By Wendell Dubberke
A Google search, using superplasticizer+concrete+soluble+alkali, got many hits. Apparently some cements are incompatible with certain sulfated superplasticizers. The problem seems to be that the sulfate from the superplasticizer goes into solution quicker than the sulfate from calcium sulfate (gypsum, etc)and is then preferentially used to make ettringite which coats the C3A particles. Consequently, the superplasticizor can not work properly if the sulfur fraction is missing. Superplasticizers seem to work well with concrete mixes made with cements containing over 0.5% soluble alkalis (potassium and/or sodium sulfate). According to one article by ASTM, the solution is to add 0.5% sodium sulfate to the concrete mix to be sure the superplasticizer works properly. Is it any wonder that the delayed ettringite (DEF) problems seem to be occurring more often recently. With the use of lignosulfonate water-reducers, sulfate based air entrainments, sulfonated superplasticizers, just how much sulfur ends up in the final concrete mix?
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The following additions made on 04/10/2008
Initially I saw no relationship between the early pcc deterioration observed in slip-formed bridge rails and in slip-formed paving, constructed in the 1980s and 1990s, but after further consideration, it would be possible to make a connection.
The early pccp deterioration observed in new highway 20 in Webster county Iowa, occurred in 4 years. A variety of researchers blamed DEF, ASR or F&T for the early deterioration. However, both the fine and coarse aggregates, used in the mix, passed the most severe testing methods and had excellent prior service records. The air entrainment was below standards, but similar to many other pcc pavements that are considered freeze/thaw durable. Before 1950, air entrainment was not used in pccp and many of those pcc pavements are still in service today.
In many of the slip-formed pcc pavements, suffering from early deterioration, longitudinal cracks appeared on the surface where vibrators were located when the pavement was placed. Improper vibrator operation during slip-form paving was investigated by Iowa DOT engineers. Upper and lower vibration limits were established and closer inspections were initiated.
Localized, severe early deterioration in the slip-formed pcc bridge rails appeared in only 2 years. Some pcc investigators blamed the coarse aggregate, used in the mix, for the problem. The coarse aggregate was of mediocre quality. However, observable, aggregate related pcc problems (D-cracking) usually can not be seen before 8 or 9 years. Also, the same aggregate was used in the bridge floor, where deicing salts would be expected to compound the problem, but early deterioration did not occur there. In addition, most of the deterioration initially occurred in the top 1/3 of the slip-formed pcc rails and remains that way to this day. The deterioration did not continue to expand in area as would be expected if this were a problem related to aggregate quality.
In both cases,(faulty pcc in slip-formed rails and pavement) pcc investigators commented on the poor consolidation of the pcc. Large vugs and voids were seen in pcc removed from deteriorated areas. Why would this occur, especially in Webster county, where investigators determined that some of the vibrators were running too fast during pccp placement.
Potassium carbonate testing results, on hardened pccp from highway 20 in Webster county, indicated lignosulphonate levels at 10 times the normal amount.
An Iowa DOT engineer said the levels of air entrainment incorporated into the pcc rail mix was very high. The mix was a low-water mix and they had a very difficult time trying to get 6% actual air in the final product.
If contractors are having difficult times getting harsh mixes through slip-form pavers, they most likely would try some changes in the field. Spraying on-board water onto the chutes of slip-form rail machines is one technique frequently used. If this technique is used in conjunction with a mix containing too much air entrainment along with aggressive vibration, couldn't foam pockets form that would later be diagnosed as empty voids or vugs? If this happens, what kind of aggregate/paste bonding could be expected? Would these areas be susceptible to freeze/thaw damage? If the aggregate/paste bond is suspect in these areas, excessive slumping would occur and that is exactly what was seen during placement.
An argument could be made that a pocket of foam, like a pocket of entrapped air, should work its way to the top and disappear during vibration. The difference is the foam is continually forming whereas the entrapped air pocket is a one-time event. It might be possible to remove all of the foam pockets with long-term vibration.
Is it possible that the foam-pocket scenario could be used to explain the early pccp deterioration on the slip-formed highway in Webster county Iowa? The mix contained too much lignosulfonate but water was not added at the paver. Air bubble volume and spacing was analyzed and found to be below target values. It does not seem logical that foam related vugs could occur and yet have bubble volumes below target values. Investigation of the hardened pccp showed air voids containing a considerable amount of ettringite. The excessive lignosulfonate in the mix probably contributed to excessive sulfur in the mix.
What happens when a mix containing too much lignosulfonate is aggressively vibrated? Did aggressive vibration knock out some of a fixed amount of air bubbles while products (foam pockets?)related to excessive amounts of lignosulfonate were still being generated? Is there a correlation between foam pockets and aggregate/paste bond? If the aggregate/paste bond is faulty, and if water gets to this gap, then a situation for enhanced activity due to DEF, ASR and F&T could occur.
Specifications should include upper limits for liquid additives (including air entrainment) added to concrete mixes. This was discussed at the Iowa DOT in the 1990s, but a senior pcc engineer said it was not needed because economic pressure would force pccp contractors to use the minimum amount of liquid additives.
Currently, the Iowa DOT specifies improved gradation limits for concrete mixes. The pcc mixes are less harsh and move through slip-form equipment more easily. Even though improved gradations may solve the problem of harsh pcc mixes, the initial cause of past pcc failures still needs to be understood.
