(Aktu Btech) Concrete Technology Important Unit-4 Concrete Production, Properties and Testing

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Q1. Explain the mixing and transporting operations of concrete in a work site.

Ans. A. Methods of Mixing: Depending on the quantity of concrete needed, either manual mixing or machine mixing is used to mix the concrete. 

• 1. Hand Mixing: 
  • i. Hand mixing concrete is only used in select situations where quality is not as crucial, either due to the unimportance of the labour or because there is less concrete needed.
  • ii. Unless it is for extremely small home projects, hand mixing should not often be employed because it generally does not produce homogenous concrete.
• 2. Mechanical Mixing:
  • i. Batch mixers and continuous mixers are the two primary categories of mechanical mixers.
  • ii. Batch mixers create concrete one batch at a time, batch by batch. The operation is sporadic. At one end, the raw materials are loaded, while at the other, the concrete is unloaded. This is a cycle of operation that is continued until there is enough concrete produced.
  • iii. Concrete is produced at a set rate by continuous mixers. At one end, the raw ingredients are continually inserted, while the delivery end is where the mixed concrete leaves.

B. Transportation of Concrete: The following methods are used for transporting concrete:  

• 1. Direct Discharge into Forms by Short Chutes:
  • i. Short chutes with a semi-circular shape and reinforced points are easy to use and cost-effective.
  • ii. Concrete shall not fall free from a height greater than 2 m.
• 2. Barrows: 
  • i. Long horizontal distances can be covered with manual wheelbarrows with a capacity of about 80 kg.
  • ii. Power barrows with an 800 kg capacity and up to 300 m hauls are employed for large-scale projects.
• 3. Dumpers and Trucks:
  • i. They are employed for great horizontal distances.
  • ii. The concrete during transit runs the risk of segregating due to jolting, especially if the terrain is rough.
• 4. Elevating Towers and Hoist: Concrete buckets are raised using elevating towers in multi-story constructions. Following that, barrows or chutes are used to disperse the raised concrete. When high lifts are necessary, this mode of transportation might be utilised.
• 5. Monorail System:
  • i. A single track is installed in tunnels and dam sites to transport a monorail power waggon that travels at a speed of 80 m/min.
  • ii. Long distance travel is possible with this mode of transportation.
• 6. Cranes and Cableways: 
  • i. When concreting is to be done in a large project covering mountains and valleys, cranes and cableways are used to provide three-dimensional transport enabling both horizontal and vertical movement. 
  • ii. Depending on the site condition, the type of crane can be chosen. It may be a derrick, crawler, or wheel mounted. 
• 7. Belt Conveyor:
  • i. It can be used when hauling concrete over long distances. It is not very much recommended because of its vulnerability to segregation. 
  • ii. The initial setting-up cost is also high. Discharge can be as high as 115 m³/h.
• 8. Concrete Bucket and Skip: The capacity of the skip varies from about 0.2 m³ to 10 m³.

Q2. What are the precautions to be taken while transporting concrete ? What are the advantages and disadvantages of concrete pump?

Ans. A. Precautions in Transporting of Concrete: Following precautions should be used during transporting of concrete:

• 1. As soon as water is added to cement, the process of hydration begins, so concrete should be delivered as quickly as possible to the formwork during the cement’s initial setting time.
• 2. In any event, the time it takes to mix, transport, place, and compact concrete should not exceed 90 minutes.
• 3. During shipping, no water may be lost from the mixture.
• 4. During transportation from the location of mixing or placement, the concrete combine should be shielded from drying in hot weather and from rain.
• 5. It is always advisable to avoid segregating concrete.
• 6. If additional time is going to be spent during transportation, the concrete must be constantly stirred in the truck mixer to prevent it from stiffening.

B. Advantages of Concrete Pump:

• 1.  Concrete pumping is a faster and easier method to complete a project. 
• 2. Concrete pumping reduces labour costs. 
• 3. It reduces site congestion as there are less construction workers. 
• 4. It provides a steady work pace, increasing productivity. 
• 5. It is effective and economical for various sized projects, including residential and commercial.
• 6. Several pumps can pour simultaneously for larger projects.

C. Disadvantages of Concrete Pump:

• 1. Possibility of a concrete pump breaking down. 
• 2. Risk of injury to construction workers and damage to property. 
• 3. During busy periods it is not always easy to find a concrete pump that is available.

Q3. Mention the different tests which are commonly adapted to measure workability and explain any one test in detail.

Ans. A. Test for Measure Workability: Following are the test used for measure workability: 

• i. Slump test. 
• ii. Compacting factor test. 
• iii. Vee-Bee test. 

B. Concrete Slump Test Procedure: 

• 1. The interior surface of the mould is first thoroughly cleaned. You can apply oil on the surface.
• 2. Next, a base plate is used to support the mould.
• 3. Three layers of fresh concrete are poured into the mould. A steel rod is used to tamp each layer 25 times.
• 4. Excess concrete should be removed from the mould once it has been filled, and the surface should be levelled.
• 5. The mould is then gradually moved vertically, at which point unsupported concrete will sag. The term “slump” refers to the height loss at the centre point, which is measured to the nearest 5 mm or 0.25 inch.

Q4. Discuss the factors affecting bleeding of concrete.

Ans. Factors Affecting Bleeding of Concrete: Following are the factors that affecting the bleeding of concrete:

• 1. Water Content and Water Cement Ratio:
  • i. More water is available for bleeding if the volume of water or the water-to-cementitious material ratio is increased.
  • ii. The bleeding rate of a typical concrete mixture can increase by more than 2.5 times with a 1/5 increase in water content.
• 2. Cement: 
  • i. Bleeding may be affected by the type, composition, and fineness of cement. The amount of bleeding diminishes as cement’s fineness rises.
  • ii. Reduces bleeding while also increasing cement content and lowering the water-to-cement ratio.
• 3. Supplementary Cementing Materials: 
  • i. By virtue of their inherent characteristics and by adding more cementitious elements to a combination, fly ash, slag, silica fume, rice husk ash, and natural pozzolanas can minimise bleeding.
• 4. Aggregate: 
  • i. Aggregate that contain a high amount of silt, clay or other material passing the75 um sieve can have a significant effect in reducing bleeding. 
• 5. Chemical Admixture: 
  • i. Air-entraining agents have been utilized frequently since it seems that the air bubbles maintain the suspension of the solid particles.
  • ii. Water reducers, which let go of retained water in the combination, help lessen bleeding.

Q5. What are the effects of bleeding on concrete properties? 

Ans. Following are the effects of bleeding on concrete properties:

• 1. Concrete loses homogeneity as a result of bleeding.
• 2. Bleeding is the cause of concrete’s permeability.
• 3. This water buildup weakens the link between the aggregate and cement paste and produces water voids. As a result, concrete loses strength.
• 4. The link between the reinforcement and concrete weakens when water collects behind the reinforcing bars, especially beneath the cranked bars.
• 5. The bleeding water spills over the pavement’s unsupported side, causing the sides to collapse.
• 6. Bleeding water hinders surface finishing and curing compound application during pavement construction.
• 7. Concrete bleeding causes number seven. Structures may lose their wear capacity and life due to the creation of laitance.
• 8. Water creates continuous channels as it flows from the bottom to the top. This channel makes concrete porous, allowing water to pass through and creating water voids in the matrix, weakening the link between the aggregate and cement paste.

Q6. What are the effect of shrinkage on concrete and how is it reduces ?

Ans. Effects of Shrinkage: Following are the effects of shrinkage on concrete:

• 1. Concrete between movement joints shrinks, which widens or opens joints. As a result, joints need to be built to accommodate the widening brought on by shrinkage.
• 2. Where other materials, such ceramic tiles, are fixed on top of a concrete surface, concrete shrinkage results in relative movement between the various components. The ensuing strains may lead to interface failure.
• 3. If shrinkage is constrained, tension is applied to the concrete, which splits when the tensional stress reaches the tensional strength.
• 4. Concrete shrinkage makes the concrete more securely hold the reinforcing bars. This makes concrete and steel more abrasive, which strengthens the binding, especially for simple bars.
•  5. Shrinkage causes flexural members to deflect more. This is because a compression zone with little reinforcement can contract more easily than a compression zone with heavy reinforcement.
• 6. The pre-stressing force decreases as a result of shrinkage.

Prevention of Shrinkage: Following are the measures to be taken to reduced shrinkage: 

• 1. To regulate the surface temperature in the event of slab construction, provide sun screens.
• 2. Before placing the concrete, dampen the subgrade because it is susceptible to water absorption.
• 3. Attempt to begin the curing as soon as you can.
• 4. To hasten the concrete’s setting period, add chemical admixtures.

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