Precast concrete is a construction material produced when concrete is cast in a reusable mold and then cured in a well-controlled environment before being transported to the construction site. Structural members like concrete frames, walls, and floors can be constructed using precast concrete to save time and resources.
The benefits of using precast concrete include quality assurance, increased time efficiency, enhanced construction safety, durability, reduced wastage, and neat working areas due to reduced clutter. Precast concrete is also resistant to fire and flooding and can be easily deconstructed.
Read on to learn more about the advantages of using precast concrete in construction. The article will also examine some of the disadvantages of using precast concrete as well.
What Is Precast Concrete?
Precast concrete derives its name from its off-site production. It is usually constructed in several stages to ensure it’s strong and capable enough to be used on site. Depending on the project’s scope, some precast concrete is prestressed with steel or cable reinforcement to increase the overall strength of the structure.
However, not all precast concrete is prestressed, so it’s best to know whether the type of structure you’re constructing requires further reinforcement.
Below are some of the main stages of preparing precast concrete.
Engineering
Before concrete is placed in a mold, it must first be designed and approved. Engineers use the latest design tools and software to make detailed drawings, which are then forwarded for approval. Once approved, the design drawings are then used as a blueprint for the rebar cage assembly and the entire precast project.
Manufacturing and Assembling the Reinforcement Cage
The first step in this process is cutting and bending steel to satisfy the measurements provided in the previous step. The rebars must be cut with utmost precision to ensure the resulting precast has the desired dimensions.
Once cut into different shapes and sizes, the bars are then assembled and carefully tied together to form the much-needed reinforcement cage.
Form Preparation and Pre-Pour Inspection
There are various steel forms used in precast factories, which usually depend on the type of project. The forms must be prepared by securing embedded items or openings cutouts before a form release agent is applied on all surfaces to ensure that the final product is easily lifted from the form.
After the form is prepared, an overhead crane then picks and places the cage in the form’s core. This step requires high-levels of precision and must be followed by pre-pour inspection from a certified quality control (QC) expert.
Concrete Placement
For the project to yield the desired success, precast concrete manufacturers must use the highest quality materials. The ingredients tend to vary depending on construction scope, but the most common raw materials include cement, sand, coarse aggregates, and chemical admixtures. These ingredients must be mixed in the correct ratio to suit the design of the final structure.Most high-quality precast factories usually have laboratories to conduct routine raw material tests and also check on the preferred quantities for each raw material. Among the many tests that concrete undergoes before placement is the spread test, whose aim is to verify the resulting mix has the right flow without any segregation.
The concrete is then carefully lifted and placed on the form, with extra care given to ensure the concrete floats without entrapping air. After the form is filled, the top part is screeded, and the specified finish is applied.
Curing and Stripping
Precast factories usually have controlled environments that allow the concrete to cure and reach full strength before transportation. Stripping will occur once a quality control technician confirms the product has achieved the desired strip strength (usually 2500-3000 psi).
To strip the precast concrete, the form’s outer jacket is removed before the inner core is carefully collapsed to allow for the attachment of lifting devices. The product is then removed and cleaned before the post pour inspection commences.
Post-Pour
Due to the little room for error when dealing with precast concrete, a thorough post-pour inspection must be conducted to ensure all design dimensions are maintained and the precast concrete is free from any visual defects.
Once the product is given a clean bill of health, arrangements can be made to transport the precast concrete to the construction site
Precast concrete has become increasingly popular in modern construction not only due to its strength but also due to its durability and ease of installation. Below are some of the main benefits associated with using precast concrete in construction projects.
1. Superior Quality
Before precast concrete is transported to the construction site for installation, it must undergo various tests in the precast factory. Quality assurance technicians constantly inspect the concrete right from mixing the cement, sand, and aggregates to preparing the mold or form for use.
Precast concrete plants are usually packed with laboratories that help to examine the chemical constituents of the ingredient. The labs ensure that the resulting concrete is well mixed and balanced, which significantly reduces the chances of error.
Moreover, before the concrete is poured onto the mold, it must be tested by quality control technicians to ensure it has the right properties requested by the client. The final product is also inspected for irregularities once the mold is removed. All these tests and inspections boost the quality level of precast concrete, ensuring it is of the highest standard.
2. Reduces Overall Construction Time
Preparing precast concrete is usually done on precast plants or factories. This allows construction engineers to focus on other projects on the site like surveys and earthwork. When the preparation of precast concrete is completed, it’s usually transported to the site, which is more often than not well prepared for immediate installation work to begin.
The beauty of working with precast concrete is that other construction tasks on site can continue simultaneously, thus saving time. By the time the precast concrete arrives at the construction site, several other tasks are usually completed to ensure construction continues as expected.
Also, since precast concrete is usually prepared and cured in precast plants, there won’t be any need to wait for the mixture to dry and strengthen. This helps to save a considerable amount of time, especially since construction continues immediately after the precast members are delivered to the site.
3. Can Use Prestressed Concrete
For heavy projects with immense loads, precast plants can use prestressed concrete to improve the overall strength and load-bearing capacities of the resulting precast. Using prestressed concrete for large construction projects helps minimize the overall cross-section size of load-bearing members, thus reducing construction costs.
The ability to integrate different concrete types in precast plants gives architects and construction engineers the flexibility and freedom to try out new designs. This makes precast concrete suitable for light, medium and heavy construction since necessary modifications can be made when preparing the mixture at the plant.
4. Enhances Safety at the Construction Site
One of the main advantages of using precast concrete is the elimination of clutter and unnecessary materials that otherwise overcrowd the construction site.
And since precast concrete only requires lifting and installation once on the site, not too many people will be needed on the site compared to when concrete is cast in place. The reduced crowding plays a huge role in increasing overall safety since there won’t be too many people or potentially hazardous tools lying around the site.
No raw materials need to be stocked on the construction site, meaning no traditional formworks or props will be brought on site. This helps to make the construction site safer and more manageable until the project is completed.
5. Highly Durable
Precast concrete is known to last long without needing too much maintenance. High-density precast concrete is usually made using lab-tested components, which translates to a more durable and corrosion-resistant product.
The impressive corrosion-resistant features of precast concrete make it an ideal option for a wall, pillar, and culvert construction. Precast concrete is resistant to moisture, thus making it an ideal option for constructing foundations.
6. Promotes Sustainable Construction
Precast concrete is manufactured using environmentally friendly procedures, thus making it sustainable in both the short term and long run. The water used in making concrete is usually recycled in precast plants to reduce overall costs.
Moreover, precast concrete is usually made from natural aggregates such as sand, rock, gravel, and water, which can be easily accessed without damaging the environment.
Cast in situ more often than not leads to environmental pollution on the construction site. The bracing and formwork used in the production of precast concrete help limit excessive usage of concrete while also eliminating the debris and packaging that usually builds up when casting in place.
Additionally, precast structures usually use fewer materials, thus reducing the total strain on the environment. The resulting structures, such as walls and slabs, are usually reusable and can be easily detached from the structure when the need arises.
Another factor that makes precast concrete production sustainable is the safe nature of the controlled factory environments. The employees working on precast factories work under better conditions compared to their counterparts mixing concrete on site. This is because precast plants can control noise pollution, air quality while also reducing safety hazards.
7. Enhances Project Efficiency
The efficiency of precast concrete stems from its manufacture in a controlled environment. Precast concrete can be produced in large quantities all year long without having to worry about weather conditions. The ability to produce precasts in scale (and in advance) means clients can place orders and store them until they’re required on site.Precast concrete also helps to save money due to the fluid assembly line techniques that require reduced labor. There’s also a reduced need to coordinate on-site labor and logistics, which contributes to the overall efficiency of precast concrete.
8. Increases Project Control
Precast concrete gives construction engineers the guarantee that construction will proceed regardless of the weather. Variables like weather usually pose a great challenge, more so in regards to project completion.
However, precast concrete ensures the construction team has complete control over climate, especially since precast concrete products are usually cured in ideal conditions. Put simply, precast concrete helps to eliminate inconsistencies associated with weather, thus ensuring that deadlines are met.
9. Versatile
Precast concrete can be molded into different shapes and sizes to suit the design of the project. This makes it highly convenient for projects with unique designs.
While some people might argue against the versatility of precast concrete, the trick is to account for the unique shape or size in the first stage (engineering) of producing precast concrete. Understanding the desired dimensions allows plant workers to cut the bars and assemble them accurately to form a suitable reinforcement cage.
The reinforcement cage’s shape and dimensions must align with the molding or form dimensions to ensure the resulting precast is of the desired size. The ability to play around with shapes, sizes, and textures make precast concrete among the most versatile concrete types.
Precast concrete’s versatility is also observable in its ability to be recycled or reused upon deconstruction. This allows owners to put the precast walls or structures into other uses if they opt to remove the structure. Also, through precast technology, construction engineers can create everything from long open spans to small sections with utmost accuracy.
10. Aesthetically Pleasing
Since precast products are usually prefabricated in a controlled plant environment, it’s possible to combine different textures and colors to complement the unique shapes. The expert assessment at every stage of production reduces the chances of error while also enhancing the chances of achieving the desired outcome. Precast concrete is behind the construction of some of the most aesthetically pleasing buildings, which has played a pivotal role in making this type of concrete popular. Schools, hospitals, offices, and residential apartments have, in recent years, become increasingly reliant on precast concrete during construction.
Disadvantages of Precast Concrete
Although precast concrete has several advantages, it does come with some flows too. Below is a brief description of precast concrete’s main disadvantages.
Must Be Transported to the Construction Site
Unlike cast in situ concrete, precast concrete must be moved from the plant to the construction site. The need to transport precast concrete translates to additional transport costs, especially if the precast factory is far from the site. This can also lead to inconsistencies in delivery time, especially when dealing with bad roads or poor weather during delivery.
Susceptible to Damage During Transportation
The risk of damage is usually high when transporting precast concrete from one location to another. Precast concrete is normally heavy once produced and cured, and excessive impact during transportation will almost certainly damage the product.
As a result, extra care must be undertaken by the production company to ensure the precast concrete gets to the location safe. And once the precast concrete arrives on site, the loaders and off-loaders must practice caution to prevent the heavy walls or slabs from slipping and falling.
Damaged precast concrete means more time will be required to complete the project, which isn’t ideal when working against tight deadlines.
Requires Special Equipment
Precast concrete is naturally heavy and needs special tools like cranes to lift more often than not. A tower crane is necessary when looking to install precast concrete on high rise buildings. Semi-trailers will be needed to transport the precast panels to the tower cranes, where they’ll be gently lifted before being placed into position.
Mobile cranes and crawler cranes might also be used to lift precast concrete, which adds to the project’s total cost. And since not too many cranes can be set up on-site, the precast pieces’ installation time might take longer than expected.
Improper Installation Will Compromise the Structural Integrity
Precast members are usually installed separately to function as part of a larger unit. And since the joints between the precast pieces tend to create structural discontinuity, the improper installation might lead to weak structures.
Expert supervision is required during installation as the precast concrete needs to transfer load pressure and forces safely and securely for the building to remain strong. The need for attention to detail means there’s little room for error when dealing with precast concrete.
The joints between adjacent precast members must be sealed with special, construction-appropriate sealants to keep them waterproof.
Factory vs. Site Precasting
Besides the factory or plant, precasting can also be done at a casting yard inside or close to the construction site. Casting yards are often preferred when there’s no precasting plant close enough to the construction site. And since construction costs tend to surge when transporting heavy loads for long distances, some contractors prefer setting up yards on site.
Of course, factory precasting is preferred to site precasting since factories usually have controlled environments and laboratories for examining the mixtures and ratios.
But for a precasting yard to be effective, the following must be observed:
- Raw materials such as aggregate, cement, water, admixtures, reinforcement bars, and formwork must be availed to the site.
- There must be a separate yard for formwork
- There must be a concrete mixing area
- There must be a steel reinforcement area for making rebar cages
- There must be a casting area
- There must be a curing area
- There must be a stacking area for placing the finished components
Precasting concrete on site doesn’t have as many benefits as doing so in the factory. Issues like inconsistent weather, which are non-issues in factory precasting, become major concerns when precasting concrete on the site. Inconsistent weather can lead to delays and even affect the curing of the concrete, thus leading to the wastage of both time and resources.
Site precasting is also known to strain the construction site, leading to clutter and different types of pollution. The tools used to make rebar cages and formwork, due to shortage of space, might pose a threat to the overall safety of construction workers.
Precast Concrete vs. Cast In Situ Concrete
Precast concrete has been compared to cast in place concrete for several years. The main difference, as suggested by the names, is that precast concrete is produced in a controlled environment inside factories. Cast in situ concrete, on the other hand, is prepared on-site.
Below are some more differences between precast concrete and cast in place concrete:
Casting
When using precast concrete, casting of slabs, beams, and columns is usually done in advance, thus saving time as they’ll only need to be set up and installed on site. Contrastingly, construction workers must allow cast in situ concrete to settle, harden and strengthen over time before proceeding with the construction.
Inconsistencies in the casting process more often than not lead to delays in critical tasks under the critical path, which makes cast in situ concrete inappropriate for fast-track construction projects.
Quality
Precast concrete is made with the highest level of precision. As revealed in the earlier sections, precast concrete is usually subjected to several tests from a certified QC technician. Even better, precast concrete is cured under controlled conditions inside the factory, ensuring that the final product is exactly what was ordered.
While cast in situ concrete can still be of the highest quality, the success of the job usually depends on the skill levels of the workers. Skilled and experienced workers are usually preferred when it comes to pouring concrete to reduce the chances of error. However, this doesn’t guarantee success since they don’t have laboratories to investigate the ingredient ratios.
Labor
Since most of the work is done off-site, precast concrete usually requires small amounts of skilled laborers to install. However, cast in situ concrete requires more work done in preparing the ingredients, mixing, pouring, and curing the concrete. Skilled workers are pivotal towards the success of cast in situ concrete, which translates to increased costs.
Speed of Construction
Once precast concrete is delivered on-site, the construction workers won’t need to allow the structures to gain strength as they’re usually ready to install. On the other hand, cast in place concrete requires time for the concrete to settle and gain strength. A minimum of 28 days is usually required for cast in situ concrete to gain full strength, which translates to more construction time.
Suitability to Weather
One of the best features of precast concrete is its weather resistance. Structures made from precast concrete aren’t subject to shrinkage, distortion, or deterioration due to weather elements and can be set in place even in rainy weather.
Cast in place concrete, on the other hand, cannot be poured in poor weather conditions. Moreover, cold weather can increase the curing time of cast in concrete, thus extending the project completion dates. Construction workers must practice patience when looking to work during bad weather.
Types of Projects That Use Precast Concrete
The use of precast concrete in modern construction has increased significantly over the years. The increased popularity is as a result of the numerous benefits that come with using precast concrete in projects.
Below are some of the main projects that use precast concrete:
Precast Concrete Structures
- Parking spaces: Precast concrete is commonly used in parking structure design due to its durability, economy, and relative ease of installation. Several members, from columns to paving slabs, stairs, and even traffic barriers, are usually made from concrete.
- Bridges: Precast concrete materials are used for girders, arches, beams, deck slaps, and caps, among other members. The durability, strength, and weather-resistance of precast concrete make it ideal for usage in bridges as not much maintenance is needed once installed.
- Culverts: Most engineers prefer precast concrete to construct culverts due to their resistance to water and temperature changes, making it ideal for wastewater and stormwater drainage. Precast concrete is also commonly used in the construction of catch basins and curb inlets due to its moisture-resistant properties.
Precast Concrete Buildings
- Multi-unit housing: Precast concrete is known for its fire resistance, which helps to lower overall fire insurance rates. The thick precast walls also help in sound absorption, making it ideal for apartment buildings, hotels, complexes, and dormitories.
- Schools: The use of precast concrete in the construction of schools allows for fast construction, which is vital in preventing delays in school schedules.
- Retail shopping centers: Precast concrete is ideal for constructing shopping malls in both rural and residential areas. The ease of installation allows investors to fast track their construction, provided expert care is undertaken in the production and installation of precast concrete.
- Medical centers: Thick precast concrete walls are great in sound absorption, a feature that is ideal in hospital settings. Moreover, fast construction time means hospitals and medical centers can be constructed without any unforeseen delays.
Winding Up
Precast concrete is among the most popular types of concrete due to its strength and durability. However, other advantages such as resistance to weather, ease of installation, quality assurance, and sustainability make precast concrete stand out as a reliable type of concrete for large and small scale construction.
Despite the numerous advantages, precast concrete is highly susceptible to damage during transportation. This means the construction team must be extra careful when offloading and installing the precast concrete structures.
Sources
- Wikipedia:Precast Concrete
- Civil Blog: What Are The Advantages And Disadvantages Of Precast Concrete
- Civil Today: Advantages And Disadvantages Of Precast Concrete
- The Constructor: Features Of Precast Concrete Construction
- Lifting Matters: Safe Work With Precast Concrete
- Concrete Network: Precast Concrete Advantages
- Hunker: Concrete Form Release
