Report No.1
RIGID PAVEMENT, DEFINITION, DESIGN PROCEDURE, COMPOSITE MATERIALS
TABLES OF CONTENT
1. Introduction 2. Definition of The Rigid Pavement 3. Principles Of Design 3.1Thickness Design 3.2Joints Design 3.3Mix Design 4. Concrete as a Composite Material
1. INTRODUCTION Concrete rigid pavements have been used for highways, airports, streets, local roads, parking lots, industrial facilities, and other types of infrastructure. When properly designed and built out of durable materials, concrete rigid pavements can provide many decades of service with little or no maintenance. In some cases, however, design or construction errors or poorly selected materials have considerably reduced pavement life. It is therefore important for pavement engineers to understand materials selection, mixture proportioning, design and detailing, drainage, construction techniques, and pavement performance. performance.
2. DEFINITION OF THE RIGID PAVEMENT Rigid pavement is the technical term for any road surface made of concrete. The largest advantages to using concrete pavement are in its durability and ability to hold a shape. There are three basic types of rigid pavement commonly used worldwide. The basic design of rigid pavement is very simple. A surface layer, made up of slabs of Portland cement concrete
(PCC), sits on top of a handful of sub-layers (See Fig 1). The layer directly under the PCC is more flexible than the concrete, but still quite rigid. This layer provides a stable base for the PCC as well as assists in drainage. Some roads have a second sub-layer under the first that is even more flexible, while some simply have the existing soil. The biggest factor in deciding whether this second layer is necessary is the composition of the existing material [1].
Figure (1) Rigid Pavement Section
The way concrete pavement deals with cracking is the main difference between the three styles of pavement. The most common style, joined plain concrete (JPC), is made up of slabs with no steel reinforcement (See Fig 2).
When cracks develop, they should occur in the cracks between slabs, making the road surface easy to repair [2].
Figure (2) JPCP – Jointed Plain Concrete Pavement
Joined reinforced concrete contains a steel mesh that reinforces the structure of the concrete slab (See Fig 3). The concrete slabs used in this style are often much larger than those used in JPC designs. The reinforcement prevents some cracks, allowing the larger slabs to be effective. The cracks, when they appear, still typically occur between slab[2].
Figure (3) JRCP – Jointed Reinforced Concrete Pavement
The third style, continuously reinforced concrete, contains a high quantity of steel reinforcement (See Fig 4). These slabs are not designed to crack at connection points — the slab itself cracks. The steel reinforcement holds cracks together so closely that they do not cause structural problems within the slab [2].
Figure (4) CRCP – Continuously Reinforced Concrete Pavement
There are two main reasons to use rigid pavement, both of which stem from its hardness. Since the surface is harder, it is also more durable over time. This keeps the road in good working order far longer than softer surfaces. The other advantage of concrete roads is in their shaping. Since the surface can withstand a lot of weight without deformation, it is possible to create groves and channels in the road to provide extra traction and move water off the road’s surface [2].
3. PRINCIPLES OF DESIGN
A pavement is built up of several layers, each having a special function. In a flexible system the load is largely distributed by the base, the main function of the surfacing being to provide a wearing surface and to protect the base. The sub-base, although it has to carry a smaller intensity of load, is nevertheless important but is usually composed of material of lower quality.
In a concrete road the slab usually provides the wearing surface as well as spreading or distributing the load. A sub-base is almost always Running surface used under a concrete slab as it serves as a useful working platform from which to construct the slab [3]. Concrete roads are sometimes surfaced with a bituminous carpet, particularly in urban areas (See Fig 5).
Figure (5) Concrete Pavement Layers
The object of pavement design is to determine the type, thickness and treatment of materials that will most economically provide an adequate wearing surface and structure to carry a given frequency and weight of vehicles on given foundation conditions [3]. 3.1 THICKNESS DESIGN
In addition to the normal design factors of thickness of slab, type and thickness of sub-base, strengthening of edges and corners of slabs and use of reinforcement, it is convenient to include the type and spacing of joints under the heading of pavement design of concrete [3]. The thickness of slab required for a given site will depend on:
(a) The properties of the soil, (b) The intensity and weight of traffic, (c) The amount of steel reinforcement present, if any, (d) The thickness of the base.
(a) Properties of the subgrade soil It can be shown by theoretical calculations using the formula due to Westergaard that the bearing capacity of the soil has little effect on the stresses in the concrete when slabs are uniformly supported, i.e. when the soil has a uniform bearing capacity. In practice, however, uniformity of support is rarely obtained due to the variable nature of soils and to changes in moisture content during and after construction. In addition non-elastic deformation of the soil occurs due to traffic. From the practical evidence available and also from the results of experimental roads, it appears that the same thickness of slab is suitable for most types of subgrade.
(b)I ntensity of traffic. Experience has shown that the intensity of traffic has by far the greatest effect on the performance of concrete roads but that light vehicles such as private cars have practically no effect structurally. In design tables, therefore, the number of commercial vehicles using the roads is the only factor taken into account. As it is not practicable or economic to strengthen a concrete slab later in life to take account of increasing traffic loads and intensity, it is particularly important to anticipate the type of traffic which the road will carry throughout its life.
(c) Steel reinforcement. Experimental work has shown that the use of reinforcement does not permit the slab thickness to be reduced, its main effect being on the slab length which can safely be laid without harmful cracking. The AASHO test [4] is a valuable reference in this context. The spacing between free joints in reinforced slabs is calculated from the concept that the amount of steel available at any cross-section must be sufficient to withstand the tensile stresses compounded from the weight of the slab and the friction between the base and underside of the slab when a crack has occurred and when the concrete is contracting. The spacing of joints in reinforced pavements is dependent, therefore, upon the thickness of the slab, the value of the friction coefficient and the cross-sectional area of reinforcement in the slab. Experience has shown that in unreinforced slabs, joints must be placed at a distance not exceeding between 15 and 20 ft if cracks are to be avoided. The use of reinforcement therefore allows joint spacing to be increased. It should be stressed that it is very desirable to avoid cracks in unreinforced slabs. If these occur they are likely to increase progressively in width and to spall with subsequent fairly rapid deterioration under heavy traffic. Cracks in reinforced slabs are not serious because the reinforcement prevents the cracks from opening, thus reducing the chance of water penetration and of sapling. Experience shows that cracks in reinforced roads suffer very gradual deterioration and often remain unchanged in width for many years [3].
(d)The provision of a sub-base. Experimental work has indicated that sub-bases fulfill very little purpose in strengthening a concrete road structure which obtains its load carrying capacity mainly from the
structural rigidity of the slab. In general, experiments have indicated that 1 in. of concrete is equivalent to about 6 in. of sub-base and unless suitable sub-base material can be obtained very cheaply it will usually be economical to increase the thickness of the slab rather than that of the sub-base in order to increase the traffic carrying capacity of the road. In practice, a sub-base is used under a concrete slab mainly for construction purposes that is to protect the subgrade soil and to facilitate the movement of construction traffic being to prevent movement or opening. 3.2 JOINTS DESIGN
Different types of joints are placed in concrete pavements to limit the stresses induced by temperature changes and to facilitate proper bonding of two adjacent sections of pavement when there is a time lapse between their construction (for example, between the end of one day’s work and the beginning of the next) [5]. These joints can be divided into four basic categories: • Expansion joints • Contraction joints • Hinge joints • Construction joints 3.2.1 Expansion Joints
When concrete pavement is subjected to an increase in temperature, it will expand, resulting in an increase in length of the slab. When the temperature
is sufficiently high, the slab may buckle or “ blow up” if it is sufficiently long and if no provision is made to accommodate the increased length. Therefore, expansion joints are usually placed transversely, at regular intervals, to provide adequate space for the slab to expand [5]. 3.2.2 Contraction Joints
When concrete pavement is subjected to a decrease in temperature, the slab will contract if it is free to move. Prevention of this contraction movement will induce tensile stresses in the concrete pavement. Contraction joints therefore are placed transversely at regular intervals across the width of the pavement to release some of the tensile stresses that are so induced. It may be necessary in some cases to install a load-transfer mechanism in the form of a dowel bar when there is doubt about the ability of the interlocking gains to transfer the load [5]. 3.2.3 Hinge Joints
Hinge joints are used mainly to reduce cracking along the center line of highway pavements [5]. 3.2.4 Construction Joints
Construction joints are placed transversely across the pavement width to provide suitable transition between concrete laid at different times. For example, a construction joint is usually placed at the end of a day’s pour to provide suitable bonding with the start of the next day’s pour [5].
3.3 PCC MIX DESIGN
PCC consists of three basic ingredients: aggregate, water and Portland cement. According to the Portland cement Association [8]: “The objective in designing concrete mixtures is to determine the most economical and practical combination of readily available materials to produce a concrete that will satisfy the performance requirements under particular conditions of use.” PCC mix design has evolved chiefly through experience and welldocumented empirical relationships. Normally, the mix design procedure involves two basic steps [8]: 1. Mix proportioning. This step uses the desired PCC properties as inputs then determines the required materials and proportions based on
a
combination
of
empirical
relationships
and
local
experience. There are many different PCC proportioning methods of varying complexity that work reasonably well. 2. Mix testing. Trial mixes are then evaluated and characterized by subjecting them to several laboratory tests.
Although these
characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during construction and under subsequent traffic loading.
Variables
PCC is a complex material formed from some very basic ingredients. When used in pavement, this material has several desired performance characteristics – some of which are in direct conflict with one another. PCC pavements must resist deformation, crack in a controlled manner, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed. In order to meet these demands, mix design can manipulate the following variables [6]: 1. Aggregate.
Items such as type (source), amount, gradation and
size, toughness
and
abrasion
resistance, durability
and
soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree. 2. Portland cement. Items such as type, amount, fineness, soundness, hydration rate and additives can be measured, judged and altered to some degree. 3. Water.
Typically the volume and cleanliness of water are of
concern. Specifically, the volume of water in relation to the volume of Portland cement, called the water-cement ratio, is of primary concern. Usually expressed as a decimal (e.g., 0.35), the water-cement ratio has a major effect on PCC strength and durability. 4. Admixtures. Items added to PCC other than Portland cement, water and aggregate. Admixtures can be added before, during or after mixing and are used to alter basic PCC properties such as air content, water-cement ratio, workability, set time, bonding ability, coloring and strength.
Objectives
By manipulating the mixture variables of aggregate, Portland cement, water and admixtures, mix design seeks to achieve the following qualities in the final PCC product [9]: 1. Strength. PCC should be strong enough to support expected traffic loading. In pavement applications, flexural strength is typically more important than compressive strength (although both are important) since the controlling PCC slab stresses are caused by bending and not compression. In its most basic sense, strength is related to the degree to which the Portland cement has hydrated. This degree of hydration is, in turn, related to one or more of the following:
Water-cement ratio.
The strength of PCC is most directly
related to its capillary porosity. The capillary porosity of a properly compacted PCC is determined by its water-cement ratio [9]. Thus, the water-cement ratio is an easily measurable PCC property that gives a good estimate of capillary porosity and thus, strength. The lower the water-cement ratio, the fewer capillary pores and thus, the higher the strength. Specifications typically include a maximum water-cement ratio as a strength control measure.
Entrained air (air voids). At a constant water-cement ratio, as the amount of entrained air (by volume of the total mixture) increases, the voids-cement ratio (voids = air + water) decreases.
This
generally
results
in
a
strength
reduction. However, air-entrained PCC can have a lower
water-cement ratio than non-air-entrained PCC and still provide adequate workability. Thus, the strength reduction associated with a higher air content can be offset by using a lower watercement ratio. For moderate-strength concrete (as is used in rigid pavements) each percentile of entrained air can reduce the compressive strength by about 2 – 6 percent [8].
Cement properties. Properties of the Portland cement such as fineness and chemical composition can affect strength and the rate of strength gain. Typically, the type of Portland cement is specified in order to control its properties.
2. Controlled shrinkage cracking. Shrinkage cracking should occur in a controlled manner. Although construction techniques such as joints and reinforcing steel help control shrinkage cracking, some mix design elements influence the amount of PCC shrinkage. Chiefly, the amount of moisture and the rate of its use/loss will affect shrinkage and shrinkage cracking. Therefore, factors such as high water-cement ratios and the use of high early strength Portland cement types and admixtures can result in excessive and/or uncontrolled shrinkage cracking. 3. Durability. PCC should not suffer excessive damage due to chemical or physical attacks during its service life. As opposed to HMA durability, which is mainly concerned with aging effects, PCC durability
is
environmental
mainly
concerned
conditions
that
with can
performance. Durability is related to:
specific
potentially
chemical
and
degrade
PCC
Porosity (water-cement ratio ). decreases
it
becomes
more
As the porosity of PCC impermeable.
Permeability
determines a PCC’s susceptibility to any number of durability problems because it controls the rate and entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing [9]. The water-cement ratio is the single most determining factor in a PCC’s porosity. The higher the water-cement ratio, the higher the porosity. In order to limit PCC porosity, many agencies specify a maximum allowable water-cement ratio.
Entrained Air (Air voids). Related to porosity, entrained air is important in controlling the effects of freeze-thaw cycles. Upon freezing, water expands by about 9 percent. Therefore, if the small capillaries within PCC are more than 91 percent filled with water, freezing will cause hydraulic pressures that may rupture the surrounding PCC. Additionally, freezing water will attract other unfrozen water through osmosis [8]. Entrained air voids act as expansion chambers for freezing and migrating water and thus, specifying a minimum entrained air content can minimize freeze-thaw damage.
Chemical environment . Certain chemicals such as sulfates, acids, bases and chloride salts are especially damaging to PCC. Mix design can mitigate their damaging effects through such things as choosing a more resistant cement type.
4. Skid resistance. PCC placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire. In mix design, low skid resistance is generally related to aggregate characteristics such as texture, shape, size and resistance to polish. Smooth, rounded or polish-susceptible aggregates are less skid resistant.
Tests
for particle shape and texture can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics. 5. Workability. PCC must be capable of being placed, compacted and finished with reasonable effort.
The slump test, a relative
measurement of concrete consistency, is the most common method used to quantify workability. Workability is generally related to one or more of the following:
Water content .
Water works as a lubricant between the
particles within PCC. Therefore, low water content reduces this lubrication and makes for a less workable mix. Note that a higher water content is generally good for workability but generally bad for strength and durability, and may cause segregation and bleeding. Where necessary, workability should be improved by redesigning the mix to increase the paste content (water + Portland cement) rather than by simply adding more water or fine material [9].
Aggregate proportion. Large amounts of aggregate in relation to the cement paste will decrease workability. Essentially, if
the aggregate portion is large then the corresponding water and cement portions must be small.
Aggregate texture, shape and size. Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult.
Tests
for particle shape and texture can identify possible workability problems.
Aggregate gradation. Gradations deficient in fines make for less workable mixes.
In general, fine aggregates act as
lubricating “ball bearings” in the mix. Gradation specifications are used to ensure acceptable aggregate gradation.
Aggregate porosity. Highly porous aggregate will absorb a high
amount
lubrication.
of
water
leaving
less
available
for
Thus, mix design usually corrects for the
anticipated amount of absorbed water by the aggregate.
Air content . Air also works as a lubricant between aggregate particles. Therefore, low air content reduces this lubrication and makes for a less workable mix. A volume of air-entrained PCC requires less water than an equal volume of non-airentrained PCC of the same slump and maximum aggregate size [8].
Cement properties. Portland cements with higher amounts of C3S and C3A will hydrate quicker and lose workability faster.
4. CONCRETE AS A COMPOSITE MATERIAL Concrete
is a composite material that consists essentially of a binding
medium, such as a mixture of Portland cement and water, within which are embedded particles or fragments of aggregate, usually a combination of fine and coarse aggregate. Concrete is by far the most versatile and most widely used construction material worldwide. It can be engineered to satisfy a wide range of performance specifications, unlike other building materials, such as natural stone or steel, which generally have to be used as they are. Because the tensile strength of concrete is much lower than its compressive strength, it is typically reinforced with steel bars, in which case it is known as reinforced concrete. A composite material is made up of various constituents. The properties and characteristics of the composite are functions of the constituent materials’ properties as well as the various mix proportions [10].
REFERENCES:-
(1) Skokie, IL, Design of Concrete Pavements for Streets and Roads, American
Concrete
Pavement
Association,
publication
No.
18184.03P, 2006.
(2) http://www.fhwa.dot.gov/engineering/geotech/pubs/05037/01.cfm
(3) D. Raymond Sharp, J. L. Raikes and D. J. Silverle, Concrete in highway engineering, 2011.
(4) Sharp, D. R. And Shacklock, B. W. A British assessment of the AASHO road test with special reference to concrete pavements, Technical Report TRA 369, London, Cement and Concrete Association, February 1963, pp. 17.
(5) Garber, Nicholas J., and Lester A. Hoel. Traffic and highway engineering . Cengage Learning, 2014.
(6) American Association of State Highway and Transportation Officials, AASHTO Guide for Design of Pavement Structures. Washington, D.C
(7) http://www.pavementinteractive.org/article/pcc-mix-designfundamentals/
(8) Portland Cement Association (PCA) (1988) Design of Heavy Industrial Concrete pavements, Information Series IS234.01P , Skokie, IL: Portland Cement Association.
(9) Mindess, S., Young, J.F., and Darwin, D. (2003) Concrete, 2nd Edition, Upper Saddle River, NJ: Pearson Education. (10)
Ortiz, Miguel, and Egor P. Popov. "Plain concrete as a
composite material." Mechanics of materials 1.2 (1982): 139-150.
