Doç. Dr. / Assoc.Dr. Hasan YILDIRIM
İ.T.Ü. Faculty of Civil Engineering – Construction Materials Group
ABSTRACT
It is seen that plasticizer admixtures are widely used in Turkey. This study focuses on activity of normal, super and new generation (hyper) plasticizers due to extensity of fields of use. The study primarily explains usage purposes, types, effect mechanisms of plasticizers and researches compliance of such type of admixtures used in Turkey. Tests performed at İ.T.Ü. Faculty of Civil Engineering Construction Laboratory with the admixtures manufactured by various companies and obtained results were analyzed for this research. According to the carried out analyses, it was seen that most of the plasticizer admixtures used in Turkey achieves the desired qualities.
1. INTRODUCTION
Today, chemical admixtures which became one of the inevitable components of concrete due to improvements they offer for concrete properties, play an effective role in time limitations of concrete mixing and placement, adverse weather conditions, problems during compaction, pumpable, high strength and high durability concrete production [1,2].
Chemical admixtures interact with the cement and enter a physical, chemical or physico- chemical reaction and modify the properties of concrete.
In general terms, defined as substances that modify the properties of mortar and concrete in fresh or hardened phase [3], admixtures are widely employed in Turkey in the recent years.
This study provides information concerning plasticizers in the general section and then emphasizes, the qualities required from these admixtures, the status of such admixtures in Turkey and compliance with the required conditions. Thus, such admixtures produced by major companies in Turkey in the recent years were evaluated for compliance with the standards according to the results obtained from the experiments carried out at İ.T.Ü. Faculty of Civil Engineering Construction Material laboratory.
2. GENERAL INFORMATION
In this section, usage purposes, types, effect mechanism of plasticizer admixtures, plasticizer admixtures used in Turkey and conditions required for plasticizer admixtures were highlighted.
2.1. Usage Purposes of Normal and Super Plasticizers
Normal plasticizers are generally used for three purposes in the application [4,5]: 1.Providing higher strength by reducing water/ cement rate provided that it offers workability equal to concrete with no additive.
2. Providing same workability in the case that cement quantity is reduced to lower hydration temperature in mass concretes. Use of the admixture in this way also ensures a more economical concrete production.
3. Increasing workability to ensure easy compaction at hard to access locations.
Usage purposes specified above with regard to normal plasticizer also covers usage purposes of super plasticizers. However, super-plasticizers are mainly used for the purpose specified in article 3 among those, in other words, such admixtures are particularly made use of for “fluid concrete” production. A secondary usage area of super- plasticizers is high strength concrete production, it is possible to obtain normal workability at low water/ cement ratios thanks to these admixtures.
While super-plasticizers have favorable effects they also have unfavorable ones. High workability feature obtained by using super-plasticizer decreases with the slump loss within 30 minutes and continues over time [6]. Loss in this feature can be extended up to two hours with the addition of special admixture at very low percentages.
Plasticizers are used in ready-mix concrete mostly for the purpose specified in above articles 1. and 2. This case improves workability, facilitates mixing in mixer, reduces adhesion to mixer walls, ensures easier pumpability, prevents segregation of concrete and facilitates compaction. Other than those, in special purpose self compacting concrete applications, new generation high rate water reducer hyper plasticizers are used. Types of plasticizers are indicated in the following.
2.2. Types and Effect Mechanisms of Plasticizers
Admixtures are grouped according to their usage purposes in various studies [7,8,9,10]. ACI Committee classified concrete admixtures as air-entraining, setting accelerator, water reducer and setting time regulator, fluid concrete admixtures and other miscellaneous special purpose admixtures in its report numbered 212 [4]. Water reducer admixtures which are of a higher range compared to normal plasticizers or super-plasticizer admixtures are included in the group of water reducer admixtures and admixtures for fluent concrete.
Normal plasticizer admixtures (ASTM C 494 Type A, TS EN 934-2) [11,12] ensure use of less water for the same workability, therefore, obtaining higher strength [13,14]. Normal plasticizer admixtures are divided under four groups as lignosulfonic acids containing Ca, Na and NH4 salts, hydrocarboxylic acids comprising of Na, NH4 or triethanolamine salts, carbohydrates and other organic compounds according to their chemical properties [15]. Lignosulfonates can be considered as a polymer formed by the combination of hydroxyl (OH), methoxyl (OCH3), phenyl chain (C6H5) and sulfonic acid (SO3H) groups [15].
In addition to those components, another component is molasses which is not included in the literature yet extensively however, used as plasticizer admixture[16]. The dark brown syrup which contains nearly 50% sugar obtained upon miscellaneous cooking and cleaning processes (at the end of centrifuging the final sugar pulp) to produce crystal sugar in sugar production, is named as molasses in sugar technology. According to the operational calculations molasses at the rate of nearly 3.8%-4% of the beet quantity is produced. Some of the sugar remaining in the molasses is free and the rest is bound as compounds with water and organic potashes. Generally, 75%-86% of molasses is dry matter and 14%-75% is water [16].
Plasticizer substance settles on the grain surface through adsorption by cement grains. The film formed by these substances settling on the grain surface is charged with negative electricity. Thus, grains charged with negative electricity repulse each other and dispersive behavior of these substances emanates. Since these substances have a lubricating effect since they prevent coagulation and facilitate slipping of the grains over each other at the same time, reduces internal friction of the concrete and this causes improvement of workability capacity of the concrete.
Super Plasticizer admixtures reduce water/cement ratios of high performance concretes in order to provide very high compressive strength. On the other hand, its fluidities are high to ensure self compacting and perfect to prevent segregation. SAs can be divided into three main groups: modified lignosulfonates, sodium naphtalene (or melamine), sulfonate-formaldehide poly-condensers (NSF or MSF) and carboxylate (or) hydroxy carboxylate) salts (HP). This third group is generally a polymeric additive which contains polyether lateral bonds (like hair comb) that are grafted to polyacrylate main polymer chain. HPs commenced to be produced after the additives in the first and second group. These are named as hyper-plasticizers or new generation SAs in the market due to the superior properties they demonstrate (higher water reduction and better plasticizing with low quantities) [17].
The most important feature of Super Plasticizers (SA) is the adsorption of long macromolecules on cement grains and their dispersion of the grains, thus, facilitating a more extensive hydratation, also, facilitating compaction by reducing the viscosity in the fluid media and the shear threshold.
Since grain surfaces are charged with the same negative electrical load, cement in the first and second group SAs is obtained through electrical repulsion of the grains between each other. In HPs, steric hindrance formed due to size and stoichiometric structure of molecules is more effective in realization of dispersion. In general, HPs in the market are not solely carboxylates, they are also added with certain amounts of NSF or MSF [18].
Inspection of SAs’ competency only in the aspect of workability is indeed insufficient. Besides this, analyses on hydratation, hardening, strengthening processes are naturally essential. Yet, SAs impact composition of interstitial liquid composition, micro-structures and morphologies of hydratation products. Cement composition and alkali content plays an important role in such changes.
2.3. Normal, Super and Hyper Plasticizers Used in Turkey
Number of companies manufacturing such admixtures or importing them from abroad and launching them in the market are increasingly growing. Plasticizer admixtures launched to the market by them can be grouped under the following titles.
• Normal plasticizers (with setting retarder or accelerator),
• The ones which are named as mid-range plasticizers in application, • Super plasticizers (also with setting retarder),
• New generator super (hyper) plasticizers (losing workability early or preserving workability for extended period).
In the recent years, total number of companies that apply to İ.T.Ü. Faculty of Civil Engineering Construction Material laboratory for researching compliance of such admixtures to the concerning standard [11] and making application for the second and third time for checking the performance in composition changes of their admixtures. The number of admixtures of the companies requested for testing and then rechecked after the previous years is 17. Number of companies which had tests done pursuant to the conditions of EN 934-2 [12] standard upon 2002 was 6. Number of plasticizer admixtures which are proposed for testing is 36. Chemical bases of the tested admixtures are Lignosulfonate based for normal plasticizers, Naphtalene formaldehide sulfonate and Melamine formaldehide based for super plasticizers. Other than those, new generation admixture types were requested to be tested not for the standard conditions but to the purposes desired in the researches. Normal plasticizers were used and tested within the range of 0.2% to 0.5% of the cement weight, super plasticizers were used and tested within the range of 0.7% ~ 2.0% of the cement weight. Recently, plasticizer admixtures which are not mentioned in the standards, however, named as “mid- range plasticizer” in our Country became frequently used. Such admixtures are mainly used within the range of 0.5% ~ 0.8% of the cement weight and generally tested according to the conditions required for normal plasticizer admixtures.
2.4. Conditions Required for Plasticizer Admixtures
Conditions required for admixtures in uniform concrete consistency in the case that they are used in concrete are shown below in Table 1 [11,12]. Standard no’s that are modified with concern to such admixtures are provided in Table 2.
3. EXPERIMENTAL STUDIES
3.1. Concrete Compositions
In the experimental studies that were carried out, cement and aggregate quantities were taken as set forth by the respective standards for mixtures produced for control series and test series with admixture [11,12]. Water quantities in concretes with and without admixture were adjusted to provide the same consistency. Material quantities used in all of the mixtures and fresh concrete properties are provided in the following Table 3.
3.2. Experiment Results
3.2.1. Experiment Results For Water Reduction
In the study 8 normal plasticizers and 7 super-plasticizers of 8 companies were tested between pursuant to ASTM standards. In the result of the tests admixture of two companies were able to reduce water at the rates of 3% and 4% respectively when used at the rate of 0.2% but could not fulfill the requirement of the standard, other than these, normal plasticizers of 6 companies which were used at the range of of 0.2% to 0.5% reduced water at the range of 8% to 10% and were able to fulfill the requirement of the standard. Only one of the 7 super plasticizers which were tested was able to reduce water at the rate of 9% with a use rate of 1%, while, the others were able to ensure water reduction at the rate of 15% to 21% with 0.7% to 1.5% use and fulfilled the requirement of the standard.
18 mid-range plasticizers, 19 super plasticizers of 6 companies were tested pursuant to EN standard. All 18 mid- range plasticizers of 6 companies used at the rates of 0.5% to 0.8% were able to reduce water at the rate of 9% to 16% and satisfied the requirement of the standard. All 19 super plasticizers which were tested were able to reduce water at the rate of 12% to 23% with a use rate of 1.0% to 1.5% and fulfilled the requirement of the standard.
3.2.2. Experiment Results For Setting Time
Effects of normal and super plasticizers on setting times were only studied on cement pursuant to TS24 [20]. According to the carried out experiments, none of the tested admixtures provide an unfavorable result in terms of setting times.
3.2.3. Experiment Results For Air Quantity
In the study, 8 normal plasticizers and 7 super-plasticizers of 10 companies were used in concrete production. In the result of the tests, no normal or super plasticizer of any company was able to achieve the maximum air entrainment quantity of 3%. 18 mid-range plasticizers, 19 super plasticizers of 6 companies were used in concrete production pursuant to EN standard. In the result of the tests, no normal or super plasticizer of any company was able to perform air entrainment more than 2% of the air quantity of the control mixture.
3.2.4. Experiment Results For Compressive Strength
In the study 8 normal plasticizers of 6 companies and 9 super- plasticizers of 8 companies were tested between pursuant to ASTM standards. In the result of the test, normal plasticizer admixture of a company and super plasticizers of two other companies were not able to achieve the required conditions with concern to strength, 7 normal plasticizer admixtures and 7 super plasticizer admixtures other than these were able to achieve the required conditions.
In the study 18 intermediate plasticizers of 5 companies and 9 super-plasticizers of 8 companies were tested between pursuant to EN standards. In the result of the test, all admixtures with normal plasticizer and super plasticizer purposes were able to achieve the required conditions in terms of strength.
Compressive strength rate of concretes with admixture compared to the control mixture is demonstrated in Tables 4 and 5 in percentages.
4. CONCLUSIONS
In the light of the above analyses, main conclusions that can be reached with this study are listed in the following.
1. Increase in the number of companies which produce concrete admixtures is an indicator of increase in use of admixtures in our country. The most remarkable one is the increase in intermediate plasticizer admixture use which is not included in the standards.
2. 2ofthe8normalplasticizeradmixturesandoneof7super plasticizer admixtures tested pursuant to ASTM C494 were not able to achieve sufficient reduction in blending water. According to TS EN 934-2 all mid-range and super plasticizer admixtures were able to achive sufficient reduction in blending water.
3. Admixtures do not demonstrate any negativity in terms of setting time.
4. None of the admixtures tested on concretes manufactured according to ASTM standard were found with air quantity in excess of 3%. None of the admixtures tested pursuant to EN standard did not exceed the admissible maximum 2% air quantity difference compared to the control mixture.
5. 8 regular plasticizers of 6 companies and 9 super- plasticizers of 8 companies were tested between pursuant to ASTM standards and one normal and two super plasticizers among them were not able to achieve the required condition in terms of strength. No negativities in terms of strength were found according to TS EN.
6. In our study; it was revealed that most of the admixtures either manufactured in Turkey or imported were able to provide the qualities required from such admixtures.
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