7212019 Concrete Durability Understanding Water Transport Mechanisms in Concrete

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Concrete DurabilityUnderstanding Water Transport Mechanisms in Concrete

Moisture migration into concrete is the leading cause of concrete degradation

worldwide There are two primary water transport mechanisms in concrete

Considering waterrsquos powerful forces and then designing concrete structures to

adequately resist the known effects of these two common water transport

mechanisms is paramount to achieving durable structures Designers contractors

and owners need to thoroughly understand the differences in the mechanisms to

ensure the structures they are building provide adequate problem-free service life

The two mechanisms listed by the magnitude of the challenge they impose are

1 Capillary absorption or sorptivity

2 Permeability

Ignoring or more commonly misunderstanding which of the mechanisms

provides the greatest threat to concrete leads to structures that fail to perform

long-term and eventually will lead to structural failure

Most degradation processes encountered by concrete require water dissolved

chemicals and the presence of oxygen Dissolved salts (chlorides) or other deleterious

chemicals can be rapidly transported to the steel reinforcement imbedded in the

concrete through the capillary network The resulting initiation of corrosion causes

rebar to expand breaking up the concrete it is embedded in Additionally in cases

where water has permeated through a concrete substrate it may damage building

interiors In each case the presence of water is detrimental

Preventing water from freely moving from the outside environment in which the

concrete is placed into service to the interior of the concrete matrix is therefore a

significant design concern worldwide

Rapid Deleterious Chemical Absorption Through CapillariesAs mix water required for concrete placement leaves concrete it leaves behind a

porous capillary structure Capillary absorption is the movement of water through

the small pores in concrete in the absence of an externally applied hydraulic head

and is the result of surface interactions between the water and the pore wall

Capillary absorption is the primary transport mechanism for water in concrete

structures

Capillary absorption is so powerful and rapid that it requires no pressure tofunction and creates far more damage potential than any of the other transport

mechanisms According to a published report written by Dr Andrew Butler of the

Transport Research Laboratory on long-term durability research titled ldquoCapillary

Absorption by Concreterdquo (UK 1998) on 50 mPa7250 psi concrete he states ldquoThe

speed of capillary absorption is on the order of 10-6 ms ndash a million times faster than

pressure permeabilityrdquo He also states ldquocapillary absorption is the primary transport

mechanism by which water and chlorides infiltrate concreterdquo and that ldquoclearly

permeability is not a good indicator of resistance to chloride penetrationrdquo

In often repeated studies and experiments even extremely dense concrete mixes

with high compressive strengths low water cement ratios and excellent pressure

permeability readings rapidly transport water through capillary absorption To ensure

350

300

250

200

150

100

050

000

0 400 800 1200 1600 2000

Time s12

Control

Hycrete

777 s12=7 days

Determination of Capillary AbsorptionASTM C1585

Absorption ()BSI 1881-122

30

20

10

0CrystalGrowth

Admixture

HycreteAdmixture

Control

Rilem Tube Absorption Test ndashAfter 24 Hours of Exposure

Control Cr ystalGrowth

Admixture

HycreteAdmixture

7212019 Concrete Durability Understanding Water Transport Mechanisms in Concrete

httpslidepdfcomreaderfullconcrete-durability-understanding-water-transport-mechanisms-in-concrete 22

Concrete DurabiUnderstanding Water Transport Mechanisms in Concr

durable concrete it is absolutely essential to adequately address capillary

absorption as the designerrsquos primary duty

Hycrete admixtures dramatically reduce capillary absorption in concrete

PermeabilityPermeability is the movement of water due to a pressure gradient such as when

concrete is under hydrostatic pressure Performance under hydrostatic pressure is a

simple function of concrete density or cementitious content Concretersquos naturally

dense matrix (of even moderate quality mixes) provides an extremely difficult

environment to push water through even under high pressure

The water pressure gradient encountered by a concrete structure is rapidly

diminished by the resistance created by its relatively dense matrix Concrete

neutralizes the pressure gradient within the concrete very quickly and then

capillary action once again becomes the primary transport mechanism and moves

the water further into the structure

Fortunately this transport mechanism is the least threatening and is inexpensive to

mitigate Most designers overcome this challenge economically by simply increasing

cementitious content (Portland cement fly ash slag silica fume) or reducing thewatercement ratio to make it more difficult for water to be forced through

concrete Relatively small amounts of cementitious material may be added (for a

small additional cost) to dramatically enhance concrete permeability performance

A Note on DiffusionDiffusion measures the slow ingress of chloride ions in saturated concrete

Although diffusion is not a primary water transport mechanism it an excellent

indicator of how deleterious materials can move through concrete structures Chloride

can penetrate concrete by diffusion which is the movement of chlorides in solution

from an area of high concentration of chlorides to an area of lower concentration

Chlorides facilitate corrosion of steel reinforcement which ultimately degrades concrete

Hycrete admixtures reduce both the surface concentration of chlorides in concrete

as well as the diffusion coefficient Also Hycrete admixtures protect steel in both

cracked as well as uncracked concrete specimens

Summary The primary water transport mechanisms in concrete are capillary absorption and

permeability Capillary absorption is by far the most important mechanism to

consider when designing durable structures as it is rapid occurs in the absence of

hydrostatic pressure and transports water long after water forced into concrete by

pressure has reached an internal equilibrium

Also as concrete permeability is simply a function of concrete density or cement

content a simple and inexpensive mix design adjustment is made to meet any

permeability requirements Furthermore because water forced into concrete

reaches equilibrium in all concretes albeit at different depths of penetration it is

not the mechanism that carries water and damaging salts

all the way through

Finally understanding the movement of chlorides through water in concrete by

diffusion is critical to ensuring your structure is protected from degradationCracked Uncracked

Protection of Steel in Concrete

ASTM G109 ndash Chloride Ponding Test

004 0015

Control

Calcium Nitrite

Hycrete

R e b a r I r o n L o s s ( ) O v e r 2 0 4 W e e k s

06

04

02

0

Control Hycrete

14

12

10

08

06

04

02

00

180E-05

160E-05

140E-05

120E-05

100E-05

800E-06

600E-06

400E-06

200E-06

000E+00 C s ( o

f w e i g h t o f c o n c r e t e )

D ( m m 2 s e c )

Cs D

Surface Concentration of Chloridesand Diffusion Coefficient

ASTM C1556

HYCRETE INC | 462 BARELL AVENUE | CARLSTADT NJ 07072 USA | PHONE 983080+1983081 2013868110 | FAX 983080+1983081 2013868155 | WWWHYCRETECOM

Copyright copy 2011 Hycrete Inc All rights reserved Hycrete Inc and the Hycrete logo are trademarks of Hycrete Inc

M o i s t u r e

PressureEquilibrium

CapillaryAbsorption

Surface interactionCapillary action

Water(Hydrostatic

Pressure)