Synthesis of design and construction practices
Table 1. Typical Properties of Rigid Bases Used in Spain
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Table 1. Typical Properties of Rigid Bases Used in Spain Rigid Base 7-Day Compressive Strength E-modulus Description Soil-Cement > 2.5 MPa
(> 360 psi) 6,000 to 10,000 MPa
(870 to 1,450 ksi)
b
Granular material + cement content 3 to 7% Gravel-Cement > 4.5 MPa (> 650 psi) 20,000 MPa (2,900 ksi) No fine material and a dense gradation Gravel-Cement Type II
> 8 MPa (> 1,160 psi) 25,000 MPa (3,600 ksi) Similar to gravel-cement, except for a higher cement content 5 to 7% Lean-Mix Compacted Concrete > 12 MPa (> 1,740 psi) 25,000 MPa (3,600 ksi) Cement content 5 to 10%. Similar to the RCC in the U.S. Compacted Concrete > 18 MPa (> 2,600 psi) 33,000 MPa (4,790 ksi) Cement content 10 to 14% a 1 MPa = 145.04 psi; b 1 ksi = 1,000 psi Composite Pavement Design
This section discusses the various methods currently in use to design composite pavement structures.
The 1993 AASHTO Guide for Design of Pavement Structures can be used to design two different composite pavements: (1) a new flexible pavement with a cement-treated (or soil- cement) base and (2) a rehabilitated PCC pavement using the section in the guide for the design of AC overlays of PCC (both jointed plain concrete pavement [JPCP] and continuously reinforced concrete pavement [CRCP]).
In the first alternative, it is critical to select a proper layer coefficient, a 2 , for the stabilized base to use the flexible SN design equation:
1 1 2 2 2 3 3
3 SN a D
m a D m a D
= + + (1)
where
SN = structural number
a 1 , a
2 , a
3 = layer coefficients
m
, m 3 = drainage coefficients D 1 , D 2 , D
3 = thickness of each layer in inches (layer 1 = HMA, layer 2 = base, layer 3 = subbase)
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A study performed by Richardson (1996) provides a general equation that could be used to determine the modulus, E c , of various cemented materials (e.g., soil cement, cement-treated bases, cement-stabilized soils) and with that, compute the layer coefficient a 2 . Once the cemented material coefficient and all other needed parameters are obtained, the composite structure can be designed.
0.7784 c u E 34.367 2006.8 q = − +
(2)
( ) 2 c a 2.7170 0.49711 Log E = −
+ × (3) where
E c = chord modulus (MPa) q u = unconfined compressive strength (MPa).
The second alternative for using the AASHTO 1993 guide is based on the procedure for designing the rehabilitation of PCC pavements with an AC overlay. In this case, the first step is to design a conventional PCC pavement, in other words, compute the thickness to satisfy the future traffic demand, D f . Once the slab thickness has been obtained, it could be assumed that placing an AC layer with a thickness of approximately 50 mm (2 in) would allow for the decrease of 25 mm (1 in) of PCC layer. This is because the guide’s “AC Overlay of PCC Pavement” procedure indicates that the required thickness, D OL , of an AC overlay of PCC is calculated using the following equation:
( ) OL f eff
D A D
D = − (4)
Where
A = factor to convert PCC thickness deficiency to AC overlay thickness D f = slab thickness to carry future traffic (in)
D eff = effective thickness of existing slab (in).
Therefore, two assumptions are made. First, in a new composite pavement design, D eff is
equal to D f because it is appropriate to assume that a newly constructed PCCP would not have any distress, thus none of the adjustment factors shown in Equation 5 would be applicable.
eff jc dur fat D F F F D = × × ×
(5) Where
D = original slab thickness (this would be equal to the thickness of the rigid base) F jc , F dur
, F fat
= adjustment factors for joints and cracks, durability, and fatigue = 1.
The second assumption involves the A factor from Equation 4. According to the guide, the A factor is computed using the following equation:
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( ) ( ) 2 f eff f eff
A 2.2233 0.0099 D D 0.1534 D D = + − − − (6)
Assuming that D f = D
eff , a conservative value of A = 2.2233 would be obtained. Lower A values, and consequently HMA thicknesses, may be obtained if using the actual D f and D eff values. For example, a 150 mm (6 in) HMA layer is required to substitute an HMA thickness of 87.5 mm (3.5 in) of PCCP in the example considered in this report. Once the overlay thickness is computed, it is typically rounded to the nearest 0.5 in.
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