Concrete masonry - UK foam-slag of early 1950's 1

[Ronnie101]

Concretemasonry

Hi, I sincerely hope I’m not infringing forum etiquette by addressing this question to you in the first instance. I confirm that I’m perfectly happy for anyone to reply, it’s just that as my question relates to your answers in two previous posts, it makes sense to me to ask you first of all.

If I’m breaching forum etiquette in any way, I apologise unreservedly.

Re: the CMU of the 1950s and 60s, having an ultimate compressive strength that was often 50 - 100% over minimum strengths. Does this scenario apply only to USA, or would this scenario apply equally well to CMU made with foam-slag (foamed blastfurnace slag) aggregate, manufactured in the UK in the early 1950’s.

(I have researched the British Standards that were in force in the 1950’s and the minimum compressive strength cited for foam slag blocks is 400 p.s.i.)

Many thanks

Ronnie

 

[concretemasonry]

The British units you are mentioning could well be over the minimums for that time and the specifications.

First, you need to identify whether you are talking about the gross or net compressive strengths. Traditionally, solid block were common in England and the industry was geared for the production of solids. A hollow block will have a higher net compressive strength than a solid block.

50% to 100% over ASTM minimums are very common in the U.S. because of the improvements in batching controls and manufacturing equipment. Most British units are not made using U.S or "knock-off" equipment and are commonly used for low strength applications.

When I was in the block business, we could make 8700 psi net compressive strengths as compared to the ASTM minimum of 1900 psi. Unfortunately most engineers did not know how to design critical structures. Internationally, engineers are better equipped or more familiar with the use, control and inspection of higher strength units/construction.

About 20 years ago, ASTM abandoned the the concept of gross strength where you could increase the equivalent (or face shell and web thicknessses) to meet the ASTM requirements using lower strength concrete. That is why the U.S. tests for net compressive strength and not gross strength.

The reason why the typical U.S. hollow strengths are so far over the minimums is that the ASTM C90 standards for compressive strength have essentially not changed for well over 60 to 80 years, but the manufacturing equipment, aggregates and processes have. Currently, with decent aggregate it is difficult to not the the minimum ASTM standards. The cheapest way to make a stronger block is to increase the amount of water in a mix, but that is in conflict with the visual properties (a high amount of exposed surfaces) and requirements by contractors for close tolerances. A block mix is a "zero-slump" mix and gradation and moisture can have a greater influence than cement (very similar to a soil density vs moisture curve for compaction).

Your slag aggregate is probably a by-product and not subjected to the controls required for a modern block plant as an aggregate. It could be very good, but it may not be acceptable for higher strengths. In the U.S. and many other countries using modern equipment, the aggregate gradation controls are far stricter than the minimal ASTM requirements. In the U.S., the cinders used to make "cinder block disappeared about 60 years and today, most lightweight aggregate is from dedicated plants that produce aggregates for both block and high strength ready-mix concrete.

I hope this clears up the differences and provides some background.

Dick

 

[Ronnie101]

Many thanks - very helpful.

Best regards

Ronnie