Air contents in concrete

As per IS 456 air contents in concrete should not exceed 5 +-1% for 20 mm aggregate and 4+-1% for 40 mm aggregate concrete. It can be checked theoretically by dividing density of concrete from batching plant to the density as per mix design multiplying by 100. or it can be checked by air pressure method. the decrees in volume in container give the %age of air by giving pressure.

However concrete being impermeable there should be 0% air voids ar as taken in mix design.

Comments

Post a Comment

Popular posts from this blog

Penetration Test for Bitumen IS 1203 1978

Hot the Bitumen in oven 90oc + Softening point and pour in in bowl, approx 10 mm more than expacted penetration. Use bowl depth 35 mm, dia 55 mm size for penetrations below 225 Then keep it in room temp for 1 – 11/2 hours (75 min) Then in water bath at temp 25oc for 1 – 11/2 hours (75 min) Use bowl depth 45 mm, dia 75 mm size for penetrations from 225 to 350 Then keep it in air for  11/2 -2 hours (105 min) Then in water bath at temp 25oc for 11/2 -2 hours (105 min) Then place it under the apparatus Wt of assembly should be 100 g (if 50 gm wt + needle + assembly) Wt of assembly should be 200 g (if 150 g wt + needle + assembly) For CRMB 60 penetration will be <50 Penetration ratio = Penetration at 4oc 200 gm for 60 sec ---------------------------------------------- x 100 Penetration at 25oc 100 gm for 5 sec
Read more

Why do we test concrete on 28th day? why not on 27th day or 29th day?

Before we go to the answer, we have to peep into the history of civil engineering. As we all know civil engineering and concrete technology were developed and practiced in England in ancient times and now US is leading in advancing the technology further. Concrete achieves its maximum material strength over its life time and depreciated by exposure and other environmental conditions. However, engineers with experience observed that about 99% of expected design strength was achieved in about 30 days and also observed that the rate of gaining strength was reducing thereafter making the concrete achieving its 100% design strength in a year or two. The Church was so powerful those days that it was compulsory for every citizen to attend Sunday prayer without fail. The defaulters were punished severely. If they had to test concrete on the 30th day, it may fall on a Sunday. In order to escape the punishment by the Church, the engineers had two choices - they have to test concrete cube o...
Read more

Cement IS 269

cement is used to bind the materials in concrete  The manufacturing process includes Proportioning of raw materials like chalk or limestone,Grinding and burning at 1400-1450OC in rotatory kiln. Due to intense heat the material becomes like spherical balls called clinkers. Gypsum is then added in 3-4%. Cement is formed by grinding the clinkers to required fineness. Tricalcium Silicate (C3S) hardens rapidly and is largely responsible for initial set and early strength. In general, the early strength of portland cement concrete is higher with increased percentages of C3S. Dicalcium Silicate (C2S) hardens slowly and contributes largely to strength increases at ages beyond 7 days. Tricalcium Aluminate (C3A) liberates a large amount of heat during the first few days of hardening and, together with C3S and C2S may somewhat increase the early strength of the hardening cement (this effect being due to the considerable heat of hydration that this compound evolves). It does affect set time...
Read more