International Concrete Abstracts Portal

This study evaluates semi-refined (SR-) kappa (κ)-carrageenan extracted from Kappaphycus alvarezii as a viscosity-modifying admixture (VMA) for cement pastes (water-cement ratio [w/c] = 0.43). Extraction parameters, potassium hydroxide (KOH) concentration (6 to 12%), temperature (104 to 176°F [40 to 80°C]), and residence time (1 to 3 hours) were varied, and the resulting materials were assessed for modified Bingham rheology (yield stress and plastic viscosity​), viscoelastic properties (storage modulus and critical shear strain), structural build-up kinetics (build-up index), forced bleeding, hydration induction period, and early compressive strength. SR-(κ)-carrageenan produced under a low-severity operating point (6% KOH, 104°F [40°C], 1 hour) and dosed at 4% (by mass of water) increased yield stress​ from 0.21 to 0.42 lb/ft2(10 to 20 Pa) and plastic viscosity​ from 0.42 to 0.80 lb/ft–s (0.63 to 1.18 Pa∙s). It enhanced early-age rigidity and build-up from 9.29 to 22.03 lb/ft2∙s–3/2 (445 to 1055 Pa/s3/2) and reduced forced bleeding relative to the reference. The induction period increased moderately (from 1.4 to 2 hours), and no early-age strength penalty was observed at 1 and 7 days. Compared with pure R-(κ)-carrageenan, SR-(κ)-carrageenan maintained (κ)-carrageenan rheology and structural build-up kinetics while producing a smaller induction-time shift in this paste class. Within the tested matrix, dosages, and protocols, the test results support SR-(κ)-carrageenan as a cement-grade VMA candidate produced through processed Eucheuma seaweed/alkali-treated cottonii (PES/ATC)-aligned unit operations.

Self-consolidating, rapid-hardening concrete is a sought-after product; however, the effects of admixtures on fresh and hardened concrete rheology are still uncertain. This study aims to create a self-consolidating concrete (SCC) using calcium sulfoaluminate (CSA) cement for its rapid-hardening properties and evaluate two set-time retarders and two viscosity-modifying admixtures (VMAs) to determine how they affect the properties of concrete. The properties studied were workability, viscosity, set time, air content, compressive strength, freezing-and-thawing resistance, and hardened air-void content of SCC. The study found that tartaric acid (TA) creates a very workable SCC with consistent results regardless of the VMA used, while citric acid (CA) only works well with hydroxypropyl methylcellulose (HPMC) as a VMA and kills viscosity-modifying properties of welan gum (WG).

In this work, novel polycarboxylate admixtures were synthesized by two different free radical polymerization systems: methacrylic acid (MAA) and methoxy polyethylene glycol methacrylate (MPEG-MA) for PC-1, and acrylic acid (AA) and isoamyl alcohol polyethylene glycol (IAA-PEG) for PC-2. Thioglycolic acid as a chain transfer agent and ammonium persulfate as an initiator were used. The synthesized carboxylic polymers were characterized using Fourier-transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), and thermogravimetric analysis (TG). The influence of the chemical structure of polycarboxylates on the rheology of the concrete, as well as the prognosis of the superplasticizer’s development, is also presented through measuring of water consistency, setting times, flow table, slump test, zeta potential, and compressive strength. The cementitious products were investigated with X-ray diffraction (XRD) and scanning electron microscope (SEM). The developed superplasticizers showed good dispersion effects and slump performance in workability and fluidity retention tests, adsorption performance, and SEM performance. Intriguingly, the PC-1 and PC-2 mixtures achieved flow table values of 230 and 200 mm, respectively. The compressive strength values at various curing ages up to 28 days exhibited double and triple values compared with the control sample. Additionally, compared to the control ordinary portland cement paste, a reduction of the water-cement ratio (w/c) of approximately 0.25 and the development of excessive hydration products give PC-1 and PC-2 extensive pastes a more dense and compact structure in XRD and SEM investigation.

This study evaluated the performance of recycled brick-concrete aggregate concrete (RB-CAC) incorporating both recycled brick aggregate (RBA) and recycled concrete aggregate (RCA). It further examined the reinforcement effects of polypropylene macrofibers (PPMF) on the composite and assessed its mechanical properties and frost resistance. The results showed that incorporating 15% RBA reduced the compressive and splitting tensile strengths of concrete by less than 20%, while the peak load decreased by 28.8%. Fiber incorporation effectively mitigated compressive strength degradation and significantly enhanced tensile strength, with the optimum fiber dosage at 0.9% by volume. However, RBA incorporation reduced frost resistance, resulting in a 37.6% strength loss and a 40.6% mass loss after 100 freeze-thaw cycles. In contrast, a 0.6% fiber admixture improved frost resistance, reducing strength loss and increasing the relative dynamic elastic modulus by 26.7%. Finally, the study established a frost-resistance durability prediction model based on PPMF and RBA content.

Crack densities obtained from on-site surveys of 74 bridge deck placements containing concrete mixtures with paste contents between 22.8 and 29.4% are evaluated. Twenty of the placements were constructed with a crack-reducing technology (shrinkage- reducing admixtures, internal curing, or fiber reinforcement) and 54 without; three of the decks with fiber reinforcement and nine of the decks without crack-reducing technologies involved poor construction practices. The results indicate that using a concrete mixture with a low paste content is the most effective way to reduce bridge deck cracking. Bridge decks with paste contents exceeding 27.3% had a significantly higher crack density than decks with lower paste contents. Crack-reducing technologies can play a role in reducing cracking in bridge decks, but they must be used in conjunction with a low-paste-content concrete and good construction practices to achieve minimal cracking in a deck. Failure to follow proper procedures to consolidate, finish, or cure concrete will result in bridge decks that exhibit increased cracking, even when low paste contents are used.