HIGH VOLUME FLY ASH CONCRETE

(The greening of concrete)

The Indian standards IS:456:1978 specify what essentially has existed for many years i.e. no more than 15%-25% fly ash as a total of cementitious content. ACI 318, till recently, allowed up to 25% fly ash for all concrete, but the latest ACI 318, just off the presses has since withdrawn this limit.

Indian RMC Manufacturers I believe add 15-18% fly ash to concrete but the comman man here still does not comprehend the benefits derived from the usage of fly ash in cement or in concrete.

The table given below is a paper presented by Dr Wilbert Langley and Dr Gordon Leaman at the sixth CANMET/ ACI / JCI International Conference, held May 31 - June 5, 1998. These are the actual mixes used in demonstration projects throughout Canada to prove the practicality of using high-volume fly ash concrete for a variety of projects. The Parklane Development in Halifax, Nova Scotia, Canada is a seven story structure and was built with 55% high-volume fly ash concrete (high strength mix given in the table below) . Cast-in-place columns and beams were poured with concrete specified to meet design strengths of between 4,350 psi at 28 days and 7,250 psi at 120 days. Actual strengths developed exceeded required strengths by 30%-40% on an average


HIGH VOLUME FLY ASH CONCRETE

All mixes contained air entraining admixtures and superplasticizers

 

Conventional Mix

 Low Strength 55% Replacement

Medium Strength 55% Replacement
  High Strength 55% Replacement

Total Cementitious Content (c+fa)
(lb/cu.yd) 

  483

374

566

660

 Cement (lb)

 483

166

250

300

 Class F Fly Ash (lb)

 0

208

316

360

 Sand (lb)

1334

1467

1250

1266

Stone (lb)

1700

1834

1834

1850

 Water (lb)

220

185

198

185

 Water to Cement Ratio

 0.46

0.49

0.35

0.28
     

 Compressive Strength (psi)

      

 3 day

4,600

1,250

2,320

3,190

 7 day

5,000

1,750

3,040

4,900

 28 day

6,500

3,350

5,500

8,300

 91 day

7,100

4,050

7,700

10,900

 365 day

7,550

7,400

10,000

-
         

 Set Time (hours:minutes)

        

 Initial

6:25

8:25

5:35

 -

 Final

7:50

11:15

7:40

 -

In the US, the State of Wisconsin has been using a 60% Class F fly ash in concrete mix since 1989. HVFA concrete has now found a commercial niche in the Sydney construction market and is being trialed for the Sydney Olympic facilities. For the Crown Casino project, Connell Wagner required highly durable and low drying shrinkage concrete for the construction of the 55,000 square meter basement that was located below the water table

Another benefit of using fly ash in concrete is that fly ash makes beautiful, "architectural" concrete. Fly ash of today is light in color and its extreme workability ensures smoother finishes. That most famous of architecturally exposed concrete buildings, the Jonas Salk Institute, was built with fly ash concrete. I have seen the NCCBM building located at Ballabhgarh & it still looks beautiful even after having weathered so many years.

Addition of fly ash in plaster virtually eliminates defects like crazing, map cracking, drying shrinkage cracks, debonding, grinning, expansion & popping. But more of it later.


sanjay agarwal
b.tech (IIT- kanpur)
cement/fly ash technologist
e-mail
abs cement website

as written for Cement India