1 Chapter5 Cement concrete. 2 Main topics Concrete Mixture Design 5.2 Technical Performances of...

1

Chapter5 Cement concrete

2

Main topics

Concrete Mixture Design 5.2

Technical Performances of Concrete5.1

Concrete for Road 5.4

Concrete Admixture 5.3

3

Technical Performances of Concrete

5.1

4

Cement concrete is a man-made stone material, which is compound that consists of cement, aggregates and water in proper proportion.

Cement concrete is widely used in civil engineering, its characteristics are:

——richness in resources and simple for construction; ——concrete and reinforcement have same linear expansi

on coefficient and fine bond strength between them, which is much beneficial to make reinforcement concrete and prestressed concrete;

——through changing the composition proportion, different feature concretes can be obtained to meet different requirements ；

——good mechanical behaviors and better durability.

Technical Performances of Concrete ——Introduction

5.1

5

Definition: ——workability is a property which means fresh ce

ment concrete mixture is easy to construction operation (including mixing, transportation, concreting, vibrating and surface treatment) , uniformity on quality and dense in structure.

Workability including: ——flowability ( 流动性 ): is easy to fill formwork in

uniformity and compactness ； ——tamping feasibility ( 捣实性 ): is likely to remov

e all air in concrete mixture to be dense.

Technical Performances of Concrete ——Construction workability

5.1

6

——cohesiveness ( 黏聚性 ): without separation and disintegration to fresh concrete depending on some cohesion;

——water-retaining ( 保水性 ): to have good water retention abilities without bleeding problem.


5.1

7

Workability test method:

——slump test( 坍落度试验 )

——Vebe consistometer test( 维勃稠度试验 )


5.1

8

Influencing factors on workability (1) internal cause: component materials: ——water and cement ratio (W/C); ——quantity of water per unit； ——sand percentage； ——raw materials: cement and cement fineness,

aggregates; ——admixture; (2) external causes ——environmental factor: temperature, humidity, w

ind speed; ——time.


5.1

9

Concrete strength

——cubic compressive strength;

——axis compressive strength;

——flexural strength;

——splitting strength.

Technical Performances of Concrete ——Strength Characteristics

5.1

150mm

150mm

150mm

Test diagram of compressive strength

300mm

150mmTest diagram of axis compressive strength

150mm

150mm 150mm 150mm

150mm

Test diagram of flexural strength

150mm

150mm

150mm

150mm

Test diagram of splitting strength

10


5.1

Cubic compressive strength( )

——definition: the compressive strength of a cubic with 150mm length on each side;

——method of calculation:

cu

Ff

A

——the standard values on the cubic compressive strength ( ): the strength satisfied the requirement of assurance rate under standard condition:

cuf

cu kf ，

, 1.645cu kf f

11

Grades of concrete strength

——there are 12 concrete strength grades in our country nowadays. Each of them can be expressed as “C” plus “standard value of strength” :

——C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60.


5.1

12

Flexural strength


5.1

150mm 150mm150mm

150mm

Test diagram of flexural strength

150mm

2

:

L span

cf

PLf

bhwhere P

—maxi mum l oadi ng(N);

— （450mm）;

b, h—wi dth and hi gth(normal l y 150mm, respecti vel y).

13

Influencing factors on concrete strength

(1) concrete composition:

——cement strength: the higher cement strength, the higher concrete strength;

——water cement ratio (W/C): concrete strength has close relationship to W/C according to a lot experience, which is the lower W/C, the higher concrete strength is.


5.1

14

——Characteristics of aggregate: ① rough surface and plentiful edges preferred; ② closely to cubic or round shape, rather than elongated o

r flat; ③ larger of maximum size is beneficial to compressive stre

ngth but unbeneficial to flexural strength, and when size of coarse aggregate is too larger, it is unfavorable to workability. Normally, nominal maximum size of coarse aggregate for concrete is no more than 31.5mm;

④ concrete mixture in continuous gradation has better workability and more dense; Comparatively speaking, gap gradation has less cement consumption compare with continuous concrete, but it is poor in workability;

⑤ Generally, sand with less specific surface area and less void ratio is preferred which has the ideal gradation.


5.1

15

(2) curing condition

——curing temperature;

——curing humidity;

——aging.


5.1

16

Evaluation on the quality of concrete strength Statistical method—Unknown standard deviation

method ：

Nonstatistcal method:

1 ,0.90cu cu kf S f


5.1

,min 2 ,cu cu kf f

,1.15cu cu kf f

,min ,0.95cu cu kf f

17

Elastic deformation

——it can be expressed by the secant modulus ( 割线模量 );

Creep deformation

——it is the deformation under sustained loading condition;

Temperature deformation Dry shrinkage deformation

Technical Performances of Concrete ——Deformation Characteristics

5.1

strain

stress

εα

σ

Diagrams of secant modulus

18

Impermeability of concrete ——it is a capability that concrete resists permeability of ga

s or water; ——according to the behavior of concrete standard specim

en without water seepage ( 渗水 ) under specified water pressure, 6 grades on impermeability are set;

Frost resistance ——it is a capability that concrete resists freeze/thaw cyclin

g ( 冻融循环 ) ； ——there are 9 grades to evaluate concrete freezing-thawi

ng durability; Resistance to chemical attack Abrasive resistance.

Technical Performances of Concrete ——Durability Characteristics

5.1

19

Asphalt Pavement Abrasive Experiment

20

Concrete Mixture Ratio Design 5.2

21

Cement ——types of cement: according to cement

adaptability which is affected by the project features and environmental condition, proper type of cement is selected;

——grades of cement: concrete grade and cement grade should be matched;

Coarse aggregate ——quality: satisfied the specified quality

requirements, such as strength, soundness etc.;

Concrete Mixture Ratio Design——Technical requirements for raw materials

5.2

22

——detrimental ( 有害的 ) impurity: including clay, sludge( 泥土 ), sulfate and organic materials etc., they are affect mostly on cohesive property between cement and aggregate;

Fine aggregate: Besides requirements on the quality and detrimental impuri

ty same as coarse aggregate, particular requirements are ——grading: based on the cumulative on the sieve size 0.6

mm, the grading of fine aggregate can be classified as I, II, III types, among them type II is better on the characteristics, because it has higher density and lower specific surface area, which gives concrete good water-retaining and less cement consumption;


5.2

23

——fineness modulus: normally medium sand shows better performance;

Water ——any domestic water can be used in

concrete mixture; Admixture ——it is a special material which uses a little

quantity but influence very much on the concrete properties.


5.2

24

Representation of concrete proportions ——unit volume method: mass of each material

per one cubic meter, for example: cement : water : sand : coarse aggregate. 300 : 150 : 600 : 1200 (kg/m3) ——relative quantity method: which means that

mass of cement considered as one unit while other materials convert into a relative units related to cement, for example:

cement : sand : c.agg., water/cement ratio = 1 : 2.00 : 4.00 , W/C= 0.50

Concrete Mixture Ratio Design——Basic concept about mixture design

5.2

25

Design requirements

——meet strength requirement: put forward a preparation strength( 配制强度 ) which is higher than the design strength( 设计强度 );

——meet workability requirement: select appropriate workability based on dimension or shape of the structure, reinforced layout situation and construction method.


5.2

,cu kf

26

——meet durability requirement: control concrete “maximum W/C ratio” and “minimum cement requirement” according to environmental conditions;

——meet economic requirement: use local materials or waste materials based on qualified requirement of workability and mechanics.


5.2

27

Design parameters ——W/C: it is a key parameter which affects

concrete performances: such as strength, workability and durability;

——water requirement: it decides quantity of cement paste under same W/C ratio;

——sand-ratio: it is closely related to concrete cohesiveness and water-retaining.


5.2

28

Design processes (1)stage of initial design: according to

requirement of the design document, initial mix proportion is found by experience way based on the characteristics of raw materials;

(2)stage of testing, adjusting and confirming: final mix proportion is determined in lab by the way of slump test and strength test;

(3)stage of work site design: the process is to find the mix proportion to construction based on moisture content of aggregate in the site.


5.2

29

Determining the preparation strength:

Types of agg. Coefficient

Crushed agg.

gravel

αa 0.46 0.48

αb 0.07 0.33

Concrete Mixture Ratio Design ——(1)Stage of initial design

5.2

Calculation of W/C:

, ,

,

,

1.645

: ( );

( )

95% ;

( ).

cu o cu k

cu o

cu k

f f

where f preparation strength MPa

f design strength MPa

assurance coefficienat in assurance

standard deviation MPa

—

— ;

1. 645—

—

,

/

: 28 ;

, : .

a ce

cu o a b ce

ce

a b

fW C

f f

where

f d actual strength of cement

regression coefficient related witht types of coarse aggregate

, 1.645cu kf f

30

Determining of water requirement ( ):

——can be get through checking the table when W/C comes within the range of 0.4~0.8;

Determining of sand ratio ( ):

——also get through checking the table by interpolation ( 插入法 );

Calculation of cement requirement:


5.2

/wo

co

mm

W C

wom

s

W/C=0.40 砂率： 27~32% =0.45 ？

=0.50 30~35% 31%

31

——Volume method ——Density method


5.2

Calculation of sand and coarse aggregate

requirements, respectively ( ):,so gom m

0.01 1

100

mm m ma

m

m m

goco wo so

c w s g

ss

s g

（%）100

m m m m

m

m m

co wo so go cp

ss

s g

（%）

: : :co wo so gom m m m

So the initial mix proportion of this stage is:

32400 ~ 2450 /cp kg m

32

Basic mix proportion

——It is the process that adjusts initial concrete design based on slump test;

——one of the possibilities for the test is shown in the following table;

——this stage result can be expressed as:

Concrete Mixture Ratio Design ——(2)Stage of adjustment

5.2

: : :ca wa sa gam m m m

33

Possibilities of Slump Test

Possibility Slump valueCohesiveness and water-retaining

Improvement measures

1 good good No improving.

2 poor goodKeeping W/C constant to adjusting quantity of cement paste.

3 good poorIncreasing sand ratio properly.

4 poor poor

Adjusting quantity of cement paste and increasing sand ratio properly.

34

Design mix proportion ——finding the relationship between strength and W/C ratio, so to

determine final W/C ratio to ensure the strength which meets the requirement:

27

29

31

33

35

0. 43 0. 48 0. 53

W/ C rati o

comp

ress

ive

stre

ngth

(MP

a)

Concrete Mixture Ratio Design ——(2)Stage of adjusting design

5.2

W/C

,cu of

35

——based on the final W/C and basic water requirement considered as , to calculate the final cement requirement again ( );

——then calculating the sand and coarse aggregate requirements under new cement and water requirements , , so this stage result can be expressed as:

wbm


5.2

cbm

wbmcbm

: : :cb wb sb gbm m m m

36

——according to correction factor of density ( ) to calculate final mix proportion:


5.2

,

,

3,

3,

:

( / );

( / )

c t

c c

c t

c c

where

actual measurement destity kg m

calculation desity kg m

—

—

——final mix proportion in lab can be expressed as:

: : :c w s gm m m m

37

Because sand and coarse aggregate materials in construction site are kept moist, so mix proportion for construction site should be adjusted according to moisture content of sand and coarse aggregate( ): %, %s gw w

Concrete Mixture Ratio Design ——(3)Stage of construction design

5.2

: (1 %)s s ssand m m w

: (1 %)g g gcoarse aggregate m m w

: ( % %)w w s s g gwater m m m w m w

: c ccement m m

38

Concrete for Road5.3

39

Ordinary concrete for road——Special requirements

Composition ——there are some special characteristics in cement compo

sition, which are higher C4FA and lower content of C3A; Workability depends very much on the method of Concrete paving, nor

mally paving method are: ——Concrete synovial paving method( 滑膜摊铺法 ): slump 2

5~30mm; ——Concrete track paving method( 轨道摊铺法 ): slump 40~

60mm; ——Concrete paving method with three roll shaft( 三辊轴摊

铺法 ) : slump 30~50mm ； ——Small machine paving method( 小型机具法 ) :slump 10~

40mm.

Mineral compositions

Chemical compositions

Ordinary Portland

cement (%)Cement for road

(%)

Tricalcium aluminate

3CaO·Al2O3 0～ 15 ≤7

Tetracalciumaluminoferrite

4CaO·Al2O3·Fe2O

3

5～ 15 ≥15

5.3

40

Durability ——Besides controlling the maximum W/C

ratio and the minimum requirement, durability of concrete for road has the requirement on air content;

——amount of air entraining content should consider normally environment condition, and nominal maximum size of coarse aggregate.

Ordinary concrete for road——Special requirements

5.3

41

Road Concrete Design——Determining preparation strength

5.3

a

1 1.04

: design strength, MP ;

: standard deviation;

: assurance coefficient;

: coefficient of variation

rc

v

r

v

ff ts

C

where

f

s

t

C

Common concrete Concrete for road

, ,

,

,

1.645

preparation strength (MPa);

design strength (MPa);

1.645 assurance coefficienat in

95% assurance;

standard deviation (MPa).

cu o cu k

cu o

cu k

f f

where

f

f

：

：

：

：

42

Road Concrete Design ——Calculation W/C

5.3

,

/

: 28d actual strength of cement;

, : regression coefficient related

witht ypes of coarse aggregate.

a ce

cu o a b ce

ce

a b

fW C

f f

where

f

a

a

1.5684/

1.0097-0.3595

(for crushed agg.)

1.2618/

1.5492 0.4709

(for gravel agg.)

: concrete preparation strength (MP )

: cement actual flexuel strength (MP ).

c s

c s

c

s

W Cf f

W Cf f

where

f

f


43

Road Concrete Design ——Find sand ratio

5.3

By table lookup method.related factors:——slump value;——type of coarse aggregate;——nominal maximum size of aggregate.


By table lookup method. Related factors:——fineness modulus; ——type of coarse aggregate;

44

Road Concrete Design ——Find water requirement

5.3

Through the look-up table method to find water requirement ；Related factors:——W/C ratio;——type of coarse aggregate;——nominal maximum size of aggregate.


Through the calculation method to find water requirement ；

104.97 0.309 11.27( / ) 0.61

(for crushed aggregate)

86.89 0.370 11.24( / ) 1.00

(for gravel aggregate)

: slump value (mm);

: sand ratio(%).

wo s

wo s

s

m SL W C

m SL W C

where

SL

45

There is no differences on the

calculation method of mix

design for cement, fine and

coarse aggregates requirements

between common concrete and

road concrete;

/wo

co

mm

W C

Road Concrete Design——Calculation requirements of cement, and aggregates

5.3

0.01 1

100

mm m ma

m

m m

goco wo so

c w s g

ss

s g

（%）

100

m m m m

m

m m

co wo so go cp

ss

s g

（%）

It should emphasize that

filling volume of coarse

aggregate is calculated,

which is not less than 70%.

(mass over tap density of

coarse aggregate).

46

Concrete Admixture 5.4

47

Concrete admixture is a special compound which mixed into concrete during mixing process.

Quantity of admixture added is not much (generally no more than 5% by weight of the cement) , but it brings about significant impact on concrete performances, such as workability, strength etc.;

The main function of admixture is reducing construction cost, obtaining desirable workability and meeting some special requirements.

Concrete Admixture ——introduction

5.3

48

Concrete Admixture ——introduction

5.4

Classification

Improving rheological behavior

Water reducer, Air entraining agent,Pumping aid,Water-retaining agent.

Adjusting speed of setting and hardening

Retarder, Early strength agent,Accelerating agent.

Adjusting air content

Air entraining agent,Foam agent, Defoamer.

Improving durability

Air entraining agent,Antifreeze agent,Anti-permeability agent,Corrosion inhibitor.

49

Reducing water agent ——it is an agent that can reduce water requirement

without affecting workability of concrete, especially flowability;

——the main functions of this admixture are: (1) because of reducing water requirement and lowering

W/C ratio, so concrete strength corresponding is improved; (2) remarkably improving workability under the condition of

no any change on concrete component and strength;

(3) without change on workability and strength, it can reduce cement consumption so to cut coast.

Concrete Admixture ——Ordinary admixtures

5.4

50

——Mechanism of action ： (1) oriented absorption and dispersion ( 定向吸附

和分散作用 ); (2) lubrication ( 润滑作用 ) ； (3) promotion hydration( 促进水化作用 ) ； ——types of reducing water agent: (1) common type: reducing water requirement abo

ut 5~15%; (2) superplasticizer: reducing water requirement a

bout over about 20%; (3) compound type: besides reducing water require

ment, also having other characteristics.


5.4

51

Air entraining agent ——this admixture can bring a larger numbe

r of microscope air-bubbles dispersed throughout the concrete;

——the benefit of this entrained is that causes disruption of the continuity of capillary pores ( 毛细管 ) , thus reducing the permeability of concrete and reducing internal stresses caused by expansion of the pore water on freezing.


5.4

52

Retarder

——it can prolong the setting time of concrete with no adverse effects on post strength;

——the mechanism of retarder is because that retarder can be attracted on the cement particles surface to form a film which is not soluble, so as to prevent cement from hydration for a period of time;

——retarder applies for mass concrete, construction in summer season and long distance translocation.


5.4

53

Early strength agent ——it is a kind admixture that promote concrete

early strength but no adverse effect on post strength;

——the mechanism relies on the catalytic role of hydration;

——it applies to the construction under room and low temperature, even under negative temperature condition. But it has to pay more attention in corrosion of steel bar because this admixture normally contains some chloride.


5.4

54

According to the following example to know how to calculate the concrete mix proportion when some reducing water agent mixed to concrete.

Known conditions: initial concrete proportions

An example of admixture application

cement : water : sand : coarse aggregate 372 : 175 : 596 : 1268 (kg/m3);Sand ratio: 32%;Admixture adding amount : 0.5% in cement;water reducing ratio: 8%.

55

After mixed 1.9kg reducing water agent ( ), finding slump value increasing obviously, from original 25mm into 120mm, and cohesiveness and water-retaining are still desirable.

3, 0.5% 372 0.5% 1.9 /ad c adm m kg m

Solution 1 Strength remains constant, while flowability of

fresh concrete increases significantly .

56

Solution 2Keep concrete flowability (slump value) and strength the same to save cement consumption.

3, (1 ) 175(1 0.08) 161 /w ad w adm m kg m

Calculating cement

requirement

Calculating water requirement when added

reducing water admixture

3, 0.5% 343 0.005 1.7 /ad c adm m kg m

Calculating Admixture quantity

Finally determining the fine and coarse

aggregate requirements

3, , / ( / ) 161 0.47 343 /c ad w adm m W C kg m

3 3: 617 / :1310 /sand kg m coarse aggregate kg m，

57

3, (1 ) 175(1 0.08) 161 /w ad w adm m kg m

Calculating water requirement when added

reducing water admixture

Cement requirement

keeps constant

Finally determining the fine and coarse

aggregate requirements

3 3: 609 / :1294 /sand kg m coarse aggregate kg m，

Calculating Admixture quantity

3, 0.5% 372 0.5% 1.9 /ad c adm m kg m

Solution 3Flowability (slump value) remains constant, while strength of concrete is improved.

3, 372 /c adm kg m

58

Summary of the example

0.471268596175372Original proportion

W/Cc.aggsandwatercement

0.431294609161372Solution 3

0.471268596175372Solution 1

0.471310617161343Solution 2

1 Chapter5 Cement concrete. 2 Main topics Concrete Mixture Design 5.2 Technical Performances of...

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Transcript of 1 Chapter5 Cement concrete. 2 Main topics Concrete Mixture Design 5.2 Technical Performances of...