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Article

Effects of Internal Curing on Inclusion in Prepackaged Cementitious Grout and Ultra-High Performance Concrete Materials

by
Naveen Saladi
1,*,
Igor De la Varga
2,*,
Jose F. Munoz
1,
Robert Spragg
3 and
Benjamin Graybeal
4
1
SES Group and Associates LLC, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA
2
Carbon Cure Technologies, Herndon, VA 20171, USA
3
Federal Highway Administration, 1200 New Jersey Avenue SE, Washington, DC 20590, USA
4
Federal Highway Administration, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(20), 13067; https://doi.org/10.3390/su142013067
Submission received: 13 September 2022 / Revised: 29 September 2022 / Accepted: 3 October 2022 / Published: 12 October 2022
(This article belongs to the Special Issue Use of Ultra-High-Performance Concrete to Promote Sustainability in Pavement and Bridge Construction)
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Abstract

:
Proprietary, prepackaged materials such as some cementitious grouts and ultra-high performance concretes (UHPC) are commonly used in the bridge construction industry due to their convenience and desirable properties. However, due to particularities of their mixture designs, grouts and UHPC are prone to exhibit shrinkage, thus resulting in potential durability issues. This paper describes a step-by-step methodology for including internal curing (IC) in these types of materials, with the main goal of addressing some of their shrinkage and durability issues. A brief analysis of the relative cost implications is also provided, along with a compilation of experimental results to show the effect that IC has on important material properties. The results from this study indicate that the inclusion of IC in cementitious grouts and UHPC has the potential to increase durability, leading to more sustainable bridge structures with longer service lives.

1. Introduction

As accelerated bridge construction techniques have been more frequently implemented over the last decade, the industry experienced a correlated increase in the use of proprietary, prepackaged cement-based materials for applications such as structural components, retrofits, and connections between prefabricated bridge elements [1,2,3,4]. Proprietary, prepackaged cement-based materials are commercially available preblended mixtures that consist of cement and cementitious materials along with a well-graded aggregate system that can be easily mixed with water and admixtures onsite. There are different types of commercially available, proprietary, prepackaged cement-based materials, including many classified as cementitious grouts and a few classified as ultra-high performance concrete (UHPC). While cementitious grouts have been extensively used in the construction industry, UHPC has begun to garner attention over the last decade [5,6]. This study focuses on some of the properties and challenges associated with proprietary, prepackaged cementitious grouts and UHPC.

1.1. Background

Cementitious grouts are generally a mixture of cementitious materials (e.g., cement, fly ash, slags), inert fillers, and powder chemical admixtures. According to some grout manufacturers, about 30 percent of the mass in the mixture consists of cementitious materials. The grout is typically supplied in a bag containing all of the solids, which are mixed with a certain amount of water following the manufacturer’s recommendations. They are commonly labeled as “non—shrink” materials, often under the premise that the presence of certain compounds results in the formation of expansive agents in the mixture (e.g., high alumina content to promote the expansive ettringite crystal formation), counteracting a portion of the shrinkage of the cementitious hydration reaction. Cementitious grouts are cost-effective materials that generally offer good workability properties and high early-age strengths. However, the effective water-to-cementitious materials ratio (w/cm) is high, in the range of 0.50–0.60 by mass, which leads to a highly porous, hardened material. Despite their “non-shrink” nomenclature, a previous study reported high early-age autogenous and drying shrinkage in a set of commercially available “non-shrink” cementitious grouts [7]. Apart from the high early autogenous and drying shrinkage, the presence of high porosity is expected to negatively affect the material’s durability [8].
UHPC is commonly available as a prepackaged material that contains a mixture of cementitious materials, inert fillers, liquid or powder chemical admixtures, and fibers (generally steel, but other types are also available). According to some UHPC manufacturers, almost half of the solids in the UHPC preblended powder consists of cementitious materials. The high cementitious, specialized inert filler and fiber contents make this material quite expensive. Thus, there has been interest by end users in the development of nonproprietary mixture designs using less expensive constituents [9,10]. The solid contents are mixed with an extremely low amount of water (in the range of 0.20–0.25 w/cm) and liquid admixtures to yield a material with superior strength and durability properties compared to conventional concretes [11]. However, because UHPC is designed with very low water and high cementitious contents, this type of material may develop early-age autogenous shrinkage that could cause microcracking, which would affect its long-term durability performance [12,13,14].
Given the potential shrinkage issues identified in both cementitious grout and UHPC materials, this paper investigates a potential solution. It is challenging to implement some of the most common shrinkage mitigation strategies, such as the addition of shrinkage-reducing admixtures (SRAs) or the use of coarser and lower amounts of cement, in prepackaged materials [15]. For example, certain types of SRAs negatively affect the performance of materials containing alkali-activated slag, which makes it challenging for the end user when employing proprietary grouts and UHPC [16]. Thus, internal curing (IC) is presented in this paper as an implementable solution to high shrinkage in proprietary, prepackaged cementitious grouts and UHPC materials.
IC is a proven technology that has been used in conventional concretes over the last decade not only in academic and industrial research, but also in field applications, where it has been successfully implemented [17,18,19,20,21,22,23]. As its name indicates, IC consists of curing the concrete from the inside. It is achieved by including highly absorptive, porous materials, such as lightweight aggregates (LWA), superabsorbent polymers (SAP), or rice husk ash, to name a few [24,25,26]. These materials, or IC agents, are typically incorporated in the concrete mixture in a prewetted state so that they can release the IC water at the appropriate time (i.e., after set, when a negative pressure forms in the cement matrix). The release of the IC water leads to a more efficient and homogeneous curing, thus, providing numerous benefits to the material performance. Key benefits include reduced total shrinkage, improved hydration, and reduced concrete capillary porosity [17,18].
Conventional IC mixture designs commonly entail a slight increase in the initial material cost because a more expensive IC agent replaces a less expensive, normal weight, fine aggregate. In the case of including IC in prepackaged materials, the IC agent is added “on top” of the ordinary mixture proportions of the material, resulting in an increased yielded volume. In other words, when extending a grout or UHPC material with IC agents to yield 1 m3 of material, the amount of the (more expensive) solid fraction of the grout or UHPC is reduced, thus decreasing the overall material unit cost. For example, cost savings of up to 30 percent were estimated in the UHPC with a sieved LWA mixture compared to the UHPC control. A detailed cost analysis of some of the mixtures included in this study can be found in these articles [27,28].
While some research groups have investigated the inclusion of IC in UHPC-type materials, the UHPCs used in those studies were based on open-source mixture designs where the UHPC constituents were known, and the IC mixture design was simplified [29,30]. The current study aims to include IC in proprietary, prepackaged grouts and UHPC materials, which should be useful to end users who will not develop their own mixture designs either due to lack of available materials or knowledge or a lack of desire to use an established, proprietary solution.

1.2. Objectives

The main goal of this paper is to describe an approach to include IC in a proprietary, prepackaged cementitious grout or UHPC material to overcome some of the shrinkage and durability issues, as previously mentioned. In particular, IC is expected to have a positive effect on the high drying shrinkage leading to potentially poor durability of the cementitious grout resulting from the high w/cm and high autogenous shrinkage of the UHPC material (due to the low w/cm), which might cause shrinkage cracking that compromises its durability performance. Including IC in cementitious grouts and UHPC is presented here as a strategy to increase the durability and, thus, sustainability of bridge structures. The objectives of the study are as follows:
  • Provide step-by-step guidelines on how to include IC in proprietary, prepackaged materials.
  • Show the effects of including IC on some of the main material properties, including fresh, mechanical, shrinkage, shrinkage cracking, and durability.

2. Materials and Methods

2.1. Proprietary, Prepackaged Materials Used

A proprietary, commercially available “non-shrink” cementitious grout (CG) was used, labeled as “CG”. The grout was supplied in a bag containing the solid contents that were mixed with a certain amount of water following the manufacturer’s recommendations to obtain an average flow of 100 percent per ASTM C1437-15 [31]. Similarly, a proprietary, commercially available UHPC material consisting of a preblended powder mixture containing all of the solids was mixed with a certain amount of water, a two-component chemical admixture system, and steel fibers following the manufacturer’s recommendations. The fibers had a nominal length of 13 mm, a nominal diameter of 0.2 mm, and a tensile strength of 3750 MPa, as reported by the manufacturer. Both of these materials were used throughout the present study.

2.2. IC Mixture Design

The most critical part of any IC mixture design process is the calculation of the amount of IC agent used to provide the IC water. Theoretically, the required amount of IC water is based on the cementitious content of the mixture and the reactivity of the cementitious system. The higher the reactivity, the higher the water demand and, thus, the more IC water is required. Bentz, Lura, and Roberts developed an equation based on these concepts to calculate the amount of IC agent in conventional concretes [32]. The equation was developed for LWA’s use because this situation is the most commonly used IC agent (Equation (1)).
M L W A · S · θ L W A = C f · C S · α m a x
However, Equation (1) does not account for the fraction of water released from the LWA. Castro, Keiser, Golias, and Weiss modified the equation developed by Bentz, Lura, and Roberts to account for the water released from the LWA, as seen in Equation (2) [33]. The term on the left corresponds to the amount of water supplied by the IC agent, whereas the term on the right is the water demanded by the cementitious system. The amount of LWA can then be easily derived using Equation (2) as follows:
M L W A = C f · C S · α m a x S · θ L W A · Ψ
where:
-
MLWA (kg/m3) is the amount of LWA per unit volume needed to provide the IC water.
-
S (unitless) is the LWA degree of saturation, taken normally as 1.
-
θLWA (g water/g dry LWA) is the LWA absorption at a particular time.
-
ψ (unitless) is the fraction of water released from the LWA at 94 percent humidity.
-
Cf (kg/m3) is the cementitious factor or content per unit volume.
-
CS (g water/g sample) is the infinite chemical shrinkage occurring in the sample, with values of 0.023 and 0.04 for the CG and UHPC materials obtained in accordance with ASTM C1608, respectively [34].
-
αmax (unitless) is the maximum expected degree of hydration in the material.
In conventional concretes, once the LWA amount is calculated, a volume of the normal weight fine aggregate is replaced by the same volume of fine LWA (both densities are needed for this calculation). However, there are some complexities that arise when trying to perform the same thing in prepackaged materials because the blended components and their volume fractions are unknown to the end user. In this case, the only possibility is to add the LWA (or any other IC agent) “on top” of the ordinary mixture proportions of the prepackaged material, thus, altering the volume fractions in the mixtures with IC compared with the control prepared without IC (see Figure 1 later in the paper). For conventional grouts, the addition of an IC constituent may be somewhat similar to the common practice of extending a grout through the use of a supplemental, inert aggregate. It is recognized that these changes in the mixture volume fractions would be expected to influence the fresh, mechanical, shrinkage, and durability properties of the internally cured materials with respect to the control.
Another difference when including IC in prepackaged materials is the time frame when the IC agent is introduced to the mix and whether the IC agent is dry or wet at that time. In conventional concretes, the IC agent is normally added in a prewetted condition during concrete mixing and after having been soaked in water for at least 24 h. One inadvisable option would be to add the prewetted IC agent to the dry prepackaged materials due to the IC water being released and hydrating the cementitious materials prior to mixing. The alternative option proposed in this research study requires the IC agent to be incorporated in the grout or UHPC mixture in oven-dry conditions. Previous research demonstrated that incorporating IC agents in oven-dried conditions resulted in improved durability and fluid transport properties [35]. In this scenario, the highly porous particles in the IC agent will quickly absorb water during the fresh stage of the mixing and will continue to absorb water until the relative humidity of the system starts to decline as the cementitious matrix solidifies (i.e., up to the mixture time of set) and then releases the absorbed water to provide the IC effect [33]. This scenario is the reason why the absorption capacity of the IC agents used in this study was not only measured at 72 h, as described in ASTM C1761-17 (which recommends adding the IC agent in prewetted conditions), but also at other times, as shown in Table 1, corresponding to the grout and UHPC times of set [36]. It is also assumed that the incorporation of IC agents would not alter the time of set significantly (which has been experimentally proven to be the case, as described in the results section of this paper). Therefore, and for convenience, IC agents in oven-dry conditions were used in this study and, in some cases, compared to prewetted conditions.
Given all these facts, the following step-by-step procedure is proposed to calculate the amount of IC agent (generally LWA) to be included in proprietary, prepackaged materials:
  • Step 1—Determine the amount of dry material from the mixture design
  • Step 2—Measure chemical shrinkage of the cementitious system via ASTM C1608-17 [34]. Normalize the results by grams of mixed sample. To estimate infinite chemical shrinkage, plot the results versus the inverse of time.
  • Step 3—Estimate the maximum expected degree of hydration in the material by diving the w/cm by 0.36, for a maximum of 1. If unknown, use an estimated cementitious content of 30 percent for grouts, and 45 percent for UHPC.
  • Step 4—Measure the absorption properties of the IC agent as described in ASTM C1761-17 [36].
  • Step 5—Measure the desorption properties of the IC agent as described in ASTM C1761-17 [36].
  • Step 6—Compute the amount of IC agent using Equation (2).
  • Step 7—Calculate the amount of IC water (right term in Equation (1)) so that it can be added to the mixing water.
In situations where SAP is used as the IC agent, this procedure should be slightly modified due to differences in the water absorption/desorption characteristics:
  • Steps 1–3—Follow steps 1–3 as described above for the use of LWA as an IC agent in proprietary, prepackaged materials.
  • Step 4—Measure the absorption properties of the SAP from comparative fresh spread using the flow test, as described in ASTM C1437-15 [31], as recommended by SAP manufacturers. The spread of the SAP-modified mixture is compared to a control (plain) mixture. The difference in water content is assumed to be absorbed by the SAP.
  • Step 5—Assume full desorption capacity (ψ = 1).
  • Steps 6–7—Follow step 6–7 as described above for the use of LWA as an IC agent in proprietary, prepackaged materials.
After the amounts of both IC agent and IC water are computed using these procedures, the internally cured grout or UHPC materials are prepared similarly to the control, with only two caveats: the IC agent should be added in a dry state to the dry components of the grout or UHPC material prior to adding the water, chemical admixtures, and fibers (if applicable), and the amount of IC water should be added to the amount of mixing water.
In this study, the following three different types of commercially available IC agents were used:
  • Two size fractions of rotatory kiln expanded shale LWA.
  • Expanded glass (EG) LWA.
  • Synthetic SAP.
They are labeled throughout the paper as LWA, EG, and SAP, respectively. Table 1 shows some of the important properties for a proper IC mixture design.
With the information presented in Table 1 and the measured chemical properties of both cementitious grout and UHPC materials used in the study, several internally cured mixtures were prepared to evaluate material properties such as fresh flow, density, and so on. Table 2 shows the mixture proportions compared to the control, and Figure 1 shows an illustration of the mixture proportioning (volume fractions) when the different IC agents were added to the prepackaged materials. As previously described, the IC agent is added on top of the ordinary mixture proportions, which alters the volume fraction of the prepackaged material in the mixtures with IC. For example, an IC agent with lower absorption capacity (e.g., expanded shale LWA) would occupy a larger fraction of the final volume because more LWA particles would be needed to provide the same amount of IC water. (Note that the volume fraction in the case of using SAP includes the IC water because the particles are not rigid and would expand, thus, occupying a larger final volume.) These changes in the mixture volume fractions would be expected to influence the properties of the internally cured materials with respect to the control.

3. Results and Discussion

3.1. Prewetted Versus Oven-Dried LWA

As previously noted, it would not be desirable to add the IC agent to the prepackaged material in prewetted conditions because it is commonly performed in conventional concretes. Ideally, the IC agent should be dry so that it can be premixed with the dry components of the prepackaged material. As such, this section aims to compare some of the material properties of the CG with LWA as the IC agent incorporated in both prewetted and oven-dried conditions. A detailed discussion on how the inclusion of IC affects the material properties is included in subsequent sections.
Table 3 summarizes fresh and mechanical properties tested on the internally cured grouts compared to the control grouts. As observed, both internally cured grouts exhibited similar performance when compared to each other. For example, fresh flow, measured as described in ASTM C1437-15, was reduced by 25 percent when including the LWA particles in the grout material, regardless of their moisture conditions (prewetted versus oven-dried) [31]. Similarly, fresh density (determined via mass measurements of a 400-milliliter volumetric cup, according to ASTM C185-20) was reduced by about 10 percent in both cases compared to the control [38]. Setting times, assessed per Vicat measurements described in ASTM C191-19, were practically unaltered with only a slight retardation compared to the control [39]. As for the mechanical properties of the internally cured mixtures, they are presented as a percentage of the control values. The compressive strength, f′c, as per ASTM C39-20, and modulus of elasticity, E, assessed via ASTM C469-14, of both LWA mixtures were assessed on 100- × 200-mm samples and were found to be consistently lower than that of the control at all ages, but similar when compared to each other [40,41]. One of the driving factors of the reduction in strength is the lower paste volume in the IC mixtures due to the addition of the LWA. The other driving factor might be the effect of IC on gel-space ratio. Previous research by Hasholt, Seneka, and Jensen reported that at high w/c ratios (w/c > 0.45), the addition of LWA affected the gel-space ratio negatively, leading to strength reduction due to increased void volume [42]. As mentioned in the introduction, cementitious grouts are characterized by high w/cm and, therefore, the observed reduction agrees with the literature. The reduction in modulus of elasticity of the IC mixtures can be attributed to the low moduli of the LWA, which leads to a reduction in density of the cementitious matrix, thus negatively affecting the stiffness of the material [43].
Shrinkage deformations were also measured in both sealed (autogenous) and drying conditions, in accordance with a modified version of ASTM C157-17 [44]. A total of six samples were cast for each of the mixtures described in Table 3, out of which three of them were used to measure shrinkage deformations in a sealed condition (samples were sealed using aluminum tape to assess shrinkage in an autogenous environment), and the other three were used to measure shrinkage deformations in drying conditions. Samples consisted of prismatic specimens with 25- × 25-mm cross-sectional area. All the CG samples were demolded at 24 h and were prepared and placed in an environmental chamber at 23 °C and 50 percent relative humidity through the duration of testing. The samples were measured for mass loss and shrinkage deformations up to 91 days. The results are presented in Figure 2. In order to account for the total shrinkage behavior, the plotted deformation curves begin at the age of one day with a starting strain value indicative of the 1-day deformation measured, according to the ASTM C1698-19 corrugated tube test (i.e., the initial expansions typically observed in this type of grout material are accounted for in the results) [45]. Despite their “non-shrink” nomenclature, cementitious grouts were observed to exhibit considerable amounts of shrinkage of as much as 1200 µε at 90 days, especially in drying conditions. LWA mixtures were capable of reducing shrinkage deformations by approximately 30 percent compared to the control, with this benefit being particularly apparent during the first days of age. The mass loss determined on all the mixtures in sealed conditions showed a mass loss of less than 1 percent, and the mass loss determined in the internal curing samples in drying conditions was 20 percent and 25 percent more than the control sample. However, the mass loss in the samples subjected to drying conditions are 13 times more than that of the samples in sealed conditions.
When using the oven-dried LWA as an IC agent, the conclusion is that similar grout material performance is obtained as that of prewetted LWA in terms of fresh, mechanical, and shrinkage properties. It is assumed that a similar outcome would be observed in UHPC materials. Therefore, only IC agents in oven-dry conditions were used in the next results sections, which evaluate the effect that the inclusion of oven-dry IC agents in proprietary, prepackaged CG and UHPC materials has on important materials properties. A more in-depth discussion of these results can be found in these articles [27,28,46].

3.2. Effect of IC on Fresh and Mechanical Properties

The effect of the inclusion of IC on some of the fresh and mechanical properties of cementitious grout and UHPC material is presented in Table 4. One caveat is that the CG control mixture corresponds to a different batch than that reported in Table 3; hence, the strength results are slightly different.
Fresh flow properties are crucial for both CG and UHPC materials. As observed, the fresh flow was reduced when including LWA as IC agents in the CG material. The reduced flow can be likely attributed to the change in surface area due to the inclusion of IC agents and the change which resulted in the modification of the designed grout granulometry when including coarser and angular-shaped LWA particles and paste volume. The goal of reducing the maximum particle size of the LWA by sieving out the coarser fraction was to minimize this flow reduction; however, this step did not seem to be very effective. Instead, the spherical shape of both EG and SAP agents seemed to be more efficient at maintaining the flow properties of the CG material. On the other hand, internally cured UHPC mixtures increased their fresh flow with respect to the control. Users should understand that rheological changes to the UHPC mixture could lead to fiber segregation issues; however, no apparent change in the (typically high) plastic viscosity of the mixtures was observed, as the three mixtures showed the same flow speed on the flow table. It is conjectured that the extra IC water added to the mixture made the system more flowable. In addition, EG particles are spherical in shape, which also likely resulted in an increase in the fresh flow. The reason why this effect was not observed in CG material might be because of its higher w/cm, estimated at about 0.55, contrary to UHPC systems designed with extremely low w/cm, where the additional IC water might be sufficient to significantly change the flow properties. More research is needed on this topic to confirm this conclusion. Additionally, fresh density was significantly reduced in all internally cured mixtures due to the lower specific gravity of the IC agents, but time of set was practically unaltered, with a slight retardation in all cases, likely attributed to a dilution effect of the systems [8].
Regarding mechanical properties, the inclusion of IC had different effects depending on the material, grout or UHPC, and the type of IC agent. Some of these differences can be explained by the differences in paste volume and also by applying the Powers model and the concept of gel-space ratio, which can be used to model the development of mechanical properties in cement-based materials [42]. The implication of the model is that at low w/c ratios (w/c < 0.40), the inclusion of IC agents can potentially increase the compressive strength at later ages by increasing the maximum degree of hydration [16,17]. This increase in reactivity can offset the larger total porosity of the IC agents and increase the gel-space ratio of the system. However, at high w/c ratios (w/c > 0.45), the increased hydration effect is less pronounced, and the enhanced hydration cannot counterbalance the increased void volume generated by the porous IC agents. In these latter cases represented by the grout systems, IC addition reduces the compressive strength and the elastic modulus [42]. Similar to what is observed in Hasholt, Seneka, and Jensen, all of the internally cured CG mixtures clearly exhibited a reduction in strength, and the UHPC mixture with sieved LWA seemed to be significantly benefited by the inclusion of IC in that regard. Isothermal calorimetry results presented elsewhere confirmed the increased degree of hydration observed in the same UHPC material with LWA as IC agent [28]. However, the UHPC mixture containing EG exhibited long-term reductions in strength and elastic modulus. This difference in mechanical performance can be attributed to the lower stiffness of the EG particles compared to the LWA and, potentially, to the differences in the void sizes of the EG particles.
Compressive strength reductions at early ages in these types of materials are not desirable, especially in UHPC. The beneficial IC effect was clearly observed in this material when using LWA, but only at later ages. As for the cementitious grout, despite the strength reduction in all of the internally cured mixtures, all of them except for the CG EG would comply with the minimum strength requirements, as described in ASTM C1107-20 [47]. Therefore, the end user should keep in mind that the inclusion of IC might not be as beneficial in field applications where high early strengths are required for CG or UHPC materials.

3.3. Effect of IC on Shrinkage Properties

The main positive effect of the inclusion of IC is expected to be observed in the shrinkage performance. Shrinkage deformations were measured in accordance with modified ASTM C157-17 in both sealed and drying conditions [44]. A total of six samples were cast for each of the mixtures described in Table 4, out of which three of them were used to measure shrinkage deformations in a sealed condition (samples were sealed using aluminum tape to assess shrinkage in an autogenous environment). The other three samples were used to measure shrinkage deformations in drying conditions. Samples consisted of prismatic specimens with 25- × 25-mm and 75- × 75-mm cross-sectional area for the CG and UHPC materials, respectively. All the CG samples were demolded at 24 h and were prepared and placed in an environmental chamber at 23 °C and 50 percent relative humidity through the duration of testing. The samples were measured for mass loss and shrinkage deformations up to 91 d. The larger UHPC specimens were selected to accommodate the presence of fiber reinforcement. Figure 3 shows the results obtained.
As observed, the inclusion of IC was found to be quite effective in reducing shrinkage deformations in sealed conditions for both material types, especially during the first 7 d of age. The inclusion of IC agents is crucial because, at these early ages, the materials are still developing mechanical properties to sustain any potential shrinkage cracking. The reduction in shrinkage deformations in sealed conditions is mainly attributed to the reduction in self-desiccation that accompanies chemical shrinkage and the reduction in paste volume fraction, as indicated in Figure 1 [15]. It is interesting to note that the autogenous shrinkage reduction in the UHPC material was not as evident as in the CG material, or even in conventional concretes, but a 100 percent reduction is commonly observed over the first day of age [16]. Other researchers have claimed that an “under designed” internally cured UHPC might be obtained when using conventional IC mixture design methods based on the chemical shrinkage. They proposed the following two reasons to explain the under design:
  • The additional space for precipitation provided by pores present in IC agents, which might be significant in very dense UHPC materials.
  • Limited travel distance of the IC water in a very dense system. A more in-depth discussion can be found elsewhere [20]. Their conclusion is that a greater amount of IC agent than that calculated using Equation (2) may be required in UHPC materials.
Regarding the total deformations in drying conditions, IC partially reduced them in the CG material, but the effect was negligible in the UHPC material, with very small reductions only over the first 28 d. However, IC agents were successful in delaying the drying shrinkage development. The larger the water content, the larger the porosity, and the larger the porosity, the faster the concrete dries [8]. Due to the higher water content of grouts, the fraction of capillary pores is higher than in the case of UHPC, which leads to higher mass loss from the capillary pores than that is observed in UHPCs. The higher reduction in mass loss leads to higher shrinkage deformations in CG material compared to the UHPC material, with a much lower w/cm. The shrinkage reductions when IC was included corresponded, generally in the same degree, to the paste volume reduction in the materials, as detailed in Figure 1. A special case was the CG material with SAP, where the presence of the IC agent did not seem to have any effect on the drying shrinkage performance. The faster drying rate of these specimens observed in another study might explain these results; however, more research would be needed to confirm this finding [27].
Given these results, it is expected that the internally cured mixtures would be less prone to develop shrinkage stresses at early ages, thus, resulting in better durability from the shrinkage cracking viewpoint. This topic is discussed in the next section. Additionally, an evaluation of water absorption rates (i.e., sorptivity) as a durability parameter is presented for both CG and UHPC materials with and without IC, given the high porosity that results from its high w/cm.

3.4. Effect of IC on Durability Properties

Large amounts of shrinkage could potentially cause cracking if the material is restrained. A dual ring test (DRT), as described in AASHTO 363-17, was used to assess the restrained autogenous shrinkage-induced stress development of some of the internally cured grouts and UHPC materials presented in the previous sections [48]. The test was executed in sealed, isothermal conditions at 23 ± 0.1 °C for 7 d, at which time the temperature of the rings was reduced at a rate of 2 °C/h down to −21 °C to induce cracking in the material being tested. During the isothermal part of the test, the shrinkage strains were recorded and used to calculate the residual stress accumulation. The induced stresses from the temperature drop after 7 d were used to show the stress reserve capacity and determine how near the specimen was to cracking. More information about these concepts can be found elsewhere [49,50]. Figure 4 shows the stresses over specimen age, where positive and negative values on the primary Y-axis indicate tensile and compressive stresses, respectively.
As observed, the control grout did not develop any substantial internal stress during the first 7 d in autogenous conditions, as expected in high w/cm systems. In contrast, both internally cured grout mixtures developed compressive stresses. This result is mainly due to the expansive effect of IC in sealed conditions, as shown in the shrinkage results in Figure 3. Once the temperature was decreased, the control grout cracked, indicated by a sudden drop in the stress reading. Both internally cured mixtures did not crack even after reaching a temperature of −21 °C. By calculating the absolute stress difference at the beginning and end of the temperature drop, the stress reserve capacity of the materials was estimated. While the control grout depicted a total stress reserve capacity of about 4.5 MPa, only a minimum stress reserve capacity of about the same magnitude was assessed for the internally cured mixtures since the materials did not crack during the temperature drop. The improvements observed in the cracking behavior of the internally cured grouts can be explained by a combination of the following three factors: less paste volume in the material, lower stiffness (i.e., elastic modulus), as shown in Table 4, which allows for a better stress relaxation, and effect of IC on suppressing the coefficient of thermal expansion [50,51,52]. It is conjectured that the IC benefits might be larger at temperature and humidity conditions different from those tested. If these materials were tested using the DRT in more realistic drying humidity conditions (e.g., 50 percent relative humidity), a larger stress development in the materials would be expected due to the high w/cm, and the IC effect could then be even more significant. Additionally, the thermal gradient typically observed in field-cast cementitious materials due to the exothermic hydration reaction would be larger in the control grout because of its higher paste volume compared to the internally cured grouts, which could be translated into more internal stresses being developed.
A similar outcome was observed for the UHPC mixtures, where the control cracked during the temperature drop part of the test that was arrested through the presence of steel fiber reinforcement. In contrast, both internally cured mixtures did not exhibit any microcracking. It is interesting to observe how the control mixture developed considerable tensile stresses during the isothermal period, as expected in low w/cm systems. This effect is thought to be the main reason why the material experienced cracking. The IC mixtures, in addition to undergoing less shrinkage deformations during the first 7 d due to the reasons explained in the previous section, can better mitigate shrinkage stresses developed within the material due to the lower modulus of elasticity compared to the control.
Finally, due to the high porosity expected in the CG material associated with high w/cm systems, sorptivity results were obtained via modified ASTM C1585-13 to estimate the resistance to freeze–thaw damage [53]. The rate of fluid ingress is a durability parameter that can be used to predict the service life of structural elements. The rate of fluid ingress was used to determine the time to reach critical degree of saturation (87 percent) which relates to freeze–thaw damage. After casting and sealed curing for a period of 28 d, six 100-mm diameter discs with a height of 50 mm were cut from standard cylinders for each mixture. Three discs were oven dried and vacuum saturated at an absolute pressure of 6 torr to determine the moisture content at saturation, MCsat. Three additional specimens were conditioned immediately after cutting at 23 ± 0.2 °C and 50 ± 3 percent relative humidity until the samples reached mass equilibrium (<0.02 percent mass change over 15 d). The specimens were wrapped with aluminum tape around the circumference and the top surface was covered with plastic and taped to prevent excessive drying. Each specimen was measured to determine the initial mass, m0. The exposed face was placed into a small amount of water, and mass measurements were conducted at a series of time intervals, m. After 8 d of testing, the specimens were oven dried to determine an oven-dry mass, mod. At the time of each mass measurement, the degree of saturation S was determined using Equation (3).
S = ( m − m 0 ) / m o d M C s a t
Results shown in Figure 5a display the characteristic bilinear behavior observed in this type of analysis, with the slopes of the respective regions termed initial sorptivity and secondary sorptivity. The CG LWA mixture showed a higher secondary water absorption rate with respect to the control mixture (approximately 36 percent increase). This behavior is a potential consequence of the higher total porosity of the IC mixtures. The degree of saturation S indicates the fraction of porosity that is fluid-filled, with 0 percent being an oven-dry state (no porosity is fluid-filled) and 100 percent being complete saturation (all the porosity is fluid-filled). For many cement-based materials, damage is observed when S meets or exceeds approximately 87 percent and the material undergoes a freeze–thaw cycle [54,55]. Figure 5b represents the water absorption in terms of degree of saturation against time. As observed, the sorptivities of the CG LWA grout mixture are below those of the control and far away from the 87 percent threshold value, indicating a better resistance to freeze–thaw damage. This result is mainly attributed to the pore refinement that occurs due to the presence of IC water. A more in-depth discussion of these results can be found in these articles [46,56].
The rate of water absorption was also measured in UHPC with and without IC, in accordance with a modified ASTM C1585-13 [53]. The samples were prepared and conditioned similar to that of CG mixtures, as described earlier. However, due to the challenges associated with UHPC, the degree of saturation was not assessed in the UHPC mixtures. Figure 6 shows the water absorbed over time in a UHPC system with and without IC agents. As seen in Figure 6, the UHPC systems did not exhibit a bilinear behavior similar to that of CG systems and can be attributed to the transport properties of the UHPC material and the absence of entrained air voids. Similar to that of the CG mixtures, the UHPC mixtures also displayed an increase in the rate of water absorption on the inclusion of IC agents.
The inclusion of LWA in the UHPC system led to an increase in rate of absorption by 25 percent. However, the rate of absorption in UHPC mixtures with and without IC is one order of magnitude lower than that of the CG mixtures. The total porosity of any cementitious system is composed of gel pores (radius < 5 nm), capillary pores (radius > 5 µm), entrapped air, and chemical shrinkage, and the typical total porosity of normal concrete is around 15 percent, while UHPCs have a porosity of 6 percent [57]. Apparent porosities of UHPC control and UHPC LWA samples were determined in accordance with ASTM C948 and were found to be 7.8 percent and 8.3 percent, respectively [58]. Including LWA, the porosity of the UHPC system increased by a factor of 1.06. Apart from the increased porosity, the change in rates of absorption can also be attributed to the change in porous matrix that occurs due to the presence of IC water. Previous research demonstrated that the gel pores contribute to most of the porosity in a UHPC system [59]. Due to the low capillary pore volume, the capillary suction of water and deicing salt solutions is effectively reduced, making UHPC a very durable material. As seen in Figure 5B, the CG LWA sample has a better freeze–thaw resistance than the CG control specimen. Even though the UHPC LWA system has a slightly increased porosity than the control system, the total porosity of UHPCs is less than 50 percent of that of a CG system. Thus, freeze–thaw damage might not be a concern in the case of UHPCs with IC agents. However, further research is needed to understand the freeze–thaw durability performance of UHPCs with IC agents.

4. Concluding Remarks

A compilation of results from different studies has been included in this paper with the main goal of proposing the inclusion of IC in proprietary, prepackaged grout and UHPC materials as a solution to potential shrinkage cracking and durability issues. IC is a proven technology that has shown many benefits when included in conventional concretes. This paper has shown that these same benefits can also be observed, to some degree, in prepackaged materials, especially in terms of reduced shrinkage cracking. Common IC agents normally used in conventional concretes have also proven to be effective in prepackaged grout and UHPC materials. While the mixture design procedure differs slightly from that of conventional concretes, it is based on the same concepts. Eccentricities of this mixture design procedure, as discussed in the paper, also imply a reduction in the initial material cost of the internally cured mixtures with respect to the control without IC. The inclusion of IC in cementitious grouts and UHPC enhances the material durability, which should lead to more sustainable bridge structures as longer service lives are expected.

Author Contributions

The authors confirm contribution to the paper as follows: conceptualization, I.D.l.V., J.F.M., and R.S.; data curation, I.D.l.V., N.S., and R.S.; formal analysis, I.D.l.V. and N.S.; funding acquisition, B.G.; investigation, I.D.l.V., N.S., J.F.M., and R.S.; methodology, I.D.l.V., J.F.M., and R.S.; project administration, I.D.l.V., N.S., and R.S.; resources, I.D.l.V., N.S., J.F.M., and R.S.; supervision, I.D.l.V., N.S., and B.G.; validation, I.D.l.V., N.S., J.F.M., R.S., and B.G.; visualization, I.D.l.V., N.S., J.F.M., and R.S.; writing—original draft, N.S., I.D.l.V., J.F.M., and R.S.; writing—review and editing, N.S., I.D.l.V., J.F.M., R.S., and B.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

No applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank Caleb Nickle, Michael Rodriguez, and Viyra Akakpo for their help with conducting the experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Illustration of the prepackaged cementitious grout and UHPC materials volume fractions when including various IC agents: (a) CG Control; (b) UHPC Control; (c) CG LWA; (d) UHPC LWA; (e) CG EG; (f) UHPC EG; (g) CG SAP.
Figure 1. Illustration of the prepackaged cementitious grout and UHPC materials volume fractions when including various IC agents: (a) CG Control; (b) UHPC Control; (c) CG LWA; (d) UHPC LWA; (e) CG EG; (f) UHPC EG; (g) CG SAP.
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Figure 2. (a) Comparison of autogenous and drying shrinkage deformations in an internally cured cementitious grout using prewetted or oven-dried LWA as IC agent; (b) comparison of mass loss during shrinkage assessment in internal cured cementitious grout using prewetted or oven-dried LWA as IC agent.
Figure 2. (a) Comparison of autogenous and drying shrinkage deformations in an internally cured cementitious grout using prewetted or oven-dried LWA as IC agent; (b) comparison of mass loss during shrinkage assessment in internal cured cementitious grout using prewetted or oven-dried LWA as IC agent.
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Figure 3. Shrinkage deformations and mass loss for CG with different IC agents: (a) CG in sealed conditions, (b) CG in drying conditions, (c) CG in sealed conditions, and (d) CG in drying conditions. Shrinkage deformations and mass loss for UHPC with different IC agents: (e) UHPC in sealed conditions, (f) UHPC in drying conditions, (g) UHPC in sealed conditions, and (h) UHPC in drying conditions.
Figure 3. Shrinkage deformations and mass loss for CG with different IC agents: (a) CG in sealed conditions, (b) CG in drying conditions, (c) CG in sealed conditions, and (d) CG in drying conditions. Shrinkage deformations and mass loss for UHPC with different IC agents: (e) UHPC in sealed conditions, (f) UHPC in drying conditions, (g) UHPC in sealed conditions, and (h) UHPC in drying conditions.
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Figure 4. Residual stress development and stress reserve capacity: (a) CG with and without IC, and (b) UHPC with and without IC.
Figure 4. Residual stress development and stress reserve capacity: (a) CG with and without IC, and (b) UHPC with and without IC.
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Figure 5. (a) Water absorption as a function of time in CG with and without IC, and (b) water absorption in terms of degree of saturation against the square root of time in hours. Data points represent experimental data and lines represent a bilinear fit model.
Figure 5. (a) Water absorption as a function of time in CG with and without IC, and (b) water absorption in terms of degree of saturation against the square root of time in hours. Data points represent experimental data and lines represent a bilinear fit model.
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Figure 6. Water absorption as a function of time in UHPC with and without IC.
Figure 6. Water absorption as a function of time in UHPC with and without IC.
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Table
Table 1. Properties of the IC agents used.
Table 1. Properties of the IC agents used.
IC AgentBulk Specific GravityMaximum Particle Size
(mm)		Water Absorption
(Percent) c		Desorption Factor at 94 Percent Relative HumidityAdded to
8 h16 h72 h
LWA1.564.016n.d.190.93CG
Sieved LWA a1.561.21616n.d.0.93CG, UHPC
EG0.601.05050n.d.0.99CG, UHPC
SAP0.600.06/0.4 b2000 d1.00 (assumption)CG
a Sieved fraction of the LWA with maximum particle size of 1.2 mm was also used to address flow concerns, as explained in the results section. b Particle size of the SAP is presented in both dry and swollen in distilled water states [37]. c Absorption capabilities were measured at different times for mixture design purposes: 72 h for prewetted conditions, 8 and 16 h for oven-dry conditions of the CG and UHPC materials, respectively. d The SAP absorption capacity was measured from comparative fresh spread using the flow test, as described in ASTM C1437-15 [31]. n.d. = No data.
Table
Table 2. Gravimetric mixture proportions of the proprietary, prepackaged cementitious grout and UHPC materials with and without IC (based on solid amount supplied in one bag).
Table 2. Gravimetric mixture proportions of the proprietary, prepackaged cementitious grout and UHPC materials with and without IC (based on solid amount supplied in one bag).
ComponentCG Mixture Proportions (kg)UHPC Mixture Proportions (kg)
ControlPrewetted LWA LWASieved LWAEGSAPControlSieved LWAEG
Solids (bag)23.023.023.023.023.023.025.025.025.0
Mixing Water3.53.53.53.53.53.52.62.62.6
IC Water- b0.50.50.50.50.5-0.90.9
Total Water a3.54.04.04.04.04.02.63.53.5
Chemical Admixtures------1.01.01.0
LWA-3.03.63.6---6.0-
EG----1.1---1.8
SAP-----0.03---
Steel Fibers------2.02.02.0
a Total water includes both mixing and IC water. b Not applicable.
Table
Table 3. Fresh and mechanical properties of grouts with and without IC, and a comparison between prewetted LWA and oven-dried LWA.
Table 3. Fresh and mechanical properties of grouts with and without IC, and a comparison between prewetted LWA and oven-dried LWA.
MixtureFlow, PercentFresh Density,
g/cc		Time of Set,
H		Mechanical Properties a
1 d3 d7 d28 d
CG Control1002.228.0f′c, MPa21.3 (0.2) b37.0 (1.1)45.1 (0.5)54.1 (0.9)
E, GPa20.6 (0.3)24.2 (0.2)27.4 (0.3)30.2 (0.3)
CG
Prewetted LWA		752.028.2f′c, percent69646976
E, percent69837980
CG
Oven-dried
LWA		751.988.5f′c, percent78706570
E, percent79747970
a Mechanical properties of the internally cured mixtures are presented as percentage of the control values. b Numbers in parentheses indicate ± one standard deviation from the average value of three specimens.
Table
Table 4. Fresh and mechanical properties of the CG and UHPC materials with and without IC.
Table 4. Fresh and mechanical properties of the CG and UHPC materials with and without IC.
MixtureFresh Flow,
Percent		Fresh Density,
g/cc		Time of Set,
h		Mechanical Properties b
1 d7 d28 d
CG Control1002.228.0f′c, MPa20.8 (0.7)39.7 (2.1)50.9 (1.1)
E, Gpa17.8 (0.7) d27.5 (1.8)30.9 (0.4)
CG LWA752.028.5f′c, percent786570
E, percent797970
CG Sieved LWA802.038.5f′c, percent586976 c
E, percent717592 c
CG EG94n.d.9.0f′c, percent334855
E, percent506570
CG SAP982.088.7f′c, percent516570
E, n.d.n.d.n.d.
UHPC Control94 a2.3416.0f′c, Mpa41.4 (3.2)107.1 (3.5)129.3 (11.3)
E, Gpan.d.32.8 (1.6)36.5 (0.3)
UHPC Sieved LWA1092.11n.d.f′c, percent2891103
E, percentn.d.9896
UHPC EG1231.9516.6f′c, percent555759
E, percentn.d.7770
a Note that the UHPC control flow is not 100 percent because this material is designed by the manufacturer with a fixed amount of water and admixtures, contrary to the CG material where a range of water is recommended by the manufacturer. The flow of the CG material was purposely targeted at 100 percent of the flow table test [21]. b Mechanical properties of the internally cured mixtures are presented as percentages of the control values. c Tested at 39 d instead of 28 d. d Numbers in parentheses represent ± one standard deviation from the average of three specimens.
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Saladi, N.; De la Varga, I.; Munoz, J.F.; Spragg, R.; Graybeal, B. Effects of Internal Curing on Inclusion in Prepackaged Cementitious Grout and Ultra-High Performance Concrete Materials. Sustainability 2022, 14, 13067. https://doi.org/10.3390/su142013067

AMA Style

Saladi N, De la Varga I, Munoz JF, Spragg R, Graybeal B. Effects of Internal Curing on Inclusion in Prepackaged Cementitious Grout and Ultra-High Performance Concrete Materials. Sustainability. 2022; 14(20):13067. https://doi.org/10.3390/su142013067

Chicago/Turabian Style

Saladi, Naveen, Igor De la Varga, Jose F. Munoz, Robert Spragg, and Benjamin Graybeal. 2022. "Effects of Internal Curing on Inclusion in Prepackaged Cementitious Grout and Ultra-High Performance Concrete Materials" Sustainability 14, no. 20: 13067. https://doi.org/10.3390/su142013067

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