The chapter delves into the application of the Modified Andreassen Particle-Packing Model in polymer modified self-leveling heavy-weight mortar, aiming to optimize properties such as porosity and segregation resistance. It begins by discussing the historical context and importance of heavy-weight concretes, particularly in radiation shielding. The study then focuses on the experimental design, materials used, and the preparation of mortar mixtures according to the modified Andreassen model. The results of various tests, including unit weight, slump flow, compressive strength, and water absorption, are presented and analyzed. Notably, the chapter highlights the significant impact of polymer admixtures and micro/nano silicas on the mortar's properties, providing valuable insights for future research and practical applications in the field of concrete technology.
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Abstract
Heavy-weight concretes are known for their high unit weight inherited from the aggregates and developed mainly for radiation shielding. Therefore, minimal porosity besides the high unit weight is a desired property. Numerous particle-packing theories were put forward to decrease the porosity by an ideal reference curve; modified Andreassen model based on the size distribution of ingredients to adjust the fineness. This research investigates the effect of the mentioned method on heavy-weight mortars. Cement, micro and nano silica combinations, and their polymer additive mixtures were used as binders in the specimens, along with barite and finely ground magnetite aggregates. In this work, the aggregate size limit selected 3 mm and the fineness factor of q was chosen as 0.22 and 0.25, depending on the mixture. To achieve a self-levelling consistency, the w/c ratio was kept constant at 0.40, and a superplasticizer was added to maintain the workability. Consecutively, the specimens were examined for unit weight, compressive strength, and capillary water absorption. The collected results were analyzed, and the difference between groups was compared according to their composition.
1 Introduction
Concrete is the most produced material due to its ability to be tailored to meet specific needs. Heavy-weight concretes have a unit weight above 2600 kg/m3 in the dry state achieved by using heavy-weight aggregates. Such heavy particles can be from natural or artificial sources. Examples of natural aggregates are barite, hematite and magnetite [1]. Heavy-weight concretes were initially introduced as a safer measurement for constructions that tended to slide, such as retaining walls. However, with nuclear technology's emergence, heavy-weight concrete production shifted to radiation shielding [2].
Particle packing models aim to fit the highest possible number of solid particles within a unit volume. The increase of the solid ratio leads to a lower ratio of the water and trapped air. Thus, if desired, the approach might lead to reduced water-to-solid and water-to-cement ratios. This impacts the various properties, including porosity, volumetric stability, and low segregation. In the early 20th century, researchers Fuller and Thomson came up with the idea of using an optimization curve for particle packing. Their curve is essentially an equation that factors in the sizes of particles used and can be tuned based on fineness with an adjustable distribution exponent (q).
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In the 1980s Japan, a durable and underwater castable concrete, Self-Leveling Concrete (SLC), was developed [3]. Even though the SLC has similar proportioning to conventional concrete, one of its unique properties is self-levelling under its own weight without external force and segregation. However, the coarse aggregate content of SLC is limited compared to traditional concrete. Furthermore, powders not only consist of cement but also mineral admixtures or filler materials, which are used to increase the segregation resistance of concrete. To reduce the water content and increase the segregation resistance, implementing superplasticizers is crucial. The ability of superplasticizers to adsorb on cement grains and repel the solid particles further apart by steric hindrance provides a higher flowability in the fresh state.
Polymer Modified Concrete (PMC) contains polymer along with cementitious particles. Depending on the polymer type, the increase in strength can continue for several years due to the continuation of curing. The PMC is preferable for its high flexural and compressive strength and high durability properties. Even though polymer is added to the cement mixture in slight amounts due to partially economic reasons, the properties are enhanced to the desired level [4]. Furthermore, the polymer in a modified cement system might lead to a decrease in permeability due to the inter-bounding of polymer particles.
The aim of this study is to evaluate the influence of various factors, including heavy-weight aggregates, micro silica, nano silica, polymer admixture and particle size distribution (PSD) of these materials in self-levelling heavy-weight mortar with unit weight over 2600kg/m3 and flowability diameter of 175 mm. As one of the main uses of this type of mortar is shielding against radiation, minimal porosity and homogeneity are essential aims. To realize these with the segregating nature of fine ground heavy-weight aggregates, modified Andreassen model is used to modify the PSD of ingredients for homogeneity, whereas self-levelling concrete technology is used to increase the segregation resistance of the mortar. The tests are carried out in both fresh and hardened states according to the standards to obtain results.
1.1 Modified Andreassen Model
The Modified Andreassen model is an optimization design that succeeds the Fuller and Andreassen models [5]. The research on the properties of concrete by determining the role of the size distribution of the aggregates is also known as particle packing models (PPM). The pursuit for optimal packing started in 1907 with Thompson and Fuller [6]. Later, the first PPM based on the void ratio of binary mixes was developed by Furnas in 1929. The initial models lacked a calculation procedure that required the determination of the theoretical packing density of a given combination of particles by the PSD of each class. Moreover, the models were insufficient to include some factors such as shape factor, interparticle forces, water demand and strength. Due to the unincorporated factors, the empirical model does not deliver the most optimized proportioning alone and requires data based on trial and error.
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The particle density can be defined as the solid volume in the total volume of the mixture. Calculating by porosity is also possible in this situation in Eq. (1) or by the void ratio in Eq. (2):
$$ \pi = 1 - \Phi $$
(1)
$$ \Phi = 1 - \varepsilon $$
(2)
where π represents porosity, Φ packing density and \(\varepsilon\) void ratio.
The modified Andreassen model is designed to bring limitations in both the lower size and distribution factor of q. The parameter q varies between 0.21 and 0.36 for optimal packing. As Eq. (3) suggests, any change in the q affects the fineness and, consequently, the flowability and workability properties of the mixture. According to Emma software guidebook [8], a q value lower than 0.25 and 0.28 are beneficial for the flowability of mortar and concrete, respectively.
In Eq. (3), d represents particle diameter considered in mm, P(d) size cumulative distribution function of considered d diameter, dmax coarsest particle diameter in the mixture in mm, dmin finest particle in the mixture in mm and the q as a parameter.
2 Experimental Studies
2.1 Materials
Heavy-weight aggregates of barite and magnetite which were selected smaller than 3 mm, fine ground CEM II 42,5R ordinary Portland cement (OPC), micro and nano silicas, water, and HR50 superplasticizer and a polymer commercially known as Bettolatex, which is a vinyl-acrylic copolymer dispersion-based polymer [7] admixtures were used in the research. In order to achieve SLC-type mortar, a distribution exponent of q in the modified Andreassen between 0.21 and 0.25 is suitable [8, 9]. Micro and nano silicas were added to mixtures to reduce the porosity and increase the solidity, as all the solid particles were considered in the PSD. The superplasticizer to obtain an SLC type is undeniable and was implemented to reduce water demand, enhance flowability and increase the mortar's strength and unit weight. Also, polymer admixture was used in the specimens to minimize water absorption of the mortar.
Table 1.
Materials, PSDs, and unit weights
Class ID
Size Class (micron)
Density, ρ (g/cm3)
Magnetite
1–600
4.9
Barite
1–3000
(8 different size groups)
4.2
Micro silica
Nano silica
Cement
1–30
0,02–0,24
1–40
1.3
2.21
3.14
2.2 Mixtures of Mortars
3 groups consisting of 6 subgroups were prepared for the study. The materials in Table 1 were used to reach close proximity to optimum curves, as shown in Fig. 1 and 2. These groups were named according to their group and subgroup. The subgroups PS and PP were the reference specimens which contained only superplasticizer but no pozzolan admixture, with PP having polymer admixture. This pattern continues in subgroups MS and MP, which stand for micro silica and subgroups NS and NP, which were micro-silica and nano-silica combinations. All mixtures have 0.05 superplasticizer to cement ratio. Figure 1 is the optimal curve of P and M groups; meanwhile, Fig. 2 is for the N group due to the involvement of nanoparticles (Table 2).
Table 2.
Volumetric usage percentage of materials in each group
Material
PS
PP
MS
MP
NS
NP
Cement
Magnetite
17,99
7,33
17,88
7,29
18,20
7,42
18,08
7,37
17,95
5,23
17,84
5,20
Barite
Water
49,51
22,45
49,22
21,48
48,24
22,26
47,92
21,20
50,62
20,42
50,32
19,50
Micro silica
Nano silica
Superplasticizer
Polymer
-
-
2,72
-
-
-
2,71
1,42
1,18
-
2,70
-
1,17
-
2,68
1,58
1,16
1,97
2,66
-
1,15
1,96
2,65
1,39
Fig. 1.
Optimization Curve for PS, PP, MS, and MP q = 0.22 with 1 micron minimum and 3000 microns maximum
Fig. 2.
Optimization curve for NS and NP q = 0.25 with 0,01 micron minimum and 3000 microns maximum
×
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3 Tests and Results
3.1 Unit Weight Test in Fresh and Hardened States
The unit weight was calculated by division of the weight of the mortar in a premeasured container in fresh state. In hardened state, it is calculated by division of the weight of samples which were 28 days cured and oven dried for 40 h at 70 ℃ by their measured volume. The test results are illustrated in Fig. 3.
Fig. 3.
Unit Weights of Mortars in Fresh and Hardened States
×
3.2 Slump Flow Tests
The slump flow test is more appropriate for the SCC rather than the slump test due to the effectiveness of superplasticizers to cause much higher flowability. The flow tests were conducted according to the guidance of the TS EN 1170–1[10]. The results are shown in Table 3.
Table 3.
Flow values of fresh mortar of each groups
Sample Group
Average (mm)
Standard Deviation
PS
192.5
4.7
PP
185
2.4
MS
195
1.2
MP
172.5
4.7
NS
190
2.4
NP
172.5
2.4
3.3 Compressive Strength Tests
4x4x4 cm3 volume of the samples were subjected to compressive strength examination. Mean values of the test results were calculated and demonstrated in Table 4.
Table 4.
Compressive strength values of hardened mortar of each groups
Sample Group
Average Compressive Strength (MPa)
Standard Deviation
PS
56.3
1.1
PP
43.6
0.6
MS
65.7
1.4
MP
50.2
0.9
NS
60.3
5.5
NP
50.5
0.4
3.4 Test for Water Absorption by Capillary
The oven dried and cooled samples were coated on the sides with paraffin to avoid any other water absorption excluding the bottom surface. The samples were placed in the water and the weight measured at predetermined times and absorption was calculated according to ASTM C1585 [11]. The results are shown in the Fig. 4.
Fig. 4.
Water Absorption Rates in mm/sn0,5
×
4 Conclusions
The aim of this study is to evaluate the influence of various factors in self-levelling heavy-weight mortar.
By standards, the classification of heavy weight is any mortar heavier than 2600 kg/m3 in oven dried state. Figure 3 demonstrates that the entirety of the sample groups was above this limit. Among the sample groups, polymer modified groups were shown lower unit weight. This could be linked to the volume that was filled with lighter polymer particles and probable air entraining properties of the polymer admixture.
The flowability of the mixtures showed that two thirds of the samples surpassed the target of 175 mm and only MP and NP groups were below. As a general trend among the polymer modified groups to have lower flowability, it can be deducted that vinyl acrylic copolymer inhibits the property.
Compressive strength values were the highest at MS and NS groups with 65.7 and 60.3 MPa, respectively, which utilize micro silica and a combination of micro silica and nano silica mixtures likely due to pozzolanic reactions and filler effect of the materials. However, polymer admixture commonly reduces the lower compressive strengths compared to ones without polymer admixture. Another study shows results where the acrylic dispersion polymer admixture increases the compressive strength [12]. Nonetheless, these results might stem from the difference of type between polymer admixtures.
Figure 4 shows that the water absorption rates decreased for both polymer and mineral additions. In the case of micro silica and nano silica addition, the absorptions decreased significantly with the latter being more effective. The fineness or structures of pozzolans might have the influence behind the superior impermeability of the mixtures. As it is intended, the polymer admixture diminished the absorption rates in PP and MP groups, whereas did not affect NP group any further. Provided that the polymeric bound network effectively fills capillary voids, the uniformity in nano silica incorporated groups indicates that the capillary voids were sealed off by reaction outputs already and did not require any further polymer supplementation. In general, polymer subgroups mostly lowered the water absorption, except for N group which shows no significant difference.
It is planned in future to carry out research on self-compacting heavy-weight mortar with further mechanical tests and various radiation permeability tests.
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