HRE 313 Highway and Railroad Engineering PDF

HRE 313 Highway and Railroad Engineering This is a property of PRESIDENT RAMON MAGSAYSAY STATE UNIVERSITY NOT FOR SALE HRE 313 – Highway and Railroad Engineering First Edition, 2021 Copyright. Republic Act 8293 Section 176 provides that “No copyright shall...

HRE 313 Highway and Railroad Engineering This is a property of PRESIDENT RAMON MAGSAYSAY STATE UNIVERSITY NOT FOR SALE HRE 313 – Highway and Railroad Engineering First Edition, 2021 Copyright. Republic Act 8293 Section 176 provides that “No copyright shall subsist in any work of the Government of the Philippines. However, prior approval of the government agency or office wherein the work is created shall be necessary for exploitation of such work for profit. Such agency or office may, among other things, impose as a condition the payment of royalties. Borrowed materials included in this module are owned by their respective copyright holders. Every effort has been exerted to reach and seek permission to use these materials from their respective copyright owners. The University and authors do not claim ownership over them. Module Overview Introduction (A short discussion of the module as to what to expect by the learners, including topics included in this particular learning module as well as the scope and coverage.) Sample: A typical flexible pavement consists of surface, base course, and subbase built over compacted subgrade. Pavement is subjected to repeated loads with variable magnitudes, tire pressures, and configurations. Distresses are developed due to traffic load repetitions, temperature, moisture, aging, construction practice, or combinations. Common distresses include fatigue cracking, rutting, roughness, and thermal cracking. Pavement fails when one or more of the distresses reach an unacceptable level. Rigid pavements are mostly found in major highways and airports. They also serve as heavy- duty industrial floor slabs, port and harbor yard pavements, and heavy-vehicle park or terminal pavements. Rigid highway pavements, like flexible pavements, are designed as all- weather, long-lasting structures to serve modern day high-speed traffic. Table of Contents Chapter/Lesson 1: Structural Design of Pavements Highway and Railroad Engineering Chapter 1 Structural Design of Pavements Chapter 1 Structural Design of Pavements Introduction: Pavements are among the costliest items associated with highway construction and maintenance, and are largely responsible for making the U.S. highway system the most expensive public works project undertaken by any society. Because the pavement and associated shoulder structures are the most expensive items to construct and maintain, it is important for highway engineers to have a basic understanding of pavement design principles. Fundamentally, a paved surface performs two basic functions. First, it helps guide drivers by giving them a visual perspective of the horizontal and vertical alignment of the traveled path – thus giving drivers information relating to the driving task and the steering control of the vehicle. The second function of pavement is to support vehicle loads, and this second function is the focus of this module 1. PAVEMENT TYPES In general, there are two types of pavement structures: flexible pavements and rigid pavements. There are, however, many variations of these pavement types. Composite pavements (which are made of both rigid and flexible pavement layers), continuously reinforced pavements, and post-tensioned pavements are other types, which usually require specialized designs and are not covered in this chapter. As with any structure, the underlying soil must ultimately carry the load that is placed on it. A pavement’s function is to distribute the traffic load stresses to the soil (subgrade) at a magnitude that will not shear or distort the soil. Typical soil-bearing capacities can be less than 50 lb/in2 and in some cases as low as 2 to 3 lb/in2. When soil is saturated with water, the bearing capacity can be very low, and in these cases, it is very important for pavement to distribute tire loads to the soil in such a way as to prevent failure of the pavement structure. A typical automobile weighs approximately 3500 lb, with tire pressures around 35 lb/in2. These loads are small compared with a typical tractor– semi-trailer truck, which can weigh up to 80,000 lb—the legal limit, in many states, on five axles with tire pressures of 100 lb/in2 or higher. Truck loads such as these represent the standard type of loading used in pavement design. In this module, attention is directed toward an accepted procedure that can be used to design pavement structures for high– traffic-volume highway facilities subjected to heavy truck traffic. 1.1 FLEXIBLE PAVEMENTS It is constructed with asphaltic cement and aggregates and usually consists of several layers, as shown in Fig. 4.1. The lower layer is called the subgrade (the soil itself). The upper 6 to 8 inches of the subgrade is usually scarified and blended to provide a uniform material before it is compacted to maximum density. The next layer is the subbase, which usually consists of crushed aggregate (rock). This material has better engineering properties (higher modulus values) than the subgrade material in terms of its bearing capacity. The next layer is the base layer and is also often made of crushed aggregates (of a higher strength than those used in the subbase), which are either unstabilized or stabilized with a cementing material. The cementing material can be portland cement, lime fly ash, or asphaltic cement. The top layer of a flexible pavement is referred to as the wearing surface. It is usually made of asphaltic concrete, which is a mixture of asphalt cement and aggregates. The purpose of the wearing layer is to protect the base layer from wheel abrasion and to waterproof the entire pavement structure. It also provides a skid resistant surface that is important for safe vehicle stops. Typical thicknesses of the individual layers are shown in Fig. 4.1. These thicknesses vary with the type of axle loading, available materials, and expected pavement design life, which is the number of years the pavement is expected to provide adequate service before it must undergo major rehabilitation.  1.2 RIGID PAVEMENTS A rigid pavement is constructed with portland cement concrete (PCC) and aggregates, as shown in Fig. 4.2. As with flexible pavements, the subgrade (the lower layer) is often scarified, blended, and compacted to maximum density. In rigid pavements, the base layer (see Fig. 4.2) is optional, depending on the engineering properties of the subgrade. If the subgrade soil is poor and erodible, then it is advisable to use a base layer. However, if the soil has good engineering properties and drains well, a base layer need not be used. The top layer is the portland cement concrete slab. Slab length varies from a spacing of 10 to 13 ft to a spacing of 40 ft or more. Transverse contraction joints are built into the pavement to control cracking due to shrinkage of the concrete during the curing process. Load transfer devices, such as dowel bars, are placed in the joints to minimize deflections and reduce stresses near the edges of the slabs. Slab thicknesses for PCC highway pavements usually vary from 8 to 12 inches, as shown in Fig. 4.2. 2. PAVEMENT SYSTEM DESIGN FOR FLEXIBLE PAVEMENTS The primary function of the pavement structure is to reduce and distribute the surface stresses (contact tire pressure) to an acceptable level at the subgrade (to a level that prevents permanent deformation). A flexible pavement reduces the stresses by distributing the traffic wheel loads over greater and greater areas, through the individual layers, until the stress at the subgrade is at an acceptably low level. The traffic loads are transmitted to the subgrade by aggregate-to-aggregate particle contact. Confining pressures (lateral forces due to material weight) in the subbase and base layers increase the bearing strength of these materials. A cone of distributed loads reduces and spreads the stresses to the subgrade, as shown in Fig. 4.3. 3. FLEXIBLE PAVEMENT LAYERS AND MATERIALS A typical flexible (or asphalt) pavement consists of surface, base course, and subbase built over compacted subgrade (natural soil) as shown in Figure. In some cases, the subbase layer is not used, whereas in a small number of cases both base and subbase are omitted. The surface layer is made of hot-mix asphalt (HMA) (also called asphalt  concrete). The material for the base course is typically unstabilized aggregates. The aggregate base could also be stabilized with asphalt, portland cement, or another stabilizing agent. The subbase is mostly a local aggregate material. Also, the top of the subgrade is sometimes stabilized with either cement or lime. 4. UNIQUE PROPERTIES OF FLEXIBLE PAVEMENTS a) Fast deterioration with time Each traffic load application contributes to some extent to pavement distresses. Different types of distress could happen and accumulate over the years such as rutting, fatigue cracking, material disintegration, roughness and bleeding. When one or more of these distresses reach a certain unacceptable level, the pavement is considered as failed. The typical life of a flexible pavement varies from case to case, with an average value of 10 to 15 years. A good method of pavement design should include the designed life, or how long the pavement is expected to last before failure. b) Repeated loads When a traffic wheel moves on the pavement surface it creates a stress pulse. This stress pulse creates a dynamic pavement response, which is harder to analyze as compared to static response. Dynamic waves propagate throughout the pavement layers and subgrade, and involve reflections and refractions at the layer interfaces. c) Variable load configuration Different vehicular axle configurations are available with a different number of wheels at the end of each axle. Axles can be single, tandem, tridem, or multiple, while wheels can be either single or dual. Passenger cars have single axles and single wheels. Different axle and wheel configurations result in stress interactions within the pavement structure, which in turn influence pavement performance. d) Variable load magnitude Traffic loads vary from light to heavy for passenger cars and loaded trucks respectively. Since pavement materials have non-linear response, doubling the load magnitude does not result in doubling the stress or strain. More importantly, doubling the load magnitude does not result in doubling the rate of pavement deterioration. In fact, increasing the load magnitude exponentially increases the rate of pavement deterioration. e) Variable type pressure Trucks have much higher tyre pressures than passenger cars. Typical tyre pressures of passenger cars are in the order of 30 – 35 psi, while trucks have tyre pressures of 100 – 115 psi. Higher tyre pressures result in higher contact pressures at the surface of the pavement and, in turn, faster deterioration of the surface layer. Truck tyre pressures have been increasing over the years, challenging pavement engineers to improve the quality of the HMA material in order to reduce premature pavement failure. f) Traffic growth Pavement is designed to carry future traffic, which usually increases over the years. Predicting future traffic rowth is not always accurate. This inaccuracy in predicting future traffic affects the accuracy of predicting pavement performance and consequently pavement designed life. g) Change of material properties with environmental conditions Environmental conditions have large effect on the properties of pavement materials. For example, HMA gets softer at high temperatures resulting in rutting, and harder at low temperatures resulting in thermal cracking. Also, rain and freeze – thaw cycles weaken the HMA materials and reduce the load carrying capacity of base, subbase and subgrade. In addition, HMA ages with time resulting in increasing its stiffness and its susceptibility to cracking.  h) Change of subgrade properties with distance Since pavement is built to cover a large distance, the same road might be built over different types of subgrade materials with different properties. Moreover, the road could be built over cut or fill subgrade sections having different material properties. The change of subgrade properties requires different thicknesses of pavement layers in order to support the same traffic load and produce the same performance. i) Channelized traffic load Traffic load is applied in the wheel path. This channelization of traffic load results in faster deterioration in the wheel path as compared to the area between wheel paths. The design process should consider the proper stress and strain distributions within the pavement structure to determine critical locations and possible deteriorations. j) Multi-layer system The pavement structure consists of several layers built over the subgrade. These layers have different materials with different properties. The distribution of stresses and strains within the multi-layer pavement system depends on the thickness and material properties of these layers. k) Unconventional failure Failure of typical civil engineering structures is defined as break or fracture. This usually happens when the applied stress exceeds the maximum allowable value, or the strength of the material. Unlike other civil engineering structures, the applied stresses in pavement are usually much smaller than the strength of the material. Therefore, one load application does not fail the pavement, but causes an infinitesimal amount of deterioration. This deterioration gradually increases until it reaches an unacceptable level, or failure. Thus, pavement failure does not happen because of a collapse of the pavement structure, but when one or more of the distresses reach an unacceptable level. 5. TRADITIONAL AASHTO FLEXIBLE-PAVEMENT DESIGN PROCEDURE A traditional and widely accepted flexible-pavement design procedure is presented in the AASHTO Guide for Design of Pavement Structures, which is published by the American Association of State Highway and Transportation Officials (AASHTO). A pavement can be subjected to a number of detrimental effects, including fatigue failures (cracking), which are the result of repeated loading caused by traffic passing over the pavement. The pavement is also placed in an uncontrolled environment that produces temperature extremes and moisture variations. The combination of the environment, traffic loads, material variations, and construction variations requires a comparatively complex set of design procedures to incorporate all of the variables. The AASHTO pavement design procedure meets most of the demands placed on a flexible pavement design procedure. It considers environment, load, and materials in a methodology that is relatively easy to use. 5.1. Serviceability Concept Prior to the AASHO Road Test, there was no real consensus on the definition of pavement failure. In the eyes of an engineer, pavement failure occurred whenever cracking, rutting, or other surface distresses became visible. In contrast, the motoring public usually associated pavement failure with poor ride quality. The performance curve is the historical record of the performance of the pavement. Pavement performance, at any point in time, is known as the present serviceability index, or PSI. At any time, the present serviceability index of a pavement can be measured. This is usually done by a panel of raters who drive over the pavement section and rate the pavement performance on a scale of 1 to 5, with 5 being the smoothest ride. The accumulation of traffic loads causes the pavement to deteriorate, and, as expected, the serviceability rating drops. At some point, a terminal serviceability index (TSI) is reached and the pavement is in need of rehabilitation or replacement. It has been found that new pavements usually have an initial PSI rating of approximately 4.2 to 4.5. The point at w/c pavements are considered to have failed varies by type of highway. Highway facilities such as interstate highways or principal arterials usually have TSIs of 2.5 or 3.0, whereas local roads can have TSIs of 2.0. 5.2. Flexible-Pavement Design Equation The basic equation for flexible-pavement design given in the 1993 AASHTO design guide permits engineers to determine a structural number necessary to carry a designated traffic loading. The AASHTO equation is W18 Automobiles and truck traffic provide a wide range of vehicle axle types and axle loads. If one were to attempt to account for the variety of traffic loadings encountered on a pavement, this input variable would require a significant amount of data collection and design evaluation. Instead, the problem of handling mixed traffic loading is solved with the adoption of a standard 18- thousand-pound–equivalent single-axle load or (with 1 kip = 1000 lb) an 18- kip–equivalent single-axle load: (ESAL). The idea is to determine the impact of any axle load on the pavement in terms of the equivalent amount of pavement impact that an 18-kip single-axle load would have. For example, if a 44-kip tandem-axle (double-axle) load has 2.88 times the impact on pavement structure as an 18-kip single-axle load, 2.88 would be the W18 value assigned to this tandem-axle load. The AASHO Road Test also found that the 18- kip– equivalent axle load is a function of the terminal serviceability index of the pavement structure. The axle-load equivalency factors for flexible pavement design, with a TSI of 2.5, are presented in Tables 4.1 (for single axles), 4.2 (for tandem axles), and 4.3 (for triple axles). ZR Represents the probability that serviceability will be maintained at adequate levels from a user’s point of view throughout the design life of the facility. This factor estimates the likelihood that the pavement will perform at or above the TSI level during the design period, and takes into account the inherent uncertainty in design. Equation 4.1 uses the z-statistic, which is obtained from the cumulative probabilities of the standard normal distribution (a normal distribution with mean equal to 0 and variance equal to 1). The z-statistics corresponding to various probability levels are given in Table 4.4. In the flexible- pavementdesign nomograph (Fig. 4.5), the probabilities (in percent) are used directly (instead of ZR as in the case of Eq. 4.1), and these percent probabilities are denoted R, the reliability (see Table 4.4). Highways such as interstates and major arterials, which are costly to reconstruct (have their pavements rehabilitated) because of resulting traffic delay and disruption, require a high reliability level, whereas local roads, which will have lower impacts on users in the event of pavement rehabilitation, do not. Typical reliability values for interstate highways are 90% or higher, whereas local roads can have a reliability as low as 50%. So The overall standard deviation, So, takes into account the designers’ inability to accurately estimate the variation in future 18-kip–equivalent axle loads, and the statistical error in the equations resulting from variability in materials and construction practices. Typical values of So are on the order of 0.30 to 0.50. SN The structural number, SN, represents the overall structural requirement needed to sustain the design’s traffic loadings. ∆PSI The amount of serviceability loss over the life of the pavement, ∆PSI, is determined during the pavement design process. The engineer must decide on the final PSI level for a particular pavement. Loss of serviceability is caused by pavement roughness, cracking, patching, and rutting. As pavement distress increases, serviceability decreases. If the design is for a pavement with heavy traffic loads, such as an interstate highway, then the serviceability loss may only be 1.2 (an initial PSI of 4.2 and a TSI of 3.0), whereas a lowvolume road can be allowed to deteriorate further, with a possible total serviceability loss of 2.7 or more. MR The soil resilient modulus, MR, is used to reflect the engineering properties of the subgrade (the soil). Each time a vehicle passes over pavement, stresses are developed in the subgrade. After the load passes, the subgrade soil relaxes and the stress is relieved. The resilient modulus test is used to determine the properties of the soil under this repeated load. The resilient modulus can be determined by AASHTO test method T274. Measurement of the resilient modulus is not performed by all transportation agencies; therefore, a relationship between MR and the California bearing ratio (CBR) has been determined. The CBR has been widely used to determine the supporting characteristics of soils since the mid-1930s, and a significant amount of historical information is available. The CBR is the ratio of the load- bearing capacity of the soil to the load-bearing capacity of a high-quality aggregate, multiplied by 100. The relationship, used to provide a very basic approximation of MR (in lb/in2) from a known CBR, is MR = 1500 x CBR The coefficient of 1500 in Eq. 4.2 is used for CBR values less than 10. Caution must be exercised in applying this equation to higher CBRs because the coefficient (the value 1500 shown in Eq. 4.2) has a range of 750 to 3000.  5.3. Structural Number The objective of Eq. 4.1 and the nomograph in Fig. 4.5 is to determine a required structural number for given axle loadings, reliability, overall standard deviation, change in PSI, and soil resilient modulus. As previously mentioned, there are many pavement material combinations and thicknesses that will provide satisfactory pavement service life. The following equation can be used to relate individual material types and thicknesses to the structural number: Values for the structural-layer coefficients for various types of material are presented in Table 4.5. Drainage coefficients are used to modify the thickness of the lower pavement layers (base and subbase) to take into account a material’s drainage characteristics. A value of 1.0 for a drainage coefficient represents a material with good drainage characteristics (a sandy material). A soil such as clay does not drain very well and, consequently, will have a lower drainage coefficient (less than 1.0) than a sandy material. The reader is referred to [AASHTO 1993] for further information on drainage coefficients. Because there are many combinations of structural-layer coefficients and thicknesses that solve Eq. 4.3, some guidelines are used to narrow the number of solutions. Experience has shown that wearing layers are typically 2 to 4 inches thick, whereas subbases and bases range from 4 to 10 inches thick. Knowing which of the materials is the costliest per inch of depth will assist in the determination of an initial layer thickness. 6. PAVEMENT SYSTEM DESIGN FOR RIGID PAVEMENT Rigid pavements distribute wheel loads by the beam action of the portland cement concrete (PCC) slab, which is made of a material that has a high modulus of elasticity,  on the order of 4 to 5 million lb/in2. This beam action (see Fig. 4.6) distributes the wheel loads over a large area of the pavement, thus reducing the high stresses experienced at the surface of the pavement to a level that is acceptable to the subgrade soil. 7. TRADITIONAL AASHTO RIGID-PAVEMENT DESIGN PROCEDURE The design procedure for rigid pavements presented in the AASHTO design guide is also based on the field results of the AASHO Road Test. The AASHTO design procedure is applicable to jointed plain concrete pavements, jointed reinforced concrete pavements, and continuously reinforced concrete pavements. Jointed plain concrete pavements (JPCP) do not have slab-reinforcing material and can have doweled joints (steel bars to transfer loads between slabs as shown in Fig. 4.2) or undoweled joints. The traverse joints between slabs are spaced at about 10 to 13 ft. Jointed reinforced concrete pavements (JRCP) have steel reinforced slabs with joints that are 40 ft or more apart. Finally, continuously reinforced concrete pavements (CRCP) do not have traverse expansion/contraction joints, necessitating the use of extensive steel-bar reinforcement in the slab. The idea with both jointed-reinforced and continuously-reinforced pavements is to permit slab cracking but to provide sufficient slab reinforcement to hold the cracks tightly together to ensure load transfer. It is important to note that faulting, which is a distress characterized by different slab elevations, was not a failure consideration in the AASHO Road Test, and thus the design of non-doweled joints must be checked with a procedure other than that presented here (more information on faulting is provided in Section 4.7.5). The design procedure for rigid pavements is based on a selected reduction in serviceability and is similar to the procedure for flexible pavements. However, instead of measuring pavement strength by using a structural number, the thickness of the PCC slab is the measure of strength. The regression equation that is used (in U.S. Customary units) to determine the thickness of a rigid-pavement PCC slab is: W18 The 18-kip–equivalent single-axle load is the same concept as discussed for the flexible pavement design procedure. However, instead of being a function of the structural number, this value is a function of slab thickness. The axle load equivalency factors used in rigid- pavement design are presented in Tables 4.6 (for single axles), 4.7 (for tandem axles), and 4.8 (for triple axles). ZR As in flexible-pavement design, the reliability, ZR, is defined as the probability that serviceability will be maintained at adequate levels from a user’s point of view throughout the design life of the facility (the PSI will stay above the TSI). In the rigid-pavement design nomograph (Figs. 4.7 and 4.8), the probabilities (in percent) are used directly (instead of ZR as in Eq. 4.4), and these percent probabilities are denoted R (see Table 4.4, which still applies). So As in flexible-pavement design, the overall standard deviation, So, takes into account designers’ inability to accurately estimate future 18-kip–equivalent axle loads and the statistical error in the equations resulting from variability in materials and construction practices. TSI The pavement’s terminal serviceability index, TSI, is the point at which the pavement can no longer perform in a serviceable manner, as discussed previously for the flexible-pavement design procedure. ∆PSI The amount of serviceability loss, ∆PSI, over the life of the pavement is the difference between the initial PSI and the TSI, as discussed for the flexible pavement design procedure. 𝑆'𝐶 The concrete modulus of rupture, 𝑆'𝐶c, is a measure of the tensile strength of the concrete and is determined by loading a beam specimen, at the third points, to failure. The test method is ASTM C78, Flexural Strength of Concrete. Because concrete gains strength with age, the average 28-day strength is used for design purposes. Typical values are 500 to 1200 lb/in2. Cd The drainage coefficient, Cd, is slightly different from the value used in flexible-pavement design. In rigid-pavement design, it accounts for the drainage characteristics of the subgrade. A value of 1.0 for the drainage coefficient represents a material with good drainage characteristics (such as a sandy material). Soils with less-than-ideal drainage characteristics will have drainage coefficients less than 1.0. J The load transfer coefficient, J, is a factor that is used to account for the ability of pavement to transfer a load from one PCC slab to another across the slab joints. Many rigid pavements have dowel bars across the joints to transfer loads between slabs. Pavements with dowel bars at the joints are typically designed with a J value of 3.2. Ec The concrete modulus of elasticity, Ec, is derived from the stress-strain curve as taken in the elastic region. The modulus of elasticity is also known as Young’s modulus. Typical values of Ec for portland cement concrete are between 3 and 7 million lb/in2. k The modulus of subgrade reaction, k, depends upon several different factors, including the moisture content and density of the soil. It should be noted that most highway agencies do not perform testing to measure the k value of the soil. At best, the agency will have a CBR value for the subgrade. Typical values for k range from 100 to 800 lb/in3. Table 4.9 indicates the relationship between CBR and k values.  The final point to be covered with regard to pavement design relates to the case where there are multiple lanes of a highway (such as an interstate) in one direction. Because traffic tends to be distributed among the lanes, in some instances the pavement can be designed using a fraction of the total directional W18. However, because traffic tends to concentrate in the right lane (particularly heavy vehicles), this fraction is not as simple as dividing W18 by the number of lanes. In equation form, AASHTO-recommended values for PDL are given in Table 4.10. As an example, suppose the computed directional W18 is an 18-kip ESAL of 10,000,000 and there are three lanes in the direction of travel. If the highway is conservatively designed, Table 4.10 shows that 80% of the axle loads can be assumed to be in the design lane (PDL = 0.8). So, the design W18 would be 8,000,000 (0.8 x 10,000,000), and this value would be used in the equations and nomographs. This design procedure applies to both flexible and rigid pavements. 8. MEASURING PAVEMENT QUALITY AND PERFORMANCE The design procedure for pavements originally focused on the pavement serviceability index (PSI) as a measure of pavement quality. However, the pavement serviceability index is based on the opinions of a panel of experts (as discussed in Section 4.4.1), which can introduce some variability into their determination. As a result, efforts have been undertaken to develop quantitative measures of pavement condition that provide additional insights into pavement quality and performance and that correlate with the traditional pavement serviceability index. Some factors that are regularly measured by highway pavement agencies now include the International Roughness Index, friction measurements, and rut depth. 8.1. International Roughness Index The International Roughness Index (IRI) has become the most popular measure for evaluating the condition of pavements. The IRI is determined by measuring vertical movements in a standardized vehicle’s suspension per unit length of roadway. Units of IRI are reported in inches per mile (in/mi). The higher the value of the IRI, the rougher the road. 8.2. Friction Measurements Another important measurement of pavement performance is the surface friction. This is critical because low friction values can increase stopping distances and the probability of accidents. Given the variability of pavement surfaces, weather conditions, and tire characteristics, determining pavement friction over the range of possible values is not an easy task. To estimate friction, a standardized test is conducted under wet conditions using either a treaded or smooth tire. Although other speeds are sometimes used, the standard test is generally conducted at 40 mi/h using a friction-testing trailer in which the wheel is locked on the wetted road surface, and the torque developed from this wheel locking is used to measure a friction number. The friction number resulting from this test gives an approximation of the coefficient of road adhesion under wet conditions (as shown in Table 2.4) and is multiplied by 100 to produce a value between 0 and 100. The friction number with a treaded tire (FNt) attempts to measure pavement microtextured, which is a function of the aggregate quality and composition. The friction number with a smooth tire (FNs) provides a measure of pavement macrotexture, which is critical in providing a water drainage escape path between the pavement and tire. A number of factors influence the friction number, such as changes in traffic volumes or traffic composition, surface age (friction has been found to increase quickly after construction, then as time passes, to level off and eventually decline), seasonal changes (in northern states, the friction number tends to be highest in the spring and lowest in the fall), and speed (the measured value tends to decrease as the test speed increases) Also, the friction number measured with the treaded tire tends to be greater than that measured with the smooth tire (usually by a value of about 20), but the difference decreases as the surface texture becomes rougher. In terms of safety, the amount of friction needed to minimize safety-related problems depends on prevailing traffic and geometric conditions. Guidelines used by some states suggest that values of FNt < 30 or FNs < 15 indicate that poor friction may be contributing to wet-weather accidents. Other state agencies have simply put in place guidelines for minimum friction requirements. For example, in Indiana, the minimum friction value is based on the smooth tire test at 40 mi/h, and a pavement with FNs < 20 is considered in need of surfacing work to improve friction (generally resurfacing). 8.3. Rut Depth Rut depth, which is a measure of pavement surface deformation in the wheel paths, can affect roadway safety because the ruts accumulate water and increase the possibility of vehicle hydroplaning (which results in the tire skimming over a film of water, greatly reducing braking and steering effectiveness). Because of its potential impact on vehicle control, rut depths are regularly measured on many highways to determine if pavement rutting has reached critical values that would require resurfacing or other pavement treatments. The critical values of rut depth can vary from one highway agency to the next. Usually, rut depths are considered unacceptably high when their values reach between 0.5–1.0 inches, indicating that corrective action is warranted. 8.4. Cracking For flexible pavements, four types of cracking are usually monitored:  Longitudinal-fatigue cracking is a surface-down cracking that occurs due to material fatigue in the wheel path. Such cracking can accelerate over time and require significant repairs to protect against water penetration into the flexible pavement structure.  Transverse cracking is generally the result of low temperatures that cause fractures across the traffic lanes (resulting in an increase in pavement roughness).  Alligator-fatigue cracking is a consequence of material fatigue in the wheel path, generally starting from the bottom of the asphalt layer. Such material fatigue creates a patch of connected cracks that resembles the skin of an alligator (as with other types of cracks, these can accelerate quickly over time and generate the need for maintenance to protect the integrity of the pavement structure).  Reflection cracking occurs when hot-mix asphalt (HMA) overlays are placed over exiting pavement structures that had alligator-fatigue cracking, or  other indications of pavement distress, and these old distresses manifest themselves in new distresses in the overlay. This results in surface cracking that increases surface roughness and the need for maintenance to protect water intrusion into the pavement structure. For rigid pavements, transverse cracking is a common measure of pavement distress. Such cracking can be the result of slab fatigue and can be initiated either at the surface or base of the slab. The spacing and width of transverse cracks, and the potential impact of severe cracking on the structural integrity of the pavement, are critical measures of rigid-pavement distress. 8.5. Faulting For traditional JPCP (Jointed Plain Concrete Pavements) rigid pavements, joint faulting (characterized by different slab elevations) is a critical measure of pavement distress. Faulting is an indicator of erosion or fatigue of the layers beneath the slab and reflects a failure of the loadtransfer ability of the pavement between adjacent slabs. Faulting is associated with increased roughness and will be reflected in International Roughness Index measurements. 8.6. Punchouts For Continuously Reinforced Concrete Pavements (CRCP) rigid pavements (those built without expansion/contraction joints), fatigue damage at the top of the slab is often measured by punchouts, which occur when the close spacing of transverse cracks cause in high tensile stresses that result in portions of the slab being broken into pieces. Punchouts are associated with increased roughness and are reflected in International Roughness Index measurements. References/Additional Resources/Readings (list down all references/additional resources/readings used; you may also provide links)  This includes all third-party materials or sources in developing the material. It shall follow the American Psychological Association (APA) Manual of Style 6th or 7th Edition. Mannering Fred, Washburn Scott, Kilaresky Walter. (2004). Principles of Highway Engineering & Traffic Analysis. John Wiley & Sons, Inc. Hwa, T. F. (2006). The Handbook of Highway Engineering. Taylor & Francis Group. Assignment Direction: Watch the given video link and then answer the question(s) below. Submit your file in PDF format. Use Font Style: Times New Roman, Font Size: 12: https://www.youtube.com/watch?v=QLCw9coHX1s 1. Explain briefly the difference between flexible and rigid pavements. Learner’s Feedback Form Name of Student: ___________________________________________________ Program : ___________________________________________________ Year Level : ____________________ Section: _________ Faculty : ___________________________________________________ Schedule : ___________________________________________________ Learning Module : Number: _________ Title : ______________________ How do you feel about the topic or concept presented? □ I completely get it. □ I’m struggling. □ I’ve almost got it. □ I’m lost. In what particular portion of this learning packet, you feel that you are struggling or lost? ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ Did you raise your concern to you instructor? □ Yes □ No If Yes, what did he/she do to help you? ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ If No, state your reason? ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ To further improve this learning packet, what part do you think should be enhanced? ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ How do you want it to be enhanced? ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ NOTE: This is an essential part of course module. This must be submitted to the subject teacher (within the 1st week of the class).