International Concrete Abstracts Portal

The presence of chloride ions is one of the most widespread causes of corrosion initiation in reinforcing steel in concrete. Trace chlorides present in cementitious materials or admixtures typically result in very low fresh chloride contents in normal-strength concrete that do not present a danger of corrosion. UHPC mixture designs, however, use much higher dosages of cementitious materials and admixtures that can result in non-negligible total fresh chloride contents. These high chloride values are likely to occur more frequently in the future as more UHPC mixtures are made with locally available materials and alternative cementitious materials and may result in concrete mixtures failing to meet specifications for fresh chloride content limits that are based on mixture proportions used in normal-strength concrete mixtures. UHPC and normal concrete samples were made without fibers and with increasing levels of internally admixed chlorides for four different levels of strength to determine chloride thresholds for internally added chlorides. The chloride threshold for fresh concrete was measured using a slightly modified version of the accelerated test EN 480-14. The water-soluble and acid-soluble chloride ion content of UHPC mixtures tested were measured according to ASTM C1218 and Florida Method FM 5-516 to determine the bound chlorides and fresh chloride limits for corrosion. The results demonstrate that the UHPC had ~ 25% higher chloride threshold than the control mixture when measured as an absolute content per unit volume of concrete. When the UHPC chloride content is normalized by mass of cementitious material, it was found that the amount needed to initiate corrosion may be lower than fresh chloride limits given in ACI-318 and ACI 222. Therefore, the ACI-318 water-soluble chloride limits as a % by mass of cementitious materials were found to be non-conservative for the two of the UHPC mixtures tested and should be re-examined for UHPC.

Maintaining workability can be a challenge when the total cement content of a concrete mixture is minimized in order to lower the carbon footprint. This is especially the case in everyday concrete where Portland cement content is mostly optimized for a targeted strength. Unlike high-performance or self-consolidating concretes (SCC) which commonly have high cement or cementitious materials contents, a minimum paste volume is generally required in normal strength concrete (NSC) mixtures to ensure adequate workability for the application and to be acceptable in the field. In this study, a new generation of rheology-modifying water-reducing admixture that improves concrete rheology is used to further reduce cement content and provide favorable workability for concrete applications. Comparisons to reference concrete are presented for their fresh and hardened properties, including plastic viscosity, dynamic yield stress, finishability, pumpability, and targeted strength. By combining concrete technology and this new rheology modifying water-reducing admixture, an economical, workable low-carbon concrete can be achieved.

Electric arc furnace slag is a byproduct generated in the steel industry, which can be categorized into oxidizing slag (electric arc furnace oxidizing slag, EOS) and reducing slag (electric arc furnace reducing slag, ERS) depending on the manufacturing process. These slags contain ions such as Ca, Si, and Mg in unstable states, allowing them to form minerals through reactions with CO₂. In this study, we investigated the carbonation efficiency of cement mortar incorporating these slags as supplementary mineral admixture and aggregates, under varying CO₂ curing conditions. CO₂ curing was conducted under a 99% concentration CO₂ at a pressure of 10 bar. The curing temperatures were adjusted to 20℃ [68°F], 60℃ [140°F], and 90℃ [194°F], and compressive strength and carbonation depth were measured according to the slag substitution rate. Cement mortar with slag shows higher in both compressive strength and carbonation depth.

Earthen construction techniques in sustainable building can offer numerous advantages. However, it comes with certain limitations, with the most notable one being the labor-intensive and time-consuming nature of the construction process. To address this challenge, self-consolidating earth concrete (SCEC) emerges as a promising solution, particularly when dealing with the presence of fine clay and silt particles, as it can help attain the desired rheological properties more efficiently. In this study, supplementary cementitious materials (SCMs) such as cement, metakaolin, and limestone filler have been used as stabilizers to evaluate their impact on the workability and rheology of earth-based mixtures. A high-range water-reducing polycarboxylate ether (PCE), either with or without the initial incorporation of sodium hexametaphosphate, was applied to various clay compositions. The presence of finer clay particles required a higher dosage of admixture to achieve the desired workability, resulting in elevated yield stress and plastic viscosity values.

Calcined clays are a very promising supplementary cementitious material. A remaining challenge for a widespread application is the rheology of calcined clay containing cementitious materials. The water and superplasticizer demand and the viscosity of calcined clay binders are higher than for normal Portland cement (PC) binders. Consequently, there is a need to improve the understanding of the rheological properties of calcined clay-based binders. Here, we report on the rheological characteristics of four polycarboxylate ether superplasticizers in Portland cement and LC³ pastes. The superplasticizer chemistry is controlled through polymer synthesis. We chose simple slump flow tests to characterize the rheology of the pastes at different superplasticizer dosages. Furthermore, we characterize the initial reactivity of the binder using in-situ calorimetry. All four polymers exhibit very similar properties in the LC³ system, while the differences are much more prominent in Portland cement. The dosage efficiency in the LC³ system is lower for all polymers, and the most dosage-efficient superplasticizer in the LC³ system is only ranked third in Portland cement. Finally, the very early heat flow of the suspensions indicates that dissolution and early hydrate phase formation of the PC are promoted in LC³ systems. We propose that the increased PC reactivity is partly responsible for the larger slump loss of LC³ binders.