Crack Control - ECS PDF
Crack Control - ECS PDF
Crack Control - ECS PDF
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
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Crack Control
Figure 1 Types
Likely Element
Slabs Reinforced Slabs Reinforced Slabs Deep Sections Top of Columns Suspended Floor Walls or Slabs
Primary Cause
Secondary Cause
Time of appearance
Reference
Plastic Shrinkage
2 3 4
Plastic Settlement
Bleeding
10 minutes to 3 hours
5 6
Restraint of thermal movement Excess temperature gradients Restraint of thermal movement Excess paste at surface Over Trowelling Inefficient Joints Preparation of sub-base Accidental overload Poor curing, poor placement Anytime after hardening Weeks, months, years Anytime Vulnerable at 1-2 days Rapid cooling 1 day to 2-3 weeks
11 8
Thick masses
12
13
Crazing
9 10 8 7 14
9 4 13 2 10 3 6 8 8
11 5 5 1 14 8 8 7 12
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Enhancing the service life of concrete structures through the control of cracking
A fundamental requirement of any concrete structure is its performance over its intended design life. Concrete must be able to withstand wear and deterioration given the environment and maintenance regime for which it was designed. If a concrete structure meets its intended design life when exposed to its anticipated environment then it can be described as being durable. The most obvious, and common, form of concrete deterioration is cracking. Once concrete is cracked it becomes vulnerable to the penetration of damaging fluids and is more prone to spalling, wear and abrasive damage. Water or other fluids transport damaging agents into the concrete and cause expansion through corrosion of reinforcing steel, freezing of water, and other effects. High strength concretes are especially vulnerable to early cracking due to the use of fine materials. They have higher cohesion, generate greater heat of hydration and have less bleed water. Careful engineering design and care during construction is critical to overcome the problems caused by uncontrolled cracking and to ensure a structure meets its design life.
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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 cracking occurs most often in summer with conditions of heat, wind, and low humidity. Concretes that are most susceptible to this form of cracking are those with: High cement content Finer cements Lower water-cement ratios including superplasticized concrete 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 windiest and/or driest part of the day start curing as soon as possible after finishing dampen form-work, subgrade and reinforcement cover with polythene prior to finishing use of plastic fibres
Craze Cracking
Plastic Settlement Cracking In plastic concrete bleed water surfaces due to gravity. If the accompanying settlement is restricted by form work or reinforcement, cracking may occur. Typical plastic settlement is approximately 6-8mm per metre depth of the concrete element (corresponding to a typical bleeding rate of 6-8 litres per cubic metre).
Crazing is the development of a network of fine random cracks on the surface of concrete caused by shrinkage of the surface layer. The cracks are rarely more than 2mm deep and typically form hexagonal shaped areas no more than 40mm across. They are more likely to occur on steel trowelled surfaces. 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 reinforcement use air entrained concrete
To prevent crazing the following precautions should be followed: dont finish concrete while bleed water exists never sprinkle or trowel dry cement into plastic concrete to absorb bleed water
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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. Concrete is most vulnerable to early thermal cracking on the day it is poured when the heat of day and the heat of hydration abates and is replaced by a cold evening. Typical thermal movements are of the order of 0.1mm per metre length (100 micro strain) per 10C 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.
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 covering concrete to slow heat loss at night or exposure to wind delay removal of formwork
Figure 2
Types of cracking
Plastic Settlement Plastic Shrinkage Early Thermal Movement Drying Shrinkage Excess Loading Corrosion Hours Days
Weeks
Months
Years
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Drying Shrinkage Cracking 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. Drying shrinkage can be defined as the "loss in concrete volume resulting from the loss of water 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 under a floor slab, or by other adjacent parts of the structure. Typical drying shrinkage movement is 0.45 to 0.80mm per metre (450-800 microstrain). This shrinkage movement represents total shrinkage of 45-80mm in a 100 metre long slab or 2.5 to 4mm per 5 metre section (isolated by saw cuts). The most important aspect in concrete mix design 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%.
The cement type will affect the rate of shrinkage. Slow hydrating cements such as slag will exhibit slow shrinkage. The size of aggregate is important. The larger the aggregate proportion of the concrete mix, the lower 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 aggregates tend to shrink less than greywacke-based concrete. 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. General remedies to control drying shrinkage: use of control joints and isolation joints use of concrete at 100mm slump or less and with low shrinkage attributes use of a specialist solution such as shrinkage compensating admixtures, post tensioning or vacuum dewatering
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 doesnt reduce drying exhibit lower shrinkage. 1 hour 1 day 1 See week 1 month 1 year 50 years shrinkage. brochure SC4 Curing.
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Construction Joints
Designing for concrete movement through jointing, saw cuts and/or a sand slip layer under a floor slab is important. Control joints can be formed by sawing, forming, crack inducers or tooling a groove in the concrete to a depth of 30% its total thickness. The joints should be no further apart than 35 times the thickness of unreinforced concrete, and 45 times the thickness for reinforced concrete. A 100mm thick concrete floor should have control joints 30mm deep and spaced 3.5 metres apart. Isolation joints enable adjacent elements such as floors meeting columns, footings or walls to move independently. These joints are through the full depth 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
Average microstrains
280kg GP Auckland 425kg GP Auckland 425kg GP Metamax Auckland 425kg GP Silica Fume Auckland
100 0
14
21
28
56
Time (days)
800
Average microstrains
GP Wellington GP Christchurch GP Auckland GP Metamax Auckland GP Silica Fume Auckland Duracem Auckland Duracem Metamax Auckland
Time (days)
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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