Keywords

1 Introduction

The significant changes have taken place since the beginning of the industrial revolution. The global warming causes the permanent temperature increase and results in new challenges in the concrete industry, which remains very important branch of the modern economy. Concrete is still the material, which is used widely to satisfy the human needs. The new types of concretes such as high strength concrete (HSC), ultra high strength concrete (UHSC), self-compacting concrete (SCC), high performance concrete (HPC) become very popular nowadays and are more often used in construction industry to achieve sustainability [1,2,3,4,5,6]. HSC is used to put the concrete into service earlier (for example opening the road traffic at 3 days), to build structures by reducing cross section area (high-rise buildings), to build superstructures such as long-span bridges and bridge desk with enhanced durability. It can be also used to satisfy the specific needs of special application such as flexural strength, modulus of elasticity and durability. Portland cements with higher compressive strength class is used as a rule to manufacture HSC. Due to the higher temperature caused by global warming, the problems related with the transportation of fresh high strength concrete in hot weather are more often risen. The transportation of high strength concrete becomes more and more complicated in such conditions, because it usually contains higher amount of cement with the higher activity in comparison with traditional concrete and as a result more heat is released when fresh concrete is delivered due to cement hydration causing its quick stiffening. Though, it is well known that ready-mixed concrete should be delivered to the construction site in plastic state to provide proper placing and compaction. That’s why the effective retarders, air entraining agent, different modifiers and both plasticizer or superplasticizer are used in contemporary concrete technologies, including HSC production, to improve the technological properties of fresh and mechanical and durability characteristics of hardened concretes [7,8,9,10,11,12]. As a result, it is compulsory to use plasticizers and retarders when such concretes are designed. There are several types of commercially available superplasticizers such as lignosulphonates, naphthalene-based, melamine-based, and modified polycarboxylates and different retarding admixtures [13].

Runova et al. [14] revealed the slump loss problem in hot weather if SNF-based superplasticizer is used for ready-mixed concrete production. Multifunctional chemical admixture was developed consisting of sulfonated naphthalene formaldehyde condensate in combination with the lignosulphonate based retarder and sodium borate to prevent rapid slump loss. Many researchers also study the efficiency of such organic retarder as sodium gluconate [15,16,17]. SG is commonly used in real concrete production resulting in the significant retarding effect and rather good compatibility with different superplasticizers. Ma et al. [15] concluded that SG results in the formation of AFt at early stage of hydration if the dosage is less than 0.03 mass%. On the contrary, SG slows down the formation of AFt if the dosage is more than 0.05 mass%. Most researchers study the setting time of cements containing retarders at normal temperature, but the influence of SG on setting time of cement at higher temperature is rarely discussed [16]. Lv et al. [17] shows that sodium gluconate has positive influence on the compressive strength at the dosage which is less than 0.15 mass% and if it is exceeded, the negative effects are observed. That’s why the aim of this researches is to study the effect of dosage different types of plasticizers and retarder on properties of fresh and hardened concrete.

2 Materials and Methods

Commercially available Portland cement CEM II/A-S 42,5R was used in this study.

The physical and mechanical properties of Portland cement are presented in Table 1.

Table 1. Physical and mechanical properties of Portland cement.
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The tests of both Portland cement and aggregates properties were carried out according to Ukrainian standards [18,19,20,21,22]. The results of aggregates’ investigations are shown in Table 2.

Table 2. Aggregates’ properties.
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Commercially available polycarboxylate (PCE-RE) and lignosulfonate (LS-RE) based superplasticizers with retarding effect as well as traditional retarder on the basis of sodium gluconate (R-SG) were used in researches.

The concrete mix designed was carried out according to DSTU B V.2.7-214:2009 [23]. The mathematical planning of experiments was carried out to determine the optimal amount of plasticizing and retarding admixtures [24]. Compressive strength of concrete was determined according to DSTU B V.2.7-214:2009 [25].

3 Results and Discussion

At first stage the researches were focused on the study of the influence of plasticizing and retarding admixtures on the properties of cement paste and both fresh and hardened concretes. As seen from the Fig. 1, cement paste containing plasticizer with retarding effect and retarder results in extending the hydration induction period and thereby lengthening the setting times. It should be noted that the use of plasticizers with secondary retarding effect is one of the most effective method to improve fresh and hardened concrete properties. The addition of LS-RE and PCE-RE delay the initial and final setting time of cement pastes by approximately 100 and 140 min respectively compared to control cement paste without admixtures. The setting time of cement paste incorporating R-SG shows the tendency of gradual significant increasing with the dosage growth from 0.2 to 0.3 mass%. Thus, the initial and final setting time for cement containing 0.3 mass% of SG extend by 380 min compared to control cement paste. However, the interval time between initial and final time is rather short and ranges between 80–110 min.

Fig. 1.
figure 1

Setting time of cement pastes.

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The compressive strength of cement paste (paste 1:0, samples-cubes 2 × 2 × 2 cm) containing plasticizing and retarding admixtures is discussed in this part and the results are given in Table 3. The water demand of cement pastes added with LS-RE and PCE-RE decreases by 8 and 9% respectively, but cement paste containing different dosages of SG present the tendency of increasing by 3%. The compressive strength of cement pastes incorporating LS-RE and different dosages of R-SG decreases at early age (after 1 and 7 days) of cement paste hardening in comparison with control paste without admixtures. The highest decrease (16 and 25.6%) is observed for cement paste with 0.3 mass% of R-SG after 1 and 7 days of hardening respectively. It should be noted that compressive strength of cement pastes incorporating LS-RE, R-SG (0.2 mass%) exceeds the compressive strength of control paste by 8 and 4% respectively, except cement paste containing 0.3 mass% of R-SG where 8% decrease is observed after 28 days of hardening. The compressive strength of cement paste containing PCE-RE has shown permanent increase for 28 days of hardening. The PCE-RE and R-SG were used for further researches such as these admixtures show the best plasticizing and retarding effect respectively.

Table 3. Effect of admixtures on compressive strength of cement paste.
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The optimization of the content of PCE-RE and R-SG was carried out using the mathematical planning of experiments. The results are presented in Fig. 2.

Fig. 2.
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Compressive strength of concretes after 1 (a) and 28 (b) days of hardening.

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The optimum dosage of PCE-RE and R-SG are found on the basis on the highest ultimate compressive strength of fine-grained concrete (Cement:Sand = 1:2). The results are presented at the age of 28 days. Dosage with lower content of PCE-RE than this optimum value reduces the compressive strength and higher is not recommended by producer, because can cause negative effect related with the over dosage of plasticizer. The content of R-SG will depend on the workability retention of fresh concrete and compressive strength of hardened concrete.

The concrete mix design was done according to Ukrainian standard to study the influence of optimal amount on plasticizing and retarding admixtures on slump loss and compressive strength of concrete. The following mix-proportion has been obtained: C = 408 kg/m3, S = 600 kg/m3, G = 1138 kg/m3, W = 246 kg/m3, PCE-RE (0,9 mass%), R-SG (0,3 mass%). Consistency class of concrete mixes was S4.

The results of slump loss determination are shown in Fig. 3. The data show the relation between dosages of R-SG and slump loss. The results show that slump reduces with time. More R-SG (0.9 mass%) than designed (0.3 mass%) was added to retain the concrete in liquid state for a longer time and, as a result, it would reduce the slump loss during the transportation of concrete to the construction site. According to Fig. 3, the designed consistency class S4 is retained during the test and the conclusion can be made that retarder R-SG (0.9 mass%) is more effective in comparison with plasticizer with retarding effect PCE-RE in retaining the slump of the high strength concrete.

Fig. 3.
figure 3

Slump loss of fresh concretes.

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The dosage of R-SG presents different behavior on the compressive strength of modified high strength concrete (Fig. 4). At early age, the addition of extra amount (0.6 and 0.9 mass%) of R-SG is not able to increase the compressive strength of concrete. On the contrary, the strength significantly reduces from 43.5 MPa (0.3 mass% of R-SG) to 18,6 MPa (0.9 mass% of R-SG), because the addition of the extra amount of retarder to the concrete delays the reaction of C3S and C3A and, as a result, the strength development is low. The situation changes only after 7 days from casting, compressive strength of high strength concrete containing R-SG slightly improves and exceeds the compressive strength of concrete incorporating 0.3 mass% of R-SG. As seen from the graph, continuous strength gain is observed with age and compressive strength of HSC containing 0.6 and 0.9 mass% of R-SG exceeds the compressive strength of concrete incorporating 0.3 mass% of R-SG by 20 and 24% respectively, because the reaction between the cement particles and water is active.

Fig. 4.
figure 4

Compressive strength of concretes.

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4 Conclusion

The efficiency of different type of chemical admixture was studied. The obtained results show that correlation between setting time of cement paste and slump loss isn’t observed. It was established that incorporation rationally selected plasticizing and retarding admixtures in high strength concrete and optimization of their dosage allows obtaining fresh concrete with designed workability retention and compressive strength. The obtained properties of fresh and hardened high strength concrete allow transporting it longer and opening up a gate towards wider market, including construction of high-rise buildings. Use of this structurally safe and environmental friendly material enables to realize concept and idea of vertical cities.