When fresh concrete mix is prepared, it ought to be transferred to the molds to get the desired shape. To do so, the concrete is to be poured into the formwork in its wet or plastic state and it begins to set and gets hardened after a certain time.
The transferring of plastic concrete from a mixing truck or hopper into the formwork or molds is termed as pouring of concrete. The process of pouring concrete requires care and caution and is generally carried out by workers or masons using chutes or shovels. They help direct the concrete into the mold without any sideway spillage.
Once the concrete is poured into the formwork, the top of it has to levelled and finished. Following this, the placed concrete is compacted to remove air voids using an internal or external vibrator (depending upon the depth of concrete in the formwork). All these steps are part of placement and finishing operations of concrete.
The pouring of concrete is a crucial stage in the overall construction work in case of concrete structures. This is because it decides the final shape and surface finishing of the placed concrete. The pouring height, the concrete’s consistency and flow are important factors to be considered for a durable finish.
It is also important to properly cure the cast concrete in order to allow it to gain strength for the required period up to 28 days.
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Methods of Pouring Concrete
The concrete can be placed into the formwork using a number of pouring methods. Some of these methods are given as follows;
Hand Pouring of Concrete
The simplest method of pouring concrete into the formwork is by manual method using shovels or buckets. However, this method is only applicable if the produced concrete quantity is small and the member to be cast is also small.
In addition, in areas that are hard to be accessed by pumps or conveyors, hand pouring of concrete may be carried out.
Concrete Pouring using Chutes
A chute is an inclined platform used for conveying things to a lower level in the direction of gravity. Their use in conveying concrete is very common for medium and small-scale construction projects.
Chutes are used to direct and convey concrete to where it is to be placed. They are an efficient way of pouring concrete into the formwork. The unidirectional path provided for concrete to flow prevents it from getting poured sideways out of the formwork.
Concrete Pouring by Pumps
When concrete is to be poured in bulk quantities for large structures, we generally prefer pouring it by pumps into the erected formwork. Mostly, reinforced concrete members are cast using pumps. This method of concrete pouring ensures continuous flow of concrete and despite being expensive, it ensures that the concrete is consistent and flows easily all around the reinforcement.
Slip-forming of Concrete
Slip forming is a process of concreting usually adopted for concrete walls where the formwork is continuously raised and the concrete is poured at each increment of formwork.
In addition, for tall structures such as silos and towers, the pouring of concrete becomes a challenging operation. In such situations, slip-forming may be done in order to continuously pour concrete into the formwork and once the underlying concrete sets, the formwork is shifted upwards and concrete is poured.
The pouring of concrete is not just dependent on the method used; it also depends on how the formwork is prepared and the quality of concrete mix to be poured. Post-pouring operations such as compacting and curing concrete are also important for the overall success and durability of the concrete structure.
Steps Involved in Pouring of Concrete
For any concreting work, the following steps are to be adopted for efficient pouring and post-pouring operations.
- The foremost step is to prepare the formwork at the place where a concrete member is to be cast. This includes erecting the molds, cleaning and oiling them and ensuring a level surface for concrete to be laid. Oiling will prevent the concrete from sticking to the mold
- The concrete is then prepared or if it is ready-mix concrete, it is delivered to the construction site. The properties of the mix should be regulated and monitored.
- The concrete is then poured into the molds using any of the above-detailed methods (buckets, chutes, pumps, etc.).
- The initial setting of concrete begins immediately after it is poured into the formwork. Therefore, the compaction of concrete after pouring should not be delayed and it can be executed using vibrators. The purpose of doing so is to remove entrapped air bubbles in the concrete that will otherwise create unsightly voids in the hardened member.
- Following this, the concrete’s top surface is levelled and made smooth by screeding. This includes using a screed board to remove any apparent irregularities on the concrete surface.
- After the screeding operation is done, the finishing of concrete is done using trowels to get the desired surface texture.
- Once the concrete hardens, it is to be cured for it to gain full strength. Different curing methods such as ponding, steam curing, chemical curing, etc. may be adopted depending upon the member size and the feasibility of curing operation at site.
Precautions in the Pouring Process of Concrete
Great care is to be exercised while pouring concrete and the following main precautions should be kept in mind while you do so;
- Before pouring the concrete, you must ensure that the formwork is level and square and is secured in place. In case of any imperfections, shims may be used to level them out.
- Care should be exercised while placing the reinforcement mesh into the formwork. Proper concrete cover at the bottom and sides should be ensured and the mesh should not be too congested to prevent concrete from flowing through it.
- The concrete mix should be homogenous and workable. Low workability mixes are difficult to flow through the steel mesh and very high workability concrete is prone to segregation, leaving excessive voids in the hardened concrete.
- The pouring height of concrete should be monitored. If you will pour concrete from large heights, it might lead to segregation.
- Once the concrete is poured into the formwork, it is to be compacted using vibrators. However, over-vibration of concrete should be avoided as it may lead to bleeding of concrete, resulting in a weak concrete structure with non-uniform strength.
- If concreting is to be carried out in high-temperature regions, the concrete will harden at a rapid rate. In such cases, it is important to protect concrete from rapid temperature changes and direct sunlight.
- Maintaining proper curing is another essential step to successful concreting. This helps the concrete gains its full strength. Curing means to prevent the escape of moisture in concrete till it develops the required strength. To do so, the concrete is kept in an environment that either ponds it or maintains a certain level of moisture and temperature around it.
- Oftentimes when the formwork is removed, the follies you make during concreting operations get unveiled. Despite a good concrete mix, you end up with a bad structural concrete element having excessive voids. The formwork should be removed after the due time and with care. Stripping off the formwork harshly may lead to disintegration of a part of concrete.
Maximum Pouring Height of Concrete
Concrete cannot be poured form any height; it must be monitored on the site. The maximum height at which concrete can be poured depends on a number of factors. These include;
- Workability or consistency of the mix
- The type of formwork used
- Method of pouring used
If you are using a chute or pump to pour concrete into the formwork, the maximum pouring height can go up to 13 to 16 feet (equivalent to 4 to 5m). However, if pouring by hand is to be done, the maximum height can be 6 to 10 feet (equivalent to 2 to 3 meters).
In some cases where drop height of concrete cannot be maintained within the above ranges owing to site restraints, specialized equipment such as tower cranes, high-rise concrete pumps, etc. may be used to pour concrete from even greater heights. However, certain regulations are to be practiced in such cases and it is better to consult concrete specialists.
Frequently Asked Questions (FAQs)
What are the features of a good concrete formwork?
The formwork used for pouring concrete should have some target properties as given below;
- It should be durable and strong and made of an impervious material. It should be capable of bearing the load of wet or plastic concrete placed on it and should not deform or buckle.
- It should be level on the inside with straight edges and a cross-section that is uniform.
- It should be easily removed without leaving any signs of deterioration on the concrete surface and should also not require too much cleaning once removed.
- It should be reusable and have a good service life.
- It should not be very costly and can easily be disposed without damaging the environment.
- It should be available or made available in different shapes and sizes and should suit the dimensions of the concrete member to be cast.
- It should be easy to clean.
What is the most effective way of pouring concrete into the formwork?
The best pouring method to be adopted varies from project to project and also depends upon other factors. These include the member size and shape, the access to the area where concreting is to be done, the quantity of concrete required, the available equipment and labor and also the local codes of practiced used in your locality.
These codes of practice impose certain safety regulations while the concreting operations are to be carried out. If you are stuck and not able to decide the pouring method, consulting a concrete specialist is the first thing you should do.
Some concreting methods such as pumping are more expensive than others such as using chutes. Therefore, apart from the feasibility of using a method, the economic viability of it should also be taken into account.
In what areas is it difficult to pour concrete?
Concrete pouring can be an arduous task in certain situations which include;
- In areas where access to the member is hard and the equipment cannot easily be maneuvered, the concreting operation becomes a challenge.
- It is also hard to cast concrete for foundations that are laid on top of unstable soils. This is because of the vulnerability to settlement and cracking because of uneven ground conditions.
- In adverse weather condition, a concrete structure is hard to be executed in time. This is because extreme high or low temperatures, heavy rainfall or other forms of precipitation not only tend to slow down the concreting work but also have a negative impact on the hydration reaction, hardening, and strength gain process of concrete.
- In areas where huge amount of concrete is to be cast at a time as in case of a concrete gravity dam, the pouring of concrete is a challenging task. This is because huge amount of heat is generated that further accelerates the setting and hardening of concrete.
- Concrete members that are designed as curved or have a complex geometry are difficult to cast and finish.
At what temperature should concrete be poured?
The range of temperature which should not be violated when pouring concrete is between 10 degrees Celsius and 32 degrees Celsius. Temperature greater than this will lead to quick setting and early onset of cracking and lower temperatures may lead to the problem of freeze-thaw damage in concrete.
When should formwork be removed?
The time for removal of formwork varies from one member to the other and also depends upon the environmental conditions (temperature, humidity, etc.) and curing time
- For reinforced concrete slab, formwork is typically removed after 1 to 7 days depending upon the thickness of the member. Thicker members require more time for gaining strength and accordingly, the formwork is removed.
- For reinforced concrete beams, the formwork may be removed at 3 to 14 days depending upon the member size and the loading it has to bear.
- Just like beams, the time for removal of formwork for reinforced concrete columns also varies between 3 to 14 days and the exact time depends on the type of column and its size, the intensity of loading it has to bear, etc.