Can you pour a footing without forms? I often see this question come up with owner-builder types who are trying their hand at construction. The DIY culture in Australia is alive and well. The weekend trip to the local Bunnings Warehouse is testament to this. At any sign of clear weather, you can be sure that the local Bunnings car park will be jam packed full or eager DIYers. So can you pour a footing without forms? Lets take a look…
Footings can be poured without forms most of the time and in some cases it is not only allowable but desirable. However there are some instances when forms may be required. As a general rule however, if the concrete mix design has been formulated with the expectation that it will be cast directly against ground and adequate cover to the reinforcing bar is provided within the footing, casting a footing without forms is acceptable. Forms may be required for footings located in sandy soils and/or locations where the footing is above natural ground level.
For those more interested in the details, lets explore instances where it may actually be preferable to pour a footing without forms and instances where it is not. We will also take a look at the effects on the structural behaviour of the footing and how it relates to whether forms were provided during construction or not.
Footing Construction in the Real World
Structural documentation and design often reflects a perfect world scenario. This is especially true for construction of footings. Footings can either be cast directly against the ground (dirt) or be provided with formwork around its edges.
In either case, a Structural Drawing may indicate a footing looking clean, sharp and pristine. Here is a typical footing detail you may find in a set of Structural Drawings (depending on what your project is)…
That same footing however constructed in the real-world may end up looking more like this…
This looks a little more rustic and less refined. However as long as the minimum structural dimensions are achieved (minimum footing depth, width, founding depth and reinforcement cover), the matter that its a bit ugly looking wont adversely affect its structural behaviour. This is an artists representation of what a footing may look li cast directly against ground (without forms).
If you are wondering what the grey hatch is immediately beneath the footing in this specific example, it is a layer of blinding concrete. To learn more about blinding concrete and why it is used in construction (and if you need it or not), take a look at THIS article.
Why Pouring a Footing Without Forms may be Desirable.
To pour a footing without forms may not only be easier to build, it may also give a better structural outcome, depending on the footings details and the load it supports.
Some footings are required to support lateral stability elements. Lateral stability elements are those elements within a building which are intended to support the horizontal loading which is applied to the structure. Horizontal loading may be applied to a structure from wind, earthquake, impact and more! (to learn more about lateral stability systems in buildings and how they work, take a look at THIS article).
For footings which support significant lateral load, they may rely upon passive resistance on the side of the footing from the adjacent soil to keep them in place. To better visualise this, lets use our previous example footing and apply a horizontal load to the supported column…
As we can see, the horizontal shear action on the column is supported through a combination of passive soil resistance on the side of the footing as well as shear friction between the base of the footing and the soil surface. (here we are ignoring the nominal overturning moment which will also be generated in this system for simplicity).
The important component is the horizontal passive soil resistance on the side of the footing. In some cases this may account for a large percentage of the lateral stability of the footing in horizontal shear. To achieve the best passive soil bearing capacity at this interface, it is ideal for the soil to be as undisturbed as possible. This is where casting a footing directly against the soil (i.e. pour the footing without forms), is desirable.
If the excavation for the footing is “over-dug” then formwork is provided, this will leave a gap between the side face of the footing and natural ground. This gap is best filled with a cement stabilised sand mix or… concrete. This would add an unnecessary step to the construction process.
When Pouring a Footing Without Forms is not Recommended.
There are some cases however where pouring a footing without forms is not desirable or practical.
An obvious case is for situations where a portion of the footing lies above natural ground level. In this case, the wet concrete will require support above natural ground level and the introduction of formwork will be required.
In some cases, forms may also be required due to safety reasons. For very large and deep footings, an over-excavation may need to be provided. This can ensure safety for the workers operating within the excavation. Due to this, it is common for very deep core raft footings to be constructed with forms (these footings can be in excess of 2m deep or 6.5 feet!). This can prevent safety issues caused by the excavation collapsing into itself during construction.
Things to Consider when Pouring your Footing Without Forms
Here is a quick checklist of things to consider if you wish to pour a footing without forms…
- If you are in doubt, your project Structural Engineer should be consulted as to what the assumptions are within the design. There may be a specific Structural reason as to whether a footing requires formwork or not.
- If forms are not used to assist in the footing construction, additional care needs to be taken to ensure minimum structural dimensions are still met (this includes, thickness, width, length, founding depth and cover to reinforcing bar).
- The concrete supplier should be notified that the concrete is intended to be placed directly against soil. This may alter their mix design assumptions.
- For very large and deep footings, safety needs to be considered. If the soil is sandy and not able to remain stable for the proposed vertical cut depth, this may pose a safety risk to those working within the footing excavation. In these instances, the site Geotechnical Engineer should be consulted for advice. If the soil can not adequately stand up during the excavation at the required depth, over-excavation may need to be considered. This may then cause a need for forms to be provided for the footing.
If you found this article interesting and would like to learn more about footings and foundations, the following articles are also a great read…
- ISOLATED PAD FOOTING DESIGN WORKED EXAMPLE TO AS3600-2018, WITH SPREADSHEET
- HOW DEEP AND WIDE DO FOOTINGS NEED TO BE
- IS TOP REINFORCEMENT REQUIRED IN ISOLATED FOOTINGS
- FOOTING DESIGN IN RAM CONCEPT
- PILE FOUNDATION TYPES AND THEIR BENEFITS
- TYPES OF PILE TESTING AND WHY THEY ARE NEEDED
- DO YOU NEED A PILE CAP FOR A SINGLE PILE