ANALYSIS OF THE MOBILITY OF PLASTICIZED CONCRETE MIXTURE

УДК 691.327.234

Abstract

This article presents a comprehensive comparative analysis of the influence of various types of hyperplasticizers (GP) on the mobility of plasticized concrete mixtures. The influence of lignosulfonate (LST), naphthalene formaldehyde sulfonate (NFS), melamine formaldehyde sulfonate (MFS) and polycarbonate ether (PCE) generations on mobility, viability, water saving and strength of concrete is compared based on laboratory experiments and research by world scientists. The results of the study showed that PCE-type hyperplasticizers, especially PCE-2 and PCE-3, have high efficiency. They were found to reduce water consumption by 36–45%, increase the cone shrinkage of concrete by 20–22 cm, and increase the 28-day compressive strength to 72 MPa. It was proved that in the dry and hot climate of Uzbekistan, an increase in temperature to 35–45 ⁰C sharply reduces the effectiveness of additives based on NFS and MFS, while PCE-2 and PCE-3 retain the necessary mobility even at high temperatures. Also, the compatibility of hyperplasticizers with local cements was assessed, and recommendations were developed for the selection of hyperplasticizers for construction practice in the dry and hot climate of Uzbekistan. The results of the study are of significant practical importance in the preparation of high-quality, durable and economically efficient concretes.

Аннотация

В данной статье представлен всесторонний сравнительный анализ влияния различных типов гиперпластификаторов (ГП) на подвижность пластифицированных бетонных смесей. На основе лабораторных экспериментов и исследований мировых ученых сравнивается влияние поколений лигносульфоната (ЛСТ), нафталенформальдегидсульфоната (НФС), меламинформальдегидсульфо-ната (МФС) и поликарбонатного эфира (ПХЭ) на подвижность, жизнеспособность, водосбережение и прочность бетона. Результаты исследования показали, что гиперпластификаторы типа ПХЭ, особенно ПХЭ-2 и ПХЭ-3, обладают высокой эффективностью. Было установлено, что они снижают водопотребление на 36–45%, увеличивают конусную усадку бетона на 20–22 см и повышают прочность на сжатие через 28 дней до 72 МПа. Было доказано, что в сухом и жарком климате Узбекистана повышение температуры до 35–45 ⁰C резко снижает эффективность добавок на основе НФС и МФС, в то время как ПХЭ-2 и ПХЭ-3 сохраняют необходимую подвижность даже при высоких температурах. Также была оценена совместимость гиперпластификаторов с местными цементами и разработаны рекомендации по выбору гиперпластификаторов для строительной практики в сухом и жарком климате Узбекистана. Результаты исследования имеют важное практическое значение при приготовлении высококачественных, долговечных и экономически эффективных бетонов.

Keywords: hyperplasticizer, plasticized concrete, cone slump, mobility, rheology, Bingham model, polycarbonate ether (PCE), naphthalene formaldehyde sulfonate (NFS), lignosulfonate (LST), water-cement ratio (W/C), pot life, dispersing effect, steric hindrance, self-compacting concrete (SCC), construction industry of Uzbekistan.

Ключевые слова: гиперпластификатор, пластифицированный бетон, конусная осадка, подвижность, реология, модель Бингама, поликарбонатный эфир (PCE), нафталенформальдегидсульфонат (NFS), лигносульфонат (LST), водоцементное соотношение (В/Ц), время жизни смеси, диспергирующий эффект, стерическое препятствие, самоуплотняющийся бетон (SCC), строительная отрасль Узбекистана.

Introduction

Concrete is the main material of the construction industry, with an annual production volume of more than 4.3 billion tons worldwide. The most important contradiction in the development of concrete technology is the struggle between plasticity and strength. Increasing the water-cement ratio (W/C) improves the workability of the mixture, but this significantly reduces the strength: according to Abrams' law, , i.e., increasing W/C by 0.05 units reduces the strength by 4–6 MPa. To solve this persistent problem in concrete technology, chemical additives - first plasticizers, and later more effective hyperplasticizers - began to be introduced into the construction industry from the middle of the 20th century [1, 9].

Plasticized concrete mix is a concrete mix prepared with the addition of chemical additives (plasticizer or hyperplasticizer) and allowing to simultaneously reduce water consumption and increase workability. Workability - the main indicator of the rheology of the mixture - is measured by cone slump (cm), spreadability diameter (mm) or setting time (sec). Hyperplasticizers can reduce water consumption by 15 - 45%, which at the same time maintains or increases plasticity and provides a significant improvement in the strength, density and durability of concrete [2, 3, 10].

In the conditions of Uzbekistan, the issue of workability is especially important: the summer temperature reaching 35 - 45 ⁰C reduces the working life of the concrete mix by 2 - 3 times (20 minutes for NFS, more than 90 minutes for PCE-2). This creates a serious problem during long-distance transportation of concrete and filling of complex structures. Therefore, a comparative analysis of hyperplasticizers around the world and their selection for the climatic conditions of Uzbekistan is a scientific issue of practical importance for Uzbek engineers [4].

Research methodology

The concrete mixture is rheologically described by the Bingham plastic fluid model: : τ = τ₀ + μ_p · γ̇, where: τ is the resulting shear stress (Pa), τ₀ is the yield stress (the minimum stress required for the medium to flow, Pa), μp is the plastic viscosity (Pa ∙ s), γ ̇ is the shear rate (the rate of deformation, (s-1). This model is the main factor in describing the concrete mixture, and reducing both τ₀ and μp increases the mobility of the mixture. Hyperplasticizers disperse cement particles, reducing τ₀ by a factor of 3–8 and μp by a factor of 2–4 [5, 6].

The cone slump test (GOST 5802-86, EN 12350-2) is a practical method for measuring the complex result of τ₀ and μp. It has been proven in experiments that the correlation between cone slump and τ₀ is R2 = 0.92 There is a correlation. For SCC, the spreading diameter is 600–850 mm, which is provided at τ₀ < 50 Pa and μ_p = 50–100 Pa·s. The rheological map of the concrete mix (in the τ₀– μ_p coordinate system) is an important tool for showing the directions of influence of different mix types and GP [7].

Unmixed cement particles aggregate under the influence of electrostatic attraction to each other, forming flocculates. Water molecules remain in this aggregate, and the plasticity of the mixture decreases. The main task of hyperplasticizers is to break up this flocculate, freeing the particles and “liberating” the internal water. This process occurs through two main mechanisms.

The first mechanism is electrostatic dispersion (for NFS, MFS, LST): GP molecules adsorb to Ca²⁺ ions on the surface of the cement particle, creating a negative charge; the particles repel each other and the flocculate breaks up. The second mechanism is steric (spatial) hindrance (for PCE): the carboxyl groups in the main chain of the PCE molecule bind to the cement surface; the PEG side chains form a “shield” between the particles, preventing them from approaching each other. The steric mechanism has a dispersive effect 3-5 times stronger than the electrostatic mechanism [8].

Adsorption kinetics determine the compatibility of the hyperplasticizer with cement. The mineral C₃A (tricalcium aluminate) rapidly “absorbs” GP, which quickly reduces plasticity. PCE-2 and PCE-3 have a “delayed release” mechanism: GP is not initially completely adsorbed, but gradually releases as the C₃A reaction proceeds - this is a crucial feature that extends the viability period in the hot climate of Uzbekistan.

Factors affecting the mobility of the concrete mixture can be divided into two groups. The first group is material-related factors: a) W/C ratio - the most important factor (every 0.05 change changes the sediment by 4-6 cm); b) Cement type and mineralogical composition - cements with high C₃A (> 8%) consume GP faster; c) Fillers – round particles flow well, needle-shaped and flat particles reduce mobility; d) Additives – fly ash (+8–12%), microsilica (-10–15%), metakaolin (-15–20%).

The second group – technological factors: a) Temperature – the most critical factor for Uzbekistan: every 10 ⁰C increase increases the hydration rate by 2–2.5 times according to the Arrhenius law; for NFS at 35 ⁰C the viability period decreases from 20 minutes to 8 minutes; b) Mixing time and method – 90 seconds in a forced mixer is optimal; c) Transportation time and conditions; d) GP dosage – exceeding the saturation point causes segregation; e) GP compatibility test with cement – mandatory for each construction project.

Results and discussion

 Table 1 shows that PCE-3 provides the highest efficiency with the lowest dosage (0.15 – 0.35 %), but the highest cost. LST is the cheapest, but the least efficient. NFS is in the middle of the price/performance ratio. In practical construction, the choice of GP should be made taking into account the cost and climatic conditions, concrete class, transportation time and cement type.

Table 1. Chemical and technical characteristics of hyperplasticizer types (main indicators)

The following important conclusions can be drawn from the results of Table 2. First, PCE-3 at a dosage of 0.25 % leads to a slump of 20 – 22 cm — which is close to the SCC limit. Secondly, in terms of the life span (K_s60), PCE-3 has 91% - significantly higher than the standard ≥ 80%; while NFS and LST have 56% and 48% - unsatisfactory. Thirdly, PCE-3 can reduce W/C from 0.55 to 0.30 and increase R28 from 28 MPa to 72 MPa - by 157%. Fourthly, bleeding (water separation) in PCE-3 is reduced to 0.5% - 12 times lower than the control.

Table 2.Influence of GP types on mobility, water saving and concrete strength (CEM I 42.5 R, 20 ⁰C, standard experimental conditions)

Table 3 provides very important data for the climate of Uzbekistan: NFS at 35 ⁰C the subsidence drops to 5 cm – within the S1 limit for practical construction. At 40 ⁰C NFS and LST cannot be used at all. PCE-2 retains 9 cm subsidence at 45 ⁰C – still in the S3 category. PCE-3 at 45 ⁰C 11 cm – in the S3 category, which can be used for summer construction in Uzbekistan. Thus, there is no technically suitable option other than PCE-2 and PCE-3 for summer construction in Uzbekistan (May – September, T ≥ 35 ⁰C).

Table 3. Change in cone subsidence (cm) of GP types under the influence of temperature (dose 0.5 % or 0.3 % for PCE, W/C=0.42, measured for 30 minutes)

The use of hyperplasticizers in the construction industry of Uzbekistan has increased sharply in recent years: in 2023, GP imports amounted to 20–25 million US dollars (Statistics Committee of Uzbekistan). The main problem: currently 60% of summer construction activities in Uzbekistan are carried out at temperatures ≥ 30 ⁰C; the distance of concrete transportation is often 30–80 km. As a result, domestic GPs based on NFS and LST (SP-1, Gidro-S) face the problem of complete loss of plasticity in summer construction - which reduces the quality, strength and safety of concrete.

Compatibility characteristics of domestic cements with GP: All PCE types work well with Kyzylkum plant CEM I 42.5 R (C₃A = 7%); the optimal dosage for PCE-2 is 0.35–0.45%. Ahangaron CEM I 52.5 R (C₃A = 9 – 10 %): PCE dosage should be increased by +15%; in this cement, NFS efficiency decreases sharply. Shirkent slag Portland cement PPC 42.5 (C₃A ≤ 5 %): The highest water saving (40%+) is achieved with PCE-3. Compliance test (Marsh cone + mini-slump) is mandatory for each new batch of cement.

Table 4 shows that PCE-2 is the most suitable option for Uzbekistan - it is temperature-stable, provides SCC, and the price-quality balance is optimal. PCE-3 is required for B55+ and UHSC. NFS is economically acceptable in winter and for short transportation. The mobility of plasticized concrete mix is one of the most complex and multifaceted issues in concrete technology.

Table 4. Recommendation matrix for selecting hyperplasticizers in Uzbekistan construction conditions

Conclusions

 Based on the results of the study, the following scientific and practical conclusions were formulated. 1) hyperplasticizers affect the mobility of concrete mix through the mechanisms of electrostatic dispersion (LST, NFS, MFS) and steric hindrance (PCE). The steric mechanism is 3-5 times more effective than electrostatic, making PCE superior in all indicators. 2) Comparative analysis of mobility indicators showed that PCE-3 at a dosage of 0.25% can bring the cone slump to 20-22 cm (3.1 times the control), reduce W/C from 0.55 to 0.30, and increase R28 from 28 MPa to 72 MPa. The service life Ks60 = 91%1 is the best indicator. 3) In the dry-hot climate of Uzbekistan (T = 35-45 0C in summer), hyperplasticizers based on NFS and MFS cannot be used in practice. PCE-2 with Ks60 = 68% at 35 0C, slump at 40 0C of 12 cm was determined as the most optimal technical and economic choice for Uzbekistan. 4) Only PCE is suitable for SCC; SF ≥ 600 mm can never be achieved with NFS. PCE-2 at 0.40% SF = 718 mm, T500 = 2.8 sec, L-box = 0.90 – all EFNARC criteria met.

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