There is a misconception that a high concrete strength means good durability. This is true until the strength reaches a level where the concrete becomes brittle and prone to cracks. Concrete in the U.S. has moved well beyond this point. Because of this misconception, a number of mistakes have been made—to the detriment of concrete durability.
The cement specification ASTM C 150 was formulated in 1940 by P. H. Bates, who designated normal cement as Type I with a seven-day strength of 2800 psi, on average. Bates wanted a cement with a higher resistance to cracking, so he created Type II with the lower seven-day strength of 2100 psi. This advantage was soon lost because contractors demanded "faster" cement and producers competed for market share by grinding more finely. As a result, early strengths have increased almost 300% over the years.
As far as the crack resistance of cement is concerned, the first mistake occurred in 1960 when the mandatory limit of 50% on tricalcium silicate was removed. The second mistake happened in 1965, when the National Bureau of Standards sent Bryant Mather to Germany to review concrete practices. "They had strength classes on all sorts of cements and had them for years," wrote Mather. "All strengths were upper- and lower-banded. I came back, advocated this, and got laughed at."
In 1979 the third mistake was made. Jack R. Benjamin and Leslie D. Long recommended that Type II cements with three-day strengths greater than 3000 psi be designated as Type III. Their complaint was ignored, so by 1995, 132 out of 147 Type II cements exceeded 3000 psi.
Later, Geoffrey Frohnsdorff, Ron Gebhardt, and Karl Hauser, of the ASTM Cement Committee, finally addressed the issue. They saw to it that in the new performance specification, ASTM C 1157 (issued in 1992), the cements were classified by strength class with minimum and maximum limits. They patterned their approach after the European cement specification ENV 197. At this point two more mistakes occurred. First, the committee would not agree with the maximum strength limits proposed and insisted that they be raised. Second, it made the limits optional.
Finally, in 2008, the final mistake was made. After Frohnsdorff and Gebhardt died and Hauser retired, the strength limits were deleted from C 1157 in order to make the specification more "user friendly." It has definitely made it more producer friendly: Now there is no distinction between general purpose cement (type GU) and high early strength cement (type HE). Every portland cement in the U.S. now can be sold as either type indiscriminately.
"Suffice it to say that the long-term durability of concrete was a primary consideration when Geoff Frohnsdorf, Ron Gebhardt, and I pushed for the strength ranges in C 1157 specification," writes Karl Hauser. "Without those, there is little need for such a performance specification, and it would become almost completely useless." Now that the limits have been deleted, "would" has become "has."
Why classify cements by strength?
ASTM C 1157 and ENV 197 classified cements by strength because the risk of cracking increases with the early strength. Certain concrete applications are prone to cracking and therefore we need slower-hydrating cements. If this were not true, we would have only one strength class—all cements would be high-early strength. This has essentially become our reality.
Short-term and long-term solutions
It took 35 years for William Lerch to get his sulfate change into C 150. It took Frohnsdorf, Gebhardt, and Hauser 15 years to produce C 1157. If the ASTM set out today to modify C 150 and C 1157 to provide crack-resistant cements, it would take years—during which time, crack-prone Type II (actually Type III) cements will be produced. The deterioration of concrete will increase as the early strength continues to creep upward due to incremental increases in fineness and C3S. A quick solution for the short term needs to be implemented immediately.
A short-term solution
I believe that the only feasible solution is a government mandate to limit both Type II (C-150) and Type GU (C 1157) cements to 4000 or 4350 psi, maximum, at seven days. The 4350 value was the Type GU upper limit in C 1157 and the 4000 value was the limit originally preferred by Frohnsdorff, Gebhardt, and Hauser.
I pointed out to one cement producer that such a such a mandate actually would be beneficial. It could grind a little coarser—saving both time and energy, sell cheaper cement at the same price, and make more money. Plus, it would be more sustainable, and they would finally be free from the game of leap frogging to higher and higher finenesses and C3Ss. If a contractor complained that the cement was too "slow," the producer could just tell them to use Type III cement if they really need a "fast" cement.
