INTERNAL CURING OF CONCRETE

Abstract

It is often said that there are two types of concrete: concrete that has cracked and concrete that is going to crack. Unfortunately, this is true all too frequently. Many of these redundant cracks develop soon after the concrete is placed and, in addition to being unprepossessing, can contribute to reduce long term durability. Internal curing has been defined by the American Concrete Institute (ACI) as "supplying water throughout a freshly 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". While external curing water is applied at the surface and its depth of penetration is influenced by the quality of the concrete, internal curing enables the water to be distributed more equally throughout the cross section. In our research we have used expanded clayey shale(LWA) as replacement for coarse aggregates in 5%, 15%, and 25 %. We have considered M20 grade of concrete for our project

NOMENCLATURE

Al₂O₃ Aluminium oxide (chemical)

α_max Expected maximum degree of hydration (ratio)

ASTM American society for testing and materials (Organization)

C_f Cement content (kg/m3 )

CoCL₂ Cobalt(II) chloride (chemical)

CRCA Crushed returned concrete aggregate (material)

CS Chemical shrinkage (g/gcement )

d Pore diameter (m)

ε_p Shrinkage strain of concrete (μm/m)

ε_p Shrinkage strain of cement paste (μm/m)

E_a Elastic modulus of the aggregate (MPa)

E_c Elastic modulus of the concrete (MPa)

IC Internal curing (method)

LWA Lightweight aggregate (material)

LWA-H Haydite lightweight aggregate (material)

LWA-K Kenlite lightweight aggregate (material)

MgO Magnesium oxide (chemical)

OPC Ordinary portland cement (material)

ν_a Poisson’s ratio of aggregate (unitless)

VFA Volume fraction of fine aggregate (%)

VLWA Volume proportions of lightweight aggregate (%)

Acknowledgment

We express our our deep sense of gratitude and indebtedness on the successful completion of our project work, which would be incomplete without the mention of the people who made it possible through their precious guidance, encouragement, supervision and helpful discussions.

Abstract

TABLE OF CONTENTS

Abstract………………………………………………………………………………………………..iv

List of Figures………………………………………………………………………………………….v

List of Tables………………………………………………………………………………………….vi

Nomenclature…………………………………………………………………………………………..x

Chapter 1 Introduction & Objectives

1.1 Introduction ......................................................................................................................................

1.1.1 Curing methods and materials .......................................................................................................

1.1.2 Internal Curing

1.2 Objectives .........................................................................................................................................

Chapter -2 Research approach

2.1 Literature review ...............................................................................................................................

2.2 Characterization of local materials ...................................................................................................

2.3 Evaluating self-curing concrete made with local materials ..............................................................

Chapter -3 Experimental investigation

3.1 Test for aggregates .............................................................................................................................

3.1.1 Specific gravity test .......................................................................................................................

3.1.2 Bulk density test ............................................................................................................................

3.1.3 Finesses modulus ...........................................................................................................................

3.1.4 Water absorption test .....................................................................................................................

3.2 Properties of cement .........................................................................................................................

3.3 Test for concrete ...............................................................................................................................

3.3.1 Workability test .............................................................................................................................

3.3.2 Compressive strength test ..............................................................................................................

CHAPTER 4 Mechanism of internal curing

4.1 Volume of water needed for IC ......................................................................................................

4.2 Ability of water to leave the LWA ................................................................................................

4.3 LWA Spacing ..................................................................................................................................

Chapter 5 Results and discussion

5.1 Introduction ....................................................................................................................................

5.2 Experimental results .......................................................................................................................

5.3 Summary .........................................................................................................................................

Chapter 6 Conclusions

6.1 Introduction ....................................................................................................................................

6.2 Future works ...................................................................................................................................

References ............................................................................................................................................

List of tables

List of figure

NOMENCLATURE

Al₂O₃ Aluminium oxide (chemical)

α_max Expected maximum degree of hydration (ratio)

ASTM American society for testing and materials (Organization)

C_f Cement content (kg/m3 )

CoCL₂ Cobalt(II) chloride (chemical)

CRCA Crushed returned concrete aggregate (material)

CS Chemical shrinkage (g/gcement )

d Pore diameter (m)

ε_p Shrinkage strain of concrete (μm/m)

ε_p Shrinkage strain of cement paste (μm/m)

E_a Elastic modulus of the aggregate (MPa)

E_c Elastic modulus of the concrete (MPa)

IC Internal curing (method)

LWA Lightweight aggregate (material)

LWA-H Haydite lightweight aggregate (material)

LWA-K Kenlite lightweight aggregate (material)

MgO Magnesium oxide (chemical)

OPC Ordinary portland cement (material)

ν_a Poisson’s ratio of aggregate (unitless)

VFA Volume fraction of fine aggregate (%)

VLWA Volume proportions of lightweight aggregate (%)

CHAPTER 1. INTRODUCTION & OBJECTIVES

1.1 . INTRODUCTION

Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a period of time immediately following placing and finishing so that the desired properties may develop. The need for adequate curing of concrete cannot be overemphasized. Curing has a strong influence on the properties of hardened concrete; proper curing will increase durability, strength, water tightness, abrasion resistance, volume stability, and resistance to freezing and thawing and de-icers. Exposed slab surfaces are especially sensitive to curing as strength development and freeze-thaw resistance of the top surface of a slab can be reduced significantly when curing is defective.

When Portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed influences the strength and durability of the concrete. Freshly mixed concrete normally contains more water than is required for hydration of the cement; however, excessive loss of water by evaporation can delay or prevent adequate hydration. The surface is particularly susceptible to insufficient hydration because it dries first. If temperatures are favourable, hydration is relatively rapid the first few days after concrete is placed; however, it is important for water to be retained in the concrete during this period, that is, for evaporation to be prevented or substantially reduced.

Fig. 1.1. Curing should begin as soon as the concrete stiffens enough to prevent marring or erosion of the surface.

1.1.1. CURING METHODS AND MATERIALS

Concrete can be kept moist (and in some cases at a favourable temperature) by three curing methods:

1. Methods that maintain the presence of mixing water in the concrete during the early hardening period. These include ponding or immersion, spraying or fogging, and saturated wet coverings. These methods afford some cooling through evaporation, which is beneficial in hot weather.

2. Methods that reduce the loss of mixing water from the surface of the concrete. This can be done by covering the concrete with impervious paper or plastic sheets, or by applying membrane-forming curing compounds.

3. Methods that accelerate strength gain by supplying heat and additional moisture to the concrete. This is usually accomplished with live steam, heating coils, or electrically heated forms or pads.

The method or combination of methods chosen depends on factors such as availability of curing materials, size, shape, and age of concrete, production facilities (in place or in a plant), aesthetic appearance, and economics. As a result, curing often involves a series of procedures used at a particular time as the concrete ages. For example, fog spraying or plastic covered wet burlap can precede application of a curing compound. The timing of each procedure depends on the degree of hardening of the concrete needed to prevent the particular procedure from damaging the concrete surface (ACI 308 1997).

1.1.2. INTERNAL MOIST CURING

Internal moist curing refers to methods of providing moisture from within the concrete as opposed to outside the concrete. This water should not affect the initial water to cement ratio of the fresh concrete. Lightweight (low-density) fine aggregate or absorbent polymer particles with an ability to retain a significant amount of water may provide additional moisture for concretes prone to self desiccation. When more complete hydration is needed for concretes with low water to cement ratios (around 0.30 or less), 60 kg/m3 to 180 kg/m3 (100 lb/yd3 to 300 lb/yd3) of saturated lightweight fine aggregate can provide additional moisture to extend hydration, resulting in increased strength and durability. All of the fine aggregate in a mixture can be replaced with saturated lightweight fine aggregate to maximize internal moist curing. Internal moist curing must be accompanied by external curing methods.

1.2. RESEARCH OBJECTIVES

The main objectives of this project are to provide information on development, manufacture, and performance of self-curing concrete made using local materials. Local materials are characterized to demonstrate which materials show the greatest potential for use in the production of self-curing concrete. High performance concrete mixtures are developed with self-curing capabilities using typical local materials. The benefits of self-curing are evaluated using laboratory testing. Finally, technology transfer has been performed to enable these materials to be developed, specified, and implemented in our project.

CHAPTER 2. RESEARCH APPROACH

The goal of this project was to congregate information on the development, manufacture, and performance of self-curing concrete made using local materials. Specific tasks considered in this project are outlined as follows:

· Task 1: Literature Review.

· Task 2: Characterization of Local Materials.

· Task 3: Evaluating Self-Curing Concrete Made with Local Materials.

2.1. LITERATURE REVIEW

The first task of this study was to perform a review of literature pertaining to the development, testing, and use of self-curing concrete. The main objective of this review was to:

· Assemble papers related to the development of self-curing concrete. This included information about previous scientific developments, mixture proportioning procedures, materials that have been used successfully, and applications of self-curing concrete. This study was expedited with information from the RILEM state of the art report on self-curing concrete.

· Assemble a complete listing of test procedures to evaluate self-curing concrete. The procedures were reviewed both based on their ability to determine theoretically fundamental properties as well as the ability to be used for performing practical field tests.

· Assemble information on the locally available constituent materials and concrete mixture proportions that would be available for the production of self-curing concrete.

Bentz.D.P. has studied that the substitution of light weight aggregate (LWA) sand for a portion of the normal weight sand to provide internal curing for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. In his study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.

Holm.T.A. has stated that for more than 80 years, shale’s, clays, and slates have been Copyright to IJIRSET expanded in rotary kilns to produce structural grade LWA for use in concrete and masonry units. Millions of tons of structural grade LWA produced annually are used in structural concrete applications.

Khokrin, N.K, discussed the unique physical characteristics of rotary kiln expanded slate lightweight aggregate for producing high performance and high strength lightweight concrete. The compressive strength, elastic modulus, splitting tensile strength, specific creep, and other properties of lightweight concrete are significantly affected by the structural properties of the lightweight aggregate used. Concrete production, transportation, pumping and placing are also affected.

Hoff. G.C. described the use of near-saturated lightweight aggregate (LWA) as a replacement for a portion of the normal weight aggregate (NWA) in high-strength/high-performance concrete in order to mitigate or eliminate the self-desiccation and autogenous shrinkage that can occur which can further lead to early age cracking and long-term durability problems. The amount of LWA used to achieve beneficial internal curing is a function of the type of LWA, its size and amount, the degree of moisture preconditioning the LWA receives, the amount and type of binder(s) in the mixture, the water binder ratio at mixing, and the amount and duration of external moist curing provided to the concrete element.

Arnon Bentura et.al . studied that the concrete with saturated lightweight aggregate exhibited no autogenous shrinkage, whereas the normal-weight concrete with the same matrix exhibited large shrinkage. The study shows that the partial replacement of normal-weight aggregate by 25% by volume of saturated lightweight aggregate was very effective in eliminating the autogenous shrinkage and restrained stresses of the normal-weight concrete. It is noted that the internal supply of water from the saturated lightweight aggregate to the high-strength cement matrix caused continuous expansion, which may be related to continuous hydration.

Ryan Henkensiefkenet. al has indicated that while internal curing may have been originally developed to reduce autogenous shrinkage and mitigate early-age cracking in high performance concretes, its application has far-reaching consequences for the performance of concrete throughout its lifetime By providing an on-demand source of extra water, internal curing can improve the slump retention, workability and finishability of fresh concrete and reduce deformations and cracking due to plastic, autogenous and drying shrinkage.

2.2. CHARACTERIZATION OF LOCAL MATERIALS

Task 2 details important physical properties of the LWA that are needed so that the LWA can be used as an effective internal curing agent. The two main physical properties of the LWA are that the LWA must be able to absorb a large volume of water and be able to desorbs (i.e., give back) that water to the concrete as self-desiccation occurs. The testing procedures used to determine these properties of lightweight aggregate; most specifically focusing on absorption procedures is presented. Highlighted in this task are the inadequacies of these testing procedures, the variability which can result, and the influence of these properties on the shrinkage performance.

2.3. EVALUATING SELF-CURING CONCRETE MADE WITH LOCAL MATERIALS

Workability and compressive strength test were utilized to evaluate self curing concrete made with local materials. This task is conducted in the experimental investigation stated in chapter 3.

CHAPTER 3. EXPERIMENTAL INVESTIGATIONS

3.1. TEST FOR AGGREGATES

3.1.1 Specific Gravity Test

AIM:-

To determine the specific gravity of given sample of fine and coarse aggregates.

APPARATUS

1. A balance or scale of capacity not less than 3 kg, readable and accurate to 0.5 g and of such a type and shape as to permit the basket containing the sample to be suspended from the beam and the weighed in water.

2. A well ventilated oven thermostatically controlled to maintain a temperature of 100oC to 110oC.

3. A wire basket of not more than 6.3 mm mesh or a perforated container of convenient size.

4. A stout water tight container of convenient size.

5. Two dry soft absorbent cloths each not less than 75×45 cm

6. A shallow tray of area no less than 650 cm.

7. An air tight container of capacity similar to that.

PROCEDURE

· Take 2 kg of aggregate. Sample larger than 10mm

· Wash the sample thoroughly to remove finer particle and dust.

· Place the sample in a wire basket and immerse it in distilled water at a temperature between 22oC and 32oC with a cover of at least 5 cm of water above the top of the basket.

· Remove the entrapped air by lifting the basket containing the sample 25 mm above the base of the tank and allowing it to drop per second, care being taken to see that the sample is completely immersed in water during the operation.

· With the sample in water at a temperature of 220C-32oC (W).

· Remove the basket and aggregate from water and allow To drain for a few minutes.

· Empty the aggregate from the basket to a shallow tray.

· Immerse the empty basket in water jolt 25 times and than the weight in water (w2).

· Place the aggregates in oven at a temperature of 100oC to 110oC for 24+- 0.5 hours.

· Remove it from the oven and cool it and find the weight. (w2)

3.1.2 Bulk density test

AIM:- Determination of unit weight or bulk density of aggregate

Apparatus

Name	Capacity
1. Balance	Sensitive to 0.5% of weight of material
2. Cylindrical metal measure	Of 3 litre capacity (for fine aggregate) Of 15 litre capacity (for aggregate upto 40mm)
3. Tamping rod	16 mm dia. and 60 cm long

Table 3.1.2

Fig. 3.1.2 cylindrical metal measure used for bulk density of aggregate.

SAMPLE PREPARATION

§ Take the required amount of test sample form the bulk sample using any sample reduction method.

§ The test is normally carried out on dry material, but when bulking tests are required, material with a given percentage of moisture may be used.

PROCEDURE FOR COMPACTED BULK DENSITY

· Measure the volume of the cylindrical metal measure by pouring water into the metal measure and record the volume “V” in litre .

· Fill the cylindrical metal measure about one-third full with thoroughly mixed aggregate and tamp it 25 times using tamping bar.

· Add another layer of one-third volume of aggregate in the metal measure and give another 25 strokes of tamping bar.

· Finally fill aggregate in the metal measure to over-flowing and tamp it 25 times.

· Remove the surplus aggregate using the tamping rod as a straightedge.

· Determine the weight of the aggregate in the measure and record that weight “W” in kg.

3.1.3 Finess Modulus

AIM:-

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by 100

PREOCEDURE

§ Sieve the aggregate using the appropriate sieves (80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron & 150 micron)

§ Record the weight of aggregate retained on each sieve.

§ Calculate the cumulative weight of aggregate retained on each sieve.

§ Calculate the cumulative percentage of aggregate retained.

§ Add the cumulative weight of aggregate retained and divide the sum by 100. This value is termed as fineness modulus

§ Refer the following example calculation

3.1.4 Water absorption test

AIM:-

To determine the water absorption of aggregates

APPARTURES

1. A balance of capacity about 3kg, to weigh accurate 0.5g, and of such a type and shape as to permit weighing of the sample container when suspended in water.

2. A thermostatically controlled oven to maintain temperature at 100-110° C.

3. A wire basket of not more than 6.3 mm mesh or a perforated container of convenient size with thin wire hangers for suspending it from the balance.

4. A container for filling water and suspending the basket

5. An air tight container of capacity similar to that of the basket

6. A shallow tray and two absorbent clothes, each not less 75x45cm.

PROCEDURE

· The sample should be thoroughly washed to remove finer particles and dust, drained and then placed in the wire basket and immersed in distilled water at a temperature between 22 and 32^oC.

· After immersion, the entrapped air should be removed by lifting the basket and allowing it to drop 25 times in 25 seconds. The basket and sample should remain immersed for a period of 24 + ½ hrs afterwards.

· The basket and aggregates should then be removed from the water, allowed to drain for a few minutes, after which the aggregates should be gently emptied from the basket on to one of the dry clothes and gently surface-dried with the cloth,transferring it to a second dry cloth when the first would remove no further moisture .The aggregates should be spread on the second cloth and exposed to the atmosphere away from direct sunlight till it appears to be completely surface-dry. The aggregates should be weighed (Weight ‘A’).

· The aggregates should then be placed in an oven at a temperature of 100 to 110^oC for 24hrs. It should then be removed from the oven, cooled and weighed (Weight ‘B’).

FORMULA USED

Water absorption = [(A – B)/B] x 100%.

Two such tests should be done and the individual and mean results should be reported.

3.2. PROPERTIES OF CEMENT

Cement is defined as the product manufactured by burning and crushing to powder an intimate and well-proportioned mixture of calcareous and argillaceous material

The cement, which is generally used for preparing concrete, is the Ordinary Portland Cement. But for special purposes other qualities of cement such as Low Heat Cement, Rapid Hardening Cement, High Alumina Cement, White Cement, Blast Furnace Slag Cement, Sulphate Resisting Cement, etc. are also use

It is always desirable to use the best cement in constructions. Therefore, the properties of a good cement must be investigated. Although desirable cement properties may vary depending on the type of construction, generally a good cement possesses following properties (which depend upon its chemical composition, thoroughness of burning and fineness of grinding).

Provides strength to masonry.
Stiffens or hardens early.
Possesses good plasticity.
An excellent building material.

1	Fineness (m²/kg)	Not less than 225 m²/kg
2	Soundness
	(a) Lechatelier expansion (mm)	Not more than 10%
	(b) Auto clave expansion (%)	Not more than 0.08%
3	Setting time (in minutes)
	(a) Initial setting	Not less than 30 minute
	(b) Final setting		Not greater than 60 minutes
4	Compressive strength (MPa)
	(a) After 73 +/- 1 hours		Not less than 27 MPa
	(b) After 168 +/- 2 hours		Not less than 37 MPa
	(c) 672 +/- 4 hours		Not less than 53 MPa

Table3.2. Physical requirement of 53 grade cement

3.3. TEST FOR CONCRETE

3.3.1 Workability test (Slump Test)

Slump test is used to determine the workability of fresh concrete. Slump test as per IS: 1199 1959 is followed .The apparatus used for doing slump test are Slump cone and Tamping rod.

APPARATUS

Slump cone,
Scale for measurement,
Temping rod (steel)

PROCEDURE

· The mould for the slump test is a frustum of a cone, 300 mm (12 in) of height. The base is 200 mm (8in) in diameter and it has a smaller opening at the top of 100 mm (4 in).

· The base is placed on a smooth surface and the container is filled with concrete in three layers, whose workability is to be tested

· Each layer is temped 25 times with a standard 16 mm (5/8 in) diameter steel rod, rounded at the end.

· When the mould is completely filled with concrete, the top surface is struck off (levelled with mould top opening) by means of screening and rolling motion of the temping rod.

· The mould must be firmly held against its base during the entire operation so that it could not move due to the pouring of concrete and this can be done by means of handles or foot - rests brazed to the mould.

· Immediately after filling is completed and the concrete is levelled, the cone is slowly and carefully lifted vertically, an unsupported concrete will now slump.

· The decrease in the height of the centre of the slumped concrete is called slump.

· The slump is measured by placing the cone just besides the slump concrete and the temping rod is placed over the cone so that it should also come over the area of slumped concrete.

· The decrease in height of concrete to that of mould is noted with scale. (usually measured to the nearest 5mm (1/4 in).

Fig. 3.3.1. Height of slump.

PRECAUTIONS

In order to reduce the influence on slump of the variation in the surface friction, the inside of the mould and its base should be moistened at the beginning of every test, and prior to lifting of the mould the area immediately around the base of the cone should be cleaned from concrete which may have dropped accidentally.

TYPES OF SLUMP

Collapse Slump
Shear Slump
True Slump

Fig. 3.3.2. Types of slump.

COLLAPSE SLUMP

In a collapse slump the concrete collapses completely. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.

SHEAR SLUMP

In a shear slump the top portion of the concrete shears off and slips sideways.

If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump.

1. If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.

2. If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix.

TRUE SLUMP

In a true slump the concrete simply subsides, keeping more or less to shape

This is the only slump which is used in various tests.
Mixes of stiff consistence have a Zero slump, so that in the rather dry range no variation can be detected between mixes of different workability.

However , in a lean mix with a tendency to harshness, a true slump can easily change to the shear slump type or even to collapse, and widely different values of slump can be obtained in different samples from the same mix; thus, the slump test is unreliable for lean mixes.

Applications of Slump Test

The slump test is used to ensure uniformity for different batches of similar concrete under field conditions and to ascertain the effects of plasticizers on their introduction.
This test is very useful on site as a check on the day-to-day or hour- to-hour variation in the materials being fed into the mixer. An increase in slump may mean, for instance, that the moisture content of aggregate has unexpectedly increases.
Other cause would be a change in the grading of the aggregate, such as a deficiency of sand.
Too high or too low a slump gives immediate warning and enables the mixer operator to remedy the situation.

This application of slump test as well as its simplicity is responsible for its widespread use.

3.3.2 Compressive strength test (Cube Test)

OBJECTIVE

The tests are required to determine the strength of concrete and therefore its suitability for the job.

REFERENCE STANDARDS

IS : 516-1959 – Methods of tests for strength of concrete

EQUIPMENT & APPARATUS

1. Compression testing machine (2000 KN).

2. Curing tank/Accelerated curing tank.

3. Balance (0-10 Kg).

Fig 3.3.2. Compressive testing machine.

PROCEDURE

· Representative samples of concrete shall be taken and used for casting cubes 15 cm x 15 cm x 15 cm or cylindrical specimens of 15 cm dia x 30 cm long.

· The concrete shall be filled into the moulds in layers approximately 5 cm deep. It would be distributed evenly and compacted either by vibration or by hand tamping. After the top layer has been compacted, the surface of concrete shall be finished level with the top of the mould using a trowel; and covered with a glass plate to prevent evaporation.

· The specimen shall be stored at site for 24+ ½ h under damp matting or sack. After that, the samples shall be stored in clean water at 27+2⁰C; until the time of test. The ends of all cylindrical specimens that are not plane within 0.05 mm shall be capped.

· Just prior to testing, the cylindrical specimen shall be capped with sulphur mixture comprising 3 parts sulphur to 1 part of inert filler such as fire clay.

· Specimen shall be tested immediately on removal from water and while they are still in wet condition.

· The bearing surface of the testing specimen shall be wiped clean and any loose material removed from the surface. In the case of cubes, the specimen shall be placed in the machine in such a manner that the load cube as cast, that is, not to the top and bottom.

· Align the axis of the specimen with the steel plates, do not use any packing.

· The load shall be applied slowly without shock and increased continuously at a rate of approximately 140 kg/sq.cm/min until the resistance of the specimen to the increased load breaks down and no greater load can be sustained. The maximum load applied to the specimen shall then be recorded and any unusual features noted at the time of failure brought out in the report.

SAFETY & PRECAUTIONS

1. Use hand gloves, safety shoes & apron at the time of test.

2. After test switch off the machine.

3. Keep all the exposed metal parts greased.

4. Keep the guide rods firmly fixed to the base & top plate.

5. Equipment should be cleaned thoroughly before testing & after testing.

CHAPTER 4. MECHANISMS OF INTERNAL CURING

Water is released from water-filled prewetted lightweight aggregate (SLWA) when it is used for internal curing. As such, the IC water needs to be described in three main ways:

1. the volume of water available for IC,

2. the ability of the water to leave the SLWA when needed for IC, and

3. the distribution of the SLWA so that it is well-dispersed and its water can readily travel

to all of the sections in the paste where it is needed.

This section provides a brief overview of each of these topics.

4.1. VOLUME OF WATER NEEDED FOR IC

In order to determine the total volume of water that is needed for IC, two terms must be defined, chemical shrinkage and autogenous shrinkage.

Chemical shrinkage, the primary cause of autogenous shrinkage and self desiccation, is a naturally occurring process that has been known for over 100 years (La Chatelier 1900; Powers 1935; L'Hermite 1960; Geiker 1983). Chemical shrinkage describes the total volume reduction of the cement system occurring during hydration of cement (Sant et al. 2006), since the volume of the hydration products is less than the volume of the reactants. It has been reported that this reduction can be as much as 8 % to 10 % by volume in a mature paste (Jensen et al. 2001a; Jensen et al. 2001b). Chemical shrinkage describes the total volume reduction of the system and is typically measured by placing cement paste in water and measuring the volume change of the cement-water system (Knudsen et al. 1982). When water is able to permeate the paste, it enables the total volume change to be measured (it is important to note that in low w/c systems the depercolation of the paste’s capillary porosity may occur and water cannot travel through the paste) (Knudsen et al. 1982). Work by Sant has shown that the depercolation occurs for a 3 mm cement paste sample with a w/c of 0.30 at approximately 24 h (Sant 2007). A typical chemical shrinkage result is shown in Fig. 4.1 for a paste with w/c of 0.30 by mass fraction (made with the same cement as used in this study). While the buoyancy measurement technique was used to obtain the data in Figure 3-1 (Sant et al. 2006), other procedures could be used such as the ASTM C1608 standard test method for the measurement of chemical shrinkage (2005).

Fig. 4.1. Chemical shrinkage and autogenous shrinkage volumes during hydration of a paste with a w/c of 0.30 (Henkensiefken et al. 2008f).

Autogenous shrinkage can be described as the external volume reduction of a cement paste as it hydrates (Jensen 2005). Before set, the volumes of chemical shrinkage and autogenous shrinkage are the same. This occurs since the system is fluid and collapses on itself as it shrinks (Barcelo et al. 2000; Bjøntegaard et al. 2004; Sant et al. 2006). At the time of set, however, the volumes of chemical shrinkage and autogenous shrinkage diverge, as shown in Figure 3-1 (Henkensiefken et al. 2008f). The solidification of the matrix prevents the bulk system from shrinking as chemical shrinkage occurs. This results in an under pressure in the fluid that eventually cavitates vapour-filled space in the pore system (Couch et al. 2006). As hydration proceeds, these voids grow and penetrate smaller and smaller pores. The internal relative humidity can be measured in these concretes to assess the pressure that develops in the fluid

The concept in using SLWA for IC is that the SLWA can be used as internal reservoirs that can reduce the pressure in the pore fluid by ‘replenishing the vapour-filled voids,’ effectively eliminating their creation within the hydrating cement paste. While strictly speaking, the volume of water that is needed is only the difference between these curves, accounting for the behaviour prior to set will generally be only a minor correction and may be unnecessary for field applications. Therefore, the volume of chemical shrinkage may be approximated as the volume of water that needs to be supplied by the IC agent.

Bentz and Snyder (Bentz et al. 1999b) used this principle to develop a method to estimate the volume of IC water that is needed and the mass of lightweight aggregate (LWA) that is needed to hold the IC water (Bentz et al. 1999b; Bentz et al. 2005).

Equation 4.1

where: MLWA (kg/m3 or lbs/yd3) is the mass of LWA (in a dry state) that needs to be prewetted to provide water to fill in the voids created by chemical shrinkage, Cf (kg/m3 or lbs/yd3) is the cement content of the mixture, CS (g of water per g of cement or lb of water per lb of cement) is the chemical shrinkage of the cement, αmax (unit less) is the expected maximum degree of hydration (0 to 1), S (unit less) is the expected degree of saturation of the LWA (0 to 1) and was taken to be 1 in this study when the LWA was soaked for 24 h, and ΦLWA (kg of water/kg of dry

LWA or lb/lb) is the absorption capacity of the LWA (taken here as the 24 h absorption). An absorption period of 24 h was chosen to represent “saturated” conditions in this study. Care must be taken when using the term “saturated” when describing LWA. Because of the pore structure of LWA, it is likely that the LWA is not in a truly saturated state. The pores of the LWA could continually absorb water for several weeks or months if left submerged. When the term “saturated” is used here, it is not intended to mean all the pores of the LWA are filled with water, but rather to a specific degree of saturation (Henkensiefken et al. 2008c). Further details can be found in chapter 2 of this thesis. A more correct approach to determining ΦLWA would be to use the measured desorption capacity of the LWA from saturation down to 92 % RH for example, as in an actual concrete, since the LWA is initially “saturated” and undergoes desorption during IC (Bentz et al. 2005).

4.2. ABILITY OF WATER TO LEAVE THE LWA

Water leaves the SLWA due to the suction (underpressure) that develops in the pore fluid within the hydrating cement paste due to its chemical shrinkage and self-desiccation. The consequence of this water movement from SLWA to surrounding paste is frequently quantified in mortar as an increase in internal relative humidity and an increase in the critical pore size that remains prewetted (Lura 2003a; Henkensiefken et al. 2008f). The basic principle for internal curing is that the largest pores will lose water first as the capillary stress that develops will be minimized when pores are emptied in this order. The LWA pores are generally larger than the pores of the surrounding cement paste. The pore size distributions of hydrated cement paste at three different ages and of two different LWA were determined using mercury intrusion porosimetry (MIP), the results of which can be seen in Fig. 4.2.1. Fig. 4.2.1. demonstrates that the pores of both the LWAs used in this study are larger than the pores of the cement paste, even at early hydration ages. It should be noted that the maximum standard deviation of the MIP results for the LWA and the cement paste (based on the total absorption) was 0.03 mL per gram of sample and 0.01 mL per gram of cement, respectively. Another important feature from the MIP pore size distribution is that the pores of the paste decrease in size as the specimen ages (or as hydration continues). As the pores in the paste decrease in size, the capillary pressure that would develop upon their emptying increases, effectively increasing the driving force that pulls water out of the LWA. This implies that as the specimen ages, the hydrating paste can pull water out of successively smaller pores of the LWA. Fig.4.2.2 illustrates desorption isotherms of two LWA (LWA-K and LWAH). The standard deviation between duplicate samples was 0.3 %. A majority of water is lost at a high relative humidity (RH>96 %) implying the pores in the LWA are large (>25 nm) and the water is available to be lost at high relative humidities, which is preferred for internal curing. It is also important to note that the samples release almost all (96 %) of their moisture when a relative of humidity of 92 % is reached, which implies that the water will leave the pores of the LWA if a large enough suction pressure (or a low enough internal relative humidity) exists. It should be noted that this favorable desorption behavior is not characteristic of all LWA (Lura 2003a).

Fig. 4.2.1. Pore size distribution for cement paste with a w/c of 0.30 at three different ages and for two different LWA measured using mercury intrusion porosimetry.

Fig.4.2.2. Desorption isotherm of two different LWA.

4.3 LWA SPACING

Knowing how much water is needed for IC (Section 3.1) and knowing how much of that water will likely leave the aggregate (Section 3.2), the last issue that needs to be understood is the distribution of the SLWA throughout the microstructure. Even if a sufficient volume of water is supplied to a system, as determined from Equation 3-1, if the distribution of the water (i.e., SLWA) is insufficient, the system will likely exhibit poor shrinkage performance. This has been shown by comparing the effectiveness of coarse SLWA and fine SLWA when the same volume of water is considered (van Breugel et al. 2000; Zhutovsky et al. 2002). Though the volume of water may be the same, the distribution of the SLWA particles will be much different, resulting in a different volume of protected paste (i.e., the volume fraction of the paste within a given distance from a SLWA particle). The coarse SLWA proved to be less effective than the fine SLWA even though they had the same volume of water because of the increased spacing between the aggregate.

The concept of protected paste volume can also be applied when different volumes of fine SLWA are considered. For a constant LWA particle size distribution, when the volume of SLWA is increased, not only is the volume of water increased, but the distribution of water is also improved, which results in a larger fraction of protected paste. The effect of different volume replacements on the protected paste volume was simulated using the hard core/soft shell model developed at the National Institute of Standards and Technology (NIST) (Lu et al. 1992; Maekawa et al. 1999; Bentz et al. 1999a). The results of simulations for the mortars investigated in this study are shown in Figure 3-5. Figure 3-5 describes the volume fraction of cement paste that is within a given distance from the surface of a SLWA particle. It is not intended to show that the paste is protected by internal water stored in the SLWA, since properties like absorption and desorption of the aggregate as well as paste permeability would have to be known to determine whether the water in the SLWA could reach the paste. As the volume replacements are increased, the fraction of paste within a specified proximity of a SLWA also increases. When lower replacements are incorporated in a mixture, there is a rapid increase in the fraction of paste within 1.0 mm of a SLWA particle. At low replacement volumes, if the water in the SLWA can travel up to 1.0 mm, a majority of the paste will be protected. When higher replacement volumes are used, a larger fraction of paste is within 0.20 mm, which may be important at later ages (RILEM 2007). This is because water cannot travel as far in a mature paste, and the distribution of the SLWA becomes important. This implies that if water is able to travel a large distance, a low replacement volume can protect a majority of the paste (assuming the SLWA can provide enough water). However, if the water cannot travel large distances, a higher replacement volume is needed to protect a majority of the paste.

Fig.4.3.1. Volume fraction of paste within a specified distance of a SLWA. This plot does not show whether water can reach the paste. Volume and travel distance of water need to be considered to determine this.

Fig.4.3.1. shows the protected paste concept when the same volume replacement of LWA is presented. Figure 3-6(a) indicates the protected paste volume when coarse aggregate is used. Figure 3-6(b) has the same replacement volume of aggregate; however the aggregate replaced is the fine aggregate. A clear distinction can be made in the protected paste volume in these two figures. Because of the better particle distribution, the fine aggregate has more potential to protect the surrounding cement paste than coarse aggregate. Using fine aggregate instead of coarse aggregate could have implications on the strength of the mixture. Replacing the coarse normal weight aggregate with coarse LWA could have detrimental effects on the strength of the concrete. When dealing with higher strength concretes, the aggregate particles will likely be the region of failure, and introducing large weak particles into the system could reduce the strength. By replacing the fine normal weight aggregate with fine LWA, the effects if including a weaker aggregate could be reduced since the fine aggregate does not affect the strength of the concrete as much as the coarse aggregate.

Fig.4.3.2. Illustrations showing the protected paste volume concept of two mixtures with similar LWA replacements volumes, but one replacement is of (a) coarse aggregate, and the other of (b) fine aggregate (Henkensiefken 2008a)

CHAPTER 5. RESULT & DISCUSSION

5.1 INTRODUCTION

A

Acknowledgment

We express our deep sense of gratitude and indebtedness on the successful completion of our project work, which would be incomplete without the mention of the people who made it possible through their precious guidance, encouragement, supervision and helpful discussions.