Research and experience have shown that
pervious concrete mixtures proportioned to have 15 to 25% air void
contents should have sufficient infiltration rates to limit storm water
surface runoff and adequate strength to avoid raveling.1 Until
recently, however, there were no U.S. standards for the verification of
air void content in fresh concrete or infiltration rates for in-place
concrete. To help producers, contractors, and owners verify that their
pavement projects will perform as needed, ASTM Committee C09.49,
Pervious Concrete, has recently introduced Standard C1688, "Standard
Test Method for Density and Void Content of Pervious Concrete"2 and
Standard C1701, "Standard Test Method for Infiltration Rate of Pervious
Concrete."3 These standards were used as part of the quality assurance
program for the construction of a parking lot at the Metropolitan
Community College (MCC) in Omaha, NE. Using test placements to develop
a compaction-density relationship, appropriate mixture properties could
be defined without guesswork. Workability tests and unit weight tests
per ASTM C1688 were used to screen loads to ensure that we placed only
workable concrete that could be consolidated to achieve a target air
void content.
UNIT WEIGHT AND AIR VOID CONTENT
Pervious concrete typically comprises a zero slump mixture with little
to no fine aggregate and uniformly graded coarse aggregate. The
workability of such mixtures can be highly sensitive to variations in
moisture content and compaction effort, leading to large variations in
the final void contents for a given pavement project. By mixing trial
batches for the contractor to use in test placements of pavement, the
producer can obtain unit weight data per ASTM C1688 and air void
content Vair (in %) from cylindrical samples according to the procedure
in Reference 4. Vair is given by
V^sub air^ = [1 - (W^sub D^ - W^sub S^)/(gamma^sub W^ . V^sub T^)] x 100 (1)
where WD is the weight of the oven-dried sample, WS is the submerged
weight of the sample (after tapping to release trapped air), gW is the
unit weight of water, and VT is the calculated volume of the sample
using its measured diameter and length.
For the MCC project,
six mixtures were prepared and samples were produced per ASTM C1688
during placement of the preliminary test panels (Fig. 1). Unit weight
and air void content for each mixture were measured and plotted, and a
linear regression analysis was used to determine the relationship
between air void content and unit weight (Fig. 2). As one might expect,
there is a linear relationship between void content and unit weight of
pervious concrete mixtures, with a maximum unit weight (about 150
lb/ft3 [2400 kg/m3]) associated with zero air void content.
It must be noted that the ASTM C1688 procedure (filling a 0.25 ft3 [7
L] cylindrical container in two lifts, with each lift consolidated
using 20 blows from a standard Proctor hammer) will not produce the
same air void content as would be produced in pavement. Our preliminary
field determination for cores removed from the test panels indicated
that a mixture with an air void content of 12% and unit weight of 133.5
lb/ft3 (2140 kg/m3) when tested per ASTM C1688 would have an in-place
air void content (found per Eq. (1) using core sample data) of 17.5%.
This in-place value was specified for the project.
QUALITY ASSURANCE PROGRAM
The owner recognized pervious concrete as a new product and thus made
it very clear that, regardless if the product was successful or not,
"we need to know why." The team was therefore expected to implement
procedures within a set quality control program, including:
* Aggregate moisture tests conducted by the concrete producer before batching operations;
* Unit weight tests per ASTM C1688 conducted at the batch plant by the producer and at the job site on every load of concrete;
* Inverted slump cone tests (described in the following section) conducted at the job site by the owner's testing agency;
* Estimated unit weight test (described in the following section) conducted on site by the owner's testing agency;
* Unit weight tests (five total) using 4 in. (100 mm) diameter cores
taken from the hardened pavement and tested using the procedure
described in Reference 4 by the owner's testing agency; and
* Permeability tests (six total) per ASTM C1701, taken at the core
locations (prior to coring) by the owner's testing agency (Fig. 3).
UNIQUE TEST PROCEDURES
Inverted slump cone test
The inverted slump cone test is qualitative, but it allows a rapid
evaluation of workability. The procedure involves resting the small
opening of a slump cone against a smooth, hard surface. The cone is
then filled with fresh concrete in one lift, with no consolidation.
Excess concrete is struck off, level with the large end of the cone,
and the cone is then lifted. The fresh concrete is observed as it flows
out of the cone. If the bulk of the concrete remains in the cone and
can only be discharged by vigorous shaking of the cone, the mixture
will be unworkable. Figure 4 shows two different mixtures after
discharge. The concrete in Fig. 4(a) was discharged after tapping of
the cone-the batch was remediated by increasing the water content. The
concrete in Fig. 4(b) flowed freely from the cone and was approved for
placement.
Estimated in-place unit weight
In this
procedure, a 0.25 ft3 (7 L) cylindrical container is filled with fresh
concrete in one lift, with no consolidation. Excess concrete is struck
off, level with the top of the container. The net weight of the
concrete is determined and the unit weight of the test sample is
calculated. The resulting value is multiplied by a compaction factor,
which is based on observations that typical consolidation methods lead
to a 1 in. (25 mm) reduction in thickness relative to the initial
placement depth. Thus, for the 6 in. (150 mm) thick pavement required
on this project, the compaction factor was 7 in./6 in. = 1.17.
Estimated unit weight values were correlated with specific regions of
the in-place pavement.
APPLICATION
Placement
The pervious concrete pavement was placed by directly discharging the
concrete from mixer trucks onto an aggregate base. Concrete was raked
into place and consolidated and finished using a hydraulic
roller-screed operating directly on top of side forms. As per ACI
522.1-08, the concrete was covered with a polyethylene sheet
immediately after finishing.5
The 5650 ft2 (525 m2) paved
area required 110 yd3 (84 m3) of concrete, which was delivered in 14
truckloads. Most of the placement was completed in 2 days, during which
the average ambient temperature was 65[degrees]F (18.5[degrees]C) and
the relative humidity was 70%.
Inverted slump testing showed
that the first truck was not workable and additional water was added
until the concrete had about 12% air void content as measured per ASTM
C1688. The second truck had too much water added at the concrete plant
and was held until the concrete had about 12% air void content per ASTM
C1688. The water content for the third truck was acceptable, so
concrete from this truck was placed while the second load was being
held. Tests of subsequent loads indicated they also had acceptable
water contents.
Pavement sections were installed with no
reports of consolidation or finishing problems. Workers with previous
experience with pervious concrete pavements reported, however, that the
mixtures would have been considered too "wet" if evaluated by visual
inspection only.
Fresh and hardened properties
ACI
522.1-08 Section 1.6.2.1 requires that the unit weight of fresh
concrete is within +-5 lb/ft3 (+-80 kg/m3) of the specified fresh unit
weight. ACI 522.1-08 Section 1.6.5.2.1.b requires that the unit weight
of the hardened concrete is within +-5% of the approved hardened unit
weight measured in test panels.
As indicated previously, the
specified in-place air void content was 17.5%. Extending ACI 522.1-08
in-place density requirements to air void content, the allowable range
would be from 12.5 to 22.5%. Air void contents measured using cores
ranged from 13.4 to 21.6%- well within the allowable range. A
comparison between estimated and in-place void contents is shown in
Table 1. Even though the estimated in-place unit weight test is highly
operator dependant, the mean of the test results was within 3% of the
mean of the values measured using cores (Fig. 5).
Table 2
compares the air void contents of the fresh concrete (measured using
ASTM C1688) and hardened concrete (measured using core samples). For
all five cores, the void contents measured per ASTM C1688 were lower
than the void contents found using the core samples.
Figure
6 shows the general relationship between void contents, as determined
per ASTM C1688, and permeability, as determined per ASTM C1701.
Permeability tests were not performed directly on the cores, as that
ASTM standard is under development. Because the same equipment and
methods were used to consolidate all pavement sections, Fig. 6 implies
that initial workability, which influences compaction, also influences
hardened permeability. The largest infiltration rate measured per ASTM
C1701 was 2016 in./hour (51,200 mm/hour) and the lowest was only 62
in./hour (1600 mm/hour). While our observation of an exponential
increase in permeability with increased void content is consistent with
observations made by others, the multi-operator reproducibility of the
test method is under evaluation.1,6 INDICATIONS
Our work for the MCC pavement project (Fig. 7) indicates that:
* Air void and unit weight tests per ASTM C1688 can be used to predict in-place air void content;
* The inverted slump cone test is a good predictor of mixture
workability and provides a rapid method for culling mixtures that will
have unacceptably low unit weights; and
* A requirement that
the in-place unit weight is within +-5% of the specified unit weight
(as per ACI 522.1-08) is appropriate and achievable.
For
workable mixtures that passed the inverted slump cone test, estimated
in-place unit weights correlated well with measured in- place air void
contents. Mixtures that met the specified unit weight of 133.5 lb/ft3
(2140 kg/m3) were very workable, although they might have been
considered too "wet" if evaluated by visual inspection only.
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