Overview of Deterioration Mechanisms in Sawn Concrete

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Tracy Lynch
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1 1 Overview of Deterioration Mechanisms in Sawn Concrete

2 About Me Name: Xin Wang (Jerry) Bachelor, Master of Science (Iowa State University) Ph.D. Iowa State University Professors: Peter Taylor, Kejin Wang

3 Problem With freeze-thaw cycles, concrete pavement can deteriorate Cause? Physical, chemical, or both

4 Aim Understanding the various deterioration mechanisms is critical to implementing appropriate solutions Following is a review

5 Winter is coming What really happens to concrete when winter comes?

6 Types of Deterioration in Slabs on Grade Physical Freeze Thaw (joints) Scaling (surface) D-cracking (joints whole) Pop-outs (surface) Chemical Oxychloride (joints) Alkali-silica reaction (whole)

7 Fundamentals Concrete is Porous Weak in tension Calcium based Sawn into smaller panels Water expands when it freezes Deicing salts generally contain chlorides

8 Freeze-Thaw Symptoms Cracks parallel to free surface Often forming thin slivers

9 Freeze-Thaw Mechanism Water penetrates and fills voids in paste (saturation) Expands when it freezes Sets up tensile stresses Cracks paste

10 Freeze-Thaw Prevention Keep the water away Prevent saturation Provide small, closely spaced air bubbles Reduce saturation Provide space for water to expand into

11 Freeze-Thaw Prevention Beware of some salts they attract water

12 Freeze-Thaw Prevention Ettringite in air voids will accelerate saturation

13 Scaling Symptoms Loss of horizontal surface Sometimes as sheets

14 Scaling Mechanism(s) Poor finishing sets up a void under the surface. Water and loading pops it off. Deicing salts crystalize in voids near surface High SCM dosage reportedly exacerbates distress

15 Scaling Prevention Only finish after bleeding has stopped Conventional wisdom is to provide air This is being reviewed Reduce permeability w/cm SCMs Curing Delay salt applications

16 D - Cracking Symptoms Cracks parallel to joints Often starts at the bottom of the joint Damage is in aggregate not paste

17 D - Cracking Mechanism Water penetrates and fills voids in aggregate Voids are sized to slow or prevent water leaving the system Water expands when it freezes Sets up tensile stresses Cracks in aggregate that grow into paste

18 D - Cracking Prevention Keep the water away Don t use at-risk aggregate Limit maximum aggregate size will delay distress

19 Popouts Symptoms Shallow holes with porous aggregate particles at the bottom

20 Popouts Mechanism Porous aggregates near surface absorb water Freeze and drive off a cone

21 Popouts Prevention Limit porous aggregates Sealants?

22 Calcium Oxychloride Symptoms Paste expansion Aggregates are separated from paste

23 Calcium Oxychloride Mechanism Calcium from cement Chlorides from salts Expands At 40 F Sutter CaCl 40 F

24 Calcium Oxychloride Prevention Minimize use of MgCl2 and CaCl2 Increase silica content of binder system with SCMs Reduce permeability Sealants

25 Alkali Silica Reaction Symptoms Extensive cracking often parallel to free surface

26 Alkali Silica Reaction Mechanism Reactive silica from some aggregates Alkali hydroxides from cement Water Forms gel that expands with water Damage starts in aggregates

27 Alkali Silica Reaction Prevention Minimize use of reactive aggregates Use low calcium SCMs Lithium compounds

Review What's critical Name What is the cause D-Cracking Water freezing in coarse calcareous aggregate containing clays and or with a pore system that holds water What does it look like Cracks near

of a joint Up to ~18" from joint Avoid deleterious aggregates.

Joints Where water is trapped A few mm Adequate air void system, low permeability Partial depth repair Oxychloride MgCl2 reacts with some paste systems at about 40F to form expansive oxychloride

28 Review What's critical Name What is the cause D-Cracking Water freezing in coarse calcareous aggregate containing clays and or with a pore system that holds water What does it look like Cracks near joints, often an a curve at intersection of joints Where does it occur Full depth, cracks are parallel to free surface, mm part Where does it start How far does it go Prevention Repair Photo Bottom of a joint Up to ~18" from joint Avoid deleterious aggregates. Reducing maximum size delays damage As per ASR Joint Related Freeze thaw Water freezing in Thin flakes parallel saturated cement to free surfaces at paste with joints inadeqaute air void system Joints Where water is trapped A few mm Adequate air void system, low permeability Partial depth repair Oxychloride MgCl2 reacts with some paste systems at about 40F to form expansive oxychloride compounds Cracks parallel to joint faces, sometimes up to 1" from previous cut or crack Joints Tops of saw cuts or in the kerf Up to ~9" from joint High SiO2 cementitious systems, low permeability, adequate air, limit use of MgCl2 Partial depth repair, Full depth repair Salt Scaling Salts or ice crystallizing below the surface, often related to poor surface finishing Flakes peeling off the surface of the slab Surface only Surface Can cover the whole slab Good finishing procedures, curing, adequate air void system, low alkali cement Grind Only Surface Popout Water freezing in low density aggregate Divot above aggregate particle At the surface Joints Whole slab surface Avoid deleterious aggregates None Chemical Reaction Alkali silica reaction Reaction Cracks mostly between some parallel to silicates in longitudinal joint aggregate, alkali hydroxides in pore solution and water Full depth Near edges Whole slab surface Avoid deleterious aggregates, low calcium SCM, Lithium comounds Remove and replace, rubblize and overlay, unbonded overlay

29 Conclusions Aggregate size and quality is a critical condition to have a long lasting concrete pavement. Finishing, curing condition is critical for air voids system, so it is critical to F-T resistance Deicers do have an impact on concrete, choose carefully Winter is coming, so know what we are dealing with is important.

30 Q&A

Delivering Concrete to Survive the Environment. Dr. Peter Taylor

Delivering Concrete to Survive the Environment Dr. Peter Taylor Outline What can go wrong? How do we prevent it? 2 What Can Go Wrong? Internal Expansion External attack Durability Ability of the concrete