Before assembling the moulds, make sure that there is no hardened mortar or dirt on the faces of the flange that prevent the sections from fitting together closely.
Cleaning the mould
These faces must be thinly coated with mould oil to prevent leakage during filling, and a similar oil film should be provided between the contact surfaces of the bottom of the mould and the base. The inside of the mould must also be oiled to prevent the concrete from sticking to it. The two sections must be bolted firmly together, and the moulds held down firmly on the base plates.
Preparing the mould
Steel cage preparation
Reinforced metal bar (rebar)
Steel (carbon or alloy) is melted down to liquid form, which requires an extreme amount of heat to achieve. Once melted, the liquid steel is pulled through small round openings to give the rebar its shape. While unfinished steel is the cheapest form of rebar available, some jobs necessitate epoxy-coated or stainless steel. The reason being that rust can occur when the rebar has prolonged exposure to salt water, which can ultimately lead to a buildup of internal pressure that can cause the concrete form to crack. Since this is not a profitable or safe option in the long-term, most developers will opt to purchase higher-grade material.
Once the steel has been properly shaped, the manufacturer will make the twists and grooves on the metal to ensure it will stay firmly in place inside the structure. Since these reinforced metal bars are highly hazardous when it comes to installation, their ends are often covered with plastic caps to prevent accidental harm to construction workers.
Figure 2.1: A bundle of rebar tied together.
Differences Between Tying and Welding Rebar
Tying rebar is the most common way of securing rebar together. This is because tying keeps the rebar cool, so structural issues down the road. It allows flexibility for the slab and the rebar to move independently to a certain extent without causing stress fractures in the product. It doesn’t require to get any particular variety of rebar unless the specs call for it and it is much faster to put together on the job site, especially when you have the proper tools for the job. Refer figure 2.2.
Not all steel can be welded. Because rebar is not as closely controlled in terms of metallurgical quality.But many inspectors won’t allow rebar to be welded, depending on their local codes, and you’ll need to remember to not quench the rebar, as this affects its ability to provide proper support in the finished product. Many people avoid welding rebar because the concrete and the rebar in the final piece will expand and contract at different rates, so having the rebar welded together creates pressure points where the concrete can crack.
Figure 2.2: Welded rebar Figure 2.3: Tying rebar
Casting of concrete
Concrete is a composite material which made up of a cement matrix with aggregates for reinforcement, that works well in compression, but not in tension. This problem can be solve by casting wet concrete around strong, steel reinforcing bars (tied together to make a cage). When the concrete sets and hardens around the bars, we get a new composite material, reinforced concrete also called reinforced cement concrete or RCC, that works well in either tension or compression: the concrete resists squeezing (provides the compressive strength), while the steel resists bending and stretching (provides the tensile strength). In effect, reinforced concrete is using one composite material inside another: concrete becomes the matrix while steel bars or wires provide the reinforcement.
The steel bars also known as rebar are typically made from twisted strands with nobbles or ridges on them that anchor them firmly inside the concrete without any risk of slipping around inside it. Theoretically, we could use all kinds of materials to reinforce concrete. Generally, we use steel because it expands and contracts in the heat and cold roughly as much as concrete itself, which means it won’t crack the concrete that surrounds it as another material might if it expanded more or less. Sometimes other materials are used, however, including various kinds of plastics.
Although reinforced concrete is generally a better construction material than the ordinary stuff, it’s still brittle and liable to crack: in tension, reinforced concrete can fail in spite of its steel reinforcement, letting water in, which then causes the concrete to fail and the rebar to rust. The solution is to put reinforced concrete permanently into compression by prestressing it (also called pretensioning). So instead of putting steel bars into wet concrete as they are, we tension (pull on) them first. As the concrete sets, the taut bars pull inward, compressing the concrete and making it stronger. Alternatively, rebars in reinforced concrete can be stressed after it starts to harden, which is known as poststressing (posttensioning). Either way, keeping concrete in compression is a cunning trick that helps to stop it cracking (and stops cracks from spreading if they do form). Another advantage is that it’s possible to use less prestressed or poststressed concrete or smaller, more slender pieces to carry the same loads, compared to ordinary, reinforced concrete.
Precast concrete is poured and molded over rebar or wire and then cured offsite. This involves pouring concrete into pre-made molds and then cured under ideal conditions within the manufacturing plant itself. Once hardened and ready for use, those preformed concrete products are shipped to the job site where they are assembled into the desired structure. As opposed to precast, site cast, sometimes known as in-situ concrete, is poured, molded and cured on site. Like precast concrete, on site concrete is formed in a mold and then lifted in place. However, one of its advantages over precast slabs is that it does not need to be moved far to be lifted into place. For a building that requires large, unwieldy and custom concrete molds, onsite is usually the way to go. Some forms are just too large to fit on the back of a flatbed truck. Conversely, the savings from precast concrete are scaled, meaning that for a small structure, in-situ concrete may be cheaper.
Figure 3.1: Precast concrete in the making.
Comparing precast and site cast concrete
Because precast concrete is mixed, poured and cured in a factory, ideal conditions and exacting measurements can be maintained throughout the process. Unfortunately, the logistics of site casting make this far more challenging. It is subjected to the humidity and temperature of the day the casting are done. The work can be done using far less precise tools. The result is an inferior quality product even under the most ideal condition
Precast is much more labor efficient. The work is done in a factory, the effort is maximized through the use of tools and machinery that simply isn’t available on the job site. Therefore, with site casting, work that might be done with machines has to be instead performed by hand. This is much more labor intensive, thus increasing labor costs and making the process costlier. Additionally, because machines aren’t involved, the labor needed for on-site casting needs to be skilled rather than unskilled. Thus, not only are more labor hours required, but those labor hours are costlier.
Curing conditions can be controlled in a factory, they can be accelerated without sacrificing strength or quality. That simply isn’t the case on site. While we can do certain things to accelerate curing on site, they are difficult and generally not worth the logistical hassle and costs. Furthermore, they run the risk of lower quality concrete, as delicate variables are hard to account for.
Concrete gains strength over time, it isn’t fully strong immediately after drying. However, with precast concrete, that hardening process takes place before arriving on the job site. That’s not the case with site cast concrete. Before you can raise the concrete into place, you have to wait for site cast concrete to harden fully. This can delay construction and increase costs. It is much more time and cost efficient to have fully hardened concrete slabs ready to be placed the moment they arrive. Furthermore, because precast receives a strength test during quality control inspections in the factory, you do not need to conduct strength tests on site. Strength tests are extremely crucial for ensuring that your building is safe, so if you are doing on site casting, then this is a step that cannot be skipped.
Cast ahead of time
With precast concrete, casting of materials can be done ahead of time, holding them until they are needed. Unfortunately, overlapping tasks and improving efficiency is all but impossible with site cast concrete. Because of the amount of space and labor required for on-site casting, we generally have to stop construction while waiting for the materials to be ready. This is a costly way to stand around and wait.
Compaction, leveling and finishing
Compaction is the process which expels entrapped air from freshly placed concrete and packs the aggregate particles together so as to increase the density of concrete. It increases significantly the ultimate strength of concrete and enhances the bond with reinforcement. It also increases the abrasion resistance and general durability of the concrete, decreases the permeability and helps to minimise its shrinkage and creep characteristics. Proper compaction also ensures that the formwork is completely filled and there are no pockets of honeycombed material and that the required finish is obtained on vertical surfaces. When first placed in the form, normal concretes, excluding those with very low or very high slumps, will contain between 5% and 20% by volume of entrapped air. The aggregate particles, although coated with mortar, tend to arch against one another and are prevented from slumping or consolidating by internal friction.
Even though the are many types of vibrators that can be used for compaction such as immersion vibrators, surface vibrators and under-vibration, Hume Concrete used form vibrators as their compaction process. Form vibrators which is normally called ‘external’ vibrators, are useful with complicated members or where the reinforcement is highly congested. They are clamped to the outside of the formwork and vibrate
it, thus compacting the concrete. The formwork will need to be specifically designed to resist the forces imposed on it.
Figure 4.1: Industrial form vibrators
Concrete levelling is a procedure that attempts to correct an uneven concrete surface by altering the foundation that the surface sits upon. It is a cheaper alternative to having replacement concrete poured and is commonly performed at small businesses and private homes as well as at factories, warehouses, airports and on roads, highways and other infrastructure.
Finishing may be defined as the process of leveling, smoothing, compacting and otherwise treating surface of fresh concrete or recently placed concrete to produce desired appearance. There are 3 different steps involved for finishing concrete, which are screeding, floating and troweling. Screeding is the process of striking off the excess concrete to bring the top surface to proper grade. While depositing concrete its thickness is kept slightly more than final finish. It is then moved by a ‘strike off’ board known as screed. A sawing motion of the screed is used as it moves on the side forms. Handles are attached to each end of the screed. When the distance between the side forms is more than 2 m, the screed is worked by two men (refer figure 4.2).
After screeding, voids left on the surface are filled with concrete and the process is repeated till uniform surface results. When the mix is dry, screed is also used as a tamper to bring mortar to the top for later finishing. Excessive tamping should be avoided. If the mix is dry, adjustments in its proportions should be made. For large jobs screeds can be fixed with rollers which move on side forms. Vibrators can also be fixed on screeds. Concrete should not be overworked. This brings an excessive amount of mortar and water on the surface. This results in low strength.
Figure 4.2: A man is screeding the concrete.
Next step is floating. Floating consists of removing the irregularities on the surface of concrete which are left after screeding. This is done with a wooden float. It is about 1.5 m long and 20 cm wide, attached with a handle. Low spots are filled with concrete and worked with float. Filling low spots with mortar should be avoided as it results in soft non-uniform surface. Finishing is done with the forward and backward motion of the float. In places where it is difficult to operate a long handle float, a wooden float 60 cm long and 10 to 12 cm wide with a handle can be used. If the area of slab is large and there are no walls and posts, bull float is used. It serves the same purpose as a float. It consists of a wooden or aluminium blade 2 cm wide and 1.0 to 1.5 m long. It is attached to a handle 5 m long. It permits floating without the operators getting on the concrete.
Figure 4.3: A man is executing floating to concrete.
Trowelling is the final operation of finishing. It provides a smoother finish which is hard and abrasion resistant. Trowelling may be necessary to finish points not finished in a satisfactory manner by floating. It should be done after all excess water has evaporated. Trowelling with a steel float when the concrete is almost dry gives a very smooth finish. The trowel is 25 to 50 cm long and 8 to 12 cm wide. The blades give better service after they have been used enough. Power trowels can also be used for very large works.
Figure 4.4: Trowelling of concrete by hand.
In Hume Concrete, there are only one identified curing process which is the steam curing. Hume Concrete produce special product such as Concrete railway sleeper which are used as the foundation of railway line. Steam curing increases the strength development of concrete is rapidly. Steam curing method a commonly used in pre-cast concrete work. In steam curing the temperature of steam should be restricted to a maximum of 75°C as in the absence of proper humidity (about 90%) the concrete may dry too soon. At this temperature, the development of strength is about 70% of 28 days strength after 4 to 5 hours. In both cases, the temperature should be fully controlled to avoid non-uniformity. The concrete should be prevented from rapid drying and cooling which would form cracks.
Figure 5.1: Concrete railway sleeper is being cured by using curing method.
During our site visit to Hume Concrete, cutting of concrete does not take place. However, there are many cutting methods in the industry such as floor sawing, remote controlled crushing and breaking, hand sawing and track sawing. The most popular cutting method is floor sawing. It commonly used diamond cutting method. It is typically used to cut horizontal flat surfaces such as floors, bridge decks, and pavement. Also called slab saws, floor saws feature a diamond blade that is mounted on a walk-behind machine requiring only one operator. Floor saws are typically used to provide expansion joints, remove damaged pavement sections, clean and repair random cracks and remove concrete sections for demolition purposes.
Figure 6.1: A man using floor saw on road.
Next, the remote-controlled crushing and breaking. The remotely operated demolition machines are the very latest in controlled demolition, featuring high power, low weight and functional design. They are highly powerful, stable machines with long reach the ideal solution to many of the environmental problems facing demolition today. These electrically powered hydraulic power packs are remotely controlled and immensely powerful. They produce no harmful fumes and can alleviate hand-arm vibration problems, traditionally associated with manual work in confined spaces.
Figure 6.2: A man controlling the remote-controlled crushing and sawing.
Besides that, methods of cutting concrete can be done by hand sawing and track sawing. Hand sawing also refers to the lighter duty use of diamond blades in hand-held power saws and chain type saws. Hand sawing provides portability, speed and accessibility at construction and demolition sites. Typical applications include sawing concrete pipes to length, creating smaller openings in masonry, eliminating overcuts associated with other types of sawing, and precision trimming. Track sawing employs a circular blade on a track-mounted machine. The track is attached to vertical walls or steep inclines or floors that will not permit the use of floor saws. Wall or track sawing is typically specified to cut precise dimensional door, vent and window openings. Straight as well as bevel cuts are possible with the wall saw. The wall saw is also an excellent choice for creating precise openings in any concrete structure.
Figure 6.3: A man using hand saw to cut to bricks Figure 6.4: A track sawing machine at site
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