This work is aimed at estimating the time available for compaction of a hot-mix asphalt mixture during construction.Laboratory tests were conducted on dense-graded asphalt and porous asphalt PA mixtures to evaluate their mechanical properties. Temperatures and mix characteristics tested in the laboratory were determined to be those typically found in an asphalt lift from initial lay down through final compaction. On the field, digital thermometers were installed at different depths to measure temperature changes to decide pavement cooling times of the same mixes as used in the laboratory. The cooling behavior of asphalt concrete could be classified into the following three stages: rapid, transition and stable zone. The air void in the PA mix was found to contribute to heat loss and result in a rapid cooling rate when compared to that in a dense-graded mix. The cooling rate at depth of 0, 2.5, and 5 cm showed an essential difference in the cooling rate initially. As the cooling process continued, the cooling rate became stable and reached a thermal equilibrium condition. An increase in lift thickness was shown to increase the compaction time. However, an increase in layer thickness more than 10 cm might not increase the time available for compaction significantly. A regression model was developed to predict the time required to cool to the minimum temperature allowed for compaction. The predicted values were in good agreement with those measured in the field. This study shows that a simplified method to predict available compaction time can be developed by considering only the significant factors affecting pavement cooling.
TABLE OF CONTENTS
Title page ii
Table of Contents viii
List of Figures xi
List of Tables xii
List of Symbols/Abbreviation xiii
CHAPTER ONE: INTRODUCTION
1.1 Preamble 1
1.2 Statement of Problem 3
1.3 Aim and Objectives 4
1.3.1 Aim 4
1.3.2 Objectives 4
1.4 Justification of Study 5
1.5 Scope and Limitation of Study 5
CHAPTER TWO: LITERATURE REVIEW
2.1 Introduction 6
2.2 Ambient Temperature 16
2.3 Base Compaction 17
2.4 HMA Asphalt 17
2.5 Asphalt Concrete 18
2.5.1 Hot Mix Asphalt Concrete 19
2.5.2 Warm Mix Asphalt Concrete 19
2.5.3 Cold Mix Asphalt Concrete 20
2.5.4 Cut-back Mix Asphalt Concrete 20
2.5.5 Mastic Mix Asphalt Concrete 20
2.5.6 Natural Asphalt 21
2.6 Performance Characteristics 21
2.7 Asphalt Concrete Degradation and Restoration 22
2.8 Prevention and Repair of Degradation 23
CHAPTER THREE: METHODOLOGY
3.1 Introduction 25
3.2 Field Tests 26
3.3 Effect of Air Voids 26
3.4 Development of Cooling Model 27
CHAPTER FOUR: RESULTS AND DISCUSSION
4.1 Mechanical Properties 29
4.2 Effect of Low Temperature 30
4.3 Effect of Air Voids 34
4.4 Effect of Lift Thickness 38
4.5 Comparison of Field and Laboratory Measurement 41
4.6 Development of Cooling Model 45
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.1 Conclusion 49
5.2 Recommendation 50
Thermal environmental conditions, to which pavements continuously are exposed in the constructionand repair phases, as well as in use, determine the temperature profile in asphaltic sections. Fluctuations in ambient air temperatures – diurnal and seasonal, intensity of solar radiation, pavement materials and geometry, convective surface conditions,and precipitation significantly impact pavement stability and therefore the long-term success of pavement design. Accurate prediction of the temperature profile in pavement construction greatly aids pavement engineers,specifically in the assessment of pavement deflection, in calculations of pavement modulusvalues, in estimations of frost action and frost penetration and seasonal heatingand cooling effects.
The top pavement layer normally is exposed to greater temperature fluctuations than the layers below it. Because of this, detailed knowledge of the temperature distribution in asphalt layers also allows for a more sophisticated specification ofasphalt binders for lower lifts through specification of less expensive asphalt binders for lower lifts, and thus it provides an economical solution to rising pavement construction costs. An assessment of the impact of pavement temperature variations on various pavement materials, such as dense and open- graded asphalt mixes is possible with a higher degree of accuracy.
This include a grading system called performance grading (PG) that proposes atwo-number system intended to ensure that the proper asphalt binder is used to resist pavement rutting in hot temperatures and to resist cracking in cold temperatures. The approachrepresents the expected maximum high and low asphalt temperatures based on
local climatic data for the hottest and coldest times of the year.
However, this raises questions with respect to pavement temperature estimations since performance grading method for asphalt binders appears to modify the asphalt operational temperature range, and thus further limits availability of asphalt that meets the prescribed criteria. One concern associated with is the cost, since both asphaltic cement and aggregate costs may be higher for pavement mixes due to limited sources or increased processing than for normal mix designs.
In this case, the cost of asphalt may increase by as much as 30 percent over conventional implementations. Thepavement performance grading requirements for lower asphalt lifts-including the binder and base courses, and the appropriate binder selection for hot mix asphalt recycling, callsfor a detailed understanding of the temperature profile in pavements construction. Many methods dealing with prediction of temperature gradients in pavements are based on statistical and probabilistic methods developed based on weather and pavement data collected through the Long-Term Pavement Performance.
However, such statistical and probabilistic methods display shortcomings in that they tend to either underestimate high pavement temperatures or overestimate low pavement temperatures raising questions about their accuracy and reliability. More detailed methods using energy balance equations to estimate pavement surface temperatures or numerical models that attempt to predicttemperature gradients in asphalt pavements are either steady-state or one–dimensional transient approaches that fail to account for thermal interaction of parallel laid asphalt pavement lifts of varying grades and binders. The uncertainties associated with the pavement algorithms call for
computationally fast tools that can accurately and reliably predict asphalt pavement temperatures at differentpavement depths and horizontal locations based on local ambient environmental conditions.
Low ambient temperatures and wind speeds create adverse conditions for hot- mix asphalt HMA paving. A HMA mix must be compacted to a specific range of air voids to control the density to achieve optimum mechanical properties. The time required for hot-mix asphalt to reach the proper compaction temperature decreases with an increased cooling rate (Chang et al., 2009). In general, the minimum temperature allowed for compaction is set at 80oC for the dense- graded mixture, AASHTO 2000.
This temperature is referred to as cessation temperature Roberts et al., 1996; Foster 1970. As the mix cools, the asphalt binder becomes stiff to prevent further reduction in air voids regardless of the applied compaction efforts. Below the minimum compaction temperature, the gain of mix density with application of additional compaction efforts becomes difficult. Any additional rolling may result in fracture of aggregate and a decrease in density. Inadequate compaction of HMA would decrease the fatigue life, reduce strength and stability, increase moisture-related damage, and hence affect long- term pavement performance.
1.2Statement of Problem
Below the minimum compaction temperature for asphalt pavement, the gain of mix-density with application of additional compaction efforts becomes difficult. At such temperature, any additional rolling may result in fracture of aggregate and a decrease in density. Inadequate compaction of HMA would decrease the fatigue life, reduce strength
and stability, increase permanent deformation, accelerate oxidation or aging, increase moisture related damage, and hence affect long term pavement performance.
Various methods for determining available compaction time have been developed using finite difference models Timm et al., 2001; Tanet et al., 1997; Tegeler and Dempsey 1973, Dickson and Corlew 1970. However, these methods use assumptions and typical values that may be inappropriate to predict the cooling rate for use in a tropical region like Nigeria. Hence the need to develop a model to adequately predict the appropriate time for asphalt laying in Nigeria
1.3Aim and Objectives
The aim of this research is to verify the appropriate time for asphalt pavement laying in Kaduna state. The time here refers to both the time for laying and temperature at the time of laying.
The objectives of this study are as follows;
i. To investigate how the cooling time of HMA is affected by thickness, temperature and air voids.
ii. To investigate the value of the Traffic opening Time (TOT) at which the temperature variation becomes negligible and the paved road is open to traffic.
iii. To propose a model for local asphalt pavement laying in Kaduna state and its environs.
1.4 Justification of Study
The outcome of this study will serve as a guide to the government, consultant and contractors in Kaduna metropolis in predicting appropriate laying temperatures for road works as well as traffic opening time. Which will immensely help in solving road deteriorating problems associated with inadequate laying temperature, compaction and premature loading of newly constructed road pavements, this will in turn prolong the life span of the roadway pavement and delay the time when heavy investment is to me made on the road for either rehabilitation or reconstruction, thus, cost savings.
1.5 Scope and Limitation of Study
The scope of this study is to develop a model that verifies time and temperature of laying asphalt concrete in Kaduna State and its environs to predict appropriate asphalt laying temperature in terms of lift thickness, air temperature and percent air voids.
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