The Concrete Truth: Building and Designing Parking Lots That Last

 Parking Lot Construction

Design Procedure

The layout of the parking lot will be performed by your Architect.

 At this time, you want to perform your Geotechnical investigation of the soil on the project. The surveyor lays out your property and marks the locations for soil borings. 

 Consult with your Geotechnical Engineer on the number of soil borings and the depth of the soil borings.

A good rule of thumb is to perform at least one boring for every 10,000 to 20,000 square feet of the proposed parking lot. However, this number can increase for larger or more complex sites. For a simple, small parking lot, a minimum of three to five borings is often recommended.

Borings should be strategically placed to provide a representative sample of the entire site. It's crucial to take borings at the corners and in the center of the proposed lot and in any areas with planned drainage systems, retaining walls, or other heavy structures. Additional borings may be necessary in areas with noticeable changes in ground slope, historical fill, or visual signs of poor soil conditions.

For a standard parking lot, borings typically extend 5 to 10 feet below the finished grade. However, the exact depth should be determined by the geotechnical engineer based on the specific site conditions. The borings need to be deep enough to determine the properties of the subgrade soil that will support the pavement and to identify any problematic layers, such as expansive clays, soft soils, or high groundwater.

For areas with heavier loads, such as truck lanes or garbage truck pads, the borings need to be deeper to ensure the stability of the underlying soil. The final depth should always be sufficient to allow the engineer to provide recommendations for pavement design, subgrade preparation, and drainage.

Your civil engineer will use the Geotechnical report to design the parking lot paving.  

What is the use of the parking lot?

Light Vehicles 

Heavy Truck Traffic

Traffic Loading

Design Index Categories for traffic

Design Index	General Character	Daily EAL
DI-1	Light traffic (few vehicles heavier than passenger cars, no regular use by group 2 or 3 vehicles)	5 or less
DI-2	Medium light traffic (Similar to DI-1, maximum 1,000 VPD, 2 including not over 10% group, no regular use by group 3 vehicles)	6-20
DI-3	Medium traffic (maximum 3,000 VPD, including not over 10% group 2 and 3, 1% group 3 vehicles)	21-75
DI-4	Medium heavy traffic (maximum 6,000 VPD, including not 15% of group 2 and 3, 1% group 3 vehicles.)	76-250
DI-5	Heavy traffic (maximum 6,000 VDP, including 25% of group 2 and 3 vehicles, 10% group 3 vehicles)	251-900
DI-6	Very heavy traffic (over 6,000 VPD, may include over 25% of group 2 or 3 vehicles)	901-3000
	Note: 1) EAL = equivalent 18 kip axle loads in design
	lane, average daily use over life expectancy of 20
	years with normal maintenance 2) VPD = Vehicles per day, all types, using design lane

A soil boing example showing fat clay under the pavement with a 64 PI (Plastic Index). This is a highly expansive soil.

Soil Analysis and Design:

1) A geotechnical engineer first takes soil samples to analyze the soil's properties, including its plasticity and moisture content. The Geotechnical Soil boring example that shows fat clay under the pavement with a 64 PI (Plastic Index). This is a highly expansive soil.

2) A geotechnical engineer would then recommend lime stabilization for the soil (hydrated lime, quicklime, etc.) and the optimal percentage to add to the soil to give it strength.

The goal of lime stabilization is to improve the properties of a soil—typically a fine-grained, clay-rich soil—to make it a more suitable material for building foundations, roads, and other structures.

The procedure is to get the unstable soil to a pH of 12.4.

 To sustain the soil's strength, you need to use a scientific, two-part process that involves testing the soil, adding the correct amount of lime, and then verifying the results. The pH of the soil is directly related to its strength because lime acts as a chemical binder that transforms the soil's properties.

---

How to obtain a pH of 12.4

1.      Initial Soil Testing: First, a geotechnical firm or lab must take samples of the soil and test its natural pH. This is done with a pH meter or pH strips. This initial test is crucial to determine how much lime is needed.

2.    Lab-Based Mix Design: The lab will then perform a mix design by adding varying percentages of lime to small soil samples. They will test each mixture to determine the exact percentage of lime required to raise the soil's pH to 12.4 and maintain it.

3.    Lime Application: Based on the lab's findings, the correct percentage of quicklime or hydrated lime is added to the soil. This is often done with a lime spreader truck to ensure an even application across the entire area.

Post-Application Testing: After the lime is mixed into the soil and allowed to hydrate, the soil is tested again to confirm that the pH has reached the target of 12.4. This final test ensures the proper chemical reaction has occurred.

  What pH Has to Do with Soil Strength

A pH of 12.4 is not an arbitrary number; it's the specific pH required to unlock the chemical reactions that give the soil its increased strength. This process is called lime stabilization.

The Standard Procedure for Lime Stabilization

The process typically involves these key steps:

1. Soil Pulverization:

The area to be treated is first pulverized to break down large soil clumps.

This is often done with a pulvimixer, rotary mixer, or a disc harrow. The goal is to prepare the soil so that the lime can be evenly mixed with it.

2.Lime Application:

The specified amount of lime is spread evenly over the surface of the pulverized soil.

This is typically done using a lime spreader truck or a similar machine to ensure a consistent application rate. This process must be done on a calm day to prevent the lime from blowing away.

On Dry Conditions, add water

Initial mixing 

Immediately after the lime is applied, it is mixed into the soil using a rotary mixer or a similar piece of equipment. 

The mixing is done in two passes. The first pass is a "dry mixing" to incorporate the lime and soil. 

Curing Period

After the initial mixing, the lime-soil mixture is allowed to rest for 24 to 72 hours. This is known as the curing period. During this time, the lime reacts with the water in the soil in a process called hydration, which increases the plasticity and reduces the swelling potential of the clay. 

Final Mixing and Compaction

After the curing period, the soil is mixed one last time to ensure the lime is completely and evenly distributed. This is a crucial step to achieve the desired soil properties. Finally, the soil is compacted to the required density using a compactor or roller. 

The subgrade preparation in the pavement areas should specify compaction of the upper 1/8" to at least 95% of maximum standard proctor density (ASTM D698) at a moisture content between optimum and +3% of optimum moisture content. The laboratory would have to take a sample of the stabilized soil to conduct a protor test. 

The standardized process ensures that the soil is properly treated to achieve the necessary strength and stability for construction. 

Lime Testing

* In-place depth tests are to be conducted every 50' using phenolphthalein. 

* Density tests every 50'. 

* Gradation tests every 50'. 

 

Steel Reinforcing
Traffic Design Index (DI)	Steel Reinforcement
DI-1	#3 bars spaced at 18" or #4 bars spaced at 24" on center both ways
DI-2	#3 bars spaced at 12" or #4 bars spaced at 18" on center both ways
DI-3	#4 bars spaced at 18" on center both ways
DI-4 and DI-5	#4 bars spaced at 12" on center both ways

Pre-pour meeting

* A mandatory on-site meeting with the contractor, engineer, inspector, and ready-mix supplier. This meeting confirms all logistics, including the mix design, pour schedule, access points for trucks, and safety protocols. 

* The contractor must submit a Mix Design to the engineer for approval. All concrete trucks supplying concrete to the site must have their concrete ticket match the concrete mix design.  

* Verify the Geotechnical Company has a technician on-site performing slump tests on the concrete. The concrete slump should be no more than 6". 

* Verify that they are making cylinders for the compression test at their laboratory. Confirm with the Geotechnical Company on the breaking schedule of the cylinders. Typically, two are broken a 3 days, two at 7 days, and then two at 28 days. 

* Confirm with the Geotechnical laboratory the results of the concrete breaks. 

Per ACI (American Concrete Institute) 318: Concrete in an area represented by core tests shall be considered structurally adequate if the average of 3 cores is equal to at least 85% of the fc (Specified compressive strength of the concrete in 28 days) and no single core is less than 75% of fc. Additional testing of cores extracted from locations represented by erratic core strength results shall be permitted. 

Allow no water to be added at the job site. 

* One gallon of water increases the slump by 1".

* One gallon of water reduces the compressive strength by 250 psi.

* One gallon of water wastes 25lbs of cement. 

* One gallon of water increases shrinkage by 10%.

* One gallon of water increases the permeability by 50%.

* One gallon of water reduces freeze/thaw resistance by 20%

* One gallon of water reduces salt scaling resistance. 

* One gallon of water increases the cracking by approximately 10%.

* One gallon of water increases the air content by approximately 1%.

* One gallon of water increases the wear damage from traffic. 

* One gallon of water increases dusting. 

* One gallon of water increases the finishing time for contractors.  

Hot Weather Concrete

Hot weather is generally not defined by temperature alone, but by a combination of conditions that accelerate the rate of moisture loss and cement hydration (the chemical reaction that hardens concrete. 

Conditions include;

* High ambient air temperature.

* High concrete temperature.

* Low relative humidity. 

* High wind speed

* Strong solar radiation

There is generally no single air temperature at which all concrete placements is universally stopped. However, most standards and industry practices recommend taking precautions when temperatures exceed a certain threshold, and there are often maximum concrete temperature limits. 

Maximum Recommended Concrete Temperature

The American Concrete Institute (ACI) recommends a maximum temperature of fresh concrete at the time of discharge not to exceed 95 degrees unless supporting field experience or preconstruction testing is available. Some local agencies may use a lower threshold, such as 85 degrees. 

Air Temperature for Precaution  

Construction professionals typically begin to implement "hot weather concreting" procedures when the air temperature exceeds 85 degrees, or when other combined factors (like high wind/low humidity) are present. 

Instead of an absolute "stop" temperature, the focus is on mitigation. When temperatures are high, concrete producers and contractors use special techniques to control the concrete's temperature and moisture loss, such as;

* Using chilled water or replacing a portion of the mixing water with ice.

* Scheduling pours for early in the morning or evening hours.

* Using chemical admixtures (like set retarders) to slow down the hydration process. 

* Protecting the concrete surface from wind and sun by applying curing compounds immediately after finishing. 

What is Cold Weather Concreting?

The main concern in cold weather is the rate of hydration (the chemical reaction between the cement and water that causes concrete to set and gain strength) and the risk of freezing the fresh concrete. 

Procedures and Precautions

To ensure proper strength and durability, concrete must be protected from freezing and maintained at an adequate curing temperature. Common cold weather practices include;

* Heating materials: Use heated mixing water and warm aggregates (sand/stone) to raise the temperature of the fresh concrete mix to at least 50 degrees upon placement. 

* Accelerating Admixtures: Use chemical accelerators (e.g. non-chloride admixtures) and/or High Early-Strength (Type III) cement to speed up the hydration process and allow the concrete to reach 500 psi faster. 

Preparing Surfaces: Remove all ice, and frozen ground from forms, rebar, and the subgrade. Placing concrete on frozen ground can lead to settlement and cracking when the ground thaws. 

temperature to be above 50 degrees for the required protection period (usually 3 to 7 days) 

* Temperature too low to pour concrete: While 40 degrees is the threshold that triggers special cold-weather procedures, the temperature generally considered too low to pour concrete is around 20 degrees. At this temperature and below, the risk and cost of providing adequate protection (extensive heating, insulated forms, full enclosures) usually become prohibitive. 

In summary, the goal is to prevent the temperature of the concrete itself from dropping below 40 degrees until it has gained sufficient strength, and it must never freeze before reaching the 500 psi threshold. 

The American Concrete Institute (ACI) defines Cold Weather Concreting as a period when the average daily temperature falls or is expected to fall below 40 degrees for more than three successive days, or when the air temperature is not expected to rise above 50 degrees for more than half of any 24-hour period. 

Control Joint Spacing

ACI recommendations indicate that control joints should be spaced at a maximum spacing of 30 times the thickness of the pavement for parking lot pavements. Furthermore, ACI recommends a maximum control joint spacing of 12.5' for 5" pavements and a maximum control joint spacing of 15' for 6" or thicker pavements. 

* Saw cut control joints should be cut within 4 to 12 hours of concrete placement to help control the formation of plastic shrinkage cracks as the concrete cures. 

* The depth of the joint should be at least 1/4 of the slab depth when using early entry saws. The width of the cut should be in accordance with the joint sealant manufacturer's recommendation. 

Control Joints

Control joints are intended to predetermine the location of cracks caused by restrained shrinkage of the concrete and by stresses induced from warping and curling. 

Construction Joints

When concrete is planned to be placed at different times, it is recommended to use a construction joint between paving areas. The construction joint should consist of a butt joint (not a key-way joint)

Isolation Joints

Concrete slabs should be separated from other structures of fixed objects within or abutting the paved area to offset the effects of expected differential horizontal and vertical movements. Isolation joints are used to separate the pavement from these structures, such as light pole bases, manhole inlets, and buildings. 

Control Curing 

* Concrete Curing materials and evaporation retardants or equivalent should be applied to the concrete surface immediately after the placement of the concrete in accordance with TxDot 2014 standard specifications item 360.

* A concrete curing compound, such as type 2 membrane curing compound to TxDot DMS-4350, "Hydraulic cement." 

A joint sealer is soft and able to accommodate the concrete slab's expansion and contraction. The sealer's purpose is to prevent water, ice, and dirt from getting into the joint (and into the subgrade) and to prevent intrusion from below the slab. 

Formed-in-Place seals

Liquid sealants- hot polymerized/rubberized asphalt materials, and also cold-poured sealants. 

Concrete Paving Checklist

1) Verify the requirements for the concrete mixes and the batching plant. 

2) Verify admixtures are as approved. 

3) Verify the base course is maintained in a firm, moist condition and is as required. 

4) Verify the reinforcement steel is not shifted or forced to the bottom of the pour. 

5) Verify joint methods and materials are provided and observed. Check the drawings and specifications. Discuss with the concrete foreman the placement of the joints. Eliminate the support stakes. 

6) Verify control joints, construction joints, and expansion joints are provided as required. 

7) Verify the curing provisions are as required and that the work is properly protected. 

8) Verify the sawed joints are made at the proper time and are properly aligned. 

9) Verify saw joints are of the proper width and depth. 

10) Verify the agency requirements are met for the design regarding sidewalks, curbs, gutters, and aprons. 

11) Verify the location and layout of parking for disabled persons. Check the drawings and specifications. 

12) Verify the wheelchair curb cut-outs, the slopes of the sidewalks, and the location of the ramps. Handicap slopes are to be 5%. 

13) Verify the Handicap ADA inspector has done their walkthrough. 

Review ASTM C94, Standard Specification for Ready-Mixed Concrete, before commencing your concrete work. 

Shettig Construction Management provides Professional Construction Management services from the inception of your project through completion. www.shettig.com mshettig@gmail.com