Link to Latest Report : Coming Soon.
Background :
UHPC for bridge elements, repair materials, and other applications often contains 2 % to 3 % by volume of steel fibers to impart resistance to plastic shrinkage, improve its resistance to abrasion and impacts, and to provide additional strength and fracture toughness. However, steel fibers are much heavier than organic fibers such as polypropylene (PP) or polyvinyl alcohol (PVA), while their stiffness and tensile strength are only modestly greater. UHPC for bridge elements could be made with a lower density and potentially higher strength-to-weight ratio if it could be reliably made with PP or PVA fibers instead of steel. PP and PVA fibers have the additional advantage that they are not susceptible to corrosion, are inert in alkaline environments, have negligible water absorption, impart greater impact resistance and abrasion resistance, and contribute less to greenhouse gas emissions than their steel counterparts. Synthetic fibers have been used to reinforce concrete for at least 40 years [Swamy 1979]. A study by Hannawi et al. [2016] compared the relative influences of steel, synthetic, and mineral fibers on the microstructure and mechanical properties of concrete. Compared to steel or mineral fibers, synthetic fibers had a more porous interfacial zone with the binder due to the hydrophobic nature of the polymer. The polymer fiber had a negligible influence on the compressive strength and elastic modulus, although it did improve the threshold stress for both initial and unstable cracking. PVA fibers provided higher cracking resistance than PP fibers, presumably due to PVA’s much greater ultimate tensile strength and stiffness.
Objectives :
The objective is to determine the compressive strength, fracture toughness, and strength-to- weight ratios of chemically pretreated PP-reinforced UHPC as a function of PP dosage. The property variations with these variables will be linked to fiber dispersion and macro flaws using lab-scale X-ray microcomputed tomography (µCT).
Scope :
Task 1 – Acquire Materials and Determine Experimental Variables.
The baseline UHPC mixture will be the published formulation for Cor Tuf UHPC, originally developed by the U.S. Army Corps of Engineers [Williams et al. 2009; Roth et al. 2010] and now commercialized. Cor Tuf typically uses hook-ended steel fibers for reinforcement, so we will be substituting untreated PP fibers at the same volume fraction as a control formulation. From that, we will do exploratory work with fiber volume fractions 20 % greater and 20 % lower than the baseline value, examining whether workable mixtures can be made. Adjustments will be made to the fiber dosages, mixing methods, or both, as needed to achieve workable mixtures that are free of macroscopic defects.
In addition, an alkaline chemical pretreatment has been shown in recent work to improve the swelling resistance and water absorption of PP fibers, greatly reducing partial hydration near the fibers and the attendant large porous defects there that compromise fiber bond to the matrix [Masoud et al. 2025]. Therefore we will examine the effects of a Ca(OH)2 alkaline pretreatment of the PP fibers prior to adding them to the mixture. The Ca(OH)2 concentration and submersion duration will be determined as part of this task.
These ranges for the experimental variables are tentative and may be adjusted based on preliminary results to achieve the maximum amount of useable data. All other mixture parameters, including w/s mass ratio, fiber length, and superplasticizer dosage, will be held constant.
Task 2 – Characterize the apparent Porosity and the 3D distribution of fibers, air voids and cementitious binder.
A lab-scale 3D X-ray micro computed tomography (Micro-CT) instrument (e.g., Bruker SKYSCAN) will be used to non-destructively interrogate the 3D microstructure with a spatial resolution of about 30-50 µm. The density differences among these three components due to their different X-ray absorptions are sufficient to distinguish their geometric characteristics within the 3D microstructure image. The mixtures described in Task 1 will be scanned to acquire representative volume elements (RVEs) that are large enough, typically about three times the largest feature size, to obtain meaningful statistical representations of the microstructures. From the 3D reconstructions obtained, stereological methods will be used to obtain not only the air void size distribution, but also the fiber and air spatial distributions (using two-point correlation functions). In addition, scanning electron microscopy will be used to examine the structure of the interfacial transition zone between the fiber and binder.
Task 3 – Measure Mechanical Characteristics of PP-reinforced UHPC mixtures.
The microstructure characterization from Task 2 will be correlated with both the mixture variables described in Task 1 and important mechanical characteristics. This task will conduct global- scale mechanical testing of UHPC specimens such as compressive strength and flexural notched bending beam testing incorporated with a digital image correlation (DIC) technique. Scanning electron microscopy (SEM) can also be used to examine fiber-matrix bonding characteristics. Test results will be investigated by integrating with porosity and pore distribution, fiber volume fraction, and fiber distribution. We anticipate that the results can also be used in a later IBT/ABC project to further investigate micromechanical properties and develop and validate a micromechanical model of plastic shrinkage and fracture of the material. Such a model can be used to efficiently explore the properties of different mixture designs of fiber-reinforced UHPC with a greatly reduced reliance on time-consuming physical testing.
Research Team:
Principal Investigator: Jeffrey W. Bullard, Ph.D.
Co-Principal Investigator: Yong-Rak Kim.