Name: Antonia Menga

Title of the project: Property development and volume changes in early age concrete for exposed environments. Experimental investigations and calculation methods.

The project is supported by a group of industrial companies including amongst others the Norwegian public road administration, Norcem, and Schwenk.

Project description

Background

Cracking of concrete structures should be avoided or limited to prevent aesthetics-, functionality- and durability-related problems. This project deals with through-cracks, which are generally of great concern since they often are permanent and involve the whole thickness of a concrete element. They may occur in the cooling phase of the hardening process, when the concrete tends to contract and the deformation is restrained by an adjacent structural system. In this case, the prevented deformation turns into tensile stress that may reach the tensile strength of the concrete, triggering the cracking.

It can be challenging to assess the development of tensile stress in a newly cast concrete element. In fact, the problem is aggravated by the influence of heat evolution, autogenous deformation, restraint conditions, development of mechanical properties and viscoelastic behaviour of the concrete. The major factors in early age cracking are illustrated in Figure 1. Thermal dilation (TD) and autogenous deformation (AD) are the ‘driving forces’, while the other parameters represent ‘the structural response’. The net result is concrete stresses, which are related to the concrete tensile strength in a ratio. This ratio is called ‘crack index’ [Bjøntegaard, 2011]:

where σ(t) and f_ct(t) are, respectively, the concrete tensile stress and tensile strength at time t. The factors in Figure 1 are subject to strong variations during the hardening phase, thus the task of estimating the crack index is a very complex one, requiring a comprehensive knowledge of the material behaviour [Bjøntegaard, 1999].

Figure: Stress development during the hardening phase [Bjøntegaard 2011]

​​​​​​​Objectives

The main aim of the project is to study the development of the most important factors involved in early age cracking of concrete structures in exposed environments. This is to help including the effects of TD, AD and creep in the analysis and design in the Serviceability Limit State (SLS).

To achieve the main aim, four main topics are going to be investigated during the research. For each topic, the following objectives are defined:

I. Volume changes.

• a. Provide test results about volume changes under realistic temperature histories.
• b. Develop a model able to describe the effect of varying temperature on AD-development.

II. Mechanical properties.

• a. Investigate the very early age (t<24 h) development of E-modulus, compressive strength and tensile strength of concrete.

III. Viscoelastic behaviour.

• a. Examine the consequences of applying a compressive load at early age loading ages and a subsequent tensile load at later ages in terms of viscoelasticity.
• b. Assess the applicability of the transient thermal creep theory [Illston and Sanders, 1973] at early ages.
• c. Update the Double Power Law introduced by [Bazant and Panula, 1978] with the results from points III.a and III.b. IV. Crack risk of self-induced strain. a. Study the crack risk and the crack width in the light of the results obtained from points I, II, III.

Expected results

According to the proposed objectives, the following results are to be expected. They are presented according to the topic of reference:

I. Volume changes.

• Database of volume change measurements.
• AD-development model under realistic temperature histories.
• Better understanding of the relative importance of TD and AD.
• Better understanding of the relationship between shrinkage and creep.

II. Mechanical properties.

• Updated mechanical models including very early age behaviour.
• Determination of software parameters for calculation software such as Crack TeSt COIN, including very early age behaviour of mechanical properties.

III. Viscoelastic behaviour.

• Transitional thermal creep theory valid for compressive and tensile creep.
• Updated DPL model. • Creep parameters for software use.

IV. Crack risk of self-induced strain

• Better understanding and descriptions of cracking of concrete at early age.

References

[Bazant and Panula, 1978] Bazant, Z. P., Panula, L. Simplified prediction of concrete creep and shrinkage from strength and mix. Structural Engineering Report, No. 78-10/6403, Department of Civil Engineering, Technological Institute, Northwestern University, Illinois, p. 24, 1978.

[Bjøntegaard, 1999] Bjøntegaard, Ø. Thermal dilation and autogenous deformation as driving forces to self-induced stresses in high performance concrete. PhD thesis, Department of Structural Engineering, NTNU, Trondheim, Norway, 1999.

[Bjøntegaard, 2011] Bjøntegaard, Ø. Basis for and practical approaches to stress calculations and crack risk estimation in hardening concrete structures–State of the art FA 3 Technical performance. SP 3.1 Crack free concrete structures, 2011.

[Illston and Sanders, 1973] Illston, J. M., Sanders, P. D. The effect of temperature change upon the creep of mortar under torsional loading. Magazine of Concrete Research, 25(84), 136–144, 1973.

Date of start of PhD: January 2021