Having worked for Boral Concrete and Readymix (Holcim), I have seen many changes in mix design, concrete batching and materials used over the years.
Many engineers today primarily rely upon concrete suppliers to provide them with the appropriate mix designs to satisfy strength and durability requirements. While the design engineer may specify the strength grade (and possibly shrinkage limit), the other decisions regarding admixture choice, type of cement, water-to-cement ratio, aggregate type and so on are often left up to the concrete suppliers. I have seen many engineering drawings specifying Type A cement in their General Notes. This type of cement was superseded in 1991 by Type GP cement, and yet most concrete suppliers use Type GB cement unless otherwise instructed.
Batching concrete over the years may have changed and even some of the materials used in concrete mixes have been improved (e.g. admixtures) but the basics don’t change. Correct water-to-cement ratio, adequate cement quantity per cubic metre, adequate barrel rotation to ensure materials are mixed thoroughly, understanding durability issues (e.g. carbonation and chloride ingress), correct concrete placement and curing, all still apply. If all these aspects are not understood, then a range of cracks will occur in concrete both early in the life of the structure and later on.
There are at least 7 forms of cracks that can occur in concrete (i) plastic shrinkage (ii) plastic settlement (iii) thermal (iv) drying shrinkage (v) crazing (vi) corrosion (vii) alkali aggregate reaction (AAR).
When the industry started moving away from water reducers and plasticizers based on lignosulphonates (LS) and started using polycarboxylate ethers (PCE), that too had some problems. The amount of PCE’s required in a concrete mix was much less than the LS liquids to achieve the same outcomes. This meant that if PCE’s were not thoroughly mixed in the truck barrel (agitator), then uneven slumps and other workability issues would result.
Another problem arose when concrete placers added extra water into the back of the premixed concrete truck. The LS liquids were more compatible with water, however, PCEs did not tolerate extra water and thus issues on-site often prevailed (the analogy being when you try and mix oil and water together). PCEs work on a steric repulsion system whereas LSs work purely on electrostatic repulsion.
Points mentioned above and more will be covered in our two (2) day ‘face-to-face’ course on Cement and Concrete Practice being held Wed 6 and Thu 7 December 2023. Day 1 will be at Rydges Hotel, Norwest (Sydney) and Day 2 will be held at the Boral Laboratory, Baulkham Hills NSW. Day 2 gives attendees an opportunity to see concrete tests being conducted.
For further information on the cost and content of this 2-day course, visit the link:
https://www.etia.net.au/event/cement-concrete-technology-practice-course-sydney
Article by Paul Uno