Core strength variation
of in-place concrete
t first, there was a lot of excite- quires only one strength test. Concrete for
An under-
ment over that high-profile the one strength test is removed from a
standing
project your ready mix compa- middle portion of a single truck. Assuming
Any was so proud to have a part 8 cubic yards per ready mix truck, it takes
of concrete
in. Then came the bad news. The cylinder 19 truckloads of concrete to fill the wall.
compressive strength test results are low. Because ACI 318 requires that cores be re-
core test-
The contractor is blaming your company. moved from the area in question, that
ing can be
He claims it was bad concrete. You think means locating the suspect concrete from
otherwise. that one truckload.
invaluable
To aid the investigation, cores will be tak- If accurate placement records are avail-
when en and tested. Knowing the in-place charac- able, the area of low strength concrete may
teristics of the concrete and how they affect be located and cores removed. However,
in-place
the measured compressive strength of the choosing core locations based on placement
cores can go a long way toward establish- records does not confirm that the one truck-
concrete
ing that the low-strength cylinders were not load of concrete tested is the only low-
quality is
the result of bad concrete. strength concrete in the wall. Although re-
moving cores from a suspected low-strength
questioned
In-place strength area satisfies ACI 318 criteria for sampling,
Concrete coring is generally deemed nec- other locations may need testing.
By Bruce A.
essary by unacceptably low laboratory- For instance, the one truckload of suspect-
Suprenant
cured or field-cured cylinder strengths. ed low-strength concrete may actually repre-
Therefore, core strengths should be ob- sent the quality of concrete in other truck-
tained from the in-place concrete that repre- loads. Using a nondestructive technique to
sents the low cylinder strengths. Unfortu- locate the suspect truckload of concrete pro-
nately, in practice, the cores aren t always vides a comparison for locating other poten-
removed from locations in the structure rep- tial low-strength areas.
resented by low cylinder strength results. Occasionally, the contractor determines
For example, if 150 cubic yards are the area of suspect concrete by pointing to
placed in a wall, ACI 318  Building Code an arbitrary location. Alternatively, the test-
Requirements for Reinforced Concrete re- ing laboratory may core concrete in a loca-
F igu re 1. Planes 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
prudent. Ideally, the engineer
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
Figure 2. Estimated within-member strength variation.
of concrete delivered. Cylinder com-
pressive strength represents the bond opens a crack, creating a tice in the industry is to neglect any
quality of concrete batching, mixing, strength-decreasing flaw. However, a effect of coring direction.
and transportation, as well as the load applied perpendicular to the
sampling, preparation, handling, cur- weak bond closes the crack, mini- Top-to-bottom strength
ing, and testing of the cylinders. mizing the effect of the bleedwater variation
Strength of cores represents the in- layer. If this theory holds true, reduc- It is generally acknowledged that
place concrete strength. In addition ing bleedwater minimizes the effect concrete strength varies within a sin-
to concrete batching, mixing, and of coring direction. Thus, any factor gle element. The strength variations
transportation, core compressive that affects bleeding, such as the shown in Figure 2 should not be
strengths represent the quality of concrete mix design, mix ingredients, considered as absolute numbers.
placement, consolidation, and cur- air content, and placement and con- Figure 2 is very useful, however,
ing, and the techniques for obtaining solidation techniques, also deter- when planning a nondestructive sur-
and testing cores. Therefore, the re- mines the strength difference of vey to determine the likelihood of a
lationship between core and cylinder cores drilled vertically or horizontal- low-strength cylinder or core loca-
strength varies because of the char- ly. t i o n s .
acteristics that each specimen repre- Most slabs and foundations are Laboratory test results indicate two
sents. cored parallel to the direction of apparent causes of the strength vari-
casting, resulting in no associated ation: strength increase at the bot-
Coring direction reduction in strength. Walls and tom attributed to greater static pres-
Cores obtained by drilling in the columns are cored perpendicular to sures caused by the concrete above
direction of concrete casting may the direction of casting, thus a re- and strength decrease at the top at-
provide a higher strength than cores duction in strength may occur. tributed to higher water-cement ra-
obtained by drilling perpendicular The data on the effect of coring tios as a result of bleedwater (Figure
to the direction of casting. The direction is contradictory. It is quite 3 ) .
strength difference due to drilling likely that the compressive strength
direction is generally attributed to of cores drilled horizontally are Consolidation
bleeding in fresh concrete, which stronger than cores drilled vertically. A contractor s consolidation effort
creates a weak paste pocket under Practical considerations, however, has a significant effect on concrete
coarse aggregate particles (Figure 1). like variations in placement, consoli- strength. It is estimated that between
Because of the bleedwater, the dation, and mix variability might ob- 5% and 20% of air is entrapped while
paste-to-coarse aggregate bond be- scure a coring direction difference placing concrete. Vibrators reduce
low the aggregate particles may be that is discernible only under pre- the amount of entrapped air by con-
w e a k e r . cise control of the mix and con- solidating the concrete. The core s
A load applied parallel to the weak struction practices. The current prac- compressive strength represents the
degree of consolidation achieved by
workers and their equipment.
Some state highway departments
studied how the spacing of im-
mersed vibrators affects core com-
pressive strength. On slipform
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-
Figure 3. Typical relative percentage strength contours for a beam (top) and for a wall (bottom).
dard cylinders. The structure s ther-
mal environment might be better or The test results presented indicate clude the top 20%, to a limit of 12
worse than that provided by labora- that for vertical members such as inches, of the lift concerned. The
tory curing. Also, most structures walls and columns, curing had little top 2 inches should not be includ-
aren t moist cured like a standard effect on core strengths. For slabs, ed in any case. The National
ASTM cylinder. Field curing is un- however, curing is critical to achiev- Ready Mixed Concrete Association
likely to be as good as moist curing. ing adequate core strength. (NRMCA) recommends against
Field concrete may be subjected drilling cores from the top layers of
to cold- or hot-weather curing con- Recommendations for core columns, slabs, walls, or footings.
ditions. High temperatures can low- locations NRMCA indicates that cores from
er concrete strength but lower tem- For a core drilled perpendicular the top layers are 10% to 20%
peratures could actually produce to a horizontal surface, ASTM C 42 weaker than cores from the middle
stronger concrete at later ages. states,  The location shall be, when or lower portion.
The methods for obtaining and possible, so that its axis is perpen-
testing a core obscure the effects of dicular to the bed of the concrete This article is based on publication
curing. Curing dramatically affects as originally placed and not near 185,  Understanding Concrete
the concrete surface, but has less of formed joints or obvious edges of a Core Testing, published by the
an effect on the interior concrete. unit of deposit. For a core drilled National Ready Mixed Concrete
The outer concrete protects the in- perpendicular to a vertical surface Association (NRMCA). For more in-
ner concrete s humidity and temper- or a battered surface, ASTM C 42 formation or to order a copy, con-
ature conditioning. When cores are states the core  shall be taken from tact NRMCA, 900 Spring St., Silver
tested, the restraint of the testing near the middle of a unit of deposit Spring MD 20910 (phone: 301-
procedure makes most concrete when possible and not near formed 587-1400, fax: [phone]).
cores fail within the middle portion joints or obvious edges of a unit of
of the core. Weak outer edges, af- deposit.
fected by curing methods, are not The Concrete Society Working PUBLICATION #J950134
usually represented by the core fail- Party recommends that  the section
Copyright © 1995, The Aberdeen Group
ure mode or the resulting test value. of core to be tested should not in- All rights reserved