Manifestation and remediation of alkali aggregate reaction

GUIDED BY, PROF: S.SIVAMOORTHY REDDY

ALKALI AGGREGATE REACTION:

Alkali aggregate reaction is a term mainly referring to a reaction which occurs over time in concrete between the highly alkaline cement paste and non crystalline silicon dioxide, which is found in many common aggregates.

This reaction can cause expansion of the altered aggregate, leading to spalling and loss of strength of the concrete.

Chemical reaction in either concrete or mortar between hydroxyl ions (OH-) of the alkalies (sodium and potassium) from hydraulic cement (or other sources), and certain constituents of some aggregates.

TYPES OF ALKALI AGGREGATE REACTIONS:

There are two main types namely,

1. ALKALI SILICA REACTION (ASR)

2. ALKALI CARBONATE REACTION (ACR)

HISTORY OF AAR :

ASR was first identified in the state of California in 1930’s.

Reported by Thomas Stanton of California state division in 1940’s .

Stanton’s studies demonstrated the expansion of mortar bars is due to alkali content of the cement.

Expansion is negligible if alkali content of cement is less than (0.60%).

In 1940’s number of agencies initiated studies on ASR in usa and other countries.

Thomas Stanton with a bridge parapet wall showing signs of ASR.

HISTORY OF AAR:

▪ Alkali carbonate reaction (ACR) was first discovered by Swensonin 1957 at Canada ,the same time when the ASR was first documented.

▪ ASR was subsequently implicated in case of degradation of concrete structures in USA (Hadley 1961).

▪ Some alleged cases of ACR has occurred in England , Bahrain, Iraq , china (ozol 2006) .

▪ However, unlike ASR, problems with ACR are still restricted to a few isolated locations worldwide.

▪ There has been comparatively little research conducted on this topic.

ICAAR:

# YEAR HOST

1 1974 Køge, Denmark

2 1975 Reykjavik, Iceland

3 1976 London, U.K.

4 1978 West Lafayette, USA

5 1981 Cape Town, S. Africa

6 1983 Copenhagen, Denmark

7 1986 Ottawa, Canada

# YEAR HOST

8 1989 Kyoto, Japan

9 1992 London, U.K.

10 1996 Melbourne, Australia

11 2000 Quebec City, Canada

12 2004 Beijing, China

13 2008 Trondheim, Norway

14 2012 Austin, U.S.A.

ALKALI SILICA REACTION:

Alkalis +Reactive

SilicaAlkali

silica gel

+

moisture

Concrete expansion and

cracking

CONCRETE FAILURE DUE TO ASR:

LOSS DUE TO ASR:

CASE STUDIES:

Hydroelectric dam built in 1938

180 mm of arch deflection due to alkali silica gel expansion

Cracking and gel flow in concrete

ASR damage on concrete retaining wall

CREATION OF ALKALI SILICA GEL:

REACTANTS:

1.ALKALIS

2.REACTIVE SILICA

3.WATER

ALKALIS:

MAIN CATIONS:

• SODIUM

• POTASSIUM

COMMON SOURCES:

• Portland cement

• Deicing agents

• Seawater

REACTIVE SILICA:

Silica tetrahedronAmorphous silica

Crystalline silica

REACTIVE SILICA:

Common reactive minerals:

• strained quartz

• opal

• obsidian

• cristobalite

• tridymite

• chelcedony

• cherts

• cryptocrystalline volcanic rocks

AMORPHOUS / REACTIVE SILICA

MOST CHEMICALLY REACTIVE


1.Siliceous aggregate in solution:


2.Surface of aggregate is attacked by OH- :

H20 + Si-O-Si

Si-OH…OH-Si


3.Silanol groups (Si-OH) on surface are broken down by OH- into SiO- molecules

Si-OH + OH-

SiO- + H20


4. Released SiO- molecules attract alkali cation in pore solution, forming an alkali-silica gel around the aggregate

Si-OH + Na+ + OH-

Si-O-Na + H20


5. Alkali-silica gel takes in water, expanding and exerting an osmotic pressure against the surrounding paste or aggregate.


6. When the expansionary pressure exceeds the tensile strength of the concrete,the concrete cracks.


7. When cracks reach the surface of a structure, “map cracking” results. Other symptoms of ASR damage includes the presence of gel and staining.

MAP CRACKS


8. Once ASR damage has begun:

4.Increased ASR damage

3.More water and external alkalis penetrate concrete

2.Increased permeability

1.Expansion and cracking of concrete

IMAGES OF ASR DAMAGE:

1.Avoid high alkali content:

• use low alkali portland cement: Na20eq < 0.69

• replace cement with low alkali mineral admixtures

2. Avoid reactive aggregate

(amorphous silica)

3. Control access to water: use low water to cement

ratio, monitor curing conditions, use admixtures to

minimize water contact.

4. Use lithium additives prior to

placement of concrete or as a

treatment in already existing

concrete

REMEDIATION TO ASR:

Alkalis + Reactive Silica + Moisture ASR Gel

We know that,

Cont….

USE OF LITHIUM TO TREAT ASR AFFECTED STRUCTURES:

Topical treatment

with lithium:

• Numerous structures have been treated by spraying the surface of structure with a solution of lithium (both LiNo3 and LiOH have been used)

• These structures have included pavements, bridge decks and other bridge components, and median barriers

• The solution has been applied by either truck-mounted spraying systems or hand-held pressurized spray bottles.

Spraying 30 percent LiNO3 solution with a tanker truck on a concrete pavement

Cont………

Typical application rates have been in the range of 0.12 to0.24 liters per square meter (L/m²) (3 to 6 gallons per square feet (gal /1000 ft²)).

The most commonly used lithium compound for this purpose is a 30 percent LiNO3 solution.

Commercially available solutions contain a proprietary surfactant to enhance penetration.

Spraying 30-percent LiNO3 solution with handheld spray applicator on barrier wall

Cont……

ELECTRO CHEMICAL LITHIUM IMPREGNATION:

• Technique is based on electrochemical chloride extraction technique.

• Electrode (anode) applied to concrete surface.

• Lithium-bearing electrolyte ponded at surface.

• D.C. voltage (~40 volts) applied between surface anode and embedded steel (cathode).

• Positively charged lithium ions are repelled by the positively charged anode and are drawn towards the negatively charged cathode (steel reinforcement).

• Duration of treatment is typically 4 to 8 weeks. Electrochemical lithium impregnation.

Cont……

irrigation tubes, wood splices, and metal strips are placed on the column. The metal strips are

attached to titanium mesh that runs inside holes drilled into the sides of the column.

A cellulose layer is applied to the side of the column

plastic sheeting is placed on all sides of the column. The gutters under the sheeting

collect excess lithium for reuse

Electrochemical lithium treatment process.

• Clean surface (e.g., road sweeper) prior to treatment.

• Do not treat if rain is forecast within 6 hours.

• Keep single application rate ≤ 0.12 L/m2 (3 gal /1,000 ft2).

• Minimum two applications.

• Applications at least 30 minutes apart.

• Ensure uniform surface coverage and no runoff.

• If precipitate forms over > 5 percent of surface, re-wet the surface to dissolve the precipitate. If surface becomes slippery, applications of water should continue until the surface is safe for vehicular traffic.

GUIDELINES FOR TOPICAL LITHIUM TREATMENT:

Cont……

Monitoring Guidelines :

• Take core samples to determine depth of lithium penetration.

• If possible, maintain untreated control section to compare performance with treated section.

• Monitor length change of concrete. There are a wide variety of techniques available; one of the simplest being to embed stainless steel reference pins and monitor the change in length between the pins using a demountable mechanical (DEMEC) strain gauge.

• Crack mapping techniques can be used to follow damage accumulation.

• Consider use of non-destructive techniques such as spectral analysis of surface waves (SASW).

• There are a number of technologies available for performing condition surveys of roads and bridges.

• Some of these have been employed to follow the progress of damage due to ASR.

IMAGES OF MONITORING TECHNIQUE:

Monitoring crack mapping of barrier

wall

measuring length changes on concrete pavement with a DEMEC gauge.

THANK U

Engineering

Manifestation and remediation of alkali aggregate reaction