The concrete bridge deck cracking problem that is common across the U.S. is due to the change in portland cement that has occurred over the last 70 years—and that is continuing. What has changed, is the early strength, a function of the rate of cement hydration. Sixty-six studies have related the hydration rate of cement to the cracking tendency of concrete. Based on 169 studies, I offer a theory: Anything that increases the early strength of concrete is detrimental to durability.
Many attempts to halt the increasing strengths of Type II cement were unsuccessful. In Europe, however, the manufacturers formulated the specification, ENV 197, to stop and reverse the trend. For control of cracking, ENV 197 has six strength-controlled Types as compared to only two for the U.S. (Types I, II and V, and Type III).
In the U.S., the slow-hardening, crack-resistant cements of the 1940s have completely disappeared, but in Europe they are still available as Type 32.5. Type 32.5 cement was obtained from Heidelberg, Germany. Tests confirmed the excellent resistance to cracking. The cement is comparable to our low-cracking cements of 1942.
ENV 197 also ensures the uniformity of each type. This feature alone has eliminated many problems. I recommend changes to our cement specifications plus a government mandate to limit the 7-day strength of Type II cement to 4000 psi.
A brief history of Type II cement
Portland cement is mainly a mixture of dicalcium (C2S) and tricalcium silicate (C3S). These compounds react with water to form a solid matrix of calcium silicate hydrates. C3S hydrates faster than C2S. The speed of this reaction is primarily controlled by the fineness.
In 1926, the fineness and the C3S began to increase. P. H. Bates, chief of the Concrete Division of the U.S. Bureau of Standards, published a paper stating his concern about the increased risk of thermal cracking associated with the increased heat of hydration of the finer-ground, higher C3S cements. In 1940, Bates issued ASTM C 150. In this specification, he acted on his concern about cracking by deviating from the normal (Type I) cement of that time which had a 7-day strength of about 2800 psi. He felt that 2800 psi was too high so he designed Type II cement by limiting C3S to 50 percent. This reduced the 7-day strength to about 2200 psi. Unfortunately, ASTM C 150 did not control the fineness and the strength has increased to as high as 5800 psi—-more than twice Bates’ original intention, as shown in Figure 1.
This happened because the large ready-mix companies used their financial clout to play one producer against another to get higher cement strengths so they could make more profit by selling leaner concrete mixtures. This was never popular with producers, since such changes always involved a substantial increase in manufacturing cost.
This phenomenon also occurred in Europe. However, in the 70s, the European manufacturers, concerned with the durability of the leaner mixtures, formulated the specification, ENV 197, to limit and reverse the upward trend of strength. The upper limit on Type 32.5 is shown in Figure 1. Note that all Type II cement met this criteria until 1960; by 2005, none of the cements available met it.
In the U.S., at least seven attempts to halt the upward trend were made. They were all unsuccessful, as described in Appendix A. A notable attempt occurred in 1992 when ASTM Committee C01, Cement, issued the new performance specification for cement, ASTM C 1157. In this specification, upper limits were placed on each strength class, as ENV 197 had done. For general construction cement, the upper limit on the 7-day strength was 4350 psi. The specification was the result of 10 years of effort by Geoff Frohnsdorff, Ron Gebhardt, and Karl Hauser. But the committee would not ratify these limits until they were made optional.
But cement users did not specify the option so there was little demand for the cement. Then, in 2010, another blunder. The strength limits were deleted in order to make the specification more user-friendly. They certainly made it more producer-friendly because, now, there is no distinction between Type II and Type III cements. One could order Type II and get Type III. One could order Type III and get Type II. There is no legal recourse.
The last attempt was my own—I proposed a low-crack cement be included in ASTM C 150 as Type VI. The proposal was balloted in 2003, 2004, 2005, and 2008 with affirmative votes of 45, 51, 78, and 69 percent. At age 92, I gave up the fight. No one has stepped up to continue the battle.
What’s wrong with strong concrete?
Intuitively, strong concrete should be durable concrete. But it is fundamental in material science that the stronger the material, the more crack-prone it is. If you drop a glass plate, it shatters. But a weak plastic or paper plate doesn’t. If you cool a rubber ball in liquid nitrogen, it becomes very hard and strong but will shatter if dropped.
The paper, “202 Observations On Concrete That Is Too-Quickly-Strong,” cites cases where concrete cracked because it was too strong from too much cement and 66 investigations that related durability problems to cement hydrating too rapidly because of high values of fineness, C3S, C3A, and/or alkalies. A summary of 169 investigations is presented in Appendix B, which proposes a theorem: Anything that increases the early strength of concrete is detrimental to durability.
Concrete is a very brittle material. If it were not able to relieve its internal stresses by creep, almost all concrete would crack. The stronger the concrete, the greater these internal stresses and the lower the creep capacity.
When a bridge deck is placed, tensile stress from autogenous shrinkage develops in the first hours. Then, as the concrete cools from the heat of hydration, stresses from thermal contraction are added. Then, in a dry climate, the concrete will lose its internal moisture and drying shrinkage adds more tensile stress. If the concrete has not yet cracked, it may crack when the ambient temperature drops or a cooling rain occurs.
How about old-fashioned concrete?
When a low-crack cement (Type VI) was balloted by ASTM Committee C01, some negative voters seemed bothered that the cement was patterned after the cement of 1942. They questioned that the distant past could have been better than the present. This prompted us to go even further into the past and describe the findings of Professor Myron Withey.
From 1923 to 1937, Professor M. O. Withey, at the University of Wisconsin, fabricated concrete specimens that were then stored outdoors for 50 years. The 1923 cement was still gaining strength after 50 years while a cement representing our current cement was regressing in strength after 10 years, as shown in Figure 2.
The 1923 cement had a 7-day strength of only 1500 psi but at the age of 50 years, it was stronger than the conventional cement. The Roman Aqueducts have a strength of about 1500 psi and they are uncracked after 2000 years.
The 1927 bridge deck shown in the slideshow is in perfect condition. It should last for hundreds of years, quite unlike our current bridge decks that often begin to crack before they are finished.
Cracking at Denver International Airport
This cracking can be blamed on the seven failed attempts to limit the early strength of Type II cement.
Concrete placed in 1991-1993 began to crack after about 8 years. In 2008, 10,000 slabs had drying shrinkage cracks. This was of great concern because of the potential for a fragment to be sucked into a jet engine. In 2008 alone, $22 million was spent replacing panels. Besides the typical map cracking from drying shrinkage, the crack patterns near the joint intersections were caused by the freezing and thawing expansion of concrete that had been severely microcracked from drying shrinkage. The basic cause was the hyperactive, crack-prone Type II cements that were used. One cement had a 7-day strength of 5477 psi. This is higher than all of the 147 Type II cements that Ron Gebhardt surveyed in 1995. His highest was 5410 psi. So this cement was arguably among the most crack-prone Type II cements in North America.
Putting the 5477 psi cement in perspective, it is worth noting that when P. H. Bates designed Type II cement in 1940, he regarded the normal cement of that time, with its strength of 2800 psi, to be too crack-prone and so he put upper limits on C3A and C3S. This reduced the 7-day strength from 2800 to 2200 psi. What would Bates think if he were resurrected and discovered that his crack-resistant Type II cement could now have a strength of 5477 psi, more than twice his intention? This strength even exceeds his Type III cements, which were 4630 psi on average. He would regard us as stupid jackasses. Our ancestors would not have tolerated the level of cracking that we now have come to accept and may even view as “normal.” In fact, when people complain about their concrete cracking, they are told, that’s what concrete does.
Cracking resistance of German versus American cements
Appendix B concludes that anything that increases the early strength, such as the rate of hydration, is detrimental to durability. The rate of hydration can be measured by an early-age adiabatic strength test, by measuring the heat of hydration with a calorimeter, or by the new chemical shrinkage test, ASTM C 1608. This is the preferred method because it is the simplest and most repeatable.
When cement and water react, the volume of the reaction products is less than the original volumes of cement and water, so the rate of the volume decrease is a measure of the rate of hydration.
A paper I wrote in 2004 concluded that a low-crack cement has a 12-hour chemical shrinkage of less than 1.05 ml of water per 100 grams of cement at 72° F. This was based on cracking tests in Germany.
This value was verified in a paper by Bentz, Sant and Weiss. They tested a special coarse-ground cement from Lehigh that had a fineness of 311 sq m/kg. This was very close to ENV Class 32.5 cement and also the proposed Type VI low-crack cement. This cement met the criteria as shown in Figure 4.
With the help of Gary Knight, the US Bureau of Reclamation obtained Class 32.5 and 42.5 cements from the Heidelberg-Berglenenfeld plant in Germany and tested them. The results, in Figure 5, show that the Class 32.5 cement met the 1.05 criteria.
Of particular interest is Figure 6. It happens that Myron Swaze made chemical shrinkage tests in 1942. His results show that the cement of 1942 met the 1.05 criteria. The curves for the year—2000 cements were provided by Gordon Sellers from tests done at the University of Texas. The Type I/II cements of 2000 failed to meet the criteria.
Is American portland cement inferior to European cement?
ASTM C 150 originally had five types of cement, but Type IV is obsolete and Types I and II have almost merged as Type I/II. For years, Type II and Type III were widely different, strength-wise, but Figure 1 showed how they have been merging as time passed. It appears that, if nothing is done, they will merge together by about 2030. All cement will then come out of the same barrel and the cement-user will have no options except to use one cement (Type I/II/III) and try to get the properties he wants by adding fly ash, slag, silica fume and chemicals. This merging together is the result of having no control of the strength, the most important factor in the control of cracking.
Now, the U.S. has only two strength classes: Type I, II and V, plus Type III. ENV 197 has six classes of cement, all strength controlled and much more uniform and predictable. ENV 197 is used by the countries that are members of CEN (Comitee Europeen de Normalisation). The countries are listed in Table 1.
A similar standard is used by China and India, who produce 75 percent of the world’s cement. In ENV 197, there are Types 32.5, 42.5, and 52.5. Then, within each type, there is the classification “N”, for ordinary early strength; and “R”, for high early strength. Class 32.5N cement, with its lower strength, makes wonderful crack-free concrete. In 1942, some of our Type II cements were as crack-resistant as 32.5N but that time has long passed.
Those who disagree with these allegations need to be reminded that the reason for classifying cements by their strength was for the control of cracking. When P. H. Bates authored ASTM C 150, he happened to classify cement by the heat of hydration-- which is the same thing.
As shown in Figure 7, American portland cement is inferior to European cement. The government should take action because ASTM Committee C01 has not and will not. This is evident from the many failed attempts to stop the 70-year upward trend of strength which is causing severe cracking problems. The integrity of our concrete infrastructures for the next 100 years is at stake.
Cost impacts of hyperactive Type II portland cement
The first cost is the cost of grinding the cement clinker to a higher fineness.
In 1970, as the fineness was increasing, some Type II cements appearing in the market place had strengths exceeding 4000 psi at 7 days. Some bridge decks were observed to be cracking. Michael Sprinkel, Associate Director of the Virginia Highway Research Council, wrote that he has not been able to made a deck without cracks since 1967.
In 1973, two costly changes were made that would not have been necessary with the low-crack cement of 1942:
- In an attempt to reduce the cracking, AASHTO increased the minimum strength of bridge decks from 3000 to 4500 psi. This actually increased the cracking, according to Howard Newlon in his survey of Virginia bridge decks. Costs were increased by the additional cement used and by the increased cracking caused by the additional cement.
- Epoxy-coated rebars began to be used. The initial cost increased because of special handling precautions to protect the coating, and now, costly stainless steel, and even fiber-reinforced polymer are being considered. The U.S. and Canada may be the only two countries to use epoxy-coated rebar. It is banned in Germany, Norway, and Sweden. It reduces the bond strength by 35 percent, increasing the cracking.
The cracking from our hyperactive cements have been very profitable for the industries producing chemical admixtures, fibers, and silica fume. These industries were minuscule in the 1950s when Colorado built 232 deiced bridges with uncoated steel rebar that did not crack.
Construction costs increased because of the necessity to install more construction joints. The traditional spacing of 10 to 15 feet was, in the case of the T-REX project, reduced to 4 feet, and the concrete still cracked.
To prevent plastic shrinkage cracking, some DOTs have had to build special misting machines to follow the finishing machines.
The nonuniformity of Type II cements have caused onsite production problems, often with air-entrainment, which has to be addressed by more laboratory testing and costly delays.
Retarding agents, originally intended only for hot days, are used all year. The above costs are in addition to the costs of our deteriorating bridge decks where estimates range from $46 to $300 million, annually.
Bridge deck cracking is followed by the corrosion of the reinforcing steel. A 2002 estimate placed the direct cost of corrosion of highway structures at $8.3 billion annually with indirect costs as much as 10 times that much.
A 1987 report by the National Materials Advisory Board found that 253,000 bridge decks were deteriorating and that number was growing at the rate of 35,000 decks every year.
A nationwide 1995 survey of 200,000 bridge decks found that over 100,000 were suffering from early-age cracking.
And the situation continues to worsen. Bridge deck surveys in Kansas indicate that bridge decks cast between 1993 and 2003 exhibited more cracking than decks cast during the preceding 10 years.
To solve this significant problem, one of the following two recommendations should be implemented:
1. ASTM C 150 should be modified to include a low-crack cement equivalent to ENV 197 Class 32.5N. The cement should have an upper limit of 3300 psi on the 7-day strength and a 12-hour chemical shrinkage less than 1.05 ml of water per 100 grams of cement at 72° F.
2.ASTM C 150 should be replaced by ENV 197. Because, historically, changing C 150 takes many years, the government should immediately mandate that portland cement for general construction should have an upper limit of 4000 psi on the 7-day strength as measured by ASTM C 109. The preferred value is 4000 because this was Frohnsdorff’s original intention that was later changed to 4350. It was also the recommendation of the eleven-man commission chaired by Long and Benjamin in 1979.
Cement producers will welcome this mandate because it will free them from the game of leapfrogging to ever higher strengths and the coarser grinding will save energy and time and be very profitable for them. Because of the many unsuccessful attempts to get ASTM Committee C01 to address the problem, the federal government needs to be involved.
The U.S. should no longer tolerate portland cement that is grossly inferior in cracking resistance to the cements of almost all other countries.
The Consequences of Using Cements with 7-day Strengths over 4350 psi.
Figure 3 relates cement strengths to the cracking of concrete projects in Denver. It also depicts the change in Type II cement from 1955 to 1995. Here’s what happened to concrete made with various cements.
A. Used in the 165 bridges that were built by C-DOT in the 1950s. The bridges are still perfect despite being built with uncoated reinforcing steel and deiced. Sixteen bridges were torn out to make room for the T-REX bridges - which are now cracking.
B. Used in the 23rd St. Viaduct. It developed transverse thermal cracks at four-foot intervals before it was finished in 1995. The C3A plus C3S content was 72%, the highest of all the Type II cements in North America, according to Ron Gebhardt’s 1995 survey of 147 Type II cements.
C. Used in the first bridge of the T-REX project at Franklin St. The bridge developed 260 cracks within three months. Concrete was torn out and replaced. The new concrete had construction joints reduced from 9 feet to only 4 feet, but it still cracked, even though fly ash was used and the concrete was placed at night.
D. Used in the T-REX bridge at Washington St. The cracking of the sidewalks was as bad as the Franklin St. Bridge even though the number of construction joints was doubled. The deck, sidewalks, retaining walls and decorative obelisks cracked. The obelisks were painted grey to hide the cracking.
E. Used in the T-REX bridge at Colorado Blvd. The 7-day cube strength was a very high 5800 psi. Blending with fly ash and placing at night did not prevent 53 transverse cracks in the deck.
F. The range of 10 cements used at the Denver International Airport where there are 10,000 panels cracked from drying shrinkage. Twenty-two million dollars was spent just through 2008 replacing cracked panels. Blending with fly ash didn’t prevent the cracking. Some cements had 7-day strengths of 5600 psi. The new concrete is expected to crack because the local cement has become even more crack-prone. The cement alkalies were 0.8% NaEq and was blended with fly ash. A bad combination that caused the cracking of Highway 520 in Iowa.