| APPLICATIONS
Designing Concrete Pavements for
Streets and Local Roads
by Ward Malisch
A new American Concrete Institute
report covers the key factors to consider when designing concrete
pavements for low volumes of traffic.
Concrete pavements built to receive low volumes of traffic will not
have the same design requirements as heavily traveled highways. But that
doesn’t mean attention to quality isn’t of equal importance. The goal
in the design and construction of any concrete pavement is the same: to
achieve the desired life span and level of performance using the most
appropriate methods and materials.
That’s the basic premise behind a new American Concrete Institute
report, Guide for Design of Jointed Concrete Pavements for Streets and
Local Roads (ACI 325.12R-02). Prepared by ACI Committee 325, Concrete
Pavements, the guide details procedures for designing concrete pavement
systems specifically for low-traffic volumes. It also emphasizes how these
procedures differ from those used for high-traffic-volume pavements, such
as highways and airport runways. Important topics covered include subgrade
support, drainage, properties of concrete paving mixtures, pavement
thickness design, jointing details, and requirements for distributed
reinforcing steel and load-transfer devices.
Dan Zollinger, chairman of the ACI 325 subcommittee that prepared the
guide, says that the document places strong emphasis on the proper spacing
and layout of joints for city streets and local roads. “The subcommittee
felt there was a great need to publish guidelines that would help
engineers improve the jointing scheme. Jointing is a big factor in the
ultimate performance of these pavements.” The subcommittee also
developed new recommendations, based on research in South Africa, for
subgrade and subbase support of low-volume roads, another important
contributor to satisfactory pavement performance.
Although ACI 325.12R is targeted to the design community, particularly
municipal engineers, it also serves as a useful reference for contractors
and inspectors.
Low-volume pavements
The term low volume refers to pavements subject to either heavy loads
but few vehicles or light loads and many vehicles. Concrete pavements for
city streets and local roads in residential areas and business districts,
and in rural areas to provide farm-to-market access, generally fit into
this category. Traffic volumes will vary depending on the street
classification (see table), but on average fewer than 100 trucks per day,
one way, travel these roads. Therefore, lower-traffic-volume pavements
usually aren’t subjected to the same load stresses or pumping action
associated with heavily loaded pavements. And in many cases, curbs tied to
the pavement edge or placed integrally with the pavement act to carry part
of the load, helping to reduce critical stresses and deflections at slab
edges.
Designing a concrete pavement system for low volumes of traffic
involves more than simply selecting the appropriate pavement thickness.
The design also must consider other variables likely to affect pavement
performance, such as jointing and construction practices and local climate
and soil conditions. Although not directly addressed in the guide,
aesthetics are an important consideration as well, since city streets and
local roads often must be integrated into the landscape and architecture
of the immediate neighborhood or business district. Designs for city
streets also must account for the presence of utilities, sewers, manholes,
drainage inlets, traffic islands, and lighting standards.
Intersections are major features distinguishing local-street pavements
from highway pavements. Because traffic moves across slabs at
intersections in more than one direction, these slabs may develop more
than a single critical fatigue location. Careful consideration must be
given to the design of the jointing system and slab layout at
intersections.
Achieving uniform support
ACI 325.12R stresses that uniform support conditions are essential to
the satisfactory performance of low-volume roads. For a subgrade to
provide reasonably uniform support, it’s necessary for design engineers
to control the following four major causes of nonuniformity:
1. Variable soil conditions and densities.
2. Expansive soils.
3. Differential frost heave.
4. Pumping (the forced displacement of fine subgrade soils and water
from slab joints, cracks, and pavement edges).
The guide explains how to minimize or eliminate the effects of these
factors through adequate design and construction of the subgrade soils,
moisture control during compaction, and the use of positive surface
drainage.
Unfortunately, it’s not always possible to build concrete roadways on
coarse-grained soils, which generally provide the best subgrade
conditions. Sometimes engineers must cope with less-desirable soils that
are subject to frost action and soil expansion. An appendix to ACI 325.12R
discusses several methods of controlling expansive soils and frost heave
effectively and economically. It also contains a table showing the general
properties of various soil types, as classified by ASTM D 2487.
With adequate subgrade preparation and appropriate consideration for
surface and subgrade drainage, concrete pavements for local streets and
roads usually can be built directly on the subgrade, without the need for
a subbase. But ACI 325.12R cites conditions that may warrant the use of a
subbase, such as to control erosion of subgrade materials, to provide a
more stable working platform during construction, to provide drainage over
subgrades that don’t drain well, and to improve joint performance under
repetitive loads. The report includes a table giving minimum recommended
subbase thicknesses for poorly drained soils.
Determining pavement thickness
For concrete pavements to perform as intended, slab thickness must be
adequate to carry the anticipated load distributions. Otherwise, cracking
and premature loss of serviceability can result. Even small changes in
concrete thickness can have a considerable impact on pavement fatigue
life. For this reason, tolerances on pavement thickness are important,
especially for thinner pavements, where small reductions in thickness
represent a significant percentage of the thickness.
ACI 325.12R presents procedures for thickness design of low-volume
pavements based on principles developed by the Portland Cement
Association. To aid in thickness determination, tables are provided giving
minimum suggested thicknesses for the design of low-volume roads (both
with and without integral or tied curb and gutter) as a function of
subgrade support and concrete flexural strength (third-point loading).
Determining design thickness using these tables requires some knowledge
of the range and distribution of traffic loads the pavement is expected to
carry. The report recommends that engineers obtain information regarding
the types of trucks that will use the pavement, the number of each truck
type, truck loads, and the daily volume anticipated over the pavement’s
design life. Just a few very heavy axle loads, such as those produced by
garbage trucks, concrete trucks, and construction vehicles, can contribute
to the cracking and faulting of thin concrete pavements. Therefore, it’s
important for engineers to anticipate future traffic loads based on an
analysis of population trends and commercial developments in the area.
Comprehensive traffic studies made within city boundaries can be useful
sources of data for the design of municipal pavements.
Joint design
Proper jointing of low-volume concrete pavements is the most critical
factor in achieving good performance, yet joint spacing and layout are
often overlooked in the design stage, according to Zollinger. For that
reason, the subcommittee that prepared ACI 325.12R gave special attention
to this topic. Guidance is given for the proper location, spacing, and
installation of transverse contraction and construction joints,
longitudinal joints, and isolation and expansion joints (if required).
These recommendations are intended to help engineers design a jointing
pattern that will limit tensile stresses caused by restrained shrinkage
and temperature differentials. In addition, an appendix presents 10 rules
of practice for joint layout, along with example illustrations of layouts
for cul-de-sacs, isolation joints for drainage structures and manhole
covers, and integral curb details.
Dowels or other mechanical load-transfer devices aren’t recommended
for the transverse joints of most city streets and low-volume roads,
particularly when passenger vehicles will make up most of the forecasted
traffic. Under these circumstances, aggregate interlock should provide
sufficient load transfer at transverse joints. However, in cases where the
average daily truck traffic could exceed 100 vehicles, engineers may find
it necessary to improve load transfer by using stabilized subgrades,
dowels, or thicker pavements.
In order for aggregate interlock to effectively transfer loads,
portions of aggregate particles from one side of the joint must protrude
into recesses in the other side. To improve aggregate interlock between
panels, ACI 325.12R recommends using close joint spacings to keep openings
at the joints small. This will also help to prevent pavement blowups by
keeping incompressible materials out of the joint.
Slab reinforcement
Distributed steel or wire mesh isn’t needed for most low-volume
pavements with closely spaced joints. The small amounts of steel normally
used for crack control won’t increase the load-carrying capacity of
these pavements or compensate for poor subgrade preparation or
construction practices. The exceptions are when irregularly shaped panels
are used, or when joint spacings exceed those that will limit mid-panel
cracking caused by concrete shrinkage. Although reinforcing steel won’t
prevent cracking caused by nonuniform support conditions, it can help
control the opening of unavoidable cracks.
Economic factors are important considerations in the design and
construction of any concrete pavement, but they can be of even greater
significance for municipal pavements because budgets for local roads often
are limited. The initial costs for materials and construction are just
part of the overall cost of a pavement. Designs also must consider
life-cycle costs for pavement maintenance and rehabilitation. Using this
new ACI guide will help engineers achieve maximum economy by ensuring the
successful performance of low-volume roads over their expected design
life.
For more information, go to www.aci-int.org.
Reprinted from Better Roads Magazine
October 2002 |
