Issue 78

C. S. Karadumpa et alii, Fracture and Structural Integrity, 78 (2026) 146-167; DOI: 10.3221/IGF-ESIS.78.09

GO was dispersed in 30% of the mixing water using magnetic stirring at 900 rpm for 30 minutes, while CNTs were dispersed in diluted PCE using a three-stage magnetic stirring protocol to prevent agglomeration. Concrete was mixed in a laboratory pan mixer following a standardized sequence of dry mixing, partial water addition, incorporation of nano-suspensions, and final homogenization. Slump was measured immediately after mixing as per IS 1199 (Part 1):2018 [27]. A total of 216 specimens were cast, including cubes for compressive strength, cylinders for split tensile strength, and beams for flexural strength. All specimens were demoulded after 24±2 hours and water-cured at 27±2°C until testing ages of 7, 28, and 56 days. Mechanical testing followed IS 516 (Part 1):2021 [28] compressive strength, flexural strength and for split tensile strength, with mean values of three specimens reported for each test. The complete list of tests performed is provided in Tab. 4. Microstructural characterization included SEM for C–S–H morphology and ITZ analysis, XRD for phase identification and portlandite quantification, and EDAX for elemental ratios (Ca/Si, (Al+Fe)/Ca, S/Ca). All microstructural samples were prepared using standardized drying, mounting, and coating procedures. Type Test Remarks Fresh Properties Slump Test Workability assessment. Used to verify consistency and flow before casting

Compressive Strength

100×100×100 mm cubes. Loading rate: 14 MPa/min. Strength at 7, 28, 56 days

Mechanical Properties

Split Tensile Strength

100×200 mm cylinders. Tensile cracking behavior

Flexural Strength

100×100×500 mm beams. Four-point bending. Span: 400 mm C–S–H morphology. ITZ thickness. Crack bridging (CNT) Phase identification. Portlandite reduction. C–S–H crystallinity Ca/Si, (Al+Fe)/Ca, S/Ca ratios. Hydration product chemistry

SEM XRD

Microstructural Tests

EDAX

Table 4: List of tests conducted in the present study.

R ESULTS AND DISCUSSIONS

T

his section presents experimental findings for fresh properties, and the mechanical performance for all the experimental mixes.

Fresh properties Tab. 5 presents slump test results for all eight mix designs. The primary objective of the mix design was to achieve the target concrete grade (M30) while maintaining the specified water-to-binder ratio (w/b = 0.48). The workability target was considered during the preliminary proportioning stage. However, the actual workability of the mixtures was intentionally evaluated as an experimental response variable to assess the influence of GO and CNT incorporation. The control concrete without nanomaterials achieved a slump of 95 mm using conventional mix proportions (w/c = 0.48), meeting the target range of 100±10 mm and indicating satisfactory workability. Nano-modified mixes (Mixes 2-8) incorporating polycarboxylate superplasticizer at 0.4% by cement weight achieved significantly higher slump values (highest value of 165 mm) despite of the reduced cement content. This enhancement is attributed to the dispersing action of polycarboxylate molecules, which provide electrostatic repulsion and steric hindrance preventing cement particle flocculation. In addition, graphene oxide (Mixes 2-4) maintained excellent workability across all dosages, with GO's oxygen-containing functional groups contributing to aqueous dispersibility. However, Mix 4 (GO 0.05%) exhibited slight stiffening compared to lower dosages, suggesting the onset of GO restacking at higher concentrations. CNT-modified concretes (Mixes 5-7) showed progressive workability reduction with increasing dosage, with Mix 7 (CNT 0.3%) displaying notably reduced flow despite superplasticizer addition. This reflects the high aspect ratio and hydrophobic nature of carbon nanotubes, which increase water demand and viscosity at elevated concentrations. The hybrid formulation (Mix 8) demonstrated balanced workability, indicating that the combined GO and CNTs at moderate dosages (0.03% + 0.1%) does not produce excessive workability loss when properly dispersed. All nano-modified mixes exhibited uniform consistency without visible segregation or bleeding during visual inspection, confirming the effective implementation of the dispersion protocol implementation. The increased slump values (165 mm) provide adequate workability for placement and compaction while the relatively high viscosity characteristic of nano modified concretes helps prevent segregation during handling—a desirable combination for structural concrete applications.

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