Menashi D. Cohen
Purdue University
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Featured researches published by Menashi D. Cohen.
Cement and Concrete Research | 1994
Douglas N. Winslow; Menashi D. Cohen; Dale P. Bentz; Kenneth A. Snyder; Edward J. Garboczi
Abstract The cement paste in concrete and mortar has been shown to have a pore size distribution different than that of plain paste hydrated without aggregate. For mortar and concrete, additional porosity occurs in pore sizes larger than the plain pastes threshold diameter as measured by mercury intrusion. Based on the assumption that these larger pores are essentially present only in the interfacial zones surrounding each aggregate, an experimental program was designed in which the volume fraction of sand in a mortar was varied in a systematic fashion and the resultant pore system probed using mercury intrusion porosimetry. The intrusion characteristics were observed to change drastically at a critical sand content. Similar results are observed for a series of mortar specimens in which the cement paste contains 10% silica fume. To better interpret the experimental results, a hard core/soft shell computer model has been developed to examine the percolation characteristics of these interfacial zone pores. Using the model, interfacial zone percolation in concretes is also examined. Finally, the implications of interfacial zone percolation for transport properties and durability of mortar and concrete are discussed.
Cement and Concrete Research | 2003
Manu Santhanam; Menashi D. Cohen; Jan Olek
This paper reports the results of an investigation on the effects of sodium and magnesium sulfate solutions on expansion and microstructure of different types of Portland cement mortars. The effects of using various sulfate concentrations and of using different temperatures are also reported. The results suggest that the expansion of mortars in sodium sulfate solution follows a two-stage process. In the initial stage, Stage 1, there is little expansion. This is followed by a sudden and rapid increase in the expansion in Stage 2. Microstructural studies suggest that the onset of expansion in Stage 2 corresponds to the appearance of cracks in the chemically unaltered interior of the mortar. Beyond this point, the expansion proceeds at an almost constant rate until the complete deterioration of the mortar specimen. In the case of magnesium sulfate attack, expansion occurs at a continually increasing rate. Microstructural studies suggest that a layer of brucite (magnesium hydroxide) on the surface forms almost immediately after the introduction of the specimens into the solution. The attack is then governed by the steady diffusion of sulfate ions across the brucite surface barrier. The ultimate failure of the specimen occurs as a result of the decalcification of the calcium silicate hydrate (C-S-H), and its conversion to magnesium silicate hydrate (M-S-H), after prolonged exposure to the solution. The effects of using various admixtures, and of changing the experimental variables such as the temperature and concentration of the solution, are also summarized in this paper. Models for the mechanism of the attack resulting from sodium and magnesium sulfate solutions will be presented in Part 2.
Cement and Concrete Research | 2000
Bing Tian; Menashi D. Cohen
Abstract Sulfate attack on Portland cement concrete is often said to arise from each of two major sulfate reactions: (1) The sulfate ions react with C 3 A and its hydration products to form ettringite with an increase in volume that results in expansion and subsequent cracking of the concrete; (2) The sulfate ions react with calcium hydroxide (CH) to form gypsum. Even though gypsum formation is generally accepted to be harmful, the specific mechanism is not well established. Especially, the idea that gypsum formation leads to any expansion is controversial. This paper covers an investigation carried out to study the gypsum formation during sulfate attack and its consequences. Two parts are included: Part 1 consists of the results of a literature review describing different theories supporting and contradicting the idea that gypsum formation is expansive. Part 2 describes the laboratory investigation carried out by the authors. The results suggested that gypsum formation during sulfate attack may cause expansion.
Cement and Concrete Research | 2001
Manu Santhanam; Menashi D. Cohen; Jan Olek
Sulfate attack research is at a critical stage. In spite of meaningful advances in the past few years, this problem is still not well understood. Due to its complicated mechanism, the reaction between cement hydration products and sulfate-bearing solutions manifests itself in a variety of ways. In order to provide adequate means for selection of materials for concrete exposed to such aggressive environments, additional research is necessary to further clarify the interaction between concrete and sulfate-bearing solutions. Specifically, the role of the cation in the sulfate solution, and the effects of formation of various products like gypsum, ettringite, and thaumasite, on the extent of damage need to be investigated. The available testing methods for sulfate attack have been subject to some criticism lately. Although these test methods can give an indication of the mechanisms involved in sulfate attack, prediction of field performance using lab studies is difficult. Efforts are needed to introduce appropriate changes in the tests in order to obtain field-like conditions in the laboratory. Combined with good monitoring methods, this would enable the prediction of service life of structures exposed to sulfate solutions. Recent advances in nondestructive testing techniques can be applied to the task of monitoring field structures, although there is a significant effort necessary to calibrate these methods for sulfate attack-related scenarios. In order to produce efficient concrete designs for service in aggressive environments, it is imperative to develop reliable models. Modeling can help in selecting the appropriate materials and their proportions, as well as in determining service life parameters. As a first step towards modeling, critical parameters, which serve as an indicator of deterioration, need to be recognized and established. This paper discusses these issues, and cites some interesting recent developments. Finally, some recommendations for future studies are provided.
Cement and Concrete Research | 2003
Manu Santhanam; Menashi D. Cohen; Jan Olek
Abstract The first paper in this two-part series [Cem. Concr. Res. 32 (2002) 915] summarized the experimental results from a comprehensive research study on sulfate attack. The current paper utilizes these results to develop models for the mechanism of attack resulting from sodium and magnesium sulfate solutions. Implications of changing the binder constituents or the experimental variables, such as concentration and temperature of the solution on the proposed mechanism, are also discussed. The potential of these mechanistic models for use in service life prediction models has also been identified. According to the proposed mechanism, the attack due to sodium sulfate solution progresses in stages. The expansion of an outer skin of the specimen leads to the formation of cracks in the interior region, which is chemically unaltered. With continued immersion, the surface skin disintegrates, and the sulfate solution is able to react with the hydration products in the cracked interior zone leading to the deposition of attack products in this zone. Now, this zone becomes the expanding zone, leading to further cracking of the interior of the mortar. In the case of magnesium sulfate solution, a layer of brucite (magnesium hydroxide) forms on the surface of the mortar specimen. The penetration of the sulfate solution then occurs by diffusion across this surface layer. As the attack progresses, the formation of attack products such as gypsum and ettringite in the paste under the surface leads to expansion and strength loss. The expansion also causes cracking in the surface brucite layer, and this leaves the mortar susceptible to direct attack by the magnesium sulfate solution. Conditions favorable for the decalcification of calcium silicate hydrate (C-S-H) are thus created, and the ultimate destruction of the mortar occurs as a result of the conversion of C-S-H to the noncementitious magnesium silicate hydrate (M-S-H).
Cement and Concrete Research | 2003
Amir Elsharief; Menashi D. Cohen; Jan Olek
Abstract This paper presents the results of an investigation on the effect of water–cement ratio (w/c), aggregate size, and age on the microstructure of the interfacial transition zone (ITZ) between normal weight aggregate and the bulk cement paste. Backscattered electron images (BSE) obtained by scanning electron microscope were used to characterize the ITZ microstructure. The results suggest that the w/c plays an important role in controlling the microstructure of the ITZ and its thickness. Reducing w/c from 0.55 to 0.40 resulted in an ITZ with characteristics that are not distinguishable from those of the bulk paste as demonstrated by BSE images. Aggregate size appears to have an important influence on the ITZ characteristics. Reducing the aggregate size tends to reduce the ITZ porosity. The evolution of the ITZ microstructure relative to that of the bulk paste appears to depend on the initial content of the unhydrated cement grains (UH). The results suggest that the presence of a relatively low amount of UH in the ITZ at early age may cause the porosity of the ITZ, relative to that of the bulk paste, to increase with time. The presence of relatively large amount of UH in the ITZ at early ages may cause its porosity, relative to that of the bulk paste, to decrease with time.
Cement and Concrete Research | 1990
Menashi D. Cohen; Jan Olek; William L. Dolch
Abstract The objective of this paper is to discuss the mechanism of plastic shrinkage in portland cement and portland cement-silica fume paste and mortar. The effects of the three delivery forms of silica fume; asreceived powder, densified powder, and slurry, on plastic shrinkage are presented. In paste, plastic shrinkage is primarily related to development of tensile capillary pressure during drying. The higher the surface area of particles, the higher the tensile capillary pressure, and consequently, the more vulnerable the system would be to plastic shrinkage cracking. In mortar, plastic shrinkage is controlled by both capillary pressure in the paste and presence of fine aggregate particles. These particles serve to reduce cracking by (a) arresting cracks, and (b) refining the size and distribution of capillary pores. Data of rate of evaporation of water can only indicate degree of drying. Charts developed by Portland Cement Association to calculate the rate of evaporation can not predict whether or not plastic shrinkage cracks occur. A quantitative method could perhaps be developed to predict plastic shrinkage cracking.
Cement and Concrete Research | 1992
David Bonen; Menashi D. Cohen
Abstract The mechanism of magnesium sulfate attack in portland cement and portland cement with silica fume paste was studied by investigating the microstructure and composition of specimens immersed for one year in magnesium sulfate solution. Results are presented in two parts. Part I, presents the microstructure of the pastes. Part II, presents the chemical and mineralogical variations across the specimen together with a proposed mechanism of magnesium sulfate attack. The magnesium sulfate attack resulted in the formation of the “surface double-layer” which was composed of brucite and gypsum layers. In addition, there was a sequential formation of inner gypsum layers resembling the occurence of lisegang bands found in rocks. The surface double-layer was composed of brucite, about 40 to 120 μm thick, and followed by a contiguous layer of gypsum, about 20 to 70 μm thick. Subsequently, up to four internal gypsum layers were deposited at depths of up to 1200 μm from the surface. These layers were parallel to the beam surfaces ranging in thicknesses from 15 to 60 μm. In portland cement with silica fume paste there were almost no inner gypsum layers, instead, massive deposition of dispersed gypsum crystals appeared up to at a distance of about 800 μm from the surface. The attack had apparently influenced the local degree of hydration of the cement constituents; near the surface only residual ferrite and belite crystals were observed, but in the middle, unreacted alite and belite were predominant.
Cement and Concrete Research | 1992
David Bonen; Menashi D. Cohen
Abstract Many of the chemical and mineralogical changes in pastes made with portland cement (PC) and with portland cement plus silica fume (PC-SF) subjected to magnesium sulfate attack occurred in a zone which was referred to as “transition zone”. This zone extended from the surface double-layer inwards where only few changes could be detected. The width of the transition zone varied according to the character of the paste, being greater for the PC paste. Among the characteristic features of the transition zone were the continuous increases in (1) the Ca/Si ratio of the CSH gel, and (2) the bulk CaO/SiO2 ratio of the paste from the surface inwards. In addition, the transition zone had a smaller amount of unhydrated cement particles and exhibited a strong differentiation in the relative amount of the residual phases. The mechanism of the magnesium sulfate attack involved the formation of two counter diffusion patterns. Hydroxide ions, (OH)−, diffused outwards from the paste, whereas SO42− ions diffused inwards from the surface. Due to the lower Ca/Si ratio of the CSH gel in the PC-SF paste and its alteration products, the PC-SF paste was more susceptible to the magnesium sulfate attack than the PC paste. A schematic model is presented which is based on the microstructure (presented in Part I) and the spatial chemical and mineralogical variations. The model attempts to provide explanation and trace the progression of magnesium sulfate attack.
Cement and Concrete Research | 2003
Manu Santhanam; Menashi D. Cohen; Jan Olek
Abstract Sodium sulfate attack was studied on C 3 S mortars, along with ASTM Type I Portland cement (PC) mortars, in an attempt to independently evaluate the effect of gypsum formation on the performance. The quantity of gypsum and ettringite, as measured by differential scanning calorimetry (DSC), increased with the time of immersion in the sulfate solution. An increase in length of the mortar specimens was also registered along with the increase in the quantity of gypsum. This result suggests that the formation of gypsum could be expansive. Indeed, considerable expansion, although delayed compared to PC mortars, was observed in the C 3 S mortars. Thus, it can be concluded that the expansion of the PC mortars occurred due to the combined effect of gypsum and ettringite formation, while the expansion of C 3 S mortars occurred as a result of gypsum formation. Thaumasite formation as small inclusions was also detected in both the C 3 S and the PC mortars, especially in regions of high gypsum deposition. The formation of thaumasite, despite the absence of carbonate bearing minerals and low temperatures, could be because of the carbonation of the surface zones of the mortars. However, it would be speculative to attribute any expansion to the formation of thaumasite, since it was detected only in minute amounts in the microstructural investigation.