If you’ve been following Alex Zamora on Instagram, you might have seen him mention a 3000 psi concrete mix in one of his posts. This medium-strength design is ideal for planters, countertops, benches, or decorative architectural elements where a high-quality, polished finish is essential. In this article, we’ll break down the recommended mix design, workability tips, and surface finishing techniques to help you replicate that signature Zamora look.
Recommended Concrete Mix Design (3000 psi / 20 MPa, Pea Gravel)
| Material | Proportion (by weight) | Per Cubic Meter |
| Cement (Type I/II) | 1 part | 350 kg |
| Pea Gravel (10mm or smaller, rounded) | 2.5 parts | 875 kg |
| Washed Sand (medium sharp sand) | 2 parts | 700 kg |
| Water | 0.5 parts (w/c ratio 0.50) | 175 kg |
| Plasticizer (optional) | as needed | per manufacturer |
Approximate Yield: 1 cubic meter (adjust as needed for smaller batches)
Target Slump: 75-100mm (3-4 inches) for ease of placement without excessive water addition.
Key Characteristics of this Mix
- Smooth Finish: The small, rounded pea gravel reduces surface voids and provides a denser, more uniform finish, ideal for polishing.
- Workability: Moderate workability with good cohesion, reducing the risk of segregation and honeycombing.
- Shrinkage Control: Lower paste content helps reduce drying shrinkage and cracking.
- Mix Flexibility: If you want a lighter color, you can replace the pea gravel with 10mm limestone aggregate. Avoid stones larger than 10mm, as they are harder to compact and can leave holes if dislodged.
Workability Tips for Different Weather Conditions
- Cold Weather: Add a rapid-setting admixture to speed up curing and reduce the risk of weak spots. This helps the concrete set properly even in low temperatures.
- Warm Weather: Use a superplasticizer or workability aid to keep the mix workable longer. This is especially useful if you are working alone and need more time to finish the surface.
- Surface Moisture: Spray water or a suitable finishing liquid on the surface while troweling to keep it easy to rub and achieve a high sheen.
Technique for Power-Float-Like Finish
Formwork Setup
- Use well-sealed, smooth-faced formwork to reduce bug holes and achieve a clean face.
- Make molds easy to strip – avoid burying screws or nails completely, so they can be quickly removed without damaging the formwork.
Initial Placement
- Pour the mix, ensuring even distribution without excessive vibration to avoid air entrapment.
Early Form Removal (Striking)
- Formwork can typically be removed 2-4 hours after pouring, depending on weather conditions. For example, a slab poured on a hot summer morning can often be walked on by the evening, while the same slab poured on a cold winter day may still be too soft to walk on 24 hours later.
Surface Working
- Use a range of trowels for different edges and textures, including bullnose trowels, corner trowels, and finishing trowels.
- If you want a rougher finish, use a sponge to rub the surface after troweling. This can help remove small imperfections and create a more textured, natural look.
Curing
- Properly cure the concrete to avoid rapid moisture loss, which can lead to surface cracking. Use a curing compound or keep the surface damp and covered for at least 7 days.
Conclusion
This mix design and finishing technique will give you a high-quality, smooth concrete surface with a dense, polished finish. With the right preparation and timing, you can achieve professional-level results on your DIY concrete projects, just like those featured by Instagram creators like Alex Zamora.