Early-age cracking, autogenous shrinkage and self-desiccation are almost inevitable problems for concrete, especially for mass high performance concrete, for the permeability of which is too low for external curing water to get in and fully hydrate the cement paste inside. The better hydration of the cement paste, the less and smaller cracks the concrete have, and the better durability and reliability the concrete structures do. In order to solve this problem, scientists have consider an unconventional methods to cure the concrete from the inside out so a better hydration can be achieved.
As early as 1957, Paul Klieger have mentioned how helpful the saturated lightweight aggregates would be to supply internal curing water and improve the long-term strength in his report. Nowadays, this method as internal curing has been fully developed and widely used to get low-cracking high-performance concrete with better hydration by replacing part of natural fine aggregates with saturated lightweight aggregates. The goal is clear and simple, but the ways to achieve the same purpose is various and have different advantages and disadvantages. In this report, several techniques and materials with different properties that could be used in internal curing, such as expanded shale clay or slate(ESCS) and Superabsorbent polymers(SAP), would be introduced and comment. Internal curing has a good performance especially in fields like mass concrete, or high performance concrete with low permeability. The properties why internal curing is a practical consideration for these instances would be discussed in this report.
1 Introduction 1.1 Background Concrete has many good properties as modern building material. It has high compression strength, fire proof, and cheap to produce. Since the using of concrete has a long history, the techniques of concrete structure construction may be fully developed. However, there still are some problems that almost inevitable for concrete, such as self-desiccation, autogenous shrinkage and chemical shrinkage. All of these issues may lead to cracking of concrete, and the chloride may penetrate through the cracks easily and cause corrosion of the reinforcement. As we all know, most of the failures of concrete structures are due to the corrosion of the reinforcement. Hence, concrete with less cracks or later to have cracks may contribute to a longer service life of a concrete structure.
What’s more, most of these unwanted cracks develop at the early-age of concrete placing. That’s why proper curing, which limits the early shrinkage and lowers the chance of early-age cracking, is so important to ensure the concrete develop the required properties and durability to reach their designed service life. Conventional curing uses methods to provide additional water to keep high relative humidity on the concrete surface, such as ponding and misting, or uses curing compounds, plastic membrane and evaporation retarder to slow evaporation. No matter water adding or moisture loss avoiding work mostly on the upper part of concrete, since the permeability of concrete is limited, the deeper inside concrete, the harder for water to penetrate.
On the other hand, self-desiccation (the reduction of the internal relative humidity in the concrete due to hydration reaction) will lead to autogenous shrinkage (concrete volume change occurring without moisture transfer to the environment) even without external moisture loss. Concrete shrinkage over time , will induce cracking that can reduce the service life expectation of concrete structure severely. In short, even proper conventional external concrete curing cannot provide perfect environment for concrete to develop its durability efficiently. Since 1980’s, the production of high-performance concrete(HPC) became more common, and to achieve their much higher strength and durability properties, lower water cement ratio and lower permeability is required.
The self-desiccation and autogenous shrinkage problem became even more severe a situation for HPC than for normal portland cement, because external curing water would be more difficult to penetrate deeply into the low permeable concrete to supply the loss water due to hydration and evaporation. When shrinkage happens, cracking is almost inevitable. Concerning a long time situation, internal relative humidity has a strong relationship with free autogenous shrinkage.
1.2 Internal curing Is there any solution that can settle down this problem? Or is there a way that can cure concrete more efficiently so can limit the cracks? The answer is yes. Since curing concrete from outside in has its limit, deeper part inside the concrete cannot be cured properly, how about cure concrete from inside out? As early as 1957, Paul Klieger  have mentioned how helpful the saturated lightweight aggregates would be to supply internal curing water and improve the long-term strength in his report. In 1991, Philleo  suggested incorporating saturated lightweight fine aggregate into the concrete mixture to provide an internal source of water to replace that consumed by chemical shrinkage during hydration of the paste. Nowadays, this method that use water-containing materials , replacing with normal aggregates to cure concrete, has been well developed and been named as internal curing. Such water-containing material could be saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers. Internal curing has been defined by the American Concrete Institute (ACI) as “supplying water throughout a fleshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation”
2 Benefits of internal curing Internal curing distributes the extra curing water throughout the 3-D concrete microstructure so that it is more readily available to maintain saturation of the cement paste during hydration, avoiding self-desiccation in the paste and reducing autogenous shrinkage. Along with this process, the main benefits bring about by internal curing may be concluded as below:
2.1 Reducing cracks due to shrinkage Concrete is susceptible to plastic shrinkage cracking at early age, especially when the evaporation rate is high. Right after placing, the concrete paste is still in a fluid state. The aggregates and cement particles tend to settle down due to gravity, while internal water is likely to bleed out onto the surface. Such layer of water will keep the evaporation of the concrete surface in a relatively constant rate. But this situation won’t last for a long time after the cement particles contact each other and start to develop strength. The rate of settlement will highly reduce along with much less water bleeding.
During this period, highly tensile stress occur inside the concrete due to surface tension of drying out internal water. Because at this time, concrete is under a plastic state but having develop enough strength to resist this tensile stress, cracks will occur. For internal curing concrete, the pre-wetted aggregates will provide water to replenish the evaporation from the surface of concrete. It makes the pores within the hydrating cement pastes fluid filled and thus helps to reduce the tensile stress. Shrinkage will be much less sever and cracks will less likely to happen. Besides limiting the happening of cracking, internal curing also contributes to delaying the age of cracking. As the volume of pre-wetted aggregates increases, the age of cracking is delayed, until an asymptote appears to be reached when sufficient pre-wetted aggregates has been added according to the research done by Schlitter et al (2010).
2.2 Long-term strength gaining Cement particles inside concrete finish most of hydration in the first 28 days, but the cement particles have not been completely hydrated after 28 days. Some unhydrated cement remain in the concrete and takes time to continue hydration. That’s the reason why as time goes by, the strength of concrete will increase even after a long period of time. For lower water cement ratio concrete, the required time to be fully hydration is longer. As to very low water cement ratio concrete, it may even be impossible for it to be fully hydrated. The hydration will stop when there is no longer capillary water available. In conventional curing, the capillary water inside the concrete will soon run out after early hydration, and the external water is not easy to penetrate into the concrete to hydrate the unhydrated cement particles. By using internal curing method, after most of the capillary water been used, the internal relative humidity drops, and the pre-wetted aggregates will provide water for cement to keep hydrating steadily for a longer time. As for the using of light-weight aggregates to provide internal curing, the reduction strength due to the light-weight aggregates can be compensate by this long-term strength gaining.
2.3 Reduction of permeability The principal contribution of internal curing results in the reduction of permeability due to a significant extension in the time of curing. It was shown that extending the time of curing increased the volume of cementitious products formed which caused the capillaries to become segmented and discontinuous.Reducing permeability leads to less penetration of aggressive agents that accelerate corrosion of embedded reinforcement. This decrease in permeability results from internal curing could obviously enhance the durability of concrete structures.
2.4 Working well with SCM Environment problem have been paid more and more attention by people today. In order to lower carbon footprint for using concrete, replacing cement with supplementary cementitious materials (SCMs i.e., silica fume, fly ash, metokaolin, calcined shales, clays and slates) is suggested as a way to use substantially less clinker. SCMs (except for silica fume) take longer time to hydrate, therefore requiring water to be present for a longer time. While more than one research has shown both internal curing and SCMs improve long term durability performance. Luckily, recent work has shown that internal curing works particularly well with SCMs. Internal curing enables the SCMs in these mixtures to react for a longer time, since the higher water content needed to support the reaction of the SCMs can be maintained.
2.5 Improving behavior of the contact zone “Contact Zone” refers to two distinctly different phenomena: (1) the mechanical adhesion of the cementitious matrix to the surface of the aggregate; (2) the variation of physical and chemical characteristics of the transition layer of the cementitious matrix close to the aggregate particle (ASTM STP 169 D  Chapter # 46 Holm & Ries). In the contact zone, the C-S-H is not evenly distributed in the outer product, and porosity is greater at aggregate surface within 15-50µm. What’s more, the obvious elastic difference between aggregate and the surrounding cementitious matrix make the transfer of stress from bulk cement paste to stiffer aggregate causes ‘softening’(microcracking) in interfacial transition zone. High incidence of interfacial cracking or aggregate debonding will have a serious effect on durability if these cracks fill with water and subsequently freeze.
All of these factors make the contact zone a weaker location in the concrete. By using internal curing, more internal water can be provide around the aggregates and lead to a better hydration at the interfacial transition zone, which decrease the porosity and increase the strength. The lower permeability also contributes to the difficulty for the chloride to penetrate. What’s more, the lower modulus of the light weight aggregate and the improved transition zone around the light weight aggregate particles due to their generally vesicular surface, helped reduce stress concentrations between the paste and the aggregate and those reductions subsequently reduced the amount of early-age cracking in the concrete.
3 Material and methods for internal curing As long as the principle is the same, different ways can be applied to achieve internal curing. Besides of light weight aggregates, the properties of other techniques and materials will be presented in this chapter. And their advantages and disadvantages will be commented.
3.1 Bentonite clay Bentonite is an absorbent aluminium phyllosilicate. It has a high specific surface, most of which more than 100 m2/g, and this property enables them to adsorb several molecular layers of water between their platelet structures . The absorbed water is held by secondary chemical bonds and the bentonite may swell up to 14 times as its original volume as a result of the water absorption. If the relative humidity in the surroundings is lowered, this water is reversibly released. Potentially, bentonite or other layered clay minerals may be used as a water reservoir for internal water curing. However, there still one important problem remain to be solved for the application of bentonite. In high ionic media, such as in cementitious materials, these clays agglomerate and form a compact structure instead of spreading out evenly . And whether inducing same charge into bentonite as water reducer do to make the bentonite particles repel from each other could solve this problem remain to be investigated.
3.2 Superabsorbent polymers A superabsorbent polymer, SAP, is a polymeric material which is able to absorb a significant amount of liquid from the surroundings and to retain the liquid within its structure without dissolving , SAPs are principally used for absorbing water and aqueous solutions. With the present polymer types the theoretical maximum water absorption is approx. 5000 times their own weight. However, the absorbency of commercially produced SAPs is around 50 g/g in dilute salt solutions such as urine, and in high ionic solutions such as cement paste pore fluid the absorbency may be below 20 g/g . The absorption of water in the SAP is based on secondary chemical bonds, and the water is so loosely held that all of it essentially can be considered bulk water. Most SAPs are cross-linked polyelectrolytes. Because of their ionic nature and interconnected structure, they absorb large quantities of water and other aqueous solutions without dissolving. SAPs have found a widespread use as a high-tech material e.g. for contact lenses, breast implants, fire fighting, drug delivery, in baby diapers and as soil conditioners. Today’s world production exceeds 500,000 tons per year of which about 85% is used for baby diapers .
Figure 5.1: Superabsorbent polymers are swellable substances which can absorb many times their own weight of liquids by forming a gel. The absorbed liquid is not released even under moderate pressure . The picture shows a dry, collapsed and a swollen suspension polymerized SAP particle. A description of the use of SAP for internal water curing can be found in the literature [2,13,15]. Compared with lightweight aggregate SAP has some peculiarities. SAP can be used as a dry concrete admixture since it takes up water during the mixing process. Furthermore, the use of SAP permits free design of the pore shape and the pore size distribution of the hardening concrete, however, the pores introduced by the SAP in the concrete may preferably be selected in the range 50-300 µm.
3.3 Crushed Returned Concrete Aggregates Recycled aggregate consists of stone particles with mortar from the original concrete attached to them. The volume fraction of this mortar may amount to 20 to 60%, and results in a significantly higher water absorption of recycled concrete aggregates compared to conventional aggregates . The relatively high water absorption of recycled aggregate, however, may be difficult to utilize for internal water curing. The cement paste fraction of recycled aggregate will, typically, have a fine and tight pore structure which cannot supply water to the coarse pore structure of a hydrating cement paste at early ages. For this reason, recycled aggregate may be less useful than normal aggregate for internal water curing. However, some experiments have shown promising results for recycled aggregate. By blending the crushed returned concrete aggregates with an appropriate lightweight aggregate sand, a substantial reduction in autogenous shrinkage will be achieved, with minimal reduction in long term compressive strength. The mortars based on light weight aggregate sand substitutions alone provided the highest compressive strengths and the greatest reductions in autogenous shrinkage. But, blending the crushed returned concrete aggregates with the light weight aggregate sand may provide the optimum mixture in terms of material costs and sustainable development.[ICwCCA]
3.4 Artificial LWA – Expanded Shale, Clay and Slate Lightweight Aggregate The Earth has been producing LWA from volcanoes since the beginning of time. This natural material, however, is inconsistent and very little is suitable for making concrete. ESCS is specially made for concrete and has been manufactured from surface‐mined raw shale, clay or slate for nearly 100 years. (ESCS raw materials typically do not have any other conventional purpose in the construction industry because they are too soft.) The raw materials for ESCS production are placed into a rotary kiln at approx. 1200°C until it turns into a strong consistent material which is called expanded shale, clay or lightweight aggregate or just ESCS for short. ESCS is a uniform, high quality, ceramic aggregate that’s about 1/2 the weight of natural aggregates.
Pores are created in ESCS during the manufacturing process as gases escape due to the application of heat. The newly created pores are ideally suited to accommodate the absorption of water, much like a sponge. ESCS’s greatest contribution is its ability to desorb water. Unlike a sponge, it does not have to be squeezed for the water to be released. This characteristic naturally permits water to egress or be desorbed from the pores of pre-wetted ESCS when the cement demands more water during the hydration process. The physical ability of the pores to manage water movement is the key to internal curing. However, the manufacturing heating process of ESCS is relatively expensive. Moreover, compared with other lightweight aggregate, ESCS has a relatively fine and less continuous pore system, a large part of the pores are closed. Some of the pore water is held down to at least RH=70%. Consequently only a part of the water held in ESCS will be useful for internal curing .
3.5 Natural LWA – Perlite and Pumice Perlite is a naturally occurring silicious, glassy rock which contains 2-6% combined water. When quickly heated to above 900°C, the crude rock expands 4-20 times its original volume as the combined water vaporizes and creates countless tiny bubbles. This results in a bulk density in the range 30-400 kg/m3, and a water absorption of 200-600%. Perlite has found multiple uses such as for filtration, as an abrasive and within horticulture to provide aeration and moisture retention. However, perlite is primarily used within the construction area for example as concrete aggregate and as a cavity-filling insulation. Disintegration of perlite particles has been observed during mixing due to their high porosity and consequently low strength . This may have adverse effects on the concrete. Fully saturated, the water content of perlite may be 4.5 kg/kg . Pumice is a porous volcanic rock which resembles a sponge, see Figure 5.2. The porous structure is formed by dissolved gases which are precipitated during the cooling as the lava hurtles through the air. All types of magma may form pumice. The connectivity of the pore structure may range from completely closed to completely open. A representative value for the absorption of pumice is 0.27 kg/kg .