Civil Engineering Materials - WordPress.com Engineering Materials SAB 2112 ... • BS 1881 Part 102...

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Civil Engineering MaterialsSAB 2112

Introduction to Concrete

Dr Mohamad Syazli Fathi

Department of Civil EngineeringRAZAK School of Engineering & Advanced Technology

UTM International Campus

August 18, 2010

Objectives of the lecture

• The main objective of this lecture is to explain to students:1. Workability, test of fresh concrete, segregation and

bleeding in concrete2. Concrete on site - method of production, concrete strength

and grade3. Concrete proportions - standard, nominal; Hardened

concrete tests-destructive and non-destructive tests

New blog: SAB2112.wordpress.comLeonard P. Zakim Bunker Hill Bridge in Boston. (Image courtesy of the Federal Highway Administration.)

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IntroductionConcrete is one of the most common

construction materials and is used in a wide variety of applications, ranging f il lti t b ildi dfrom piles, multistory buildings, dams, foundations, pavements, storage tanks and many other structures.

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Bridge over the Sungai Selangor, Kuala Selangor, Malaysia

• The asymmetric cable stay bridge has a main

f 232 d 114span of 232m and 114m back span along a 1000m radius.

• It is a pre-stressed concrete structure with a 25m wide deck comprising a glued segmental spine and precast side frameprecast side frame.

• The deck is supported from an inclined tower on a single plane of pre-stressed stays in the median which are anchored on the back span to a precast boom. An outrigger beam is employed to stabilize the tower with transverse stays anchored to the beam below deck level.

Sources: http://www.bandraworlisealink.com/abroad-pro.html

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Introduction• It is one of the most economical materials of construction and is very

versatile in nature and application.

This is a hostel in Austria, designed by art college graduate Andreas StraussSource: http://slog.thestranger.com/2008/08/future_concrete

The cube-shaped building consists of an irregular concrete structure enveloped in glass. Sources: http://www.dezeen.com/2008/12/07/nordwesthaus-by-baumschlager-eberle/

Sources: http://www.core.form-ula.com/2008/04/16/c-space-adrlten-pavilion/

Advantages of Concrete1. Economical in long run compared to other engineering material (low cost)

2. Possesses a high compressive strength 3. Corrosive & the weathering effects are

minimal.4. Fresh concrete can be easily handled &

moulded in any shape / size. 5. Formwork can be re-used6. Can be sprayed on & filled the cracks

for repairing process.7. Durable & fire resistant & requires little

i tmaintenance.8. Can be pumped and hence it can be laid

in the difficult position9. Protection of embedded steel10. Variety of finishes obtainable.

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Introduction• Concrete is made by mixing: Cement, water, course fine aggregates

and admixtures (if required). • The aim is to mix these materials in measured amounts to make

concrete that is easy to: Transport, place, compact, finish and which will set, and harden, to give a strong and durable product.

• The amount of each material (ie cement, water and aggregates) affects the properties of hardened concrete

Sources: http://www.transport.vic.gov.au/DOI/Internet/transport.nsf/AllDocs/74164766E0CEAF95CA25700500122952?OpenDocument

Properties of Concrete

A. Properties of Fresh Concrete

B. Properties of Hardened ConcreteConcrete

1. Workability, 2. Segregation, 3. Bleeding, 4. Heat of Hydration

Hardened Concrete1. Strength2. Deformation3. Creep4. Shrinkage

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Properties of Fresh Concrete

Workability

1 Generally implies the ease1. Generally implies the ease with which a concrete mix can be handled from mixer to it’s finally compacted shape– Consistency - fluidity– Mobility - ease of flow– Compactability - ease of compaction

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2. Internal work required to produce full compaction.

Tests to Measure Workability

Four widely used testsFour widely used tests

1.1.Slump TestSlump Test2.2.Compacting factor testCompacting factor test3.3.VebeVebe time testtime test

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44 Flow testFlow test

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Slump Test• Developed in 1913 in

US by Chapman

100100

US, by Chapman• BS 1881 Part 102• Required

– Slump cone– Tamping Rod

Ruler

300300

13

– Ruler• Suitable for normal

mixes of med to high workability

200200

14

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Sources: http://www.concrete-curb.com/tag/concrete-structure/

Slump Test (cont)Slump Test (cont)

• MethodSlump (mm)Slump (mm)

– Concrete put in cone in 3 layers, each layer tamped 25 times

– Top struck off– Cone carefully lifted

offSl d

True slumpTrue slump

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– Slump measured• Not suitable for dry

mixesShear slumpShear slump Collapse slumpCollapse slump

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Adding more water to a mix will create a weaker hardened concrete!

Source: http://hia.com.au/HIA/Content/IS/BPS/NAT%20Change%20in%20slump%20helps%20straighten%20your%20back.aspx

ReferenceCement Concrete & Aggregates Australia (2008) Premixed concrete delivery to domestic house floor slabs and footings –changes to the Australian standard: Important information for concrete placers, www.ccaa.com.au

Compacting Factor TestStandards: BS 1881-103, BS 5075

• The Apparatus enables a check to be made on the weight of concrete when it is falls from fixed heightswhen it is falls from fixed heights into a cylindrical container of standard capacity.

Comprises:• The Apparatus consists of two

conical hoppers each with a hinged

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conical hoppers each with a hinged trap with quick release mechanism to allow free flow of theconcrete sample.

• A cylindrical mould is fitted beneath the hoppers.

Sources: http://geotechnical-equipment.com/Concrete.html

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Compacting Factor Test

• Thought to be more sensitive than the slump test

• Suitable for all mixes• Method

– mixed concrete put in top hopper– allowed to fall into 2nd hopper then

cylinder

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– cylinder stuck off, concrete weighed and compared with weight of fully compacted cylinder

VebeVebe testtestStandards: EN 12350-3, ASTM C1170

•The vebe test is a variation of the simple slump test where the concrete is subject to vibration after removal from the slump cone to determine the workability of fresh concrete.

•A plastic disc is placed into contact with the upper surface of the concrete, the vibrating table underneath operates at a fixed

lit d ll i th l f f th

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• Vebe test - time for standard cone to be compacted flat by glass plate on vibrating table eg for workable concrete the Vebe time = approx 3s

amplitude allowing the lower surface of the disc to be completely coated with cement.


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Flow Flow teststestsStandards: EN 12350-5, BS 1881-105

• The Flow Table test is used for determining the consistency and workability of fresh concreteconcrete.

• The conical mould for casting the flow specimen and the calliper for measuring the diameter of the mortar after it has been spread on the flow table.

• The apparatus consists of a double steel t bl th t bl i 700 700 S h // h i l i /C h l

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table, the upper table measuring 700x700 mm and hinged at one side to the lower table.

••Flow test Flow test -- the measured spread in mm of a standard cone on a the measured spread in mm of a standard cone on a dropping table (40mm, 15 times)dropping table (40mm, 15 times)••Neither Neither VebeVebe nor Flow test popular on sitenor Flow test popular on site


Flow Test

22

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Comparisons Between TestsComparisons Between Tests

3,63,6

Vebe

Tim

e (s

ec)

Vebe

Tim

e (s

ec)

1010

2020

Slum

p (m

m)

Slum

p (m

m)

00

505033

66

6633

23

Slump (mm)Slump (mm) Compacting FactorCompacting Factor

0000 5050 100100 0.60.6 0.80.8 1.01.0

100100

Compacting FactorCompacting Factor0.60.6 0.80.8 1.01.0

For aggt/cement ratio = 3,6For aggt/cement ratio = 3,6

Effect of Time and Temperature Effect of Time and Temperature on Workabilityon Workability

??100100

2002001:2:4 mix 1:2:4 mix with with w/c=0.775w/c=0.775

24Time (h)Time (h)

0000 22 44

Temp (Temp (ooC)C)55 2020 4040

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Segregation

• The tendency for – sand-cement mortar

to separate from coarse aggregates

– cement mortar to separate from fine aggregates

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Segregation

• Caused by1. Excessive vibration2. Dropping fresh concrete

from a height3. Poor grading4. High workability5 Mixes with no air

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5. Mixes with no air entrainment

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Concrete handling techniques to prevent segregation.

• Sources: https://rdl.train.army.mil/soldierPortal/atia/adlsc/view/public/9708-1/accp/en5466/lsn4.htm



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Bleeding

• Tendency for t t i t thwater to rise to the

surface• This will cause

weakness or dustiness of the surface of the finished concrete, or a

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line of weakness between pours

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Bleeding

• Bleeding affected largely by thelargely by the properties of the cement.

• Avoided by– a finer cement

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– high C3A content– richer mix

Aggregate Grading

• Although the aggregate di h littl di tgrading has little or no direct

affect on strength, it does affect workability, and hence w/c ratio.

• The fundamental idea is that finer stones fill up the gaps b t l t d

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between larger stones, and so on, and the remaining space is filled by cement paste.

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Typical Grading Curves (BS882)

100100

6060

8080

4040

Grading MGrading Mfine aggtfine aggt 20 mm20 mm

coarsecoarseaggtaggt

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7575 150150 300300 600600 1.181.18 2.362.36 55 1010 2020 37.537.500

2020

aggtaggt

mmmmμμmm

Proportioning and mixing concrete

• A concrete mix is designed to produce concrete that can be easily placed at theconcrete that can be easily placed at the lowest cost.

• The concrete must be workable and cohesive when plastic, then set and harden to give strong and durable concrete. Th i d i t id th• The mix design must consider the environment that the concrete will be in; ie exposure to sea water, trucks, cars, forklifts, foot traffic or extremes of hot and cold.

Sources: http://www.enco.ch/pns.htm

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Proportioning and mixing concrete

1. PROPORTIONING Concrete is a mixture of Cement Water Coarse andmixture of Cement, Water, Coarse and Fine Aggregates and Admixtures.

2. The proportions of each material in the mixture affects the properties of the final hardened concrete.

3. These proportions are best measured by i htweight.

4. Measurement by volume is not as accurate, but is suitable for minor projects.

Batching, Mixing, Placing and Compaction of Concrete

BatchingI th f i t i i di t ith b• Is the process of measuring concrete mix ingredients either by volume or by mass and introducing them into the mixture.

• Traditionally batching is done by volume but most specifications require that batching be done by mass rather than volume.

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Batching of Concrete

Sources: http://www.crusher2.com/machine-concrete_batching_plant.htm

Batching, Mixing, Placing and Compaction of Concrete

MixingTh i i ti i t f t ti ti i th bj ti• The mixing operation consists of rotation or stirring, the objective being to coat the surface the all aggregate particles with cement paste, and to blind all the ingredients of the concrete into a uniform mass; this uniformity must not be disturbed by the process of discharging from the mixer.

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Type of Mixer – Batch MixerThe usual type of mixer is a batch mixer, which means that

one batch of concrete is mixed and discharged before any more materials are put into the mixer. There are four types of batch mixer.1. Tilting drum mixer:

• A tilting drum mixer is one whose drum in which mixing take place is tilted for discharging. The drum is conical or bowl shaped with internal vanes, and the discharge is rapid and unsegregated so that these mixers are suitable for mixes of low workability and for those containing large size aggregate.

2. Non tilting drum mixer:• A non tilting drum is one in which the axis of

the mixer is always horizontal, and discharge take place by inserting a chute into the drum or by reversing the direction or rotation of drum. Because of slow rate of discharge, some segregation may occur.

Type of Mixer – Batch MixerThe usual type of mixer is a batch mixer, which means that one

batch of concrete is mixed and discharged before any more materials are put into the mixer. There are four types of batch mixer.3. Pan type mixer:

• A pan type mixer is a forced–action mixer, as distinct from drum mixer which relies on the free fall of the concrete inside the drum. The pan mixer consist of a circular pan rotating about its axis with one or two stars paddles rotating about vertical axis of pan.

4. Dual drum mixer:• A dual drum is sometimes used in highway

construction. Here there are two drums in series, concrete being mixed part of the time in one and then transferred to the other for the remainder of the mixing time before discharging.

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Type of Mixer

Continuous mixers:– These are fed automatically by a continuous weigh-batching system.y y g g y

Batching Plant

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Batching Plant

Source: http://www.hi-unique.com/productdetail.aspx?cpid=98

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Mobile Concrete Mixing Plant

Uniformity of mixingMixing time:• It is important to know the minimum mixing time necessary to

produce a concrete of uniform composition, and of reliable strength.• The mixing time or period should be measured from time all the g p

cementing materials and aggregates are in mixer drum till taking out the concrete.

• Mixing time depends on the type and size of mixer, on the speed of rotation, and on the quality of blending of ingredients during charging of the mixer. Generally, a mixing time of less than 1 to 1.25 minutes produces appreciable non-uniformity in composition and a significant p pp y p glower strength; mixing beyond 2 minutes causes no significant improvement in these properties.

• Table: Recommended minimum mixing times

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Recommended minimum mixing times

Capacity of mixer (yd³) Mixing time (Minutes)

Up to 1 1

2 1.25

3 1.5

4 1.75

5 2

6 2.25

10 3.25

Prolong mixing:• If mixing take place over a long period, evaporation of

water from the mix can occur, with a consequent decrease in workability and an increase in strength.

• A secondary effect is that of grinding of the aggregate• A secondary effect is that of grinding of the aggregate, particularly if soft; the grading thus becomes finer and the workability lower.

• In case of air entrained concrete, prolong mixing reduces the air content.

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Ready mixed concrete:• If instead of being batched and mixed on site, concrete is

delivered for placing from a central plant. It is referred to as ready-mixed or pre-mixed concrete. This type of concrete is used extensively abroad as it offers numerous advantages inused extensively abroad as it offers numerous advantages in comparison with other methods of manufacture:

• Close quality control of batching which reduces the variability of the desired properties of hardened concrete.

• Use on congested sites or in highway construction where there is little space for a mixing plant and aggregatethere is little space for a mixing plant and aggregate stockpiles;

Ready mixed concrete:

• Use of agitator trucks to ensure care in transportation, thus prevention segregation and maintaining workability

• Convenience when small quantities of concrete or intermittent placing is required.

• There are two categories of ready-mixed concrete:1. central-mixed - mixing is done in a central plant and then

concrete is transported in an agitator truck. 2. transit mixed or truck mixed - the materials are batched at a

central plant but are mixed in a truck.

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Placing and compaction

• The operation of placing and compaction are interdependent and are carried out simultaneously.

• They are most important for the purpose of ensuring the requirements of strength, impermeability and durability of hardened concrete in the actual structure.

• As for as placing is concerned the main objective is to deposit the• As for as placing is concerned, the main objective is to deposit the concrete as close as possible to its final position so that segregation is avoided and the concrete can be fully compacted. The aim of good concrete placing can be stated quite simply.

• It is to get the concrete into position at a speed, and in a condition, that allow it to be compacted properly.

Placing and compactionTo achieve proper placing following rules should be kept in mind:

1. The concrete should be placed in uniform layers, not in large heaps or sloping layers.

2. The thickness of the layer should be compatible with the method of vibration so that entrapped air can be removed from the bottom of each layerso that entrapped air can be removed from the bottom of each layer.

3. The rate of placing and of compaction should be equal. If you proceed too slowly, the mix could stiffen so that it is no longer sufficiently workable. On no account should water ever be added to concrete that is setting. On the other hand, if you go too quickly, you might race ahead of the compacting gang, making it impossible for them to do their job properly.

4. Each layer should be fully compacted before placing the next one, and each subsequent layer should be placed whilst the underlying layer is still plastic sosubsequent layer should be placed whilst the underlying layer is still plastic so that monolithic construction is achieved

5. Collision between concrete and formwork or reinforcement should be avoided.6. For deep sections, a long down pipe ensures accuracy of location of concrete

and minimum segregation.7. You must be able to see that the placing is proceeding correctly, so lighting

should be available for large, deep sections, and thin walls and columns.

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Placing Concrete

PlacingPlacing

• By skip, wheelbarrow, shute, h l tshovel or concrete pump

• Place at final position - do not vibrate into position

• Vibrate using poker - approx 10 seconds at 0.5 metre intervals

• Level with wooden float leave

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• Level with wooden float, leave for a while, then finish with steel float

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Concrete placing techniques

Sources: https://rdl.train.army.mil/soldierPortal/atia/adlsc/view/public/9708-1/accp/en5466/lsn4.htm

Compaction• Once the concrete has been placed, it is ready to be compacted. • The purpose of compaction is to get rid of the air voids that are

trapped in loose concrete.

• Why It Matters?

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CompactionIt is important to compact the concrete fully because:

1. Air voids reduce the strength of the concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and 7%. This means that concrete containing a mere 5% air voids due to incomplete compaction can lose as much as one third of its strengthlose as much as one third of its strength.

2. Air voids increase concrete's permeability. That in turn reduces its durability. If the concrete is not dense and impermeable, it will not be watertight. It will be less able to withstand aggressive iquids and its exposed surfaces will weather badly.

3. Moisture and air are more likely to penetrate to the reinforcement causing it to rust.

4 Air voids impair contact between the mix and reinforcement (and indeed4. Air voids impair contact between the mix and reinforcement (and, indeed, any other embedded metals). The required bond will not be achieved and the reinforced member will not be as strong as it should be.

5. Air voids produce blemishes on struck surfaces. For instance, blowholes and honeycombing might occur.

Compaction• Summing up, fully compacted concrete is dense, strong and

durable; badly compacted concrete will be porous, weak and prone to rapid deterioration. Sooner or later it will have to be repaired or replaced It pays therefore to do the jobbe repaired or replaced. It pays, therefore, to do the job properly in the first place.

• Stiff mixes contain far more air than workable ones. That is one of the reasons why a low-slump concrete requires more compactive effort than one with a higher slump - the compaction needs to continue for a longer time, or morecompaction needs to continue for a longer time, or more equipment has to be used.

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Compaction1. Even air-entrained concrete needs to be compacted to get rid of

entrapped air voids. 2. The difference between air voids and entrained air bubbles should be

noted at this stage. 3. The air bubbles that are entrained are relatively small and spherical in

shape, increase the workability of the mix, reduce bleeding, and increase frost resistance.

4. Entrapped air on the other hand tends to be irregular in shape and is detrimental to the strength of the mix.

5 It is to remove this air that the concrete must be properly compacted5. It is to remove this air that the concrete must be properly compacted. 6. There is little danger that compaction will remove the minute air

bubbles that have been deliberately entrained, since they are so stable.

Compaction - Vibration• Vibration:

– To compact concrete you apply energy to it so that the mix becomes more fluid. Air trapped in it can then rise to the top and escape. As a result, the concrete becomes consolidated, and you are left with a good dense material that will, after proper curing develop its full strength and durabilityafter proper curing, develop its full strength and durability.

– Vibration is the next and quickest method of supplying the energy. Manual techniques such as rodding are only suitable for smaller projects. Various types of vibrator are available for use on site.

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Compaction – Poker Vibrators• Poker Vibrators

– The poker, or immersion, vibrator is the most popular of the appliances used for compacting concrete. This is because it works directly in the concrete and can be moved around easily.Sizes:– Sizes:

• Pokers with diameters ranging from 25 to 75mm are readily available, and these are suitable for most reinforced concrete work. Larger pokers are available - with diameters up to 150mm - but these are for mass concrete in heavy civil engineering.

Compaction – Poker VibratorsRadius of action:

– When a poker vibrator is operating, it will be effective over a circle centred on the poker. The distance from the poker to the edge of the circle is known as the radius of action.

– However, the actual effectiveness of any poker depends on the workability of the concrete and the characteristics of the vibrator itself. As a general rule, the bigger the poker and the higher its amplitude, the greater will be the radius of action. It is better to judge from your own observations, as work proceeds on site, the effective radius of the poker you are operating on the concrete you p y p g yare compacting.

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Compaction – Poker VibratorsThe length of time it takes for a poker vibrator to compact

concrete fully depends on:1. The workability of the concrete: the less workable the mix, the

longer it must be vibratedlonger it must be vibrated.2. The energy put in by the vibrator: bigger vibrators do the job

faster.3. The depth of the concrete: thick sections take longer.

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Heat of HydrationHeat of Hydration

•• The exothermic reaction during setting canThe exothermic reaction during setting can•• The exothermic reaction during setting can The exothermic reaction during setting can cause a significant temperature rise in large cause a significant temperature rise in large concrete pours. concrete pours.

•• This causes expansion, then setting, then This causes expansion, then setting, then contraction.contraction.

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•• If the pour is restrained, or has a If the pour is restrained, or has a temperature differential, cracking may temperature differential, cracking may occuroccur

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Heat of Hydration (large pours)Heat of Hydration (large pours)Te

mp

(Te

mp

(oo C)

C)

2020

4040

6060 InteriorInterior

SurfaceSurface

InteriorInterior

SurfaceSurface

2020ooCC 2020ooCC

71

00

00

00 11 22 33 00 11 22 33Time (days)Time (days) Time (days)Time (days)

Hardened Concrete

StrengthP d i tl ff t d b th

ress

ive

Stre

ngth

ress

ive

Stre

ngth VibrationVibration

Hand CompactionHand Compaction

Fully CompactedFully CompactedConcreteConcrete

Predominantly affected by the amount of pores in the hardened concrete.

As this is strongly affected by the non-hydrated water remaining after hardening, the water cement ratio is

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WaterWater--Cement ratioCement ratio00 0.50.5 1.01.0

Com

prC

ompr InsufficientlyInsufficiently

CompactedCompacted

the water-cement ratio is taken to be the main determinant of strength.

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Fineness of Cement

• The finer the mm--11

))

1 year1 year• The finer the cement, the faster it sets, and the faster compressive strength is gained.

sive

Stre

ngth

(Nm

msi

ve S

treng

th (N

mm

2020

4040

6060

1 day1 day

7 days7 days28 days28 days

1 year1 year

73Fineness of Cement (mFineness of Cement (m22kgkg--11))

400400 800800600600

Com

pres

sC

ompr

ess

00

2020

Different Cement Composition

A - High C3S, C3A m-1

)

B - Med high C3S, C3AC - Med low C3S, C3AD - Low C3S, C3A

Cements C and D may i 50% f th i si

ve S

treng

th (N

mm

20

40

60

A B

C

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gain 50% of their strength after 28 days

Com

pres

s

0

20

Age (log days)1 10 365

CD

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Curing

• If left in contact with water, t ill ti t iconcrete will continue to gain

strength for many months• Otherwise all free water

evaporates or is used up in the hydration process, and no further hydration can continue

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• Curing ensures that water for hydration is available as long as possible

Sources: http://www.larsenbuildingproducts.com/products/range/2/category/8/-/0/product/66/

Concrete curing

Concrete's Age-Compressive strength with different curing temperature.Source: http://commons.wikimedia.org/wiki/File:Age-Compressive_strength_(concrete-curing).png

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New Technology: Internal Concrete Curing

• There's a renewed interest in developing methods to incorporate internal curing as a tool to reduce cracking.

• More info:http://www.theconcreteproducer.com/industry-

news.asp?sectionID=1423&articleID=466478

CuringCuring

Moist-cured, dry at testAir-cured after 1 and 3 months,dry at test

100

50Air-cured, saturated at testAir-cured, dry at test

Moist cured, moist at test

Water curing after 9 monthsAir-cured after 1 month,dry at test

78Age (months)0 6 12

0

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Compression Testing

• Two compression tests are used hh

– The most common in the UK is the cube test

– However a cylinder with a height-diameter ratio of 2 can be used

– cylinder strength = cube strength x 0.87

1 01 0

Rel

ativ

e St

reng

thR

elat

ive

Stre

ngth

79Height/diameterHeight/diameter11 22

1.01.00.870.87

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Compression testing on cube and its various failure modes

• Compression test of “concrete specimen” is the most widely used test to measure its compressive strength. Two types of concrete specimen: Cubes & Cylinders are used for this purpose:

• Cubes of size 150mm are more common in Asia, Russia & European countries while Cylinders of 150mm in diameter & 300mm high are common in U.S and Australia.

Sources: http://buildingresearch.com.np/services/mt/mt1.php

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Deformation of Concrete

• To sense the deformation of concrete, cement or rock specimens under compression.

Standards ASTM C469ISO 6784DIN 1048UNI 6556ASTM D2664 (Rock testing)ASTM D2938 (Rock testing)ASTM D3148 (Rock testing)ASTM D5407 (Rock testing)

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Applications 1. For determining the Elastic Modulus and strength

characteristics.2. To be directly glued to the specimens for the

measurement of deformation as alternative to the Electronic Compressometer.

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Deformation of Concrete

E=Stress/StrainBut the stress-strain curve

for concrete is non-linear

E varies from 15 to 40 Mpa, but the tangent

Stre

ssSt

ress

85

modulus at high stress gets much lower

StrainStrain

Creep

Load sustainedLoad sustained Load removedLoad removed

CreepCreep

ImmediateImmediateelasticelasticrecoveryrecovery

CreepCreeprecoveryrecovery

oad susta edoad susta ed oad e ovedoad e oved

86TimeTime

Elastic deformation Elastic deformation on loadingon loading

PermanentPermanentdeformationdeformation

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Creep

• Concrete creep is defined as: deformation of structure under sustained load.

Basically, long term pressure or stress on concrete can make it change shape. This deformation usually occurs in the direction the force is being applied. Like a concrete column getting more compressed, or a beam bending.

Creep does not necessarily cause concrete to fail or break apart. Creep is factored in when concrete structures are designed.

CreepCreep

• Concrete naturally shrinks as it cures. If the pour is large enough, cracks WILL occur.

• Structural designers introduce so-called "control joints" in large pours so that cracking either does not occur, or occurs in predictable areas.

• If you look at a typical concrete driveway for instance, you will notice lines scored across the concrete every few feet. Any stress cracks that occur are more likely to happen in the joint, where they will be less noticeable.

• This can be a huge problem in certain structures such as dams or the i b ildi l l

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containment building at a nuclear power plant.• Concrete for these types of structures has to be carefully engineered and layed.

Concrete gives off heat as it cures (an 'exothermic' chemical reaction) and this can make the problem much worse.

• It is common to mix the concrete for a dam with chilled water and pour it in many thin layers to minimize creep and shrinkage caused cracks and flaws.

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CreepCreep

• This can be a huge problem in certain structures such as dams or the containment building at a nuclear power plant.

• Concrete for these types of structures has to be carefully engineered and layed. Concrete gives off heat as it cures (an 'exothermic' chemical reaction)

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and this can make the problem much worse.

• It is common to mix the concrete for a dam with chilled water and pour it in many thin layers to minimize creep and shrinkage caused cracks and flaws.

CreepCreep

• The magnitude of creep is affected by– More cement in mix - more creepMore cement in mix more creep– Higher w/c ratio - more creep– Higher relative humidity - lower creep– Greater age - lower creep– Rapid Hardening - lower creep

90Source: http://www.ndt.net/article/ndtce03/papers/p028/p028.htm

Time relationship of creep coefficients on bended and pressed specimens.

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Standard creep test frames.

ASTM C512 - Compressive Creep• The standard creep test consists of a frame• The standard creep test consists of a frame

and hydraulic loading system to applyconstant stress to 150X300 mm (6x12 in.)cylindrical specimens (Figure).

• Deformation is monitored periodically overtime and compard to compansion unloadedspecimens to obtain the creep strain of theconcrete, which can then be used to calculatethe creep compliance, or “specific creep” ofthe material

Standard creep test frames. (Courtesy of CTLGroup)

the material.• Tests are typically started at 7 or 28 days of

age, but this test can be modified for earlyage by starting the test as early as 24 hours.

• Sealed tests are used to evaluate “basic”creep and unsealed tests incorporate thePickett Effect, or “drying” creep.

Source: http://www.cement.org/tech/cct_cracking.asp

ShrinkageShrinkage

3 principal types of shrinkage/expansion:p p yp g p1. Plastic Shrinkage - caused by settlement of

solids and loss of free water from plastic concrete.

2. Autogenous Shrinkage - Cement gel has a lower volume than the water and cement that makes it So at a constant water content

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makes it. So at a constant water content shrinkage takes place.

3. Drying Shrinkage - Loss of water from cement gel, after loss of water from pores and capillaries. Can be as high as 1.5E9.

Image Source: http://www.inspectapedia.com/structure/FoundationOccur2.htm

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Drying ShrinkageDrying Shrinkage

1. Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink.

2. Restraint to shrinkage, provided by the subgrade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete.

3. Restraint to drying shrinkage is the most common cause of

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concrete cracking. 4. In many applications, drying shrinkage cracking is inevitable. 5. Therefore, contraction (control) joints are placed in concrete to

predetermine the location of drying shrinkage cracks.

More info: http://www.cement.org/tech/faq_cracking.asp

Avoidance of Cracked Concrete

•• If concrete is If concrete is restrained,restrained, movement joints or antimovement joints or anti--crack crack i f t t b d H t f h d ti d d ii f t t b d H t f h d ti d d ireinforcement must be used. Heat of hydration, and drying reinforcement must be used. Heat of hydration, and drying

shrinkage must be minimised.shrinkage must be minimised.•• If concrete is not restrained, differential heat of hydration If concrete is not restrained, differential heat of hydration

and drying shrinkage should be minimised.and drying shrinkage should be minimised.

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