Civil Engineering ETDs

Publication Date

Fall 8-22-2019

Abstract

With the deterioration of highway pavements across the country, more emphasis is being laid on the rehabilitation of existing pavements. Unbonded concrete overlay (UBCO) is a cost-effective technique used to rehabilitate damaged concrete pavements. The design and performance of UBCO rely on various properties of concrete, of which coefficient of thermal expansion (CTE) is an important one. Concrete CTE has a direct impact on the design and performance of rigid pavements and overlays. CTE regulates the magnitude of curling and related stresses that impact the performance of overlays with regards to cracking, faulting and pavement roughness. Previous research revealed that thermal expansion of concrete is caused by re-distribution of water between capillary pores and gel pores in the cement paste with a change in concrete temperature. The volume of these pores changes with the ongoing hydration process in the cement paste and thus the CTE of concrete also changes with age progression. Several researchers worked on the effects of concrete age on CTE but found different conclusions with some researchers opined that CTE decreases with age, some said that CTE increases with age, and some said that CTE remains constant. Almost all of the previous studies had some limitations i.e. use of old test method, short term CTE testing, and not following the test protocol.

With the above in view, it is a necessity to study the effects of age progression on the CTE of paving concrete. The present study involved testing of 7 concrete paving mixes for long term CTE ranging from 7 days to 360 days to obtain accurate laboratory test data for further analysis. These concrete mixes were collected from different districts of New Mexico (NM), prepared from different coarse aggregates with varying mineralogy. The CTE value of these mixes varies over a fairly large range i.e. from 3.71 to 5.95 μԐ/˚F. The long term CTE test data showed significant effects of age progression on CTE value of paving concrete with CTE increasing in the range of 0.33 μԐ/˚F to 0.55 μԐ/˚F with a percent increase of 6.4% to 12.6% between 28 days and 360 days. The increase in CTE is different for different paving mixes which can be attributed to the difference in mix design proportions and different (mineralogy) coarse aggregates being used. This variation in CTE may affect the performance of UBCO and use of 28 days CTE may give inaccurate design.

The impact of coarse aggregate mineralogy on the CTE value was also evident from the test data in this study. The CTE values of CA-ID-1, 2, 4, and 7 i.e. granite, dolomite, and quartzite are consistently higher than that of CA-ID-3 and 5 which are limestone mixes. This confirms the previous research/literature that concrete with limestone aggregate has the lowest CTE value as compared to other minerals.

The effects of aged CTE on the design and performance predictions of UBCO were evaluated by conducting simulations in Pavement ME (Mechanistic-Empirical) Design software version 2.3. Time series data for concrete mechanical properties including compressive strength, elastic modulus and modulus of rupture (MOR) were obtained by laboratory testing of the beam and cylindrical specimens at the age of 7 days to 90 days. Inter-conversion models were developed to convert compressive strength to elastic modulus and MOR. Analysis showed that these models work better for NM paving mixes as compared to the Pavement ME default models. The effects of aged CTE were quantified by conducting UBCO design simulations incorporating time series data and using 28 days CTE value for each mix and then repeating the process with 360 days CTE value with other design variables. The results show that there is a significant impact of CTE variation on the performance of UBCO with regards to transverse cracking with a percent increase of 1.6 to 9.7% while the percent increase of joint faulting is 10% to 19.9%. Further analyses were conducted to determine the reduction of overlay service life with an increase in CTE from 28 days to 360 days. It is shown that the reduction in overlay service life ranges between 4 to 13 years. It is evident from these results that a UBCO designed with 28 days CTE value may not be able to perform up to the designed service life as the distresses increase with an increase in CTE value which may result in the early failure of overlay pavement.

A prediction model was developed to determine long term CTE incorporating mixture volumetrics, concrete strength properties, concrete age and 28 days CTE value. The analysis of the developed model showed that the model worked well in predicting aged CTE when compared with long term CTE test data. This model can be incorporated in Pavement ME Design software to better predict pavement performance and enhance the effectiveness of UBCO design.

A temperature gradient exists between the top and bottom of the concrete pavement slab which results in thermal curling, producing thermal stresses in the pavement. Previous practice was to assume a linear temperature gradient in the pavement slab, but later researchers found that the thermal gradient is non-linear. In the present study, numerical modeling was conducted in finite element software ABAQUS 6.14 to evaluate the impact of aged CTE on the stresses in pavement slab incorporating nonlinear temperature gradient. As an initial step, pavement slab was modeled with a linear temperature gradient with constant CTE value to determine the state of bending stresses through the thickness of the pavement slab. The results were compared with previous analytical and numerical studies and good match was found. With this analysis, it was deduced that the FE idealization used in this study works well and can be further used for nonlinear thermal modeling. As a next step, the pavement slab was modeled with a nonlinear temperature gradient and the results of bending stresses through the thickness of the slab were determined. These results were again compared with the previous studies and matched well with the stress profiles.

FE modeling was conducted to evaluate the effects of aged CTE (at 360 days) in comparison to CTE at 28 days on the stress profiles in concrete pavement slab with a nonlinear thermal gradient. The FE idealization used earlier was re-employed for this purpose. The results showed that the longitudinal bending stresses at the top and bottom of the pavement slab increase significantly with an increase in CTE values. The increase in stress values ranges from 6.4% to 12.5%. This increase in stresses may result in increased distresses and early deterioration of concrete pavements and UBCOs. It became evident from these results that UBCO designed with CTE value at 28 days may not perform well through its designed service life.

Keywords

Concrete, Unbonded overlays, Coefficient of thermal expansion, Finite element simulations, Transverse cracking

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Committee Member (Chair)

Dr. Rafiqul A. Tarefder

Second Committee Member

Dr. John C. Stormont

Third Committee Member

Dr. Yu-Lin Shen

Fourth Committee Member

Dr. Fernando Moreu

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