Sustainable Concrete in the Construction Industry of Kurdistan-Iraq through Self-Curing

Abstract

The improper curing of concrete can seriously affect its hardened properties. However, a large quantity of water is required to cure concrete after casting. Water is a valuable resource and its availability is posing a particular challenge in the Middle East including the Kurdistan region of Iraq. Self-curing concrete may be considered a novel curing method in that the water inside the concrete mix is retained so that hydration can continue without the supply of additional water after casting. Therefore, the aim of this study was to include a self-curing agent, named Polyethylene glycol-400 (PEG-400), as one of the concrete mix constituents in order to save water that is normally required after casting. Six concrete mixes were cast with a constant W/C ratio of 0.5; two of them were ordinary concrete mixes whereas the other mixes contained 0.5%, 1%, 1.5%, and 2% of PEG-400 by weight of cement. All concrete ingredients, except the PEG-400, were provided locally. Three different curing regimes were employed: air curing under ambient laboratory conditions, water curing, and self-curing using different dosages of PEG-400. Testing included compressive strength, ultrasonic pulse velocity (UPV), and water absorption. The results showed that 1% of PEG-400 is the optimum dosage to be used for self-cured concrete.

1. Introduction

Globally, concrete is the most commonly utilised civil engineering material. However, to attain the desired and specific engineering properties, concrete needs to be cured properly by retaining the water inside the concrete mix for hydration to take place [1,2].

There are numerous methods for curing concrete but the most widespread curing type in Kurdistan-Iraq is water curing. There are three basic approaches to concrete moisture control to consider; the first one is adding water to the surface, the second one is preventing moisture loss, and the final approach is providing water internally or by internal-curing [3]. In the internal-curing or self-curing method, the hydration process happens due to the availability of extra internal water retention that is not part of the integrated water. Normal curing occurs from the outside to the inside. However, internal curing allows for curing from within to without via inner reservoirs [4,5]. Currently, concrete plants utilise two techniques to produce self-curing concrete. The first is the use of a natural and/or artificial lightweight aggregate (LWA) and concrete will become a lightweight aggregate concrete (LWAC). LWA has a high and rapid water absorption rate due to the use of saturated porous pours to provide internal water retention that can be replaced with the water used by chemical shrinkage during the hydration of cement [6,7]. The second technique is the use of an admixture as a self-curing agent such as chemical polyethylene glycol in the concrete. The purpose of such an admixture is to decrease the evaporation rate of water and increase the concrete internal water retention capacity as reported by researchers earlier [8,9,10].

The use of self-curing concrete in the construction industry is of extreme importance as freshwater resources are limited not only in Kurdistan-Iraq but in the whole of planet Earth. Developing self-curing concrete is very important in order to save water in the environment, as water usage by the construction industry has recently been the focus of much research; only 1% of the Earth’s clean water is available to us (USA-EPA). In the construction industry, 350 litres of water can be consumed for each 1 m² of a masonry wall, approximately. This amount of water is used during the pre-wetting materials (e.g., bricks), mixing and re-mixing of mortar, washing masonry tools, curing of walls, etc. [1,11]. It has also been reported that 1 cubic meter of normal concrete requires 3 cubic meters of water from the mixing stage to the curing process. Therefore, self-curing is a type of curing that can be used to save water in the construction industry and make construction economic and more environmentally friendly [1,11].

The aim of this study is to produce self-curing concrete in order to contribute to sustainable development in Kurdistan-Iraq. The specific objectives are to evaluate the effect of PEG-400 as a self-curing agent on concrete engineering properties. Although there is similar work in the literature, the main aim of this study was to encourage the local government, concrete production plants, and construction companies to produce and use self-curing concrete, incorporating local resources in their projects in order to reduce the quantity of water used in the construction industry, thus contributing to sustainable development (i.e., saving water) in Kurdistan region. Producing self-curing concrete using local materials would play a significant role in encouraging the construction industry to use it in different projects. The outcome of this research would be communicated to construction professionals (e.g., concrete producers, engineers, and local government officials) in order to obtain concrete with adequate performance while saving large quantities of water. This research will also be beneficial to other areas in the world where there is an acute shortage of fresh water. Due to the current effects of climate change and global warming, water resources are getting extremely valuable globally and more so in Middle Eastern countries including the Kurdistan region of Iraq.

2. Experimental Methodology

2.1. Materials

Portland cement Type I produced by the Tasluja Company [12] located in Selemany, Kurdistan, Iraq, which complied with Iraqi Standard IQ. S 5:1984 Type I [13] and EN 197-1:2011 grade 42.5 R [14], was used for the present investigation of its physical and chemical characteristics presented in

Table 1. Physical properties of Portland cement [13,14].

Test	Requirement	Result
Fineness Specific Surface (m2/kg)	260 Minimum	317
Setting Time (Minutes)
Initial	45 Minimum	190
Final	375 Maximum	255
Soundness Autoclave Expansion (%)	0.8 Maximum	0.04
Compressive Strength 50 mm Mortar cubes (MPa)
3 days	12 Minimum	19
7 days	19 Minimum	24
28 days	Not Applicable	34.1

and

Table 2. Chemical characteristics of Portland cement using XRF technique [13,14].

Composition	Requirement	(%)
SiO2	No limit	21
IR	5% Max	0.28
Al2O3	No limit	5
Fe2O3	No limit	3.8
CaO	No limit	63.4
MgO	6% Max	2.3
SO3	3.5% Max	2.5
Na2O	No limit	0.22
K2O	No limit	0.5
LOI	3% Max	1.6
C3S	No limit	45.2
C2S	No limit	26
C3A	No limit	7
C4AF	No limit	11.4

IR—Insoluble Residue; XRF—X-ray Fluorescence.

.

The coarse aggregate used was crushed rock with a maximum size of 20 mm and the fine aggregate was river bed sand with particles passing through 5 mm, both available in the Kurdistan-Iraq region.

Table 3. Physical properties of fine and coarse aggregates.

Physical Properties	Values
Specific Gravity	2.79
Water Absorption	1.3%

presents the physical properties of aggregates whereas Figure 1 and Figure 2 show the particle size distribution of fine and coarse aggregates, respectively. The aggregates conform to BS EN 933-1:1997 [15]. The water used for mixing and curing was potable water. The self-curing agent used was poly-ethylene glycol PEG-400 and its properties are presented in

Table 4. Properties of PEG-400 (Source: alpha-chemika, 2019).

Description	Properties
Density (g/cc)	1.128
Specific gravity	1.2
Molecular weight	400
Appearance	Clear Fluid

.

2.2. Mix Proportions

The proportions (by weight) and the water-to-cement ratio (W/C) used in this experimental study for all mixes were 1 (cement): 2 (fine aggregate): 4 (coarse aggregate) and 0.5, respectively. The mix design was based on the absolute volume method. Five mixes were cast for this investigation. Mix 1 is the control mix without the use of PEG-400. In mixes 2 to 5, 0%, 0.5%, 1%, 1.5%, and 2% of PEG-400 was added, respectively. Three cube specimens of 100 mm in size were prepared. Each cube was cast in two layers and each layer is compacted 25 times using a steel rod. After casting, specimens were kept under laboratory conditions and covered by plastic sheets for 24 h. After this period the specimens were demoulded and placed in air at ambient laboratory temperature (20 ± 2 °C) and relative humidity (60 ± 10%). Mixes with PEG-400 were referred to as self-cured concrete although they are cured in air at ambient temperature. For the control mix (Mix 1), some specimens were placed in water. The compressive strength and ultrasonic pulse velocity (UPV) tests at 7 days and 28 days were conducted. The total water absorption test was conducted at 28 days only.

Table 5. Details of concrete mixes.

Mix No.	Quantities (kg/m3)
	PC	FA	CA	Water	PEG-400 *	W/C	Curing Code	Curing Condition
1	362	724	1448	181	0.0	0.5	Water-0.0	Water-Curing
							Air-0.0	Air-Curing
2	362	724	1448	181	0.5	0.5	Air-0.5	Self-Curing
3	362	724	1448	181	1.0	0.5	Air-1.0	Self-Curing
4	362	724	1448	181	1.5	0.5	Air-1.5	Self-Curing
5	362	724	1448	181	2.0	0.5	Air-2.0	Self-Curing

* % (by weight of cement).

illustrates the mix proportioning of concrete and the curing method of the present study.

2.3. Testing Methods

2.3.1. Workability

The workability was assessed using the slump test. The test procedure was carried out according to the BS EN 12350-2 [16].

2.3.2. Compressive Strength

This test was carried out on cubes of 100 mm in size at 7 days and 28 days of age under different curing conditions of air, water, and by self-curing according to BS EN 12390-3:2009 [17]. A compression testing machine of 2000 KN capacity as shown in Figure 3 was used [18] with a loading rate of 0.5 MPa/s. Three specimens were tested at each curing age and condition. Further details about the compressive strength test are given elsewhere [19,20].

2.3.3. Ultrasonic Pulse Velocity (UPV)

The UPV test was conducted on the concrete-hardened state at different ages of 7 and 28 days according to BS EN 12504-4:2004 [21]. The UPV test was carried out by measuring the time taken by ultrasonic pulse to get through the specimens (Figure 4). It was conducted in order to determine the quality of the concrete so that a correlation could be made with compressive strength. Velocity criteria for engineering quality of concrete grading are shown in

Table 6. Velocity criteria for concrete quality grading [2].

UPV (Km/s)	Quality
>4.5	Excellent
3.5–4.5	Good
3–3.5	Medium
<3	Doubtful

. This test was also conducted on cubes of 100 mm in size. Two readings were taken from each cube and three cubes were tested at each age, so each UPV value is the average of six readings.

2.3.4. Water Absorption

The water absorption (WA) test was conducted on cubes of 100 mm in size at 28 days of curing. The test procedure for all mixes was the same. At 28 days, the specimens were placed in an oven at 90 °C until a constant dry mass was achieved (W₂). This process took 48 h. After cooling, the specimens were totally submerged in water for 24 h. After this time the specimens were taken out from the water tank and wiped with a cloth and their saturated surface dry (SSD) condition (W₁) was determined. Water absorption is expressed as percentage difference between total wet mass (W₁) and dry mass (W₂). Further details about water absorption test of concrete are given elsewhere [22,23,24]. Equation (1) was used for the WA as follows:

$$\mathrm{WA}(\%)=\frac{{\mathrm{W}}_1-{\mathrm{W}}_2}{{\mathrm{W}}_2}\times 100$$

(1)

3. Results and Discussion

3.1. Workability

The slump test results are presented in Figure 5. The control mix had a slump of 25 mm which was the lowest of all mixes. As the percentage of PEG-400 in concrete increased the slump was found to increase. The maximum dosage of PEG-400 in concrete was 2% where a slump of 52 mm was obtained. This slump is more than twice that obtained for the control mix (M1).

3.2. Compressive Strength

The compressive strength values of the concrete mixes are shown in Figure 6. According to the results obtained, all mixes with PEG-400 had higher compressive strength than the control exposed to air curing. Increasing the PEG-400 from 0% to 1% caused an increase in compressive strength compared with the control. However, it is interesting to see that the further increase in PEG-400 content beyond 1% resulted in a reduction in compressive strength at both curing ages of 7 and 28 days. The increase in compressive strength for 1% PEG-400 at 7 and 28 days of age was 7.4% and 15.3%, respectively, compared with control concrete subjected to air-curing. Therefore, the optimum percentage of PEG-400 in concrete for 7 and 28 days of age was 1% where a maximum compressive strength was obtained. The lowest compressive strength at 28 days was observed for the control concrete subjected to air curing which is likely to be due to the incomplete process of hydration caused by the non-availability of water. Unlike the results reported earlier [1], in the present study and at 7 and 28 days of curing the compressive strength of water-cured concrete was higher than those of other mixes except for the concrete with 1% PEG-400. This is an indication of the efficiency of PEG-400 admixtures at a 1% dosage in keeping the water inside the concrete so hydration continues, thereby causing an increase in strength. At 28 days of age, the compressive strength of concretes containing different percentages of PEG-400 ranged from 32.46 to 37.42 MPa. It is worth mentioning that the ACI 318 Standard [25] indicates a minimum specified compressive strength of 14 and 17 MPa for filling purposes and structural concrete. The majority of studies reported that the maximum compressive strength can be achieved at a dosage of 1% PEG-400 [1,5,26,27,28]. They also noticed that if a dosage of PEG was higher than 1% the compressive strength was slightly reduced. However, it was observed that by adding 1.5% of PEG to concrete, the compressive strength will reach its maximum value for M-25 concrete [29]. Another study conducted a few years ago showed that the optimum dosage of PEG-400 was 0.5% for M-40 concrete [30]. The relative compressive strength of mixes according to control mixes of air and water curing at 28 days of age are shown in Figure 7. It can be clearly seen that the concrete containing 1% PEG-400 has shown the highest relative strength of 115.3% and 104.1% under air and water curing conditions, respectively. Self-curing compounds such as PEG-400 consist of polymer resulting in the formation of a hydrogen bond with the water molecules that reduces the rate of evaporation from the concrete surface [26,27]. Therefore, the increase in the compressive strength in mixes containing PEG-400 compared to the control mix exposed to air curing is due to the availability of water which allows the hydration to continue. The optimum dosage of PEG-400 that can cause maximum enhancement in compression would depend upon the mix constituents. In this investigation, the optimum PEG-400 dosage was 1% (by weight of cement) where the hydrogen bond with the water molecule may be the strongest amongst other dosages. The results for higher dosages are different as reported by other studies [26,31,32].

3.3. Ultrasonic Pulse Velocity (UPV)

The UPV values for all mixes are shown in Figure 8 for specimens cured for 7 and 28 days. According to the results obtained, the UPV increased as the percentage of PEG-400 increased up to 1%, then decreased when the percentage exceeded 1%. However, it was reported that the maximum UPV was obtained by a dosage of 0.5% [33]. This means that as the hydration process continues, the internal structure of concrete will be densified, thus causing an increase in UPV. Similar results have been reported elsewhere [34,35,36]. In the present study, by adding 1% of PEG-400 the UPV increased by 3.8% and 5% at 7 days and 28 days, respectively, compared with control concrete under the air-curing condition. All UPV measurements ranged from 3.8 to 4 Km/s, which indicates “good” quality concrete.

3.4. Water Absorption (WA)

Figure 9 shows the WA results of concrete mixes. The obtained results indicate that the PEG-400 in concrete as a self-curing agent resulted in a reduction in the WA compared with air- and water-curing conditions. As reported in the literature [37], the reduction in the WA of concrete indicates a decrease in porosity. The application of the self-curing method using 1% PEG-400 in concrete contributed to a reduction in the water absorption level by almost 22% compared with normal concrete under air curing condition. According to the CEB assessment criteria [38], concrete with 5% WA at an initial stage of 30 min can be considered “good” quality concrete. Therefore, concrete containing %1 PEG-400 with 3.95% WA (lowest absorption) indicates “good” quality concrete.

3.5. Correlation between Different Properties

3.5.1. Correlation between Compressive Strength and UPV

The 28-day compressive strength–UPV relationship of concrete mixes is shown in Figure 10. An increase in compressive strength is associated with an increase in UPV. Further details about the strength and UPV relationship are given elsewhere [39,40].

3.5.2. Correlation between Compressive Strength and Water Absorption

Figure 11 shows that the relationship between compressive strength and water absorption is inversely proportional. If a linear relationship is fitted to the data obtained, Equation (2) is obtained as follows:

Y = −4.4907X + 55.142

(2)

where X represents the water absorption value (%) and Y represent compressive strength in (MPa). The correlation coefficient R² = 0.9942 indicates a strong linear correlation. Further details about compressive strength and water absorption are given elsewhere [41].

4. Conclusions

This experimental investigation was conducted to determine the effect of different percentages of PEG-400 (a chemical admixture that is a self-curing compound) on compressive strength, UPV, and the water absorption of concrete. Based on the experimental results, the following conclusions can be drawn:

• As the dosage of PEG-400 in concrete increases, workability is also increased. At 2% of PEG-400 addition, the slump is more than double compared with the control mix.
• The self-curing agent of PEG-400 at 1% addition yielded the highest compressive strength compared with other mixes. The 28-day compressive strength increased by 13.3% and 4.0% compared with air- and water-cured concrete, respectively.
• For UPV measurements, the observed results were found to be satisfactory for all mixes. Using 1% of PEG led to an increase in the UPV by almost 5% compared with air-cured concrete and increased by 1% compared with water-cured concrete.
• Using 1% of PEG-400 as a self-curing agent reduced the water absorption of concrete compared with air curing and water curing. Adding 1% of PEG-400 reduced the absorption by almost 22% and 6% compared with air- and water-cured concretes, respectively.
• According to the results obtained, using PEG-400 in concrete can enhance the curing process of concrete by water retention and drastically reduce the requirement for water. This technique can contribute to sustainable development in the construction industry in the Kurdistan region of Iraq.

Further mechanical and durability tests should be conducted on concrete containing the self-curing agent PEG-400. These include split tensile strength, flexural strength, modulus of elasticity, shrinkage, permeability, chloride penetration, and chemical attack.