Compaction of concrete
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This document discusses properties of concrete and compaction methods. It covers the importance of compacting concrete to remove air voids and increase strength. Methods of compaction include manual techniques like rodding and tamping as well as mechanical vibration using internal and external vibrators. Improper vibration can lead to defects like honeycombing or segregation. Newer techniques like self-compacting concrete use superplasticizers to reduce the need for external vibration during pouring and placement.
Compaction of concrete
- 1. PROPERTIES OF CONCRETE Lecture No.4
- 2. Lecture Contents • Compaction of Concrete • Importance of Compaction • Methods of compaction • Manual Compaction • Mechanical Compaction • Consequences of improper vibrations • Defects of under vibration • Defects of over vibration • Self Compacting Concrete • Super Plasticizer
- 3. Compaction of Concrete • 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.
- 4. Compaction of Concrete Purpose: • To remove entrapped air bubbles in concrete • To achieve high density • To improve strength and durability • To eliminate honey comb and other defects.
- 6. • It is important to compact the concrete fully because, Air voids reduce the strength of the concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and 7%. This means that concrete containing about 5% air voids due to incomplete compaction can lose as much as one third of its strength. Importance of Compaction
- 7. • Air voids increase concrete's permeability. That in turn reduces its durability. • If the concrete is not dense and impermeable, it will not be watertight. It will be less able to withstand aggressive liquids. • Moisture and air are more likely to penetrate to the reinforcement causing it to rust. • Proper compaction also ensures that the formwork is completely filled i.e. there are no pockets of honeycombed material and that the required finish is obtained on vertical surfaces Importance of Compaction
- 9. Methods for concrete compaction Concrete compaction can be carried out either by hand or machines. There are many factors that should be considered to choose compaction methods such as reinforcement quantities and spacing, concrete paste consistency and formwork complexity. Manual compaction 1) Roding 2) Tamping 3) Ramming
- 10. Methods for concrete compaction Mechanical compaction 1) Internal vibrator (pokers) 2) External vibrators 3) Formwork vibrator 4) Table vibrator 5) Platform vibrator 6) Surface vibrator (Screed vibrator) 7)Vibratory Roller
- 11. Methods for concrete compaction Manual compaction Manual compaction is used for ordinary and unimportant structures. Unimportant Concrete work, small works. Places where reinforcement is more & does not permit use of vibrating equipment. Workability should be decided in such a way that the chances of honeycombing should be minimum.
- 12. Methods for concrete compaction Manual compaction 1) Rodding: Rodding is done continuously over the complete area to effectively pack the concrete and drive away entrapped air . The Rodding is done using a 2 m long rod with 16 mm dia. The thickness of layers for rodding is 15-20 cm.
- 13. Methods for concrete compaction (Manual compaction) 2). Ramming: This method is used to compact Plain Cement Concrete (P.C.C.) on ground level and around foundations. It is not used in RCC or upper floors.
- 14. 3). Tamping: It is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm to squeeze voids and make the surface plain. Both compaction and leveling are achieved simultaneously. It is mainly used for roof slabs and road pavements. Methods for concrete compaction (Manual compaction)
- 15. Compaction by Tamping Methods for concrete compaction (Manual compaction)
- 16. Mechanical compaction • In mechanical method vibrations are imparted to remove air bubbles and for compaction. • When concrete is vibrated the internal friction between aggregates particles is temporarily destroyed and concrete behaves like a liquid. • It settles under gravity action and air bubbles move upward to the surface. • Internal friction reestablished as soon as vibration stops • It is modern method and now a days normally used. Methods for concrete compaction (Mechanical compaction)
- 17. Internal vibrators: This is one of the most common and promising method used for concrete compaction. The diameter (20–180 mm) and length (25–90 mm) of the head of vibrator varies according to the requirement at the site. Methods for concrete compaction (Mechanical compaction)
- 18. Internal vibrators: Energy for compaction is supplied through oscillatory motion of vibrations. Internal vibrators are used for cast-in-place pours. but there are exceptions where reinforcement is very intense. Poker can easily be removed & moved from point to point Methods for concrete compaction (Mechanical compaction)
- 19. Internal vibrators: To aid in the removal of trapped air the vibrator head should be rapidly plunged into the mix and slowly moved up and down. The actual completion of vibration is judged by the appearance of the concrete surface which must be neither rough nor contain excess cement paste. Methods for concrete compaction (Mechanical compaction)
- 20. Internal vibrators: • Parts of internal vibrator 1) motor 2) flexible shaft 3) head (poker) • Inside the head there is an unbalanced weight which rotates at high speed causing head to move in circular motion • The motor can be powered by electricity, gasoline or air. • Diameter of head: 20mm to 180mm Methods for concrete compaction (Mechanical compaction)
- 21. External vibrators When heavy reinforcement is used in a construction component or if the section is very thin, it gets difficult to use an internal vibrator to compact concrete. In such cases, concrete is vibrated externally, which in turn is much more expensive (heavy electricity consumption) and requires tightly bound formwork to bear the vibrations. External vibration is less effective and it consumes more power as compared to the internal vibration. Methods for concrete compaction (Mechanical compaction)
- 22. External vibrators External vibrators rigidly clamped to formwork so that form & concrete are subjected to vibrations. Considerable work needed to vibrate forms. Forms must be strong and tied enough to prevent distortion and leakage of the grout. Suitable for thin members or Pre-Cast units Frequency range – 1000-5000 cycles per min Finish quality – Good Consumes more power – less efficient than internal Methods for concrete compaction (Mechanical compaction)
- 23. External vibrators Two categories are • When external vibrators are attached or applied to formwork i.e form vibrators, vibratory screeds, rammers • When formwork is attached to external vibrators i.e vibrating tables, shock tables etc. Methods for concrete compaction (Mechanical compaction)
- 24. Form vibrators (external vibrator) It is a vibrator which is clamped to the formwork so that both the form and concrete are vibrated. It is used 1) where concrete members are very thin and congested with reinforcement. 2) to supplement internal vibration. 3) stiff mixes where internal vibrators cannot be used
- 25. Vibrating table ( external vibrator ) Used for small quantities of concrete (lab & some precast elements)
- 26. Vibrating table ( external vibrator ) • In vibrating table form work is clamped to vibrator • It can vibrate horizontally as well as vertically • They are used for pre cast concrete • It is mainly used for laboratories where concrete is put on the table.
- 27. Surface vibrators These are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth. Types : • Vibratory screeds • Vibrating rollers • Trowels
- 29. Vibratory screeds • Used to consolidate concrete in floors and other flat works • Concrete should not have slump more than 75 mm ( 3 inch ). • For slump greater than 75 mm, accumulation of fine aggregates and mortar will occur • They should not applied after concrete has been sufficiently consolidated.
- 30. Vibratory screeds • Vibratory screeds are used for consolidating non reinforced slabs or light reinforced slabs • Combination of internal vibrators and surface vibrators are used for reinforced slabs.
- 31. Other surface vibrators • Vibratory rollers , electric hammer , and trowels are some other surface vibrators • A vibratory roller is used for consolidating thin slabs. • Electric hammer is used for compacting test cubes.
- 32. Consequences of improper Compaction There are various problems and defects that could arise when concrete is not vibrated adequately. • Honeycomb • Excessive entrapped air voids • Sand streaks • Cold joints • Placement lines • Subsidence cracking
- 33. Honeycomb in Concrete due to poor Compaction or Vibration Consequences of improper Compaction
- 34. Cold Joints due to poor Compaction of Concrete Consequences of improper Compaction
- 35. Defects of under vibrations • Excessive entrapped air voids: Under vibration leaves a lot of entrapped air in concrete which reduces strength • Sand streaks: When heavy bleeding washes mortar then a harsh mixture lefts behind that lacks workability. It is also caused by insufficient fine aggregates
- 36. Defects of under vibrations Placement lines: • These are dark lines between adjacent layers of concrete batches. it occurs when vibrators did not penetrate through under lying layers.
- 37. Defects from over vibration Segregation: Heavier aggregates settle while lighter aggregates rise Bleeding: water comes out at surface due to excessive vibrations. Form damages Over vibration may damage the formwork. Under vibration is more often a problem than over vibration
- 38. Self compacting concrete • It is a concrete which do not need vibrations or external compaction for settling down. • It settles under its own weight and gravity. • Super plasticizers are added to improve its workability. • As its workability and strength both are acceptable so it is also called HIGH PERFORMANCE CONCRETE.
- 39. SUPER PLASTICIZERS • They are also known as high range water reducers and are used as admixtures to make self compacting concrete. • They increases the flow characteristics of concrete by reducing water cement ratio of concrete with out disturbing workability. • Now a days there use become very important as they save cost for compaction and also save time.