Q.
Discuss the main advantages and disadvantages on in-situ concrete frames, steel frames and pre-cast concrete frames.
(25 marks, 2019 Q2)
A.
In-situ Concrete vs Prefabricated Steel frame
In situ concrete
Also known as the most traditional below-ground construction method, main contractors and construction teams have been using in situ for the past 80 – 100 years. Very little is fabricated or constructed off-site, although sometimes the reinforcement or shuttering can be modular and delivered part-assembled.
This is typically a complete on-site approach that involves excavating a large hole, with timber shuttering and steel reinforcements all taken to site, pieced together with steel fixers and carpenters. The concrete is then poured between the shuttering.
Once the structure is built the waterproofing is then applied.
This method of below-ground construction may be traditional, but it can be very challenging. Often, the environment can be tricky, plus, you’ll need operatives to enter the excavation to complete the works. And that’s not even mentioning the weather conditions in the typical good old UK!
Not forgetting the logistics involved in moving the materials, and general site constraints. All of this can impact on the quality of the build, which can mean increased risk, stress and issues for you and your team.
TIP: In situ is probably still the best solution if your substructure must cover a large area – for instance (super basement).
The advantages of in situ concrete:
- It’s typically the cheapest method
- It can be very effective when properly constructed
- It’s a well-established method used for over 100 years
- It works well on large-scale substructures
The disadvantages of in situ concrete:
- There’s a chance it can leak once constructed – often caused by reduced skill levels and on-site quality control when being constructed
- It’s very labour intensive on-site work – and it can take a long time. You also have to wait for the concrete to cure before you can can load it
- This method can easily be out of tolerance (at around +-20mm)
- In order for your team to work safely, you’ll need a large excavation site
- Quality control on the finish is not assured
- It only lasts for around 30 years in the ground
- It tends to create a lot of waste
- Problems can occur if batches of concrete are mixed up. I.e. the wrong concrete mix is supplied during the concrete pour by the plant. This issue is only discovered after 28 days when the cube tests are completed and may lead to sections of concrete having to be broken out.
Ideally used for: Large-scale scenarios make in situ an effective choice – in fact, it’s often the only way large-scale projects can be completed.
Prefabricated steel
You might be starting to get a good idea why a more innovative method might be a better alternative. This comes in the form of prefabricated steel. It’s manufactured entirely out of steel, off-site in a factory environment, with strict quality control, and constant monitoring and controlling.
Unlike in situ, this newer, almost revolutionary method won’t leak or crack, due to the unique way it’s constructed out of steel plates which are then welded together. Think of it as similar to the hull of a boat.
It’s a particularly good solution if your project is in a high water table area, or if time constaints are absolutely vital to your construction programme – as time on-site is reduced by 70 – 80%. It can also be built to very, very high tolerances (+-2mm/3mm), and lowers long-term maintenance costs.
TIP: Want to know more about steel prefabrication? Read my recent blog: ‘What is Steel Prefabrication Below-Ground and How Does it Work?’.
To sum it all up:
Advantages of prefabricated steel:
- Jobs can be completed 70 – 80% faster (typically in a matter of days compared to weeks/months)
- This method will not leak
- It’s built to very, very high tolerances (+-2mm/3mm)
- The long-term costs are much cheaper, with less long-term maintenance required
- Prefabricated steel substructures can last for over 100 years in the ground
- It comes with a full design package from your subcontractor
Disadvantages of prefabricated steel:
- It can seem expensive
- It’s an innovative method that very few people are aware of yet. People who are unfamiliar with a concept are generally sceptical about it
Ideally used for: When your substructure is on the critical path of the programme, must be built to tight tolerances/precision engineering, or if a project is located in a high water table area and long-term water ingress is a concern.
Advice from a specialist subcontractor
Still weighing up your options? It may be a good idea to seek out a subcontractor who specialises in prefabricated steel, so they can talk you through your options, and give you a better idea of what’s possible. It won’t always be that prefabricated steel is the best option – for instance, if your project is on a large scale.
However, it’s still worth having a chat. Most people tend to be skeptical of prefabricated steel as a method of below-ground construction until they’ve seen the product being made – or have installed and used them for themselves.
So, you could ask to see examples of their work, and even get in touch with some previous clients to ask them about their experiences.
Advantages and disadvantages of Pre-Cast Concrete Frames
The form of construction where concrete is casted in a re-usable mould and then cured in a controlled environment (precast plant) is called precast concrete. The casted structural member is then transported to the construction site and then erected. Structural members such as concrete frame, concrete wall and concrete floors etc. can be constructed using precast concrete.
Advantages of Precast Concrete
There are many advantages of precast concrete. They are discussed below.
- Saves Construction Time: Precast Concrete construction saves time, the risk of project delay is also less. The precast concrete casting can be carried on simultaneously with other works on site such as earthwork, survey, etc. and thus saves time.
- Quality Assurance: The key factors which regulate the quality of construction such as curing, temperature, mix design, formwork, etc. can be monitored for Precast Concrete. So, improved quality construction can be performed.
- Usage of Prestressed Concrete: By using pre-stressed precast, structural materials of high strength and load-bearing capacity can be achieved, which can result in greater clear span, reduced size of the cross-section of structural members, etc.
- Cost-effective: The simplified construction process reduces the time, increases the productivity, quality and safety and thus the cost is reduced.
- Durability: Precast Concrete structure has a longer service time period and minimal maintenance. The high-density Precast Concrete is more durable to acid attack, corrosion, impact, reduces surface voids and resists the accumulation of dust.
- Aesthetics: As the structures are prefabricated in a controlled factory environment, several combinations of colors and textures can be used. A wide range of shapes and sizes are available to choose from with smooth finishing and thus the aesthetical value of products are increased.
- Safe Construction Platform: No raw materials have to be stocked in site for Precast Concrete construction. It reduces the requirement of traditional formworks and props, wastage, workers, etc. and thus provides a safe working platform.
Disadvantages of Precast Concrete
There are some disadvantages to precast concrete. They are discussed below.
- High Initial Investment: For installing a Precast Concrete plant, heavy and sophisticated machines are necessary which requires a high initial investment. A large scale of precast construction projects must be available to ensure sufficient profit.
- Transportation Issue: The construction site can be at a distant location from the Precast Concrete plant. In that case, the precast members must be carried to the site using trailers. In many cases, the reduced costs of Precast Concrete is compensated by the transportation cost.
- Handling Difficulties: Proper care and precaution have to be taken for handling precast concrete. Usually, precast members are heavy and large which makes it difficult to handle without damage. Generally, portable or tower cranes are used to handle precast members.
- Modification: Limitation In case of precast structures, it is difficult to modify the structure. For example, if a structural wall is to be dismantled for modification it will impact the overall stability of the structure.
- Sensitive Connection Works: Assembling of the precast members is one of the key points for ensuring strong structural behavior. Connections between several structural members must be supervised and done properly to ensure the intended behavior of the connection such as simple, semi-rigid or rigid connections. Besides this, faulty connections may lead to water leakage and fail sound insulation.
Ref:
Earlier post.
https://civiltoday.com/civil-engineering-materials/concrete/232-advantages-and-disadvantages-of-precast-concrete