Colloidal nano silica is a highly reactive pozzolanic material that can refine the microstructure and improve the strength of concrete. Its effect has mostly been evaluated at the material level, while its influence on the flexural behavior of reinforced concrete beams and on finite element prediction of this behavior has received less attention. This paper presents an experimental and numerical investigation on plain and reinforced concrete beams made with 2.5% colloidal nano silica as a partial cement replacement. Beam specimens of 100 × 150 × 1360 mm were cast for plain and reinforced cases, using a control mix and a colloidal nano silica mix, with three specimens per condition. All beams were water-cured for 28 days and tested to failure under four-point bending. Finite element models were built in Abaqus using the concrete damaged plasticity model, with inputs from cylinder test data. The reinforced colloidal nano silica beam showed about 10% higher yield load, 5.8% higher ultimate load, and 22% greater pre-yield energy absorption than the control, while the plain colloidal nano silica beam carried about 10% more load at failure. However, the reinforced colloidal nano silica beam reached its ultimate load at a smaller deflection, indicating reduced ductility. The finite element predictions showed differences of 0.44% to 5.23% in ultimate load and 3.03% to 9.31% in mid-span deflection. The results show that colloidal nano silica improves the pre-yield stiffness and delays cracking in reinforced concrete beams but reduces their ductility. The model was preliminarily verified using the results obtained from the experimental study and is limited to the tested configurations.
