Concrete Petrography: A Handbook of Investigative Techniques VOL II

369 Pages·146.848 MB·English

by  Poole A.B.,Sims I.

Chapter 6
Examination of deteriorated
and damaged concrete
6.1 I NTRODUCTION
6.1.1 B ackground to concrete durability
The durability of concrete is a broad, extensively researched subject and there is much
literature available. Codes of practice to assist engineers in designing durable concrete have
been published by the British Standards Institution (previously BS 8110 1985 BS 5328 1990
and now BS EN 206-1 2000 and BS 8500-1, 2 2006), the American Concrete Institute
(annual publication), the Comité Euro-International du Beton (1992) and the Commission
Internationale des Grands Barrages (1989). Good general reviews of concrete durability
include Kropp and Hilsdorf (1995), Glanville and Neville (1997), Hobbs (1998), Walker
(2000), Neville (2001), Rendell et al. (2002), Newman and Choo (2003), Sims 2003, Page
and Page (2007) and Neville (2006).
A discussion of the approach used in the USSR to deal with aggressive fluids has been given
by Ivanov (1981) and in South Africa by Alexander et al. (1994). Concrete Society Technical
Report 22 provides a valuable review of the types and causes of non-structural cracks in con-
crete (1992 and 2010). Monographs by Eglinton (1987) and Mays (1992), intended for the engi-
neer, are useful texts that deal with many aspects of the durability of concrete. Robery (2009a)
has provided guidance for ensuring concrete durability in extreme exposure conditions.
The durability of concrete involves both intrinsic and extrinsic reactions. Intrinsic
attack occurs from deleterious substances or properties incorporated into the concrete
mix, although an intrinsic component such as alkali may be later augmented from external
sources. Extrinsic attack takes place from agents outside the concrete and, with the excep-
tion of physical agencies, usually involves deleterious solutions. This latter type of attack
produces a zone of deterioration that moves in from the outer surface of the concrete. From a
textural aspect, this leads to two important differences for the petrographer. Intrinsic attack
will affect a considerable portion of the concrete texture, while extrinsic attack is restricted
to zones adjacent to outer surfaces. This helps to explain why laboratory testing has on the
whole been more successful in predicting durability for an intrinsic reaction such as alkali–
aggregate reaction (AAR) than for an extrinsic problem such as sulphate attack (Cohen and
Mather 1991; Mehta 1993), although also some forms of deterioration leave clearer diag-
nostic signatures in the concrete for later identification by the petrographer.
The concept of an external layer of deterioration is important when investigating the
effects of external agents on concrete. In many cases, this outer layer consists of three zones:
a an outer corrosion zone where the attack has gone to completion; an intermediate zone
where attack on calcium hydroxide, the hydrates and clinker occurs and salts are deposited;
and an inner zone where leaching of calcium hydroxide and possibly other materials from
the hardened paste takes place.
385
386 Concrete petrography
When examined petrographically, the outer zone usually consists of amorphous forms of
hydrated silica, alumina and iron in which are embedded aggregates. In most cases, only
hydrated silica and aggregate remain as both the iron and alumina compounds are more
readily removed. It is rarely intact as it often suffers damage during sampling, and in some
cases, such as attack by citric acid, the zone is of minimal width because silica is dissolved
by this acid. The outer zone merges into the intermediate zone. It is the most complex of
the zones as recrystallisation of salts, removal of calcium hydroxide and decomposition of
the cement hydrates all occur together in this zone. It is highly variable in depth depend-
ing on the reaction, often contains carbonated material and may be stained by amorphous,
hydrated iron compounds. Observable calcium hydroxide is rare or absent, indicating that
the pH will be below that required for the stability of the cement hydrates. Where salt
recrystallisation has occurred, the intermediate zone may be extensively exfoliated. The
intermediate zone grades into apparently sound hardened cement paste, but careful exami-
nation will often detect an inner zone where the paste appears to be darkened due to a dimi-
nution of calcium hydroxide. This is the chemical gradient which invariably occurs where
hardened paste is undergoing dissolution; it has been described in detail by Pavlik (1994).
In many cases, lack of durability in concrete is a chemical problem that arises during manufac-
ture or later, although to the engineer it is the changes in volume and loss of mechanical integrity
that are of importance. The hardened cement paste and many aggregates are stable only under
certain conditions, of which pH is by far the most important. Mehta et al. (1992), in review-
ing the ‘performance and durability of concrete systems’, summarised some relevant data from
Reardon (1990) and Gabrisova et al. (1991) on the stability of the cement hydrates as follows:
In the pH range 12.5 to 12.0, calcium hydroxide, calcium monosulphate hydrate and
aluminate hydrates, dissolve in sulphate solution from which ettringite precipitates out.
Next, gypsum precipitates out in the pH range 11.6 to 10.6; below pH 10.6, ettringite is
no longer stable and will start decomposing. Note that the lowering of the pH below 12.5
will also cause the C-S-H phase to be subjected to cycles of dissolution and reprecipitation
(Ca/Si ratio is 2.12 at pH 12.5, and 0.5 at pH 8.8) which continue until it is no longer
stable at or below pH 8.8. All the foregoing chemical changes in a hardened Portland
cement product must be taken into consideration when attempting to understand their
physical manifestations, such as expansion, cracking, and loss of strength or mass.
Low permeability in the surface zone is of crucial importance to reducing the rate of attack by
corrosive agents. Products formed by the attack may be beneficial or deleterious depending
on circumstances. For instance, carbonation may make a concrete more impermeable while
reducing the pH, but if the concrete still remains sufficiently permeable after carbonation,
oxygen and moisture can penetrate and corrode the reinforcing steel. On the other hand, the
formation of a product such as ettringite can be expansive and disrupt the concrete. When
considering whether an increase or decrease in permeability is likely to occur, comparison of
molar volumes of reactants and reaction products provides useful information.
Intrinsic reactions sometimes occur between aggregates and the pore solution of a con-
crete because of internal sulphate or salt attack. Unsoundness due to hard burnt lime (CaO)
and/or magnesia (MgO, periclase) is now rare and, as cement manufacturers now control
production to avoid these problems, they will not be discussed in detail in this chapter.
A concrete or mortar made with a cement containing excess hard burnt lime or magnesia
can expand for months or years after setting, leading to the development of cracks. The
expansion is due to the slow hydration of the hard burnt lime or magnesia/periclase leading
to volumetric expansion. Deng Min et al. (1995) have proposed a mechanism for the expan-
sion that occurs when hard burnt lime hydrates in hardened cement paste, and McKenzie
Examination of deteriorated and damaged concrete 387
(a) (b)
~50 µm
Hydrated
magnesium
oxide
Iron oxides
(c) (d) (e)
Figure 6.1 Concrete damaged by the gradual hydration of periclase (MgO): (a) surface pop-outs on a con-
crete floor surface; (b) close-up of pop-out pit and its ‘lid’; (c) close-up of pop-out pit, highlighting
the expanding particle at its base, and in a separate case; (d) periclase in a steel slag aggregate,
in thin sections; (e) periclase altering to brucite (Mg(OH)), in SEM.
2
(1994) has described the microscopic methods for studying lime burning and quality. The
textures of hard burnt lime illustrated will be applicable to lime in cement.
Similar problems can be caused by impurities of lime or periclase in aggregates, especially
some waste/by-product materials. The authors have investigated several cases of concrete
damage caused by periclase, variously as contamination of otherwise inert aggregate and as
a constituent of steel slag material (Figure 6.1).
6.1.2 D urability investigation and classification
While the concept of extrinsic and intrinsic reaction (see Section 6.1.1) is useful to the
petrographer, some form of classification of concrete deterioration, for example, that pro-
posed by Popovics (1987), is of more practical use for engineering purposes. Popovics’
classification is a useful summary of the types of attack likely to be encountered and is
reproduced in Table 6.1.
The petrographic investigation of durability requires an assessment of the quality of the
in situ concrete. If the problem involves the materials, it is necessary to identify cement
type, mineral additions and mineralogy of the aggregates; to estimate the cement content,
water/cement ratio and air content; and to make a qualitative assessment of aggregate
quality and concrete compaction. The data from such an assessment will show that, in
a surprisingly large number of cases, the concrete was not mixed and placed accord-
ing to specification and/or accepted practice, although the deviations may not be large.
388 Concrete petrography
Table 6.1 Classification of concrete deterioration
Class I Leaching by soft water
Class II Non-acidic reactions
Class IIA Base exchange, that is, magnesium salts
Class IIB Saponification reactions from fats and oils
Class IIC Reactions with sugars
Class III Reactions involving excessive expansion
Class IIIA Sulphate attack
Class IIIB AAR
Class IV Reactions with acidic water
Class V Physical processes other than mechanical
Class VA Salt solutions causing efflorescence and/or spalling or cracking
Class VB Freezing and thawing
Class VI Mechanical deterioration
Class VIA Abrasion and wear
Class VIB Excessive shrinkage, uneven thermal expansion, overload, repeated loading, etc.
Source: Popovics, S., A classification of the deterioration of concrete based on mechanism, Concrete Durability – Katherine
and Bryant Mather International Conference, ACI SP-lOO, American Concrete Institute, Detroit, MI, 1987, Vol. 1, pp. 131–42.
Where severe or unusual service conditions apply, non-compliance with accepted practice
can lead to deterioration of the concrete.
The second part of the petrographic investigation involves identification of the conditions
that are causing the materials to deteriorate. In most cases, identification of the cause of the
aggression and the extent of deterioration is possible provided that the investigative meth-
ods used are adequate. However, an estimate of the remaining useful life of the materials in
a structure requires rates of deterioration to be determined. In many cases, it is difficult, if
not impossible, to estimate the rate of attack, and any prediction of the remaining useful life
of the concrete will need to be based on previous experience.
Given the potential complexity of investigating problems of durability, it is pertinent
to define the scope of concrete petrography. Where the in-service conditions are unusual,
petrographic examination combined with chemical investigation of field concretes can pro-
vide information that may lead to the better design of concretes. In cases where the mecha-
nism of deterioration is still not fully understood, such as with expansive processes in large
structures such as dams, engineering and petrographic field data are vital and often might
take precedence over laboratory testing. The applicability of these field data is largely depen-
dent on being able to characterise the condition of the concrete, and an important part of
this characterisation will be provided by examination of microtextures in thin section.
Often, the most obvious manifestations of concrete distress and/or deterioration will
be cracks, some of which will have structural causes, but others of which will have non-
structural origins that relate variously to the nature, quality and behaviour of the concrete
material. An excellent explanatory and diagnostic guide to non-structural cracking is pro-
vided in Concrete Society Technical Report 22 (1992, 2010), including a schematic diagram
illustrating typical crack patterns and locations (see Figure 6.2) and a chart demonstrating
that the time to the appearance (‘age’) of cracks can be a helpful diagnostic element
(see Table 6.2). Practising petrographers will find that exceptions to these ‘rules of thumb’
will sometimes be encountered, but the guidance is nonetheless sound and helpful. Fookes
(1977) prepared a similar earlier guide to concrete cracking in hot arid climatic conditions,
and Fookes and Walker (2011) provided a useful simplified chart (Figure 6.3).
Sibbick (2011) has correctly commented that, in practice, the petrographer will also and
typically be confronted with combinations of mechanisms, variously occurring concurrently
Examination of deteriorated and damaged concrete 389
Example of non-structural cracks in
hypothetical concrete structure
Type of cracking
A J
A
Plastic settlement A, B, C
B
I Plastic shrinkage D, E, F
I
E
Early thermal contraction G, H
K
Long-term drying shrinkage I
C
Shear Crazing J, K
cracks F
Corrosion of reinforcement L, M
N Top of kicker Alkali-silica reaction N
B
G
H B
Tension bending cracks
“Bad”, that is ineffective, joint
H L
Cracks at I
kicker joints
D
M Plus rust stains
Figure 6.2 D iagram illustrating non-structural crack types and typical locations. (From Concrete Society,
Non-structural cracks in concrete, Report of a Concrete Society Working Party, Concrete
Society Technical Report No 22, 4th & current edition, The Concrete Society, Camberley,
U.K., 2010.)
Table 6.2 Time to appearance of various types and causes of non-structural cracks
(simplified and developed from Concrete Society Technical Report No 22 [2010])
Crack pattern Time to appearance Primary cause Secondary cause
Type (see Figure 6.2) (UK conditions) (simplified) (simplified)
Plastic settlement A, B, C 10 min to 3 h Excess bleeding Rapid early drying
Plastic shrinkage D, E 30 min to 6 h Rapid early drying Low bleeding rate
F Rapid early drying
and near-surface
steel
Early thermal G 1 day to 2 to 3 Excess heat Rapid cooling
contraction weeks generation
Early shrinkage Hours to days Very low water/
by self- cement ratios
desiccation
Long-term drying I Weeks or months, Inefficient stress relief High shrinkage
shrinkage up to several years concrete
Surface crazing Days to months Impermeable Poor quality
formwork concrete
Reinforcement L More than 2 years Inadequate cover, Poor quality
corrosion M carbonation, salts concrete
Calcium chloride
Delayed Can be similar Months to years Excess heat Other factors and
ettringite to ASR generation subsequent
formation (DEF) wetting
Alkali-carbonate Can be similar Months to years Reactive carbonate Mechanism not
reaction (ACR) to ASR aggregate fully understood
Alkali-silica N More than 5 years Reactive aggregates Other factors and
reaction (ASR) and high alkalis subsequent
wetting
390 Concrete petrography
Appearance of cracks
Loading, service
conditions
AAR/ASR
(aggregate mineral
reactions)
Corrosion
Drying shrinkage
Early thermal
contraction
Plastic shrinkage
Plastic settlement
1 h 1 day 1 week 1 month 1 year 50 years
Time from placing concrete
Figure 6.3 C hart showing the relationship between time and the appearance of cracking from various
causes. (From Fookes, P.G. and Walker, M.J., Geol. Today, 27(4), 141, 2011.)
or sometimes as synergistic interactions between mechanisms. Common examples would
include freeze–thaw cycling and salt scaling, freeze–thaw cycling and surface abrasion, AAR
and calcium leaching processes, the thaumasite form of sulphate attack (TSA) and aggressive
carbonation, AAR followed by seawater and/or sulphate attack (Sibbick 2009), alkali–silica
reaction (ASR) and delayed ettringite formation (DEF) (Martin 2010; Martin et al. 2012a,b)
and AAR followed by de-icing salt scaling and reinforcement corrosion. These various mecha-
nisms and others will be separately discussed in the succeeding subsections of this chapter,
but the petrographer must remain alert to combinations and will sometimes need to ascertain
which mechanism has been the main causal and/or initiating factor. In the case of the some-
times almost ‘symbiotic’ relationship between ASR and DEF, for example, Thomas et al.
(2007, 2008a) have devised a testing procedure to help determine the primary mechanism.
6.1.3 Q uantification of deterioration and damage
Petrography is sometimes employed to diagnose a particular issue, such as the cause of sur-
face degradation or the cause of cracking and/or manifestations of expansion, but on other
occasions, the examination will form part of a routine, pre-purchase or pre-change-of-use
condition survey. Whichever applies, some degree of quantification of the findings will be
helpful to the structure managing engineer, variously for comparative purposes, and/or to
gain insight into the degree to which a concrete is affected by the mechanism(s) concerned
and/or to act as a benchmark for future surveys, but no standard procedures have been
developed. Sims et al. (1992) suggested a method for quantifying evidence of ASR in con-
crete, but the quantitative system that has gained the most international recognition was
developed by Grattan-Bellew (1995) and this ‘damage rating index’ (DRI) is described in the
immediately following text, with kind assistance from Paddy Grattan-Bellew.
The ASTM C856-11 (2011) standard method for the petrographic evaluation of concrete
provides a sound basis for determining the condition and mineralogical composition of con-
crete cores taken from a structure. However, this method does not provide a procedure for
Examination of deteriorated and damaged concrete 391
quantifying the amount of deterioration that has occurred in the concrete. The DRI method
was developed in the 1990s to address this shortcoming in ASTM C856 (Grattan-Bellew
1995). The DRI method was developed initially and primarily to evaluate concrete affected
by ASR, but can also be used to evaluate concrete affected by other mechanisms of deteriora-
tion, such as alkali–carbonate reaction (ACR), DEF, damage caused by cycles of freezing and
thawing and sulphate attack (Grattan-Bellew and Mitchell 2006). The DRI method consists
essentially of counting the numbers of features indicative of concrete deterioration on the pol-
ished surface of a concrete core.
6.1.3.1 Methodology
A series of cores should be taken from the structure under investigation so that an estimate
of the mean damage to the structure can be obtained. The minimum core diameter should be
10 cm (100 mm). The surface area of the sawn surface of the core should be 200 cm2, so that a
reasonably representative area of the concrete from each core is examined. The core should be
sawn in two axially and the sawn surface ground using a series of grits down to about 40 μm,
or its diamond equivalent, and/or polished. In many concretes, polishing has been found to
be unnecessary. The finished surface of the core is then coated with uranyl acetate, let stand
for two to three minutes, and rinsed off with water (Natesaiyer and Hover 1988). Uranyl
acetate is adsorbed preferentially by any alkali–silica gel (and some other compounds; see also
Section 6.8), which then fluoresces when viewed in ultraviolet (UV) light. This greatly assists
in observing narrow cracks filled with gel that might otherwise be missed.
The polished surface of the core is mounted under a stereo-binocular microscope on a
mechanised stage, if available, and the entire surface is scanned, one field of view at a time
at a magnification of ~16×. The numbers of features indicative of damage to the concrete
are counted in each field of view and then modified by the factors shown in Table 6.3. The
purpose of applying factors to the raw numbers is to attempt to relate, on the basis of expe-
rience, the measured features to the actual damaging effect that they have on the concrete.
For example, reaction rims are indicative that ASR has occurred, but usually do not actu-
ally contribute to the damage to the concrete; for this reason, a relatively low factor of 0.5
is applied. In the case of coarse aggregate particles, cracking almost always occurs even in
freshly cast concrete, and for this reason, the number of cracks is discounted by a factor of
0.25. By contrast, open cracks, peripheral spaces at aggregate/cement interfaces (indicative
of debonding) and gel deposits associated with cracking in the cement paste matrix are all
considered especially important and allocated higher weighting factors of 3.0 or 4.0.
Upon completion of the factoring of the numbers of damage features, the results are nor-
malised for an area of 100 cm2. The DRI is the normalised sum of the factored numbers
of damage features. If a mechanical stage is not available, a one centimetre grid, that is
Table 6.3 DRI damage features and their weighting factors
Damage feature measured Factor
Cracks in coarse aggregate 0.25
Cracks with gel in coarse aggregate 2.0
Open cracks in coarse aggregate 3.0
Debonding around coarse aggregate 3.0
Reaction rims around aggregates 0.5
Cracks in the cement paste 2.0
Cracks with gel in the cement paste 4.0
Gel in air voids 0.5
392 Concrete petrography
approximately the area observed in one field of view at a magnification of 16×, can be drawn
on the surface and the numbers of damage features recorded in each square. Following the
practice adopted by Grattan-Bellew, the results of the DRI measurements are shown on a
bar chart. Depending on the type of aggregate in the core, additional damage features can
be added. For example, corrosion occurs in some particles of sandstone from Norway. It
was assumed that the corrosion of the particles would contribute to the deterioration of the
concrete.
An actual example of the application of DRI to cores from a Canadian dam structure
has been provided by Paddy Grattan-Bellew and is illustrated in Figures 6.4 and 6.5. Some
images of the key damage features observed under the microscope are shown in Figure 6.4,
and the resultant DRI bar charts, for cores from various different parts of the dam, are
shown in Figure 6.5.
In this case, as in most, there is a wide variation in the DRIs of cores taken from differ-
ent parts of the structure. Assuming that ASR is the main mechanism responsible for the
observed deterioration, the variation in the DRIs can be related to a number of possible
causes, including variations in the cement content of the concrete in different components of
Agg
Limestone
P
Paste
1000 μm
(a) (b)
Paste
QZT
1 mm granite Paste
1 mm
(c) (d)
Figure 6.4 Photographs of a polished, uranyl acetate-treated surface of a core viewed under a stereo-
binocular microscope showing (a) reaction rims around coarse aggregate particles indicated by
white arrows, while the black arrows point to cracks in the aggregate and in the cement paste,
(b) an open crack at the aggregate/paste interface caused by debonding of the aggregate, (c) an
open crack in a coarse aggregate particle indicated by white arrows and (d) a crack filled with
alkali–silica gel in a quartzite coarse aggregate particle and in the cement paste (in UV light).
Examination of deteriorated and damaged concrete 393
G DRI 85
CA/fracture
CA/crack/gel
F DRI 71
CA/open crack
CA/debond
E DRI 58
CA/rims
CEM/crack
D DRI 102
CEM/crack/gel
C DRI 40
B DRI 310
A DRI 225
0 50 100 150 200 250 300 350
DRI
Figure 6.5 DRI charts for the cores taken from different parts of a dam, showing the determined variation
in damage ratings.
the structure, variations in the alkali content of the cement and variations in the moisture
content of the concrete.
There is gel in cracks in the coarse aggregate particles in all the cores, but in only a few
of the cracks in the cement paste. It should be noted that in cores A and B with the highest
DRIs, 225 and 310, respectively, gel was not observed in the many cracks in the cement
paste. In cold climates, such extensive cracking in the cement paste can usually be attributed
to cycles of freezing and thawing of concrete above the waterline in the dam. Further evi-
dence that the cracking in the cement paste is mainly due to cycles of freezing and thawing
is the presence of multiple cracks subparallel to the surface of the core (see Figure 6.6).
6.1.3.2 I nterpretation and reproducibility
Shrimer (2006) found a reasonable correlation between a visual damage rating of struc-
tures in the field and the DRI values obtained separately for cores taken from them (see
Figure 6.7). However, it is not known how much expansion has occurred in the structures.
The AAR expansion occurring in structures from which cores have been taken for DRI
measurements is generally not known. Therefore, instead, DRIs were determined for con-
crete prisms stored at 38°C and ~100% relative humidity (RH), to assess AAR expansion (see
also Section 6.8), and the findings were compared. Similar assessments were made using DRI
and expansion data from a number of authors (Rivard and Ballivy 2005; Dunbar et al. 1996
and an unpublished report by Grattan-Bellew 2011). The resultant correlations are shown
in Figure 6.8. The following expansions were calculated from the trend lines fitted to graphs
of expansion versus DRI for a DRI of 100: Rivard and Ballivy (2005), 0.10%; Dunbar et al.
(1996), 0.09%; and Grattan-Bellew, 0.18%. The results from the first two authors were
essentially the same, while those from Grattan-Bellew were significantly higher. The reason
394 Concrete petrography
Figure 6.6 C racks subparallel to the surface of the core caused by frost action. The core diameter is
~10 cm (100 mm).
9
y=0.007x+1.110
8 R2=0.767
n
o7
ati
v
er6
s
b
o
n 5
o
rati4
o
eri
et3
d
d Field deterioration
Fiel2 Linear (field deterioration)
1
0
0 200 400 600 800 1000
DRI
Figure 6.7 Chart showing the correlation between the measured DRI values and the visually observed
deterioration ratings of assorted concrete structures. (Drawn using data from Shrimer, F.H.,
Development of the damage rating index method as a tool in the assessment of alkali-aggregate
reaction in concrete: A critical review, Fournier, B. (ed), Marc-André Bérubé Symposium on Alkali-
Aggregate Reactivity in Concrete, Montreal, Canada, May 2006, pp. 391–401.)
for the differences between the calculated expansions has not been established, but may be
related to the types of aggregates tested.
The single operator reproducibility for the DRI method is very good. A coefficient of
variation of 6% was obtained for 6 repeat measurements on a polished core. However, when
multiple operators (again 6) made measurements on the same core using the same equipment
and after discussion on what should be counted, the coefficient of variation rose to 22%