Concrete formwork - best approach to counter uplift forces ...

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I'm casting a 4m long triangular-profile reinforced gravity retaining wall against soil movement due to a 1 metre jump in ground level.

The lateral forces and moments on the formwork have been easy to handle, but I'm trying to find a good approach to the vertical uplift hydrostatic force on small-scale/DIY formwork while concrete is poured, which is a general problem for which I can't seem to find a good solution.

Details -

I'm using a very fluid self-consolidating (SCC) mix since access for compaction would be hard once poured (slump-flow ~ 550mm, full spec available if needed). The form has one vertical face and one face with a 2:1 slope; it's this second face that creates uplift forces. The actual height of the visible wall above ground is about 92 - 100 cm but I've dug down to about 1.2 - 1.4m to allow a "heel" and more mass under the "soil" side, and also because then it can be braced against slipping by the other concrete under the pathway itself. The wall has about 0.5 m2 cross-section area. I've gone for an "over engineered" approach - it's also being laterally braced at both ends by perpendicular concrete works, and will have 10mm (A393) welded mesh and 12+16mm rebar and "L" bends added along the length and at the corners. The form is lined with polythene for easy removal of the formwork. I'm indifferent to surface finish.

I'm confident in the actual formwork - plywood backed by C16 2x4's (50x100 mm), which are in turn backed by 3x6's (70x150 mm) held together with heavy duty structural screws, 12mm s/steel threaded bar between the formwork timbers of the two faces, all the entire form ultimately being braced laterally against solid ground on both sides nearby.

My concern is countering the vertical uplift forces from the hydrostatic pressure of the fluid concrete when it's poured, which will be about 4 - 5 tons equivalent (40 - 50kN), ie the weight of the "missing" concrete above the slanted face of the form. Basically, I want it not to be lifted off the ground by the fluid concrete. I've thought of several possible approaches to offset this force, but the reality is that I'm just not sure which is best or most foolproof, or if there are standard solutions used in civil engineering that I'm unaware of.

Possible solutions + sketches I've come up with -

1. Reduce the footprint and hence uplift, by making the wall slimmer. Perhaps with strong lateral restraint, rebar, and concrete under an adjacent path, the usual 50% rule of thumb is excessive and I can reduce the width of the wall to ~ 40cm (and therefore the upward force by ~ 30-50%)?
2. Allow the concrete to fill the full vertical space of the wall's footprint, removing the excess pour when set. Precisely zero hydrostatic pressure guaranteed as all concrete is at the same top level; the extra volume can be cast with plywood dividers so it comes away easily in sensible sized slices.
3. Cast in 2 parts, using a smaller 1st cast steel-linked to the form, as a deadweight for a larger 2nd cast. Cast the bottom 1/4 only, but with steel rod embedded to link it to the form, and vertical rebar for the join. Then cast the top 3/4 about 48 hrs later. The bottom 1/4, attached to the form itself, acts as a deadweight to hold down the form against upward pressure when casting the narrower but taller top 3/4. Extra weight still needed but only 1/4 as much; hopefully the vertical rebar linking the pours will ensure long term integrity.
4. Add a 'false floor' in and fixed to the sides of the formwork so the weight rests on the formwork not the ground; remove and underfill this space beneath the wall once the 1st pour is set. 5 tons of concrete along a 4m length can easily be supported on 80 side-by-side 2x4 (50x100mm) timber (weight per 50mm 'slice' about 62kg which is small for a 0.8m clear span). Because the 'floor' is itself supported by the formwork, there is no net uplift force - it's replaced by vertical tension in the formwork which is much easier to handle. After the form has set, remove alternate 50cm sections of these 2x4 timbers, and pour concrete into the void below, to properly support the wall. When that's set, remove the other half of the timbers and pour under the rest of the wall. Needs 3 pours but again, guaranteed to work since there is zero net uplift force and the weight spread across many 50mm slices is small for each slice. Also simple to set up.
5. Brute force. The simple solution. Add about 5 tons of weight. For stability it's added to the heavy members at the base of the form, not on top. problem is that 5 tons is a heck of a lot to move.

How can I easiest ensure the formwork doesn't rise at the base and the pour escape, when it's poured?

Things to Consider When Choosing Concrete Formwork for Construction

The concrete&#;s properties can affect the formwork and the entire project. That is why it is critical to understand the concrete&#;s characteristics.

In Brisbane, concrete is one of the most often utilized building materials. This is because it combines strength, durability, and lifespan with affordability and flexibility, to name a few advantages.

However, while concrete has seen many improvements and new variants throughout the years, it is far from a novel material. According to many historical sources, it has been in use for about 2,500 years. Nowadays, it is utilized in many ways, from laying strong foundations to constructing pools and hardscapes and adding beautiful finishing touches.

Basics of Concrete Forming

Concrete forming is the act of securing liquid concrete in place as it chemically cures into a solid material utilizing prefabricated structures, ranging from plants to polymers. To achieve this, the forms must be robust, flush with the floor to prevent spillage, and, in most instances, detachable and reusable.

Concrete must be poured into an enclosed area and allowed to solidify enough to retain its form. Newly poured concrete can be kept in form by existing features such as walls and edgings. Alternatively, temporary shuttering, commonly known as formwork, may be required.

Formwork construction on vertical buildings can be challenging and is thus often performed by experienced formwork erectors. On the other hand, ground-level slab work is often less complex and requires just basic formwork.

In all instances, whether vertical constructions or ground-level work, the formwork must be robust. It must be strong enough to withstand the forces generated by the wet concrete, as well as the weight of the vibration-generating equipment. In addition, the formwork joints must be securely secured to prevent the wet concrete from leaking during vibration and curing.

Any expert formwork contractor from Brisbane will follow all the safety regulations and provide expert services. You can visit sites like www.formworkcontractorsbrisbane.com to get a better understanding of how such companies work.

But a thing to remember is that if you are from Brisbane or any other part of Australia, you need to comply with the Australian concrete formwork standard.

Standards for Concrete Formwork

There is a general guideline called the Australian standard AS - formwork for concrete.

You must comply with this standard if you plan to engage in any of the following activities, regardless of whether you live in Sydney, Brisbane, Melbourne, or any other city in Australia. They are:

• The design, erection, fabrication, and stripping of formwork.
• The assessment, specification, and repair of the formed concrete surface. 
• And the design and construction of an on-site concrete structure.

Importance of Formwork

1. Formwork is essential for a variety of reasons. However, one of the most compelling reasons is that any other technology cannot replace it. 
2. Concrete buildings can be built rapidly and affordably using fiberglass reinforcement.
3. Throughout the building process, formwork provides adequate access and working platforms that significantly improve scaffold safety for employees.
4. By reducing the floor-to-floor building cycle time, formwork significantly reduces the project&#;s schedule and cost, allowing more projects to meet their financial requirements.
5. Formwork enables construction managers to provide precise and on-time mobilization and demobilization of formwork resources, increasing project effectiveness and resource utilization.
6. Formwork is linked with the surface finishing of concrete structures. The higher the quality of the formwork employed, the better the surface finish.
7. Formwork ensures structural safety by providing solutions for all overlay loads, resulting in safe and practical construction.

Things to Consider for Concrete Framework

Materials

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Concrete formwork can be constructed using a variety of materials. Steel and aluminum are two of the most prevalent. Steel is usually less expensive and offers the strength necessary for some building projects, but it is also heavier and bulkier. On the other hand, aluminum is lighter and more adaptable than steel, but it is usually costlier.

Scale

The scale should also be taken into account in the calculation. Numerous factors, such as the kind of formwork that will work best, are determined by the size and complexity of the building project. It will also affect the total cost of developing the formwork and other variables.

Strength

Additionally, strength is critical. The Formwork and Falsework must be robust enough to support both the weight of wet concrete and dried concrete. It should also be capable of withstanding any extra structural components that can be needed, as well as the pressure placed on it by your employees, equipment, and other factors. But it is advised to avoid too much pressure when placing concretes.

Cost

Cost is a significant consideration for all projects as well as each component. Formwork is no exception. It will affect the project&#;s cost regarding the materials and labor required to construct the formwork. Additionally, it can result in additional costs if it is not appropriately constructed and planned according to your particular requirements. Finally, if the formwork is heavy and difficult to handle, you can incur extra costs by renting the necessary equipment.

Texture and Appearance 

Different materials used in formwork have different textures. As such, they can affect the final concrete&#;s surface. If you need a smooth surface, make sure to choose the appropriate formwork materials. When different materials are used to cover the surface, the textures and effects of the formwork may not be an issue.

Usability

Whatever project you are working on, it is critical to have simple formwork to handle, install, and remove. This will assist you in staying on schedule and within budget for your project. On the other hand, difficult-to-manage formwork can result in expensive delays, unanticipated costs, and other complications.

Joints

Joint strength is essential in formwork from various perspectives. For one thing, the strength of the joints affects the formwork&#;s total strength. Second, suppose the joints are not strong and secure. In that case, you risk wasting a lot of material and producing a subpar final product.

Safety

Formwork should add to, not detract from, the safety of your building site. This feature is influenced by the materials used to construct the formwork, its strength, the strength of its joints, and its simplicity of usage. Strong materials that are well-assembled, suitable for the task at hand, and easy to deal with will contribute significantly to job site safety.

Types of Concrete Forms

A square foot of ordinary concrete weighs about 150 pounds, and a typical concrete project may need the placement of hundreds to thousands of square feet of concrete at once. Concrete forms must support all weight, so most forms are constructed of sturdy wood or metal. Although advances in concrete forms made of plastic, fiberglass, and resins have occurred in recent years, the cost and strength of these materials have not yet surpassed the established performance of metal and wood.

Concrete Wall

Pre-manufactured forming systems are often utilized for pouring walls or bigger structures such as piers or foundations. These wall systems, usually constructed of engineered wood with a metal frame or entirely metal, connect through a pin or latch mechanism. Additionally, these kinds of forms use a tie system to attach reinforcing bars inside the form and connect adjacent sections during the pouring of the walls. These forming sections are available in various sizes and forms. The majority of manufacturers can accommodate special sizes for particular applications.

Wood

Concrete slabs are most often constructed using hardwood planks bolted or fastened to wood or metal pegs. Contractors drive stakes into the prepared subbase and then level the forming boards using leveling equipment (manual, laser, or string lines). Following that, more boards are utilized to secure the places where one board interconnects with another.

When turns, rounded edges, or free-form designs are needed, thinner cross-section boards are used. To keep concrete from sticking to the forms, they are often coated with a low-grade oil or form-release agent. This also keeps the forms cleaner and allows them to be used multiple times before being discarded.

Insulated Concrete (ICFs)

The drive toward more energy-efficient house building has resulted in a meteoric rise in insulated concrete forms (ICFs) popularity. ICF systems are made out of hollow insulation blocks (often expanded Styrofoam) that fit together similarly to children&#;s construction blocks. ICF systems are built on the foundation slab and then stay in place to serve as both the foundation and outer wall system.

A network of metal reinforcing bars is installed within the block wall construction. The walls are subsequently filled with high-slump concrete. The resulting sandwich of foam and concrete is highly energy-efficient. The foam serves as both an interior and exterior building surface. Using ICFs avoids the requirement for form removal since the hollow block walls stay in place after the concrete is poured.

Conclusion

These are just a few variables to consider when planning a building project and choosing the kind of formwork that will best suit your requirements. Formwork should always be seen as a tool to assist you in effectively completing a project. If you need assistance in determining the precise requirements for the formwork you will require, feel free to contact the professionals.

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