A special rack fitted to a digital micrometer was used at the Iowa DOT. Between measurements, the 5 cyclinders from each concrete sample are stored in their own container filled with distilled water. At the conclusion of the measurements (20 days), the high and low measurements are discarded and the middle three are averaged to obtain the final test result expressed in percent growth.
Throwing out the high and low readings was a change from what Duggan recommended. If the polished end on a concrete cylinder contains a void or has a reactive particle (chert or shale) at the point of measurement, it can have a disproportionate effect on test results. The samples were read in a horizontal position.
The overall duration of the test is thirty days. Evaluation of the test at the DOT indicated that the test could be completed in twenty days (10 day read instead of 20) and still maintain adequate accuracy.
Duggan suggested that any expansion over .05% should be cause for concern. The North American railway system uses a .075% limit. For concrete that is used for structures that will always be high-and-dry, a higher limit could be used. For concrete that will come into contact with water, a Duggan test result over .075% should initiate a change in mix design or a change in concrete constituents. In some cases, an increase of water, in the mix design, could solve the problem.
A few years after the Duggan test was proposed, it was evaluated by concrete researchers from Canada. They found that the primary cause of concrete expansion, using this test method, was related to the generation of excessive ettringite in the concrete matrix. About this same time, some concrete investigators at the Iowa DOT were becoming concerned about the significant amount of fiberous, amorphous, hydrous, calcium sulfoaluminate (called ettringite gel for lack of a better name) filling the air voids in the early deteriorating Portland cement concrete pavements (PCCP) built during the last 10 years.
An evaluation of the Duggan test method was initiated at the Iowa DOT in the early 1990s.
The evaluation of the Duggan test method proved successful (using concrete fabricated in the laboratory), and it was used to evaluate cements and water reducers in laboratory mixed concrete.
Other Iowa PCCPs, built during the last 20 years, and still performing adequately, also contained this ettringite gel in the air voids, but to a lesser degree. Older, durable Iowa PCCP contains only an extremely small amount of ettringite that is difficult to locate. Quantifying and qualifying ettringite gel is difficult because x-ray diffraction (XRD) will not work.
The scanning electron microscope can show the elements in in ettringite gel as well as silica gel and can give indirect information about mineralogy. Concrete investigators at the Minnesota DOT have developed other techniques for measuring ettringite in concrete.
Is it OK to have air voids partially full of ettringite gel if the pavement is able to perform adequately? What is an adequate service life for PCCP? In Iowa, the best concrete aggregates are expected to perform for 30-40 years, even when deicing salts are used on the PCCP. Should air voids also function as reservoirs for excessive ettringite gel? Do you go after the source of the problem or do you use remedial techniques?
At DOT meetings (relating to early PCCP deterioration) some cement manufacturing representatives indicated that thier type I cement formulations were now semi shrinkage compensating (called expansive in the old days). Type K (and Type S, now obsolete) is a shrinkage compensating cement. The generation of substantial amounts of ettringite (gel?), at the proper time during concrete hydration, is the mechanism that makes it work. If all of the ettringite is formed while the concrete is still plastic, then cracking of the matrix will not occur because the expansive ettringite, even if it is excessive, will make room for itself before the concrete becomes brittle.
A low water/cement (W/C) ratio can hinder the total development of ettringite gel while the concrete is still plastic. If water becomes available at a later time and if it can get to the ettringite gel building components, ettringite gel will form. If the void system in the concrete matrix is insufficient for the volume of ettringite gel, cracking can occur.
During the 1960s, a Portland Cement Association (PCA) concrete researcher fabricated laboratory concrete that was expansive (due to delayed ettringite) by using a very low W/C ratio in the mix.
It is possible that the Duggan test is identifying those cements that have a high potential for developing large amounts of ettringite due to- (1) excessive use of grinding aids and/or (2) high amounts of ettringite building components (sulfur and aluminum) in their cement formulation. To be durable (long-term), maybe these cements should not be used with a low W/C ratio PCCP mix.
Back in the 1930s, W/C ratios of 0.6 or more were commonly used and even recommended. Recent trends are towards low W/C ratios that relate to higher early strengths. There seems to an assumption ("everybody knows") that high early strength equals long-term durability in concrete. This assumption may be true for structures that are high-and-dry (maybe).
Many concrete investigators say heat, applied during the hydration of concrete, is a requirement for the development of excessive ettringite that can crack the matrix. They dismiss the Duggan test method because it uses heat during the pretreatment phase of the test. Therefore, they say, expansions should be expected and consequently mean nothing.
They are unconcerned about other concrete tests that use heat in their operation. In our meetings, cement company representatives also indicated a concern about the difficulty of formulating a cement that would consistently pass the test. Iowa DOT Duggan test results showed that some cement sources consistently failed the test and other cement sources consistently passed the test. One of the sources that consistently failed was used in failing Iowa PCCP. The failing PCCP also contained a lignosulfonate water reducer in the mix.
If the application of heat, during cement hydration, is required for the generation of excessive, expansive ettringite, how then can type K and type S expansive cement work without the application of heat?
There are many stories floating around the concrete construction industry concerning the use of type S cement in construction work. Type S cement was formulated with extra sulfur and aluminum. The generation of substantial amounts of ettingite was intentional.
The use of type S cement was abandoned because of poor performance. Additional heat was not applied to hydrating type S cement concrete, and yet it failed due to an ettringite related problem. The Portland Cement Association (PCA) initially promoted type S cement during the 1960s. Locating projects where type S cement was used along with its related service life is difficult to obtain.
Many concrete researchers also say that heat destroys ettringite. In a technical sense, it may be true, but realistically, if all that is happening is dehydration of ettringite, is this really destruction? Most likely, if water returns, ettringite will form again.
Work at the Iowa DOT and Iowa State University (ISU), using the scanning electron microscpe (SEM), showed that heat (both dry and steam) did not alter the physical appearance of the ettringite gel in concrete air voids.
It might be thought that dehydration would at least cause the ettringite to shrink, but it did not. Using QXRD to quantify crystalline ettringite before and after heating should show a difference. QXRD can not quantify the amorphous ettringite found in PCCP air voids.
In Brazil, there is no known preventive guide or standards of procedures of execution of structures in concrete geared explicitly to the DEF.
Only the NBR 11709:2010 - Dormentes de Concreto - Projeto, materiais e componentes specifies various characteristics for the concrete used in the manufacture of sleepers, for example the maximum temperature reached by the concrete together with the maximum content of SO3, however, not specifies be the DEF the main cause for this specification (ABNT [21]).
Having seen the lack of normative standards for prevention of DEF in concrete structures, the LCPC recently established a technical guide for prevention of the DEF. The finality of this guide is providing recommendations of preventive measures for confection of concrete elements in order to mitigate the risks associated with DEF, over the service life of the structure.
The guide is based on the realization of cross-references between the category that describes the structure (or part thereof), as well as the level of acceptable risk and environmental actions that affect the structure (or part thereof) over its service life.
This cross-reference step is to establish the necessary level of prevention, which determines the set of preventive measures to be implemented.
Such measures depend sorely of the maximum temperature limit reached in the core of the structural elements, during the hardening of concrete and of choice of a better conception of concrete that is satisfactory.
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