“The road to success is paved with the hot asphalt of failure.”
Craig D. Lounsbrough
Hello word...!!
As it was mentioned in my previous
post, here we will go through some concepts regarding the deterioration of
pavements. Feel free to express your point of view. The aim is to share
knowledge and to understand how different people sees pavement engineering so
let's start.
It is well known that pavement
deterioration is caused by the interacting damaging effects of traffic loading,
environment, materials and failure criteria. Could it be that we are missing
something between these colossal factors? What are the consequences of not
applying maintenance plans to our pavements and perhaps more important, who
will suffer the after-effect? The aim of this post is to provide an
overview of the damaging effects that pavement engineers need to face in everyday
situations and propose alternative solutions to overcome them. Traffic loads and their impact on
pavements are quantified in terms of axle loads, timing of axle passes, axle
lateral placement, tyre inflation pressure and number of repetitions. Axle loads,
axle configuration and load magnitude are deeply connected between themselves.
Axle configuration is defined by the number of axles sharing the same
suspension system and the number of tyres in each axle which can be referred to
as single, tandem, triple, or quad. Besides the single axle, all the other
configurations need to be treated differently since the transmitted load in
terms of stresses and strains will overlap in the pavement. The timing of axle
passes is important for both flexible and rigid pavements, mainly because of
the seasonality in pavement layer properties and the time dependency of thermal
stresses, respectively. The vehicle speed, axle speed, is relevant mainly to
flexible pavements due to the viscoelastic behaviour of the asphalt concrete as
well as the lateral distribution of the accumulated damage because of the
weaving effects. When reviewing the number of repetitions there are two
well-defined paths that can be followed, firstly, the widely accepted procedure
which develops equivalent factors and converts each load group into an
equivalent single-axle load (ESAL – 80 kN) and secondly, the consideration of
multiple axles which is not a simple matter as it could lead out to an unsafe
procedure if the tandem and triple axles are treated as a group and considered
as one repetition or on the other hand lead to a too conservative procedure if
each axle is treated independently and considered as one repetition. All in
all, traffic loads, primarily those from heavy trucks, cause stresses/strains
in pavement structures, whose effects accumulate over time resulting in
pavement deterioration such as plastic deformation in asphalt concretes or
fatigue cracking in Portland concretes.
Regarding how the environment
influences the pavement, let me refresh the concepts of climate and weather
before we get into detail. Climate is defined as the regular weather conditions
for an area where the weather is the day to day manifestation of this climate.
History has demonstrated, sometimes with pathetic examples, that weather has
always been one of the primary factors that affect the pavement. Factors such
as temperature, frost and thaw, moisture and precipitation play major roles
affecting the elastic moduli of the different layers within the structure. In
hindsight, every pavement structure has been designed based on historical
climate however, during their design life a very different condition could
occur therefore not taking this into consideration could be vast in terms of
pavement performance, disruption to traffic and public safety just to name a
few. There are also less obvious effects on pavement deterioration caused by
the increase in average temperature and changes in rainfall patterns therefore
the use of adaptive maintenance practices such as permeable pavements, polymer
modified binders and improved routine maintenance of joint seals is encouraged.
In the near future, in order to minimize the disruption caused by weather
changes, it will be crucial to plan and adapt to the changing climate rather
than base decisions on the climate we experienced in the past. Figure
1, categorize the different variables that can be found in the design per
importance degree; as it was stated before, temperature, precipitation, wind
and soil moisture are the main variables that as pavement engineers need to
consider.
Figure 1: Climate variables relevant to highway maintenance
Henceforth, materials will be
analysed and most importantly, the impact that they cause on the performance of
our structures will be discussed. When relating materials to pavement
deterioration, it is almost automatic to think about the construction process
and to even agree that in most of the occasions, the issues related to the
early appearance of distress come from poor construction quality. Going a
little bit deeper, the after-effects that we face in our pavements namely
distress can easily come from an improper grading of aggregates for base or
subbase or from a poor subgrade soil of low bearing capacity, which for some
readers could not even be imaginable. Another interesting point of view is the
use of marginal or second-rate base materials for pavement construction due to
geological conditions of the area which may provoke or accelerate deterioration
resulting in rutting, cracking and shoving or combinations of these problems.
All in all, failure to obtain proper compaction, improper moisture conditions
during construction, quality of materials and accurate layer thickness after
compaction will deteriorate over time the performance of a pavement. Figure 2, shows a
general overview of these problems.
Figure 2: Climate variables relevant to highway maintenance
The objective with the design of a
pavement structure is to avoid the excessive flexing of any layer, failure to
achieve this will result in the over stressing of a layer, which ultimately
will cause the pavement to fail. In both flexible and rigid pavements, the load
distribution pattern changes from one layer to another due to the strength
differences. The top layer is considered the least flexible therefore will be
the strongest material since the load is applied to a small area (surface of
the wheel) resulting in high stress levels; based on the same analogy, the
lower layer will be the most flexible therefore the weakest material because
the load is applied in a larger area resulting in lower stress levels,
see Figure 3.
In the past, empirical pavement
design methods were use based on observations of performance in pavements with
known dimensions and materials under specific climatic, geologic and traffic
conditions. The outcome seen by many engineers was that this method led to
overly conservative designs. Nowadays, mechanistic qualities have been added to
the empirical design philosophy resulting in the Mechanistic-Empirical (M-E)
design methods to determine the stresses, strains and deflections a pavement
will experience from external influences and an empirical relationship to
connect pavement response with pavement deterioration. The implementation of
M-E will allow pavement engineers to design our structures with the right
thickness for the specific conditions in each geographic area. It is fundamental
that we recognize that pavement design and the type of selection process is
dynamic and will change as more data is gathered and more lessons are learned.
Figure 3: Load distribution of flexible and rigid
pavements
To conclude, if knowing the damaging
effects that the exposed factors produce to our pavements, why do we still see
early distress in our projects? Is it because the gap between the practical-theoretical
knowledge and the constructional process is that big, that even with
understanding the behavior, we cannot place it on the field and that the only
way to overcome this situation is through intensive research both from the
Academia and the Private sector. Limitations such as not using high-quality
materials due to the location of the project should not be a barrier to our
designs, at least not anymore since we could overcome either by modifying
existing materials with recycling techniques or by creating new materials where
most of the mechanical, physical and chemical properties could be designed to
withstand the adverse weather and loading conditions. In the near future, we
should not allow poor construction processes to be a factor that can deter the
development of our structures; it would not matter to have the most resistant
materials if we cannot place them correctly on site. The beauty, as I see it,
is that most of the lessons have been learnt from past experiences; we are
entering a new era where new techniques will come into play and we need to be
prepared to face them and to use them to our best interest; which as pavement
engineers, we should not forget the end goal, minimize the total cost to
society.
That's all from today. Stay tuned and see you soon!
