1 4 3 Design for Early Thermal Cracking(2)

P Bamforth, J Shave, S Denton 1

DESIGN FOR EARLY AGE THERMAL CRACKING P Bamforth, independent consultant, London, UK J Shave, Parsons Brinckerhoff, Bristol, UK S Denton, Parsons Brinckerhoff, Bristol, UK

Abstract

The design of concrete members subject to early age thermal cracking in the UK has

previously been carried out to BD28[1]

for bridges and BS8007[2]

for water retaining

structures. These standards have now been replaced by the respective parts of BS EN 1992.

CIRIA C660[3]

„Early Age Thermal Crack Control in Concrete‟ was published in February

2007 to accommodate changes that had occurred in the UK in relation to the wider range of

cements and combinations being used; the development of high performance concretes; and

the introduction of the Structural Eurocodes. The use of CIRIA C660 in combination with BS

EN 1992 represents a significant step forward in the design methodology for early thermal

cracking in the UK.

The principal objective of CIRIA C660 is to support both designers and constructors by

providing procedures to limit the extent of cracking to an acceptable level and, where

appropriate, to avoid cracking altogether. The report and the associated spreadsheets provide a

means for estimating all of the input variables for the design process, including thermal

effects and profiles, shrinkage strains, restraint factors and concrete properties, and they set

out processes for minimising the risk of cracking and the control of crack widths. These

processes are aligned with the Eurocode requirements, extended for use in the UK.

This paper focuses on critical changes in design for early-age thermal cracking in the UK

resulting from the introduction of BS EN 1992. These changes relate specifically to the way

in which the reinforcement ratio is estimated for calculating the minimum steel ratio and the

crack spacing and width.

BS EN 1992 does not fully address design for early-age thermal cracking. CIRIA C660 has

been developed to complement the BS EN 1992 design process and provide estimated crack

widths that reflect more reliably those observed in practice, so that a robust and serviceable

design may be achieved. In the absence of these recommendations, BS EN 1992 could be

interpreted to lead to a significant increase in minimum reinforcement, particularly in sections

thicker than about 800mm. Conversely, in some other situations the use of BS EN

1992without CIRIA C660 could lead to significantly less reinforcement than required by

previous standards BS 8007 or BD28 for controlling crack widths.

The use of CIRIA C660 is therefore recommended in the UK for the design of early thermal

cracking effects in combination with BS EN 1992.


Introduction Early-age thermal cracking (EATC) occurs as the result of restraint to contraction as concrete

cools from its peak hydration temperature. In the UK, early-age thermal cracking has

previously been dealt with using BD28[1]

for bridges and BS 8007 [2]

for water retaining

structures, supported by CIRIA 91[4]

which provided the background to the design method

and data for use in the design process. CIRIA 91 was replaced in 2007 by CIRIA C660[3]

to

take account of the use of a wider range of concreting materials and strength classes; and

changes in the design process arising from the introduction of the Structural Eurocodes, in

particular BS EN 1992-1-1 and BS EN1992-2 which have replaced BS 8110[5]

and BS 5400-

4[6]

as the general design codes; and BS EN 1992-3 which has replaced BS 8007 for water

retaining structures. CIRIA C660 has been cited as Non-Contradictory Complementary

Information in the UK National Annex to BS EN 1992-3.

In addition to reflecting changes in the design process (dealt with in this paper) CIRIA C660

also differs from CIRIA 91 as follows;

Values of temperature change (T1) for CEM I have been revised and additional

information is provided on concretes containing fly ash and ground granulated

blast-furnace slag (ggbs)[8]

.

Information is provided on autogenous shrinkage which BS EN 1992 now

requires „should be considered specifically when new concrete is cast against old

concrete‟.

Additional information is given on different forms of restraint, how they may be

calculated and how they affect crack width.

Tensile strain capacity is dealt with more comprehensively

A method for reinforcement design has been developed to deal with cracking

caused by temperature differentials in thick sections

Guidance is given on methods for minimising the risk of cracking

Advice is provided on specification, testing and in situ monitoring.

In addition, spreadsheet calculators are provided on a CD for estimating temperature rise and

temperature differentials; autogenous and drying shrinkage to BS EN 1992-1-1:2004; edge

restraint; and reinforcement for crack control.

Review of Previous Design Practice Prior to the implementation of the Eurocodes in March 2010, design for the effects of early

thermal cracking in UK bridges was carried out to BD28.

BD28 firstly required that sufficient minimum reinforcement should be provided so that the

force required to first crack the section would not yield the tension reinforcement, as follows:


c

y

ct

s Af

fA

*

(1)

The area of concrete was taken as the gross cross sectional area unless the section was thicker

than 500mm, in which case a surface zone of 250mm at each face is taken.

A further check was then made, to satisfy crack width requirements.

ultthshc

b

ct

s Rw

Af

fA

5.0

2

*

(2)

Although given directly as a required area to control cracking, the equation contains elements

relating to crack spacing and crack widths, combined into a single formula. By rearranging

Equation (2) to give crack width expressed as the product of crack spacing and crack inducing

strain as in Equation (3), comparisons with other standards may be more readily made.

crrSw max;

(3)

Where:

b

ct

s

c

b

ct

rf

f

A

A

f

fS

**

max; 5.02

(4)

and cturctuthshcr R 5.05.0

(5)

For comparison, BS8007 had the following calculation procedure:

maxmax Sw

(6)

Where:

2max;

b

ctr

f

fS

(7)

and

610100 tecs or

TR

(8)

Once expressed in this form it can be seen that the requirements of BS8007 and BD28 were

reasonably well aligned, with BS8007 presented in a similar style to the method in the

Eurocodes and CIRIA C660. There were, however, some differences in the way in which

creep and restraint were accounted for in BD28 and BS8007. Notwithstanding these

differences, the principles and the overall approach of BS8007 and BD28 were similar. For

completeness, comparisons have been drawn in this paper with both BD28 and BS8007 where

possible, relative to Eurocodes.


The Eurocode Design Approach The Eurocode design method comprises two stages. Firstly, the magnitude of free contraction

free is estimated and a restraint factor Rax is applied to determine the restrained-strain which,

if of sufficient magnitude, may result in cracking. The way in which the restrained-strain is

distributed as cracking is then estimated based on the volume and distribution of

reinforcement and the nature of the restraint.

BS EN 1992-3 deals with two forms of restraint; continuous edge restraint and end restraint.

The nature of the restraint is assumed to influence the way in which cracking develops and

different approaches are adopted to estimate the magnitude of crack-inducing strain, i.e. that

component of strain which is relieved and exhibited as cracking.

Estimating the Risk of Cracking

Estimating restrained contraction Similarly to the previous standards, BS EN 1992-3:2006 uses a strain based approach and

assumes that all compressive stresses induced during heating are relieved by creep. The

restrained contraction r is estimated using the expression;

r = Rax free (9)

where Rax is the degree of external axial restraint and εfree is the free contraction assuming no

restraint. For early-age deformation εr is estimated using the expression (10);

r = (c. T1 +. ca) K. Rax (10)

T1 is the temperature drop; c is the coefficient of thermal expansion of concrete; ca is

autogenous shrinkage; and K is a coefficient for creep. A spreadsheet model for predicting T1

is provided in CIRIA C660 which derives adiabatic temperature rise curves for a variety of

UK concretes. It is based on extensive testing at the University of Dundee [7]

and was

validated against in situ measurements [3]

. Autogenous shrinkage is calculated using the

expression of BS EN 1992-1-1.

Estimating the risk of cracking The risk of cracking is estimated by comparing the restrained strain εr with the tensile strain

capacity of the concrete εctu; for no cracking εctu > εr . εctu is estimated from the ratio of the

mean tensile strength, fctm(t) and the modulus of elasticity Ecm(t) at early-age (the 3-day value

is recommended if the specific time of cracking is not known). fctm(t) and Ecm(t) are estimated

using the expressions provided in EN 1992-1-1 and coefficients are applied to the ratio

fctm(t)/Ecm(t) which take account of both creep (0.65) and the effect of sustained loading (0.8).

The net effect of these coefficients is to increase εctu under short term loading by 0.8/0.65 =

1.27.

Estimating Minimum Area of Reinforcement The minimum area of reinforcement As,min is that which ensures that, if all of the tension in the

concrete prior to cracking is assumed to be transferred to the steel immediately after cracking,


then the stress in the steel will be below its yield strength. Expressions used by BS 8007 and

BS EN 1992-1-1 are shown in Table 1.

In the design approach of BD28 and BS 8007 it was assumed that cracking is initiated from

the surface [8]

. In practice however, it is more likely that, under conditions of external restraint

in which there is tension across the full section, cracking will be initiated at the point where

the temperature drop is the greatest, i.e. at the centre of the section (see Figure 1). Stress will

therefore be transferred from the full section to the reinforcement when a crack occurs. Hence

the underlying assumption regarding the surface zone may not be valid.

BD28 BS 8007 EN 1992-1-1

y

*

ctcs

f

fAA crit c

y

ctcs ρA

f

fAA )ρA k(k

f

fA kkA crit ct c

ky

effct,ct cmins,

Ac is the gross

cross section or a

surface zone of

250mm at either

edge

Ac is the gross cross

section or a surface

zone of 250mm at

either edge

Act is the area of concrete in tension

7.0.0 cuct f12f fct is the tensile

strength of the

concrete

fct,eff is the tensile strength of the

concrete

fy is the yield

strength of the

steel

fy is the yield

strength of the steel

fyk is the yield strength of the steel

k allows for non-uniform and self-

equilibrating stress which leads to a

reduction in restraint forces

k = 1 for h ≤ 300mm

k = 0.65 for h ≥ 800mm

intermediate values are interpolated

kc takes account of the stress

distribution in the section = 1 for

pure tension

Table 1. Expressions for estimating the minimum area of reinforcement

Restra

int

Restra

intR

estr

ain

tR

estr

ain

t

Cracking propagated

from the centre where

temperature change is

greatest

t0

t1

t2

t3 Temperature profile


Figure 1. Cross-section through a thick wall subject to external restraint

This revised view of crack development is reflected in the change in EN 1992-1-1 and has

been extended for the case of EATC in CIRIA C660 which has increased the coefficient k

(see Table 1) from 0.65 to 0.75 for h ≥ 800mm to take account of both the (generally)

parabolic temperature profile and the fact that in practice some compressive stresses must be

relieved by a drop in temperature before tensile stress are generated. A comparison of the

effective surface zones is shown in Figure 2.

Figure 2. Surface zones used in estimating the minimum area of reinforcement in sections that are dominated by external restraint

The difference is most significant in thick sections that are at risk of cracking due to external

restraint, where the methods of EN 1992-1-1 and C660 assume that stress is transferred from

a much higher proportion of the section thickness (i.e. a much higher effective surface zone)

and hence lead to the requirements for more minimum reinforcement compared with BS 8007

and BD28.

Estimating Crack Spacing and Crack Width The characteristic crack width wk (expected to be about 30% higher than the mean value

[9],

[10]) is estimated from the product of the crack-inducing strain εcr and the crack spacing Sr,max.

Crack spacing The expressions for calculating crack spacing are given in Table 2. The same expressions

apply for both edge restraint and end restraint. The second term in the BS EN 1992-1-1:2004


expression appears to be very similar to that of BS 8007. However, the way in which ρe,eff is

calculated leads to very different results. Consider a 500mm thick wall; if c = 40mm and φ =

20mm, the effective surface zone, he,ef = 2.5(40 +20/2) = 125mm. For a 500mm wall this is

only half the value of 250mm used by BD28 and BS 8007. As the value of ρp,eff is inversely

proportional to he,ef this will result in ρp,eff being double the value used by BD 28 and BS8007,

thus halving the value of the second term in the crack width expression. This difference is

partially offset by a cover term (3.4c) but the net effect is that, in this example, the crack

spacing estimated using EN 1992-1-1 will be significantly lower than the crack spacing

estimated using BS 8007. With no other changes this would lead to a significant reduction in

crack control reinforcement compared with BS 8007 as shown in Figure 3 (a).

BS 8007 and BD28/87 EN 1992-1-1

ρf

φf0.5S

b

ct

maxr, effp,

1

maxr,ρ

φk0.4253.4cS

NO cover term c is the cover (mm)

fct/fb is the ratio of the

tensile strength of the

concrete to the bond

strength, which for type 2

deformed bars = 0.67

k1 is a coefficient which takes account

of the bond properties of the

reinforcement = 0.8 (and increased in

C660 to 1.14)

φ is the bar diameter (mm)

ρ is the steel ratio based

on a surface zone of

250mm or h/2, whichever

is less

ρe,eff is the effective steel ratio based

on an effective surface zone he.eff to a

depth of 2.5 (c + φ/2) or h/2,

whichever is less

Hence, ρ

φ0.335S maxr, and,

effp,

maxr,ρ

φ0.343.4cS

Table 2. Expressions for the calculation of crack spacing

30 mm

40 mm

50 mm

60 mm

70 mm

0

20

40

60

80

100

120

140

160

180

200

300 400 500 600 700 800 900 1000

Thickness (mm)

Perc

en

t ste

el re

lati

ve t

o B

S8007

Cover

30 mm

40 mm

50 mm

60 mm

70 mm

0

20

40

60

80

100

120

140

160

180

200

300 400 500 600 700 800 900 1000

Thickness (mm)

Pe

rce

nt

ste

el

rela

tiv

e t

o B

S8

00

7

Cover

(a) k1 = 0.8 (EN 1992-1-1) (b) k1 = 1.14 (CIRIA C660)


Figure 3. The ratio of reinforcement requirements for design to EN 1992 and BS8007 (C30/37 concrete; plywood formwork; limiting early-age crack width to 0.15 mm; cover

as shown)

Observations by the authors suggest that the requirements of BD28 and BS 8007, while

having been generally applicable, have occasionally led to excessive crack widths and that it

would be unsafe to adopt a design that significantly reduces the current requirements. The

design parameters were therefore investigated. BS EN 1992-1-1:2004 recommends a factor

of 0.7 is applied to bond stress in cases when “good” bond cannot be guaranteed. In C660,

this factor has been considered applicable to k1 in cases of EATC, so the bond coefficient k1 is

increased from 0.8 to 1.14, since 0.8/0.7 = 1.14. Calculations using the increased value of k1

are shown in Figure 3 (b) and lead to steel requirements that are closer to those of BS 8007

within the normal range of cover. Higher steel ratios than those suggested by BS 8007 are

generally associated with high cover.

Crack width Up to this point in the design the nature of the restraint has not been considered. However, in

estimating crack width, BS EN 1992-3:2006 uses different expressions for estimating the

magnitude of crack-inducing strain εcr.

For continuous edge restraint informative Annex M of BS EN 1992-3:2006 assumes εcr = εr

i.e. the restrained-strain. CIRIA C660 proposes the expression εcr = εr – 0.5εctu i.e. εr less

the residual strain in the concrete after cracking. (This approach is also taken by BD28.) In

each case the assumption is that the crack width is strain limited.

For end restraint only a different expression is used as follows;

ρα

11

E

..k.f.k0.5α

es

effctce

cr

Es is the modulus of elasticity of the steel; αe is the modular ratio; fct,eff is the tensile stress in

the concrete immediately prior to cracking; ρ is the steel ratio based on the full area of

concrete in tension [N.B. This is not the same as ρp,eff used in the calculation of crack

spacing]; k and kc are area coefficients described in Table 1. This expression assumes that the

crack width is limited by the stress transferred to the steel.

Under conditions of end restraint, even when the minimum steel ratio is exceeded, crack

widths may be significantly wider than achieved under conditions of edge restraint, although

fewer cracks may occur. For example, in a 400mm section with 16mm bars at 250mm centres

using C30/37, the crack width resulting from end restraint is estimated to be 1mm, while

under conditions of edge restraint the estimated crack width is in the order of 0.15mm.

The Influence of Cover The net effect on crack width of cover alone is shown in Figure 4. This has been recognised

for many years. For example, Campbell-Allen & Hughes [11]

recommended that “the placing

of such reinforcement shall be as near to the surface of the concrete as is consistent with the


requirements of adequate cover”. However, in relation to control of EATC, the effect of

cover has not previously been quantified. Furthermore, recognising that the crack profile may

differ significantly from that of a crack developed by an element in flexure, it may be

inappropriate to adopt a similar expression to derive crack spacing and further research is

recommended in this area to avoid unnecessarily high volumes of reinforcement being used

when high cover is specified.

0.00

0.05

0.10

0.15

0.20

0.25

30 40 50 60 70

Cover (mm)

Cra

ck w

idth

(m

m)

0

500

1000

1500

2000

30 40 50 60 70

Cover (mm)A

rea o

f re

info

rcem

en

t (m

m2)

(a) Effect of cover on crack width (b) Area of reinforcement (mm

2/m/face)

required to achieve a crack width of 0.15mm

Figure 4. The Effect of Cover in a 300mm Wall Subject to a 30oC Temperature Drop and 70% Restraint

Comparison Between Estimated and Observed Crack Widths Comparisons between observed crack widths and predictions using current methods are

shown in Figure 5. These graphs are based on back analysis of measured data, as explained by

Bamforth[12]

. It is clear that both BS 8007 and BS EN 1992 (without extension as

recommended in C660) lead to unsafe predictions of crack width for many of the examples.

In some cases the difference was as much as 50%. The method of CIRIA C660, which is

based on the method of BS EN 1992 but extended as described, shows a much better

correlation with the reported crack widths.

BS8007

0.0

0.1

0.2

0.3

0.4

0.5

0.2 0.25 0.3 0.35 0.4

Measured crack width (mm)

Esti

mate

d c

rack w

idth

(m

m)

EN1992-3

0.0

0.1

0.2

0.3

0.4

0.5

0.2 0.25 0.3 0.35 0.4


Esti

mate

d c

rack w

idth

(m

m)


CIRIA C660

0.0

0.1

0.2

0.3

0.4

0.5

0.2 0.25 0.3 0.35 0.4


Esti

mate

d c

rack w

idth

(m

m)

Figure 5. Comparison of observed and predicted crack widths using the methods of BS 8007, BS EN 1992-3:2006 (without extension for EATC) and CIRIA C660[12]

Conclusions The design approach for early-age thermal cracking adopted by BS EN 1992-3:2006 is

broadly similar to that of BD 28 and BS 8007 but there are some significant and important

differences as follows;

1) Different values of surface zone are used to estimate the minimum area of

reinforcement

2) Different surface zones are used to estimated the steel ratio for calculating crack

width

3) BS EN 1992-1-1:2004 includes cover in the expressions for crack spacing and width.

This was not included in BS 8007 of BD 28

4) The term fct / fb (tensile strength/bond strength) has been replaced by the coefficient

k1

5) Crack development and crack widths depend on whether the element is subject to

edge restraint or end restraint and this is reflected in different expressions for

calculating crack width

6) Autogenous shrinkage is assumed to occur in all grades of structural concrete

CIRIA C660 has recognised these changes and has proposed modifications to the design

parameters to complement EN 1992 and ensure that estimated crack widths reflect more

reliably those observed in practice.

Acknowledgements The authors wish to thank the Construction Industry Research and Information Association

for their permission to publish this paper.

References [1] BD28/87 Early Thermal Cracking of Concrete, Design Manual for Roads and Bridges

Vol 1 Section 3, Incorporating Amendment No 1, 1989, TSO

[2] British Standards Institution, 1987, Design of Concrete Structures for Retaining

Aqueous Liquids, BS8007:1987.

[3] Bamforth, P B, 2007, Early-age Thermal Crack Control in Concrete, CIRIA Report

C660, Construction Industry Research and Information Association, London.


[4] Harrison, T A, 1992, Early-age Thermal Crack Control in Concrete, CIRIA Report

91, Construction Industry Research and Information Association, London.

[5] British Standards Institution, 1985, Structural Use of Concrete. BS8110:1985

[6] British Standards Institution , 1990, BS 5400-4:1990, Steel, concrete and composite

bridges. Code of practice for design of concrete bridges.

[7] Dhir, R K, Paine, K A and Zheng, L, 2006, Design data for low heat and very low heat

special cements, University of Dundee, Research Contract No. CTU/I53), Report

CTU/4006, September 2006

[8] Anchor, R D, Hill, A W and Hughes, B P, 1979, Handbook on BS 5337:1976 (The

structural use of concrete for retaining aqueous liquids) Viewpoint Publications,

Cement & Concrete Association, Slough.

[9] Narayanan, R S and Beeby, A W, Designers’ Guide to EN 1992-1-1 and EN 1992-1-2

Eurocode 2: Design of Concrete Structures. General rules and rules for buildings and

structural fire design, Thomas Telford.

[10] Beeby, A W, 1990, Fixings in cracked concrete – The Probability of Coincident

Occurrence and Likely Crack Width, CIRIA Technical Note 136, Construction

Industry Research and Information Association, London.

[11] Campbell-Allen, D and Hughes, G W, 1981, Reinforcement to Control Thermal and

Shrinkage Cracking. Transaction of the Institution of Engineers, Australia, Civil

Engineering, August, Vol. CE23. No. 3.

[12] Bamforth, P B, 2008, A revised approach for the design of reinforcement to control

cracking in concrete resulting from restrained contraction - Case studies and

validation, ICE Research Project 0706, Project Report ICE/0706/007. Institution of

Civil Engineers, London. Project Report ICE/0706/007.

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