| Major Topics on this Page | ||
| 9.1 | Workability | |
| 9.2 | Strength | |
| 9.3 | Durability | |
| 9.4 | Early Age Behavior | |
| 9.5 | Summary | |
When aggregate, water and portland cement paste are combined to produce a homogenous substance, that substance takes on new physical properties that are related to but not identical to the physical properties of its components. Thus, several common mechanical laboratory tests are used to characterize the basic mixture and predict mixture properties. Unlike HMA, it is difficult to draw a clean distinction between characterization tests and performance tests. Typically, PCC is characterized by slump, air content and strength. However, these characteristics can also be used as performance predictors for workability, durability and strength respectively. Therefore, this section does not distinguish between mixture characterization tests and performance tests.
Whereas HMA tests are often scale simulations of actual field conditions (such as rut tests), PCC tests are directed more at the basic physical properties of PCC as a material.
The challenge in PCC testing is to develop physical tests that can satisfactorily characterize key PCC performance parameters and the nature of their change throughout the life of a pavement. These key parameters are:
Although there are many different PCC tests, only those typically used on pavement PCC are discussed in this Guide.
Workability is a general term used to describe the basic rheological aspects of fresh PCC (e.g., PCC in a wet, plastic state). Workability is instrumental in the proper placement and compaction of fresh PCC. In general, excessively stiff (or harsh) fresh PCC can be difficult to place and compact resulting in large void spaces and a honeycomb-like structure that can quickly fracture and disintegrate. This is especially true in and around reinforcing steel. Pavement PCC, especially that used for slip form paving, is usually quite stiff and must be vibrated into place. Excessively fluid fresh PCC is easy to place but may not be able to hold the coarse aggregate in place resulting in segregation and bleeding.
Slump Test
The slump test (see Figure 5.40) is the most common test for workability. The slump test
involves hand placing an amount of fresh concrete into a metal cone and then
measuring the distance the fresh PCC falls (or "slumps") when the cone is
removed.
The slump test is meant to be a basic comparative test. Variation in slump measurement on the same PCC can be as much as 50 mm (2 inches). The American Concrete Pavement Association (2001) says the following about slump:
"The bottom line is that the slump test is useful only as a comparative tool. If changes in slump are greater than 2 inches on a given job, one can conclude that there was likely a change in the mix. Variation in slump less than 2 inches is more than likely from a combination of the testing and typical concrete variability. No conclusion can be drawn from slump tests to the quality of the material. Strength measurements must be used to indicate quality."
The standard slump test is:
Strength is probably the most well-known PCC performance parameter. Compressive and tensile strength are fundamental to any building material in order to properly proportion and design structural items made from that material. Although PCC is most often known for its compressive strength, it is typically its tensile strength (or more exactly, its flexural strength) that governs its use in rigid pavements. However, given the popularity and relative ease of the compressive test, both tests are typically used in pavement applications. Strength concepts covered are:
A Note on Age vs. Strength
Since PCC continues to gain strength over time, it is important to specify a
particular age at which a certain strength is measured. Most often, 28-day
strength is specified although other strengths such as 1-day, 7-day and 90-day
strength can be used as well. For pavement applications, strength at a
particular age is quite important because typically, rigid pavements cannot be
opened to traffic until the PCC reaches a certain strength.
Curing methods
can play a major role in PCC strength gain. Often, PCC
maturity is used to
estimate strength at a particular time.
PCC is most often known by its compressive strength. This is because PCC is much stronger in compression than it is in tension and thus, is often used in compression. The ACI Concrete Code gives some rough rules-of-thumb for converting compressive strength to tensile and flexural strength:
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where: |
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compressive strength |
Compressive strength is most often measured by forming 150 mm diameter, 300 mm long (6 inch diameter, 12 inches long) test cylinders and then breaking them at a specified age (typically 28 days) although it can also be performed on specimens of different sizes and origins (such as field cores or the remnants of a flexural test).
Some state agencies use compressive strength as a field quality assurance measurement of a flexural strength specification. Flexural strength is first correlated to compressive strength based on mix design test results. Then, using this correlation, quality assurance field tests can use the easier and more widely known compressive strength test, which can be converted back to flexural strength through the previously determined correlations.
| WSDOT Use of PCC Compressive Strength |
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WSDOT correlates PCC compressive strength to flexural strength and then uses compressive strength in acceptance testing. |
Most pavement PCC has a compressive strength between 20.68 and 34.47 MPa (3000 and 5000 psi) (ACPA, 2001). High-strength PCC (usually defined as PCC with a compressive strength of at least 41.37 MPa (6000 psi)) has been designed for compressive strengths of over 137.90 MPa (20,000 psi) for use in building applications.
The standard compression tests are:
Although PCC is not nearly as strong in tension as it is in compression, PCC tensile strength is important in pavement applications. Tensile strength is typically used as a PCC performance measure for pavements because it best simulates tensile stresses at the bottom of the PCC surface course as it is subjected to loading. These stresses are typically the controlling structural design stresses. Tensile strength is difficult to directly measure because of secondary stresses induced by gripping a specimen so that it may be pulled apart. Therefore, tensile stresses are typically measured indirectly by one of two means: a splitting tension test or a flexural strength test.
A splitting tension test uses a standard 150 mm diameter, 300 mm long (6-inch diameter, 12" long) test cylinder laid on its side. A diametral compressive load is then applied along the length of the cylinder until it fails (see Figure 5.41). Because PCC is much weaker in tension than compression, the cylinder will typically fail due to horizontal tension and not vertical compression.
Figure 5.41: Split Tension Test (Click picture to animate)
The standard split tension test is:
Flexural strength (sometimes called the modulus of rupture) is typically used in PCC mix design for pavements because it best simulates slab flexural stresses as they are subjected to loading. Because the flexural test involves bending a beam specimen, there will be some compression involved, and thus flexural strength will generally be slightly higher than tensile strength measured using a split tension test. Usually, mix designs are typically tested for both flexural and compressive strength; they must meet a minimum flexural strength, which is then correlated to measured compressive strengths so that compressive strength (an easier test) can be used in field acceptance tests.
There are two basic flexural tests: the third-point loading (Figure 5.42) and the center-point loading (Figure 5.43). For maximum aggregate sizes less than 50 mm (2 inches), each test is conducted on a 152 x 152 x 508 mm (6 x 6 x 20 inch) PCC beam (see Figures 5.44 and 5.45). The beam is supported on each end and loaded at its third points (for the third-point loading test) or at the middle (for the center-point loading test) until failure. The modulus of rupture is then calculated and reported as the flexural strength. The third-point loading test is preferred because, ideally, in the middle third of the span the sample is subjected to pure moment with zero shear (Mindess and Young, 1981). In the center-point test, the area of eventual failure contains not only moment induced stresses but also shear stress and unknown areas of stress concentration. In general, the center-point loading test gives results about 15 percent higher (ACPA, 2001).
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Figure 5.45: Casting Flexural Beam Test Specimens in the Field |
The standard flexural strength test is:
Durability is a measure of how PCC performs over time. Durability is one factor in PCC pavement performance. Typically, the two major factors that affect PCC pavement durability are freeze-thaw cycles and chemical attack. Fortunately, steps can be taken to mitigate these factors and tests are available to determine PCC vulnerability to them.
Freeze-thaw resistance is important in order to avoid excessive cracking, scaling and crumbling. As water freezes it increases in volume by about 9 percent. Thus, as the water in PCC freezes and expands it exerts osmotic and hydraulic pressures on capillaries and pores within the cement paste. If these pressures exceed the tensile strength of the cement paste, the paste will dilate and rupture (PCA, 1988). As this process repeats itself over a number of freeze-thaw cycles, the result can be cracking, scaling and crumbling of the PCC mass.
In the late 1930s it was discovered that purposefully increasing PCC air content (called "air entrainment") mitigates the effects of freeze-thaw damage. This occurs because the greater air content provides extra void space within the PCC into which the freezing water can expand. Thus, hydraulic and osmotic pressures on the cement paste are minimized, which effectively prevents dilation and rupture. The total air content of the mortar (cement paste + fine aggregate) required to give optimum freeze-thaw protection is about 9 percent, which results in an air content by volume of PCC of between 4 and 8 percent (Mindess and Young, 1981). In addition to the total volume, the distribution of air within the cement paste is also important for freeze-thaw resistance. A properly air-entrained PCC contains a uniform dispersion of tiny bubbles throughout the cement paste. As these bubbles get larger and farther apart, it becomes more difficult for the freezing water to migrate through the cement paste into them. In general, the smaller the bubbles and more uniform their distribution, the better. Actions such as excessive vibration or pumping can adversely affect both total air volume and air distribution. Today, most PCC for exterior use (this includes pavements) is entrained with air to mitigate freeze-thaw effects.
Laboratory testing of PCC freeze-thaw resistance involves subjecting a specimen to a series of rapid freeze-thaw cycles, then reporting a durability factor. First, specimens are created such that they are between 75 - 125 mm (3 - 5 inches) in width and depth or diameter and between 280 - 400 mm (11 - 16 inches) long (see Figure 5.46). Specimens are then subjected to a number of freeze-thaw cycles in the following manner (AASHTO, 2000a):
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Figure 5.46: Beam Specimens for Use in Freeze-Thaw Tests |
The durability factor is then calculated as:
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where: |
DF |
= |
durability factor |
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P |
= |
relative dynamic modulus of elasticity at N cycles (in percent) |
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N |
= |
number of cycles at which P reaches the specified minimum value for discontinuing the test (usually 60 percent) or the specified number of cycles at which the test is to be terminated (usually 300 cycles), which ever is less |
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| M | = | specified number of cycles at which the exposure is to be terminated (usually 300) |
Typically, a DF < 40 indicates a PCC that may have poor freeze-thaw resistance, while a DF > 60 indicates a PCC that has good freeze-thaw resistance (Mindess and Young, 1981). However, there are several limitations to this test. First, it uses 2 - 4 hour freeze-thaw cycles, which are much more rapid than will be experienced in the field. ASTM C 671 solves this issue by using only one freeze-thaw cycle every 2 weeks. Second, even though these cycles are rapid when compared to field conditions, the test can take between 600 and 1200 hours to complete (if the full 300 cycles are tested).
Standard freeze-thaw tests are:
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Although it is actually the air content within the mortar (cement paste + fine aggregate) that is of concern, cement paste air content is usually what is measured. This air content can be measured in several ways, the most common of which is the pressure method. Using the pressure method, a sample of fresh PCC is placed in a pressure vessel (see Figure 5.47). The remaining volume of the vessel is filled with water and then the vessel is pressurized. The water level is read once, then the vessel is depressurized and the water level is read again. Finally, using Boyle's law (The principle that at a constant temperature the volume of a confined ideal gas varies inversely with its pressure) the difference in water levels (which corresponds to a volume) is converted into a volume of air.
Standard air content tests are:
PCC can deteriorate over time due to its interaction with various chemicals. Chlorides are of the greatest concern for pavement PCC because they are often contained in deicing compounds. Chloride ions can corrode steel components within PCC such as reinforcing steel or dowel bars. One standard test used for pavement PCC (AASHTO T 259) is described here. In this test, multiple slabs of at least 75 mm (3 inches) thick and 300 mm (12 inches) square are formed then abraded using grinding or sandblasting in order to simulate vehicular wear. Small dams are then built around all but one slab (designated the control slab) and subjected to continuous ponding of a 3 percent sodium chloride (NaCl) solution to a depth of 13 mm (0.5 inches) for 90 days. After 90 days the NaCl solution is removed and the slabs are wire brushed to remove any salt buildup. Slab samples are then taken and measured for chloride ion content at two depths:
These chloride ion concentrations are compared to the average chloride ion concentration of the control slab to determine the amount and extent of chloride ion penetration. Critical chloride ion concentrations for reinforcing steel corrosion are on the order of 0.6 - 1.2 kg Cl-/m3 (1.0 - 2.0 lb Cl-/yd3) of PCC.
Although sulfate attack is a PCC concern, it is generally not an issue in PCC pavement.
Some standard tests for chemical attack are:
The service life of PCC pavements is highly dependent upon their early-age behavior. Rigid pavements are significantly affected by temperature and moisture changes during the first 72 hours following placement. Stresses in the PCC build up primarily due to the combined effects of curling and warping and restraint to axial movements at the slab-subbase interface. These stresses may be of sufficient magnitude to cause cracking because PCC strength is relatively low during this early-age period (see Figures 5.48 and 5.49). Pavement stresses during this time are extremely important to long term pavement performance.
The FHWA and the Transtec Group, Inc. have produced a software package, termed HIgh PERformance PAVing (HIPERPAV), that is capable of assessing the influence of mix design, structural design, construction methods and environmental conditions on the early-age behavior of rigid pavements. HIPERPAV was originally produced for an FHWA study of fast-track rigid pavements. The goal of this project was to develop high early strength rigid pavements that could be rapidly opened to traffic upon construction completion. What this project discovered was that rapid-setting high early strength PCC created a new set of concerns including: uncontrolled slab cracking, spalling and excessive plastic shrinkage. HIPERPAV addresses these issues and others by modeling early-age PCC pavement performance (see Figure 5.50).
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Figure 5.48: PCC Early Age Crack in Palmdale, CA |
Figure 5.49: Close-Up of Early Age Crack |
Figure 5.50: One Output of HIPERPAV Showing Early Age Tensile Strength vs. Time
(screen shot courtesy of
Transtec Group,
Inc.)
All pavements can be described by their fundamental characteristics and performance. Thus, PCC tests are an integral part of mix design because they can describe PCC characteristics and provide the means to relate mix design to intended performance. Typically, PCC performance tests concentrate on basic physical properties such as strength and durability. Early age behavior modeling can also be beneficial in predicting early strength gain, excessive plastic shrinkage, cracking and spalling. PCC performance modeling provides the crucial link between laboratory mix proportioning and field performance.
