Rachel J. Detwiler

Water permeability and chloride ion diffusion in portland cement mortars: Relationship to sand content and critical pore diameter

The pore structure of hydrated cement in mortar and concrete is quite different from that of neat cement paste. The porous transition zones formed at the aggregate-paste interfaces affect the pore size distribution. The effect of the sand content on the development of pore structure, the permeability to water, and the diffusivity of chloride ions was studied on portland cement mortars. Mortars of two water-to-cement ratios and three sand volume fractions were cast together with pastes and tested at degrees of hydration ranging from 45 to 70%. An electrically-accelerated concentration cell test was used to determine the coefficient of chloride ion diffusion while a high pressure permeability cell was employed to assess liquid permeability. The coefficient of chloride ion diffusion varied linearly with the critical pore radius as determined by mercury intrusion porosimetry while permeability was found to follow a power-law relationship vs. this critical radius. The data set provides an opportunity to directly examine the application of the Katz-Thompson relationship to cement-based materials.

Reaction kinetics of portland cement mortars hydrated at different temperatures

Abstract This work is a portion of a larger study of the microstructure and properties of concrete cured at temperatures of 5° to 50°C, a range chosen to reflect temperatures encountered in field curing. This paper discusses the overall hydration reaction kinetics of a 0.5 water/cement ratio portland cement mortar at ages up to 91 days. The results are correlated with microstructural observations of companion cement paste specimens. The apparent activation energy is found to be 11.2 kcal/mol for degrees of hydration between 20 and 30%, indicating that at this stage the rate of reaction is controlled by a chemical process. Beyond about 30% hydration the apparent activation energy gradually decreases.

Resistance to chloride intrusion of concrete cured at different temperatures

This paper desribes a preliminary investigation of the ability of concrete to protect against the corrosion of reinforcing steel. Two types of tests measured the rate of chloride diffusion. An accelerated corrosion test compared the ability of the concrete to protect against the corrosion of reinforcing steel. Both test methods are described. The results of both tests indicate that at a given water-cement ratio, elevated curing temperatures reduce the ability of portland cement concrete to protect against chloride diffusion and the consequent depassivation of reinforcement. This effect is more pronounced at lower water-cement ratios. These findings should be taken into account in the construction of concrete structures for which durability is a concern.

Use of Supplementary Cementing Materials to Increase the Resistance to Chloride Ion Penetration of Concretes Cured at Elevated Temperatures

It has long been known that elevated curing temperatures, while accelerating the early strength gain of concrete, reduce the ultimate strength. Recent research has shown that elevated curing temperatures can also reduce the resistance to chloride diffusion of plain portland cement concretes. This paper describes an investigation of the chloride penetration of 0.40 and 0.50 water-cement ratio (w/c) concretes containing either 5 percent silica fume or 30 percent blast furnace slag (substitution by mass) cured at elevated temperatures. Plain portland cement concretes were used as controls. The concretes were cured at constant temperatures of 23, 50, or 70 deg C to a degree of hydration of approximately 70 percent. Supplemental tests were performed on concretes cured overnight using a steam-curing regime. Both the silica fume and slag concretes performed better than the controls in these tests.

Backscattered electron imaging of cement pastes cured at elevated temperatures

Abstract The microstructure of cement pastes containing 5% silica fume or 30% ground granulated blast furnace slag was investigated in comparison with plain portland cement pastes. The pastes were cured at constant temperatures of 23 °C and 70 °C to degrees of hydration of 30 and 70% as indicated by the nonevaporable water content. Backscattered electron images indicate that in general, elevated temperature curing results in a coarser, more continuous pore structure. Both silica fume and slag are effective in reducing the size and continuity of the pores. These results are consistent with measurements of the rate of diffusion of chloride ions through comparable concretes subjected to the same curing conditions.

Water permeability and microstructure of three old concretes

Measurement of the permeability of concrete to water is complicated by the self-sealing phenomenon, the progressive reduction of flow during the test. Many researchers have attributed self sealing to the hydration of previously unreacted cement on exposure to water. This paper describes permeability tests on concretes continuously hydrated for 26 years. Backscattered electron images show that virtually no unhydrated cement remains in these specimens, yet they exhibit self-sealing behavior.

Texture of calcium hydroxide near the cement paste-aggregate interface

Abstract Several researchers have been using an X-ray diffraction technique to examine the orientation of calcium hydroxide crystals at the interface between cement paste and aggregate. Recently, however, the use of X-ray diffraction for this purpose has been called into question because the goniometer sweeps through only a single arc, thus missing any crystals which are not oriented with either a (0001) plane or a (10 1 1) plane parallel to the interface. This paper describes the degree of preferred orientation of calcium hydroxide crystals using pole figures. The results confirm the validity of the X-ray diffraction technique and provide new information about the nature of the cement paste-aggregate interface.

SUBCRITICAL CRACK GROWTH IN THE CEMENT PASTE-STEEL TRANSITION ZONE

Abstract The shape of the stress-strain curve for most concretes may be explained in terms of subcritical crack growth in the transition zone between coarse aggregate particles and the cement paste matrix. This paper discusses an investigation of subcritical crack growth in composite double torsion specimens made of steel and cement paste containing silica fume, carbon black, or plain portland cement. At age 7 days, the specimens containing carbon black proved to be less resistant to crack propagation in the transition zone than the other specimens. This result is consistent with the stress-strain behavior of concretes containing carbon black. An examination of the microstructure showed that the carbon black cement paste contained larger and more numerous voids than a comparable silica fume cement paste.

ETTRINGITE DEPOSITS IN AIR VOIDS

Investigations of pavements suffering from freeze-thaw damage in the presence of deicing salts have found significant deposits of ettringite in the air voids. Concern has been expressed that the deposition of ettringite in air voids reduces their effectiveness in protecting the concrete. A series of experiments intended to reproduce the process of deterioration in the laboratory allowed the authors to observe the sequence of events. A suite of cements was selected to provide a range of C3A contents, sulfate contents, and sources of sulfate within the cement (that is, either the clinker or the gypsum). Specimens were subjected to a modified version of ASTM C666 in which salt solutions with and without added gypsum were substituted for the water, and the specimens were allowed to dry periodically. Sacrificial companion specimens were examined petrographically, and the observations were correlated with mass loss, fundamental transverse frequency, and length change. The results demonstrate that the deposition of ettringite does not adversely affect the ability of the air voids to protect the concrete. The sequence of events (microcracks first, then ettringite deposits), the morphology of the ettringite (long, loosely packed needles), and the lack of association of air voids containing significant ettringite deposits with cracks all indicate that the deposition of ettringite is opportunistic. The observed large deposits of ettringite are a consequence of saturation and freeze-thaw damage, not a cause.