Internal curing of concrete using lightweight aggregates

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Internal curing of concrete using lightweight aggregates Amin K. Akhnoukh To cite this article: Amin K. Akhnoukh (2018) Internal curing of concrete using lightweight aggregates, Particulate Science and Technology, 36:3, 362-367, DOI: 10.1080/02726351.2016.1256360 To link to this article: https://doi.org/10.1080/02726351.2016.1256360

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PARTICULATE SCIENCE AND TECHNOLOGY 2018, VOL. 36, NO. 3, 362–367 http://dx.doi.org/10.1080/02726351.2016.1256360

Internal curing of concrete using lightweight aggregates Amin K. Akhnoukh Construction Management & Civil and Construction Engineering Department, University of Arkansas, Little Rock, AR, USA ABSTRACT

KEYWORDS

Internal cured concrete (ICC) has been recently used in the local and international construction markets. ICC contains surplus amount of water to compensate the shrinkage of the mix and the volumetric changes which result in early-age cracking of concrete. Concrete cracking is a direct result of the shrinkage of the water–cement paste during early stages of the hydration process and continues for a significant amount of time during the life span of the concrete section. Early-stage shrinkage, prior to the concrete hardening, is associated with volumetric changes, until final setting is achieved. Afterward, the reduction in cement paste particle size results in increased voids within the concrete structure. These voids result in increased permeability, additional sulfate and chloride attacks on steel reinforcement, and internal tensile stresses in concrete, which result in significant cracking. ICC uses the additional water added to the mix in counteracting the reduced volume of the concrete. Several techniques are used for internal curing (IC). In this research, water-saturated lightweight aggregates (LWAs) are used in partial replacement of normal weight aggregate as a source of additional water. LWA is submerged in water prior to concrete mixing to absorb a significant amount of water, which is stored within the LWA particles. Once added to the mix, the water is gradually desorbed and compensates the water losses during hydration. Hence, it counteracts the shrinkage induced. Different ICC mixes are developed in this research using two different sizes of LWA, and supplementary binding materials are used to improve compressive strength. ICC compressive strength and reduced shrinkage attained are presented. ICC mixes developed in this research can be successfully used in pouring highway segments and bridge decks with lower cracks and reduced life cycle cost due to reduced maintenance.

Cracking; curing; expansion; internal cured; shrinkage

Introduction Rotary kiln lightweight aggregates (LWAs) have been used for several decades in the production of different types of lightweight concrete. LWAs are available in nature in a random particle structure as a result of volcanic emissions. The “as-exist” shape of LWAs makes it impossible for use in any concrete mix design. To date, expanded shale, expanded clay, and expanded slate are manufactured by heating the raw “as-exist” lightweight volcanic emissions in rotary kilns at a temperature that exceeds 1000°C. The resulting aggregate is characterized by its high quality, strength, durability, light weight, and high absorption capacity. The expanded shale, clay, and slate produced, termed as ESCS, is increasingly used in producing lightweight structural concrete, geotechnical asphalt concrete, sustainable structures, and internal cured concrete (ICC). ESCS is shown in Figure 1. For almost a century, ESCS has been used in lightweight wall construction, high-performance marine platforms, and lightweight bridge decks. Currently, the research projects focus on using the ability of LWAs in absorbing, retaining, and desorbing water to provide concrete mixes with a post-hardened water supply. This process is known as internal curing (IC) of concrete and results in a reduced volumetric changes and/or reduced tendency to cracking at early age. ICC, also known as self-cured concrete, is increasingly used in the United States

construction market in pouring highway and lightweight bridge decks, with longer life span expectancy due to minimized shrinkage and lower reinforcing steel corrosion.

Background and literature review The ability to use LWAs as an IC agent for concrete mixing was advantageous in modern construction projects due to the increased demand for high-performance concrete (HPC). In HPC mixes, low water–cement ratios are used to increase the mix strength. Low water content results in increased shrinkage and early cracking (Shah, Weiss, and Wang 1998; Villareal 2006). Different approaches were used to mitigate the effect of concrete shrinkage including the use of expansive admixtures and/or shrinkage compensating admixtures (Collepardi et al. 2005), or through IC of concrete (ACI-231 2010). The behavior and role of IC agents (LWA) was reasonably explained in the early 1990s (Philleo 1991). LWA was pre-wet and incorporated in the concrete mix. It was assumed that the water absorbed within the pre-wet LWA will start to desorb into the surrounding cement paste after 24 h of mixing, mainly due to the decrease of the paste water content, which is consumed during the hydration process. LWA efficiency in concrete IC is dependent on several parameters: (1) the amount of the LWA incorporated in the mix, (2) the

CONTACT Amin K. Akhnoukh [email protected] University of Arkansas at Little Rock, 2801 South University Ave., Little Rock, AR 72204, USA. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/upst. © 2017 Taylor & Francis

PARTICULATE SCIENCE AND TECHNOLOGY

Figure 1. ESCS size and density compared to normal weight aggregates (ACI-231 2010 Expanded Shale, Clay, and Slate Institute).

degree of saturation of LWA, and (3) the distribution of LWA particles within the mix. Since the LWA acts as the IC agent, the even distribution of the LWA particles substantially improves the IC efficiency. Similarly, increased degree of saturation of the LWA improves the IC efficiency as it allows for additional water to desorb into the dehydrating paste. In a recent study, the degree of saturation of the LWA introduced to the mix was studied, and its effect on the IC concrete final properties was quantified (Golias, Castro, and Weiss 2012). In current practices, the IC concrete produced in research laboratories incorporates pre-wet aggregates that were submerged in water for 24 h prior to the mix. On the contrary, IC concrete produced and supplied by batch plants incorporate LWA that was sprayed with water at the time of batching. Sprayed LWA will provide the concrete mix with IC water; however, due to the lesser degree of saturation, IC process using sprayed aggregates are not as efficient as laboratory IC concrete. Recent research projects focused on evaluating the effect of LWA porosity on the mass of produced concrete and the final compressive strength of concrete mix. In a relevant research, LWA was used to replace 25% of the total aggregate volume. LWA used had a dry density of 88.6 pcf and a porosity of 50%, and the moisture content of submerged aggregates was 20% by mass. The produced concrete was cured using different temperatures and different relative humidity chambers. The research concluded that the lightweight concrete produced had properties that were insensitive to the external curing regimen due to the continuous effect of IC performed by water-saturated LWA (Weber and Reinhardt 1997). In a different research, saturated LWA was used in partial replacement of normal weight aggregate. The percentages of aggregate replacement were 10%, 17.5%, and 25% by volume. The research concluded that a properly soaked LWA will have no negative effect on the final compressive strength of the concrete mix (Van Breugel and De Vries 2000). In Saudi Arabia, researchers produced lightweight concrete using Pozzolanic scoria to counter the effect of high temperature and the potential internal cracking due to hot weather. In this research, LWAs were submerged in water for 24 h to allow most of

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the pores to become water-filled. Prior to batching, the LWA was spread on burlap outside the laboratory for 30 min to attain a saturated surface dry (SSD) condition. Cubic specimens of dimension 6 inch were poured to be tested at 28 d. One half of the specimens was cured in water and the other half was cured in air under a relative humidity of 43%. Test results showed that dry-cured specimen performance was similar to wet-cured specimens when pre-wetted coarse aggregate was used. The researchers attributed the result to the continuous IC performed by the wetted LWA (Van Breugel and De Vries 2000). The effect of the LWA type on the reduction of autogenous shrinkage was investigated in Japan. Three different LWAs were selected. First aggregates were a crushed coated coarse aggregate made from expanded shale, and the other two aggregates, denoted as HLA1 and HLA2, are pelletized and coated, made from finely ground perlite powder. The main objectives of this research paper are to: (1) develop IC concrete mixes using different sizes of LWA as internal agent for curing and (2) determine the effect of pre-wetting procedure and duration on the mechanical properties of the produced concrete.

Materials The mix design is accomplished by using local construction materials available in the local construction market in the State of Arkansas. Type I/II portland cement will be used as the sole binding material (specific gravity ¼ 3.15), limestone with maximum size of 0.25 inch (6.5 mm) will be used as normal weight coarse aggregate, and normal weight fine sand will be used as fine aggregate. Both limestone and fine sand gradation conforms with the designated ASTM specification (Shah, Weiss, and Wang 1998). Coarse LWA was used. The LWA used is expanded clay, with a specific gravity ¼ 1.25 and water absorption rate ranging from 15% to 20%. In order to maintain the coarse LWA in a SSD condition, the LWA was presoaked for several hours and left in room temperature for a total duration of 2 h until surface moisture was drained. A presoaked LWA is required to mix the sand–lightweight concrete mix. Type I/II Portland cement was used in developing the lightweight concrete mixes in this research, and the chemical composition of Portland cement are shown in Table 1. The coarse aggregate used in this research project was crushed limestone, obtained from local quarries in the State of Arkansas. The coarse aggregate selected for mix proportioning is in compliance with the requirements of ASTM C136-06, as shown in Table 2. The coarse aggregate used had a specific gravity of 2.68 and an absorption capacity of 0.4%. The fine aggregate used in producing the concrete mixes to be investigated was obtained from local suppliers at the State of Arkansas. The sand used had a normal weight and complies to AASHTO T 27 requirements as specified by the Table 1. C3S 60.3%

Portland cement chemical composition. C2S C3A C4AF Free CaO SO3

MgO

Blaine fineness

18.2%

1.3%

351 (m2/kg)

5.4%

11.3%

0.9%

2.6%

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Table 2. Limestone gradation according to sieve analysis. Sieve size AHTD specification, coarse aggregate % passing 1.2500 1.000 0.7500 0.500 0.37500 #4 #8

100 60–100 35–75 – 10–30 0–5 –

Table 3. Fine aggregate gradation. Sieve size AHTD specification, fine aggregate % passing 0.37500 #4 #8 #16 #30 #50 #100

100 95–100 70–95 45–85 20–65 5–30 0–5

Table 4. Lightweight coarse aggregate. Coarse aggregate Absorption capacity (Percent) Expanded clay Expanded shale

15.0 12.9

Specific gravity 1.25 1.41

Arkansas Highway and Transportation Department in State Highway Construction projects. Sand gradation is shown in Table 3. Two different types of coarse LWAs were used for producing lightweight concrete mixes. The LWAs used were expanded clay and expanded shale. Expanded clay was produced by a local provider in the State of Arkansas, while expanded shale was imported from the State of Kansas. The lightweight coarse aggregate properties are as shown in Table 4. Expanded clay and shale are shown in Figure 2.

Experimental work Concrete mixing LWA was presoaked in water for a 24-h period to fill the voids of the expanded clay. LWA saturation is necessary to provide

Figure 2. Expanded clay (left) versus expanded shale (right).

water to the cement paste after the hydration process starts. The presoaked aggregates, once saturated, act as an IC agent that desorbs water to the cement paste once the water in the paste is consumed through the hydration reaction. The LWA water starts to desorb after final setting is achieved and compensates the autogenous shrinkage developed due to the shrinking volume of the hydrating paste. To quantify the effect of different amounts of LWA, a control mix was poured using normal weight aggregates. The same mix was redesigned using stepwise replacement of course aggregates (limestone) with coarse LWAs. Three different amounts of coarse LWAs were tried ranging from 100 lbs (42 kgm) to 300 lbs (146 kgm). Different mix designs, using expanded shale LWAs, are shown in Table 5. Mixes with expanded shale included a replacement of coarse aggregate with percentages of 10%, 20%, and 30% respectively. A different set of mixes were poured using expanded clay as a LWA. Expanded clay LWA was used in partial replacement of coarse weight aggregate at percentages of 12.5%, 25%, 37.5%, and 50%, respectively. The expanded clay mixes were designed to attain self-consolidating concrete properties (spread diameter greater than 20 inch). Mix proportions are as shown in Table 6. The control mixes and lightweight mixes were batched using a high-speed paddle mixer similar to the mixers used in current industry practices. High-speed paddle mixers are selected due to their capability of producing concrete mixes with shorter mixing duration to avoid water dissipation due to high temperature. High-speed paddle mixer used in batching concrete is shown in Figure 3.

Experimental investigation Concrete mixes are produced in batch sizes of 3 cubic foot to investigate the different concrete properties and compare the experimental results of lightweight concrete mixes with the control mix. Experimental investigation included the measurement of mix density, flowing ability (slump and spread), compressive strength, and modulus of rupture. The results of concrete mechanical properties are shown in the following section.


Table 5. Mix designs (control mix and LWA mix with expanded shale) (ASTM C33 2002). IC1 IC2 IC3 lbs (kg) lbs (kg) lbs (kg) Control lbs (kg) (10% LWA) (20% LWA) (30% LWA) Cement Limestone LWA Sand Water W/CM

611 1750 0 1400 270 0.44

611 1600 100 1350 270 0.44

611 1500 200 1265 270 0.44

611 1400 300 1180 270 0.44

Table 6. Mix designs (control mix and LWA mix with expanded clay) (ASTM C33 2002). Unit weight per unit volume Mixture Control Clay1 (12.5%) Clay2 (25%) Clay3 (37.5%) Clay4 (50%)

Water

Cement

Coarse aggregate

LWA

Sand

w/c

325 325 325 325 325

570 570 570 570 570

1100 960 825 690 550

0 65 130 195 255

2000 2000 2000 2000 2000

0.57 0.57 0.57 0.57 0.57

Figure 3. High speed paddle mixer.

Density of produced concrete The density of fresh mixed concrete was measured to quantify the reduction of concrete weight associated with the incorporation of saturated LWAs. The reduction in concrete weight is advantageous in construction due to the reduced dead loads of structural sections. The concrete unit weight for the control mix as compared to different types and percentages of LWA mixes is shown in Figure 4. Slump of concrete mixes Due to its higher absorption capacity, LWAs result in dry concrete mixes. A special consideration is required to proportion LWAs. In addition, presoaking of aggregates is required to avoid moisture absorption and to ensure that LWA used is in a SSD condition. Slump test was performed to mixes with expanded shale lightweight mixes and spread test was performed for the expanded clay mixes as they were designed as self-consolidating concrete mixes. Slump and spread tests performed showed an average slump of 3 inches for expanded shale mixes and a spread diameter of 22 inches of selfconsolidating expanded clay mixes. The use of LWAs did not alter the predesigned slump of the mix due to the incorporation of the aggregate in a presoaked SSD condition. It was noticed that lightweight concrete mixes were homogenous and no indication for visual segregation was noticed, as seen in Figure 5. Compressive strength of concrete Compressive strength tests were performed on concrete specimens poured on 4 inch × 8 inch cylinders. An average of three cylinders were tested to failure at the age of 1, 7, and 28 d.

Figure 4. Unit weight of concrete mixes when expanded shale and clay are used as lightweight aggregate.

Figure 5. Slump test of lightweight expanded clay mixes.

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Figure 6. Compressive strength test results for expanded shale and expanded clay lightweight aggregate mixes.

Figure 7. Steel molds for concrete shrinkage measurements.

Figure 8. Length comparator with digital indicator.

LWA mixes displayed a relatively lower compressive strength at different ages as compared to the control specimen poured using regular coarse aggregate. The lowered compressive strength did not violate the minimum strength required for highway construction projects of 3000 psi. The strength reduction of mixes produced using expanded shale is lower than the strength reduction in expanded clay mix. This is attributed to the lower water–cement ratio used for expanded clay mix designs. Compressive strength specimens were cured by submergence in water tub at a curing temperature of 22°C. Specimens were left in curing tubs until compressive strength test days at ages of 1, 3, 7, 14, and 28 d. The results of compressive strength tests are shown in Figure 6. The shown figures present the average of three specimens tested for compressive strength at each age. Due to the nature of concrete mixes, compressive strength tests conducted at each age showed a standard deviation of 60 psi. Concrete shrinkage Shrinkage was tested according to ASTM C 157. Four 4 × 4 × 10 inch steel molds were cast for each mixture, and gage studs were embedded into each end to the concrete specimens, as shown in Figure 7. The inside surface of the steel molds was oiled to facilitate demolding. After casting, the molds holding the fresh concrete were immediately placed in an environmental chamber with a relative humidity of 50 � 4% at a temperature of 73 � 3°F. The first measurement was recorded at 23 � 0.5 h, then shrinkage was recorded at 1, 7, 14, and 28 d for each of the four prisms cast with each experimental mixture. The linear shrinkage of the specimens

Figure 9. Strain measured for LWA specimens poured using expanded clay versus control specimens.


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Figure 10. Strain measured for LWA specimens poured using expanded shale versus control specimens (Weber and Reinhardt 1997).

was measured using a length comparator with a digital indicator accurate to 1/10,000 of an inch, as shown in Figure 8. The shrinkage test results indicated that control mix specimens displayed a higher strain compared to lightweight concrete mixes. The reduction in measured strain was directly correlated to the amount of LWAs used in the mix. Results, shown in Figures 9 and 10, indicate that LWAs can be successfully used as an IC agent to reduce concrete strain due to drying shrinkage. The recorded micro-strain represents an average of three readings at each age of the concrete specimen. Micro-strain readings had an average deviation of 3% compared to the recorded mean value.

Conclusion The results of the experimental investigation showed that different types of lightweight coarse aggregates can be successfully used in producing lightweight concrete mixes, without a substantial reduction in concrete slump, spread, and/or compressive strength. LWAs can be successfully used in replacing up to 50% of normal weight coarse aggregates. The advantages of using LWAs include the production of concrete mixes with lower weight for a reduced dead load of structures, in addition to the positive impact of presoaked LWAs in reducing the shrinkage and subsequent cracking due to the LWA capability of acting as an IC agent to the hardened concrete mix.

Acknowledgments The author would like to acknowledge the effort of the graduate students Mohammed Al-Mansouri and Magued Hanna for their help with the experimental phase of the project. The author also appreciates the kind

support of Professor W. Micah Hale for his helpful technical advice during the project steps.

References ACI-231. 2010. Report on early-age cracking: Causes, measurement and mitigation. Farmington Hills, MI: American Concrete Institute. ASTM C33. 2002. Standard specifications for concrete aggregates. Annual Book for ASTM Standards, American Society for Testing and Materials. Collepardi, M., A. Borsoi, S. Collepardi, J. Olagot, and R. Troli. 2005. Effects of shrinkage reducing admixture in shrinkage compensating concrete under non-wet curing conditions. Elsevier Journal of Cement and Concrete Composites 27 (6):704–708. doi:10.1016/j.cemconcomp. 2004.09.020 Expanded Shale, Clay, and Slate Institute. http://www.escsi.org/Content Page.aspx?id=53. Golias, M., J. Castro, and J. Weiss. 2012. The influence of the initial moisture content of lightweight aggregate on internal curing. Elsevier Construction and Building Materials Journal 35:52–62. doi:10.1016/j. conbuildmat.2012.02.074 Philleo, R. 1991. Concrete science and reality. In Materials science of concrete II 1–8, ed. J. Skalny and S. Mindess. Westerville, OH: American Ceramic Society. Shah, S., J. Weiss, and W. Wang. 1998. Shrinkage cracking – Can it be prevented? Concrete International 4 (20):51–55. Van Breugel, K., and H. De Vries. 2000. Potential of mixture with blended aggregates for reducing autogenous deformation in low water-cement ratio concrete. Proceedings of the International Symposium on Structural Light Weight Aggregate Concrete:463–472, Norway. Villareal, V. 2006. Internal curing – Real world ready mix production and applications: a practical approach to lightweight modified concrete. ACI Special Publication SP-256:45–56. Weber, S., and H. W. Reinhardt. 1997. A new generation of high performance concrete: Concrete with autogenous curing. Advanced Cement Based Materials 6:59–68. doi:10.1016/s1065-7355(97)00009-6