UFC 3-260-02

30 June 2001

must be provided by a stabilized base, keyways, mechanical load transfer devices or aggregate

interlock.

d. Structural Slab Cracking from Aircraft Loadings. The cracking of a nonreinforced jointed

concrete slab with relatively short joint spacing is controlled by:

(1) The magnitude of flexural stress caused by aircraft traffic.

(2) The flexural strength of the concrete.

(3) The number of stress applications.

The number of allowable stress applications to crack the concrete slab is controlled by the ratio of critical

stress to flexural strength of the concrete. The relationship used in this design procedure to relate

stress/flexural strength ratio to the number of stress applications to cracking was developed by the PCA

and is shown in Table 12-2. The lower the ratio of critical stress to flexural strength, the larger the

number of load applications that the slab can carry before cracking occurs.

e. Structural Slab Cracking and Mixed Aircraft Loading. When two or more aircraft will utilize a

given pavement, each may cause a certain amount of fatigue damage in the concrete slab. The effect of

mixed traffic can be provided for in the pavement design by using Miner's cumulative fatigue damage

procedure. Fatigue damage is defined as the ratio of the number of loading cycles actually applied (at a

given stress level) to the number of allowable load applications to cracking failure (at the same stress

level). The resulting fraction represents the proportion of the useful life of the concrete that is consumed

by repeated loading.

'j

(12-1)

where

ni = number of applied loads (coverages) at a given stress level (as denoted by i)

Ni = number of allowable loads (coverages) at the same stress level to cracking of the

concrete

The fatigue damage can be accumulated over any number of stress levels (or different aircraft loadings)

as indicated by the summation sign.

f. Thickness Design Inputs. Five key design inputs are needed to determine the required slab

thickness.

(1) Design concrete flexural strength. The 28-day third-point loading flexural strength is used

for pavement design. The design flexural strength should be as high as practicable and economical but

not less than 4.48 MPa (650 psi). The actual mean flexural strength in the field will be greater than the

design flexural strength.

(2) Value of k at top of base. The k value on the subgrade and at the top of the base layers will

be determined using the procedure presented in Chapter 5. The value used for design is that obtained

at the top of the base. The combined base and subgrade should have a minimum design k value of

12-7

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