Crack Control
Anchor Dairy Factory Factory,, Te Rapa, 1999
Enhancing the service life of concrete structures through the control of cracking Why the control of concrete cracks is important Tensile Cracking of Concrete Preparation of Subgrade and Formwork Construction Joints Technical data t e
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Crack Control
Figure 1 Types
Plastic Shrinkage
Plastic Settlement
Early thermal Contraction
Crazing
Structural location of different types of cracks Location or Direction
Likely Element
Diagonal
Slabs
Random
Reinforced Slabs
Over Reinforcement
Reinforced Slabs
3
Over Reinforcement
Deep Sections
4
Arching over columns
Top of Columns
Change of depth
Suspended Floor
External Constraint
Walls or Slabs
Restraint of thermal movement
Internal Constraint
Thick masses
Excess temperature gradients
Mirrors underlying reinforcement
Appears over reinforcement
Restraint of thermal movement
Against Form-work
“Fair-faced” concrete
Excess paste at surface
Flat-work
Slabs
Over Trowelling
Slabs and walls
Inefficient Joints
Weeks, months, years
8 7
Drying Shrinkage
Primary Cause
Secondary Cause
Time of appearance
Reference
1
30 minutes to 6 hours
Low rate of bleeding
Rapid Evaporation
10 minutes to 3 hours
Bleeding
5 6 11 8
Rapid cooling
1 day to 2-3 weeks
Poor curing, poor placement
Anytime after hardening
Sub-grade Movement
Slabs, footings
Preparation of sub-base
Anytime
Accident overload
Exposed surfaces
Typically Slabs
Accidental overload
Vulnerable at 1-2 days
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9 4 8
2
8
10 6
3
11 5 5
12 1
14 8
12
13
Construction Movement
13
2
8
7
9 10
14
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ECS Crack Control 2
Enhancing the service life of concrete structures through the control of cracking A fundamental requirement of any concrete structure
Water or other fluids transport damaging agents into
is its performance over its intended design life.
the concrete and cause expansion through corrosion
Concrete must be able to withstand wear and
of reinforcing steel, freezing of water, and other effects.
deterioration given the environment and maintenance regime for which it was designed. If a concrete
High strength concretes are especially vulnerable to
structure meets its intended design life when exposed
early cracking due to the use of fine materials. They
to its anticipated environment then it can be described
have higher cohesion, generate greater heat of
as being durable.
hydration and have less bleed water.
The most obvious, and common, form of concrete
Careful engineering design and care during
deterioration is cracking. Once concrete is cracked it
construction is critical to overcome the problems
becomes vulnerable to the penetration of damaging
caused by uncontrolled cracking and to ensure a
fluids and is more prone to spalling, wear and abrasive
structure meets its design life.
damage.
Why the Control of Cracks in Concrete is Important The New Zealand Concrete Structures Standard
The type, width and orientation of cracking in a
NZS3101:1995 defines durability as "the ability to
structural element normally provides an indication of
withstand the expected wear and deterioration
the risk of corrosion. Cracks wider than 0.30mm seldom
throughout the intended life of the structure without
heal, on the contrary they tend to enlarge through
the need for undue maintenance". The Standard
spalling, leaching and stresses. Concretes that contain
applies to the detailing and specifying of concrete
slag cements and/or high reactivity pozzolans (such
structures with a design life of 50 years.
as silica fume) have less capacity to self-heal due to
The corrosion of steel reinforcement through the
the reduced quantity of calcium hydroxide.
penetration of chloride ions is recognised within this
The American Concrete Institute (ACI) recommends
standard as the most common and obvious form of
0.15mm as the maximum limiting crack width at the
durability failure. Uncontrolled cracks lowers chloride
tensile face of a reinforced concrete structure subjected
resistance.
to wetting and drying cycles or sea water spray.
Tensile Cracking of Concrete Concrete cracks when its tensile strength is unable
Crack control is an integral part of reinforced concrete
to withstand the forces which the concrete is subjected
design and construction. Primary reinforcement will
to. The tensile strain capacity of concrete varies with
control crack widths but excessive amounts of
the age of the concrete and the rate of application of
reinforcing steel can increase the risk of cracking.
the strain.
Reinforcement does not eliminate or reduce shrinkage
It must be recognised that concrete has an inherent potential to crack because of its low tensile capacity.
cracking in concrete. Rather, it transforms a few wide cracks into many fine cracks and micro-cracks.
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Plastic Shrinkage Cracking
Plastic shrinkage cracks occur on the surface of freshly placed concrete during finishing or soon after. These types of cracks occur when the rate of evaporation of surface moisture exceeds the rate at which bleed water is rising through the concrete. Plastic shrinkage
Precautions to avoid plastic shrinkage include
use of anti-evaporation spray on solutions after screeding or floating and before finishing avoiding adverse conditions through early morning or afternoon pours that avoid the
cracking occurs most often in summer with conditions
windiest and/or driest part of the day
of heat, wind, and low humidity.
start curing as soon as possible after finishing
Concretes that are most susceptible to this form of
dampen form-work, subgrade and
cracking are those with: • High cement content • Finer cements
reinforcement cover with polythene prior to finishing use of plastic fibres
• Lower water-cement ratios including superplasticized concrete
Craze Cracking
Plastic Settlement Cracking
Crazing is the development of a network of fine
In plastic concrete bleed water surfaces due to gravity.
random cracks on the surface of concrete caused by
If the accompanying settlement is restricted by form
shrinkage of the surface layer. The cracks are rarely
work or reinforcement, cracking may occur.
more than 2mm deep and typically form hexagonal
Typical plastic settlement is approximately 6-8mm per
shaped areas no more than 40mm across. They are more likely to occur on steel trowelled surfaces.
metre depth of the concrete element (corresponding to a typical bleeding rate of 6-8 litres per cubic metre).
These cracks are unsightly but rarely compromise structural integrity of the concrete. Crazing occurs when good concrete practice is not followed, eg poor curing, wet mixes, rapid surface drying or when concrete is finished too early while bleed water is still present.
Measures to reduce the possibility of plastic settlement cracking, include
revibrate concrete where necessary control concrete slump (80-100mm) to restrict bleed water provide sufficient concrete cover to
To prevent crazing the following precautions
reinforcement
should be followed:
use air entrained concrete
don’t finish concrete while bleed water exists never sprinkle or trowel dry cement into plastic concrete to absorb bleed water
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ECS Crack Control 4
Early Thermal Cracking
As concrete hardens the cement hydration process produces heat and the concrete element expands. The element then contracts as it cools. If contraction is restrained the resulting tensile stresses may cause cracking.
To reduce the risks of early thermal cracking:
start curing as soon as possible use of grooved jointing tool, crack inducers, early age cutting and isolation joints
Concrete is most vulnerable to early thermal cracking
covering concrete to slow heat loss at night
on the day it is poured when the heat of day and the
or exposure to wind
heat of hydration abates and is replaced by a cold evening.
delay removal of formwork
Typical thermal movements are of the order of 0.1mm per metre length (100 micro strain) per 10ºC change in temperature. Thermal cracks are common and the practice of joints not being cut for up to 48 hours leaves concrete vulnerable to this mechanism. In mass concrete pours the resulting thermal effects need special treatment. Typical solutions involve the use of a concrete that generates low thermal heat (via a slag cement such as Duracem), and/or the use of insulating form work. Insulating form work controls the release of heat so avoiding excessive thermal gradients between the core and the surface.
Figure 2
Time of appearance of cracking
Types of cracking
Time
Plastic Settlement Plastic Shrinkage Early Thermal Movement Drying Shrinkage Excess Loading Corrosion Hours
Days
Weeks
Months
Years
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The cement type will affect the rate of shrinkage. Slow
Drying Shrinkage Cracking
hydrating cements such as slag will exhibit slow Drying shrinkage cracks can be a significant cause of damage to a concrete structure. These rarely appear earlier than 5-7 days following placing. Shrinkage occurs over a prolonged period and typically 70-80% of total drying shrinkage is reached after 12 months.
shrinkage. The size of aggregate is important. The larger the aggregate proportion of the concrete mix, the l ower the paste content tends to be. The type and stiffness of aggregate can also influence the amount of concrete shrinkage. Tests show that concrete using basalt
Drying shrinkage can be defined as the "loss in
aggregates tend to shrink less than greywacke-based
concrete volume resulting from the loss of water
concrete.
from the concrete after hardening". The extent of cracking that can result from drying shrinkage depends primarily on the amount of restraint that exists to stop movement. The degree of drying shrinkage, strength and elasticity of the concrete will all have some influence. All concrete is restrained to some extent, often by friction with the subgrade
The tensile strain capacity of concrete at early days is typically no more than 100-250 micro strain. Therefore, with expected final shrinkage in excess of 500 microstrains, no matter how low the concrete slump, or how low its water-cement ratio, the concrete cannot withstand the stresses due to drying shrinkage.
under a floor slab, or by other adjacent parts of the General remedies to control drying shrinkage:
structure.
use of control joints and isolation joints Typical drying shrinkage movement is 0.45 to 0.80mm per metre (450-800 microstrain). This
use of concrete at 100mm slump or less and
shrinkage movement represents total shrinkage of
with low shrinkage attributes
45-80mm in a 100 metre long slab or 2.5 t o 4mm
use of a specialist solution such as shrinkage
per 5 metre section (isolated by saw cuts).
compensating admixtures, post tensioning or
The most important aspect in concrete mix design
vacuum dewatering
to control drying shrinkage is the total amount of mix water. Water is required for hydration purposes and also to provide for workability. This "water of convenience", needs to be kept to a minimum. A general rule is that for each 1% increase in water content drying shrinkage increases by 2%.
Curing
Curing is important for controlling all forms of cracking. Curing prolongs the cement hydration process as
Drying shrinkage increases for concretes at higher
concrete hardens thereby assisting in strength
water-cement ratios. Concrete with a low volume
development. Curing also retains moisture in the
of mixing water and a low water-cement ratio will
concrete which slows but doesn’t reduce drying
exhibit lower shrinkage.
1 hour
1 day 1 See week 1 month 1 year shrinkage. brochure SC4 Curing.
50 years
Preparation of Subgrade and Formwork For slabs on grade all top soil and soft spots should
Formwork must be constructed in a manner that will
be removed in site preparation. The remaining soil
enable it to withstand the pressure of the fresh
destined to be below the slab should be well
concrete without any movement. Vapour barriers can
compacted by rolling or tamping and the subgrade
increase bleeding and increase cracking of high slump
should be sloped in the natural drainage direction.
concrete.
Smooth, well compacted bases will help to prevent cracking due to movement in the finished slab.
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ECS Crack Control 6
Construction Joints Designing for concrete movement t hrough jointing,
A 100mm thick concrete floor should have control
saw cuts and/or a sand slip layer under a floor slab
joints 30mm deep and spaced 3.5 metres apart.
is important. Control joints can be formed by sawing, forming, crack inducers or tooling a groove in the
Isolation joints enable adjacent elements such as floors meeting columns, footings or walls to move
concrete to a depth of 30% its total thickness. The
independently. These joints are through the full depth
joints should be no further apart than 35 times the thickness of unreinforced concrete, and 45 times the thickness for reinforced concrete.
of the concrete and are constructed by using a barrier to prevent bond or interlock occurring between elements.
Technical Data Plastic shrinkage cracks occur when the rate of evaporation exceeds the rate at which water rises to the surface of recently placed concrete. A useful formula for calculating evaporation is provided by Uno (ACI Materials Journal, July-August 1998). E=5([tc+18]2.5 - r x [Ta=18]2.5)(v+4)x10-6 Where:
E= evaporation rate Tc=kg/m2 /hr Tc=concrete (water surface) temperature, C Ta=air temperature, C
r= (relative humidity %)/100 v= wind velocity, kph
An evaporation rate in excess of 0.50 kg/m2 /hr is considered to expose concrete to an increased risk of plastic shrinkage cracking. Above 1.0 kg/m2 /hr cracking will almost always occur. 800
1998 BRANZ Concrete Drying Shrinkage Test Results 700
280kg GP Auckland 425kg GP Auckland
Test Method AS 1012.13-1992
s n i a 600 r t s o r 500 c i m e 400 g a r 300 e v A 200
425kg GP Metamax Auckland 100
425kg GP Silica Fume Auckland
0
7
14
21
28
56
Time (days)
800
350kg GP Wellington 350kg GP Christchurch 350kg GP Auckland 350kg GP Metamax Auckland 350kg GP Silica Fume Auckland 350kg Duracem Auckland 350kg Duracem Metamax Auckland
1998 BRANZ Concrete Drying Shrinkage Test Results 700
s n i a r t s o r c i m e g a r e v A
Test Method AS 1012.13-1992 600
500
400
300
200
100
0 7
14
21
Time (days)
28
56
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Other topics in this series of brochures include: ECS 1 Marine & Coastal ECS 2 Chemical Resisting ECS 4 High Strength Concrete ECS 5 Industrial & Commercial Floors ECS 6 Abrasion Resisting
Also Site Concrete series: SC 1 Ordering Ready Mixed Concrete SC 2 Moving Concrete SC 3 Placing & Compacting Concrete SC 4 Curing of Concrete
Phone: 0800 ECS DATA 0800 327 328 The information presented in this brochure is offered in good faith, however, due to differences in specific conditions, environments and materials no responsibility
Fax:
0800 ECS FAX 0800 32 7 32 9
procedures discussed. For advice on your particular
Email:
[email protected] www.concrete.co.nz
project call these numbers.
Edition: August 1999
can be taken for the application of the principles and
