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International Concrete Abstracts Portal

Showing 1-5 of 10 Abstracts search results

Document: 

SP267-05

Date: 

October 1, 2009

Author(s):

J. Schoepfer and A. Maji

Publication:

Symposium Papers

Volume:

267

Abstract:

The development of nanotechnology has led to the ability to produce silicon dioxide in nano-sized particles of predictable size ranges. In this study, concrete mixtures were developed using silicon dioxide of various sizes. Compressive strength testing showed significant increase in strength with decrease in particle size of the silicon dioxide down to 12 nm (4.7 × 10–7 in.). However, the mixture under 12 nm (4.7 × 10–7 in.) had a slightly lower increase in strength. High vacuum SEM analysis was performed on the samples. High-resolution images at magnifications of 5000× to 60,000× were achieved. The photographs suggest that only the surface of silicon dioxide particles is involved in chemical reactions. The particles then appear to become nucleation sites for the development of CSH crystals. Fine silicon dioxide particles provide numerous and small nucleation sites. Silicon dioxide particles smaller than 12 nm (4.7 × 10–7 in.) do not appear to generate additional nucleation sites for CSH. SEM photos of the 7 nm (2.8 x 10–7 in.) mixture reveal a structure similar to that of the 150 nm (59.1 × 10–7 in.) mixture.

DOI:

10.14359/51663282


Document: 

SP267-02

Date: 

October 1, 2009

Author(s):

Z.S. Metaxa, M.S. Konsta-Gdoutos, and S.P. Shah

Publication:

Symposium Papers

Volume:

267

Abstract:

In this study, the development of cementitious nanocomposites reinforced with multiwall carbon nanotubes (MWCNTs) at water-cement ratios of 0.3 and 0.5 was investigated. The effect of carbon nanotubes at low concentrations on the fracture properties, nanoscale properties and microstructure of the nanocomposite materials was studied. The morphology and the microstructure of nanocomposite samples were investigated using an ultra high resolution field emission scanning electron microscope. A special type of nanoindenter, along with in-place scanning probe microscopy imaging, was used to determine the local nanoscale mechanical properties. The results show that the mechanical properties of cementitious matrices can be increased by the incorporation of very low amounts of CNTs. Nanoimaging of the fracture surfaces of cement nanocomposites have shown that CNTs reinforce cement paste by bridging the nanocracks and pores. Additionally, nanoindentation results suggest that CNTs modify and reinforce the nanostructure of cement paste by increasing the amount of high stiffness C-S-H and reducing the porosity.

DOI:

10.14359/51663279


Document: 

SP267-03

Date: 

October 1, 2009

Author(s):

A. Yazdanbakhsh, Z.C. Grasley, B. Tyson, and R.K. Abu Al-Rub

Publication:

Symposium Papers

Volume:

267

Abstract:

Due to their excellent mechanical characteristics, carbon nanofibers (CNFs) and nanotubes (CNTs) are expected to enhance properties such as strength, ductility, and toughness in cementitious composites. However, such enhancements cannot be achieved unless the fibers are uniformly distributed in the composite and properly bonded to the matrix. CNT/Fs tend to agglomerate due to their high level of van der Waals interactions, and typically form a weak bond with hardened cement paste matrix. This work first presents a summary of the efforts made in the past to overcome these two problems. Some typical methods of qualifying/quantifying the dispersion of CNT/Fs either in the hardened cement paste or the mix water are discussed. To demonstrate the challenges associated with CNFs and their dispersion and interfacial bond with cementitious matrices, some of the results from an ongoing experimental program are presented. The experiments investigate the effect of surfactants on dispersion and their benefits and shortcomings when cementitious composites are concerned. It was shown that mixing cement and a well dispersed water-surfactant-CNF solution may not result in a uniform distribution of CNFs in the paste or an optimal CNT-matrix interfacial bond. However, it was also found that the interfacial bond can reach to a level high enough to prevent fiber pullout.

DOI:

10.14359/51663280


Document: 

SP267-06

Date: 

October 1, 2009

Author(s):

J.A. Jain and N. Neithalath

Publication:

Symposium Papers

Volume:

267

Abstract:

The effects of small dosages of nano-silica as a partial cement replacement material on the Ca ion leaching resistance of cement pastes exposed to deionized water is reported in this paper. Plain and modified cement paste specimens (containing either 6% or 9% of silica fume, or 0.5% or 1.5% of nano-silica) are subjected to leaching in deionized water for different durations after 56 days of curing in saturated limewater. The mass loss, change in porosity, and the changes in calcium hydroxide (CH) and C-S-H contents from thermogravimetric analysis between the specimens cured under saturated limewater for the entire duration and the specimens leached for different times are used to bring out the beneficial effects of these cement replacement materials when pastes are exposed to a leaching medium. The nano-silica modified cement pastes are observed to demonstrate lower mass loss and a lower increase in porosity when subjected to leaching. Using the changes in CH and C-S-H contents between the saturated and the leached pastes, it is shown that leaching and continuing cement hydration and/or pozzolanic reaction are essentially coupled, especially for the modified pastes. The paste with higher nano-silica content is seen to demonstrate increased C-S-H contents when undergoing leaching. The net Ca ion loss from both CH and C-S-H phases are seen to be lower for the pastes incorporating nano-silica as compared to those containing silica fume. The plain paste is seen to suffer the highest amount of Ca ion loss. A simplified method of calculating the apparent depth of the CH dissolution front is also reported, which is seen to highlight the influence of nano-silica and silica fume in improving the leaching resistance of pastes.

DOI:

10.14359/51663283


Document: 

SP267-01

Date: 

October 1, 2009

Author(s):

E.A.B. Koenders, J.S. Dolado, K. van Breugel, and A. Porro

Publication:

Symposium Papers

Volume:

267

Abstract:

Recent developments in nanotechnology paved the way for deepening the modeling level of science-based kernels and enabling new opportunities in terms of understanding the formation of hydration products and their contribution to the microstructure. Based on these developments next generation models are considered to have the potential to design new advanced materials that contribute to sustainable construction. One of the activities currently running in this field is the Codice (Computationally Driven design of Innovative Cement-based materials) project which is a multi-scale modeling research project established within the European research arena FP7. The project has the ambition to bridge the nanoscale to the microscale by applying advanced computational simulation modeling techniques for cementitious materials. Nano-based models will be connected to the microscale level within the framework of the Hymostruc model. It is the objective of the CODICE project to provide more insight into the role of the fundamental building blocks of CSH gel (basically 5nm sized nanoparticles) and the mechanisms that govern their aggregation into high-density (HD) and low-density (LD) C-S-H varieties. Thus, the project aims to refine the microstructure of the Hymostruc model (Breugel 1991) so as to recognize the two types of C-S-H aggregates. The new computational scheme is expected to be a perfect tool to design new cementitious materials with improved mechanical properties.

DOI:

10.14359/51663278


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