Type II cement was born in 1940 with the publication of the “Standard Specification for Portland Cement,” ASTM C 150. The father, Pharon H. Bates, was a brilliant scientist with the National Bureau of Standards. Bates sired Type II cement for the purpose of reducing thermal cracking by limiting the tricalcium silicate (C3S) and tricalcium aluminate (C3A). For a time, Type II cement performed very well. In 1943, the magazine Concrete announced, “Low heat cements have been developed very successfully and the reduction of shrinkage cracks has been greater than expected.”
In 1940, there was an 8% limit on C3A and a 50% limit on C3S. But in 1960, Type II cement changed. Contractors realized they would profit if Type II cement's early strength could be increased. Even though the fineness was increasing, contractors pressured the cement producers who, in turn, influenced ASTM Committee C01, Cement to remove the mandatory limit of 50% on C3S. As a compromise, the committee offered the moderate heat option where the sum of C3A plus C3S was limited to 58%. Unfortunately, very few specifiers had been educated to use this option, and so, by 1994, only 33 moderate heat cements remained of the 147 Type II cements in North America. There are even fewer now.
Competing for higher early strengths, cement producers continued to increase both the fineness and the C3S. In 1965, the late Bryant Mather visited Europe and found that they had had maximum limits on early strength for years. When he recommended this to the ASTM committee, he was laughed at. Thus, the early strength of Type II cement continued to increase, but it could not be tolerated without adverse effects.
Jack R. Benjamin and L.D. Long knew that something was wrong. They, with nine other experts selected by the American Concrete Institute, began a study of problems in the concrete industry. In 1979, they concluded that the primary problem was the change that was taking place in Type II cement—it was getting stronger faster and causing cracks. Long and Benjamin deplored the lack of maximum limits on early strength and said any Type II cement with a three-day cube strength greater than 3000 psi should be considered a Type III cement regardless of the producer's claim. In 1954, all Type II cements were under 3000 psi. But by 1994, 89% of them were over 3000 psi and should have been considered Type III cements. Despite the efforts of the ACI task force, ASTM again failed to respond and fineness, C3S, and early strength continue to increase. If this persists, Type II cement will expire in the year 2030 at the age of 90.
In defense of Type III cements, they are ideally suited for the precast/pre-stressed industry. Prestressed girders do not crack as the concrete does not go into tension. Precast panels made with Type III cement also have a history of good performance. But bridge deck concrete is another story.
What will concrete technologists say when Type II cement completely disappears? Most of them will remain complacent with their flawed theory that hyperactive Type II cements can be tamed and safely used by adding more inert materials like fly ash and slag. This has been proved incorrect by a number of investigations and by recent projects in Denver. Portland cement is the glue that holds the rocks together. If you have bad glue, adding relatively inert materials to it will not magically transform it into good glue.
In 1987, Adam Neville wrote that the deterioration of concrete was due to the failure to put maximum limits on fineness, tricalcium silicate, and early strength. More than 66 studies support the principle that anything that increases the rate of hydration of the cement decreases the durability of the concrete.
Although it is difficult to accept that very strong concrete is prone to cracking, as a rule materials become more crack-prone as they get stronger, and concrete is no exception. But with concrete, there are additional factors. Concrete that gains strength too quickly may crack from self-stress—the cumulative internal stresses from autogenous shrinkage, thermal contraction, and drying shrinkage. And strong concrete has less creep capacity to relieve these stresses. Its higher modulus also contributes to cracking.
The marked change in Type II cement between 1954 and 1994 is shown in Fig. 1, as well as the wide range in the early strengths. When one specifies a Type II cement, one can be lucky and get a crack-resistant cement with a strength of 3500 psi or be very unlucky and get a crack-prone cement with a strength of 5000 psi. This has repeatedly happened to the Colorado DOT.
Slow-hydrating cements were used in the 165 still-perfect bridges that were built in Colorado in the 1950s, such as the Washington Street Bridge, which was still crack-free after 50 years. However, cement used in the new 23rd Street Viaduct, which cracked before it was finished in 1996, had a combined C3A and C3S of 72%—higher than all the Type II cements in North America in 1994. The fineness was also very high.