Mixing lean concrete: A step-by-step guide

• What is lean concrete?
• What is lean concrete used for?
• The correct mixing ratio for lean concrete
• How do you mix lean concrete correctly?
• Tips for optimum results when mixing lean concrete

Lean concrete is a building material consisting of cement and fine gravel with a grain size of 0/8 (the largest gravel has a diameter of 8 mm). Water is added to it until its consistency becomes earth-moist after mixing. Compared to conventional concrete, lean concrete contains a lower proportion of cement. This makes it less pressure-resistant, but more cost-effective and ideal for specific applications, e.g. in gardening and landscaping.

Due to the weight of the material and its thick consistency, the use of a mixer or mixing machine is highly recommended for mixing the mortar. Mixing by hand is physically demanding, time-consuming and often has qualitative defects that affect the durability of the structure.

Due to its lower load-bearing capacity, lean concrete is mainly used as a filling material behind pool and basement walls. It is ideal for laying paving slabs and bedding layers and as a clean layer for garden paths. It is also used for setting kerbstones or concreting in fence posts, flagpoles and palisades. It also stabilises smaller embankments and slopes and provides a stable foundation for garden walls and patio slabs.

This building material is particularly suitable for foundations and substructures that do not require high structural loads. Typical applications are foundations for lightweight structures such as tool sheds, garden sheds, carports or pergolas. Lean concrete is also used for levelling layers and as a filling material for levelling surfaces and cavities as well as for stable, but not heavy-duty supporting structures in gardening and landscaping. It is also used for temporary traffic routes and construction roads or, to provide a uniform surface to the foundation concrete and to prevent the direct contact from the soil.

Lean concrete is often used for setting kerbstones, as it is specially adapted to the requirements of this area of application. The lower load-bearing capacity of the material makes it possible to create a stable base that is also flexible enough to adapt to different ground conditions. This ensures long-term stability of the kerbs by minimising settlement or displacement that could be caused by ground movement or weathering. In addition, lean concrete offers good adhesion, which fixes the kerbstones securely in place and reliably secures their position. These properties make it a preferred choice for the design of pavements, driveways and gardens, where the safety and durability of the kerbs are crucial.

The typical mixing ratio for lean concrete is around 1 part cement to 5 to 8 parts sand/gravel. This can vary depending on the specific requirements and intended use.

• Foundations and substructures:   1 part cement, 8 parts sand/gravel, 0.4 - 0.5 parts water
• Kerbs and borders:   1 part cement, 5 parts sand/gravel, 0.4 - 0.5 parts water

Lean concrete with sand
Sand is used in lean concrete when a finer texture or better filling of narrow cavities is required. It is ideal for thinner layers, levelling layers, joints and for applications where a smooth surface is required, such as pavements, paving stones or tile underlays.

Gravel for lean concrete
Gravel is used as an aggregate in lean concrete when a coarser structure or a higher load-bearing capacity is required, such as for foundations, substructures and temporary traffic routes or construction roads. The ideal gravel for lean concrete is characterised by its uniform grain size (typically 4-32 mm), purity free of impurities such as clay or organic substances, sufficient hardness and strength for structural loads, good bonding properties with cement mortar and regional availability.

1. Prepare your workplace: Before you start mixing lean concrete, thorough preparation of the workplace, tools and materials is crucial for a successful result. Ensure that the work area is clean and free of obstacles and that a water connection is accessible.

2. Measure the materials: Prepare the required quantities of cement, sand/gravel and water. Precision is crucial for the quality of the concrete. Based on the amount of ready-mix mortar required, you can estimate whether you should mix the lean concrete by hand or with a machine. If you only need a small quantity (approx. 20 kilos), you can mix the lean concrete by hand. For larger construction projects where efficiency is required and larger quantities are needed, the use of an electric mixing device or a concrete mixer is highly recommended.
 

3. Mixing lean concrete: When mixing lean concrete by hand, start by mixing sand or gravel thoroughly, favouring coarse sand for fine-grained concrete that should fill narrow cavities well. The cement is then added and mixed again until it is distributed evenly. Water is added slowly while mixing continuously to achieve the desired consistency.
 

When mixing lean concrete with a machine, use a mixer with a suitable mixing attachment. Then pour approx. 2/3 of the required amount of water into a clean mixing bucket. Then add the components such as sand or gravel and cement. Start the mixing process and work the dry material through thoroughly with the water. Once the mixture has been worked through, add the rest of the water and mix until you have a uniform, earth-moist mortar. The mixing process with the mixer usually takes around 3 minutes. For larger quantities, the use of a compact or forced-action mixer is recommended; these do the job quickly and without physical effort. Our Collomatic TMX forced-action mixer or the Xo 55 R Duo double-spindle agitator are also ideal for mixing lean concrete.

Thorough and complete mixing of the components with the Collomatic TMX - effortlessly and faster than with a conventional concrete mixer

Tips for optimum results when mixing lean concrete

The following tips should help you to achieve an optimum end result. You should therefore pay particular attention to these factors:

• Quality of the materials: Use only high-quality materials to ensure the best concrete quality.
• Cleanliness: Keep your tools and machines clean to avoid contamination.
• Correct storage: Store cement and other materials in a dry and protected place to maintain their quality.
• Pay attention to temperature: In extreme weather conditions, it may be necessary to adjust the mixing time or the water ratio.

When mixing lean concrete, you should also be on the lookout for a few potential sources of error: An incorrect mixing ratio, especially too little cement, can reduce the quality of the concrete. It is important to adhere strictly to the recommended weight proportions. Uneven mixing leads to weak spots in the concrete. Make sure that all materials are mixed thoroughly and evenly to ensure a homogeneous mixture. The right amount of water is crucial. Too much water weakens the concrete mix, causing cracks to form, too little water makes work more difficult and leads to uneven distribution of the components, resulting in unmixed dry patches. After installation, ensure that the concrete is compacted by tamping to avoid cavities.