with SCM and admixtures used. In general, cements with a higher fineness will tend to hydrate faster and develop a high early strength, although the strength development at later ages may be lower when compared to coarser cements. Furthermore, it has been observed that fine ground cements are associated with durability problems.(5) A similar effect on the strength development occurs when the concrete is cured at high temperatures. A rapid strength increase is observed at early ages, and a less steeper strength increase is noted on the long-term strength.
It is also important to mention the effect of moisture on concrete strength. For concrete pavements, pronounced moisture profiles may occur as a consequence of climatic conditions. Whenever the internal relative humidity in the concrete drops below 80 percent, the strength development is affected significantly.(6)
The primary mode of failure during early-age behavior is tensile stress; therefore, the tensile strength of the concrete is commonly the primary concern when evaluating the behavior of concrete pavements.
Maturity information can be obtained from laboratory tests as outlined in American Society for Testing and Materials (ASTM) C 1074.(50) With this approach, concrete strength is predicted by relating the strength obtained in the laboratory to a particular maturity of the concrete in the field according to its temperature history.
As with strength, the hardening process of concrete contributes to its stiffness or modulus of elasticity. The concrete modulus of elasticity is directly related to the concrete strength, and it also depends on the type of aggregates and its volume in the concrete mix. In general, a higher stiffness of the concrete leads to higher stresses induced to the pavement.
Climatic parameters typically have the greatest variability of all the HIPERPAV II inputs. Actual weather conditions are unpredictable, so caution should be used when selecting these inputs. "Average" values of the climatic input may be satisfactory for most simulations, but the user should also test the effect of extreme conditions. The necessary climatic data for HIPERPAV II is available from the National Weather Service, from the meteorological department of the local media, or from a weather almanac. HIPERPAV II has the added advantage of storing this information in an internal database.
No one has direct control over the environmental conditions at the time of construction, but indirect control is available by selecting the time of day and the season. Five key climatic parameters directly influence the early-age performance of concrete pavements:
Cloud cover is a necessary input to the guidelines. The cloud cover corresponds to the solar radiation and heat that the pavement is subjected to at the time of construction. The cloud cover can be estimated based on the knowledge of regional weather patterns for the specific construction season. For hot weather, sunny is the critical condition, compared to partly cloudy and overcast. On the other hand, overcast conditions may be more critical during cold weather concreting, as this condition may affect the strength gain. Again, these inputs can be pulled from the electronic database in the HIPERPAV II software.
Windspeed also influences early-age pavement behavior. Wind can cause convective cooling of an unprotected or minimally protected pavement. This can contribute to undesirable behavior, such as thermal shock. In addition, high windspeeds remove moisture from the surface of the pavement, increasing the potential for excessive shrinkage. Measures can be taken to reduce the effect of wind by various curing methods, such as blankets or polyethylene sheeting.
In the long term, influential climatic conditions are primarily:
Similar to pavement behavior at early ages, air temperatures will affect the thermal gradient in the slab in the long term, influencing the curling and warping stresses. Expansion and contraction of the concrete is also affected by changes in temperature in the long term. Slab contraction will increase the crack or joint width and will affect the load transfer capacity at those locations. In addition, infiltration of water and incompressibles are more likely to occur at wider cracks or joints.
The effect of precipitation on pavement performance will depend, in large part, on the efficiency of the pavement drainage. Water from rainfall usually infiltrates through cracks or joints, shoulders, and pavement edges. The presence of water in the pavement, combined with the repetition of heavy loads, commonly results in the pumping of fines, degradation of subbase material, and loss of support, thus affecting pavement performance. Prolonged periods of rainfall, even of low intensity, may be more damaging to the pavement than short periods of rainfall with high intensity. Moisture absorption by the soil will be more pronounced under the former condition.
Frost heave in a pavement structure is a result of freezing temperatures combined with presence of water in the pavement, either from precipitation or other sources such as groundwater and movement of water table in liquid or vapor form. Frost heave is the raising of the pavement due to formation of ice crystals in a frost-susceptible subgrade or subbase.(51) Unlike HMA pavements, where damage occurs due to loss of subgrade support during frost melting, for concrete pavements, the capacity to distribute loads over larger areas make this type of pavement less susceptible during the thawing period.(52) However, the differential heave under the pavement slabs during freezing temperatures and presence of moisture at the subgrade increases stresses when combined with the action of traffic loads.
The freeze-thaw action also affects the durability of concrete. During freezing temperatures, water in the cement paste pores tends to expand, generating cumulative damage in the concrete with time. Lower w/cm ratios and air entrainment admixtures generally are used in freezing areas to reduce concrete damage. Low w/cm ratios usually result in fewer pores in the cement paste. Air entrainment in the concrete provides room for the water that is freely available in the cement paste to expand during freezing temperatures.
Factors affecting the moisture or temperature state of the concrete pavement during construction will also affect its early-age behavior. Construction methods for JPCP and CRCP provide some of the most critical parameters that influence the pavement's early-age behavior and long-term performance. They are also the most flexible parameters, because these factors often can be modified onsite. This can, in most cases, prevent early-age damage and ensure a long-lasting pavement. In this section, five construction issues are discussed: curing, time of day of construction, initial PCC mix temperature, sawcutting methods and timing, and initial subbase temperature.
Curing methods and timing usually affect the development of concrete stresses and strength. Curing procedures control the moisture loss in the pavement, affecting drying shrinkage stresses. In addition, moisture loss due to poor curing conditions can decrease concrete strength. Several curing methods are readily available for PCCP construction. Selecting the proper curing method is key to constructing a high-performance concrete pavement. Because of the complexity of the pavement construction process, sound judgment should be used in addition to the general recommendations made here.
The most common curing method is the application of a liquid membrane, which is commonly white. The purpose of this type of curing compound is to minimize shrinkage cracking by minimizing excess moisture loss through evaporation. One of the two common methods of applying this type of compound is using a sprayer bar, which traverses the width of the pour and applies the membrane by sprayer jets evenly spaced across the pavement width. The second method is using a wand or other hand-held device, which is directed by human control. Neither of these application methods is fail-safe. The application rate is usually uneven, subjecting some areas of the pavement to greater moisture loss than others. A liquid membrane is beneficial in minimizing moisture loss, when used correctly. A double or triple application of liquid membrane can lead to an even lower potential for moisture loss-related distresses (e.g., plastic shrinkage cracking).
A second type of curing method is polyethylene sheeting. If used properly, this method is very beneficial in retaining moisture. By applying the sheeting, the moisture is trapped beneath the nearly impervious layer. This minimizes evaporation, thus drying shrinkage. However, if the sheeting is not securely fastened to the surface by edge weighting, wind can enter through the openings and increase the potential for early-age damage. In addition, because the sheeting cannot be placed until the PCC has hardened enough to sustain the disturbance from the sheeting application, a liquid membrane is usually applied first, with the sheeting placed later. Polyethylene sheeting also acts as a thermal insulator. This property can be beneficial if the construction site is subjected to rapid cooling following construction. However, it can be detrimental if used improperly. The insulation, in concert with the excess heat produced by fast-track PCC mixes, can result in too great a curing temperature, causing damage after placement.
Cotton mats or burlap are the third most common curing method used on PCC pavements. The benefits of this method are similar to that of polyethylene sheeting. However, in addition to blocking moisture loss, the cotton mats or burlap are usually wetted, providing free moisture that may counter the negative effects of drying on the surface of the slab. The cotton mats or burlap also serve as thermal insulators, so they are subject to the same benefits and drawbacks described for polyethylene sheeting.
The thinner the pavement, the more critical is the curing of the pavement, such as for overlays or whitetopping. The effects of drying shrinkage are limited to the surface; therefore, the large surface area of bonded concrete overlay (compared to the volume of concrete) subjects it to a greater sensitivity towards moisture loss than full depth paving. This, coupled with the greater heat loss, increases the probability of delamination or other types of early-age distress. Using an insulating curing method such as polyethylene sheeting, cotton mats, or burlap can minimize these distresses in many instances.
Unpredictable climate conditions at the time of construction can create a high level of uncertainty in the pavement's early-age behavior. However, the time of construction can be controlled, which can help offset some climatic uncertainty. By adjusting the time of day of construction, the pavement's temperature buildup due to hydration can be controlled. The concrete's strength gain can be maximized, and the pavement's early-age damage can be minimized. This control is especially critical for fast-track mixes. However, other factors, such as traffic, will often dictate construction schedules. Adjusting the time of construction should still be considered as a relatively inexpensive way to minimize early-age pavement damage. The optimal time of construction for hot and cold weather concreting is discussed below.
For hot weather concreting (ambient temperature greater than 32 °C), paving in the early evening and into the night generally is recommended. Delaying the time of paving to evening takes advantage of nighttime cooling. This offsets the concrete's heat gain during hydration. The subsequent warmup the following day also counters the effects of thermal shock, caused by the loss of hydration heat. This strategy minimizes early-age damage and also means that construction will take place during off-peak traffic periods. The user costs associated with construction will be minimized.
For cold weather concreting (mean daily temperatures less than 4 °C for 3 successive days), early morning paving is recommended. Construction in the early morning takes advantage of daytime solar radiation and heat generation to promote increased concrete strength. Cold weather retards the rate of hydration as well as the concrete's strength gain. For this method of construction, insulating curing methods such as cotton mats and/or polyethylene sheeting should be used in the evening for heat retention. This will minimize early-age damage.
The temperature of the air during construction, the hydration characteristics of the cement, and the subsequent increase or decrease of the air temperature after placement all govern the concrete set. The development of stresses in the concrete is a function of the temperature at set and the subsequent temperature changes. As a general rule, the most critical placement time is that which results in a peak temperature due to hydration of the cement coinciding with the maximum air temperature during the day. When the pavement is placed at higher temperatures, larger temperature drops are expected. Therefore, higher stresses in the concrete develop.
Because concrete temperature control is critical in constructing high performance pavements, the initial mix temperature is important. Several methods have been developed in the past to reduce its temperature by cooling one or more of the mix ingredients. Five of them are discussed below. The influence of cooling each ingredient on the mix temperature is related to the temperature, specific heat and quantity of each material. These techniques can be used during hot weather concreting.
Reducing the temperature of the cement portion of the concrete mix is not commonly done, because a temperature change of 6 °C generally will change the temperature of the concrete mix by only 0.6 °C.
For cold weather paving, it may be necessary to heat the mix. This also can be an issue for thin pavements or overlays, because heat is dissipated faster from them than from newly constructed pavements. Some of the techniques currently used are:
Joints in concrete pavements create vertical weakening planes in the concrete pavement to induce the cracks along their controlled axis, thus facilitating pavement maintenance and crack sealing. To minimize the potential for uncontrolled cracking in a PCCP, proper joint sawing procedures should be established. In general, the time of joint sawing should consider the following limiting criteria:
Ideally, the temperature of the subbase should be as close as possible to the temperature of the concrete when placing the concrete during cold weather concreting. The ground should not be frozen, but could be thawed by steaming, coving with insulation, or spreading a layer of hot sand, gravel, or another material.
Traffic is one of the most important factors influencing pavement performance. In fact, pavements are designed to resist traffic and climatic loads under specified levels of safety and comfort. Primary traffic characteristics include load configuration, traffic volume, traffic classification, traffic distribution, growth rate, and traffic wandering.
Vehicular loads for highway pavements are conformed with varied axle configurations, including single, dual, tandem, and tridem axles. The type of axle typically will depend on the type of vehicle and the load that vehicle is intended to carry. In addition, vehicle axles are designed to resist specific loads according to the pressure at the tires. The stress imparted on the pavement at the surface will depend on the magnitude of the load and wheel contact area. The pressure imparted by the wheel on the pavement is practically equal to the tire pressure. The damage to which a pavement is subjected by a traffic load will depend on the axle configuration and tire pressure. In addition, because the elastic modulus is influenced by the loading rate at which a given material is subjected, the vehicle speed is also a significant factor in determining damage to the pavement.
Because traffic typically consists of varied wheel configurations and loading magnitudes, it is common to group traffic in these terms. In particular, for mechanistic methods of design or analysis, different groups of axle configuration and distribution of magnitude of load per axle configuration (vehicle load spectra) are used more commonly to characterize traffic. Other methods of traffic characterization include fixed traffic or fixed vehicle methods. For fixed traffic methods, the most damaging load anticipated is considered for design. This method typically is used for airport pavement design. On the other hand, the fixed vehicle method involves considering a standard vehicle or axle load (typically 80-kilonewton (kN) equivalent single axle load (ESAL) for highway pavements). For this case, the number of repetitions of that load is considered, and traffic factors are used to convert axles with other configurations and load magnitudes. In more practical terms, traffic commonly is classified according to the type of vehicle such as automobiles, recreational vehicles, buses, trucks, and trailer trucks. This simple classification may provide an estimate of the axle configuration and magnitude of load per vehicle category.
Knowing the amount of traffic loads expected during the design period is fundamental for long-term pavement analysis. The volume of traffic expected is usually provided in terms of annual average daily traffic and in terms of percentage per each vehicle category.
In addition, to predict the traffic expected for a given facility accurately, it is important to accurately estimate the rate of growth of traffic. For a given region, the traffic growth rate typically is affected by land development, construction of new facilities, economic growth, and other factors. A common practice of estimating traffic growth is by using methods similar to estimating the worth of money in the future with simple, exponential, or logistical growth functions.
Highway pavements typically are designed according to the traffic directional distribution (direction with greater percentage of traffic) and lane distribution (lane with greater percentage of traffic) for the case of facilities with more than one lane per direction. In addition, because pavement conditions are different depending on the time of the day (curling and warping) and season (soil support), the distribution of traffic through the day and throughout the year significantly affects pavement performance. Traffic distribution usually depends on the type of facility in question. For example, the traffic through the day for a city street typically concentrates early in the morning and in the afternoon (rush hours), while traffic for rural roads is typically less concentrated, and rather, spread throughout the day.
