Showing posts with label SSC JE Previous Year Paper. Show all posts
Showing posts with label SSC JE Previous Year Paper. Show all posts

 

Hot Weather Concrete Construction

 When the temperature of freshly mixed concrete approaches approximately 77 degrees Fahrenheit adverse site conditions can impact the quality of concrete. Ambient temperatures above 90 degrees Fahrenheit and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in producing quality concrete.


 The precautions required to ensure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 77 degrees Fahrenheit a plan should be developed to negate the effects of high temperatures.

 The effect of high ambient temperatures and high temperature concrete component materials have on the setting time of concrete mixtures is a topic of concern due to the reduced time in which concrete must be placed, consolidated and finished; increased potential for plastic shrinkage cracking, thermal cracking and cold joints; potential strength reduction due to high water demand and high curing temperatures; difficulty in controlling air content; and increased urgency for applying appropriate curing method at an early age.

 As a general rule of thumb an increase of 20 degrees Fahrenheit will reduce the setting time of a concrete mixture by as much as 50 percent. As an example, a concrete mixture that reaches final set in three hours at 60 degrees Fahrenheit may reach final set in as little 1½ hours at 80 degrees Fahrenheit. As the concrete temperature increases the setting time is further reduced. The actual temperature of the concrete mixture as delivered is effected by the temperature of the materials used in the mixture, the cementitious content of the mixture, the temperature of the equipment used to batch and transport the concrete, and the ambient temperature and conditions at the project site. Concrete applications may be considered hot weather concrete at temperatures ranging from 77 to 95 degrees Fahrenheit depending on the specific application. Precautions should be planned in advance to counter the effects of high temperature well in advance of execution to counter these effects.

 Precautions may include use of materials with a good performance history in high temperature conditions, cool concrete materials or concrete mixture, provide concrete consistency and placement equipment and crew for rapid placement, reduce time of transport, schedule placement to limit exposure to atmospheric conditions (night time placement or more favorable weather), plan to limit rapid moisture loss (sun screens, wind screens, misting, or fogging), and consider the use of an evaporation retarder. Schedule a preconstruction meeting including all of the participants to discuss the plan to control the effects specific to the project and expected conditions. 



 The precautions may include some or all of the following:

  •  Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
  •  Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
  •  Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature. The effect of hot cement on concrete temperature is only minimal. 
  • Use a concrete consistency that allows rapid placement and consolidation.
  •  Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
  •  Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.
  •  Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
  •  Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
  •  In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.
  •  The use of liquid nitrogen is one option to reduce concrete’s temperature during hot-weather concreting.
  •  Modify concrete mix designs as appropriate. Retarders, moderate heat of hydration cement, pozzolanic materials, slag, or other proven local solutions may be used. Reduce the cement content of the mixture as much as possible, while ensuring the concrete strength will be attained
  • Limit the addition of water at the job site.








Cold Weather concreting

  1.  WHAT is Cold Weather? 

 Cold weather is defined as a period when the average daily temperature falls below 40°F [4°C] for more than three successive days. These conditions warrant special precautions when placing, finishing, curing and protecting concrete against the effects of cold weather. Since weather conditions can change rapidly in the winter months, good concrete practices and proper planning are critical.

2. Why Consider Cold Weather?

 In its plastic state, concrete will freeze if its temperature falls below about 25°F [-4°C]. If plastic concrete freezes, its potential strength can be reduced by more than 50% and its durability will be adversely affected. Concrete should be protected from freezing until it attains a minimum compressive strength of 500 psi [3.5 MPa], which is about two days after placement for most concrete maintained at 50°F [10°C]. Low concrete temperature has a major effect on the rate of cement hydration, which results in slower setting and rate of strength gain. A good rule of thumb is that a drop in concrete temperature by 20°F [10°C] will approximately double the setting time. The slower rate of setting and strength gain should be accounted for when scheduling construction operations, such as form removal.Cement hydration is a chemical reaction that generates heat. Newly placed concrete should be adequately insulated to retain this heat and thereby maintain favorable curing temperatures. Large temperature differences between the surface and the interior of the concrete mass should be prevented as cracking may result when this difference exceeds about 35°F [20°C]. Insulation or protective measures should be gradually removed to avoid thermal shock .

 3. How to place concrete in cold weather?

  1. The ready mixed concrete producer can control concrete temperature by heating the mixing water and/or the aggregates 
  2. Placing concrete in cold weather provides the opportunity for better quality, as cooler initial concrete temperature will typically result in higher ultimate strength. 
  3. Slower setting time and strength gain of concrete during cold weather typically delays finishing operations and form removal.
  4.  Chemical admixtures and other modifications to the concrete mixture can accelerate the rate of setting and strength gain. Calcium chloride is a common and effective accelerating admixture, but should not exceed a maximum dosage of 2% by weight of cement.
  5.  Accelerating the rate of set and strength gain can also be accomplished by increasing the amount of portland cement or by using a Type III cement (high early strength).
  6. The relative percentage of fly ash or ground slag in the cementitious material component may be reduced in cold weather but this may not be possible if the mixture has been specifically designed for durability.
  7. Concrete should be placed at the lowest practical slump as this reduces bleeding and setting time.
  8. Adequate preparations should be made prior to concrete placement. Snow, ice and frost should be removed and the temperature of surfaces and metallic embedments in contact with concrete should be above freezing. This might require insulating or heating subgrades and contact surfaces prior to placement.
  9.  Materials and equipment should be in place to protect concrete, both during and after placement, from early age freezing and to retain the heat generated by cement hydration. Insulated blankets and tarps, as well as straw covered with plastic sheets, are commonly used measures.
  10.  The concrete surface should not be allowed to dry out while it is plastic as this causes plastic shrinkage cracks.Subsequently, concrete should be adequately cured. Water curing is not recommended when freezing temperatures are imminent. Use membrane-forming curing compounds or impervious paper and plastic sheets for concrete slabs.
  •  Cold Weather Concreting Guidelines 

  1.  Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
  2.  Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior to placement.
  3.  Place and maintain concrete at the recommended temperature.
  4.  Place concrete at the lowest practical slump.
  5.  Protect plastic concrete from freezing or drying
  6.  Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
  7.  Limit rapid temperature changes when protective measures are removed


 

 

SSC JUNIOR ENGINEER EXAMINATION

Candidates may refer to the notice of Junior

Engineers(Civil/Mechanical/Electrical and Quantity Surveying and Contract)Examination 2014 published in the Employment News/Rozgar Samachar dated 01-03-2014 and the Corrigendum issued on 28-04-2014.


Written examination of Paper-II will be held on 18-01-2014 Forenoon Session

As every one of you are searching the previous year conventional Papers of SSC JE,
I am hereby attaching the papers of Civil Engineering.


Kindly click on the link to download the papers