Concrete Work for DIY Builders

Bags or ready-mix · Reinforcement and release oil · Joints and surface treatment · Step by step

Concrete is the most widely used building material in the world — and one of the most common tasks for DIY builders. Whether you are pouring a garage floor, a foundation for a garden building, stair footings or a column, the principles are the same: correct volume, correct strength class, correct reinforcement and correct curing. This guide covers the entire process from planning to finished surface.

Layer build-up — from bottom to top

A correctly constructed ground-bearing concrete slab consists of several layers. Each layer serves a purpose — skip one and the result is compromised.

1. Subgrade / compacted fill — load-bearing ground with a frost-safe solution for the actual site
2. Gravel / aggregate, 150–200 mm — capillary break layer, prevents moisture wicking
3. PE membrane / construction membrane to suit the system — vapour or moisture barrier against the ground, with overlaps and taping in accordance with the manufacturer's instructions
4. Expansion strip along edges — isolates slab from foundation wall, absorbs movement
5. Reinforcement mesh on spacers — position and cover must follow the design or documented system solution
6. Concrete with selected strength class and thickness — strength class, exposure class and slab thickness must be chosen from load, environment and the designed solution
7. Levelled surface / self-levelling compound if required — ready for floor covering

Bags vs. ready-mix concrete

• Under 0.30 m³ — bagged concrete (25 kg = approx. 0.0125 m³) is simplest and cheapest
• 0.30–1.0 m³ — consider bulk bag concrete (IBC) or bagged depending on access
• Over 1.0 m³ — ready-mix truck delivery (minimum order typically approx. 3 m³)

Concrete strength classes

Concrete is classified by its characteristic compressive strength after 28 days of curing (C[cylinder]/[cube] MPa). For DIY work, C20/25 and C25/30 are the most common classes.

Reinforcement

Plain concrete is strong in compression but weak in tension. Reinforcement is used where the design requires it to carry tension, limit cracking and provide the required robustness and capacity. For load-bearing structures, reinforcement and detailing must always be designed or otherwise documented.

Reinforcement mesh — for slabs

Reinforcement mesh consists of welded steel mats supplied in standard sheets (typically 2.35 × 5.85 m = 13.7 m²) or rolls. They are faster to lay than individual bars and provide uniform reinforcement distribution.

• Place mesh on plastic spacers (25–30 mm) — ensures correct cover
• Overlap between sheets: min. 2 mesh spacings = approx. 300 mm
• Tie overlaps with binding wire
• Calculate net area + 15–20% for overlaps when ordering

Rebar — for foundations and columns

Strip foundations and columns use longitudinal bars and stirrups. Longitudinal bars carry load; stirrups hold them in position and prevent buckling.

• Strip foundations and columns must be reinforced in accordance with the project design. Bar count, diameters and stirrup spacing cannot be fixed generically in a simplified guide
• Stirrup dimensions and cutting lengths must be calculated from the actual geometry, bend radius, cover and reinforcement system
• Cover: 25 mm against internal formwork, 50 mm against soil
• Lap splice length: min. 40 × bar diameter (e.g. ø12 → 480 mm)

Formwork and release oil

Formwork holds the concrete in place while it cures. Adequate application of release oil is essential so the formwork can be stripped without damaging the concrete surface.

Formwork materials

• Formwork board 22×100 mm — economical, can be reused 3–5 times
• OSB sheet 18 mm — for larger flat formwork and walls, smoother surface finish
• Column formers / shuttering tubes — for columns and some prefabricated edge solutions
• Stakes and wedges — keep formwork straight and stable against concrete pressure; bracing requirements depend on height, geometry and pour rate

Release oil

• Approved release oil (vegetable or mineral-based) — apply by brush or spray
• Typical consumption: 0.10–0.20 L/m² of formwork area
• Use only approved release agents. Used engine oil and other unsuitable oils should not be used
• Allow 15–30 minutes for the oil to be absorbed before pouring

PE membrane and capillary break layer

Under all ground-bearing concrete slabs, a capillary break layer and a moisture barrier must be installed. Without these, ground moisture will wick up into the slab and damage floor coverings and indoor air quality.

• Gravel/aggregate 8–32 mm, min. 150 mm thick — breaks capillary rise from the ground
• Construction membrane to suit the system — installed as the moisture / vapour barrier in accordance with the project or documented system
• Overlap at joints: min. 200 mm, tape all joints
• Turn membrane up at edges and clamp in place with the expansion strip
• Calculate area + 10% for overlaps when ordering

Expansion joints and edge strip

Concrete moves due to temperature, shrinkage and restraint. The joint layout should limit uncontrolled cracking and must be adapted to slab thickness, geometry, reinforcement and drying conditions.

Expansion strip along edges

• Installed around the full perimeter against the foundation wall — 10 mm thickness is standard
• Also acts as thermal insulation and prevents cold bridging
• Available in EPS (white) or mineral wool — EPS is most common for outdoor use
• Calculate: perimeter of slab = 2 × (L + W) lm

Construction joints in large slabs

• The need for control joints must be assessed from slab thickness, geometry, restraint and reinforcement. A common rule of thumb is joint spacing of about 24–36 times the slab thickness, but the designed solution takes precedence
• Saw-cut control joints when the concrete is ready for cutting without raveling, and with depth and timing suited to the chosen method and documented practice
• Any sealing or joint filling must be selected for the movement, exposure and intended use, in accordance with the sealant manufacturer's instructions

Admixtures

Admixtures are mixed into the concrete to modify its properties. For bagged concrete, follow the manufacturer's mixing instructions; admixtures are most relevant for ready-mix orders.

Surface treatment and curing

Fresh concrete is porous and grey. Surface treatment protects against moisture, frost and chemicals, and improves appearance.

Curing compound

• Sprayed onto fresh concrete immediately after finishing
• Prevents premature evaporation of water — critical in hot and/or windy weather
• Consumption: approx. 0.15–0.20 L/m²
• Alternative: cover with plastic sheeting for 7 days

Concrete sealing and coating (after curing)

• Impregnation — a breathable water-repellent treatment may be suitable outdoors. Select the product for the exposure class and follow the data sheet for coverage and application
• Epoxy coating — dense and wear-resistant for some interior floors, but only where residual moisture, substrate preparation and primer all meet the system requirements. Follow the supplier's data sheets
• Concrete paint / floor paint — lighter treatment where the selected system is suitable for the substrate and the intended use

Step-by-step guide

Step 1 — Planning and excavation

• Mark out corners with a string line — check diagonals for square (equal diagonals = right angle)
• Excavate to the designed depth and frost-safe solution for the site + slab thickness + gravel layer
• Lay drainage pipe at the base if groundwater is a concern, then compact gravel in layers

Step 2 — PE membrane and expansion strip

• Lay PE membrane with 200 mm overlap and tape all joints
• Turn membrane up at edges — tape to existing wall or foundation
• Install expansion strip (10 mm EPS) around the full perimeter

Step 3 — Formwork

• Fix formwork boards along the outer edges to the correct level — this defines the finished slab height
• Brace the formwork sufficiently for its height, geometry and the expected concrete pressure
• Apply 2 coats of release oil to the inside face — allow 15–30 min to absorb

Step 4 — Reinforcement

• Place suitable spacers so the reinforcement stays at the designed level and maintains the required cover throughout the pour
• Lay reinforcement mesh with 300 mm laps — tie with binding wire
• The reinforcement mesh must be placed at the designed level. For crack-control reinforcement in slabs-on-ground it is often placed in the upper half / upper third, but the final position depends on the structural solution
• For double reinforcement, both the upper and lower layers must have the specified cover and the correct spacing between layers

Step 5 — Mixing and pouring

• Mix bags per instructions — typically 3.0–3.5 L of water per 25 kg. Never add more water than specified
• Pour from the lowest point and work upwards — do not drag concrete sideways with a shovel. Place a slab in one full-depth operation; deep pours such as walls and columns are placed and compacted in layers (lifts) so each layer is properly compacted before the next
• Compact the concrete with suitable equipment and method without over-vibration. Execution depends on consistency, thickness, reinforcement density and the selected concrete mix
• Strike off the surface continuously with a screed board

Step 6 — Screeding and finishing

• Draw a screed board across the top of the formwork for final levelling
• Steel-float the surface after approx. 1–2 hours (once bleed water has been absorbed)
• For outdoor use: broom-finish for a slip-resistant surface
• Spray on curing compound immediately after floating

Step 7 — Curing and stripping

• Cover with plastic sheeting immediately — alternative to curing compound
• Wet the surface 1–2 times per day for the first 3–7 days in hot weather
• Cold weather: do not pour onto frozen ground, and do not let fresh concrete freeze before it has gained enough strength — use cold-weather measures (protection, insulated covers, a suitable mix) and follow the supplier's minimum-temperature guidance. Hot, dry or windy weather needs the opposite care so it does not dry out too fast
• Strip formwork only when the concrete has reached sufficient strength for the geometry, temperature conditions and expected loading
• Loading and follow-on work must be adapted to the concrete's achieved strength, temperature conditions and the designed solution
• Carry out any control joints when the concrete is ready for saw cutting in accordance with the chosen method and documented practice

Rules of thumb and reference figures

Common mistakes

• ✗Adding too much water — makes mixing easier, but reduces strength; always follow the water ratio on the bag
• ✗Pouring in freezing conditions without protection — concrete below +5 °C will not cure and can be destroyed by frost
• ✗No vibration around tight reinforcement — voids and pores weaken the structure and create pathways for water
• ✗Pouring layers that are too thick — pour a maximum of 300 mm at a time and vibrate each layer separately
• ✗Insufficient cover over reinforcement — minimum 25 mm indoors, 40–50 mm against soil or moisture
• ✗Stopping curing too early — concrete needs to be kept moist for at least 7 days to achieve full strength
• ✗Pouring against dry formwork — without wetting or release agent the formwork draws water from the concrete, leaving a weak surface

Calculate your materials

Use the calculator to find the number of bags, reinforcement, membrane and extras: