Chapter 7 – Concrete Mixture
QUALITY ALERT
Workability for Slipformed Concrete:
The concrete mixture design process should not focus solely on meeting the strength and slump require- ments. Achieving acceptable workability is equally critical. Workability related factors include the follow- ing:
1. Segregation during transport and placement.
2. Ease of consolidation that will result in a well-distributed concrete matrix.
3. Well-formed slipformed edges with little or no edge slump.
4. Minimum hand finishing required behind the paver to manipulate the surface for tightness and smoothness.
Obtaining the desirable workability for a given mixture requires consideration of the following items:
1. Aggregate – Size, grading, particle shape, water demand, variability.
2. Cement – Cement content, water demand.
3. Fly ash (if used) – Effect on initial set, water demand, effect on finishing.
4. Slag cements and granulated ground blast furnace slag (GGBFS) – Effect on finishing and saw cutting.
5. Water – Total water demand.
6. Admixtures – Air-entrained concrete exhibits better workability; water reducers reduce water de- mand while improving workability.
7. Lab mixture designs should be prepared and tested for workability at the anticipated temperature conditions that will be present during construction – laboratory mixtures are commonly prepared and tested at room temperature (approximately 70°F), while mixing temperatures in the field com- monly range from 55°F to 90°F (or warmer) depending upon the placement season. Ambient field temperatures are also highly variable.
Chapter 7 – Concrete Mixture
QUALITY ALERT
Flexural Strength versus Percent within Limits (PWL) Requirements:
The FAA P-501 guide specification uses flexural strength acceptance criteria in terms of PWL statistical criteria. Under this procedure, the payment for meeting the specified flexural strength requirement is greatly influenced by both the lot average flexural strength and the lot standard deviation. The lot aver- age strength needs to be sufficiently higher than the specified strength and/or the lot standard deviation needs to be significantly low to qualify for full or bonus payment. This requires good control over the concrete production process and the flexural beam testing process. P-501 requires the PWL to be 90 to qualify for full payment for a lot. The pay factor has a potential benefit to the contractor above a produc- tion quality level (PWL) of 96.
An example computation is given below to illustrate how the lot pay factor is affected by the strength sta- tistics.
Specified flexural strength: 650 psi (4.5 MPa)
Lower flexural strength tolerance limit (93 percent): 604.5 psi (4.2 MPa)
Standard deviation: 50 psi (350 kPa) (representing good quality control) based on 4 tests
Lot Average Strength, psi Lot PWL Lot Pay Factor
625 (4.31 MPa) 64 77.6
650 (4.48 MPa) 81 95.5
675 (4.65 MPa) 97 106.0
As seen in the above example, in order to qualify for the 100 percent pay factor for a lot, the target flex- ural strength of the concrete needs to be about 660 psi (4.55 MPa) or higher versus the specified strength of 650 psi (4.48 MPa), even when the standard deviation is limited to 50 psi (350 kPa) (repre- senting good control over the production and testing processes). Therefore, it is important that the con- tractor considers both the average flexural strength and the concrete uniformity/consistency. The contractor can achieve a lower standard deviation (reduce variability) by controlling concrete production at all of the different phases, by producing concrete with consistent properties from batch to batch, and by ensuring that proper procedures are followed to fabricate, handle, store, and test specimens.
Air Entrainment
Concrete that is subject to freezing must contain a well-distributed system of finely divided air voids of the appropriate size to protect it from frost damage. While the specifications typically provide a required volume of air as measured by ASTM C231 (pressure method) or ASTM C173 (volumetric method) these methods do not dis- tinguish between a good air void system and a poor one. The following items need to be con- sidered:
1. Trial batches need to be made to deter- mine the correct dosage of the admixture for the conditions, including temperature, expected on the job site.
2. Typical air content requirements for pave- ments range from 5 to 7 percent, depend- ing on exposure.
3. The volume of air required for frost pro- tection increases with decreasing aggre- gate size because of the corresponding increase in paste content.
4. All other factors being equal, an increase in air content results in a decrease in con- crete strength.
5. Although not commonly specified, testing the air-void system parameters on the hardened concrete according to ASTM C457during the mixture design stage is a best practice.
a. An air-void spacing factor of 0.008 in.
or less is optimum.
QUALITY ALERT
Fast Track Concrete Requirements:
Refer to IPRF’s Report IPRF-01-G-002-02-3, “Accelerated Practices for Airfield Concrete Pavement Construction—Volume I: Planning Guide,” for detailed guidance.
• Although fast track paving does not necessarily mean high early-strength concrete, there are many situations when fast track concrete or high early-strength concrete may be specified or is necessary.
• Fast track concrete is best suited for bridging the areas incorporating cross taxiways or high traffic volume apron areas.
• The production of high early-strength concrete can be achieved using normal locally available con- crete ingredients and conventional construction methods.
◦ Typically, a conventional high early-strength concrete mixture incorporates a higher cement factor, optimized w/cm ratio, uniform aggregate gradation, and admixtures as needed. A Type III cement may also be considered, but is not a necessity and can sometimes lead to performance issues.
◦ There are no specific or unique mixture designs for achieving high early-strength concrete. A wide range of mixtures can be designed to meet project needs.
◦ High early-strength concrete can be produced using proprietary cements and admixtures.
• When high early-strength concrete is specified, the early-age strength requirement is typically defined in terms of compressive strength, as follows:
◦ About 750 to 1,000 psi (5 to 7 MPa) in about 4 to 6 hours.
◦ About 2,000 to 3,000 psi (14 to 21 MPa) in about 24 hours.
• There may also be a specified flexural strength requirement at 14, 28 or 90 days.
Chapter 7 – Concrete Mixture
6. The concrete for the trial batch should be allowed to sit for a length of time repre- sentative of the haul time and then be measured at the end of that period to en- sure that testing accounts for loss of air.
a. When concrete is delivered to the site in non-agitating trucks, the loss of air can range from 1 to 2 percent- age points. Target air contents for tests taken at the concrete plant should consider the loss of air in transport.
b. Research has shown that certain non-Vinsol Resin air entraining ad- mixtures may lead to a clustering of entrained air bubbles around the pe- riphery of coarse aggregate particles (Figure 7.1). This has been shown to occur when additional water is added to transit mix trucks (re-tempering).
The resulting air void clustering can significantly reduce the concrete strength. Research is yet to be per- formed to evaluate whether this ef- fect also occurs with central mixed concrete; thus, caution should be ex- ercised when re-tempering any con- crete mixture.
7. In a no-freeze environment, if air is en- trained solely to facilitate workability, the minimum air contents required for frost damage protection do not apply.
8. Typical slipform paving operations reduce air content by 1 to 2 percent during con- solidation.
a. Stability of the entrained air system should be checked at least twice daily by testing the air content of fresh concrete both in front of and behind the paver. Excessive air loss through the paver (>3%), may indi- cate an unstable air void system.