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6
Section 7. Materials Science
Narov Rustam Alixanovich, candidate of technical sciences, Tashkent Institute of Architecture and Construction, Tashkent, Republic of Uzbekistan E-mail: [email protected],
THE EFFECT OF AFR-3M AND MARA ADDITIVES ON TECHNOLOGICAL AND TECHNICAL PROPERTIES OF CONCRETE
Abstract: The article presents the results of a study of the effect of acetone-formaldehyde resin (AFR)-3M and methyl-acetone resin with ammonia (MARA.) additives on technological and technical properties of concrete. It is stated that at expedient ratio of sand and crushed stone, the flowability factor of the standard slump increases by 2-2.5 times with the introduction of 0.15% of the binder weight of AFR -3M additive and by 2-4 times with the introduction of 0.10% of MARA additive. Concrete shrinkage without the additives develops faster by 32% and 38%, respectively. The introduced additives contribute to the change in the nature of porosity: the total volume of open pores is reduced by about 1.5 times and the average pore size by 5-6 times.
Keywords: workability, flowability, vibro-molding, impermeability to water, frost resistance, shrinkage, durability, plasticizing additives.
Introduction
As is well known, technological and technical properties of concrete mix, such as the workability, unmixing and water separation are determined by the amount ofbinder, water content, characteristics of aggregates and their ratio, the amount of plasticizing agent, the temperature and humidity of the environment, the method of transportation and the duration of holding the mix before laying, the nature and the mechanism of the plasticizing additive effect, the structure of the adsorption layer, the dispersion of the fillers, the concentration and adsorption activity of the aggregate.
One of the most important properties of concrete mix is the workability, characterized (in accordance with current regulatory requirements) by the flowability factor of the standard slump and rigidity. It should be noted that for vibro-compacted mixes with plasticizing additives, the assessment ofworkability by flowability factor does not correspond to the actual behavior of concrete mix under dynamic effect. Therefore, the workability of concrete mix with a plasticizing additive is more appropriate to be characterized by vibro-molding, which in practice can have a significant technical and economic effect.
Materials and research methods
At the first stage of the study, the issue of the effect of AFR-3M and MARA additives on the workability of concrete
mix is considered. As is well known, the additives demonstrate the most effective plasticizing effect at a certain sand-crushed stone ratio. Therefore, for a start, the design of concrete mix with rational sand-crushed stone ratio is determined [1; 2].
For example, for concrete mixes with flowability factor of 4-6 cm, consumption of cement 300-450 kg/m3, crushed stone of 10-20 mm fraction and sand of normal size with expedient ratio 0.33 of fine and coarse aggregate, the flowability factor of concrete mix increases by 2-2.5 times in proportion to the content of AFR-3M additive and by 2-4 times in proportion to MARA additive.
Studies have shown that AFR-3M and MARA additives have a positive effect on strain properties of concrete due to the self-stressing effect of the cement stone. Experiments on the effect of additives on long-term shrinkage deformations of concrete with cement consumption of350 kg/m3 and water/ cement ratio = 0.47 have been conducted at a temperature 20 + 5 °C and relative humidity 55-60% [3; 5].
It was stated that the most intense increase in shrinkage is observed for the first 40-50 days. The shrinkage of concrete without the additives develops faster than with the introduction of AFR-3M and MARA additives: in absolute values of concrete shrinkage strain by 32% less with the introduction of AFR-3M additive and by 28% less with MARA additive.
THE EFFECT OF AFR-3M AND MARA ADDITIVES ON TECHNOLOGICAL AND TECHNICAL PROPERTIES OF CONCRETE
This is obviously due to the fact that the additives can reduce the amount of water by 15-18% and cement by 18-20%. As a result, reducing the amount of cement paste provides a significant reduction of the contraction phenomena.
Numerous laboratory studies have shown that due to the plasticizing and air-entraining effect, the water demand of concrete mix is considerably reduced and the porosity of micro- and macrostructure of concrete is improved.
The true porosity of concrete with addition ofAFR-3M is the same, while with MARA, additive it is slightly higher than the reference composition due to the air-entraining effect of the surfactant. The character of porosity with the introduction of additives varies considerably. Thus, the total volume of open pores is reduced by 1.5 times, and the average pore size by 5-6 times under more uniform distribution of pores in size [4] with the formation in concrete of conditionally closed pores of much smaller sizes, which contributes to improving the impermeability to water and frost resistance of concrete.
The indicators of impermeability to water in concrete with additives, depending on the type of surfactant and curing conditions, increase by 2-2.5 times. The main factor of curing and hardening of cement stone is the reduction of water content [6] in the binder due to the plasticizing effect ofAFR-3M additives. An additional strengthening effect associated with specific features of the additive effect on the process of hydration and hardening during the early structure-formation of cement stone should be noted as well. A determinant factor in structure formation in the early period of hydration, and in density and strength of the cement stone in the process of long-term hardening is the velocity and content of forming calcium hydrosulfoaluminate.
Usually the re-crystallization process with a change in phase structure, the morphology of hydrated new formations causes destructive phenomena in the cement stone, which can lead to a decrease in strength and durability of concrete. In this regard, it can be assumed that the occurrence of this process at an early stage, when the cement stone is characterized by high plastic strains, has advantages over the process occurring in the subsequent periods in a more brittle cement stone.
The original feature of the effect of AFR-3M is to simultaneously increase the flowability of concrete with 0-6 cm slump and the strength of concrete in rational batch limits (0.1-0.15). The extreme nature of the change in strength properties is observed; the increase in strength at 0.15% will reach 20-25% of surfactant on the binder weight.
Introduction into concrete structure 0.15% of AFR-3M additive allows one to get uniform plastic concrete mix with 4-6 cm of initial slump at15-17% reduce in water content. The overall increase in concrete strength with cement consumption of 270-430 kg/m3 is 35-38%.
The hardening effect of small dosages ofAFR-3M additive is associated with the features of the formation of concrete microstructure, with decrease in contact porosity of the macrostructure, which indicates an increase in adhesion strength of the cement stone with the aggregate and is confirmed by 28-35% increase in tensile strength of the samples under bending.
At rational dosage of AFR-3M additive, the resulting increase in strength can save up to 17-20% of cement. Using the criterion of workability on standard slump for concrete mix with flowability of 2-3 cm, depending on the initial consumption of the binder, one can save 50-75 kg/m3 of cement at the introduction of MARA additive. Saving the binder for the same mixes in the evaluation of workability, equal to 10-15 seconds, can reach 50-100 kg/m3, or 18-22%.
Plasticizing air-entraining additives reduce the rate of increase in strength of concrete, which adversely affects the duration of monolithic concrete curing till the removal of form and the loading time of structures under maturing in normal conditions. The AFR-3M and MARA additives are free from these disadvantages. The most intensive increase in strength of concrete with additives is observed at the initial period of hardening up to 7 days. So, if the increase in strength of the reference concrete after 3 days of maturing in air and humid conditions at a temperature of 25-30 °C is 30%, the strength of concrete with MARA and AFR-3M additives reaches 70% [7]. Later on, the increase in concrete strength is stabilized and after 90 days of normal maturing it is 15-18% higher than that of the reference concrete. AFR-3M additives should be used to intensify the maturing of concrete and to reduce the modes of heat and moisture treatment of products. Using the AFR-3M additive as an example, the possibility of significant reduction of the duration of concrete heat treatment is shown on condition of optimizing concrete mixes with an additive based on vibro-molding, which corresponds to the workability of a conventional concrete mix based on characteristics of a standard slump.
Introduction of 0.15% of AFR-3M and 0.1% of MARA additives into concrete mix allows one to reduce water demand by 14-18% and 18-22%, respectively and to obtain a full-strength mix based on vibro-molding. This reduction in water demand provides an increase in strength of steamed concrete with AFR-3M and MARA additives from 30-35% to 38-40%, respectively. To obtain the handling strength of concrete with AFR-3M and MARA additives it is possible to reduce by 2-3 hours the duration of heat treatment at standard temperature or to reduce to 60-65 °C the temperature of isothermal heating at constant duration of steaming.
The use of solar energy in production of reinforced concrete is very effective in conditions of a dry hot climate in
summer period. An effective way to accelerate the maturing of concrete is proposed; it is simple and does not require capital investment on the installation of special helio-poly-gons. It can be used in ordinary polygons. The essence of this method is to use the heat accumulated by individual components of concrete mix and the heat taken by concrete at the hardening process in the open area, combined with plastici-zation of concrete mixes by AFR-3M and MARA additives and subsequent curing of the products under waterproof film. Freshly molded products are covered by a polymer film and kept in open area for 1 day. The proposed method for the intensification of concrete hardening involves the selection of concrete mixes with AFR-3M and MARA, additives according to the criterion of vibro-molding, taking into account the temperature of the initial components of the mix. We should proceed from the fact that the required strength of concrete in the product after 1 day of curing in natural conditions of the hot period of year at a temperature of concrete mix 35~45 °C will be provided at the time of its laying into molds.
Frost resistance of concrete with the introduction of AFR-3M, MARA additives increases by 1.5-2 times. An increase in frost resistance of concrete with the additives is explained by the formation of closed pores, which play the role of reserve volumes, contributing to a further decrease in the crystallization pressure [6; 8] On the other hand, the air involved in concrete mix partially blocks the capillaries and,
by reducing the water content in general, the number of capillary pores decreases.
It should also be noted that to increase the frost resistance of concrete, it is very important to accelerate the binding of Ca(OH) with gypsum to hydrosulfoaluminate, and the duration of free lime transition to calcium silicate. As noted above, the AFR-3M and MARA additives accelerate the hydration, increase the period of calcium silicate formation and thereby contribute to the formation of strain structure of the cement stone.
The AFR-3M and MARA additives contribute to the implementation of a mechanism of influence on the macrostructure, which leads to a change in the kinetics of water saturation of concrete. Indeed, when obtaining concrete mix of the same flowability with 0.15% of additives and without the additive, an increase in water saturation of concrete samples is observed. Obviously, these features of the structuring of cement stone contribute to a significant increase in frost resistance of concrete with AFR-3M and MARA, additives.
Conclusions
The introduction of AFR-3M and MARA additives into concrete mix has a positive effect on workability and strain properties of concrete. The proposed additives introduced into concrete mix increase the impermeability to water by 2-2.5 times and the frost resistance of concrete by 1.5-2 times. The introduction of 0.10-0.15% (ofbinder weight) ofAFR-3M and MARA additives into concrete mix reduces the water demand by 15-18% and the consumption of cement by 18-20%.
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