plik

t first, there was a lot of excite-
ment over that high-profile
project your ready mix compa-
ny was so proud to have a part

in. Then came the bad news. The cylinder
compressive strength test results are low.
The contractor is blaming your company.
He claims it was bad concrete. You think
otherwise.

To aid the investigation, cores will be tak-

en and tested. Knowing the in-place charac-
teristics of the concrete and how they affect
the measured compressive strength of the
cores can go a long way toward establish-
ing that the low-strength cylinders were not
the result of bad concrete.

In-place strength

Concrete coring is generally deemed nec-

essary  by  unacceptably  low  laboratory-
cured  or  field-cured  cylinder  strengths.
Therefore,  core  strengths  should  be  ob-
tained from the in-place concrete that repre-
sents  the  low  cylinder  strengths.  Unfortu-
nately,  in  practice,  the  cores  aren’t  always
removed from locations in the structure rep-
resented by low cylinder strength results.

Fo r example, if 150 cubi c yards ar e

placed in a wall, ACI 318 “Building Code
Requirements for Reinforced Concrete” re-

quires only one strength test. Concrete for
the  one  strength  test  is  removed  from  a
middle portion of a single truck. Assuming
8 cubic yards per ready mix truck, it takes
19  truckloads  of  concrete  to  fill  the  wall.
Because ACI 318 requires that cores be re-
moved  from  the  area  in  question,  that
means  locating  the  suspect  concrete  from
that one truckload.

If accurate placement records are avail-

able,  the area  of low  strength concrete may
be  located  and  cores  removed.  However,
choosing core locations based on placement
records does not confirm that the one truck-
load  of  concrete  tested  is  the  only  low-
strength  concrete  in  the  wall.  Although  re-
moving cores from a suspected low-strength
area  satisfies  ACI  318  criteria  for  sampling,
other locations may need testing.

For instance, the one truckload of suspect-

ed low-strength concrete may actually repre-
sent the quality of concrete in other truck-
loads. Using a nondestructive technique to
locate the suspect truckload of concrete pro-
vides a comparison for locating other poten-
tial low-strength areas.

Occasionally, the contractor determines

the area of suspect concrete by pointing to
an arbitrary location. Alternatively, the test-
ing laboratory may core concrete in a loca-

An under-
standing
of concrete
core test-
ing can be
invaluable
when
in-place
concrete
quality is
questioned

By Bruce A.
Suprenant

Core strength variation
of in-place concrete

F i g u r e 1 .

Pl a nes of

weakness under coarse
aggregate particles due
to bleeding.

tion  accessible  to  its  equipment.
While  accurate  placement  records
are  beneficial,  verification  by  a
nondestructive  testing  technique  is
pr ud ent.   Id eall y,  the  engin eer
should be involved in determining
the location for core testing.

Low cylinder strengths may be

due to  errors  in sampling  or  testing
and not due to inadequate concrete.
Engineers  must  decide  whether  the
low cylinder strength indicates poor
testing, a bad truckload of concrete,
or  a  bad  placement,  then,  if  neces-
sary,  plan  an  appropriate  core  test-
ing program.

Cores vs. cylinders

Cores do not serve the same pur-

pose  as  cylinders.  Strength  of  stan-
dard  cylinders represents  the quality
of concrete delivered. Cylinder com-
pressive  strength  represents  the
quality of concrete batching, mixing,
and  transportation,  as  well  as  the
sampling, preparation, handling, cur-
ing,  and  testing  of  the  cylinders.
Strength  of  cores  represents  the  in-
place  concrete  strength.  In  addition
to  concrete  batching,  mixing,  and
transportation,  core  compressive
strengths  represent  the  quality  of
placement,  consolidation,  and  cur-
ing, and the techniques for obtaining
and  testing  cores.  Therefore,  the  re-
lationship between core and cylinder
strength  varies  because  of  the  char-
acteristics  that each specimen repre-
sents.

Coring direction

Cores obtained by drilling in the

direction  of  concrete  casting  may
provide a higher strength than cores
obtained  by  drilling  perpendicular
to  the  direction  of  casting .  The
strength  difference  due  to  drilling
direction  is  generally  attributed  to
bleeding  in  fresh  concrete,  which
creates  a  weak  paste  pocket  under
coarse aggregate particles (Figure 1).
Because  of  the  bleedwater,  the
paste-to-coarse  aggregate  bond  be-
low  the  aggregate  particles  may  be
w e a k e r .

A load applied parallel to the weak

bond  opens  a  crack,  creating  a
strength-decreasing flaw. However, a
load  applied  perpendicular  to  the
weak  bond  closes  the  crack,  mini-
mizing  the  effect  of  the  bleedwater
layer. If this theory holds true, reduc-
ing  bleedwater  minimizes  the  effect
of  coring  direction.  Thus,  any  factor
that  affects  bleeding,  such  as  the
concrete mix design, mix ingredients,
air content,  and  placement and  con-
solidation  techniques,  also  deter-
mines  the  strength  difference  of
cores  drilled  vertically  or  horizontal-
ly.

Most slabs and foundations are

cored  parallel  to  the  direction  of
casting,  resulting  in  no  associated
reduction  in  strength.  Walls  and
columns are cored perpendicular to
the  direction  of  casting,  thus  a  re-
duction in strength may occur.

The data on the effect of coring

direction  is  contradictory. It is  quite
likely  that  the  compressive  strength
of  cores  drilled  horizontally  are
stronger than cores drilled vertically.
Practical  considerations,  however,
like variations in placement, consoli-
dation, and mix variability might ob-
scure  a  coring  direction  difference
that  is  discernible  only  under  pre-
cise  control  of  the  mix  and  con-
struction practices. The current prac-

tice in the industry is to neglect any
effect of coring direction.

Top-to-bottom strength
variation

It is generally acknowledged that

concrete strength varies within a sin-
gle  element.  The  strength  variations
shown  in  Figure  2  should  not  be
considered  as  absolute  numbers.
Figure  2  is  very  useful,  however,
when planning a nondestructive sur-
vey to determine the likelihood of a
low-strength  cylinder  or  core  loca-
t i o n s .

Laboratory test results indicate two

apparent causes of the strength vari-
ation:  strength  increase  at  the  bot-
tom  attributed  to  greater  static  pres-
sures  caused  by  the  concrete  above
and  strength  decrease  at  the  top  at-
tributed  to  higher  water-cement  ra-
tios as a result of bleedwater (Figure
3 ) .

Consolidation

A contractor’s consolidation effort

has  a  significant  effect  on  concrete
strength. It is estimated that between
5% and 20% of air is entrapped while
placing  concrete.  Vibrators  reduce
the amount of entrapped air by con-
solidating  the  concrete.  The  core’s
compressive  strength  represents  the

Figure 2.

Estimated within-member strength variation.

degree of consolidation achieved by
workers and their equipment.

Some state highway departments

studied  how  the  spacing  of  im-
mersed  vibrators  affects  core  com-
pr essive  str eng th .  On  sli pfor m
pavers  the  vibrators  are  fixed  at  a
set spacing. Concrete directly in the
path of the vibrators is consolidated
better  than  concrete  between  the
vibrators.  Vibrator  spacing  is  cho-
sen  based  on  the  radius  of  influ-
ence, usually 24 inches.

Cores removed from the path of

the vibrator are stronger and denser
than cores removed between vibra-
tors.  Cores  removed  from  the  bot-
tom  are  stronger  and  denser  than
those removed from the top. Work
by  several  highway  departments
shows that a reasonable maximum
decrease in a pavement core’s unit
weight compared to the unit weight
of  an  ASTM  cylinder  is  4%.  This
corresponds  to  a  loss  in  compres-
sive strength of about 1200 psi.

Effects of curing

The thermal history and curing of

cores is quite different than for stan-
dard  cylinders.  The  structure’s  ther-
mal environment might be better or
worse than that provided by labora-
tory  curing.  Also,  most  structures
aren’t  moist  cured  like  a  standard
ASTM  cylinder.  Field  curing  is  un-
likely to be as good as moist curing.

Field concrete may be subjected

to cold- or hot-weather curing con-
ditions. High temperatures can low-
er concrete strength but lower tem-
peratures could actually produce
stronger concrete at later ages.

The methods for obtaining and

testing a core obscure the effects of
curing.  Curing  dramatically  affects
the concrete surface, but has less of
an  effect  on  the  interior  concrete.
The  outer  concrete  protects  the  in-
ner concrete’s humidity and temper-
ature  conditioning.  When  cores  are
tested,  the  restraint  of  the  testing
procedure  makes  most  concrete
cores  fail within the middle portion
of  the  core.  Weak  outer  edges,  af-
fected  by  curing  methods,  are  not
usually represented by the core fail-
ure mode or the resulting test value.

The test results presented indicate

that for vertical members such as
walls and columns, curing had little
effect on core strengths. For slabs,
however, curing is critical to achiev-
ing adequate core strength.

Recommendations for core
locations

For a core drilled perpendicular

to a horizontal surface, ASTM C 42
states, “The location shall be, when
possible, so that its axis is perpen-
dicular  to  the  bed  of  the  concrete
as  originally  placed  and  not  near
formed joints or obvious edges of a
unit of deposit.” For a core drilled
perpendicular  to  a  vertical  surface
or  a  battered  surface,  ASTM  C  42
states the core “shall be taken from
near the middle of a unit of deposit
when possible and not near formed
joints or obvious edges of a unit of
deposit.”

The Concrete Society Working

Party recommends that “the section
of core to be tested should not in-

clude the top 20%, to a limit of 12
inches,  of  the  lift  concerned.  The
top 2 inches should not be includ-
ed  in  any  case.”  The  Nati ona l
Ready  Mixed  Concrete  Association
(NRMCA)  r ecommends  again st
drilling cores from the top layers of
columns,  slabs,  walls,  or  footings.
NRMCA  indicates  that  cores  from
the  top  layer s  are  10%  to  20%
weaker than cores from the middle
or lower portion. ✥

This article is based on publication
185,  “Understanding  Concrete
Core  Testing,”  published  by  the
National  Ready  Mixed  Concrete
Association (NRMCA). For more in-
formation or to order a copy, con-
tact NRMCA, 900 Spring St., Silver
Spring  MD  20910  (phone:  301-
587-1400, fax: 301.585.4219).

Figure 3.

Typical relative percentage strength contours for a beam (top) and for a wall (bottom).

PUBLICATION #J950134