Questionable concrete?
Let cores prove your point!
By Bruce A. Suprenant
on’t tear out that concrete yet. Just
D because a cylinder breaks low
doesn’t mean that you have bad con-
crete. Occasionally, cylinders aren’t
properly made or tested. Cores re-
moved from the structure also can be
used for acceptance. Contractors must
know about coring to protect their in-
place concrete investment. Cores can
get you out of trouble.
Cores for acceptance
ACI 318 “Building Code Require- The compressive
ments for Reinforced Concrete” (Ref. strength results of
three cores can be
1) requires three cores removed from used for accepting
the concrete area in question. If the concrete. Don’t take
average of the three cores is equal to cores from the top
at least 85% of the 28-day compres- layers of columns,
sive strength and no core is less than walls, or footings.
75% of the 28-day compressive These top layers will
strength, then the concrete is consid- be 10% to 20%
weaker than layers
ered to be structurally adequate. from the middle or
These concrete acceptance criteria lower portion.
based on core testing are quite com-
mon. However, be careful; some own-
ers require the average core strength
results to be equal to the required 28-
day compressive strength.
ASTM requirements the ASTM standard, check with the minimum core diameter would be 3
Never let a testing laboratory obtain engineer and the owner to make sure inches and preferably 4 1⁄ 2 inches.
and test a core that isn’t in confor- they will consider the core a satisfac- Be careful — a 3-inch-diameter
mance with ASTM C 42, Standard tory test. core barrel will provide a core with a
Test Method for Obtaining and Testing Core diameter. The core diameter smaller diameter. Core barrels are
Drilled Cores and Sawed Beams of must be at least twice the maximum sized by outside diameter, not inside
Concrete (Ref. 2). If the core doesn’t nominal size of the coarse aggregate. diameter. Select a core barrel greater
meet this ASTM standard, the engi- However, it is preferable for the core than 3 inches to obtain a minimum 3-
neer or owner could question the va- diameter to be three times the maxi- inch-diameter core.
lidity of the test and make the contrac- mum nominal size of the coarse aggre- Core length. The length of the
tor pay for another test. If for some gate. For 11⁄2-inch coarse aggregate the core, when capped, should be about
reason a core is taken that violates twice the diameter. If after capping
End preparation and capping.
Table 1. Length-to-diameter Strength Correction Factor (Ref. 2) Don’t forget end preparation and cap-
Length/Diameter Ratio Strength Correction Factor ping. Sloppy capping and unplaned or
misaligned ends can affect core
1.75 0.98 strength. Cores, like cylinders, must
1.50 0.96 be properly prepared and capped. The
1.25 0.93 provisions for capping a core are the
1.00 0.87 same as for a cylinder.
the core height is less than the Practical considerations
core diameter, then the core When to core? For new
shouldn’t be tested. ASTM construction, a low 7-day or
permits a core length from 28-day compressive strength
about equal size to a maxi- break usually initiates the
mum of twice the size of the coring process. Don’t core
core diameter. too early; bond strength de-
Moisture conditioning. velops slower than compres-
The core should be submerged sive strength. ASTM C 42
in lime-saturated water at recommends waiting 14 days
73.4° F ± 3.0° F for at least 40 until coring to avoid disturb-
hours. Test the core promptly ing the bond between the
after removing it from the coarse aggregate and mortar.
lime-water. If necessary, pro- For new concrete cured in
tect the core with a wet burlap cold weather the time period
cover until the test. before coring may be longer.
If directed by the engineer Where to core? A low
or owner of the project, a core cylinder break usually leads
can be preconditioned in an- to drilling a core, but
other manner (dry), under where? The National Ready
ASTM C 42. ACI 301 (Ref. 3) Mixed Concrete Association
and ACI 318 (Ref. 1) both pro- (Ref. 4) says to avoid coring
vide guidance for conditioning the top layers of columns,
cores. walls, or footings. These top
Length-to-diameter ratio. layers will be 10% to 20%
The length of the capped core weaker than cores from the
and the core diameter must be Figure 1. Cores from the top of vertical pours can be middle or lower portion (see
measured to the nearest 0.01 much weaker than those from the middle or bottom of
Figure 1). For vertical struc-
inch. Use the average of two the concrete deposit. Also, cores obtained by drilling in tures, ASTM C 42 recom-
core diameter measurements at the direction of the casting test about 8% higher than mends taking the core from
mid-height. These measure- cores obtained by drilling perpendicular to the direction the middle of the deposit
ments are used to compute the of concrete placement.
length-to-diameter ratio.
ASTM provides length-to-di-
ameter correction factors that are used
to reduce the compression strength Table 2. Dry Versus Wet Compressive Strength
measure by the testing machine. Tested Wet Tested Dry
These correction factors are appli-
cable for concrete strengths ranging Compressive Unit Weight Compressive Unit Weight
from 2000 to 6000 psi, lightweight Strength (psi) (pcf) Strength (psi) (pcf)
(100 to 120 pounds per cubic foot 3380 142.4 4400 141.4
[pcf]) and normal-weight concrete, 2820 138.8 4240 143.0
and cores tested wet or dry. A core 2970 143.8 3660 142.7
with a length-to-diameter ratio from 3280 141.9 —— ——
1.94 to 2.10 requires no correction 2530 140.8 —— ——
factor. Correction factors not provided
by Table 1 can be determined by inter- Avg. 3000 142.0 4100 142.0
polation.
and away from formed joints or edges of the concrete The cost of coring
placement. Cores aren’t free. The cost of a two-man drilling crew
Cores should be removed from the location in the con- runs about $65 an hour. Don’t forget travel time, core barrel
crete structure that is considered to have low strength. Most wear, and finally testing. Besides compressive strength,
testing laboratories use a rebound hammer for a relative most test laboratories also check the density and absorption
comparison of concrete strength to locate the potentially of the core. Don’t be surprised if the cost of obtaining and
weak concrete. ACI 301 (Ref. 3) requires the engineer/ar- testing three cores hits $500.
chitect to determine the location of the cores to minimize Who pays for the cores? This varies from region to re-
damage to the structure. If possible, avoid coring through gion. In some areas the contractor always pays for the
reinforcement. It decreases the life of the core barrel, cores. In other areas, who pays depends upon the strength
makes core interpretation difficult, and may decrease the results. If the strength results indicate good concrete, then
strength of the member. the owner pays. If the strength results indicate bad or
Test wet or dry? ASTM C 42 requires the core to be marginal concrete, then the contractor pays.
submerged in lime-saturated water for a minimum of 40 Coring questionable concrete usually is a good business
hours prior to testing, and only the project engineer or own- practice. Most specifications automatically require cores if
er can alter the conditioning requirements of the core. Both the cylinders break low. Following ASTM C 42 and know-
ACI 301 and ACI 318 allow the core to be air-dried before ing when, where, and what to test, usually provides strength
testing if the concrete in the structure will be dry under ser- results that make contractors happy. Don’t have good quali-
vice conditions. ty concrete rejected—take cores to prove your point.
Most engineers consider the difference between dry ver-
sus wet compressive strength to be around 10%. However, References
this may not always be true. Reference 5 reports that air-
1. ACI 318, “Building Code Requirements for Reinforced
dried specimens can show a 20% to 25% higher strength Concrete and Commentary,” American Concrete Institute
than corresponding specimens tested in a saturated condi- (ACI), P.O. Box 19150, Detroit, MI 48219.
tion. On one project in Colorado, the compressive strength
2. ASTM C 42, Standard Test Method for Obtaining and
of air-dried cores was 1100 psi higher than the compressive Testing Drilled Cores and Sawed Beams of Concrete, ASTM,
strength of saturated cores (Table 2). The average unit 1916 Race St., Philadelphia, PA 19103.
weight of the cores for both test conditions was the same,
3. ACI 301, “Specifications for Structural Concrete for Build-
142.0 pcf.
ings.”
If the concrete will be dry in service, contractors must
make sure that their cores will be tested dry in accordance 4. “What, Why & How? Strength of In-place Concrete,” Con-
with ACI 301 and ACI 318. Testing wet, when not appropri- crete in Practice, CIP 10, National Ready Mixed Concrete
Association, 900 Spring St., Silver Spring, MD 20910.
ate, could provide strength results leading to the rejection of
good quality concrete. 5. P. K. Mehta, Concrete: Structure, Properties, and Materi-
als, Prentice-Hall, New York, 1986.
Low-strength cores 6. D. L. Bloem, “Concrete Strength Measure m e n t — C o re s
Cores drilled through cracks or other defects shouldn’t be Versus Cylinders,” Proceedings, ASTM, 1965, Vol. 65.
tested for compressive strength and neither should a core
that breaks during drilling. Also, ACI 318 (Ref. 1) permits Bruce A. Suprenant is a consulting engineer, an associate
the testing of additional cores extracted from a location rep- professor at the University of Colorado in Boulder and a
resented by an erratic core strength result. contributing editor for Concrete Construction.
Occasionally, be prepared for low-strength cores. Howev-
er, this doesn’t automatically require the concrete to be re-
moved and replaced. Ask the engineer to review his calcula-
tions to consider the effect of low-strength concrete. The
load capacity of many reinforced concrete members isn’t PUBLICATION #C920437
controlled by concrete but rather by the reinforcement. Copyright © 1992, The Aberdeen Group
Quite often, the concrete can stay in place. All rights reserved
