CONCRETE WORKS
Concrete works include the following basic processes: preparation of the concrete mix; delivery of the mix to the construction site; feeding, distribution, and compaction of the mix in the formwork (molds); curing of the concrete as it hardens; and quality control of the concrete work.
Concrete is a hardening building material made of cement, fine aggregates (sand), and coarse aggregates (gravel) combined with water. The most prevalent type of cement used in the manufacturing of concrete is Portland cement.
Concrete technology is concerned with the study of concrete characteristics and their practical applications.
Concrete is used to construct foundations, columns, beams, slabs, and other load-bearing elements in building construction. Apart from cement, several types of binding materials are employed, such as lime in lime concrete and bitumen in asphalt concrete for road building.
CONCRETE MIXTURE
Lean Concrete
When the cement content of concrete is less than 10% of the overall components, it is referred to as lean concrete.
It’s widely used in foundations and flooring.
Lean concrete is exemplified by P.C.C. (1:4:8) and P.C.C. (1:6:12).
The cement content of PCC (1:4:8) is about 7%, whereas PCC is 5%. (1:6:12)
Advantages of Lean Concrete
- Lean concrete gives the foundation a consistent surface and keeps the foundation concrete from coming into direct contact with the soil.
- Underneath the foundation is lean concrete.
- It creates a strong concrete substrate on which steel reinforcing for the foundation work may be easily placed.
- It keeps the foundation concrete from coming into direct contact with the ground.
- Since moisture and other chemicals in the soil, such as sulfates, may damage and weaken the concrete, it protects the main foundation from the soil below.
Rich Concrete
Rich concrete contains more than 15% cement. Rich concrete with an 18% cement concentration is made from a 1:1.5:3 PCC mix. This concrete is used to create small structural elements that can withstand heavier loads.
Concrete Mix
A concrete mix is made up of five primary components in varying proportions: cement, water, coarse aggregates, fine aggregates (i.e. sand), and air. Additional components, such as pozzolanic minerals and chemical admixtures, can be added to the mix to give it desired characteristics.
Checklist before Pouring of Concrete
- Conduct a pre-slab meeting to iron out plans. This should be done about a week before the pour. The crew foreman and project superintendent should ideally be present to address issues like as necessary equipment, manpower requirements, rebar pulling/chairing, mix design, and so on. In addition, any contract-related issues must be evaluated and discussed. Things that appear to be insignificant, such as whether a contract’s weather-related concrete installation and batching plans have been agreed, should not be disregarded. At this point, it’s critical to pay close attention to the smallest details.
- Inspect formwork thoroughly. This is critical because it will help you avoid the expenses associated with an inefficient pour and the need to rework. Make sure that the forms are in the right place and have the proper grade and alignment. You should also ensure that they have been fitted and braced in accordance with previous designs. It is also critical to keep the formwork free of dirt, debris, and garbage.
- Check reinforcing steel. Before a pour can commence, rebar must be sampled and authorized. It must be clean and free of corrosion, cracking, breaking, or pitting. And it must be put in the proper, pre-determined position, using the proper size of rebar. There must also be the proper and precise amount of space between the rebar and the formwork. Furthermore, every reinforcing steel must comply with the prescribed cover, and all reinforcing steel must be knotted and fastened.
- Make sure you have the right materials and equipment. You must deploy the appropriate vehicles to the jobsite. Before pouring, the appropriate concrete supplies must be on hand, examined, and authorized. Water, cement, aggregates, and additives are all included. It’s a good idea to contact your concrete provider the day before the pour. Furthermore, protecting and curing products must be kept on hand and conveniently accessible. This may include insulated blankets, damp mats, tarps, and external heaters, depending on the job.
- Double-check the materials and jobsite to ensure readiness. This refers to what you should do just before a pour begins. Slump and air tests must be performed on trucks, and test cylinders must be ready to travel. In order to prepare for the pour, the formwork must be hosed off and moist. And, before the pour, the concrete must be appropriately vibrated. All of these last-minute processes may seem apparent if you’ve been pouring concrete for a while, but it’s not unheard of for seasoned workers to miss or ignore one of these critical duties.
In hot weather that surpasses 77 degrees Fahrenheit, it’s critical to prepare ahead if you’re pouring concrete. Use the following tips to mitigate the negative impacts of heat, humidity, and wind:
- During the warmest sections of the day, don’t pour.
- Protect yourself with a sunshade or windbreak.
- Before utilizing, store bags of concrete in the shade, garage, or other cool place.
- Before laying your concrete slab, wet the subgrade.
- To help bring down the temps, add ice to your concrete water mix.
- Reduce the mixing time once the water has been added to the mix.
- Make sure you have help with everything from pouring to supervising the curing process.
It’s critical to keep the temperature of the concrete curing process at the right level. Before exposing concrete to cold conditions, it must set. The following tips might help you handle cold weather issues.
- Keep concrete materials in a dry, warm location.
- With heaters, you can thaw frozen earth, snow, or ice.
- Use treatments that are designed to cure quickly in the cold.
- Pour your concrete into a mixing bowl filled with hot water.
- Using more cement will result in a more intense reaction (e.g. 100lb. per cubic yard).
- To remove bleed water rapidly, use squeegees or a vacuum.
- Framings should not be removed until the concrete has hardened.
Concrete Setting Time at Various Temperatures
ADVANTAGES AND DISADVANTAGES OF USING CONCRETE IN CONSTRUCTION
The advantages of concrete are as follows:
- Easy access to concrete components.
- Concrete is simple to handle and mould into any shape.
- Easy transit from the mixing station to the casting station before the initial set.
- Ability to pump/spray into cracks and tunnel linings.
- Reinforced buildings may be anything from a simple lintel to massive flyovers.
- A monolithic construction has a better look and more rigidity.
- Since concrete has a high compressive strength, it is less expensive to build a concrete structure than a steel construction.
The disadvantages of concrete as follows:
- To avoid fractures, concrete must be strengthened due to its poor tensile strength.
- If there is a significant temperature difference in the area, expansion joints are necessary in lengthy buildings.
- To prevent cracks caused by drying shrinkage and moisture expansion, construction joints are provided.
- If moisture combines with soluble salts in concrete, efflorescence occurs.
- Typical Portland cement concrete is integrated in the presence of alkalies, sulfates, and other substances.
- Structures that are subjected to sustained loads undergo creep.
Difference between a construction joint, cold joint, contraction joint, isolation joint, and a expansion joint
A construction joint is the point where two concrete placements meet, which is done intentionally to make building easier.
A cold joint is a joint or discontinuity formed by a significant delay in placement to prevent material intermingling and bonding, or when mortar and plaster rejoin or meet.
A contraction joint is a groove in a concrete structure that is created, sawed, or tooled to produce a weakened plane that controls the position of cracking caused by dimensional changes in various areas of the structure.
An isolation joint is a gap between adjacent parts of a concrete structure that allows relative movement in three directions while interrupting any bonded reinforcing.
In a concrete construction, an expansion joint is a space between adjacent sections that allows movement due to dimensional changes in the adjacent sections while also interrupting some or all of the bonded reinforcement. It is a gap between slabs filled with a compressible filler material on pavement slabs on the ground.
Strengthening Techniques for Reinforced Concrete Columns
It’s one of the techniques for improving or restoring the strength of reinforced concrete columns. The jacket’s size, as well as the quantity and diameter of steel bars used in the jacketing process, is determined by the structural study of the column.
The Jacketing Process for Reinforced Concrete:
- If it’s necessary, temporarily lower or remove column loads. Mechanical jacks and other props are used between the floors to accomplish this.
- Remove the concrete cover and clean the steel bars using a wire brush or a sand compressor if the reinforcements are discovered to be corroded.
- The steel bars should next be coated with a corrosion-resistant epoxy coating.
- The jacketing operation begins by attaching steel connections to the existing column if decreasing loads and cleaning reinforcement are not required.
- Steel connections are installed in the column by drilling holes 3–4 mm bigger than the diameter of the steel connectors used and at a depth of 10–15 cm.
- The new stirrups of the jacket should be spaced no more than 50 cm apart in both vertical and horizontal orientations. Fill the holes with a suitable epoxy substance before attaching the connections.
- Using the same two previous techniques, add vertical steel connectors to secure the vertical steel bars of the jacket.
- Install the jacket’s new vertical steel bars and stirrups according to the dimensions and diameters specified in the design.
- It would be impossible to coat the existing column with an epoxy substance that would provide a strong connection between the old and new concrete.
- Before the epoxy substance dries, pour the concrete into the jacket. To prevent shrinkage, the concrete used should be made up of tiny particles, sand, cement, and extra ingredients.
This technique is used when the weights applied to the column will be raised while expanding the cross-sectional area of the column is not permitted.
Steel Jacketing Process
- Removing the concrete cover
- Using a wire brush or a sand compressor, clean the reinforcing steel bars.
- Corrosion might be avoided by coating the steel bars with epoxy.
- Installing the steel jacket to the appropriate size and thickness as specified in the design and creating openings to allow the epoxy material to be poured through them ensures the necessary connection between the concrete column and the steel jacket.
- Use a suitable epoxy material to fill the area between the concrete column and the steel jacket.
Where the column is required to carry bending moments and properly pass them through the floors, a steel collar should be installed at the column’s neck using bolts or appropriate bonding material.
To improve or enhance the capacity of reinforced concrete columns, fiber-reinforced plastic (FRP) axial strengthening systems are used. It works for both circular and rectangular columns, although the former is more effective.
Fiber-reinforced polymer (FRP) axial strengthening is generally conducted by wrapping reinforced concrete columns in FRP. When the column is circular, this method of strengthening is especially effective.
Nevertheless, if the reinforced concrete column is rectangular and the depth-to-width ratio is higher than 2, or if the smallest side of the column is longer than 900mm, ACI 440.2R-08 is not used for this strengthening technique.
The ineffectiveness of rectangular or square column confinement might be attributable to non-uniform stress distribution and stress concentration at the section’s corner. This might result in the strengthened component failing prematurely.
It is important to fully wrap reinforced concrete columns with FRP in order to adequately restrict and enhance the element. In contrast to the flexural and shear strengthening of reinforced concrete beams, the FRPs that surround the column are activated only when the member is extended laterally and imposes loads on the FRPs. This indicates that beam reinforcement is an active system, whereas column reinforcement is a passive system.