Concrete Pixels

For more than 15 years of my career in construction, I have only known the heavyweight concrete from the literatures such as textbooks, publications, etc. But now I have a close encounter with this concrete as I was involved in the design and testing of such type of concrete for our project. This concrete is made on the basic principle that since concrete is composed mainly of aggregates (about 70%) and so to make it heavy, the aggregates to be used must be heavy. Aggregate of very high unit weight, such as barium, iron ore, steel shot or punchings are normally used. For our case we used the iron ore. Heavyweight concrete has been widely used as bridge counterweights and as radiation shielding in nuclear plants or other radiation related application in medical establishment. Selection of water-cement ratio for heavyweight concrete is crucial as the cement dosage should be both high enough to allow for radioactive impermeability and low enough to prevent splits originating from shrinkage. A w-c ratio of around 0.40 is a good target. The grain size distribution of the used aggregates has to be carefully looked into as well that it is close enough & within the required sieve envelopes, and the distribution is smooth without much of kinks. The types and dosage of admixtures has to be tested well inorder to achieve the required properties of the fresh concrete. Small amount of microsilica and flyash would also be good. For the iron ore and a 20mm maximum size aggregates , a density of more than 3700 kg/m3 can be easily achieved (normal concrete is only about 2400 kg/m3) and an incredible early strength of more than 40 MPa in just 3 days for a K40 designed concrete. So you can imagine how much the 28-days strength will be.