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Mix Design

            The mix design is a very important aspect of every engineering job performed with concrete. There are a number of admixtures, different aggregates, and other materials that can be added to help improve the cement. All of these factors greatly impact the water/cement ratio. The criterion for any concrete mix design of this nature includes Flexural Strength, Thickness, Smoothness, Grade, Edge Slump, and Dowel Bar Alignment.

Before the specifications are listed, it should be noted that the engineer should designate the design strength. The minimum strength for airports is 600 psi. The flexural strength that is specified by the engineer should be significantly larger then the largest airplane loading allowable at that specific site. In the case of mixing concrete for airport runways, a mix design is required to achieve a 28-day flexural strength of 90 (PWL) [1] or higher. The lower specification tolerance limit (L) is (0.93 x the above-specified strength).

In general, a minimum cement content of 500 lbs. should be specified. A minimum water/cement ratio is 0.50. The water/cement ratio can be adjusted as necessary to incorporate the use of admixtures and the like. The mix that is determined should have a slump between 1 and 2 inches (25mm to 50mm). For vibrated slip form concrete the slump should be between ½ and 1½ inches.

The thickness of the cement material should be accepted if the PWL of the lot meets or exceeds 90 percent. Once the concrete has hardened, the smoothness should be tested using a sixteen-foot (5-meter) straight edge. There must not be a deviation in height greater than ¼ of an inch along the straight edge. If there is a greater rise than ¼ inch and less than ½ inch, then the surface should be ground off to appropriate levels. If the defect is in excess of ½ inch, then the slab should be removed and replaced at the owner’s expense.

The grade tolerances in lateral deviation from the assigned alignment of the pavement edge should not exceed .10 feet in any lane. The vertical deviation from the assigned grade given in the paving process should not exceed .04 feet (30mm).

The dowel bar and dowel bar assemblies should be checked for position and alignment. The alignment of each dowel bar should not exceed more than ¼ inch per foot.

The concrete can be mixed at the site of application or at the mix plant. In truck mixtures, the mixing time should be measured from the time all ingredients are added together with the exception of water. The maximum amount of time after mixing occurs should be no more than 90 minutes in agitating trucks, or no longer than 30 minutes in non-agitating trucks. Also the concrete should not be mixed, placed or finished without the proper amount of natural or artificial light.

            As can be expected, the mixtures must be free of any frozen areas such as water and aggregate. The concrete mix should not be placed if the temperature falls below 40 degrees F. in shaded areas. It is also unacceptable for the concrete mix to be placed on any frozen material. If the temperature exceeds 85 degrees F. all forms should be sprinkled with water immediately before use.

            Once the concrete is put into place it can be machine finished, hand finished, or a floating device may be used for the final finish. The project engineer must approve any method that is to be used.

            There are several types of curing that may be used such as: the Impervious Membrane method, Polyethylene Films, Waterproof Paper, White Burlap-Polyethylene sheets, and curing in cold water. All of the curing methods can be found in the provided link to the AC 150/5370-10A standard; however, it should be noted that the project engineer must disapprove of any curing methods that might not be feasible around aircraft jet blasts.


 

[1] Flexural Strength and thickness shall be evaluated using the method of estimating percentage of material within specification limits. (PWL)