effect of duration of curing on compressive strength of concreeete

KIGALI INSTITUTE OF SCIENCE AND TECHNOLOGY

INSTITUT DES SCIENCES ET TECHNOLOGIE DE KIGALI

Avenue de l’Armée, BP3900 Kigali- Rwanda

FACULTY OF ENGINEERING

DEPARTMENT OF CIVIL ENGINEERING AND ENVIRONMENTAL TECHNOLOGY

A PROJECT REPORT

ON

Submitted by:

NGENDAHINYERETSE Alexandre (REG. NO: GS20050638)

Under the guidance of

Mr.TWUBAHIMANA Joseph Desire

Submitted in partial fulfillment of requirements for the award of

BACHELOR OF SCIENCE DEGREE IN CIVIL ENGINEERING AND ENVIRONMENT TECHNOLOGY

SEPTEMBER, 2009

“EFFECT OF DURATION OF CURING ON COMPRESSIVE STRENGTH OF CONCREEETE”

PROJECT ID: CEET/09/11

i

KIGALI INSTITUTE OF SCIENCE AND TECHNOLOGY

INSTITUT DES SCIENCES ET DE TECHNOLOGIE DE KIGALI

Avenue de l'Armée, B.P. 3900 Kigali, Rwanda

FACULTY OF ENGENEERING

DEPARTMENT OF CIVIL ENGINEERING AND ENVIRONMENTAL

TECHNOLOGY

C E R T I F I C A T E

This is to certify that the Project Work entitled “EFFECT OF DURATION OF

CURING ON COMPRESSIVE STRENGTH OF CONCRETE” is a record of the

original work done by NGENDAHINYERETSE Alexandre (REG.No: GS20050638)

in partial fulfillment of the requirement for the award of Bachelor of Science Degree in

Civil Engineering and Environmental Technology of Kigali Institute of Science and

Technology during the Academic Year 2009.

………………………….. …………………………

TZUBAHIMANA Joseph Desire G. Senthil KUMARAN

Project Supervisor HEAD, Dept. of CE&ET

Submitted for the Project Examination held at KIST on September 2009

ii

DECLARATION

I, NGENDAHINYERETSE Alexandre hereby declare that this research “EFFECT

OF DURATION OF CURING ON COMPRESSIVE STRENGTH OF CONCRETE”

for the award of Bachelor of Science Degree in Civil Engineering and Environmental

Technology and is my original work. All sources I have used and quoted have been

acknowledged as complete references.

…………………………..

NGENDAHINYERETSE Alexandre

REG NO 20050638

iii

DEDICATION

This research project is dedicated to:

Almighty God

Family members

Colleagues and friends

iv

ACKNOWLEDGEMENT

I am deeply intended to almighty God who has guided me through the whole period of

my studies. My sincere thanks are due all friends and colleagues who helped me in one-

way or another. I am very grateful to all members of my family for their support and

advice.

My special thanks are addressed to the Government of Rwanda for its appreciable

policy of promoting education at all levels.

Again my sincere acknowledgements go to entire administration of KIST and the whole

academic staff.

My sincere gratitude goes to my supervisor, Mr.TWUBAHIMANA Joseph Desire for

his technical and wise advice, suggestions and corrections that made this research

project fruitful.

Finally I express my gratitude to each one who directly and indirectly contributed to

make my studies successful today.

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ABSTRACT

The general objective of my study is “the verification of effect of curing duration on

compressive strength of concrete.” To achieve the goal I used the laboratory tests by

testing 66 cubes at different ages. I found the average compressive strength of concrete

stayed in air its entire time is 16.9 MPa ; 18.9 MPa for that stayed in air after 1 day ;21

MPa for that stayed in air after 2 days;26.2 MPa for that stayed in air after 3 days; 30.5

MPa for that stayed in air after 7 days; 33.2 MPa for that stayed in air after 14 days;

35.1 MPa for that stayed in air after 28 days. These results show that after 7 days the

compressive strength is 87 per cent of the design compressive strength of concrete; I

assumed that the minimum number of days required for curing concrete is equal to 7

days. It recommended to all construction companies not cure their concrete within

under 7 days in order to get the high compressive strength and not use a number of days

more than 7 days for preventing the lost.

vi

TABLE OF CONTENT

DECLARATION ......................................................................................................... ii

DEDICATION ............................................................................................................ iii

ACKNOWLEDGEMENT ........................................................................................... iv

ABSTRACT..................................................................................................................v

TABLE OF CONTENT .............................................................................................. vi

LISTE OF TABLES AND FIGURES............................................................................x

LIST OF ABBREVIATIONS ...................................................................................... xi

NOMENCLATURES AND SYMBOLS LIST ........................................................... xii

CHAP 1 : GENERAL INTRODUCTION......................................................................1

1.1 Introduction .........................................................................................................1

1.2 Problem statement ...............................................................................................1

1.3 Objectives of the study ....................................................................................1

1.3.1 Main objective ..............................................................................................1

1.3.2 Specific objectives ........................................................................................1

1.4 Scope of study .....................................................................................................2

1.5 Justification of the project ...................................................................................2

CHAP 2: LITTERATURE RIVIEW..............................................................................3

2.1 What is concrete? ................................................................................................3

2.1.1. Definition.....................................................................................................3

2.1.2 Composition .................................................................................................3

2.1.2.1 Cement....................................................................................................3

2.1.2.2 Water ......................................................................................................3

2.1.2.3 Aggregates ..............................................................................................4

2.1.2.4 Chemical admixtures ...............................................................................4

2.1.3 Water- cement ratio ....................................................................................4

2.1.4 Properties ......................................................................................................5

2.1.4.1 Strength ..................................................................................................5

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2.1.4.2 Elasticity .................................................................................................5

2.1.4.3 Cracking .................................................................................................5

2.1.4.4 Creep ......................................................................................................6

2.1.4.5 Physical property ....................................................................................6

2.2 Curing of concrete ...............................................................................................6

2.2.1 Definition .....................................................................................................6

2.2.2 Three phases of curing concrete ....................................................................7

2.2.3 Influence of curing on properties of concrete.................................................7

2.2.4 Duration of curing .........................................................................................8

2.2.4.1 introduction .............................................................................................8

2.2.5 Moist curing..................................................................................................9

2.2.6 Curing conditions ..........................................................................................9

2.2.7 Maturity of concrete .................................................................................... 10

2.2.8 Methods of curing concrete ......................................................................... 10

2.2.8.1 The methods which replenish partly the loss of water by interposing a

source of water, or prevent the evaporation. ...................................................... 10

2.2.8.1.1 Ponding of water over the concrete surface after it has set ............... 10

2.2.8.1.2 Covering the concrete with straw or Damp Earth ............................ 10

2.2.8.1.3 Covering the concrete with Burlap .................................................. 10

2.2.8.1.4 Sprinkling of water ......................................................................... 11

2.2.8.1.5 Covering the surface with waterproof paper .................................... 11

2.2.8.2 The methods preventing or minimizing the loss of water by interposing

an impermeable medium between the concrete and the surrounding environment

......................................................................................................................... 12

2.2.8.2.1 Leaving the shuttering or Formwork on .......................................... 12

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2.2.8.2.2 Membrane curing of the concrete .................................................... 12

2.2.8.2.3 Chemical curing .............................................................................. 12

2.2.8.3 Methods involving the application of artificial heat while the concrete is

maintained in a moist condition are used in plant curing where the curing of

concrete is accelerated by raising its temperature. ............................................. 13

2.3 compressive strength of concrete ....................................................................... 13

2.3.1 Introduction ................................................................................................ 13

2.3.2. How is compressive strength determined? .................................................. 14

2.3.3. Why do we test the compressive strength of concrete? ............................... 14

CHAP 3 MATERIALS AND METHODOLOGY ....................................................... 16

3.1 materials used .................................................................................................... 16

3.2 Methodology ..................................................................................................... 16

3.2.1 Making cubes .............................................................................................. 16

3.2.2 Crushing cubes ........................................................................................... 17

3.2.3 Tabulation of results ................................................................................... 19

3.2.4 Calculation.................................................................................................. 19

3.2.4.1 Determination of the strength ................................................................ 19

3.2.4.1.1 Introduction .................................................................................... 19

3.2.4.1.2 Derivation of the unit of the strength ............................................... 19

3.2.5 Graphing ..................................................................................................... 20

3.2.6 Analysis of results obtained ........................................................................ 20

CHAP4. DATA PRESENTATION, ANALYSIS AND INTERPRETATION ............. 21

4.1 Results at 3 days age .......................................................................................... 21

4.1.1 Diagrams of results for 3 days age............................................................... 21

4.1.2 Results discussion ....................................................................................... 22

4.2 Results at 7 days age .......................................................................................... 23

4.2.1 Diagrams of results for 7 days age............................................................... 23


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4.3.1 Diagrams of results for 14 days age ............................................................. 25

4.3.2 Results discussion: ...................................................................................... 26

4.4 Results at 28 days age ........................................................................................ 27

4.4.1 Diagrams of results for 28 days age ............................................................. 27


4.5 Figure summarizing all results ........................................................................... 29

CHAP 5. CONCLUSION AND RECOMMENDATIONS .......................................... 30

5.1 Conclusion ........................................................................................................ 31

5.2 Recommendations ............................................................................................. 31

REFERENCES ........................................................................................................... 32

APPENDICES ............................................................................................................ 33

x

LISTE OF TABLES AND FIGURES

Tab. 1 compressive strength of concrete at 3 days age ................................................. 21

Diag. 1 Variation of compressive strength of concrete with curing duration (for 3 days

age) ............................................................................................................................. 21

Fig. 1 variation of compressive strength with curing duration (for 3 days age) ............. 22

Tab. 2 Deficit and Increase in compressive strength percentages ................................. 22

Tab. 3 compressive strength of concrete at 7 days age ................................................. 23

Diag. 2 Variation of compressive strength of concrete with curing duration (for 7 days

age) ............................................................................................................................. 23

Fig. 2 Variation of compressive strength with curing duration (for 7 days age) ............ 24


Tab. 5 Compressive strength of concrete at 14 days age .............................................. 25

Diag. 3 Variation of compressive strength with curing duration (for 14 days age) ........ 25

Fig. 3 Variation of compressive strength with curing duration (for 14 days age) .......... 26


Tab. 7 Compressive strength of concrete at 28 days age .............................................. 27

Diag. 4 Variation of compressive strength with curing duration (for 28 days age) ........ 27

Fig. 2 Variation of compressive strength with curing duration (for 28 days age) .......... 28


Fig. 5 Strength of concrete dried in air after preliminary moist curing.......................... 29

Tab. 9 Effect of curing duration on compressive strength of concrete .......................... 30

xi

LIST OF ABBREVIATIONS

ASTM: American Society for Testing and Materials

CIMERWA: Cimenterie du Rwanda

KIST: Kigali Institute of Science and Technology

OPC: Ordinary Portland cement

RCC: Reinforced cement concrete

RWF: Rwandan franc

SFAR: Student Financing Agency in Rwanda

W/C: Water Cement ratio

xii

NOMENCLATURES AND SYMBOLS LIST

A: Column of compressive strengths at each curing duration

A-B: Column of difference in compressive strength of A and B

B: Column of compressive strength for the curing duration equal to 7 days

Cm : Centimetre

CO2 : Carbone dioxyde

GPA: Giga Pascal

KN: Kilo Newton

m: meter

MPa: Mega Pascal

m2: meter square

NaC: Sodium carbon

N: Newton

Psi: Pounds

%: percentage

OC : temperature

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CHAP 1 : GENERAL INTRODUCTION

1.1 Introduction

Concrete properties and durability are significantly influenced by curing since it greatly affects

the hydration of cement. Curing is the process of keeping concrete under specific environmental

condition until hydration is relatively complete. Because the cement used in concrete requires

time to fully hydrate before it acquires strength and hardness, concrete must be cured once it has

been placed. As the curing of concrete increases the compressive strength of concrete, the

duration of curing has also a certain effect on this compressive strength. In my research I have to

verify how the compressive strength of concrete varies with different duration of curing. At the

end of my work I am able of finding the minimum number of days required to achieve the

desired strength of concrete.

1.2 Problem statement

On various sites of construction the curing process is done without thinking on the effect of

curing duration on the compressive strength of concrete. It is a big problem because they use the

short time while curing the concrete which reduces the probability of getting the concrete of

good and desirable strength.

1.3 Objectives of the study

1.3.1 Main objective

The main objective of my work is:

-to check the effect of duration of curing on the compressive strength of concrete.

1.3.2 Specific objectives

The specific objectives of my work are:

To know the minimum number of days for curing the concrete up to the required

compressive strength of concrete. And

To increase the ability of getting the concrete of good strength

2

1.4 Scope of study

In my work I must deal with testing the compressive strength of 84 cubes. These cubes are in 7

categories: 12 cubes that have to stay in air entire time, 12 cubes that must be cured within 1day,

12 cubes to be cured within 2 days, 12 cubes that must be cured within 3 days, 12 cubes that

have to be cured within 7 days, 12 cubes that might be cured within 14 days and 12 cubes that

might be cured entire time (28days). The compressive strengths must be tested at the age of

3,7,14 and 28 days.

1.5 Justification of the project

This project will be the key of improving the behavior of the construction companies towards the

effect of duration of curing on the compressive strength of concrete. This must increase the

probability of getting the concrete of good strength.

3

CHAP 2: LITTERATURE RIVIEW

2.1 What is concrete?

2.1.1. Definition

Concrete is a construction material composed of cement (commonly Portland cement) as well as

other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse

aggregate such as gravel, limestone, or granite, plus a fine aggregate such as sand), water, and

chemical, admixtures. The word concrete comes from the Latin word "concretus" (meaning

compact or condensed), the past participle of "concresco", from "com-" (together) and "cresco"

(to grow). (From www.wikipedia.org/wiki/concrete)

2.1.2 Composition

2.1.2.1 Cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of

concrete, mortar, and plaster. English engineer Joseph Aspdin patented Portland cement in 1824;

it was named because of its similarity in color to Portland limestone, quarried from the English

Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of

calcium, silicon and aluminum. Portland cement and similar materials are made by heating

limestone (a source of calcium) with clay, and grinding this product (called clinker) with a

source of sulfate (most commonly gypsum). The manufacturing of Portland cement creates about

5 percent of human CO2 emissions.

2.1.2.2 Water

Combining water with cement forms a cement paste by the process of hydration. The cement

paste glues the aggregate together, fills voids within it, and allows it to flow more easily. Less

water in the cement paste will yield a stronger, more durable concrete; more water will give an

easier-flowing concrete with a higher slump. Impure water used to make concrete can cause

problems when setting or in causing premature failure of the structure.

4

2.1.2.3 Aggregates

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and

crushed stone, are mainly used for this purpose. Recycled aggregates (from construction,

demolition and excavation waste) are increasingly used as partial replacements of natural

aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag

and bottom ash are also permitted. Decorative stones such as quartzite, small river stones or

crushed glass are sometimes added to the surface of concrete for a decorative "exposed

aggregate" finish, popular among landscape designers.

2.1.2.4 Chemical admixtures

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete

to give it certain characteristics not obtainable with plain concrete mixes. In normal use,

admixture dosages are less than 5% by mass of cement, and are added to the concrete at the time

of batching/mixing. (From www.wikipedia.org/wiki/concrete)

2.1.3 Water- cement ratio

Water-cement ratio is the ratio of weight of water to the weight of cement used in a concrete

mix. It has an important influence on the quality of concrete produced. A lower water-cement

ratio leads to higher strength and durability, but may make the mix more difficult to place.

Placement difficulties can be resolved by using plasticizer. The water-cement ratio is

independent of the total cement content (and the total water content) of a concrete mix.

Often, the water to cement ratio is characterized as the water to cement plus pozzolan ratio,

w/(c+p). The pozzolan is typically a fly ash, or blast furnace slag. It can include a number of

other materials, such as silica fume, rice hull ash or natural pozzolans. The addition of pozzolans

will influence the strength gain of the concrete.

The concept of water-cement ratio was developed by Duff A. Abrams and first published in

1918.Concrete hardens as a result of the chemical reaction between cement and water (known as

hydration)

5

2.1.4 Properties

2.1.4.1 Strength

Concrete has relatively high compressive strength, but significantly lower tensile strength. It is

fair to assume that a concrete samples tensile strength is about 10%-15% of its compressive

strength. As a result, without compensating, concrete would almost always fail from tensile

stresses – even when loaded in compression. The practical implication of this is that concrete

elements subjected to tensile stresses must be reinforced with materials that are strong in tension.

Reinforced concrete is the most common form of concrete. The reinforcement is often steel,

rebar (mesh, spiral, bars and other forms). Structural fibers of various materials are

available.Concrete can also be prestressed (reducing tensile stress) using internal steel cables

(tendons), allowing for beams or slabs with a longer span than is practical with reinforced

concrete alone. Inspection of concrete structures can be non-destructive if carried out with

equipment such as a Schmidt hammer, which is used to estimate concrete strength.

2.1.4.2 Elasticity

The modulus of elasticity of concrete is a function of the modulus of elasticity of the aggregates

and the cement matrix and their relative proportions. The modulus of elasticity of concrete is

relatively constant at low stress levels but starts decreasing at higher stress levels as matrix

cracking develops. The elastic modulus of the hardened paste may be in the order of 10-30 GPa

and aggregates about 45 to 85 GPa. The concrete composite is then in the range of 30 to 50 GPa.

2.1.4.3 Cracking

All concrete structures will crack to some extent. One of the early designers of reinforced

concrete, Robert Maillart, employed reinforced concrete in a number of arched bridges. His first

bridge was simple, using a large volume of concrete. He then realized that much of the concrete

was very cracked, and could not be a part of the structure under compressive loads, yet the

structure clearly worked. His later designs simply removed the cracked areas, leaving slender,

beautiful concrete arches. The Salginatobel Bridge is an example of this.

6

Concrete cracks due to tensile stress induced by shrinkage or stresses occurring during setting or

use. Various means are used to overcome this. Fiber reinforced concrete uses fine fibers

distributed throughout the mix or larger metal or other reinforcement elements to limit the size

and extent of cracks. In many large structures joints or concealed saw-cuts are placed in the

concrete as it sets to make the inevitable cracks occur where they can be managed and out of

sight. Water tanks and highways are examples of structures requiring crack control.

2.1.4.4 Creep

Creep is the term used to describe the permanent movement or deformation of a material in order

to relieve stresses within the material. Concrete which is subjected to long-duration forces is

prone to creep. Short-duration forces (such as wind or earthquakes) do not cause creep. Creep

can sometimes reduce the amount of cracking that occurs in a concrete structure or element, but

it also must be controlled. The amount of primary and secondary reinforcing in concrete

structures contributes to a reduction in the amount of shrinkage, creep and cracking.

2.1.4.5 Physical property

The coefficient of thermal expansion of Portland cement concrete is 0.000008 to 0.000012 (per

degree Celsius) .The density varies, but is around 2400 kg/m³.

(From www.wikipedia.org/wiki/concrete)

2.2 Curing of concrete

2.2.1 Definition

Curing is the process of keeping concrete under a specific environmental condition until

hydration is relatively complete. Because the cement used in concrete requires time to fully

hydrate before it acquires strength and hardness, concrete must be cured once it has been placed.

Good curing is typically considered to use a moist environment which promotes hydration, since

increased hydration lowers permeability and increases strength, resulting in a higher quality

material. Allowing the concrete surface to dry out excessively can result in tensile stresses,

7

which the still-hydrating interior cannot withstand, causing the concrete to crack. Also, the

amount of heat generated by the chemical process of hydration can be problematic for very large

placements. (From www.wikipedia.org/wiki/concrete)

2.2.2 Three phases of curing concrete

There are three phases of curing and the length of time each lasts depends on the concrete and

the environmental conditions:

When concrete is first placed for a slab, bleed water rises as the concrete mixture settles.

During this period (initial set), if the bleed water is evaporating from the surface faster

than it is rising out of the concrete then you need to do some initial curing or else you are

likely to end up with plastic shrinkage cracks.

Between initial set and final set, intermediate curing would be needed if the finishing (or

stamping) is complete prior to final set.

After final set, you need to do final curing. (From www.wikipedia.org/wiki/concrete)

2.2.3 Influence of curing on properties of concrete

Curing has a strong influence on the properties of hardened concrete such as durability,

strength, water tightness, abrasion resistance, volume stability, and resistance to freezing and

thawing and deicer salts. Exposed slab surfaces are especially sensitive to curing. Surface

strength development can be reduced significantly when curing is defective.

Curing the concrete aids the chemical reaction called hydration. Most freshly mixed concrete

contains considerably more water than is required for complete hydration of the cement;

however, any appreciable loss of water by evaporation or otherwise will delay or prevent

hydration. If temperatures are favorable, hydration is relatively rapid the first few days after

concrete is placed; retaining water during this period is important. Good curing means

evaporation should be prevented or reduced.

(From www.concretenetwork.com/curing-concrete)

8

2.2.4 Duration of curing

2.2.4.1 introduction

During the curing period from five to seven days after placement for conventional concrete the

concrete surface needs to be kept moist to permit the hydration process. Hydration and

hardening of concrete during the first three days is critical. The early strength of the concrete

can be increased by keeping it damp for a longer period during the curing process. Minimizing

stress prior to curing minimizes cracking. High early-strength concrete is designed to hydrate

faster, often by increased use of cement which increases shrinkage and cracking. In practice,

this is achieved by spraying or pounding the concrete surface with water, thereby protecting

concrete mass from ill effects of ambient conditions. In around 3 weeks, over 90% of the final

strength is typically reached though it may continue to strengthen for decades.


2.2.4.2 Minimum number of days for curing concrete

To develop design strength, the concrete has to be cured for up to 28 days. As the rate of

hydration, and hence the rate of development of strength, reduces with time, it is not

worthwhile to cure for the full period of 28 days. IS: 456-1978 stipulates a minimum of 7-day

moist –curing, while IS: 7861(part I)-1975 stipulates a minimum of 10 days under hot weather

condition. High-early-strength cements can be cured for half the periods suggested for OPC.

For pozzolana or blast furnace slag cements the curing periods should be increased. There are

many opinions on the length of curing period. Periods varying from 10 to 30 days are

specified for highway pavements. There cannot be a definite mandate on this matter as there

are too many variables involved, such as the type of cement, ambient temperature, nature of

the product, method of curing adopted, etc. generally increasing curing periods are desirable

for high-quality concrete products, concrete floors, roads and airfield pavements. (By ML

GAMBHIR)

9

2.2.5 Moist curing

Concrete to be moist-cured shall be maintained continuously wet for the entire curing period,

commencing immediately after finishing. If water or curing materials used stain or discolor

concrete surfaces which are to be permanently exposed, the concrete surfaces shall be cleaned

as approved. When w o o d e n forms are left in place during curing, they shall be kept wet at

all times. If steel forms are used in hot weather, non supporting vertical forms shall be broken

loose from the concrete soon after the concrete hardens and curing water continually applied

in this void. If the forms are removed before the end of the curing period, curing shall be

carried out as on unformed surfaces, using suitable materials. Surfaces shall be cured by

pounding, by continuous sprinkling, by continuously saturated burlap or cotton mats, or by

continuously saturated plastic coated burlap. Burlap and mats shall be clean and free from any

contamination and shall be completely saturated before being placed on the concrete. The

Contractor shall have an approved work system to ensure that moist curing is continuous 24

hours per day.


2.2.6 Curing conditions

Proper curing is one of the important steps in making high quality concrete. A good mix

design with low water-cement ratio alone cannot ensure good concrete. The favorable

conditions to be set up at early hardening periods for best results are:

a) Adequate moisture within concrete to ensure sufficient water for continuing hydration

process, and

b) Warm temperature to help the chemical reaction. In addition, the length of curing is

also important. On an average, the one year strength of continuously moist cured

concrete is 40 per cent higher than that of 28-days moist cured concrete, while no

moist-curing can lower the strength to about 40 per cent. Moist curing for the first 7 to

14 days may result in a compressive strength of 70 to 85 per cent of that of 28 day

moist-curing. (By ML GAMBHIR)

10

2.2.7 Maturity of concrete

Since the strength of concrete depends on both the period of curing (i.e. age) and temperature

during curing, the strength can be visualized as a function of period and temperature of curing.

The product (period× temperature) is called the maturity of concrete. Here the temperature is

reckoned from -10˚C which is a reasonable value of the lowest temperature at which an

appreciable increase in strength can take place and the period in hours or days. The maturity

of concrete is measured in ˚C hours or ˚C days. The strength of concrete is found to increase

linearly with its maturity.

(By ML GAMBHIR)

2.2.8 Methods of curing concrete

2.2.8.1 The methods which replenish partly the loss of water by interposing a source of

water, or prevent the evaporation.

2.2.8.1.1 Ponding of water over the concrete surface after it has set

This is the most common method of curing the concrete slab or pavements and consists of

storing the water to a depth of 50 mm on the surface by constructing small puddle clay bunds

all around. Ponding may promote efflorescence by leaching.

2.2.8.1.2 Covering the concrete with straw or Damp Earth

In this method the damp earth or sand in layers of 50 mm height are spread over the surface of

concrete pavements. The material is kept moist by periodical sprinkling of water.

2.2.8.1.3 Covering the concrete with Burlap

The concrete is converted with burlap (coarse jute or hemp) as soon as possible after placing,

and the material is kept continuously moist for the curing period. The covering material can be

used a number of times and, therefore, tends to be economical.

11

2.2.8.1.4 Sprinkling of water

This is a useful method for curing vertical or inclined surfaces of concrete where in the earlier

methods cannot be adopted. The method is not very effective as it is difficult to ensure that all

the parts of concrete be moist at the time. The spraying can be done in fine streams through

nozzles fixed to a pipe spaced at set intervals. Flogging is done in the same way except that

the flogging nozzles produce a mist-like effect, whereas spraying nozzles shed out fine sprays.

(By ML GAMBHIR)

2.2.8.1.5 Covering the surface with waterproof paper

Waterproof paper prevents loss of water in concrete and protects the surface from damage.

The method is satisfactory for concrete slabs and pavements. A good quality paper can be

often reused. The paper is usually made of two sheets struck together by rubber latex

composition.

Plastic sheeting is a comparatively recent innovation as a protective cover for curing concrete.

Being light and flexible, it can be used for all kinds of jobs, effectively covering even the most

complex shapes. Several types of sheets, which are guaranteed to give excellent results

consistent with economy and can be used over and over again, are available. Most plastic

sheetings used in the concrete industry are milky or white in appearance, and this helps keep

the concrete temperature at a reasonable level. Plastic sheeting can be can be welded at the site

instead of resorting to large overlaps and made airtight to prevent moisture evaporation from

concrete.

12

2.2.8.2 The methods preventing or minimizing the loss of water by interposing an

impermeable medium between the concrete and the surrounding environment

2.2.8.2.1 Leaving the shuttering or Formwork on

The thick watertight formwork also prevents the loss of moisture in concrete and helps in

curing the sides and the base of the concrete

2.2.8.2.2 Membrane curing of the concrete

The process of applying a membrane forming compound on concrete surface is termed

membrane curing. Often, the term membrane is used not only to refer to liquid membranes but

also to solid sheeting used to cover the concrete surface. The curing membrane serves as a

physical barrier to prevent loss of moisture from the concrete to be cured. A curing liquid

membrane should dry within 3 to 4 hours to form a continuous coherent adhesive film free

from pinholes and have no deleterious effect on concrete. Curing with a good membrane for

28 days would give strengths equivalent to two weeks moist-curing. Membrane curing may

not assure full hydration as in moist-curing but is adequate and particularly suitable for

concrete members in contact with soil.

The different sealing compounds used are:

Bituminous and asphaltic emulsion or cutbacks,

Rubber latex emulsions,

Emulsions of resins, varnishes, waxes, drying oils and water-repellant substances, and

Emulsions of paraffin or boiled linseed oil in water with stabilizer.

2.2.8.2.3 Chemical curing

Chemical curing is accomplished by spraying the sodium silicate (water glass) solution. About

500 g of sodium silicate mixed with water can cover 1 m2 of surface and forms a hard and

insoluble calcium silicate film. It actually acts as a case hardener and curing agent. The

application of sodium silicate results in athin varnish like film which also fill pores and

13

surface voids, thus sealing the surface and preventing the evaporation of water. (By ML

GAMBHIR)

2.2.8.3 Methods involving the application of artificial heat while the concrete is

maintained in a moist condition are used in plant curing where the curing of concrete is

accelerated by raising its temperature.

The accelerated process of curing has many advantages in the manufacture of precast concrete

products since,

The moulds can be reused within a shorter time

Due to reduced period of curing the production is increased and the cost reduced, and

Storage space in the factory is reduced.

The temperature can be raised in practice by:

a) Placing the concrete in steam,

b) Placing the concrete in the hot water, and

c) Passing an electric current through the concrete.

(By ML GAMBHIR)

2.3 compressive strength of concrete

2.3.1 Introduction

Compressive strength is the measure of the capacity of a material to withstand axially

directed crushing forces. The forces may be caused either by 'live' or 'dead' loads. It is an

indication of the maximum compressive stress that a material is capable of developing.

The compressive strength depends on the type of material in question. A brittle material fails

in compression by fracturing. In such cases, the compressive strength has a definite value.

However, in the case of materials which are ductile, malleable or semi viscous, the value

denoting the compressive strength depends on the levels of distortion of the material.

14

The compressive strength of concrete is very important, as it is used more often in

compression than in any other way. It is rather difficult to give average values of the

compressive strength of concrete as it is dependent on so many factors. The available

aggregates are so varied, and the methods of mixing and manipulation so different, that tests

must be studied before any conclusions can be drawn. For extensive work, tests should be

made with the materials available to determine the strength of concrete, under conditions as

nearly as possible like those in the actual structures. (From

www.concretenetwork.com/curing-concrete)

2.3.2. How is compressive strength determined?

The compressive strength of concrete is determined using small specimens like 15×15×15cm

cubes. The test cubes are cast in steel mould in three layers, each layer being compacted by 25

strokes of a steel rod. The moulded specimens are stored at specified time and condition until

the prescribed age of testing, usually 28 days. The compressive strength of the cube is the

maximum uniaxial load recorded divided by the cross sectional area of the cube when tested

using hydraulically operated compression machines. The cubes give the potential strength of

the concrete.

The compressive strength is influenced by the curing conditions, specimen preparation, age at

testing, mode of testing, and mode of failure of the specimen.

2.3.3. Why do we test the compressive strength of concrete?

There are many ways to test the strength of a batch of concrete. The tests used can be

categorized as destructive and nondestructive tests. Usually when a batch of concrete is

ordered on a job site it is specified to be of a specific compressive strength -- 4000 psi, for

instance. When the concrete comes to the job site in a ready-mix truck, the contractor places

some of the batch in cylinders which are 6 inches in diameter and 12 inches in height. These

cylinders are cured for 28 days and tested by compression until they are crushed. This will

give the contractor or the engineer the compressive strength for that batch of concrete. He or

she can then compare that value to the design value used to make sure that the structure was

constructed properly.

15

Once the concrete has been placed for a particular structure, there is a nondestructive test

which can be performed to estimate the strength of the concrete. This method uses a Schmidt

hammer (also called a Swiss hammer). This method of testing is based on the inertia of a ball

inside the Schmidt hammer testing apparatus that is "bounced off" of the concrete. (From

www.concretenetwork.com/curing-concrete)

16

CHAP 3 MATERIALS AND METHODOLOGY

3.1 materials used

The materials to be used during this project are:

a. Cement: the cement to be used is the Portland cement of CIMERWA of density

350 kg/m³

b. Aggregates: Fine aggregates are 0/5 and coarse aggregates are 5/30.

c. Water: it is the natural water with cement ratio of 0.5

d. Vibrator: the vibrator is used for compacting the concrete in moulds

e. Tamping load: it will be used for compacting the concrete in moulds manually

f. Motorized shaker machine: it is used for sieving the sand

g. Moulds

h. Scoop

i. Trowel

j. Balance

k. Compressive strength measuring machine

3.2 Methodology

In order to achieve this study the laboratory test must be used to measure the compressive

strength of concrete

3.2.1 Making cubes

In my research I have to make 66 cubes (15×15×15 cm) as follows,

12 cubes to stay in air entire time,

12 cubes to be cured within 1 day,

12 cubes to be cured within 2 days,



6 cubes to be cured within 14 days, and 3 cubes to be cured within 28 days.

17

The cubes are obtained by mixing cement, water, gravel and sand. The mixing proportions are

1: 2.27:4.42 which is ratio that can be used in beams and column. After 24hours the cubes are

removed in moulds then cured

Pic.1 Cubes Pic.2 Curing in water

3.2.2 Crushing cubes

The cubes remain in water until the required age:

At the age of 3 days I crush 12 cubes,

3 cubes that stayed in air their entire time,

3 cubes that cured within 1 day,

3 cubes that cured within 2 days,

3 cubes that cured within 3 days.


3 cubes stayed in air their entire time,





18





3 cubes that cured within 3days,











Pic3.crushing machine for compressive strength

19

3.2.3 Tabulation of results

At every crushing day I record results in tables

Before graphing I make other small tables summarizing results at each age

3.2.4 Calculation

3.2.4.1 Determination of the strength

3.2.4.1.1 Introduction

The strength of each concrete cube is obtained by determining the ratio between the load

applied for crushing the cube and its cross sectional area.

Strength=

3.2.4.1.2 Derivation of the unit of the strength

The load unit was KN

The cross section unit was cm2

Derivation, = =107Pa=10 MPA

E.g. the average load for the 3cubes stored continuously in air and crushed after 28 days is

405 KN and the cross sectional area is 225 cm2, the strength is calculated as followed:

The strength= =405×103 N / 225×10-4 m2 = ×107 Pa = ×10 MPA

20

3.2.5 Graphing

After recording all results obtained I make graphs:

Bar diagram at each age showing the relationship between the compressive strength

and the duration of curing.

Scatter lines graph at each age showing the relationship between the compressive

strength and the duration of curing.

3.2.6 Analysis of results obtained

This is the last step of my work where I have to discuss about the results obtained before

concluding and recommending.

21

CHAP4. DATA PRESENTATION, ANALYSIS AND INTERPRETATION

4.1 Results at 3 days age

4.1.1 Diagrams of results for 3 days age

Tab. 1 compressive strength of concrete at 3 days age

Age(days) Curing duration(days) Compressive strength(MPA)

3

0 9.9 1 12 2 12.9 3 15.6 7 15.6 14 15.6 28 15.6

Diag. 1 Variation of compressive strength of concrete with curing duration (for 3 days age)

22

Fig. 1 variation of compressive strength with curing duration (for 3 days age)

4.1.2 Results discussion

The results show that the compressive strength of concrete increases quickly for the first 7 days of curing and after it increases at a low rate. The curing duration of 7 days is the reference. The table below shows the deficit and excess in compressive strength for each curing duration compared to the compressive strength for the curing duration of 7 days. This excess or deficit is expressed in percentage.

Tab. 2 Deficit and Increase in compressive strength percentages

Curing duration(days)

Compressive strength(MPA) -A-

Reference(compressive strength for curing duration of 7 days) -B-

Deficit in compressive strength(MPA) (A-B)

Increase in compressive strength (MPA) (A-B)

Percentage (٪) (AB)× 100/B

0 9.9 15.6 5.7 36.54 1 12 15.6 3.6 23.08 2 12.9 15.6 2.7 17.31 3 15.6 15.6 0 0 7 15.6 15.6 0 0 14 15.6 15.6 0 0 28 15.6 15.6 0 0

23



Tab. 3 compressive strength of concrete at 7 days age


7

0 13.54 1 13.9 2 15.41 3 19.4 7 21.2 14 21.2 28 21.2

Diag. 2 Variation of compressive strength of concrete with curing duration (for 7 days age)

24

Fig. 2 Variation of compressive strength with curing duration (for 7 days age)


The results show that the compressive strength of concrete increases quickly for the first 7 days of curing and after increases at a low rate. The curing duration of 7 days is the reference. The table below shows the deficit and excess in compressive strength for each curing duration compared to the compressive strength for the curing duration of 7 days. This excess or deficit is expressed in percentage.


Curing duration (days)

Compressive strength (MPA) -A-

Reference(Compressive strength for curing duration of 7 days)(MPA) -B-

Deficit in compressive strength (MPA) (A-B)


Percentage (٪) (A-B)×100/B

0 13.54 21.2 7.66 36.13 1 13.9 21.2 7.3 34.43 2 15.41 21.2 5.79 27.31 3 19.4 21.2 1.8 8.49 7 21.2 21.2 0 0 14 21.2 21.2 0 0 28 21.2 21.2 0 0

25



Tab. 5 Compressive strength of concrete at 14 days age


14

0 15.3 1 16.32 2 17.68 3 23.5 7 26.4 14 28.3 28 28.3

Diag. 3 Variation of compressive strength with curing duration (for 14 days age)

26


4.3.2 Results discussion:




Reference(Compressive strength for curing duration of 7 days) (MPA) -B-



Percentage (٪) (A-B)×100 B

0 15.3 26.4 11.1 42.04 1 16.32 26.4 10.08 38.18 2 17.68 26.4 8.72 33.03 3 23.5 26.4 2.9 10.98 7 26.4 26.4 0 0 14 28.3 26.4 1.9 7.2 28 28.3 26.4 1.9 7.2


27



Tab. 7 Compressive strength of concrete at 28 days age


28

0 16.9 1 18.9 2 21 3 26.2 7 30.5 14 33.5 28 35.1

Diag. 4 Variation of compressive strength with curing duration (for 28 days age)

28







Reference(Compressive strength for curing duration of 7 days)(MPA) -B-



Percentage(٪) (A-B)×100 B

0 16.9 30.5 13.6 44.6 1 18.9 30.5 11.6 38.03 2 21 30.5 9.5 31.15 3 26.2 30.5 4.3 14.1 7 30.5 30.5 0 0 14 33.2 30.5 2.7 8.85 28 35.1 30.5 4.6 15.1

29

4.5 Figure summarizing all results

Fig. 5 Strength of concrete dried in air after preliminary moist curing.

30

The design compressive strength of concrete is achieved when concrete is cured within 28 days the following results show the effect of curing duration on the compressive strength of concrete, the results are expressed in percentages. (See tab. 7)

Tab. 9 Effect of curing duration on compressive strength of concrete

Curing duration(MPA)

Compressive strength at 28 days age(MPA) -A-

Compressive strength of concrete cured within28 days at 28 days age(MPA) -B-

Difference in compressive strength (MPA) (B-A)

Deficit in compressive strength(٪) (B-A)×100 B

0 16.9 35.1 18.2 51.8 1 18.9 35.1 16.2 46.1 2 21 35.1 14.1 40.2 3 26.2 35.1 8.9 25.3 7 30.5 35.1 4.6 13.1 14 33.5 35.1 1.6 4.5 28 35.1 35.1 0 0

31

CHAP 5. CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

The general objective of my study was of verifying the effect of duration of curing on

compressive strength of concrete. At the end of my study I found that the duration of curing of

concrete increases the strength of concrete. I also discovered that 7 days are sufficient for curing

concrete because more than 75٪ of the design compressive strength is achieved when concrete is

only cured for 7 days. In addition with the fig.5 it is easy to estimate the minimum number of

days required for curing concrete up to the desirable compressive strength.

5.2 Recommendations

The compressive strength and other engineering properties of concrete would be thoroughly

studied so as to check its suitability for various applications in the construction.

It recommended the Ministry of education to encourage the final year students by helping

them executing what they have discovered in order to improve our technology in

Rwanda.

It recommended to the government authorities to supervise the construction companies’

activities for checking whether the duration of curing used on their sites is proportional to

the design compressive strength of concrete required.

It recommended to all construction companies that the minimum number of days for

curing concrete is equal to 7 days.

Finally, I recommended to SFAR (Student Financing Agency in Rwanda) to increase

project found since 100000 RWF is not sufficient for the research.

Future research:

The effect of curing duration of concrete should also be studied up to the age equal to 1 year, for

verifying the relationship between the compressive strength of concrete cured within 7 days and that

cured within 28 days at that age.

32

REFERENCES

1) Concrete technology second edition , Second Edition by ML GAMBHIR

2) Concrete technology (theory and practice)

M.S SHETTY

S.CHAND

3) www.cement.org/basics/concrete basics_curing.asp

4) Wikipedia.org/wiki/concrete

5) www.concretenetwork.com/curing-concrete

6) www.tpub.com/content/construction

33

APPENDICES

34

TABLES OF RESULTS IN DETAILS

Table10 .Results of cubes stored continuously in air

Age(days) Load(KN) Cross Section(cm2) Strength(MPA) 3 235 225 10.44 3 240 225 10.67 3 193 225 8.58 Average 222.67 225 9.9 7 300 225 13.33 7 290 225 12.89 7 324 225 14.4 Average 304.65 225 13.54 14 348 225 15.47 14 335 225 14.89 14 350 225 15.55 Average 344.3 225 15.3 28 390 225 17.3 28 425 225 18.9 28 326 225 14.49 Average 380.4 225 16.9

Table 11 .Results of cubes cured within 1 day

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 290 225 12.89 3 310 225 13.78 3 210 225 9.33 Average 270 225 12 7 350 225 15.55 7 286 225 12.71 7 303 225 13.47 Average 313 225 13.9 14 350 225 15.55 14 381 225 16.93 14 370 225 16.44 Average 367.2 225 16.32 28 481 225 21.38 28 395 225 17.55 28 400 225 22.2 Average 425.33 225 18.9

35

Table.12 Results cubes cured within 2 days

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 301 225 13.38 3 300 225 13.33 3 270 225 12 Average 290.3 225 12.9 7 332 225 14.75 7 343 225 15.24 7 365 225 16.22 Average 346.7 225 15.41 14 458 225 20.35 14 396 225 17.6 14 340 225 15.11 Average 398 225 17.68 28 467 225 20.75 28 420 225 18.67 28 530 225 23.55 Average 472.5 225 21

Table.13 Results of cubes cured within 3 days

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 294 225 13.07 3 359 225 15.95 3 400 225 17.78 Average 351 225 15.6 7 560 225 24.89 7 370 225 16.44 7 380 225 16.89 Average 436.5 225 19.4 14 497 225 22.09 14 459 225 20.4 14 630 225 28 Average 528.75 225 23.5 28 668 225 29.69 28 520 225 23.11 28 560 225 24.89 Average 589.5 225 26.2

36

Table.14 Results of cubes cured within 7 days

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 294 225 13.07 3 359 225 15.95 3 400 225 17.78 Average 351 225 15.6 7 529 225 23.51 7 416 225 18.49 7 486 225 21.6 Average 477 225 21.2 14 772 225 34.31 14 550 225 24.3 14 460 225 20.44 Average 594 225 26.4 28 759 225 33.73 28 740 225 32.89 28 560 225 24.89 Average 686.25 225 30.5

Table.15 results of cubes cured within 14 days

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 294 225 13.07 3 359 225 15.95 3 400 225 17.78 Average 351 225 15.6 7 529 225 23.51 7 416 225 18.49 7 486 225 21.6 Average 477 225 21.2 14 645 225 28.67 14 730 225 32.44 14 535 225 23.78 Average 636.75 225 28.3 28 864 225 38.4 28 612 225 27.2 28 765 225 34 Average 747 225 33.2

37

Table. 16 of cubes cured within 28 days

Age(days) Load(KN) Cross section(cm2) Strength(MPA) 3 294 225 13.07 3 359 225 15.95 3 400 225 17.78 Average 351 225 15.6 7 529 225 23.51 7 416 225 18.49 7 486 225 21.6 Average 477 225 21.2 14 645 225 28.67 14 730 225 32.44 14 535 225 23.78 Average 636.75 225 28.3 28 805 225 35.78 28 745 225 33.11 28 820 225 36.44 Average 789.75 225 35.1

effect of duration of curing on compressive strength of concreeete

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