Concrete Design and Formwork Fundamentals

The reinforcing bars run parallel to the length of the footing and are typically placed at the bottom for maximum support.

According to the American Concrete Institute, the design load for an elevated slab formwork includes the weight of the slab itself plus an additional allowance of 50 lbs per square foot (lbs/ft²) for construction activities.

Control tests for slump, air content, compressive strength, and yield should conform to the established standards and specifications outlined by the American Concrete Institute (ACI) and other relevant industry codes.

Too much air in concrete weakens its strength, reduces its durability, and can lead to increased shrinkage and cracking. It also affects the ability of the concrete to resist water penetration.

The design procedure that accounts for the anticipated downward deflection of formwork when the concrete fills the form is called a 'deflection allowance' or 'deflection design.' This design allows the concrete to cure and harden while the formwork deflects slightly, preventing excessive stress on the concrete.

The best type of nail for formwork bracing is a 'ring shank nail' because it holds well for the duration of the pour and can be easily removed once the concrete has hardened. Ring shank nails provide more holding power than straight shank nails and also minimize damage to the wooden forms when removed.

The best way to reduce lateral pressure on concrete forms is to ensure proper placement of concrete within the form. Using a concrete pump or placing the concrete in layers can help minimize lateral pressure. Also, using properly designed and braced forms is crucial.

Calcium chloride is added to concrete in cold weather to accelerate the curing process. It speeds up the chemical reaction of hydration, which generates heat and helps the concrete set even in low temperatures.

Lightly dampening the subgrade (the soil beneath the concrete slab) before placing the concrete helps minimize water absorption from the concrete. This improves the bond between the concrete and the subgrade, preventing cracking and ensuring a strong foundation.

Fly ash is an admixture used to cool down hot concrete that has too much water. Fly ash is a fine, powdery byproduct of coal combustion, and its addition to concrete helps reduce the temperature of the mixture.

High-range water-reducing admixtures, also known as superplasticizers, are best for finishing concrete that has a high temperature because they allow for improved workability and flowability. This enhanced workability helps to spread the concrete more easily, allowing proper finishing even in high-temperature conditions.

Using admixtures can enhance the concrete's performance properties, alter its setting time, improve workability, and increase durability. However, it's NOT a reason to use admixtures for increasing the compressive strength of the concrete. To achieve higher compressive strength, one should use a higher proportion of cement or select a higher strength concrete mix.

The American Concrete Institute (ACI) recommends that higher temperatures, such as those experienced during hot weather conditions, require faster finishing times to prevent the concrete from setting too quickly. This is due to the accelerated rate of hydration at higher temperatures.

Laminated fiber forms for columns should be braced if they are over 40 feet tall. This is to prevent the forms from collapsing and ensure safety during the concrete pour.

High-range water-reducing admixtures are also known as superplasticizers. They are potent chemicals that reduce the amount of water needed in a concrete mix without sacrificing workability, allowing for stronger, more durable concrete.

Cold-weather concreting requires special precautions when the mean daily temperature is below 40°F (4°C) or expected to fall below that point. The concrete may not cure properly and may be susceptible to damage.

Slip forms offer good reuse of equipment because the forms are moved as the concrete is poured, enabling continuous pouring without having to dismantle and reassemble the forms. This results in fewer forms needed, larger concrete placements (allowing for faster pouring), and decreased erection time.

The depth of the control joint for a slab that is 5 inches thick should be approximately 2.5 inches. This depth allows for controlled cracking at the joint, preventing random cracking elsewhere in the slab.

Concrete that was originally cast below 40 degrees Fahrenheit will have its strength compromised, potentially causing a premature cracking and/or a failure of the structure.

Too much air in the concrete can lead to it becoming less dense and less strong. This is because the air pockets fill the voids in the mix, reducing the actual amount of concrete material.

A slump test is vital for checking the consistency of the concrete mix. It measures the amount of slump or sagging in a cone-shaped sample of fresh concrete, indicating its workability. A higher slump indicates a more fluid and workable concrete mix.

The compressive strength of concrete can be affected by several factors, including the water-cement ratio, the type of cement used, the curing conditions, and the presence of admixtures. A higher water-cement ratio generally leads to lower compressive strength.

To effectively minimize the risk of cracking in concrete, it is vital to incorporate proper control joint placement. These strategic cuts regulate the direction and pattern of cracking, ensuring a more controlled and predictable outcome.

Calcium chloride is generally avoided in concrete for construction because it is known to accelerate the setting time, but it also increases the risk of cracking and corrosion in steel reinforcement if used in excessive amounts.