sábado, 11 de junho de 2011

VIDA E MORTE DO CIMENTO TIPO II DA ASTM



The great change in Type II cements.


The new bridge at Washington St. and I-25 has cracks on the sidewalks, the deck, and the barriers. Photos:
Credit: Richard Burrows

Map cracking is clearly evident on the deck of the new bridge at I-25 and Washington Street.
Credit: Richard Burrows 







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.


In 2002, 15 of the still uncracked 1950s-vintage bridges were replaced with wider bridges, which are cracking. The first, at I-25 and Franklin Street, developed 260 cracks within three months even though fly ash and night placement were used. The sidewalks had to be torn out and replaced with the construction joint spacing reduced from 9 feet to only 4 feet. The problem was then thought to be solved for all the later bridges but after several years, the new sidewalks began to crack as well as the other 14 bridges.
Concrete used in the bridge at I-25 and Colorado Boulevard, placed in 2004, has developed 53 transverse cracks. The cement's seven-day cube strength was 5600 psi.
Cracking of the runways at the Denver International Airport has precipitated a major investigation. It will be some months before the investigation is completed, but the cracks are from self-stress, not alkali silica reaction. A sample of 10 cements used at the airport had high seven-day cube strengths of between 4520 and 5380 psi, with an average of 5069 psi.


In 1940, P. H. Bates, the creator of ASTM C 150, was concerned with the cracking tendency of normal (Type I) cement, which, at that time, had a seven-day cube strength of about 3000 psi. Therefore, he created Type II cement with the lower strength of about 2500 psi. If Bates was worried about the cracking tendency of 3000 psi cement, what would he think about our modern Type I/II cements with strengths of 5000 psi? In reading accounts of the early work on the cracking of concrete, one thing is obvious. We have become much more tolerant of cracking than our predecessors—probably because of our obvious inability or unwillingness to solve the problem.
Based on Fig. 1, the concrete specifier who is selecting Type II cement for an application prone to cracking should impose a maximum limit of 4350 psi on the ASTM C 109 seven-day strength. The limit of 4350 psi was chosen because that is the maximum limit for Type GU (general construction) cement in ASTM C 1157, “Performance Specification for Hydraulic Cement.” An attempt is being made to remove this limit, which would be analogous to the 1960 removal of the mandatory limit on tricalcium silicate. History, does, in fact, tend to repeat itself.
Another solution is to invoke the moderate heat of hydration option (Table 4 of ASTM C 150) which limits tri-calcium aluminate plus tricalcium silicate to 58%. In either case, finding a cement producer who can and will supply slower-hydrating Type II cement will be difficult.
For three years the 75-member ASTM Committee on Cement has been voting on a slower-hydrating and very crack-resistant cement which would be called Type VI. It is currently stalled with nine negative votes. However, the concrete cracking problem at the Denver International Airport may turn out to be of such significance as to finally impress the committee with the need for a more crack-resistant portland cement.
Another impressive development is that the proposed Type VI cement is essentially identical to the special low-crack cement used in the largest dam the world—the first dam of the Three Gorges Project in China. This cement was designed by Professor Li Wenwei, vice director of concrete operations. Wenwei used ACI Monograph No. 11, “The Visible and Invisible Cracking of Concrete,” as a guide.
— Richard W. Burrows, a graduate of the Colorado School of Mines, joined the Bureau of Reclamation in 1946 and published his first paper on the durability of concrete in 1951. He is the recipient of the 1960 and the 2001 Wason Medal for Most Meritorious Paper, awarded by the American
http://www.concreteconstruction.net/concrete-construction/the-life-and-death-of-type-ii-cement.aspx?page=1


Referência com texto traduzido pelo Prof .Eduardo Thomaz
http://xa.yimg.com/kq/groups/1622770/489578303/name/CimentoTYPE%20VI%20%20nos%20USA%2Epdf

http://www.ime.eb.br/~webde2/prof/ethomaz
Pode-se observar a grande importancia dos teores de C3S e de C3A do cimento na propensão ou não do concreto à fissuração.

Alguns sugerem limites para reduzir a fissuração, entre eles o Prof. Aitcin.

Outros propoem a criação de um novo cimento TYPE VI , semelhante ao cimento usado na China com baixo calor de hidratação.
 


Na discussão do artigo nota-se um enfoque totalmente diferente por parte de quem executa as obras.




Fissuração do concreto. 
A influência dos teores de  

C3S e C3A dos cimentos. 

Autor: Richard Burrows - 2007


Nenhum comentário:

Postar um comentário