UNIT IV---R.C.CONCRETE ,bc 5th sem breif notes

UNIT IV---R.C.CONCRETE 

MIXING CONCRETE 

Concrete is mixed either by hand or machine. No matter which method  is  used, you  must  follow  wellestablished procedures if you expect finished concrete of   good   quality. 

MIXING BY HAND 

 A   batch   to   be   hand   mixed   by   a   couple   of crew members should not be much larger than 1 cubic yard.  

 The equipment consists of a water tight metal or wooden platform, two shovels, a metal-lined measuring box, and a graduated bucket for measuring the water.  

 The mixing platform does not need to be made of expensive materials. It can be an abandoned concrete  slab or concrete parking lot that can be cleaned after use. 

 A wooden platform having tight joints to prevent the  loss  of  paste  may  be  used.  Whichever  surface  is used, you should ensure that it is cleaned prior to use and level. 

 Mix the sand and cement together first, using the following procedure: 

 Dump 3 cubic feet of sand on the platform first, spread it out in a layer, and dump a bag of cement over it. 

 Spread out the cement and dump the rest of the sand (2.5 cubic feet) over it.

  Dump the second sack of cement on top of the lot. This  use  of  alternate  layers  of  sand  and  cement reduces    the    amount    of    shoveling    required    for complete mixing 

 When  the  cement  and  sand  are completely  mixed,  the  pile  should  be  leveled  off  and the  coarse  material  added  and  mixed  by  the  same turning method.  

 The  pile  should  next  be  troughed  in  the  center.  
 The  mixing  water,  after  being  carefully  measured should be poured into the trough.  

 The dry materials should then be turned into the water, with great care taken to ensure that none of the water escapes. 

 When all  the  water  has  been  absorbed,  the  mixing  should continue  until  the  mix  is  of  a  uniform  consistency.

MIXING BY MACHINE 

 For machine mixing,all the materials including water are taken in a revolving drum and then the drum is rotated for a certain period. 

 The features of machine mixing are as follows- 

 Machine mixing is more efficient as it produces better quality concrete in a short time 

 Mixers of various capacities are available. They are of tilting and non-tilting type. 

 For small works, a mixer capable of producing concrete of one bag cement is used. For works such as roads, aerodromes, dams etc., special types of mixers are used. 

 Water  should enter the drum at the same time as the other materials. This ensures proper mixing. 

 The concrete mixer should be washed and cleaned thoroughly after use. If this is not done, the efficiency of the mixer is affected.

 The inside should be inspected regularly;the damaged or broken blades should be replaced. 

 The mixing time should be one to two minutes. 

 The concrete discharged should be used within 30 minutes. 

CAUSES & REPAIR OF FAILURE IN CONCRETE 

DETERIORATION:- 

 The most prevalent forms of concrete deterioration are:

 spalling and de-lamination due to corrosion of embedded steel reinforcement from chloride contamination or carbonation.  
 Freshly placed concrete has a high internal pH that protects embedded uncoated steel from corrosion by forming a protective layer on the surface of the steel.  
 chlorides and carbon dioxide break down this protection and create conditions conducive to corrosion.  
 Corrosion of the steel will occur under these conditions if water and free oxygen are present.
 Corrosion by-products, which form rust, expand the outer surface of the steel, creating large internal bursting stresses that crack the concrete. 

SPRAY REPAIR METHOD 

 Sprayed concrete is a method of applying concrete that is generally self supporting without the need for form work. Sprayed concrete is often referred to as Gunite (dry mix) or Shotcrete (wet mix) 

 Material is conveyed in a dry or semi dry state using compressed air to the nozzle where water is added to hydrate and mix the concrete before application with velocity onto the substrate. 

MIXING DRUM FRAME TURNING HANDLE WHEELS 

 The concrete mix is supplied in the wet form and is pumped to the spraying nozzle where air is added to project the concrete onto the substrate.
  
 The use of accelerators added at the nozzle can assist in applying the concrete in overhead situations or to increase early strengths gain requirements in ground support and tunneling. 

PRESSURE GROUTING: 

 Pressure grouting involves injecting a grout material into generally isolated pore or void space and is often referred to simply as grouting.

 The grout may be a cementitious, resinous, or solution chemical mixture. 

  The greatest use of pressure grouting is to improve geomaterials (soil and rock).  

 The purpose of grouting can be either to strengthen or reduce water flow through a formation. 

  It is also used to correct faults in concrete and masonry structures.  

 grouting has been performed on the foundation in order to reduce the amount of leakage through the rock, and sometimes to strengthen the foundation to support the weight of the overlying structure, be it of concrete, earth, or rock fill.  

METHOD OF PRESSURE GROUTING: 

 The purpose of Pressure Grouting is to control the amount of grout going into the pipe. As the process is repeated at additional elevations, the soil is compacted. 

 The process begins with the pumping of grout at the lowest elevation. The pipes are withdrawn slightly and the grout is allowed to set. 

 The process is repeated at the next elevation. Upon completion, the pipe is again withdrawn slightly to the next elevation. The grout is allowed to set. Each level of grout pumping will push the soil upward.

 The number of grout pumpings at these different elevations that are needed to compact the soil will vary. Once the soil is compacted, it causes lift. This lift then pushes the footing and wall upward. 

 Circumstances under which Pressure Grouting can be used include stabilizing soils under existing footings and slabs to alleviate settlement   Pressure grouting can be used for the filling of voids or cavities below grade without excavation.   Filling voids in soil or rock to control water seepage or leakage can also be accomplished with Pressure Grouting.