Concrete and Mortar: Classic Mixing Ratios
Concrete and mortar are the most commonly self-mixed construction materials. The correct mixing ratio determines the strength, durability and workability of the finished material. Cutting corners or working carelessly risks cracks, insufficient load-bearing capacity and expensive remedial work.
Concrete follows standardized classes per DIN EN 206. The most common for home use are C12/15 (lean concrete for curbs and edging), C20/25 (standard concrete for foundations and floor slabs), C25/30 (for more heavily loaded foundations) and C30/37 (for heavy-duty structures).
The mixing ratio is given in parts: cement : sand : gravel. For C20/25, the ratio is 1:3:5 -- one part cement, three parts sand (0-4mm) and five parts gravel (4-16mm). Approximately 300 kg of cement is needed per cubic meter. The water-cement ratio (w/c value) is 0.55 -- about 165 liters of water per cubic meter.
Mortar differs from concrete by the absence of coarse aggregates (gravel). Masonry mortar has a ratio of 1:4 (cement:sand), interior plaster mortar is mixed with lime additive (1:2:8 = cement:lime:sand), making it smoother and easier to work with.
A common mistake: adding too much water. This makes the concrete easier to pour but significantly reduces final strength. Each percent of water above the optimal w/c ratio reduces compressive strength by approximately 5%. The consistency should be earth-moist -- the concrete holds its shape when squeezed in your hand.
Epoxy Resin: Mixing Resin and Hardener Correctly
Epoxy resin is a two-component system consisting of resin (Component A) and hardener (Component B). Unlike concrete, where small deviations are tolerable, the mixing ratio for epoxy must be exact. Even 5% deviation can cause the resin to never fully cure.
The most common mixing ratios are 2:1 (standard, by volume), 3:1 (slow curing for larger pours), 5:1 (for coatings) and 100:45 (by weight, European standard). The exact ratio is always printed on the manufacturer packaging and must be followed meticulously.
For mixing, use a kitchen scale with 0.1g accuracy. Volumetric measuring (with cups) is less precise and should only be used as a last resort. The two components are thoroughly stirred in a clean container for at least 3 minutes, scraping the sides and bottom.
Pot life (working time after mixing) ranges from 20 to 45 minutes depending on the product. Larger quantities increase temperature through the exothermic reaction and drastically shorten pot life. Always work in small batches for large areas and pour the mixture flat -- do not leave it standing in a tall bucket.
Primer and Basecoat: Dilutions and Coverage
Primers prepare the substrate for subsequent paint or flooring. They bind dust, regulate absorbency and improve adhesion. Coverage varies considerably depending on substrate and primer type.
Deep primer is the standard for mineral substrates (plaster, concrete, drywall). On normally absorbent surfaces, one liter covers approximately 6 sqm. On highly absorbent substrates (old plaster, aerated concrete), coverage drops to 3-4 sqm/L. A pre-coat of 1:1 diluted primer followed by an undiluted coat is recommended.
Bonding primer (quartz primer) is used before applying plaster to smooth surfaces. The gritty surface gives plaster mechanical grip. Concrete contact is the stronger variant for particularly smooth surfaces like concrete ceilings.
Barrier primer blocks discolorations (nicotine, water stains) and prevents them from bleeding through the new paint. Applied undiluted with coverage of approximately 8 sqm/L.
Filler and Leveling Compound: Powder-Water Ratio
Fillers and leveling compounds are supplied as powder and mixed with water. The correct powder-water ratio is critical: too much water leads to cracks during drying and reduced strength. Too little water makes the compound hard to work with and produces a rough surface.
Gypsum filler: 25 kg powder to approximately 12.5 L water (2:1 ratio). Sprinkle the powder into the water (never the reverse!) and stir smooth after 2 minutes of soaking. Working time: approximately 45 minutes. Drying time: 4 hours at 3mm thickness.
Leveling compound: 25 kg powder to approximately 6 L water. The thin liquid is poured onto the primed floor and distributes self-leveling. Maximum layer thickness: 10mm. For larger irregularities, work in multiple layers, each after the previous layer has dried.
Tile Adhesive and Grout: Consumption per sqm
Tile adhesive consumption primarily depends on the trowel notch size. The notch determines the height of the adhesive ridges and thus the quantity per square meter. Typical values: 6mm notch = 2.5 kg/sqm, 8mm = 3.5 kg/sqm, 10mm = 4.5 kg/sqm, 12mm = 5.5 kg/sqm.
For large format tiles (over 60cm edge length), the buttering-floating method is used: adhesive on both the floor and the tile back. This increases consumption by approximately 50% but ensures void-free installation.
Grout is calculated using a formula that considers tile size, joint width and tile thickness: kg/sqm = (length + width) / (length x width) x joint width x tile thickness x density. For 60x60cm tiles with 3mm joint and 10mm thickness, this yields approximately 0.27 kg/sqm.
General Tips: Weigh Accurately and Mind the Pot Life
Tip 1: Invest in a good scale. For epoxy, a kitchen scale (0.1g); for concrete and mortar, marked buckets or a wheelbarrow as measuring unit will suffice.
Tip 2: Mind the temperature. Most materials should be processed between 5 and 30 degrees Celsius. Cold delays curing, heat accelerates it (and shortens working time).
Tip 3: Always add powder to water, never the reverse. This prevents lumps and produces a more consistent mixture.
Tip 4: Plan 10% reserve. Unforeseen losses from spillage, tool adhesion and substrate irregularities are normal.
Tip 5: Document your mix. Note quantity, ratio and ambient temperature. If problems occur, you can narrow down the cause.