International Concrete Abstracts Portal

Many of the papers presented in this volume were included in the two-part session Nanotechnology for Improved Concrete Performance, sponsored by ACI Committee 241, Nanotechnology of Concrete at the ACI Convention in Philadelphia, PA, on October 26, 2016. In line with the practice and requirements of the American Concrete Institute, peer review, followed by appropriate response and revision by authors, has been implemented.

A  detailed  microstructural  and  micromechanical  study  of  a  fly  ash‐based  geopolymer  paste  including: (i) synchrotron x‐ray tomography (XRT) to characterize the pores (size > 0.74 m) that are influential in fluid transport, (ii) mercury intrusion porosimetry (MIP) to capture the volume fraction of smaller  pores,  (iii)  high  resolution  scanning  electron  microscopy  (SEM)  combined  with  a  multi‐label  thresholding method to identify and characterize the solid phases in the microstructure, and (iv) nanoindentation  to  determine  the  component  phase  elastic  properties  using  statistical  deconvolution  techniques, is reported in this paper. The 3D pore structure from XRT is used in a computational fluid transport  model  to  predict  the  permeability  of  the  material.  The  pore  volume  from  XRT,  solid  phase  volumes from SEM, and the phase elastic properties are used in a numerical homogenization framework to determine the homogenized macroscale elastic modulus of the composite. The homogenized elastic moduli are in good agreement with the flexural elastic modulus determined on macroscale paste beams. It  is  shown  that  the  combined  use  of  microstructural  and  micromechanical  characterization  tools  at  multiple scales provides valuable information towards the material design of fly ash‐based geopolymers.

The current state of the art in fiber-reinforced cement-based materials indicates that adding multiple fiber types or sizes primarily creates a superpositioned behavior state: the behavior from each fiber type separately is added to the composite behavior of the material. Carbon nanofibers (CNFs) and milled carbon microfibers (MCMFs) can increase cracking resistance in cement-based materials by bridging cracks, although CNFs bridge cracks significantly smaller than cracks bridged by MCMFs. This research suggests that multi-scale fiber reinforcement (CNFs with MCMFs) might add compounded benefits to cracking resistance. Tests evaluating cracking resistance were performed utilizing a restrained-ring drying shrinkage test with Portland cement mortars. The CNFs and/or MCMFs were pre-mixed with cement using a sonication/distillation technique and/or rotary tumbling. Concentrations of CNFs and MCMFs were tested up to 5% and 6% by mass of cement, respectively. Restrained ring tests on mortar with high concentrations of CNFs or MCMFs reveal delayed cracking time by factors up to 6.4 or 2.6, respectively. Combining CNFs with MCMFs delayed cracking by a factor of at least 52. The increase in cracking resistance is attributed to the combined effects of bridging cracks of multiple sizes.

The piezoresistive response and self-sensing ability of carbon nanotube reinforced mortar sensors have been investigated. The study aims on optimizing the development of a self-sensing nanoreinforced cement-based sensor for monitoring and evaluating the condition of concrete elements, in real time applications. It has been shown that the piezoresistive response of the nanomodified mortars was substantially enhanced just by adding a low amount of carbon nanotubes (CNTs), 0.1 wt%. Resistance measurements, using direct current (DC) and alternating current (AC), were conducted under the application of cyclic or monotonic compressive loading. The results show the sensor’s great ability to detect crack propagation and damage accumulation at all stages of deformation up to failure.

Fiber additions in portland cement composites is a regular practice for crack prevention and for increasing the flexural strength. In this research, fiber-reinforced composites (FRC) with polyvinyl alcohol (PVA) fibers and carbon nanofibers (CNF) or carbon nanotubes (CNT) were investigated. Specimens were tested to measure their flexural strength, water absorption and electrical conductivity in water or sodium chloride solution. It was found that the developed composites, depending on applied stress and exposure to chloride solutions, exhibit some electrical conductivity. These dependencies can be characterized by piezoresistive and chemo-resistive coefficients demonstrating that the material possesses self-sensing capabilities. The sensitivity to strain,  crack formation, and chloride solutions can be enhanced by incorporating small amounts of CNF or CNT into a composite structure. Conducted research has demonstrated a strong dependency of electrical properties of the composite on crack formation in moist environments. The developed procedure is scalable for industrial application in concrete structures that require nondestructive stress monitoring, integrity under high service loads and stability in harsh environments.