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Effects of the second generation of superplasticizers on concrete properties
CBI - Cement- och betonginstitutet.
1995 (English)Report (Refereed)
Abstract [en]

Since the first superplasticizer was used for the first time work has been done to obtain more efficient products. The superplasticizer used today may be said to belong to the second generation of products. The concrete is said to have smaller slump losses and a higher short-term and final strength using the new generation of superplasticizer. An other aim of the development was to make the superplasticizers innocuous from a health point of view by minimizing or completely removing the formaldehyde content. _x000D_ _x000D_ The aim of the research project was both to test some of the second generation of superplasticizers and to try to express the concrete's rheology in fundamental parameters, such as yield stress and plastic viscosity. These parameters were measured using various viscometers. For concrete, the CBI's BML viscometer was used and for mortar (Dmax = 0.25 mm), the Institute for Surface Chemistry (YKI) tested with their paste viscometer, the Bohlin Controlled Stress Rheometer. The results were compared with those obtained using more traditional methods, such as slump, remoulding value and spread. Evaluation of slump losses was done using interpolation between slump test values at different times after mixing. Testing was also done of the air content, compressive and splitting strength, shrinkage and, in the case of concrete with a water/cement ratio of 0.60, also the temperature development and the time necessary before power floating/trowelling. _x000D_ _x000D_ Five different superplasticizers and a water-reducing product were tested and compared. The superplasticizers are based on different active substances, such as melamine, naphthalene, polymers and one with a mixture of copolymer and melamine. The water-reducing agent is based on lignosulphonate. _x000D_ _x000D_ Concretes with three different water/cement ratios, 0.45, 0.55 and 0.60, were tested. The concrete material was proportioned so that the initial slumps for the three were about 70 mm. Two different mixing orders of components and admixtures were tested. Mixing alternative A involved coarse aggregate and cement being dry mixed, after which water and half the amount of admixture were added and then the gravel. This was then mixed for two minutes and allowed to rest for two minutes, after which the second half of the dose of superplasticizer was mixed in. The concrete was then mixed for a further two minutes before testing was started. Mixing alternative B involved all components and superplasticizer being poured into the mixer at the same time and mixed for three minutes. _x000D_ _x000D_ The various consistency tests and air content tests were performed five minutes after mixing. At this time, samples were also taken for strength and shrinkage tests. The concrete that was tested was poured back into the mixer, which was run some revolutions. The concrete was covered with plastic and permitted to rest until the time for the next test. _x000D_ _x000D_ The test batches were made using three different cements. Most were made using cement types called S and D. The different cements vary as regards chemical composition and particle size distribution. Cement S is the most finely ground, with a Blaine fineness of 460 m2/kg, a C3A content of 8% and alkalinity expressed in equivalent Na2O of 1.10%. Cement D is a coarse, low- alkali cement with a Blaine fineness of 300 m2/kg, C3A content of 1.6% and equivalent Na2O of 0.47%. Cement FD is a more finely ground variant of cement D, with a Blaine fineness of 400 m2/kg, C3A content of 1.0% and equivalent Na2O of 0.47%. _x000D_ _x000D_ The results for the traditional consistency testing of concrete with a low water/cement ratio shows that the superplasticizers are generally 20 to 35% more effective with cement D than with cement S. The necessary doses for a certain consistency are lower when cement D is used. That is because cement C has a considerably higher C3A content which adsorbes the superplasticizer molecules more rapidly. Cement D is also more coarsely ground which gives a higher molecule concentration on the surface of the cement grains. _x000D_ _x000D_ Different superplasticizers exhibit different effects depending on the dose and cement sort. The completely or partly polymer-based superplasticizer has lower slump losses at normal dose than the others. If the slump loss is counted as the time for reduction of the initial slump by 30 mm, these products in combination with cement S have about 15 to 20 minutes longer time than the other. This implies almost a doubling of the time. In combination with cement D, the corresponding time is 8 to 17 minutes, or 44 to 130% longer. The differences are not as great at half the normal dose. The slumps five minutes after mixing with cement S varied widely. _x000D_ _x000D_ In testing the "building concrete" with cement S, at a water/cement ratio of 0.60, the slumps were controlled to 180 and 230 mm respectively. This enabled comparison of the superplasticizers on the basis of the same initial slump. These tests exhibited even more clearly that the wholly or partly polymer-based superplasticizers provide the best effect on both consistency and slump losses. _x000D_ _x000D_ The complementary tests on retardation also showed that these polymer-based products retard the setting of the concrete to a certain extent. For example, the time before power trowelling increased by about 2-2.5 hours for these products, for the dose to achieve full flow. But it must be said that the other products also provided a longer hardening time, varying between 1 and 2 hours with doses for full flow. _x000D_ _x000D_ The short-term strength was measured after 24 hours. No greater strength was generally noted in concretes with superplasticizers. _x000D_ _x000D_ The strength after 28 days varies basically with the measured air content in such a way that the cases in which the air content was lower than for the reference the strength was also higher. In the case of cement D and a normal dose of superplasticizer, all the measured strengths were higher than the reference. In corresponding case with cement S, the results were not the same. With cement S, superplasticizers appear to increase the air content slightly at normal dosing. _x000D_ _x000D_ Measurement of shrinkage shows that concretes containing superplasticizer shrink less than the corresponding reference concrete. The reason may be that the superplasticizer's liquid phase is included in the water/cement ratio as contributing to the cement's hydration. If the liquid phase does not contribute to the hydration, the calculated water/cement ratio is overestimated. The concrete's shrinkages also decreases with a smaller water content. _x000D_ _x000D_ In all cases in which mixing alternative A were compared with mixing alternative B, alternative A exhibited the better effect, both as regards the initial slump and the change in slump. _x000D_ _x000D_ This investigation of superplasticizers shows that a major change of positive effects on concrete properties as to be called second generation of superplasticizers can only be those containing polymers partly or completely. _x000D_

Place, publisher, year, edition, pages
Cement och Betong Institutet , 1995. , p. 80
Series
CBI rapport, ISSN 0346-8240 ; 1995:2
Keywords [en]
Bingham model, Concrete, Rheology, Shrinkage, Slump loss, Superplasticizer, Trowelling hardness, Viscosity, Workability
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:ri:diva-3001OAI: oai:DiVA.org:ri-3001DiVA, id: diva2:960606
Note
284 Går att beställa: kontakta eva.lundgren@cbi.seAvailable from: 2016-09-07 Created: 2016-09-07Bibliographically approved

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