This document discusses the development of a multiphysics model to predict alkali-silicate reaction (ASR) in concrete. ASR causes expansion in concrete over time due to a reaction between alkalis in cement and silica in aggregates, producing an expansive gel. The researcher aims to generate virtual concrete microstructures and develop a thermo-chemo-mechanical model using governing equations to predict the kinetic reactions of ASR and resulting expansion. The model will be validated by studying the time-dependent results and analyzing concentration of ASR gel, expansion, and intermediate products over time.

1. MicroMechanics & Smart Infrastructure Group
The concern over general alkali-aggregate
reaction (AAR) in Portland cement concrete
(PCC) was raised first by Stanton in 1940’s.
Many researchers have quantified
expansion associated with ASR gel
product. Most existing research however
just focused on one or few particular
aspects or stages of ASR without
considering ASR as a multi chemo-physics
process. My research focus is to develop a
multiphysics model for predicting the kinetic
reactions of ASR and the incurred
expansion of alkali-silicate gel in concrete.
A coupled thermo-chemo-mechanical
transient model can predict service life of
concrete as well as on time maintenance
decision.
Goal:
To generate concrete microstructure in the
virtual cement concrete testing lab.
To develop multiphysics model with
governing equations and then validate that
model.
Methodology:
Analyzing concrete microstructure and
develop 2D & 3D images from it.
Generating time dependent equation
based model using concrete microstructure
images as a geometry.
Defining parameters and implementing
governing equations.
Studying time-dependent model and
analyzing the results.
Validating the model.
Governing Equation for ASR Modeling
∂Ci/∂t+ (-Di Ci)+u Ci=Ri (1)∇ ∇ ∇
ρ(Cm/ρg+SeS) ∂p/∂t+ ρ(-Ks/ Kr p)=Qm (2)∇ ∇
u= ρ(-Ks/ Kr p) (3)∇ ∇
Qm = R[H2O] * 18 g/mol (4)
R[siloxane] = -k1.[siloxane].[H2O]
R[silanols] = 2.k1.[siloxane].H2O-k2.[silaxols].[ROH]
R[ROH] = -k2.[silanols].[ROH]
R[H2O] = -k1.[siloxane].[H2O] + k2.[silanols].[ROH] –k3.[silanols].[ H2O]
R[AS gel] = k2.[siloxane].[ROH] - k3.[silanols].[ H2O]
R[expanded products] = k3.[silanols].[ H2O]
Concentration of ASR gel increases with time. Higher concentration of alkali-silicate gel is in the space between aggregate particles.
Expansion increases as a function of volume with time and large enough to develop crack on the concrete block.
I. Introduction II. Solution III. Result & Discussion
Coupled Thermo-Chemo-Mechanical Prediction of ASR Expansions
Md Asif Rahman, Md Aminul Islam, Dr. Yang Lu*
Department of Civil Engineering
ASR Process
Step 1: Silica in aggregates reacts with
alkali in cement to produce a gel.
Step 2: The gel absorbs water, causing
expansion and hydraulic pressures
sufficient to fracture and break apart the
concrete.
For chemical reaction
alkali-silicate gel is
generated first around
the rim of reactive
aggregates and all voids
are filled up with alkali-
silicate gel that will
expand by absorbing
water.
Observation
Expansion
(percent)
Aggregate
Reactivity
0.0 – 0.04 Non-reactive
> 0.04 Potentially reactive
Figure: ASR gel at two weeks stage Figure: ASR gel after two years
ASR gel absorbs water and causes expansion. The expansion curves exhibits a significant increase. But, the
expansion curves tend to level off at the depletion of alkali ions. Silanol is an intermediate product in ASR and
therefore it increased first and decrease later due to the formation of alkali-silicate gel.
mol/m3