Manufacturing of Concrete:
Good quality concrete is essentially a homogenous mixture of cement, coarse and fine aggregates, and water which consolidates into a hard mass due to chemical action between cement and water.
Each of the four constituents has a specific function:
(i) The coarse aggregate acts as a filler.
(ii) The fine aggregate fills up the voids between the paste and the coarse aggregate.
(iii) The cement in conjunction with water acts as a binder.
The aim of quality control is to ensure the production of concrete of uniform strength from batch to batch. This requires some rules to be followed in various stages of concrete production and are discussed as follows.
The Stages of Concrete Manufacturing Process:
1. Aggregates, cement, and water should be measured with an accuracy of ± 3 percent of the batch quantity and the admixtures by 5 percent of the batch quantity.
There are two prevalent methods of batching materials are:
(a) Volume batching
(b) Weight batching
2. For most important works weight batching is recommended whereas, volume batching is generally recommended for small jobs only.
(a) Volume Batching:
- The amount of each solid ingredient is measured by loose volume (not compacted).
For example, the volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand.
- Therefore correction for bulking of sand is done if volume batching is adopted.
(b) Weight Batching:
- Cement is always measured by weight, irrespective of the method of batching.
- Water is measured in kg or liters where the density of water is 1
- The volume of 1 bag of cement is 0.035 m³ (or sometimes also said 35 liters)
- The objective of mixing is to obtain homogenous, uniform colour and consistent concrete of desired strength.
- Mixing time depends on the type and capacity of the mixer but IS-456 suggests approximately mixing time as 2 minutes.
- Generally, 20 revolutions of concrete in the mixture provide sufficient mixing.
- If mixing time is increased up to 2 minutes the compressive strength of concrete produced is enhanced and beyond this time the improvement in compressive strength is insignificant and prolonged mixing may cause segregation as due to longer mixing periods the water may get absorbed by the aggregates or evaporate resulting in loss of workability and strength.
- The mixing is done in two ways i.e. 1. Hand mixing and 2. Machine mixing (mixture)
Hand mixing is adopted for small jobs where the quantity of concrete involved is small and the approximate time is 2 minutes and should never exceed 3 minutes.
- When a large quantity of concrete of the desired quality is to be produced, the machine mixing becomes imperative as Concrete can be produced at a faster rate with better quality.
- Concrete mixers are specified by the volume of mixed concrete discharged after mixing each batch expressed in m³ (such as 0.25, 0.38, 0.57, 0.75, 1.5, 2.25, and 3 m³).
- Sometimes the total volume of the unmixed ingredients in m³ is given as a prefix. i.e. 1.0/0.75 the mixer takes 1 m³ of unmixed material and gives 0.75 m³ of mixed concrete in each batch.
The machine mixing is done by using
- Tilting type mixture
- Non-tilting type mixture
- Batching plant
1. Tilting type mixture:
- In this mixed concrete is discharged by tilting the drum about the horizontal axis.
- Tilting mixers are useful for large construction works.
- It gives better results even will dry concrete.
- It can be used for aggregate sizes of more than 75 mm.
- Tilting mixers are easier to clean and can discharge the mix quickly and with minimum segregation.
- The tilting type mixtures are represented as 85T, 100T, 140T, and 200T (where 85, 100, and 140 are in litres.)
2. Non-tilting type mixture:
- Non-tilting mixers are suitable for small works.
- A non-tilting mixer is equipped with a drum rotating about a horizontal axis.
- Non-tilting mixers cannot be used when the aggregate size is more than 75 mm.
- Non-tilting mixers are represented as 200 NT, 280 NT, 340 NT, 400 NT, and 800 NT.
NOTE: Sometimes the mixers are specified by two quantities the total volume of ingredients added and the volume of concrete produced for example 285/200 liters mixer takes 285 liters of ingredients and yields 200 liters of concrete.
- The specification states that the process of mixing transporting placing and compacting the concrete should not take more than the initial setting time of cement (30 minutes using OPC)
- It must also ensure that segregation does not take place.
The transporting of concrete can be done by the following methods
1. Pans: Recommended only for small jobs.
2. Power Buggies: These have sped up to 24 km/h.
3. Chutes: When concrete is to be deposited below ground level at a higher depth, it can be discharged through a steel shaft called a chute.
4. Concrete Pumps: It is used commonly for tunnel works and on locations that are not easily accessible where concrete can be pumped for a distance of about 400 m. horizontally and 80 m vertically having slump values of 50 mm to 100 mm and the pipe used in the concrete pump has a diameter of 10 cm to 20 cm.
5. Transit Mixer: Transit mixer is a truck on which a concrete mixer is mounted and is useful in built-up areas.
6. Belt-conveyer: A belt conveyor is used when the concrete is to be transported continuously and to an inaccessible area.
- Research has shown that delayed placing of concrete results in a gain in ultimate compressive strength provided the concrete can be adequately compacted.
- For dry mixes in a hot weather delay of half to one hour is allowed whereas for wet mixes in cold weather it may be several hours.
- As per IS456 maximum permissible free fall of concrete may be taken as 1.5 m
The process of removal of entrapped air and of uniform placement of concrete to form a homogeneous dense mass is termed compaction.
The density and consequently the strength and durability of concrete depends upon the quality of compaction.
The presence of even 5% and 10% voids in hardened concrete left due to incomplete compaction may result in a decrease in compressive strength by about 30% and 60% respectively.
The various types of vibrators used are:
1. Internal Vibrators
- These vibrators consist of a metal rod that is inserted in fresh concrete.
- Skilled and experienced men should handle internal vibrators. These vibrators are more efficient than other types of vibrators.
- These vibrators can compact up to 450 mm from the face but have to be moved from one place to another as concrete progresses.
- The frequency of vibration is about 4000 to 12000 rpm.
- The needle diameter varies from 20 mm to 75 mm and its length from 25 cm to 90 cm.
2. Surface Vibrators
- These vibrators are mounted on platforms or screeds.
- They are used to finish concrete surfaces such as bridge floors, road slabs, station platforms, etc.
- It is placed directly on the concrete mass for the compaction of shallow elements (where internal vibrators cannot be applied) i.e. depth not greater than 150 mm. Ex: Road surfaces, plain concrete floors, etc.
3. Form Vibrators or Shutter Vibrators
These vibrators are attached to the formwork and external centering of walls, columns, etc. The vibrating action is conveyed to concrete through the formwork during the transmission of vibrations. Hence they are not generally used.
But they are very much helpful for concrete sections which are too thin for the use of internal vibrators.
4. Vibrating Tables:
It is very efficient in compacting stiff and harsh concrete mixes required for the manufacture of precast elements.
- The test specimens should be stored in a place free from vibration in most air of at least 90% relative humidity and at a temperature of 24-30°C for 24 hours from the time of addition of water to the dry ingredients.
- After this period the specimens are marked and removed from the moulds and unless required for test within 24 hours immediately submerged in clean fresh water and kept there until taken out just prior to the test.
- The specimens are not to be allowed to become dry at any time until they have been tested.
- Cement gains strength and hardness because of the chemical action between cement and water.
- The water in a concrete mix takes one of the following three forms as a consequence of hydration are:
1. Combined water: Combined with hydration products (C3A, C₂S, C4AF) it’s not evaporable.
2. Gel water: The water prevails over the cement Gel Surface Area.
3. Capillary water: Which “occupies capillary pores” (Evaporable).
- The increase in strength of concrete is very rapid from 3 to 7 days and continues slowly for an indefinite period.
- It has been observed that moist cured concrete for 7 days is nearly 50% stronger than that which is exposed to dry air for the entire period.
- If concrete is cured for one month, its strength is nearly double that of concrete exposed to dry air.
Objective of Curing
To prevent the loss of moisture from concrete due to evaporation or any other reason supply additional moisture or heat and moisture to accelerate the gain of strength.
To keep capillary pores saturated to ensure hydration of cement; to increase durability, and impermeability of concrete, and reduce the shrinkage.
As per IS: 456 concrete members shall be kept under curing for a minimum period of 7 days for OPC at 90% humidity and at least 10 days where mineral admixtures and blended cement are used.
NOTE: Lower temperature reduces the rate of setting and higher temperature reduces the ultimate strength. Therefore curing temperature need to be within 5 to 30°C.
For concrete mixes with a water-cement ratio ranging from 0.3 to 0.7 the increased rate of strength development can be achieved by resorting to steam curing.
This method of curing is also known as accelerated curing since an increased rate of strength development can be achieved.
Concrete members are heated by steam at 93°C either at low pressure or high pressure.
By low-pressure steam curing, about 70 percent of the 28-day compressive strength of concrete can be obtained in about 16-24 hours and high-pressure steam curing is usually applied to precast. concrete members and gives 28-day compressive strength at 24 hours.
It reduces the shear strength of concrete.
It also results in increased resistance to sulphate action and to freezing and thawing.
The rate of increase or decrease of temperature should not exceed 10 to 20°C per hour to avoid
“Infra-Red Radiation” is also a helpful method of curing for Rapid Gain of Strength.
Finishing is defined as the process of leveling and smoothing the top surface of freshly placed concrete to achieve the desired appearance is done as follows:
1. Screeding: Striking off the excess concrete to bring the top surface up to proper grade is called screeding.
2. Troweling: Final operation of finishing be done after all excess water has evaporated by steel float in conical shape giving a very smooth finish.
Maturity of Concrete
The strength of concrete depends upon both the time as well as temperature during the early period of gain in strength. The maturity of concrete is defined as the summation of the product of time and temperature.
Maturity = ∑(Time x Temperature)
Its units are °C hours or °C days.
The temperature is reckoned from -11°C as origin in the computation of maturity, since hydration continues to take place about up to about this temperature.