Selection of raw materials

Natural aggregates from quarries were used. Table 1 shows the characteristics of the coarse and fine aggregate.

The cement used was CPO, or general-purpose cement, whose characteristics are shown in Table 2.

Preparation of mixtures by MCV and MSM

The mixtures were made at the Construction Laboratory of the Universidad Nacional de Colombia, Medellín. Two concrete mixtures were designed: one called conventional manual mixing (MCV), and the other called manual mixing with mechanical suspension (MSM). A slump in the range of 5 cm to 7.5 cm was sought, using an A/C ratio of 0.52 in the two mixtures. The slumps are shown in Table 3.

Preparation of conventional manual mixing

The MCV mixture is the procedure used worldwide for manufacturing mortars and concretes at a small scale. It has the following stages:

The aggregates are prepared and mixed in a dry state until a uniform mixture is obtained, as shown in Figures 1 and 2.

Then a kind of volcano is made, to pour the cement in the center of the aggregates, and it is stirred until obtaining a homogeneous dry mixture with a predominant gray color, as seen in Figure 3.

Finally, water is poured over the aggregate-cement combination and stirred until a wet mixture is obtained, which is poured into the cylindrical specimens, as shown in Figures 4, 5 and 6.

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Figura 1 Materials for mixtures.  

Source: own elaboration (2020).

Figura 2 A mixture of coarse and fine aggregates in the dry state.  

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Figura 3 Mixing cement with aggregates.  

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Figura 4 Mixture of aggregates, cement and water.  

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Figura 5 Concrete mixture in wet state.  

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Figura 6 10 × 20 cm cylindrical specimens (MCV).  

Manual mixing with mechanical suspension

The MSM mixture is the proposal of this work. It modifies the mixing process of the components and, as will be seen in the section corresponding to the results and their discussion, has an impact on the behavior of the concrete, both in the fresh and hardened states. Its stages are:

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Figura 7 Preparation of materials.  

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Figura 8 Mixing cement with water: mechanical suspension.  

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Figura 9 Pouring of fine aggregate into the mechanical suspension.  

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Figura 10 The coarse aggregate is mixed with the paste obtained by mechanical suspension.  

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Figura 11 10 × 20 cm cylindrical specimens (MSM).  

The same design and dosing of materials was used for both mixtures since the study was not on the substitution of conventional material, but on the mixing methodology. Material consumptions are shown in Table 4.

Strength and durability tests

Cylindrical samples of 10 cm in diameter and 20 cm in height were made and then immersed in a curing tank with lime-saturated water at a temperature of 23 + 3 ²C, as shown in Figures 12, 13, and 14. The samples were taken to the hydraulic press to determine their resistance to compressive stress at ages of 7, 14, 28, and 56 days. Hardened concrete samples were also analyzed to determine their behavior in terms of absorption, bulk density, and porosity. For these analyses, ovens according to ASTM C-642, electronic press for compressive failure, and ultrasonic pulse velocity equipment were used.

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Figura 12 Preparation of lime water for curing.  

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Figura 13 Immersion of the specimens for curing.  

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Figura 14 MCV and MSM specimens for seven-day failure.  

Compressive strength tests

Fifteen concrete samples were made for each of the mixtures, for a total of 30, using cylindrical specimens 10 cm in diameter and 20 cm in height. Three specimens were taken per mixture for each failure age, according to NTC 1377, equivalent to ASTM C192M (Icontec, 1994). The results are shown in Table 5.

The MCV mixture was taken as a reference, to make a percentage comparison in terms of compressive stress behavior, assuming, then, that the MCV mixture is 100%, as shown in Table 6.

An average of the compressive strength at the four failure ages showed that the MSM mixture was 100.39%, compared to the control mixture, which was the MCV, indicating a slightly higher performance or in favor of the MSM; however, statistically, they are considered identical. The positive aspect of this result is that the new mixing methodology behaves following the requirements of the mixture design parameters, without negative effects, and reaching the performance resistances of ordinary concrete made in popular sectors, which are in the order of 21.0 MPa, according to NSR-10 (Decree 926 of 2010).

This result is consistent with the slump test of the mixtures in the fresh state, since, as shown in Table 3, the MSM mixture showed a higher slump compared to the MCV mixture. These slumps were 7.20 cm and 6.90 cm, respectively, which leads to infer that there is an optimal accommodation of the materials in the concrete matrix (^{Figueroa & Palacio, 2010}). In this sense, the workability of the mixtures in a fresh state for small volumes was better in the case of the MSM mixture, where better lubrication of the components was noted at the time of stirring the materials, represented by a lower effort on the part of the participants in the manual mixing activity.

Durability tests

Although there are no significant differences between the two types of mixtures in terms of compressive strength, as shown in the previous section, this research is especially interested in their behavior in terms of durability, since the constructions referred to in this study, and the small volumes, between 0.25 m³ and 0.50 m³, are located in cities such as Bogotá, Medellín, Caracas, Mexico, and Santiago de Chile, which have high levels of atmospheric C02 (^{Rosas et al... 2010}, p. 12), 2010, p. 12); thus, the performance of the material in a hardened state in these carbonate environments is an important aspect for the service life of CPO-based structures and coatings.

The tests performed were: standard test method for density, absorption, and voids in the concrete according to ASTM C642-06, as shown in Table 7; MBE, and ultrasonic speed, as shown in Table 8. The results are shown below.

It can be observed that the MSM mixture has better behavior, or performance, than the MCV reference mixture. In terms of dry bulk density, the MSM was also superior. Regarding the values of absorption and porosity of the hardened concrete, both mixtures presented a behavior under the ranges exposed by different researchers; among them, D. K. Nekrasov, who asserts that concretes register porosities that oscillate between 20% and 30%, or Apórtela, who obtained average porosities of 23.3 % with limestone aggregates and an A/C ratio of 0.50. As for absorption, the values obtained for the two mixtures also fall within the range described by D. K. Nekrasov, which is between 10 % and 20 % (^{Olivares et al., 2004}).

Scanning electron microscopy

As noticed in the MBE, (figures 15 - 20)the MSM mixture shows different behavior from that of the reference mixture MCV, since a more homogeneous, less porous surface topography is observed, to the extent that a better covering of the aggregates by the paste is evident. This means that the cement particles present a greater dispersion, and therefore the mechanical suspension covers, in a homogeneous manner, the inert filler material of the cementitious matrix. As the level of microscopic observation increased, noticeable differences were identified regarding the presence of pores and the exposure of aggregates in the hardened concrete, and to corroborate these findings, a reading was made using the chemical probe for micro-areas of the MBE, as shown in Figures 21 and 22.

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Figura 15 500X MBE of the MCV.  

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Figura 16 MBE at 500X of the MSM.  

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Figura 17 MBE at 2 000X of the MCV.  

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Figura 18 MBE at 2 000X of the MSM.  

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Figura 19 MBE at 5 000X of the MCV.  

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Figura 20 MBE at 5 000X of the MSM.  

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Source: own elaboration (2019). CC BY

Figura 21 Chemical probe by MBE in the MCV mixture.  

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Figura 22 Chemical probe by MBE in the MSM mixture.  

According to Figures 21 and 22, the presence of the components reported by the chemical probe for micro-areas coincides with the quality of the national cement used for the manufacture of CPO-based concrete (^{Tobón & Montoya, 2006}). The presence of potassium (K) in the MSM coincides with a stage or phase of maturation of the cement paste of the concrete in a hardened state, but which is generally obtained by mechanical mixing, and not by manual mixing; this is an important achievement of the present research, since more complete hydration of the cement perhaps is being reached (^{Franus & Panek, 2015}, p. 105).

Ultrasonic pulse rate

The results for the MCV and MSM mixtures are shown below. All cylindrical specimens were cured for 28 days, immersed in water saturated with lime; they were then dried for seven days at room temperature. The percent moisture content of the air-dried specimens was measured as well as the VPU, as shown in Table 8.

It is observed that the moisture content of the specimens is uniform in both mixtures. The MSM presented a higher PVU than the MCV, an aspect that is consistent with the characteristics of workability and initial slump, with the compressive stress resistance and with the porosity tests. If the value of the PVU of the MCV is taken as reference, then it is found that the MSM reaches 104.93%; that is, it is 192.50 m/s higher.

According to Malhotra, concrete with a PVU between 3660 m/s and 4575 m/s is considered good; below this range, it is considered fair and above this, it is considered excellent (^{Cruz et al., 2014}). For the normal use, mixtures referred to in this research, which are between 21.0 MPa and 23 MPa, a performance in the range of 3660 m/s and 4575 m/s is considered optimal.