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6Technobius, 2023, 3(2), 0038, DOI: https://doi.Org/10.54355/tbus/3.2.2023.0038
Technobius
https://technobius.kz/
Article
Fine-grained concrete for repair and restoration based on complex modifiers
Kazakhstan Institute for Reconstruction and Development, Karaganda, Kazakhstan 2Department of Technology of Industrial and Civil Engineering, L.N. Gumilyov Eurasian National University, Astana,
Kazakhstan Correspondence: [email protected]
Abstract. Nowadays ensuring normal operation of buildings made of monolithic reinforced concrete is of great relevance. Bearing building structures in the process of operation or erection can have defects and damages of various origins. There is a need to develop repair compositions to ensure the durability and reliability of monolithic reinforced concrete buildings. In this paper we have conducted research and developed a repair composition of fine-grained concrete (FGC) on the basis of modification and improvement of its structure. In the composition of complex modifier on the basis of analysis were chosen: microsilica MCU-95 as a highly active pozzolanic mineral admixture; superplasticizer C-3, hydrophobizing component - soapstock. Cements of Novo-Karaganda, Ust-Kamenogorsk, Shymkent cement plants M400 were chosen as binders. In order to study the influence of modifiers on the setting time of cement dough and concrete structure, several concrete compositions were developed. The performance of the developed FGC was improved in terms of basic physical and mechanical parameters. Cement consumption was reduced by 15% (from 450 kg to 382.5 kg). The influence of the main factors and dependence of performance indicators on physical and mechanical properties of cement stone and the studied FGC has been established. Production waste and local construction materials were used to save binder.
Keywords: microsilica, concrete, repair, repair, composition, defects, damage.
Currently, construction in Kazakhstan is developing rapidly. There is an increase in the construction of low-rise and high-rise residential buildings, various commercial and cultural centers. Monolithic construction of higher storey with load-bearing stone structures or reinforced concrete space frames has been widely used. In the monolithic buildings constructed in recent years, there is a significant number of defects of load-bearing structures [1-3] (Figure 1). The nature of defects and damages in buildings made of monolithic reinforced concrete depends on many factors and reasons: the quality of engineering surveys and design documentation, the quality of materials and products, compliance with the quality control system during construction, as well as violation of the technology of construction works, including the use of poor-quality formwork [4]. Physical deterioration of reinforced concrete structures under the factor of time: creep and shrinkage of concrete, exposure to aggressive external environments, mechanical loads lead to cracking of concrete, corrosion of steel, peeling of concrete coating, and, as a consequence, loss of serviceability and safety of the structure [5]. This has caused a huge need to improve the performance of existing reinforced concrete structures by repairing and reinforcing them in order to extend their service life. According to the results of the authors' research [6] - the main cause of defects in buildings and structures is the inability to ensure the necessary characteristics of concrete, both at the stage of concrete mixture preparation and at the stage of erection of the finished monolithic structure.
Sarsenbek Kozhasov1, Aigul Kozhas2*
1. Introduction
Ensuring reliability, durability and increasing the service life of buildings made of monolithic and prefabricated reinforced concrete structures is an urgent issue in the construction industry not only in Kazakhstan, but also in the whole world. The construction industry of all industrially developed countries spends about 42 % of financial injections for repair and operating costs on objects built of reinforced concrete. In [7], a repair cost analysis using PSLF (probabilistic service life function) is proposed, where changes in initial and extended service life due to repair are considered.
In view of the relevance of the use of concrete mortars for the repair of concrete and reinforced concrete structures, the works and research work of many researchers have been studied. Since concrete on coarse aggregate is not quite acceptable for its application in thin layers, it was decided to use crushed stone of 5...10 mm size as an aggregate. The concrete obtained on such aggregate is classified as fine-grained concrete.
In the present work, the research and development of repair composition of fine-grained concrete (FGC) based on modification and improvement of its structure, which consists in improving its physical-technical and hydraulic properties. To obtain FGC with high repair properties, the existing materials were reviewed and compared.
It is well known that the repair mortars used are made from ordinary Portland cement (OPC), which were developed to repair old concrete to achieve better compatibility with concrete bases [5]. To obtain concrete with improved technical and performance characteristics, one of the most promising directions in construction is to modify concrete with a complex of modifying additives and production wastes, which is economically feasible and develops domestic house building. The paper [8] presents the results of research work on the development of fine-grained polymer concrete modified with microsilica. The authors of [9] used 5%, 7.5% and 10% (by weight) of microsilica to improve the mechanical properties of mixtures as a partial replacement of cement, and the results show that the mixture with 25% recycled aggregates and 5% microsilica showed characteristics comparable to conventional concrete, while the use of a higher content of microsilica negatively affected the rheological properties.
In order to obtain concrete with enhanced performance characteristics under the condition of minimizing raw material, energy and labor costs in the present work, studies on the development of the optimal composition of FGC were carried out. The following materials were used: microsilica MCU-95, superplasticizer - C-3, hydrophobizer - Soapstock and curing gas pedal - sodium sulfate.
The purpose of the work is to study and develop an effective composition of concrete FGC for repair and restoration of reinforced concrete structures in the construction and operation of residential buildings.
The purpose of the work should be achieved by solving the following tasks:
- introduction of microsilica, hydrophobizer and curing gas pedal in addition to superplasticizer into the concrete composition to obtain an effective FGC composition;
- to find out the optimal ratio of components to obtain FGC with high performance characteristics, effectively used in the repair of residential buildings made of concrete and reinforced concrete;
- to establish the influence of the main factors and dependence of performance indicators on physical and mechanical properties of cement stone and the studied FGC.
2. Methods
In the course of research and practical work the following materials were used.
Cements of Novo-Karaganda, Ust-Kamenogorsk, Shymkent cement plants M400 were chosen as a binding material. Characteristics on mineralogical and chemical composition of cements are given in Tables 1 and 2.
Table 1 - Mineralogical composition of cements
Name of manufacturing plant Mineral content, %
C3S C2S C3A C4AF
Karaganda 58.16 18.92 8.61 13.52
Ust-Kamenogorsk 54.88 22.96 9.70 12.31
Shymkent 51.92 15.06 8.18 13.96
Table 2 - Chemical composition of cements
Name of manufacturing SiO2 M2O3 Fe2O3 CaO MgO SO3 CaOcB
plant
Karaganda 19.87 6.09 4.48 61.53 1.50 1.26 0.58
Ust-Kamenogorsk 21.68 6.29 4.02 67.60 0.80 0.44 0.67
Shymkent 19.42 6.05 4.83 61.67 1.63 1.74 0.71
The cements used in the work were tested in accordance with [10] and [11]. The results are attached in Table 3.
Table 3 - Cement test results
Name of plant, Bulk Specific Grinding Normal Ultimate strength at the
type and grade of density, surface fineness by density of age of 28 days, MPa
cement kg/m3 area, residue on cement Compressive Flexural
cm2/g sieve No. dough, %
008, %
Karaganda Portland 1095 3100 8.4 27.0 43.0 5.9
cement M400
Ust-Kamenogorsk 1100 3200 8.2 27.0 42.0 6.0
Portland cement M400
Shymkent Portland 1100 3200 8.2 26.0 41.0 5.8
cement M400
The tested cements, according to the obtained results, meet the requirements of [12].
Water for testing of cements and concretes corresponded to [13]. The analysis of total hardness of water used in the technology showed 12.8...100 mg-eq/l, from slightly brackish (dry residue up to 2.3 g/l) to strongly brackish (dry residue 3.0+19.5 g/l) and by acidity from slightly alkaline to slightly acidic (pH - 6.7...7.3).
Sand and crushed stone used in concrete mixtures meet the requirements of [14-15]. Characteristics of the used sand are given in Table 4.
Table 4 - Sand test results
Sand mine Characteristics of sand
Coarseness modulus Bulk density, kg/m3 Impurity, %
Karagandanerud, JSC 2.6 Gauhartas, LLP 2.2 SBS Group, LLP 2.4 1450 1420 1480 2...3 0.5...1 1.5
The characteristics of the coarse aggregate used are given in Table 5.
Table 5 - Test results of coarse aggregate
Fraction of aggregate, mine Name of indicators
Density, kg/m3 Water Sparseness, absorption, % % Impurity, %
5-10 mm, Karagandanerud, JSC 1400 5-20 mm, Bektas Group, LLP 1420 5-10 mm, TechnoIndustry, LLP 1370 0.85 8.06 0.96 9.24 0.41 3.1 0.75 2.06 1.86
The composition of organomineral admixture of fine-grained concrete (FGC) for repair was developed on the basis of the following materials. In the composition of the complex modifier were chosen: microsilica MCU-95 as a highly active pozzolanic mineral admixture; superplasticizer C-3, contributing to the effective liquefaction of concrete mixtures [16], hydrophobizing component -soapstock [17-18], obtained by refining oils; curing gas pedal sodium sulfate (Na2SO4), meeting the requirements of TS 38.10742-84.
Requirements for superplasticizer C-3 and composition are given in Table 6.
_Table 6 - Requirements for superplasticizer C-3_
_Name of indicators_Normal value_
Appearance The liquid is brown in color. A
precipitate is allowed. Active substance content in terms of dry product, % at least 69 Water content, % not exceeding 68 Ash content in terms of dry product, % exceeding 38 _pH of 2.5 % aqueous solution_7-9_
Superplasticizer C-3 is 20...40 % water concentrate and is supplied in tanks or drums. Percentage of water in the solution is regulated by passport data.
Soapstock is a production waste obtained during processing of vegetable oils, consisting of 41 % of fatty acids and 50 % of inactive inclusions (Table 7). Soapstock has the consistency of viscous dark brown paste, well and stable emulsifying with water.
_Table 7 - Characteristics of soapstock (in accordance with TS-10-04-0280-91)_
Name of indicators _Characterization_
Light oil soapstock Cottonseed oil Animal fat soapstock
Color
Consistency at 20°C
Smell
External solid impurities
Yellow to dark yellow with a grayish tint
soapstock
Light to light brown Brown to dark brown with a tinge of the color of the original oil Liquid or ointment Ointment Ointment
specific, characteristic of soapstock from various oils and fats fats and greases, a slight odor of decomposition products of organic substances is allowed, no odor of petroleum products is allowed, odor of decomposition products of organic substances is allowed, petroleum product odor is not
allowed
Absence Absence Absence
Sodium sulfate (SN) is a powdered or granular product supplied in packaged form. According to the known method of concreting, its main application was to accelerate the processes of setting and hardening of concrete [19-22]. In the developed composition of FGC with modifying additives SN, stabilizing the action of soapstock, is simultaneously a reagent that actively hardens the components of the liquid structure on the surface areas of mineral particles.
The International Union of Experts and Laboratories for Testing of Building Materials, Systems and Structures RILEM proposes to classify mineral additives from industrial waste by assessing their best applicability, primarily in terms of pozzolanic and hydraulic activity.
Microsilica is an ultradisperse material consisting of spherical-shaped particles, obtained in the process of gas cleaning of furnaces in the production of silicon. The main component of the material is amorphous silicon dioxide.
Microsilica is supplied in saleable forms and is labeled accordingly: compacted, MCU-90, MCU-95.
The numerical index in the labeling indicates the minimum allowable amount of silica (SiO2). Technical specifications TS 5743-048-02495332-96 for condensed condensed microsilica are presented in Table 9.
_Table 9 - Specifications for condensed microsilica_
Name of indicators Standards for grades of condensed
_compacted microsilica_
MCU-90 MCU-95
Appearance Fine-grained powdery material of gray
color with aggregate size up to 0.5 mm
Mass fraction of water, %, not exceeding 5 5
Mass fraction of loss on ignition, %, not exceeding 5 5
Mass fraction of silicon dioxide (SiO2), %, at least 90 95
Mass fraction of sulfur dioxide, %, not exceeding 0.6 0.6
Grinding fineness (specific surface area), m/g, at least 12 12
Bulk density of microsilica dry forms, kg/m3 80-500 280-500
The chemical composition of microsilica is presented in Table 10.
_Table 10 - Chemical composition of microsilica_
Manufacturer _Content, %_
_SiO2 C Fe2O3 Al2O3 CaO Impurities other
Tau-Ken Temir, LLP 96.9 1.82 0.05 0.16 0.22 2.84_0Z8_
3. Results and Discussion
In order to study the effect of modifiers on the setting time of cement batter and structure of concrete, several concrete formulations were developed as shown in Table 11.
Table 11 - Experimental compositions of concrete mixtures
No. Composition, kg/m3
Portland Sand Crushed Water Superplastisizer Microsilica Hydrophobicizer Hardening
cement stone C-3 of vegetable oil accelerant
soapstock sodium
sulphate
1 (Reference) 450 660 1060 167 3.6 (0.8%) - -
2 455.5 650 1060 167 3.6 (0.8%) 4.5 (5%) - 5.4 (1.2%)
3 382.5 650 1060 167 3.6 (0.8%) 67.5 (15%) - 6 (1.3%)
4 382.5 650 1060 167 3.6 (0.8%) 67.5 (15%) 2.1 (0.46%) 9 (2%)
In the course of laboratory studies it was possible to obtain the most optimal ratio of aggregates, filler and modifiers in relation to the mass of binder, the distribution of which is as follows: Portland cement - 382.5 kg/m3, Crushed stone - 1060 kg/m3, Sand - 650 kg/m3, Water -167 kg/m3 at W/C of 0.43; Microsilica - 67.5 kg/m3 (15%); Superplastisizer C-3 - 0.5.0.9% (in fact - 0.8%); Soapstock - 0.2.0.5% (in fact - 0.46%); Sodium sulphate - 1.2% (in fact - 2%).
To determine the effect of the complex of modifying additives on the processes of structure formation, the influence of their dosage on setting time was studied.
Table 12 shows the results of cement dough tests to determine the normal density and setting
time.
Table 12 - Effect of complex modifiers on normal density and setting time of cement dough
No. Composition Quantity by weight of Normal Setting time, h
cement, % consistency, _
% Start End
1 Superplastisizer 0.8 22.2 4.1 6.1
2 Superplastisizer +Sodium sulphate 2 (0.8+1,2) 21 3.8 5.6
3 Superplastisizer +Sodium sulphate 2.1 (0.8+1.3) 20 2.6 3.5
4 Superplastisizer + Soapstock+ S odium sulphate 3.26 (0.8+0.46+2) 18.5 2.3 3.1
From the data obtained, it can be seen that the normal density of cement dough decreased from 22.2 % to 18.5 % when the dosage in composition No. 4 was changed. It is also observed that the onset of setting time decreased from 4.1 hours to 2.3 hours and the completion accelerated from 6.1 hours to 3.1 hours. Based on the results obtained, it is assumed that the optimum balance in dosage of superplasticizer C-3 and curing gas pedal sodium sulfate has been found. Applying only Superplastisizer (C-3) separately, it is impossible to simultaneously correct and comprehensively affect the normal cement dough density and setting time.
It should be noted that the acceleration of the beginning and end of setting time is of great importance for the use of complex modifiers of concrete during repair works. According to the results of the experiments shown in Table 13 and Figure 2, it can be observed that the densest structure is characterized by the composition No. 4 containing Microsilica, Superplastisizer, Soapstock and Sodium sulphate with an open porosity of 6.1% and corresponding water absorption of 2.2%.
Also, from the obtained experimental results it can be concluded that Superplastisizer more densely packs different-sized grains, and in the process of cement hydration mineral additives and Microsilica react with other materials. All these processes lead to the fact that the surfaces of particles are enveloped by a thin layer of hydration products and the whole structure is united into a single whole.
Table 13 - Main microstructure indicators
No. of Age of Average Water Overall Volume Average Uniformity
compositions concrete, day density, kg/m3 absorption of concrete, % by weight porosity (Po), % of open pores, Pop,% size index of capillary pores, dimensions index of open capillary pores, a
1 2140 4.2 16.8 9.0 0.74 0.61
2 28 2188 3.8 16.3 7.4 0.60 0.68
3 2260 2.9 13.2 6.4 0.54 0.76
4 2272 2.2 12.8 6.1 0.56 0.77
20
18
16
14
% 12
it si 10
o
r o 8
P
6
4
2
0
16.8
1
16.3
2
Overall porosity (Po), %
Volume of open pores, Pop,%
Figure
3 4
Composition No.
2 - Effect of modifying additives on density indices of repair concrete structure
It should be noted that the composition modified by the complex of additives has a lower capillary porosity in relation to the control composition, and, therefore, has less shrinkage, which is also an important property for repair mortars.
Determination of shrinkage deformation and corrosion resistance of the developed modified MZB formulation.
Before the developed modified concrete composition is the task of using it mainly in the repair of structures, which means that the prepared concrete mortar will need to be applied in layers with a thickness of 10 ... 40 mm. Due to the relative thinness of the layers there is a high probability of formation of shrinkage cracks in them. Repairs made in this way will be short-lived and will not provide proper protection of the structure from corrosion and destruction. In order to determine the shrinkage performance of concrete, the compositions below were tested:
- the reference composition - plasticized concrete of class B40 on coarse aggregate (crushed stone 5-20 mm), without additional modifiers;
- composition No. 1 - plasticized FGC class B40, without additional modifiers;
- composition No. 4 - developed by FGC for repair, with a modified composition (Microsilica + Superplastisizer C-3 + Soapstock + Sodium sulphate). The results obtained are shown in Table 14.
Table 14 - Shrinkage strain values of compositions
Curing Reference composition Composition No. 1 Composition No. 4
age, Shrinkage Shrinkage Shrinkage
days Absolute, mm Relative, % Absolute, mm Relative, % Absolute, mm Relative, %
1 0.119 0.09 0.108 0.08 0.064 0.03
2 0.169 0.10 0.153 0.10 0.085 0.04
3 0.200 0.14 0.178 0.12 0.094 0.05
5 0.210 0.17 0.184 0.12 0.097 0.05
7 0.210 0.17 0.188 0.13 0.097 0.05
14 0.220 0.17 0.194 0.13 0.100 0.05
28 0.220 0.17 0.205 0.14 0.100 0.05
42 0.220 0.17 0.205 0.14 0.100 0.05
The results in Table 14 show that compared to the samples of the control mortar and mortar No. 1, the developed mortar No. 4 is the least subject to shrinkage, which allows this concrete to be placed in relatively thin layers. This means that the developed composition can be recommended for use as a repair mortar or even for cladding, since shrinkage cracking is minimized.
As a fine-dispersed mineral additive in the developed composition of FGC is used microsilica from Karaganda plant of Tau-Ken Temir, LLP, which is, in fact, a waste product of ferrosilicon production. Soapstock is also a waste product of Karaganda margarine plant, obtained during processing of vegetable oils. Cement, sand and crushed stone are supplied by local Karaganda producers. All this gives grounds for significant savings on raw materials and transportation costs.
4. Conclusions
The conducted research allows us to conclude that a composition of fine-grained concrete with high performance characteristics has been developed, which is recommended to be used for repair and facing works of residential structures made of concrete and reinforced concrete.
The experimental results confirm that:
- Improvement of characteristics of the developed FGC on the basic physical and mechanical indicators at application in its composition of additives-modifiers of concrete, such as: superplasticizer C-3 on a naphthalene formaldehyde basis, highly active microsilica as a fine-dispersed pozzolanic mineral admixture, hydrophobizing component - soapstock, i.e., a by-product of oil refining and gas pedal of hardening sodium sulfate (Na2SO4);
- Reduction of cement consumption by 15% (from 450 kg to 382.5 kg);
- The influence of the main factors and the dependence of performance indicators on the physical and mechanical properties of cement stone and the studied FGC was established.
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Information about authors:
Sarsenbek Kozhasov - MSc, Researcher, Kazakhstan Institute for Reconstruction and Development, Karaganda, Kazakhstan, [email protected]
Aigul Kozhas - Candidate of Technical Sciences, Senior Lecturer, Department of Technology of Industrial and Civil Engineering, L.N. Gumilyov Eurasian National University, Astana, Kazakhstan, [email protected]
Author Contributions:
Sarsenbek Kozhasov - resources, testing, modeling, interpretation, editing, funding acquisition. Aigul Kozhas - concept, methodology, data collection, analysis, visualization, drafting.
Received: 12.05.2023 Revised: 06.06.2023 Accepted: 06.06.2023 Published: 30.06.2023
