Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Abla Zayed, Ph.D.

Committee Member

Manjriker Gunaratne, Ph.D.

Committee Member

Gray Mullins, Ph.D.

Committee Member

Aydin Sunol, Ph.D.

Committee Member

Abdul Malik, Ph.D.

Committee Member

Kyle Riding, Ph.D.

Keywords

cracking mitigation, full-depth repiar, High early-strength concrete, metakaolin, slag

Abstract

Over the past years, prevention of cracking in concrete structures, especially at early age, has been a challenge facing transportation agencies. During early age, restraint of thermal and moisture related volume changes causes cracking in concrete. This research focuses on evaluating the effectiveness of different cracking mitigation strategies that can be implemented, in different concrete elements, to reduce early-age cracking incidents and therefore enhance concrete service life. To this end, a battery of field and laboratory testing were conducted, and finite element analysis was performed to better understand the mechanisms of primary significance on minimizing cracking risks commonly associated with concrete pavement, repair slabs and bridge decks.

Typically, rapid repair materials such as high early-strength (HES) concrete mixtures are used when full depth repairs are required in jointed plain concrete pavements (JPCPs). HES concrete are characterized by high cement content and low water-to-cement (w/c) ratio, which can result in higher temperature rise and higher autogenous shrinkage, and ultimately higher cracking potential. This study investigated several strategies which could minimize these effects and subsequently lower the early-age cracking risks. Those cracking mitigating measures include modifications to mixture design; namely, reduction of paste content, inclusion of lightweight aggregate (LWA), shrinkage-reducing admixture (SRA), and incorporation of polypropylene fiber. Additionally, the effectiveness of base restraint minimization measures using different base friction-reducing mediums (geotextile and double layers of polyethylene sheets) at the slab-base interface were also examined. The effect of each of these measures was evaluated in-situ by measuring the stress and temperature development in concrete test slabs instrumented with concrete stressmeters and thermocouples. Additionally, laboratory experiments were also conducted in order to further assess the cracking mitigating strategies by determining the tensile stress to strength ratio.

The findings indicate that at early-age, it is possible to achieve higher tensile strengths and lower tensile stresses with low paste mixtures. Similarly, incorporation of lightweight aggregates (LWA), decreased the induced tensile stresses at early age and demonstrated promising results in minimizing cracking risks. However, SRA and fibers, showed minimal effects on stress development at early age. In terms of the two base-friction mediums examined here, differences in their performance during the first 24 hours was believed to be due to their different effects on the moisture migration to the base, rather than their friction reducing effect; polyethylene sheets at the slab-base interface prevented moisture loss while the geotextile augmented it. Therefore, it is important to moisten the base prior to concrete placement to prevent moisture loss and resulting initial tensile stresses.

In addition to assessing performance at early-age, analysis of the different mitigating strategies was extended to concrete performance at longer ages. Based on the temperature and stress evolution within the slabs at 50 days, some of the mitigating measures indicated behavior similar to that at early-age. The paste-reduced mixtures showed enhanced tensile strength and lower induced tensile stresses proving their effectiveness at early and extended ages. In regards to base friction reducing mediums, the geotextile medium demonstrated promising results at the hardened stage more than during early age. If the geotextile is adequately moistened prior to slab placement, to avoid moisture absorption from concrete at early age, it can be recommended as an effective method of minimizing cracking risks.

Field testing and laboratory experiments were further supported by the finite element analyses performed using DIANA finite element software, to identify the physical phenomena controlling early-age cracking in JPCP repair slabs. While the analyses indicated the importance of concrete hydration kinetics and viscoelastic behavior on the early-age stress development, FE analysis confirmed that the moisture loss to the base is the most significant phenomenon. Moreover, numerical modeling of concrete slabs was found to be useful in predicting the stress development in advance of costly field trials. The modeling approach adopted in the finite element analysis is proposed as a method which can be applied to evaluate the performance of concrete mixtures prior to slab placement and thus improve and economize the current rigid pavement maintenance practices followed in the pavement industry.

Apart from the aforementioned cracking mitigating strategies, incorporation of supplementary cementitious materials (SCMs) in concrete would also contribute to enhancing concrete durability. However, inclusion of SCMs such as MK, increase autogenous shrinkage in concrete due to their effect on pore size refinement. Incorporation of pre-wetted LWA was found to minimize the autogenous shrinkage and associated cracking risks in concrete. Therefore, LWA addition, is a promising cracking mitigating measure for MK blended concrete, which are highly susceptible to increased autogenous shrinkage due to the higher reactivity of MK. However, in order to accurately proportion the required LWA content, chemical shrinkage coefficient of MK has to be known. Thus, a limited study was performed to determine the shrinkage coefficient associated with MK to mitigate autogenous shrinkage associated with MK blended concrete. Using Thermodynamic modeling (Gibbs Energy Minimization Software (GEMS)), a shrinkage coefficient of approximately 26 ml/100g MK was determined which can then be used in proportioning LWA for MK blended concrete.

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