Introduction
Whether for a pergolaa terrace or a bicycle shelterMany landscaping projects fail because of an unstable foundation. There's nothing more frustrating than seeing a structure sag or loosen over time, especially after investing hours of work and resources.
This problem is often due to a poor choice of foundation, or to errors in making the concrete blocks. Poorly prepared ground or poorly poured concrete can lead to cracks, movement in the structure, or even partial collapse.
Fortunately, there is a simple solution available to everyone: make solid concrete studs by following a precise method. In this article, we guide you through 9 key steps to pouring your own concrete blocks, ensuring lasting stability for your outdoor landscaping. Follow this guide and say goodbye to settling problems!
Part 1: Soil analysis
Part 1: Soil analysis
Before pouring concrete blocks for your pergola, terrace or shelter, it's essential to understand the nature of the ground you'll be working on. What's the point? Because the stability of your structure depends entirely on the soil's ability to support the weight of the concrete blocks.
Why carry out a soil analysis?
Soil that's too loose or unstable may settle over time, causing structural problems such as cracks or movement. Soil analysis enables you to determine whether your land is capable of supporting the load of your construction, and to determine the depth and size of the concrete blocks required.
Soil types and their load-bearing capacity
Each type of soil can support a different load, and this capacity is expressed in bars (unit of pressure measurement). Here's a simple table of the main types of soil and their load-bearing capacity:
| Soil type | Capacity (Work rate) |
|---|---|
| Plateau silt | 1.5 to 3 bar |
| Millstone earth | 3 to 4.5 bar |
| Clay | 0.3 to 3 bar |
| Sand and gravel | 6 to 9 bar |
| Chalk | 9 to 10 bar |
| Coarse limestone | 18 to 45 bar |
Example If you have limestone soil, it can bear much more weight than clay soil. This means that the studs on limestone soil can be smaller, whereas on clay soil they'll need to be wider to distribute the load evenly.
How can you analyze your soil yourself?
You don't need to be an expert to test the stability of your floor. Here are a few simple methods:
- Compaction test Take a metal bar (like a crowbar) and push it into the ground in different places. If it sinks in easily, the ground is loose and therefore less stable.
- Visual observation Dig a hole about 50 cm deep. Look at the composition of the soil: is it sandy, clayey or full of stones? These elements will give you clues as to the stability of the soil.
- Drainage test Dig a 30-50 cm hole and fill it with water. If the water doesn't run off for a long time, the soil is poorly drained, which could be a problem. In this case, we recommend adding gravel or drains to improve water drainage.
What to do depending on the type of soil?
- Soil too soft or fresh (e.g. after building work) The soil: Wait a few weeks for the soil to stabilize before starting work. Otherwise, dig deeper until you reach a more stable layer.
- Clay soil Add a layer of gravel or sand to improve load-bearing capacity before pouring the blocks.
- Wet soil Drainage: Remember to install drains to prevent water from stagnating under your concrete blocks, which can weaken the structure over time.
Simple tools for soil analysis
- Hand auger Ideal for digging holes and examining soil in depth.
- Shovel and spade Used to remove surface soil and dig to the required depth.
- Pencil A good tool for testing soil compactness at different depths.
Part 2: Choice of concrete stud type
Once you've analyzed the soil, it's time to choose the right type of concrete stud to support your shelter, pergola or small structure. The choice of plot depends on several simple criteria: the load each plot will have to support, the type of ground you're working on, and the stability required for the structure.
The different types of concrete blocks
There are three main types of concrete blocks, each with its own advantages depending on the structure you wish to install:
- Cylindrical studs :
- Description Easy to dig and pour, they are ideal for lightweight structures such as a small garden shed or pergola.
- When to use them : If your structure is light and does not have a strong wind hold, these studs are often sufficient.
- Cylindrical studs with extended base :
- Description These studs are similar to cylindrical studs, but with a wider base to distribute the weight more evenly.
- When to use them Ideal for heavier structures, such as terraces or shelters with additional elements (furniture, shelves, etc.). The wide base provides greater stability.
- Cubic studs :
- Description Square or rectangular studs are often used for heavier constructions or structures requiring greater stability.
- When to use them Recommended for foundations that have to support heavy loads or very sturdy shelters, especially on soft ground.
How to choose the right type of stud?
To choose the right stud, you need to consider three essential elements:
- The weight of the structure :
- For a small shelter or pergola, simple cylindrical posts are often sufficient. On the other hand, if you're installing a terrace or shelter with heavy furniture (e.g. a Jacuzzi), opt for blocks with a wider base or cubic blocks.
- Soil type :
- If the soil is stable (e.g. limestone, sand), smaller or fewer studs will suffice.
- If the soil is clayey or loose, you'll need to increase the size or number of studs to distribute the load more evenly and prevent subsidence.
- The shape of the structure :
- Structures with a high wind load (e.g. open pergolas) require wider or deeper studs to ensure stability.
Specific recommendations for heavy loads (such as a Jacuzzi)
If you're planning to add heavy loads (e.g. a Jacuzzi on a terrace, or heavy furniture in a shed), you'll need to adapt the size of the studs or add more to distribute the weight. For example, for a terrace with a Jacuzzi, it's advisable to choose blocks with a wider base, or to combine several blocks to distribute the load more evenly and prevent sagging.
Part 3: Calculating dimensions and number of studs
To ensure that your pergola, terrace or garden shed rests on a solid foundation, it is essential to calculate both the size of the foundation and the amount of space required. number and the size concrete blocks. Here's a simple method for making these calculations with complete peace of mind.
Standard dimensions for different types of projects
Here is a simplified table to help you determine the number and size of studs for your project:
| Structure type | Approximate surface area | Recommended number of studs | Minimum stud diameter |
|---|---|---|---|
| Pergola | 3×3 m | 4 to 6 studs | 30 to 35 cm |
| Wooden terrace | 4×4 m | 9 to 12 studs | 25 to 30 cm |
| Garden shed | 3×2,5 m | 6 studs | 25 to 30 cm |
These dimensions are indicative and may vary according to the type of floor and the total weight of the structure.
Depth of studs
The depth of the studs is just as important as their surface. It must take two main factors into account:
- The frost-free zone The minimum depth required to prevent foundations from shifting in the event of frost is generally between 60 and 80 cmdepending on your region. This information is available online or from local authorities.
- Moisture protection The studs must protrude 15 cm above ground to protect the structure from moisture. This prevents groundwater from rising by capillary action and damaging your foundations.
Quick tip for estimating the number of studs
A simple rule: for small structures, such as a garden shed or a small pergola, divide the length and width by 1 meter to estimate the number of studs. For example, for a 3×2,5 mthis gives approximately 6 studs (3 studs lengthwise and 2.5 studs widthwise, rounded up to 6).
Estimate the size of your concrete blocks
Use our tool to calculate the dimensions of your concrete blocks in just a few seconds.
Part 4: Location and spacing of studs
Once you've analyzed your floor and decided on the type and number of studs you'll need, the next step is to define their use. location and determine thespacing optimal. This ensures that your structure, whether it's a pergola, terrace or garden shed, will be stable and durable over time.
What's the point of spacing the studs?
Incorrectly spaced studs can lead to uneven weight distribution, which in turn can cause the structure to sag or warp. By following a few simple recommendations, you can ensure that your construction rests on a solid, well-balanced base.
Recommended spacing according to structure
- Wooden terrace For a wood deck, we recommend spacing the concrete blocks by 50 to 60 cm from each other. This provides stable support for the joists (pieces of wood under the deck boards), while avoiding any risk of sagging under the weight of users or furniture.
- Pergola or garden shed For these structures, the studs must generally be installed under each post foot. If, for example, you're building a 4 x 4 metre pergola, you'll need four studs, one at each corner. If your structure is larger or more complex, you'll need to adjust the number of studs according to the distribution of the posts.
How do I mark out the location of the studs?
Precisely marking out the location of the studs is a crucial step in guaranteeing the structure's stability and alignment. Here's a step-by-step guide to success:
- Using the chalk line :
- Stretch a chalk line between the corners of your future structure (for example, the corners of the pergola or terrace). This will give you a straight line to follow to mark the location of the studs.
- If you're installing several rows of studs, make sure that each line is either parallel or perpendicular, depending on the layout of your structure.
- Marking locations :
- Use a chalk or spray paint to mark the exact center of each stud on the ground. This will serve as a reference point for digging later on.
- If you're installing post feet, don't forget that the center of post foot must correspond to the center of the concrete stud.
- Checking with a level :
- Before digging, use a spirit level to ensure that all your lines are properly aligned. Proper alignment will avoid placement errors that could cause imbalances in the final structure.
- Remember to check that the ground is level, or if not, plan to compensate for the slope when laying the studs.
What should I do if I make an investment mistake?
It's possible that, despite your precautions, a stud may be slightly out of position in relation to its intended location. Here are some solutions if this happens:
- Move the stud If you notice the error before the concrete is completely dry, you can slightly adjust the position of the formwork or stud to correct the alignment.
- Adjustment shims If the error is minor (a few centimetres), you can use shims to adjust the position of the post foot on the stud.
- Reworking after drying If the offset is too large, wait for the stud to dry, then consider drilling and adjusting with an anchor stud.
Part 5: Setting up the formwork
Formwork is a key stage in the production of concrete blocks, as it enables the concrete to be given the desired shape, while guaranteeing its stability during drying. Without proper formwork, you run the risk of obtaining irregular or misaligned blocks, which would compromise the solidity of your structure.
What type of formwork should I use?
There are several types of formwork, each with its own advantages depending on the type of structure to be built.
- Wooden formwork Very common and economical, it is ideal for square or rectangular studs. It's easy to make, with planks adjusted to the dimensions of the studs.
- Metal formwork More expensive but also quicker to set up, this type of formwork is modular and easy to dismantle after drying. It's perfect for structures requiring several regular blocks.
- Cardboard formwork (or formwork tube): Simple to use for cylindrical studs, it is often employed for pergolas or small shelters. Simply place the cardboard tube in the hole and fill with concrete.
How to lay formwork precisely?
To guarantee straight, solid blocks, the formwork must be set up with care. Here are the essential steps to follow:
- Level check :
- Use a spirit level to ensure that the formwork is level. If the formwork is not level, your stud could be tilted, making the structure unstable.
- Also check that all formwork is level with each other. This is particularly important if you're pouring several blocks for one structure, to avoid imbalances.
- Formwork stabilization :
- Use clamps to hold the various parts of the wooden formwork together and prevent the concrete from moving them.
- In the case of cardboard formworks posts or pebbles can be placed around the tube to prevent it shifting under concrete pressure.
Tips for successful installation :
- If you're using wooden formwork, make sure it's securely in place. waterproof consolidating corners with clamps. This will prevent concrete escaping through the gaps.
- When using cardboard formwork, we recommend that you moisten slightly before pouring the concrete. This prevents the cardboard from absorbing water too quickly from the concrete, which could compromise setting and drying.
What to do if the formwork moves?
It can happen that, despite all precautions, the formwork shifts slightly under the weight of the concrete. Here are some solutions:
- Reposition immediately If you notice any movement before the concrete begins to dry, adjust the formwork and stabilize it again with stakes or clamps.
- Grind after drying If the formwork has moved and the concrete has taken on an irregular shape, you can lightly sand the concrete once dry to rectify surface defects.
Part 6: Concrete preparation
Good concrete preparation is essential to ensure strong, long-lasting studs. Whether you use a concrete mixer or mix concrete by hand, follow these simple steps to obtain quality concrete that will stand the test of time.
What type of concrete to use?
You have two options for preparing your concrete:
- Ready-mix concrete Easy to use, just follow the instructions on the packaging to mix with water. A good solution for small projects or if you don't want to measure out the ingredients yourself.
- Do-it-yourself concrete You can also prepare the concrete yourself by mixing cement, sand, gravel and water. This method is ideal for larger projects, as it's often more economical.
Precise dosing for solid concrete
If you choose to prepare your own concrete, here are the recommended proportions:
- 1 volume of cement
- 2 volumes of sand
- 3 volumes of gravel
- 1/2 volume of water
Tip If the gravel or sand is already damp, you can reduce the amount of water slightly to avoid excessively liquid concrete.
Good concrete should be thick enough to hold in place without flowing too quickly. If the concrete is too liquid, it will take longer to dry and be less resistant in the long term.
Preparing concrete: manual or concrete mixer?
- With a concrete mixer If you have several blocks to pour, or if your project is large, the concrete mixer is ideal. It allows you to mix larger quantities of concrete evenly. Simply add cement, sand, gravel and water to the concrete mixer and let it run until a uniform mixture is obtained.
- By hand For smaller projects, you can mix the concrete by hand in a wheelbarrow or on a flat surface. First add the dry ingredients (cement, sand, gravel) and mix well. Then add the water a little at a time, mixing constantly with a shovel or trowel, until you reach the desired consistency.
Practical tips for proper preparation
- Don't mix too much at once If you're working on your own, it's best to prepare the concrete in small batches, one batch at a time, to avoid the mixture setting before pouring.
- Check consistency The concrete should be thick enough to hold in place without sinking. If you can make a compact ball with the mixture in your hands, it's ready to pour.
Weather-related precautions
- In case of rain If the weather is wet or rainy, protect the concrete and forms with a tarpaulin to prevent too much water from mixing with the concrete. Excessive water could weaken the strength of the concrete.
- In hot weather High temperatures can cause concrete to dry too quickly and crack. In this case, dampen the concrete slightly after pouring to prevent it from drying too quickly and to allow it to set evenly.
Part 7: Pouring the concrete
The pouring of concrete is a crucial step in ensuring that your studs are solid and well-formed. This step requires precision to ensure that the concrete is evenly distributed, free of air bubbles and perfectly level. Here's how it's done, whatever type of formwork you use.
1. Prepare formwork before pouring
Before you start pouring concrete, make sure your forms are properly installed and stabilized:
- Wooden formwork Check that the boards are properly adjusted and held in place with clamps.
- Metal formwork Make sure the formwork is securely assembled and locked to prevent any leaks.
- Cardboard formwork Place the form tube in the hole, securing it with stakes or pebbles to prevent it from shifting under the weight of the concrete.
2. Pouring concrete
- Casting method Pour the concrete slowly into the formwork, taking care not to pour it all at once to avoid creating air pockets. You can use a shovel or bucket to spread the concrete more easily.
- Progressive filling Fill the formwork in successive layers (10-15 cm at a time). This method ensures better compaction at each stage and guarantees even distribution.
3. Compacting and vibrating the concrete
Tamping and vibration are essential to avoid air bubbles in the concrete. These bubbles can weaken the structure and lead to long-term cracking. Here's how to do it right:
- Tamping by hand After each layer of poured concrete, use a trowel or a metal bar to lightly stir the concrete. This eliminates any air bubbles inside.
- Vibration If you have a concrete vibrator at your disposal, use it to compact the concrete and make sure it's evenly distributed throughout the formwork. If not, tap the formwork (wood or metal) lightly with a hammer to encourage vibration.
Tip Always move upwards from the bottom when tamping or vibrating. This ensures that air bubbles gradually rise and exit the concrete.
4. Check level
Once the formwork has been filled, it's essential to check that the concrete is level. This will ensure perfect stability for your structure.
- Use a spirit level Place a spirit level on the concrete surface to make sure everything is level. If necessary, add or remove concrete to adjust.
- Trowel finishing Smooth the surface with a trowel for a clean, even finish. This also allows you to check the level of your plot one last time.
5. Final checks and precautions
Before moving on to the drying stage, here are a few points to check:
- Stable formwork Make sure the formwork has not moved during pouring. If it has, reposition it before the concrete starts to set.
- Eliminate remaining bubbles : Run a trowel or bar through the concrete one last time to remove any air pockets that may still be present.
Part 8: Drying and stripping
Once the concrete has been poured, it is essential to respect the drying times to guarantee the solidity of the blocks. Stripping the formwork must be carried out with care to avoid damaging the freshly hardened concrete. Here are the steps you need to follow to successfully complete this crucial phase.
Drying time depending on formwork type
Concrete drying time varies slightly depending on the type of formwork used, but here is a general estimate:
- Wooden formwork : At least wait 7 days before stripping the formwork. Wood absorbs a small amount of moisture from concrete, which slows drying slightly.
- Metal formwork Metal does not absorb moisture, so drying time is similar to that for wood, approx. 1 week.
- Cardboard formwork If you're using a cardboard tube, it can be left in place to simplify the work. However, if you wish to remove it, please wait at least 7 days.
In general, we recommend waiting 1 to 2 weeks before installing a heavy structure on the concrete blocks, to allow the concrete to harden completely.
Weather conditions during drying
Concrete drying also depends on weather conditions. Here are some tips on how to manage the drying process depending on the weather:
- In case of rain Fresh concrete is highly vulnerable to weathering, especially in the first 24 hours. If rain is forecast, be sure to cover your blocks with a tarpaulin to prevent water from seeping into the concrete and compromising its setting.
- In hot weather : Drying too quickly under high temperatures can cause damage to the skin. cracks in the concrete. To avoid this, you can dampen your studs slightly with water after pouring (once they have taken on a solid shape) or protect them from direct sunlight with a tarpaulin.
Stripping: when and how?
Stripping must be done with care to avoid damaging the concrete, which is still consolidating:
- Wooden formwork : Remove the boards after 7 daysStart by unscrewing or loosening the loosest parts. If the wood sticks to the concrete, you can dampen it slightly to facilitate removal. Be careful not to hit the corners of the studs.
- Metal formwork : Metal formwork is often modular and easy to remove. You can remove the formwork after 7 daysby unscrewing the modular parts and gradually removing them.
- Cardboard formwork If you've used a cardboard tube, it's often best to leave it in place, especially if it's to be buried. If you wish to remove it, cut it gently with a box cutter afterwards. 7 days.
Protection during drying
During drying, concrete must be protected to prevent cracking or environmental damage:
- Covering your studs If you're expecting heavy rain or excessive heat, cover your blocks with a tarpaulin to protect them. This helps control humidity around the concrete.
- Light humidification In very dry or hot weather, you can lightly water the blocks once or twice a day to slow down drying and prevent cracking.
By following these recommendations, your concrete blocks will have all the time they need to dry properly and reach their maximum strength before supporting the structure you have planned.
Mistakes to avoid
Even with the best of intentions, some common mistakes can compromise the strength and durability of your concrete blocks. Here are the mistakes to avoid at every stage, along with solutions for correcting any problems after the fact.
1. Neglecting soil analysis
One of the most common mistakes is not paying enough attention to soil analysis. Poorly stabilized or excessively loose soil may not support the weight of the structure properly, leading to long-term subsidence or movement.
❌ To avoid:
- Do not check the composition of the soil or neglect to test its load-bearing capacity.
✅ Solution:
- If your soil is too loose, you can dig deeper to reach a more stable layer or add gravel to reinforce the foundation.
2. Incorrect dimensioning and spacing of studs
The size and spacing of the studs are essential to ensure even weight distribution. Studs that are too small or spaced too widely can cause parts of the structure to sag.
❌ To avoid:
- Use studs that are too small for the total weight of the structure.
- Spacing the studs too far apart can weaken the structure.
✅ Solution:
- If you've already poured the studs and they're spaced too far apart or too small, it's sometimes possible to reinforce the structure with shims under the post feet, or to add extra studs, if the structure allows.
3. Do not check level and alignment of studs
Another common mistake is not to check the level of the studs after pouring. A misaligned or tilted stud can cause stability problems for the structure (pergola, shelter, terrace).
❌ To avoid:
- Do not check the level of the studs once the concrete has been poured.
- Placing forms without checking their alignment with each other.
✅ Solution:
- If a stud is slightly inclined, you can sand the concrete surface to adjust the height. If the error is greater, you can wedge the foot of the structure with suitable materials.
4. Neglect concrete tamping and air bubble management
When concrete is poured, it's crucial to pack or vibrate it well to eliminate air bubbles. These bubbles weaken the structure and can cause cracks over the long term.
❌ To avoid:
- Do not compact or vibrate the concrete properly after pouring.
✅ Solution:
- If air bubbles are discovered after drying, it is difficult to correct the problem without breaking and remaking the stud. On the other hand, if the error is minor, the stud can still be used for light loads.
5. Poor management of weather conditions
Concrete is sensitive to humidity and temperature for the first 24 to 48 hours. Concrete exposed to rain or excessive heat may lose strength and crack.
❌ To avoid:
- Leave concrete exposed to rain or direct sunlight without protection.
✅ Solution:
- If the concrete has been exposed to adverse weather conditions before drying, it is advisable to wait and watch for cracks. If necessary, you can reinforce the structure by adding shims or reinforcements.
By avoiding these common mistakes and following our corrective advice, you can ensure that your concrete blocks will be strong and capable of supporting the structure for a long time to come.
Examples of structures on concrete pads
Conclusion
Laying solid concrete blocks is a key step in guaranteeing the stability and durability of your pergola, terrace or shelter. By following these 9 steps, from soil analysis to form removal, you'll have everything you need for a successful foundation. Every detail counts: from the meticulous preparation of the concrete to the correct spacing of the studs, not forgetting the management of weather conditions. By avoiding common mistakes such as incorrect dosing or imprecise installation, you can ensure that your structure will stand the test of time, whatever the external constraints. If you follow the advice in this guide, your concrete blocks will provide a stable, resistant base for all your outdoor landscaping projects.
Now all you have to do is use these techniques to bring your project to life!
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