International Concrete Abstracts Portal

This experimental study evaluated the correlation between measured concrete expansion from a modified version of the miniature concrete prism test (MCPT) with the concentration of chemical markers leached from the prisms into an alkaline soak solution. Fifteen concrete mixture designs were tested for expansion and soak solution concentrations over time. The changes in expansion and soak solution concentrations were found to correlate well even with variations in alkali loading and substitution of cement with Class F fly ash. A model was developed to estimate the expansion potential of concrete based on an expansion reactivity index (ERI) that incorporated the concentrations of silicon, sulfate, calcium, and aluminum. The relationship between ERI and expansion was then used to identify potentially expansive concrete mixtures using the ERI of cores taken from a structure exhibiting potential alkalisilica reaction (ASR) expansion and concrete cylinders matching the mixture designs of the MCPT specimens.

Alkali-silica reaction (ASR) is one of the most harmful distress mechanisms affecting concrete infrastructure worldwide. ASR is a chemical reaction that generates a secondary product, which induces expansive pressure within the reacting aggregate material and adjacent cement paste upon moisture uptake, leading to cracking, loss of material integrity, and functionality of the affected structure. In this work, a computational homogenization approach is proposed to model the impact of ASR-induced cracking on concrete stiffness as a function of its development. A representative volume element (RVE) of the material at the mesoscale is developed, which enables the input of the cracking pattern and extent observed from a series of experimental testing. The model is appraised on concrete mixtures presenting different mechanical properties and incorporating reactive coarse aggregates. The results have been compared with experimental results reported in the literature. The case studies considered for the analysis show that stiffness reduction of ASR-affected concrete presenting distinct damage degrees can be captured using the proposed mesoscale model as the predictions of the proposed methodology fall in between the upper and lower bounds of the experimental results.

Ettringite-based binders are used in niche applications that require a high compressive strength in a very short period of time to minimize construction times and disruption to the traveling public or user. High early strength is achieved due to the formation of ettringite within the first few hours of hydration, which results in a non-expansive system capable of reaching strengths of approximately 20 MPa (2900 psi) in the first 3 hours of hydration. Ettringite-based binders also carbonate at a faster rate than ordinary portland cement (PC)-based systems as a result of the limited amount or absence of portlandite (CH). Carbonation results in the conversion of ettringite into products that occupy less space, resulting in a loss of strength and increased porosity. The formation of ettringite is achieved through the use of systems composed of calcium aluminate cement (CAC, main phase CA) and calcium sulfate (CS), or calcium sulfoaluminate cement (CSA, main phase C4A3S) interground with calcium sulfate. In many cases, ettringite-based binders are used to accelerate ordinary PC-based systems to achieve a relatively high compressive strength and a suitable working time while still maintaining the hydration characteristics of PC. This study compared the performance of carbonated and noncarbonated concrete in terms of the resistance of the near-surface concrete to deicer-salt scaling and chloride ion penetration. Chloride binding tests were also conducted on the carbonated and noncarbonated cement paste samples and mechanical properties conducted on mortar specimens. The results show that CSA-based binders carbonate at a much faster rate than both CAC and PC based systems as a result of the increased ettringite content within the system. A decrease in the mechanical properties of carbonated ettringite-based binders is also observed as a result of the conversion of ettringite.

To improve the early-age cracking resistance of self-consolidating concrete (SCC), this paper investigated the effects of an expansive agent (EA), fibers, and the interaction between EA and fibers on the cracking behavior of restrained SCC caused by plastic shrinkage based on the slab test. Twenty-one types of samples were prepared, including one control group, two EA contents (6 and 8% of the mass fractions of cementitious materials), three steel fiber contents (0.25, 0.50, and 0.75% by volume), three polypropylene fiber contents (0.05, 0.10, and 0.15% by volume), three hybrid fiber contents, and nine combinations of EA (8% of the mass fraction of cementitious materials) and fibers. The initial cracking time and propagation of cracks over time were both observed. Test results indicate that an increase of EA dosage presents no significant improvement on early-age cracking resistance capability. Compared with steel fiber (SF), polypropylene fiber (PP) with equivalent fiber factors was particularly effective in reducing the nominal total crack area. In general, crack reduction factors of fiber-reinforced expansive self-consolidating concrete (FRESCC) are 70% greater than that of SCC containing fiber only. It indicates that the combination of EA and fibers enable SCC to present better early-age cracking resistance.

Corrosion-related cracking in reinforced concrete is caused by expansive corrosion products and the resulting tensile stresses. While the amount of corrosion to cause cracking has been studied for uncoated conventional reinforcement, significantly less is known about the corrosion loss at cracking for galvanized reinforcement. Conventional and galvanized bars were cast in chloride-contaminated concrete. Clear cover to the bar ranged from 0.5 to 2 in. (12.7 to 51 mm). Specimens were tested both with and without the use of impressed current to drive corrosion. It was found that galvanized reinforcement requires greater corrosion losses to crack concrete than conventional steel reinforcement. Visual observations at autopsy suggest that the cracking of the concrete specimens containing galvanized reinforcement was not due to zinc corrosion products, but rather to corrosion products from intermetallic iron-zinc layers or from the underlying steel. Further study is needed to determine the exact nature of these corrosion products. Tests using impressed current may be used to establish the corrosion loss required to cause cracking.