Question: We are having problems entraining air and maintaining slump. Our sand gradation has changed, but our supplier says it still meets ASTM C33. Shouldn’t the concrete stay the same if the sand still meets the requirements?
Answer: How we purchase and sell aggregates is quite different, and it can lead to trouble. Most specifications require coarse and fine aggregate to conform to the requirements, including the gradation, of ASTM C33. Concrete producers then use that material to make concrete, the properties of which are quite dependent on the aggregate gradation, both coarse and fine. This presents a conflict — if the aggregate gradation changes, then the concrete proportions may need to be adjusted.
Many times the structural engineer or the contractor will object to a change in proportions, sometimes suggesting that a new submittal and a new set of testing data is required. This can be a tough position to be in — where the installer is not happy with the performance and to fix it you need to resubmit the mixture.
Even when the industry used simple methods like the absolute value method found in ACI 211, the aggregate gradation had an effect. The dry rodded unit weight and the fineness modulus of the sand, both important inputs in that design method, will change as the gradation changes.
Buying and selling
The problem originates in this variance between purchasing and selling. In ASTM C33 the allowable variation in the sieves can be quite large.
For example, the No. 30 screen can differ between 25% and 60% passing and still meet the ASTM C33 gradation requirements. If normal content is 35 and it changes to 55, the concrete will not perform the same as before. The increased surface area will make the water demand increase and the mixture will be sticky, and may begin to entrain air if the change is due to a decrease in the 50 and 100 sieve size.
There is an easy way to address this: Increase the cement content. This will increase the water content and make the mixture less sensitive to the change in surface area that occurs when the fine aggregate changes. Of course, the cost will increase, and the shrinkage and temperature of the concrete will increase.
The pressure to use more pozzolans and lower total cementitious contents is leading the industry to use more sophisticated mix designs. Most ready-mix concrete producers use their own methods, hire a testing lab, or rely on a vendor. These methods often include a concept of “ideal gradation.”
From Fuller’s maximum density curve, the .45 power curve, 8-18, gap grading to the latest “tarantula,” the aggregate gradation has a major impact on performance.
The aggregate suspension method, recently published by ACI 211, illustrates this clearly, as the dry rodded bulk density of blended coarse aggregate is used to determine the paste volume required to infill the voids in the mineral aggregate, and a quantity of “excess paste” is used to float the particles and prevent point-to point aggregate content. If there are swings in gradation there could be too little or too much paste to the detriment of the properties of the concrete.
These methods need a job mix formula approach, where the deviation on each sieve is limited to a few percent. If you are making concrete where the properties are sensitive to the gradation a purchasing agreement that allows less variation in the gradation than in ASTM C33. Setting this as a purchasing requirement can prevent this problem, and help to reject the concept that concrete is just a blend of ingredients. More on that in the next issue.
Kevin MacDonald is president of Beton Consulting Engineers. Contact him at firstname.lastname@example.org or visit www.betonconsultingeng.com.