foundation system of burj al arab

Plastic Shrinkage Cracking in Concrete

When rate of evaporation exceeds the rate at which the bleeding water rise to the surface, plastic shrinkage cracking is likely to occur. Sometimes cracks also form under a layer of water and merely become apparent on drying. The critical evaporation rate is > 1.0 Kg/m2 per hour.

Details of plastic shrinkage cracking

Size of cracks

Plastic shrinkage cracking can be very deep, varying in width between 0.1 and 3 mm, and can be quite short or as long as 1 m.
Encouraging environment
A drop in ambient relative humidity encourage this type of cracking, so that, in fact, the cause of it appear to be rather complex. According to ACI R 305 R-91 the risk of plastic shrinkage cracking is the same at the following combinations of temperature and relative humidity.

410 c (1050 F) and 90 percent.

350 c (950 F) and 70 percent.
Plastic Shrinkage Cracking in Concrete

240 c (750 F) and 30 percent.

Wind velocity in excess of 4.5 m/s ( 10mph) aggravates the situation.


Location of appearance

Plastic Shrinkage Cracking in Concrete
The loss of water from by suction by the underlying dry concrete or soil may result cracking. Diagonal cracking is observed in pavements and slabs. Over reinforced section especially steel very near to surface of slab are frequently subjected to such cracking. Random cracking may occur normal reinforced concrete slabs.

Causes

Rapid early drying generally produces this type of cracking. Low rate of bleeding as well can result cracking. As discussed previous, temperature, ambient relative humidity and wind velocity exceeding specified range results crack friendly environment. It should be remembered that evaporation is increased when the temperature of the concrete is much higher than the ambient temperature, under such circumstances, plastic shrinkage can occur even if the relative humidity of the air is high.


Remedy 
Complete prevention of evaporation immediately after casting eliminates cracking. The best practice is to protect the concrete from sun and wind, to place and finish fast, and to start curing very soon thereafter. Placing concrete on a dry subgrade should be avoided.

Arrival time

They are generally appeared within 30 min to 6 hr of concrete placement.

3 comments:

  1. Fully appreciate the value you are providing here.
    thanks for posting this blog. its really very helpful for us
    plastic forms for concrete

    ReplyDelete
  2. Woah, that's amazing! Thanks for sharing this amazing how-to guide.
    Excellent post, thanks.
    repair and design concrete

    ReplyDelete
  3. Dear Author, Please confirm whether the second photo is of a topping concrete thin layer on some dense substrate. Thanks.

    ReplyDelete

Followers