Concrete Cracking - Theory

Concrete Cracking Theory

Flexure, Early Thermal Contraction, Long Term Drying Shrinkage and Seasonal Thermal Contraction

Introduction

Structural Engineering Is THE ART OF USING MATERIALS That Have Properties Which Can Only Be Estimated TO BUILD REAL STRUCTURES That Can Only Be Approximately Analyzed TO WITHSTAND FORCES That Are Not Accurately Known SO THAT OUR RESPONSIBILITY WITH RESPECT TO PUBLIC SAFETY IS SATISFIED. (Adapted From An Unknown Author) Introduction

Introduction There are numerous causes of cracking in concrete, but most instances are related more to concrete specification and construction practices than by stresses due to induced forces. The four primary causes of cracking that the designer can help to prevent are: • • • •

Flexural Cracking Early Thermal Contraction Cracking Long Term Drying Shrinkage Cracking Seasonal Thermal Contraction Cracking

This presentation is aimed primarily at thin suspended slabs, but the basic principles can be applied to other forms of construction. Introduction

Flexural Cracking Flexural cracking is a well known and well documented phenomenon and as such will only be briefly discussed. As a structure deflects to support the applied design loads, tension forces are induced in one face of the concrete which leads to the formation of cracks. In Limit State design, the tension capacity of concrete is ignored and the steel reinforcement is designed to support the entire tensile strain. To ensure that durability of the concrete is not compromised, maximum crack widths are specified depending on the exposure conditions, purpose of the structure and the visual acceptability of cracking. Typically for non aesthetic, non water-retaining structures, crack widths are kept below 0.3mm. (RCC spreadsheet RCC14 calculates the crack width based on equations similar to those on the following page) Flexural Cracking

Excerpts from BS 8007

Flexural Cracking

Cracking due to flexure is additive to cracking due to the effects discussed in the remainder of this presentation: • Early Thermal Contraction Cracking • Long Term Drying Shrinkage Cracking • Seasonal Thermal Contraction Cracking

Flexural Cracking

Restraint •

For lightly reinforced sections, as is typical for suspended slabs, contraction of the concrete results in induced stress only if the concrete is subjected to some form of restraint.

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Typically, restraint is primarily provided parallel to walls, previously poured slabs and between cores or other rigid restraints resulting in potential cracks perpendicular to the line of restraint.

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Restraint parallel to walls and previously poured slabs is only significant for Early Thermal Strains. For Shrinkage and Seasonal temperature variations both the slab and the restraint are deemed to contract together. This is not always true but is a good approximation in most cases.

Restraint

Restraint

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Restraint parallel to walls and previously poured slabs extends in a o triangular pattern at 45 from the ends of the wall or slab. This is due to the relative in plane stiffness of the walls and the previously poured slab edge preventing the new slab from contracting. The effect of restraint parallel to previously poured slabs can be reduced by ensuring that the time between casting of the adjacent sections is kept to a minimum. Minimizing the total number of pours also reduces the amount of restraint by reducing the number of restrained edges.

Restraint

Restraint

Restraint

Contraction Cracking Contraction cracking, as the name suggests, is due to the contraction of concrete, primarily due to: • • •

Early Age Thermal Strains Long Term Drying Shrinkage Seasonal Thermal Strains

Intro to Contraction Cracking

Concrete Life Cycle Concrete typically goes through the following life cycle: • •

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Within the first couple of days after casting new concrete, the hydration process causes the concrete to heat up. From an initial casting temperature, it rises to a peak temperature at around three days old, coupled with a small expansion in the volume of the concrete. As the concrete cools down to equalize with the ambient temperature, it begins to contract as it simultaneously sets. This contraction, if restrained, leads to tensile stress forming in the concrete. If the induced tensile stress exceeds the tensile strength of the concrete, the concrete cracks. Intro to Contraction Cracking

Intro to Contraction Cracking

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In the following three to four weeks, the tensile and compressive strength of the concrete increases rapidly. In its first year of service, the concrete will be subjected to its first cycle of temperature and humidity, the degree of which depends on the exposure conditions of the concrete. Over a longer, sustained period of 30 years, water within the initial concrete mix will be lost to the environment by evaporation and drying by surface winds. Again, the degree of loss is dependent upon the exposure conditions to which the concrete is subjected. Similarly to early thermal contraction, the change in temperature in the slab and the loss of water to the atmosphere will lead to contraction of the concrete and tensile stresses leading to potential cracking. Intro to Contraction Cracking

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Creep acts to reduce the effects of the various forms of contraction over a number of years.

Intro to Contraction Cracking

Early Age Thermal Contraction The principal of Early Age Thermal Contraction has already been discussed. As the fresh concrete contracts upon cooling and begins to harden, restraints to that contraction induce tensile stresses in the concrete. The amount by which the concrete contracts and the capacity the concrete has to sustain that contraction before cracking occurs is down to many variables, all of which are difficult to define with accuracy without testing.

Early Age Thermal Contraction

Early Age Thermal Strain: ξeth = T1αkR T1 α k R

is the temperature drop from the peak concrete temperature to the ambient temperature Is the thermal coefficient of expansion for the aggregate used in the concrete Is a modification factor allowing for inaccuracies in the assumptions Is a factor representing the degree of restraint (where R = 1 represents a rigid restraint and R = 0 represents no restraint)

Temperature Drop: T1 is governed by the balance between the heat of hydration and the rate of heat loss to the atmosphere of the concrete. Factors affecting the temperature drop include: the placing temperature of the concrete; the ambient temperature; the thickness of the section; the type of formwork used; the cement content and the presence of any concrete replacement materials.

Early Age Thermal Contraction

Early Age Thermal Contraction

Early Age Thermal Contraction

Long Term Drying Shrinkage As the concrete matures, the excess water used in the initial mix is lost through a variety of means, such as evaporation and the drying effects of wind. The concrete mix design has a large part in defining the degree to which the concrete is affected by long term drying shrinkage. The principle factors affecting drying shrinkage are: • Ambient humidity – the higher the ambient humidity, the less the amount of water that is lost by the concrete • Surface Area – the higher the relative surface area, the higher the amount of water that is lost from the concrete • Water/Cement Ratio – the higher the water/cement ratio, the more water there is in the mix to be lost

Long Term Drying Shrinkage

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The following method for estimating drying shrinkage strain is from BS 5400-4

Long Term Drying Shrinkage Strain:

ξcs = kLkckekiR kL kc ke ki

R

is the basic value of shrinkage related to ambient humidity is a coefficient to account for the composition of the concrete mix (cement content and water content) is a coefficient to account for the effective thickness of the section is a coefficient to account for duration of the applied strain is a factor representing the degree of restraint (where R = 1 represents a rigid restraint and R = 0 represents no restraint)

Long Term Drying Shrinkage

Relative Ambient Humidity for Dubai is fairly constant year round at 60% (expressed as 0.6 RH)

Long Term Drying Shrinkage

Long Term Drying Shrinkage

Seasonal Thermal Contraction/Expansion

As previously discussed, concrete volume changes with temperature. As its temperature rises, the concrete expands and conversely, the concrete contracts as its temperature drops. It is important to note that the temperature of the concrete is relatively insensitive to rapid changes in the surrounding air temperature. This is particularly true where the concrete is not subject to direct solar gain, is subject to internal or partially internal environments or the concrete is insulated. The temperature to be used in most instances relates to mean daily temperatures.

Seasonal Thermal Strain

Seasonal Thermal Strain:

ξth = T2αR T2 α R

is the temperature difference from the ambient temperature at the time of casting to the expected maximum and minimum mean daily temperatures over the year (N.B. this can be positive or negative) is the thermal coefficient of expansion for the aggregate used in the concrete is a factor representing the degree of restraint (where R = 1 represents a rigid restraint and R = 0 represents no restraint)

Seasonal Thermal Strain

Seasonal Thermal Strain

Conclusion •

The calculations required to determine the stresses induced in concrete due to contraction are laborious and the parameters for use in the design equations are difficult, if not impossible, to accurately determine.

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At the end of the day, those stresses that are induced result in cracking, for which reinforcement is required in order to control the size and spacing of the cracks.

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It is strongly advised that, wherever possible, movement joints at approximately 70m centers (maximum) are incorporated into concrete structures. With such movement joints, the majority of the thermal contraction strains described herein can effectively be ignored.