Impact of Rolled and Crushed Aggregate with Natural Pozzolan on the Behavior of HPC

Recent studies conducted all over the world have proven that the quality of the aggregate strongly affects the fresh and hardened concrete, especially in terms of durability and economy. Therefore, the selection of aggregates is a very important process. Current trends in the formulation of high performance concrete and low environmental impact show that the solid volume fraction is increasingly high. However, increasing the volume fraction of solid presents serious effects on the workability of new concrete. The high performance concrete (HPC) is highly fluid concrete which requires high dosages of sand and fine elements compared to ordinary concrete (OC) [1, 2]. Although some variation in aggregate properties is expected, particle shape, surface texture, unit weights, absorption voids, and surface moisture are among the characteristics that should be considered when selecting aggregates.

Many studies have focused on improving certain characteristics of concrete, such as lightness and thermal and sound insulation [3]. HPC is usually produced using high quality materials its use is widely in the construction of huge building such as bridges, tall buildings, tunnels, nuclear structures ... etc. The improved properties of the HPC can be obtained only by several simultaneous dispositions:

- The use of specific additives, in particular water-reducing super plasticizers, as well as mineral additions such as pozzolan, fly ash, blast furnace slag, silica fume and other.

- The aggregates must be of good quality, the strength of the concrete Being limited by that of the aggregates themselves,

- The use of high grade cements at dosages between 400 and 550 kg / m³ [3].

Navarrete et al. demonstrated that coarse aggregates of the same content, type and surface texture, but with different aspect ratios and angularity indices, have an impact on the mechanical behavior of hardened concrete [4]. Other researchers shown that the strength, stiffness and fracture energy of concrete depend on the type of aggregate, especially for high-strength concrete [5].

Brito et al. explained that the shape regularity of the aggregate is one of the major factors that affect the influence of aggregates on concrete quality [6].

Neuwald et al. stated that the aggregates’ characteristics should be taken into consideration for the performance required for fresh and hardened concrete [7]. The shape and size of coarse aggregate has a vital influence on the necessary mortar and paste volume to cover all particles. Naturally uncrushed gravel often needs less mortar or paste than does limestone. Crushed aggregate tends to reduce flow because of the interlocking of the angular particles, whilst rounded aggregate improves the flow because of lower internal friction [8].

The incorporation of mineral additives is currently an important technology in improving concrete and mortar properties such as fluidity, strength and durability. However, to take full advantage of these materials and choosing the best solutions to improve the formulation, it is necessary to know the properties of these new components and the extent of their impacts on mortar and concrete [9]. It was agreed that the effect of pozzolanic materials in a cement system is significant, especially for development of calcium silicate hydrate framework [10].

Aitcin [2] has been noted that the characteristics of the paste, regardless of the cementitious additions (CA) that compose it in terms of shape and texture, as well as minerals and strength may result in different concrete strengths. From this aspect, the impact of the additives properties on the strength of concrete becomes very important in terms of improving its quality and performance, as in the case of HPC [11].

Sahin et al. [12] observed that the increase in strength for a given increase in cement content depends on the cement content itself and the type of aggregate used. On the other hand, it is not enough to just relate the compressive strength to w/c of HPC, but there are other factors that have a direct impact on the development of the mechanical properties of concrete especially HPC. In such a situation, the coarse aggregate should be carefully chosen relenting on its physical and mechanical properties [13, 14].

The aggregate properties depend on the properties of the parent rock (e.g., chemical and mineralogical composition, petro graphic classification, hardness, strength, and pore structure). All these characteristics have an important influence on the properties of both fresh and hardened concrete [15, 16].

Celik et al. considered that the crushed stone is more adapted for the HPC production compared to natural gravel and sand [16]. Other researchers studied some types of CA using percentages of 36%, 40% and 44% by volume of concrete for producing HPC with an E/C of 0.28 [13]. They confirmed that the optimum coarse aggregate ratio ranged between 36% and 40%, and that the compressive strength was reduced with an increase of more than 40% of the additives. While other works have also founded that the using an aggregate block with a maximum aggregate size of limestone of 16 mm gives a higher values of mechanicals strength were obtained [17].

Based on a literature review, natural pozzolanic aggregate is chosen as a replacement material for coarse aggregate. The variety and content of pozzolan aggregate have a significant impact on the properties of concrete, primarily caused by the characterization of pozzolan aggregate and the density of concrete decreases with the addition of pozzolan aggregate, which decreases depending on the type of pozzolan aggregate used [18]. A pozzolanic material relies on the secondary reaction following the hydration of cement, whereby it reacts with the free calcium hydroxide to form the calcium silicate hydrates (C-S-H) phase which is the major contributor to strength in concrete [1]. Belouadah et al. [19] have reported that the addition of 10% of marble powder in the concrete mixture enhances its resistance in the time.

  Several studies on mortar and concrete high performance were developed for the purpose of high lighting the effect of the morphology of coarse aggregate and CA on the properties of concrete and HPC, However, there are few studies that have focused on the effect of both the nature and distribution of the granules, as well as the size of rolled and crushed aggregates on the characteristics of concrete [20, 21]. These studies showed the importance to inclusion of rolled coarse aggregate of 3/8 mm fraction at 40% substitution level and 60% of 8/15 mm coarse aggregate fractions in concrete effectively improved the mechanical properties. Hence, the objective of this study is to investigate the impact of the particle size distribution of rolled gravel and crushed gravel on the compressive strength of HPC using several combinations of granular fractions at different percentages with pozzolan addition combined with admixtures. In order to reach the desired goals of this study, we have taken the following steps:

- Firstly, investigate the physical properties of the aggregates (sand aggregate and coarse aggregate) used in terms of granular distribution, strength, relative density, porosity and surface texture;

- Then, the density and the porosity for fractions mixtures rolled gravel and crushed gravel in various percentages (10% to 90%) in order to have the best results of the porosity;

- In this state, three combinations (C1, C2, C3) has been chosen with the proportions of RG (3/8) and CG (8/16) (%) as follows: (35 – 65), (40 – 60), (45 – 55);

- In order to complete the granular skeletons of HPC’s, four percentages of pozzolan addition were tested (0, 5, 8 and 10%) by weight of cement;

- Finally, after selecting the assembly with the lowest porosity, twelve mixtures were formed with 5%, 8% and 10% of pozzolana in order to study the:

1. Development of compressive strength as a function of time;
2. Effect of pozzolan addition on the strength concrete;
3. Effect of rolled gravel on the strength concrete;
4. Effect of the sand /gravel ratio (S/G) on the strength concrete.

3.1 Materials

Cement: The Portland cement type NA 42 - CEM 152,5 R was used in this experimental study. It has an absolute density, consistency and fineness values of 3.1 g/cm³, 28% and 4000 - 5400 cm²/g, respectively.

Pozzolan: pozzolan powder was come from BeniSaf cement plant (north-west of Algeria). In this study, the pozzolan presented a specific gravity of 2670 kg/m³ and 960 m²/ kg for the specific surface. The properties composition of the cement and the pozzolan are given in Table 1.

Aggregates: Two types of coarse aggregates are used, the 8/15 fraction of crushed gravel and 3/8 fraction of rolled gravel, Table 1 shows the chemical and mineralogical compositions, the crushed gravel (CG) have rough and angular surfaces, rolled gravel (RG) have smooth and rounded surfaces. The two types of gravel have a volumetric coefficient greater than 0.2, which complies with the standard conforming to the studies [22, 23].

Sand: A mixed sand is used in our concretes in equal proportions (according to a study presented in a previous publication), 50% natural river sand (DS) which is characterized by siliceous granules, round and smooth shape and a coefficient of fineness, estimated at 1.92, as for the other 50% of sand was made of crushed natural sand (DC) with angular and rough surfaces and a modulus of finesse 2.25 [24].

Superplasticizer: it was used the superplasticizer reducer water manufactured under the name of MEDAFLOW 30. It is designed based on Ether Polycarboxylate which considerably improves the properties of concrete. It is recommended to use 0, 5 to 2% weight of the cement [25].

The properties of the CG, RG and superplasticizer are represented in Table 1.

Table 1. Chemical composition of cement, pozzolan and the coarse aggregate (crushed gravel CG and rolled gravel RG) used

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Figure 1. Distribution size of aggregates used

The particle size analysis of aggregates used is shown in Figure 1.

The mineral phases of the RG investigated by the x-ray diffraction (XRD), in which identified by crystalline mineral phases which are essentially composed of the quartz, calcite and anorthite. Result indicated the presence of amorphous materials.

Figure 2 shows the fraction 3/8 of rolled gravel smooth and rounded surfaces, this type of gravel facilitates rearrangement of coarse gravel and improves the workability of the concrete. The different aggregates properties employed in this study are presented in the Table 2.

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Figure 2. Grain size of aggregates used

Table 2. Properties of aggregates used

3.2 Mixture composition and granular proportioning

Table 3 provide the density and the porosity for fractions mixtures rolled gravel and crushed gravel.

Table 3. Density and porosity of fractions mixtures

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Figure 3. Porosity of mixtures gravel fractions

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Figure 4. Density of mixtures of gravel fractions

Table 4. Combinations of granular fraction used

According to the Figures 3 and 4, on can be seen that the mixing combination with 35, 40 and 45% fraction 3/8 rolled gravel give a maximum bulk density with minimum porosity.

Three granular systems were used as basic groups with many percent gravel fractions were selected based on preliminary bulk density and porosity calculation tests, examining the porosity of the different granular fraction combinations. The three combinations chosen in this work gave the best results of the porosity. It was selected and studied with the combination of two types of gravel, namely the fraction 8/16 of crushed gravel and the fraction 3/8 of rolled gravel, in order to better understand their impact on HPC properties. Four pozzolan percentages were tested in this research (0, 5, 8 and 10%) by weight of cement in order to find the best percentage of pozzolan is added, 1.5% by weight of cement of the super plasticizer was added. These percentages were found by saturation tests.

All concrete mixed in accordance of Dreux-Goris method. Table 4 presented the combinations of granular fraction used in the concretes.

3.3 Testing method

The value of the slump in all mixtures was determined by value of about 70 ± 10 mm [26, 27]. Twelve forms of HPC were prepared from the three combinations of granular fraction C1, C2 and C3 and four percentages of pozzolan (0, 5, 8, and 10% based on the weight of cement). The concrete mixtures were labeled using the following codes: CX-PY where the first letters CX represents the combined fraction of CA followed by the number 1, 2, or 3, the letters PY represents the percentage of pozzolan (0, 5, 8 and 10%). The required proportions of dry materials were mixed. Then followed by the stage of adding water mixed with super plasticizer (Medaflow 30) until we get homogeneous concrete mix. Cubic steel moulds (100 × 100 × 100) mm³ have been used for the compressive strength test according to NF EN 12390 – 4 [27]. At the end of mixing, concrete is poured into the molds in three layers, as specified in the standards, using a vibrator plate to compact the concrete into the molds. Then it kept in a laboratory environment at (20℃ ± 2℃ and 55% ± 5% HR) for the first 24 h. The samples, after being removed from the molds, were stored in water until the day of the test, which was determined at: 7, 14 and 28 days.

Table 5 shows the various combinations of coarse gravel with different percentages of pozzolan.

Table 5. Compositions of concretes mixture

This study proved its significance method to made concrete with binary gravel (natural river rolled gravel and manufactured crushed gravel) in order to correct the physical properties of coarse aggregates. In addition, the enhancing of mechanical responses using these two combined types allows a high strength concrete to be produced. Thus, the test results obtained in the present study allow the following conclusions to be drawn;

• 3/8 mm fraction rolled gravel up to a percentage of 40% is found to improve the physical properties of binary gravel (grading, low porosity, high compactness.) as well as the mechanical strengths. The rate of the strength as the percentage of rolled gravel replacement increase is around 10% for HPC tested mixes overall fractions used in this program.
• Pozzolan incorporation matched with enhances the concrete resistance of HPC mixes. It reaches a significant increase of more than 70% for its compressive strength at the percentages of 5% pozzolan and 1.5% superplasticizer by weight of cement compared to control concrete (without pozzolana).
• Through the obtained results, it was found that the compressive strength is related to the variables of coarse aggregate (content, fine to coarse aggregate ratio and grain size distribution) of the concrete mixture).
• Again rolled (uncrushed) coarse aggregates and pozzolan are cheaper than crushed coarse aggregates in Algeria. So from the study, it can be concluded that the use of rolled gravel and manufactured crushed gravel are appropriate in concrete mixture for better performances in terms of strength and economy.