Evaluation of As-Built Airfield Hangar Slab Capacity under - EJGE

12 Pages·2013·0.95 MB·English

Evaluation of As-Built Airfield Hangar
Slab Capacity under Wide Body Aircraft
1 2
G.G. Marino, Ph.D., P.E., E. Tutumluer, Ph.D.,
3 4
M.M. Elgendy, P.E. and B. Armaghani, P.E
1: Marino Engineering Associates, Inc., 1101 E. Colorado Ave., Urbana, IL
61801; PH: [phone]; FAX [phone]
e-mail: [email protected]
2: Professor and Corresponding Author, Department of Civil and Environmental
Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801,
PH: [phone]; [email protected]
3: Marino Engineering Associates, Inc., 1101 E. Colorado Ave., Urbana, IL
61801; PH: [phone]; FAX [phone]; e-mail:
[email protected]
4: Marino Engineering Associates, Inc., 1101 E. Colorado Ave., Urbana, IL
61801; PH: [phone]; FAX [phone]; email: b-
[email protected]

ABSTRACT
The paper describes an investigation of the structural loading performance of a newly
constructed concrete slab in an airfield hangar facility under wide body aircraft. It was
determined during this investigation that the prepared soil subgrade did not meet the design
requirements. This in-turn became problematic when installing the unbound aggregate subbase
which required a very high compaction effort in order to meet the design modulus of subgrade
reaction. To assess the as-built capacity of the slab, a detailed investigation was undertaken
which included measuring the deflection of the slab and joint under parked wide body aircraft
such as the Boeing 777, as well as a series of subgrade soil and pavement testing activities. The
measured field evaluation data were used in numerical analyses to determine the ability of the
reinforced concrete slab to handle repeated loading from housed aircraft. The analysis findings
demonstrated that the concrete pavement would perform satisfactorily.
INTRODUCTION
This paper describes a construction investigation of an as-built reinforced concrete slab and its
substrate in an airfield hangar facility. The hangar slab was intended to support wide body aircraft
for maintenance. The investigation was necessary as concern existed that the constructed slab
condition might be below that required to support the intended aircraft. A plan showing the hangar
slab is provided in Figure 1.
The 16-in. reinforced hangar slab design was in accordance with the FAA specifications (see
Figure 2). Based on the design requirement to achieve a composite modulus of subgrade reaction
of 200 pci, 1.0 ft of unbound aggregate subbase compacted to 100% modified Proctor at ± 2%
optimum moisture content (OMC) was specified. The subbase design assumed that the soil
modulus of subgrade reaction would be 125 pci based on a tested CBR value of 7.7. To achieve
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this modulus of subgrade reaction (k) value, at least the top 1.0 ft of the native surface and all
structural fill was to be compacted to 90% modified Proctor at ± 2% OMC.

Figure 1: Plan showing the variation of the subgrade soils across the hangar slab

Figure 2: The design cross-section of the hangar slab
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SOIL PROFILE CONDITIONS
Figures 1 and 3 show the plan and cross-section of the soil subgrade conditions. As shown in
cross-section A-A in Figure 3, the hangar slab area was in a cut. Across the slab area, a cut of
about 5 to 25 ft. west to east was required. However, at the time of the slab construction, the site
was already graded to close to subgrade level. These more recent grades indicated that up to
several feet of fill would be required in the western portion of the slab area to bring the subgrade to
design level.

Figure 3: Cross-section A-A of soil subgrade conditions
Using all the data available, soil subgrade conditions in plan (including fill area) are shown in
Figure 1. As depicted in Figure 1, there are three main soil types across the slab: lean clay, sandy
clay, and clayey sand. There were also subgrade inspection records which indicated the presence
of subgrade soil consisting of red to yellow sand. These compaction reports were only for the
backfill of utilities. Also an accurate location of these lines and therefore these materials was not
known.
Based on the boring information, the soil subgrade soils (to a depth of at least 2 ft.) consist of a
brown silty clay trace to some medium fine sand (loess) to Bay 9 (see Figure 1). To the east of
Bay 9, the subgrade soils appear to grade from sand with possibly some fines, to a clayey sand to a
sandy clay. The more sandy material appears to be present in the east central portion of the slab
area. The eastern subgrade soils are multicolored, mainly yellow, orange brown, red brown, and
brown.
For the brown silty clay with trace to some medium fine sand, the Standard Penetration Test
(SPT) values range from 6 to 17 blows per foot (bpf) (11 bpf average from 11 tests) except for one
outlier of 39 bpf which was affected by subgrade compaction. The Liquid Limit and Plasticity
Index in these loessial soils ranged from 30% to 43% (37% average from 10 tests) and 12% to
24% (17% average from 10 tests), respectively.
One of the eastern subgrade soils is a fluvial sandy clay. During the design investigation
program, SPT values in this material near subgrade elevation ranged from 11 to 29 bpf (18 average
from 8 tests excluding one outlier). After the slab was poured, the SPT values taken were 10 and
14 bpf and were overall lower than prior to construction. The Atterberg Limits for this material
had Liquid Limits of 26% to 34% (29% average from 5 tests) and Plasticity Indices of 10% to 20%
(14% average from 5 tests), respectively.
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As discussed above, in other subgrade location clayey sand is encountered. SPT values in this
material were found to range from 11 to 17 bpf (14 bpf average from 3 tests). The clayey sand had
Liquid Limits of 25% and 39% and Plasticity Indices of 7% and 23%. Further, the other eastern
subgrade soil was red or yellow sand. SPT values in these sand layers closest to the finished
subgrade elevation were generally between 30 to 40 bpf. Based on the available data, the
groundwater table appeared to be about 4 ft. below final slab subgrade elevation.
AS-BUILT SUBGRADE CONDITIONS
During the period of time when subbase was placed over the subgrade from Bay 1 to Bay 15,
there is no record of the specified compaction that the underlying subgrade had taken place except
for in utility trenches. Field compaction reports indicated that the specification applied to the
trench fill was a minimum 95% standard Proctor compaction used with essentially no moisture
control. The dearth of the compaction testing was discovered after visual observation of pumping
of the subbase. This initiated the subgrade remediation operations.
During the remediation phase, the compaction of the subgrade soils was checked by
uncovering points per bay. Assuming the surrounding area was representative of these test points,
these areas were determined if they met specifications. If a test point did not meet specifications, it
was processed and re-compacted. The compaction standard used for all the subgrade was 90%
compaction of an assumed maximum density of 119.8 pcf (or a minimum dry density of 107.8 pcf)
and without moisture control. Subsequently, the subbase installed and the 16-in. reinforced slab
was poured.
AS-BUILT SUBBASE CONDITIONS
The original design required the FAA P-209 subbase aggregate layer to be compacted to at
least 100% the modified Proctor (ASTM D1557) maximum dry density, and within 2% optimum
moisture content. However, it was discovered during the period of investigation of the constructed
conditions, that when attempting to compact the subbase to 100% compaction, the prepared
subgrade would eventually breakdown and all densification efforts would be lost. From field tests,
it was determined that only compaction efforts to reach at least 95% compaction (D1557) were
practical.
Construction records indicated that when the subbase was finally compacted, passing tests
were from 95% to 97% the modified Proctor compaction. Passing moisture contents were 3.7%
below to 0.3% above optimum moisture with a range of 2.3% to 6.3%. The P-209 subbase was
tested to have a maximum density of 144.2 pcf and an optimum moisture content of 6.0%. Post-
construction borings indicated that the in-place subbase was typically 10 in. to 12 in. but was
somewhat contaminated with subgrade soil towards the base, despite the presence of underlying
geofabric (see Figure 1). This can be explained by the stockpiling and reworking of the P209
aggregate material during the remediation process.
SUBGRADE DETERIORATION EFFECTS
Moisture Exposure. Because the hangar roof was constructed after slab construction, the
substructure was exposed to direct precipitation after the preparation of the subgrade and subbase
and prior to pouring the hangar floor slab. Using the as-built schedule for the hangar slab and the
precipitation records, the amount of rain which fell on various slab areas was estimated as
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summarized in Figure 4. It was assumed in preparing Figure 4 that the subgrade surface was
drained and any soft/wet materials were removed prior to the placement of the subbase, thus
limiting precipitation effects. Bays 1 and 2 were completed before subbase and subgrade problems
were discovered and were consequently exposed to limited precipitation of less than 0.1 in. (see
Figure 4). The time to pour these two bays was less than one month after subgrade preparation.
This slab area was not removed for remediation since no aircraft wheel loads were expected in this
area.

Figure 4: Approximate exposure of subbase and subgrade to precipitation
After placing the subbase to the start of the remediation, there was 10.3 in. to 17.3 in. which
fell on the aggregate material in each bay. Soil subgrade remediation was reported for Bays 3 to 8.
Note that this includes an area of fill where the brown silty clay subgrade existed (see Figure 1),
which, as presented later, has egregious moisture sensitivity. After the start of the placement of
subbase less than 0.5 in. fell on Bays 3 and 4 but the remainder of the bays the precipitation totals
were 1.5 to 3.5 in. From Bays 9 to 15, the subbase covered the subgrade for a long duration with
no subgrade remediation and in turn where significant rain accumulations occurred to the time the
slab was poured. Total precipitation for these eastern bays ranged from about 16 in. to 54 in. It
should be noted that although Figure 4 provides the rain fall per bay, this related groundwater will
drain laterally and under already poured sections of slab. Evidence of pooled water on the
subgrade was found during a post-construction investigation.
Moisture Stability. To assess the post-construction condition of the subgrade, CBR testing
was performed on the various subgrade materials as depicted in Figure 1. The CBR tests were run
at ranges of compacted moistures and densities. The results of these tests are shown in Figure 5.
The test results showed that when considering the design of at least 90% modified Proctor
compaction within 2% of OMC, there is a dramatic drop in strength in the finer grained subgrade
soils once they are soaked. The most egregious loss within the specified range was for the silty
clay and the sandy lean clay soils. For these materials, the CBR strengths dropped to about 1 to 3
(generally dry to wet) from unsoaked strengths were from about 20 to 85 (wet to dry). Even the
clayey sand subgrade soil showed poor moisture stability where the strength dropped to about 1 to
4 (dry to wet) upon soaking from unsoaked values of 21 to 124 (wet to dry). For the subgrade sand
only soaked tests were run with CBR values in the range of 9 to 19 (wet to dry).
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To better understand the strength lost, one dimensional swell-consolidation tests were
conducted at a vertical load of 150 and 300 psf. These results are summarized in Table 1. The
results show that when the finer grained soils were compacted on the dry side, they exhibited a
swell potential when soaked exacerbating the strength loss and explaining lower soaked CBR
strength trends from wet to dry. Except for the subgrade sand, swell pressures and preload strains
were typically 3 to 30 ksi and 3 to 8%, respectively, when compacted within the specified range.
Post-prepared subgrade swell can also explain the subbase or concrete thickness in places as
observed below the specified. The inconspicuous swelling upon exposure to moisture raises the
grade and in turn results in cutting the stone and for concrete thickness in order to meet the
specified floor surface elevation.
SLAB DEFLECTION TESTING
Because of the exposure of the slab subgrade to significant moisture, as discussed above, the
soil subgrade soils were considered to be in a soaked state, or in other words, have essentially
reached their weaker state, which is the ultimate capability of the slab and substrate to handle the
expected aircraft loads that may be present. Consequently, the realistic ultimate slab support
potential could be assessed by performing a number of Heavy Weight Deflectometer (HWD) tests
on the slab and slab joints. There were 180 slab and 186 load transfer efficiency (LTE) tests
performed across the hangar slab.
The deflection results for the HWD tests are depicted in Figure 6. As can be seen in Figure 6,
the load deflection results were normalized to a standard drop weight of 25 kips. Measured slab
deflections ranged from 1.98 mil to 6.62 mil (standard deviation σ = 0.86). LTE ranged from 44%
to 100% and averaged 79% (standard deviation σ = 12.9).

Figure 5: Continues on the next page
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Figure 5: Continues from the previous page

Figure 5: CBR test results for various hangar slab subgrade soils
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Table 1: Swell potential of subgrade soils at dry side moisture contents and various dry
densities
Before Test Swell Data
DD @ Swell
Subgrade Soil DD(pcf % Compacti MC ∆ Preload Max. Pressure Swell
OMC
) Compacted on Type (%) (psf) Swell (psf) (%)
(%)
(pcf)
119.6 90 Mod 6.1 -2.30 300 114.0 6300 4.7
118.5 89 Mod 7.1 -1.30 150 110.0 12000 7.2
116.1 88 Mod 6.3 -2.10 300 110.9 7800 4.5
Clayey Sand
124.9 94 Mod 7.1 -1.30 300 116.4 40000 6.8
(SC)
125.2 94 Mod 6.9 -1.50 150 115.1 32000 8.0
118.8 99 n/a 8.7 -0.30 300 115.0 6000 3.2
120.5 100 STD 6.6 -5.30 300 114.0 18000 5.4
111.6 90 Mod 7.2 -2.20 300 111.7 n/a -0.03
Sand, Little Silt 117.5 95 Mod 7.4 -2.00 300 117.6 n/a -0.01
and Clay (SC) 118.6 99 n/a 8.8 -0.20 300 118.5 500 0.05
120.2 101 STD 5.3 -5.00 300 120.1 700 0.1
108.3 90 Mod 8.2 -2.00 300 102.5 3200 5.4
108.2 90 Mod 8.4 -1.78 150 104.0 2400 3.9
114.5 95 Mod 8.0 -2.20 300 107.1 7100 6.5
Sandy Lean
114.2 95 Mod 8.3 -1.90 150 104.2 12000 8.8
Clay (CL)
26000(es
118.5 99 n/a 12.9 -0.10 300 114.0 3.8
t.)
110.6 100 STD 11.3 -5.20 300 105.8 2000 4.3
106.0 90 Mod 11.0 -2.30 300 102.8 9500 3.0
40000(es
111.3 95 Mod 11.4 -1.90 300 108.5 2.7
Silty Clay Trace t.)
Sand (CL) 111.2 95 Mod 11.5 -1.82 150 106.7 31000 4.0
118.3 99 n/a 13.0 0.00 300 116.3 17000 1.7
109.4 100 STD 11.0 -5.20 300 106.2 12000 2.9
DD = Dry density MC = Moisture Content ∆ OMC = Deviation from optimum moisture

ANALYSIS OF SLAB PERFORMANCE
The induced stresses from the anticipated wide body aircraft loading on the hangar slab was
analyzed using an FEM program called ISLAB2000. The assumptions followed by the results are
provided below.
Subgrade and Subbase Assumptions. From analyzing the project conditions discussed
above, the subgrade can be summarized into the western (Bays 1 to 8) and eastern (Bays 9 to 16)
areas. In the western area, there is primarily a silty clay (loess) subgrade with soaked CBR values
of 1 to 4. As the makeup of the eastern portion of the slab subgrade ranges from a sandy clay to a
sand with little fines, the soaked CBR values again range from 1 to 4 but can be as high as 19.
Using ACI (1997) and considering 0.05-in deflection of a 30-in. plate, the modulus of subgrade
reaction (k) ranges from less than 25 to 100 pci (for CBR = 1 to 4) up to 290 pci (for CBR = 19).
The constructed thickness of the subbase was found to be on the order of 10 in. to 12 in. but
the lower portion of the P209 subbase was found to be contaminated with subgrade fines.
Considering 6 in. of clean stone, the composite modulus of subgrade reaction, K, would to range
from 40 pci to 150 pci (soil CBR = 1 to 4) up to 325 pci (soil CBR = 19). The AC 150/5320-6D
CH61 was used to determine K.
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Figure 6: Normalized Deflection Values from HWD Tests
Slab Assumptions. Based on the construction reports of the hangar slab, it was determined
that a 16-in. reinforced concrete slab can be assumed. Also from the numerous concrete test
results, the average concrete compressive strength (f′c) was 6,286 psi with the average modulus of
rupture (Mr) of 721 psi. Therefore, f′c and Mr were considered to be 6,200 psi and 720 psi,
respectively. The reinforcing steel was assumed as specified with top and bottom layers in the
jacking pad areas.
Aircraft Load Assumptions. The hangar slab had to support a Boeing 777-200LR aircraft. This
was fortunate, and made the use of the as-built hangar slab feasible, as the design called for
support of the heavier and more robust A380 airbus. The as-built slab was evaluated for various
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gear locations on the slabs considering the empty weight of the plane (326,000 lb) and a tire
pressure of 218 psi. The six gear locations analyzed are depicted in Figure 7. Also examined were
the loads applied when the plane is jacked off the ground. Each jack aircraft tripod that would be
utilized had 315-in. diameter pads equally spaced in a 67-in. radius. Based on the analysis of the
plane gear and jack loading conditions, the plane gear represented the more critical loading
conditions and was used in the FEM analysis.
Slab Performance Analysis. The layout of the joint system and floor drains in the hangar
slab are depicted in Figure 6. Because of presence of the floor drains, the hangar slab was
simulated using two rows of slab sections with unrestricted boundaries on each of the long ends of
the model (or at the drains locations). The jointed slabs were taken as 370 in. by 292 in. (see
Figure 7).
To assess the range of slab deflection and in-place stress conditions which could exist in the
slab at each load location given above, K was considered at 50, 100, 150, and 200 pci and a LTE
of 50, 65, 80, and 95%. The computed results from these analyses are provided in Table 2. For
each case, the maximum and minimum, deflection and in-plane stress values are highlighted in
bold. An example of the deflection and stress distribution for the case of loading 2 adjacent slabs
(LTE = 50% and K = 50 pci) is provided in Figure 8. As can be seen in Table 2, the most critical
case was at Load Location 1 with K=50 pci where the wheel loads are concentrated at the free
corner and adjacent to the floor trench drain (see Figure 7). Considering these results, the ratio of
the maximum in-plane stress to the Modulus of Rupture (σ/Mr) is 0.63. According to Huang
(2004) this is equivalent to at least 10,000 wheel repetitions to concrete failure, and is below the
number of repetitions expected across the life span of the hangar slab.

Figure 7: Axle locations used in analysis for maximum slab deflection
and maximum stresses