Microstructural and Plastic Shrinkage Studies on M-Sand and Coconut Shell Used Self-Compacting Concrete

Abstract

This study investigates the plastic shrinkage behavior of self-compacting concrete made from coconut shells (CS) and blended with manufactured sand (M-sand). The study used the crushed coconut shell as a coarse aggregate (CA) and M-sand as river sand. M-sand was replaced at 25, 50, 75 and 100% in place of river sand and studied the microstructural and plastic shrinkage characteristics of self-compacting coconut shell concrete (SCCSC). To confirm the SCCSC, slump flow, T500 flow time, V-funnel test, L-box and GTM screen stability experiments were conducted. Using M-sand with the combination of coconut shells decreased the formation of plastic shrinkage cracks. Microstructural studies such as SEM and XRD show that there was a decrease in the pore presence and anhydrous compounds and does not affect the formation of hydrated compounds. This study proved that using coconut shell and M-sand in the manufacturing of SCCSC to achieving sustainable development goals, such as “responsible consumption and production”.

1 Introduction

Concrete is a manmade substance of cement, aggregates, and water bonded together as a single material. After water, concrete is the second most widely used material in the world. It is also the most often used building material globally [1]. Due to its versatility and durability, self-compacting concrete (SCC), which came after concrete, became more popular. Since this concrete is so easily flowable, complete compaction can be achieved without the use of vibration. It has a number of advantages, including low labor costs, reduced noise pollution, quick casting times, well-finished surfaces, design flexibility, simple placement in out-of-reach places, and a uniform concrete matrix free of honeycombing [2]. SCC is, therefore, highly sought in the heavy construction sector. Ecological imbalance results from the diminution of natural resources brought on by using natural aggregates [3, 4]. As a result, in an effort to protect the environment, people are using a variety of aggregates made from waste generated by various domestic, industrial, and agricultural sources. Coconut shell (CS) is the agricultural wastes utilized as coarse aggregate in the making of concrete [5]. In addition, people are looking for alternatives for river sand since it is not widely accessible everywhere in the world. One of the materials used as a substitute for river sand by many studies is M-sand [6, 7]. According to the literature, M-sand has been used very rarely used in the production of SCC. This study recommended using M-sand as the fine aggregate and CS as the coarse aggregate when making concrete. Volume changes in fresh concrete are common. One of the most detrimental characteristics of concrete that affect its durability is the occurrence of volume change. The volume deviation in concrete leads to cracks formation of surface of the concrete. The volume modification brought on by concrete innate characteristics is called plastic shrinkage. The presence of cracks in concrete is one of the least appealing flaws. One significant factor that contributes to concrete cracking is contraction. Manufacturing concrete that does not shrink and crack is challenging. The only factors that must be considered are quantity, the width of crack, and length of crack propagation. This study evaluated the plastic shrinkage behavior and microstructural properties of SCC manufactured with a combination of CS and M-sand.

2 Materials Used

53 grade ordinary Portland cement was utilized according to Indian Standard 12,269:2013 [8]. The coarse aggregate of size 10 mm and fine aggregate of less than 4.75 mm were used. River sand and M-sand confirmed to zone-III grade according to IS 383:2016[9]. Figure 1 shows the M-sand sample. A superplasticizer, Conplast SP430 was used as a water-reducing admixture in accordance with IS 9103:1999 [10]. In this investigation, the coarse aggregate was replaced with waste CS gathered from a local market. The raw CS was crushed to the desired size using a coconut shell crusher machine located in SRMIST. The crushed CS is shown in Fig. 2.

Fig. 1

M-sand sample

Fig. 2

Crushed coconut shell

2.1 Mix Proportion

For this investigation, three SCC mixes were prepared according on the already designed mix proportion by the earlier study [11]. Table 1 displays the mix proportion of three SCC mixes. In the case of SCCSCM, M-sand was used as a replacement for 25, 50, 75, and 100% of the river sand.

Table 1 Mix proportion

3 Experimental Investigation

The following experiments examined the SCCSC, including workability, plastic shrinkage, and microstructural study.

3.1 Workability Test

The slump flow, T500, and V-funnel experiment were used to study the filling capacity behavior of all compounds in accordance with the standard set by EFNARC (2005) [12]. The L-box experiment was performed to determine the SCC ability of passing through reinforcement bars. The slump flow is divided into three categories such as SF1, SF2, and SF3, by EFNARC 2005. According to the designations SF1, SF2, and SF3, the slump flow ranges are 550–650 mm, 660–750 mm, and 760–850 mm, respectively. The passing ability is divided into two categories: PA1 and PA2. PA1 stands for 0.8 with two rebars, while PA2 stands for 0.8 with three rebars. The GTM screen stability test was performed to measure SCC stability. According to EFNARC 2005 guidelines, a segregation ratio between 5 and 15% of the sample weight is satisfactory.

Fig. 3

Screeding

Fig. 4

Bull float application

Fig. 5

Trowelling

Fig. 6

Air circulation through fan

Fig. 7

Plastic shrinkage crack

Fig. 8

Crack length measurement

Fig. 9

Plastic shrinkage crack test result

3.2 Plastic Shrinkage

The plastic shrinkage test was carried out on a slab of size 533 × 838 × 40 mm suggested by the previous study [13,14,15]. The required size of slab mold was cast with fresh-state concrete with proper tamping to expel the air voids in the fresh concrete. After that, aluminum straight-edge floating was used to remove the air voids in the concrete; this method was applied to the surface of the slabs to ensure that the water bleeding had vanished entirely. Bull floating and trowelling was used to make a smooth and dense surface, which helps to spot elimination on the slab surface. It was observed that the atmospheric temperature ranged between 27° + 2 °C and 27°–2 °C, with the relative humidity ranging between 42 and 88%, during the experiment. The cast slab is exposed to atmospheric conditions, which helps to evaporate excess accumulated surface water. Air circulation was done with the help of a pedestal fan. The fan was turned off and removed when the complete air circulation was done. The width of the plastic shrinkage crack was observed using a handheld microscope with an ocular magnification of 40 × and a sensitivity of 0.01 mm. Then, the length of the shrinkage crack was observed with the help of scale and thread. This procedure was done as many cracks formed in the slab as possible. The average crack width was multiplied by its length; the average crack area was observed. The crack measurement procedures are displayed (Figs. 3, 4, 5, 6, 7, 8, 9).

Fig. 10

SEM image of SCCC

Fig. 11

SEM image of SCCSC

Fig. 12

SEM image of SCCSCM25

Fig. 13

SEM image of SCCSCM50

Fig. 14

SEM image of SCCSCM75

Fig. 15

SEM image of SCCSCM100

4 Results and Discussion

4.1 Slump Flow

The test findings of slump flow for all the mix proportions are displayed in Table 2. The slump flow diameter for all the mixes ranged between 702 and 748 mm. As per EFNARC guidelines, these slump flow values satisfied the SCC flow requirements and fall under SF2 slump flow class (660–750 mm). The maximum flowing ability of the slump was achieved for the mix SCCSCM100. This enhancement of flow properties happened due to replacing of M-sand and the use of superplasticizer (SP) in SCCSC.

Table 2 Rheological properties of different mixes used

4.2 T500 Test

The T500 time experiment was also performed to evaluate the filling ability of SCC. The test outcomes of SCCSC with varying combinations of M-sand are depicted in Table 2. The T500 time flow of all the compound ranged from 6.45 s to 4.92 s which falls under VS2 class according to EFNARC. The replacement of M-sand in SCCSC increased the flow diameter steadily.

4.3 V-Funnel Test

V-funnel experiment also identifies the viscosity and filling capacity of the SCC. Table 2 shows the V-funnel test outcomes. According to EFNARC guidelines, the V-funnel time of all mixtures was within the VS2 class (8–12 s) which satisfied the flow requirements of SCC. The mix SCCSCM 100 exhibits deferred flow time when compared to SCCSC mixes. It is possible that this is due to the SP dosage and the addition of M-sand to composite, both of which rise the flowability of SCC.

4.4 L-Box Test

The L-box experiment evaluated the passing capacity of all the mixes. Table 2 presents the experiment outcomes of all the mixes. EFNARC guidelines recommend that the ability to pass value be equal to or greater than 0.8 to ensure that the SCC will not be blocked during the placement process. The L-box passing capacity outcomes for all mixes varied from 0.88 to 0.96, which is fell within the limits. The test findings indicated that the mix SCCSCM100 achieved more viscous at higher levels of M-sand substitution.

4.5 GTM Screen Stability Test

The GTM screen stability experiment was employed to assess the mixture stability. Table 2 shows the test findings of all the mixes. The outcome demonstrated 100% M-sand replacement enhanced the stability and segregation resistance (less than 15%) of all the mixes. The developed mixes can be concluded from all of these that they satisfy and confirm the requirements of say, SCC.

4.6 Plastic Shrinkage

The plastic shrinkage experiment was performed to observe the width, length of the crack, area of the crack, and the number of cracks and crack area percentage. The experimental test results of all the mixes blended with various proportions of M-sand replacement were displayed in Table 3 and depicted in Fig. 9. The formation of cracks number and crack area identified in SCCSCM 100 was 7 and 68.27 mm², which is 22.22 and 9.34% lower than SCCSC. According to test results, 100% replacement of M-sand instead of natural sand and 100% replacement of CS in place of conventional coarse aggregate reduced the total shrinkage crack area. It has been observed that the CS fibrous nature was controlled and prevented the development of shrinkage cracks. The same declining trend was followed in previous studies [13,14,15].

Table 3 Plastic shrinkage test

4.7 Scanning Electron Microscope (SEM) Analysis

The SEM picture of all the mixture blended with various combinations of M-sand is displayed in Figs. 10, 11, 12, 13, 14 and 15. The internal microstructure of all the mixes was analyzed and compared with SCCSC. The captured SEM images visualized the formation and distribution of hydration products of the mix. The SEM images visualized the development of calcium silica hydrated (C–S–H) gel, portlandite (Ca (OH)2), quartz (SiO2), and ettringite. When M-sand is used as a 100% substitution for natural sand in concrete production, SEM images demonstrate that the hydration products are consistently finer and more quietly packed, resulting in denser concrete with greater strength. These formations of elements were one of the reasons for the strength performance of the mix. The incorporation of M-sand in place of river sand has enhanced strength and altered the distribution of minerals.

4.8 X-Ray Diffraction (XRD)

The XRD analysis of all the mixes with various combinations of M-sand in SCCSC is shown in Figs. 16, 17, 18, 19, 20 and 21. The XRD patterns analysis was taken to detect the crystalline phase of the mixture. The crystalline phases identified in the mixes are calcium silica hydrated (C–S–H) gel, quartz (SiO2), portlandite (Ca (OH)2), calcite (CaCO3), and ettringite. The C–S–Hand SiO2 are the major crystalline phases in all the mixtures.

Fig. 16

XRD image of SCCC

Fig. 17

XRD image of SCCSC

Fig. 18

XRD image of SCCSCM25

Fig. 19

XRD image of SCCSCM50

Fig. 20

XRD image of SCCSCM75

Fig. 21

XRD image of SCCSCM100

5 Conclusions

This study examined the effects of M-sand on self-compacting concrete. Based on the findings of the study, it is possible to draw the following specific conclusions:

• 1. Using coconut shell and M-sand with superplasticizer in SCC enhances the rheological properties significantly when the replacement rate increases.

• 2. According to the results of the plastic shrinkage tests, the overall crack area decreases as the proportion of M-sand raises. It can be advised to substitute M-sand for river sand to lower the plastic shrinkage crack area compared to conventional concrete because the usage of M-sand in SCCSC reduces shrinkage overall.

• 3. The SEM micrograph confirms that CS and M-sand impact the microstructure of mixes, which visualizes the formation of hydration products. Adding M-sand improves the integrity of SCC mixes by densifying the pore structure.

• 4. XRD phase analysis pattern identifies the crystalline phases formed in SCC mixes. Calcium silica hydrated (C–S–H) gel, quartz (SiO2) is the prominent peak identified in all the mixes.

Further investigations are being conducted to evaluate its durability and structural element behavior for its appropriateness in practical implementations.