Kimberly E. Kurtis

Time to failure for concrete exposed to severe sulfate attack

Abstract In the 1940s, the U.S. Bureau of Reclamation (USBR) began a long-term, nonaccelerated laboratory test program to determine the influence of a variety of concrete-mix parameters on resistance to severe sulfate exposure conditions. This paper reports the time of failure of these samples as influenced by their water-to-cement (w/c) ratio, cement composition, and percent replacement of cement with fly ash. The analysis indicates that there is a “safe zone” for concrete made with w/c ratio lower than 0.45 and cement with unhydrated tricalcium aluminate (C 3 A) content lower than 8% where failure did not occur within the 40-year exposure period. As expected, concrete samples cast with high amount of C 3 A failed after a relatively short time of sulfate exposure. Expansion tests indicated that cements containing high amounts of C 3 S may lead to premature failure of concrete, even when moderate w/c ratios are used. Samples prepared with 25% and 45% replacement of cement with fly ash showed significantly less expansion than comparable mixtures containing no pozzolans.

Imaging of ASR Gel by Soft X-ray Microscopy

Abstract The soft x-ray transmission microscope XM-1 was used to examine alkali-silicate reaction (ASR) gel morphology in an experimental investigation of the alkali-aggregate reaction (AAR). The XM-1 microscope is operated by the Center for X-ray Optics on beamline 6.1 of the Advanced Light Source, a third generation synchrotron radiation facility operated by the Ernest O. Lawrence Berkeley National Laboratory. The instrument is unique as samples can be observed wet, with high resolution (43 nm), over time, as chemical reactions proceed. Soft x-ray microscopy was used to examine the in situ reaction of ground ASR gel, obtained from a large dam, and solutions of sodium hydroxide, calcium hydroxide, and combined sodium and calcium hydroxide. From this investigation, it appears that ASR gel combines with alkalis present in pore solution to produce a reaction gel capable of swelling, while the reaction of the ASR gel in the presence of calcium hydroxide and no alkalis results in the formation of a structure resembling C-S-H. It is theorized that the formation of C-S-H or a related compound will decrease the degree of swelling that would otherwise result from the formation of an alkali-aggregate reaction product. The C-S-H-like structure may also contribute strength. These hypotheses are currently under investigation.

Proposed mechanism of C-S-H growth tested by soft X-ray microscopy

Abstract The reaction of silica particles in solutions saturated or supersaturated with respect to portlandite is observed by transmission soft X-ray microscopy, and a model for calcium silicate hydrate (C-S-H) growth is proposed which corresponds with the morphology observed in the X-ray images. These images show a product that consists of bundles of dendrites that appear to diverge as the growth process continues, resulting in the formation of a characteristic morphology which is narrowest in the middle and broadened at the end — the “sheaf-of-wheat” morphology. In many cases, the dendrites appear to be oriented about a common axis, and it is proposed that this axis corresponds to a common origin in a single crystalline nucleus.

Utilization of Savannah Harbor river sediment as the primary raw material in production of fired brick

A laboratory-scale study was conducted to assess the feasibility of the production of fired bricks from sediments dredged from the Savannah Harbor (Savannah, GA, USA). The dredged sediment was used as the sole raw material, or as a 50% replacement for natural brick-making clay. Sediment bricks were prepared using the stiff mud extrusion process from raw mixes consisted of 100% dredged sediment, or 50% dredged sediment and 50% brick clay. The bricks were fired at temperatures between 900 and 1000 °C. Physical and mechanical properties of the dredged sediment brick were found to generally comply with ASTM criteria for building brick. Water absorption of the dredged sediment bricks was in compliance with the criteria for brick graded for severe (SW) or moderate (MW) weathering. Compressive strength of 100% dredged sediment bricks ranged from 8.3 to 11.7 MPa; the bricks sintered at 1000 °C met the requirements for negligible weathering (NW) building brick. Mixing the dredged sediment with natural clay resulted in an increase of the compressive strength. The compressive strength of the sediment-clay bricks fired at 1000 °C was 29.4 MPa, thus meeting the ASTM requirements for the SW grade building brick. Results of this study demonstrate that production of fired bricks is a promising and achievable productive reuse alternative for Savannah Harbor dredged sediments.

Influence of Additions of Anatase TiO2 Nanoparticles on Early-Age Properties of Cement-Based Materials

The performance and properties of cement-based materials can potentially be altered by the addition of nano-sized inclusions. In this study, the effect of chemically nonreactive anatase TiO2 nanoparticles on early-age hydration of cement was investigated. First, the effects of different percentage addition rates of TiO2 to portland cement on early-age behavior were examined through isothermal calorimetry and measurements of chemical shrinkage. On the basis of accelerations in hydration observed in TiO2 portland cements, additional experiments were performed with tricalcium silicate (C3S), the main strength-giving mineral component of portland cement, to determine whether the influence of TiO2 could be adequately described by a kinetic model that relies on boundary nucleation theory. Comparison of the experimental results and the modeling showed that (a) an increase in addition rates of TiO2 accelerates the rate of cement hydration and (b) the heterogeneous nucleation effect rather than the dilution effect was dominant. The result of the boundary nucleation model reinforces the concept of the heterogeneous nucleation effect and demonstrates that the surface area provided by nano-TiO2 particles increases the rate of hydration reaction. This research forms the foundation for future studies aimed at optimizing photocatalytic and other nanoparticle-containing cements.

Effect of Nano-sized Titanium Dioxide on Early Age Hydration of Portland Cement

The effect of nano-scale non-reactive anatase titanium dioxide (TiO2) on early age hydration of cement was experimentally studied. Isothermal calorimetry was performed on cement pastes with two different particle sizes of TiO2 at replacement levels of 5, 7.5 and 10%. The addition of TiO2 to cement increased the heat of hydration and accelerated the rate of reaction at early stages of hydration. This increase was found to be proportional to the percentage addition and the fineness of TiO2. These results demonstrate that the addition of non-reactive nano-scale fillers could affect the rate of cement hydration by heterogeneous nucleation.

Chemical additives to control expansion of alkali-silica reaction gel: proposed mechanisms of control

Calcium chloride, lithium chloride, and acetone have previously been shown to affect expansion caused by alkali-silica reaction (ASR), a deleterious reaction occurring between reactive siliceous minerals present in some aggregate and the strongly alkaline pore solution in concrete. Here, the effect of these chemical additives was examined by transmission soft X-ray microscopy and a quantitative elemental analysis, using ICP-OES. In examining the effect of calcium chloride on ASR gels, the formation of a calcium silicate product, believed to be non-expansive, was identified by X-ray microscopy. Additionally, the elemental analysis results suggest that the concentration of calcium ions in the pore solution, which is dependent upon the solubility of the chemical additive and the percent addition, relative to the concentration of silica species in solution is an important parameter for effective control of expansion associated with ASR. In examining the effect of lithium chloride, quantitative elemental analysis showed silica dissolution in solutions of 0.7 M NaOH + 0.1 M LiCl, but with silicon present in slightly lower concentrations than in 0.7 M NaOH solutions alone. However, X-ray microscopy showed less evidence of repolymerization of the dissolved silica into an expansive gel in the presence of lithium chloride as compared to the reaction of the ASR gel in alkaline (0.7 M NaOH) solutions without lithium. With acetone, the results, including X-ray images showing the formation of repolymerized gel in 0.7 M NaOH solution containing 10% v/v acetone, indicate that the use of acetone as a chemical additive may not be as effective as once believed in preventing expansion caused by ASR. It is proposed that any reduction in expansion by use of acetone is temporary and diminishes over time.

EMPIRICAL MODELS TO PREDICT CONCRETE EXPANSION CAUSED BY SULFATE ATTACK

The development of empirical models to predict concrete expansion produced by sulfate attack is described. Data collected by the U.S. Bureau of Reclamation over more than 40 years of nonaccelerated testing form the basis for the model. In the nonaccelerated test program, concrete cylinders were continuously submerged at room temperature in 2.1% sodium sulfate solution, which corresponds to severe field exposure conditions. Expansion measurements were made periodically. Over 8,000 expansion measurements were collected for 114 specimens cast from 51 different mixtures. Analysis of the data showed the significance of water-to-cement ratio (w/c) and C3A content of the cement, with the data revealing distinct behavior for mixtures containing cements with low C3A content (less than 8%) and high C3A content (greater than 10%). As a result, two models are proposed to predict expansion by sulfate attack as a function of w/c, duration of exposure, and C3A content. The random effects method was used to capture unobserved heterogeneity in the data set, and the results from a simple regression and a regression including the random effects method are compared.

Theoretical and experimental study of the nonlinear resonance vibration of cementitious materials with an application to damage characterization

This paper presents a theoretical and experimental study of the nonlinear flexural vibration of a cement-based material with distributed microcracks caused by an important deterioration mechanism, alkali-silica reaction (ASR). The general equation of motion is derived for the flexural vibration of a slender beam with the nonlinear hysteretic constitutive relationship for consolidated materials, and then an approximate formula for excitation-dependent resonance frequency is obtained. A downward shift of the resonance frequency is related to the nonlinearity parameters defined in the constitutive relationship. Vibration experiments are conducted on standard mortar bar samples undergoing progressive ASR damage. The absolute nonlinearity parameters are determined from these experimental results using the theoretical solution in order to investigate their dependence on the damage state of the material. With the progress of the ASR damage, the absolute value of the hysteresis nonlinearity parameter increases by as much as six times from the intact (undamaged) state in the sample with highly reactive aggregate; this is in contrast to a change of about 16% in the linear resonance frequency. It is demonstrated that the combined theoretical and experimental approach developed in this research can be used to quantitatively characterize ASR damage in mortar samples and other cement-based materials.

Nonlinear Wave Modulation Spectroscopy Method for Ultra-Accelerated Alkali-Silica Reaction Assessment

In order to predict potential aggregate alkali-silica reactivity, there was development and assessment of an ultra-accelerated testing method using advanced nonlinear ultrasonic techniques. There was observation of the nonlinear interaction of propagating acoustic waves being affected by very early alkali-silica reaction (ASR) gel formation during standard accelerated mortar bar exposure (ASTM C1260 or AASHTO T 303). There was observation, furthermore, of a clear distinction of varying reactivity in the nonlinearity parameter for three different aggregates. Compared to a 14-day standard accelerated mortar bar test method test period, aggregate reactivity could be distinguished in as early as four days through the spectroscopic technique. Study results suggests that further development of this method could result in very rapid screening of aggregates for alkali reactivity and very early detection of ASR damage in the laboratory.