What is Concrete?| Concrete Mix | Concrete Workability
What is Concrete?
A building material obtained by the mixture of cement, water, fine aggregate, and coarse aggregate is known as concrete. There are primarily four concrete types namely;
- Plain Cement Concrete Known as PCC.
- Reinforcement Cement Concrete Known As RCC.
- Pre-Cast Concrete.
- Pre-Stressed Concrete
Plain Cement Concrete.
Plain Cement Concrete, PCC It is a low-cost concrete type and does not contain a reinforcement. It is suitable as a levelling base for RCC. PCC Concrete helps distribute the load evenly to the ground as the ground is not even after installation of sub base
It also helps separate RCC and soil, which prevents cement slurry loss.
Reinforcement Cement Concrete, R C C.
Reinforcement Cement Concrete R C. C. Stands for reinforced cement concrete. It is obtained by adding reinforcement into a section of concrete. A combination of steel and cement strengthens the components so that they can be used to build strong supporting structures like columns, beams, and slabs in buildings.
Advantages of RCC.
- RCC Concrete structure becomes very rigid because the concrete develops a perfect bond with the steel bars.
- It is fire-resistant.
- RCC Concrete structures do not deteriorate and have a life span of a few decades.
- Ingredients of RCC are locally available
- RCC is a very resilient and durable building material.
- It can be molded into any shape and size.
Pre-cast concrete means concrete is poured into steel mould with wire mesh or rebar. It is cured in a controlled environment. Once finished, the pre-cast concrete is transported to a construction site and put into place.
Pre-cast concrete provides benefits from the versatility of shape and size. Time-saving portability can be made to order and is reusable.
Pre Stressed Concrete
Pre-stressed concrete is a unique variety of RCC Concrete in which compressive stress is applied to the concrete. This reinforcement is replaced by high strength tendons, which are high tensile wires.
The tension in the tendons produces the compression. This results in improved structural capacity and serviceability compared with conventionally reinforced concrete in many situations. The salient features of pre-stressed concrete are:
- Requires lesser Concrete Mix.
- Has Greater Strength and Support.
- Size of Structure Member Reduces
- Results in Crack Free Concrete.
Other cement composition mixes:
Grout. This free-flowing, self-compacting material, which feels small gaps. They come in many varieties, and common types are tiling flooring, structural, and non-shrink.
Many types of concrete are available, distinguished by the proportions of the main ingredients. By substitution for the cementations and aggregate components, the finished concrete can be tailored to its application.
Strength density, as well as chemical and thermal resistance, are variables.
Workability of concrete is one of the physical parameters of concrete that affects the strength, durability, cost of labour, and the finished product’s appearance.
It is workable when easily pleased and compacted homogeneously, for example, without bleeding or segregation.
Methods to Increase Workability,
- Increase water-cement ratio,
- Increased size of aggregate
- Use well-rounded aggregate instead of a regular aggregate
- Increase mixing time and temperature
- Use Admixtures.
Tests for Workability
There are two primary tests are used to check workability are;
- Slump test
- Compaction factor Test
The apparatus which used in the slump test is:
- The mould having 300 millimetres or say 12 inches of height days is 200 millimetres or eight-inch in diameter and has a smaller opening at the top of 100 millimetres or, say 4 inches.
- A 600 millimetres long bullet nosed metal rod, which is 16 millimetres in diameter.
Procedure for SumpTest
- Place the mold on a flat, moist, non-absorbing surface like a steel plate.
- Fill them all to one-third full by volume and compacted it.
- Fill the mould to two thirds full and compact it.
- Each layer shall be tempted 25 times with a standard 16 millimeters diameter steel rod.
- Strike off the top surface of the concrete from the top of the mould to make it even
- Removed the mould carefully take about five seconds
- Immediately invert and place the mould beside the slumped concrete and place the rod horizontally across the mould and measure the slump.
Types of Slumps
Collapse slump is when concrete collapses completely indicates the mix is too wet and is not appropriate.
When the top portion of concrete shears off and slipped sideways indicates harsh mixes like the mix’s lack of cohesion.
True, when the concrete simply subsides.
Slump Test Application
Slump Test is used to ensure uniformity for different patches of the similar concrete grade under field conditions
Helps to check on day to day or hour to hour variation in the materials being fed into the mixer
- About 50 millimeters when mixed manually,
- About 75 to 100 when required to pump the concrete.
Compaction Factor Test.
- First, the concrete is placed gently at the upper hopper. The upper hopper’s bottom door is then released, and the concrete falls into the lower hopper.
- The bottom door of the lower hopper is then released, and the concrete falls into the cylinder.
- Cut off the excess of concrete above the top level of a cylinder using trowels and level it.
- Weight the cylinder with partially compacted concrete, which will be W1.
- Empty the cylinder and then refill it with the same concrete mix in layers, when each layer is heavily running to obtain full compaction.
- Weight the cylinder with fully compacted concrete, note it as W 2.
- Find the way to empty cylinder, which will be W.
Now to find compaction factor = (W1-W2/W2-W)
Advantages of the compaction factor test
- It is suitable for testing workability in laboratories
- Suitable for concrete of low workability
- Suitable to detect the variation in workability over a wide range.
- The results are more precise and sensitive.
Joints in Concrete Flooring.
Joints are planned breaks and concrete, which allows the RCC to move and prevent random cracking.
There are four types of joints;
- Expansion of Concrete Joints
- Isolation Joints
- Contraction Joints
- Construction Joints.
Construction Concrete Joints
Construction joints are provided at the location where concrete work is stopped temporarily. Tie bars are provided for good bonding between abutting sections of concrete.
Rails are used to support the edge of the concrete joint is normally running through the joint.
Expansion of Concrete Joints.
Expansion joints are provided to permit thermal contraction and expansion without inducing stresses into the elements. The building, longer than 45 m, is generally provided with one or more expansion joints.
Recommended center to center spacing is 30 m. The joints are formed by providing material such as thermal/bitumen filled with joint fillers
Isolation Concrete Joints.
These joints totally separate a concrete element from another concrete element or a fixed object such as a wall or column, so that each can move and not affect the other. The joint feeling should be in full depth.
It can be made of cock foam, rubber, or some other flexible material.
Contraction Concrete Joints,
Contraction joints also called control joints, and joints are introduced in concrete structures to localize shrinkage movement as the structure will shrink the joints open slightly. Cracks will develop in the joints rather than in a random manner.
These are grouped, formed are starting to sidewalks, driveways, pavements, floors,
Joint fillers are hard, semi-rigid materials typically used to fill joints in concrete floors.
The joint filler must be able to transfer a load of heavy wheel traffic across the joint on protecting the joint edges from damage, commonly used to joint fillers are bitumen, Epoxy is polyurethane-based.
Their main purpose is also to prevent water other liquids and debris from entering the joint. They also need to improve the appearance of the floor.
Timing of Joint Filling and Sealing
The joint filing is postponed as long as possible to minimize joint filler, splitting, and cracking. This condition occurs because the concrete slab shrinks and the joint itself widens.
Semi-rigid Epoxy and polyurethane fillers are not flexible enough to withstand joint widening on may split when concrete shrinkage has occurred.
Compaction is a process that expels air from freshly placed concrete and packs the aggregate particles together to increase concrete density. The placing and compaction of concrete is a simultaneous step.
However, importance should be given to compaction. Concrete compaction is an important step in getting a durable structure.
Methods of Compaction.
The Rodding step involves poking with a long rod, greater than 16 in diameter, and the thickness of layers should be less than 200 millimeters.
Ramming is compaction on the ground usually done over PCC, and it should not be performed over RCC. Damping: It is a method in which a crossbeam beats the top surface
Mechanical Method of Compaction,
- Internal compaction- needle vibrator
- External compaction- Formwork vibrator
- Table or platform vibration
- Surface vibrators such as screed machines,
These have motion type machines. They are commonly known as needle or poker vibrators. It consists of a power unit with a flexible shaft and the head with the vibratory machine operated electrically over a diesel motor. The vibrator needle is immersed in concrete to compact it.
The needle size varies commonly used sizes up 25 to 60 millimeters in diameter. It produces vibrations in the range of 3000 to 6000 revolutions per minute. The needle should be immersed for about 30 seconds to two minutes, and the concrete layers should not be more than 600 millimeters high.
Guidelines for Poker Vibrator
- These vibrators are heavy and need handling with a couple of operators from time to time. The vibrations produced might be inadequate to compact, low slum concrete.
- There is a loss of power and capacity of compaction if the shaft is damaged. It is easily damaged, double due to streams and rough handling.
- Ensure this cement slurry does not enter the shaft, and the flexible hose does not have any cuts.
- Pokers should penetrate using their own weight to the bottom of the concrete layer while withdrawing. Be slow so that the concrete is filled in the gap.
- It should be left for about 5 to 10 seconds, depending upon the slum. Stagger the insertion points so that every corner is compacted.
- Avoid touching formwork with the poker to avoid damage to both the poker on formwork tightness.
- If there is a heap of concrete, do not insert poker in the center. Move inwards, gradually starting from the bottom of the heap.
- Avoid sharp bends to prevent damages to the hose. The drive motors should be placed such that there is minimal handling required.
- Ensure extra needles are available at the site as they’re easily damageable.
- Do not insert the poker more than the head length. Hold the needs about 2 to 3 ft from the top of the needle’s head while not a news and show, it does not come in the way while handling concrete.
Internal Vibrator Capacity.
The vibrators are usually 4 to 6 m in length. The radius of actions is usually 2 to 4 times the needle’s diameter, depending upon the slump.
External vibrators originally climbed to the formwork so that both the form and concrete are subjected to vibration. They are used for thinner sections or heavy reinforcements, such as for precast panels.
The general range of vibrations is 3000 to 9000 revolutions per minute. Precautions need to be taken to ensure formwork does not open up, during vibration.
In order to obtain a really flat and strong concrete floor or flat surfaces, screed vibrators are used. Their action is similar to that of camping. The effective depth of vibrations is only 150 millimeters. They should not be used when the depth of the concrete is more than 250 millimeters.
Internal vibrators should also be used in-depth greater than 200 millimeters in concrete pours of slump higher than 75 millimeters of cement mortar might rise on the top, weakling the top surface and causing loss of strength.
It helps provide a level on a smooth surface, improving the durability of the concrete pavement.
Precautions in using vibrators
- Make it handle for the operators by recruiting a support person who holds the chord of the vibrator and operates it on an offline directive.
- While placing concrete wear safety goggles, long pants, rubber boots, and a long sleeve shirt,
- The concrete splatters may burn the eyes and may irritate and can dry out the skin
When to stop compaction.
Compaction helps remove air points as the concrete settles down, entrapped air is removed, and the pin slurry floats on the top. Move to the next spot as the air bubble stops emerging out, and a slurry is formed, hiding the cause aggregate.
The poker’s noise also smoothens out as the concrete is compacted while compacting the concrete makes a harsher noise. An experienced person can make out the compaction quality from changes in the noise of the poker.
Over vibration causing segregation. The cement slurry floats on the top, and the coarse aggregate settled to the bottom, causing cracks or weak joints in columns or walls. It will lead to non-uniforms, strength, and concrete structures affecting durability.
The concrete loses more than 25% of strength if it is not properly compacted. The quality of material proper batching and mixing compaction is critical in getting a durable concrete structure with good aesthetics.
Finishing of concrete is the screeding, floating, or troweling of the concrete surface to densify and further compact the surface of the concrete and give it a look you want. Finishing takes place in two stages.
- Initial Concrete Finishing
- Final Concrete Finishing
Initial Concrete finishing
Initial finishing includes screeding, also commonly called master finishing concrete is first to screed it to the level of the formwork and left to set.
In some cases, screeding leaves a good enough finish, especially if floor coverings are to be used over the concrete.
Bleed water is the water that appears on the concrete’s surface while finishing, ensure bleed water has dried up and under no circumstances finished tools be used before accumulated water is removed.
Mixing bleed water with the surface-based will weaken the top surface, possibly resulting in a dusty surface.
Excess bleed water can be removed by dragging an ordinary garden horse pipe across the concrete’s surface.
Once the bleed, water dries up and concrete can support a person’s weight with only a slight marking to the surface, the final finishing can begin.
Final Concrete finishing.
This involves floating, troweling, edging jointing, or patterning the concrete. Special finishes, such as booming, colouring, hardening, or patent finishes, can be applied to the surface.
Troweling leaves or dense, hard, smooth, adorable surface. The surface should be troweled twice and a well-troweled surface will be very smooth and Can be slippery when wet troweling can be done by hand or power trowel.
Don’tsDon’ts and Dos for Concrete Finishing
All the edges of a slab should be finished with a special edging tool. This gives a neater and stronger edge less prone to chipping. Joints should be planned before placing. Once any surface has been finished, it must be cured.
The concrete used should be of the lowest practical slump. Ensure proper internal vibration has been performed.
Finishing should be performed within the time period in which concrete can be worked with. For example, when it is in the plastic state. Avoid manipulating concrete as little as possible to produce the desired result.
The addition of water to make concrete workable or additional dry cement to absorb bleed water should be avoided.
Ensure a good proportion of concrete with proper consistency should be used.
Repairing of Concrete.
After the striking of the formwork, all concrete work shall be inspected for defects. Areas of concrete segregation, stone pockets, and other damaged areas shall be chipped and all loose material removed with compressed air or washing.
This and other repair work shall be performed as soon as possible so that the concrete has not hardened completely.
Bonding of old concrete with the repair materials shall be achieved by wetting the chipped-out surface sprinkling, dry cement, or by brushing rich cement grout.
The cement mortar/ concrete shall have the same proportions as those of the parent concrete. The repaired areas should be cured for 24 hours.
Cement Wash surfaces which are not to be plastered nor receive any other treatment may be provided one coat of cement wash.
The quality of the concrete floor is highly dependent on the timing and procedures followed during finishing operations.
Failure to address this issue will lead to cracking, low resistance to wear, dusting and skilling.
Concrete Defects and Deterioration.
Some common concrete defects are:
- Concrete honey coming
- Concrete segregation,
- Concrete bleeding,
- Concrete leaching,
- sulfite or acid attack
- corrosion of reinforcement steel
Concrete honeycombing can be defined as hollow space and cavities left on the surface or inside of the concrete mass. The surface is coarse and stony, as shown below.
Causes of Concrete Honeycombing
- Concrete honeycombing is a result of insufficient fine material in the mix.
- Leakage of mortar fraction from the concrete at construction or formwork joints
- In proper vibration during concrete and
- Use of very stiff concrete.
Damages by Concrete Honeycombing
- The damages can occur like small, shallow areas cosmetically unappealing.
- Deeper areas lead to a reduction in the protection to the reinforcement,
- The possible durability problems in the future.
Rectification of Concrete Honeycombing
Segregation is the separation of the constituent materials of concrete, resulting in a non-uniformed mix. For instance, separation of course aggregates from cement paste,
Causes of Concrete Segregation
- Generally caused by the high water-cement ratio in concrete
- Pour gradation due to bad concrete mix design.
- Improper mixing.
- The most common reason is to place concrete from height, especially for underground foundations.
Remedies for Concrete Segregation
- The possible solution is to alter sand and water content.
- Using finer sand
- Proper handling during transportation and placing it may result in honeycombing, which will reduce strength and your ability of concrete member
Concrete bleeding occurs when water in the concrete tends to rise to the surface of freshly placed material.
Causes of Concrete Bleeding
- Concrete Bleeding is predominantly observed in a highly wet mix.
- Badly proportioned fine aggregates on insufficiently mixed concrete
Effects of Concrete Bleeding
- Makes top surface porous dusty on weak
NB: to avoid bleeding inshore removal of water through the watering or evaporation before finishing.
Leaching occurs when water flows through cracks dissolving the minerals. It produces a sandy appearance on concrete surfaces and causes erosion of internal concrete.
Remedies to Concrete Bleeding
- It can be controlled by reducing permeability,
- converting the soluble calcium hydroxide into insoluble calcium silicate Hydrate CSH
Sulfite or Acid Concrete Attack
Sulfate Acid attacks can possible if high concentrated sulfates, sulfuric, hydrochloric, and nitric acids are found in groundwater and soil.
Sulfates cause chemical changes to cement disintegrating concrete in only a few years. Resistance can be improved by converting calcium hydroxide to the more chemically stable calcium silicate hydrate CSH.
Corrosion of Reinforcement Steel
corrosion leads to internal stresses that cause the formation of cracks and disrupts the concrete structure.
Ensures rebars have been properly installed, for example, not located too close to the concrete surface in n contact with the air. The solution can be properly covering the bottom and sides.