Design of Concrete Parking Lots
The thickness design of the concrete used for the parking lots is determined by combining many factors including:
The designer of a parking lot must be aware of what type of vehicles are going to use the facility once it is put into service. Usually, the pavement will be subjected to predictable but variable loadings in their lifetime. These loadings can be predicted by knowing if the lot is intended for passenger cars, light trucks, or heavy trucks. The number of trips that each vehicle type makes daily, vehicular loads, and the daily volume of vehicles is also important in design. Vehicles are classified into traffic categories which are used in determining the suggested thickness (ACI 330R). The following are the traffic categories as defined by ACI 330R.
Table 2.3 - Traffic Categories (Copyright ACI)
1. Car parking areas and access lanes - Category A (autos,pickups, and panel trucks only)
2. Truck access lanes - Category A-1
3. Shopping center entrance and service lanes - Category B
4. Bus Parking areas, city and school buses
Parking area and interior lanes - Category B
Entrance and Exterior Lanes - Category C
5. Truck Parking areas
Single units - Parking areas and interior lanes - Category B
-Entrance and exterior lanes- Category C
Multiple Units - Parking areas and interior lanes - Category C
- Entrance and exterior lanes - Category D
The subgrade is the surface that will lie underneath the pavement. The required thickness of the pavement depends largely upon the strength and uniformity of the subgrade. The ability of the subgrade to uniformly support loads that are applied to it from the pavement is very important. This is the proper goal of proper site preparation. Two important properties of the subgrade, the modulus of subgrade reaction, k, and the California bearing ration, CBR are used in a table to determine the recommended thickness.
The California bearing ratio and modulus of subgrade reaction are defined by the type of soil. Table 2.1 and Table 2.2 from ACI 330R give ranges for these values depending on soil type. These tables and definitions of the values involved in the tables from ACI 330R are found below:
modulus of subgrade reaction- stress per 1 in penetration of a circular plate into the subgrade and determined from the stress required to cause .05 in penetration of a 30 in diameter plate.
California bearing ratio - a bearing value for a soil that compares the load required to force a standard piston into a prepared sample of the soil, to the load required to force the standard piston into a well graded crushed stone.
R, Resistance value- The stability of the soil, as determined by the Hveem Stabilometer, which measures the horizontal pressure resulting from a vertical load
SSV, Soil Support- An index number that expresses the relative ability of a soil or aggregate mixture to support traffic loads through a flexible pavement structure
Table 2.1 - Subgrade soil types and approximate support values
The k value from this chart is then used in conjuction with Table 2.2 of ACI 330R to determine the modulus of subgrade reaction. Table 2.2 is found below.
Table 2.2 Modulus of subgrade reaction k (Copyright ACI)
The loads applied to concrete parking lots produce both compressive and flexural stresses in the slab; but the flexural stresses are more critical. These stresses can almost reach the flexural strength of the concrete, while the compresses stresses are small compared to the compressive strength of concrete. The flexural strength of concrete is determined by the flexure strength test, ASTM C78. A specimen is prepared in the lab or field with accordance to ASTM C192 or C31 respectively. The sample must have a square cross section and a span of three times the specimen depth. After molded, specimens are kept in the mold for the first 24 +- 8 hours, and then removed. They are cured at either 23+- 1.7°C in saturated lime-water or in a moist cabinet with a relative humidity of 95% or higher until tested. When tested, the specimen is turned on its side and centered in a three point loading apparatus. The load is continuously applied at a specified rate until rupture. The modulus of rupture is then calculated as R =(PL)/(bd)2 where P is the maximum load, L is the span length, b is the average width, and d is the average depth of the specimen.
The design of pavements is usually based upon the modulus of rupture, but it is more practical to use compressive strength testing. For large projects, a correlation between the two stresses should be found in laboratory tests, but on small scaled projects, a relationship between the modulus of rupture, MR ,and compressive strength, fc, can be computed by the formula MR=2.3 fc2/3 (ACI330R).
Once the modulus of subgrade reaction, modulus of rupture, California bearing ratio, and traffic category are determined; the recommended thickness for the parking lot could then be determined. Table 2.4 of ACI 330R relates all of these values, along with the average daily truck traffic or ADDT. This table can be found below.
Table 2.4 Twenty-year design thickness recommendations, in. (Copyright ACI)
Relationships between traffic categories, modulus of rupture, modulus of subgrade reaction, and the California bearing ratio can be deduced.
modulus of rupture -As the modulus of rupture decreases, the thickness of the concrete must increase to account for the loss in flexural strength.
California bearing ratio- As the California bearing ratio decreases, the thickness of the concrete must subsequently increase. This relationship exists because a higher CBR value indicates a more sturdy subgrade. Therefore as the CBR value decreases, more concrete must be present to distribute the load.
modulus of subgrade- As the modulus of subgrade reaction decreases, the thickness of the concrete must subsequently increase. This relationship occurs because the lower the modulus of subgrade reaction, the less the amount of stress that is required to cause a penetration into the soil
traffic category- The traffic category plays an obvious role in the thickness of the concrete. The higher the traffic category and average daily truck traffic, the thicker the concrete must be to support these larger loads.
Joints are used in concrete pavement to reduce random cracking and to facilitate construction. There are three types that are commonly used in concrete pavement: contraction joints, construction joints, and expansion or isolation joints. During its lifetime, concrete is subjected to physical changes in height, length, width, volume, and shape. These changes come from a variety of sources, such as shrinkage, carbonation, creep, chemical reactions, or just the application of a particular load on the pavement. The joints relieve the internal stresses and minimize the appearance of self-formed cracks by accounting for the movement without loss of the integrity of the structure ("Joints and..). Except for isolation joints, all joints provide a way to connect slabs mechanically. This helps with the distribution of the loading on one slab to the surrounding slabs, decreasing the stress in the slab and increasing the lifetime of the concrete (American Concrete Pavement Association) The different uses for the three types of joints are described below.
Contraction joints are predetermined cracks placed in the concrete during construction. Hardened concrete shrinks almost 1/16 in for every 10 ft of length while drying. If this shrinkage is restrained, tensile stresses form which can near the tensile strength of the concrete, producing cracks. This is where the contraction joints role comes in. Contraction joints create planes of weakness in predetermined locations that will form cracks as the concrete shrinks (ACI 330R). They are made during construction either by forming the joint with a strip of wood, metal, or plastic, or they can be cut with a saw after hardening (Joints and...). The depth of the joint depends on the thickness of the concrete, but should generally be a quarter of the slab when using a saw.(ACI 330R)
Contraction joints are further subdivided in name depending on the direction in which they run. In parking areas, longitudinal joints are those that are parallel with the direction of paving, while transverse joints are those with divide the paving lanes into separate panels. The pattern of the contraction joints are usually square and should be continuous across lanes(ACI 330R). Some agencies do skew transverse contraction joints away from being perpendicular to decrease the dynamic loading across the joints by getting rid of the simultaneous crossing of the two wheels on one axle of a car ("Joints").
Construction joints are used to join concrete that was placed at different times. They are also designed for in very large jobs, where interruptions in placement is unavoidable ("Joints and..). These joints can be keyed or butt type. Butt-type joints do not aid in load transfer, but this is usually not needed in parking lots that serve light vehicles, but should be considered under heavy traffic. Keyways do provide load across construction joints, and proper dimensions are important if they are used to avoid creating a weak joint. Transverse construction joints are used for interruptions in paving, whether it be the end of a day, weather, mechanical failure, or any other reason.
To separate pavement from other structures or objects and allow independent movement of the pavement and structure without any connection that could cause damage, isolation joints are used. They are used where pavement meets manholes, buildings and sidewalks, and drainage fixtures ("Joints"). These are vertical joints that run the full depth, and compressible material is used to fill them. They are also referred to as expansion joints, but are rarely necessary to accommodate the expansion of concrete.
Pre-molded joint fillers prevent new slabs from bonding to other objects during and after the placing of the concrete. This filler extends through to the subgrade and is recessed below the surface of the pavement so it can be sealed with a joint-sealant material. Types of materials used to fill joints include bituminous mastic, bituminous impregnated cellulose, or cork, sponge rubber, and resin-bound cork.
This type of joint is not generally used as a regularly spaced joint. Construction and maintenance is difficult, they provide no load transfer, and can actually lead to pavement distress, distortion, and premature failure. Also, their role to accommodate expansion is not needed when contraction joints are properly placed throughout (ACI 330R).
"Guide for Design and Construction of Concrete Parking Lots." ACI 330R. American Concrete Institute, 2001.
"Joints and Their Functions." United States Army Corps of Engineers. 2 April 2005. <http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2102/c-2.pdf>.
"Joints." American Concrete Pavement Association. 2 April 2005. <http://www.pavement.com/PavTech/T ech/Fundamentals/fundjoints.html>.
Villemagne, Maurice and Yves Charonnat, eds. Cement Concrete Pavements. Paris: A.A. Balkema Publishers, 1996.