Use a mix that’s tried in the field – a 1:2:3 cement to sand to gravel ratio with a high‑strength cement, about 400–500 kg/m³, works well. Add 10‑15 % fly ash to cut cost and improve ductility, and use a proper seismic reinforcement layout – vertical bars at 150 mm spacing, bend at 45° on corners. Keep the water‑cement ratio low, around 0.45, to keep strength up. That’s the recipe that holds up in earthquakes and keeps the bill down.