Mixing Time. Mixing time starts when the water is added to the mix and should be adequate but not excessive. Minimum mixing times vary with the size and type of the mixer and range from 1 to 3 minutes. Maximum mixing times can range from 3 to 10 minutes. For stationary mixers, minimum mixing time can be established by tests on mixer performance. For truck-mixed concrete, complete mixing requires from 50 to 100 revolutions of the drum at mixing speed. Check the manufacturer’s plate on the mixer. If, after mixing, drum speed is reduced to agitation speed or stopped, the drum should be rotated at mixing speed for from 10 to 15 revolutions just before concrete is discharged.
Elapsed Time. For normal temperatures, the total time from start of mixing to discharge should not exceed about 11/2 hours and should be reduced as temperatures increase.The mix should be discharged before 300 revolutions of the drum.
Slump. Slump tests should be made periodically in accordance with ASTM C143, Standard
Method of Test for Slump of Portland Cement Concrete, to ensure that concrete has the specified slump for proper placement in pile casings, shells, or holes. The slump for concrete as delivered to the top of the pile casing or hole should be 5 in (127 mm) for conventional concrete or 4 in (101.6 mm) for reduced coarse aggregate concrete, both with a tolerance of +2 in, -1 in (+50.8 mm,-25.4 mm). Special-type piles may require concrete having different slumps. See Special-Type Piles.
Sometimes it is advisable to check the slump just before adding the final water at the jobsite to avoid too high a slump or a wet mix.
Slump Loss. Slump loss can be caused by overmixing, hot weather, pumping through long lines, or delays in delivery and placement of concrete. Overmixing can and should be avoided. If necessary, all the mix water can be added and all mixing done upon delivery at the jobsite. This could prevent overmixing and may help in eliminating slump loss due to hot weather. If concrete is to be pumped to the pile locations, the slump should be increased without changing the water-cement ratio or concrete strength to compensate for slump loss during pumping. All preparations should be made for depositing concrete upon delivery, and delivery schedules should be arranged to eliminate delays in placing concrete.
Retempering. The addition of water to the concrete mix to compensate for slump loss resulting from delays in delivery or placing is permissible provided the design water-cement ratio is not exceeded and the concrete has not attained its initial set. Initial set is not to be confused with a false set, when the concrete appears to stiffen but can be made workable with agitation.
Delivery Tickets. A delivery ticket must accompany each load or batch of concrete. The delivery ticket is for the purchaser, but the inspector should be furnished a copy. It should include sufficient data to identify the producer, project, contractor (purchaser), truck mixer used, and specified concrete mix or strength. Other information which should be on the delivery ticket is the date of delivery, type and brand of cement, maximum aggregate size, weights of cement, sand, and coarse aggregate, type and amount of admixtures, quantity of water, time batched, reading of revolution counter and time when water was first added, volume of batch, and amount of water added by the receiver. The inspector should note the times of delivery and placement and the air temperature.
Concrete Strength. Standard cylinders for compression tests should be made periodically or as specified in accordance with ASTM C31, Standard Method for Making and Curing Concrete Test Specimens in the Field, to ensure that concrete of required strength is being furnished. The frequency for making test cylinders will vary with the job size and other factors, but generally a test set (minimum of two cylinders) should be made for each daily pour or for every 50 yd^3(38.2 m3) placed.
Also a test set should be made for each age at which compression tests are to be run. The inspector should ensure that cylinders are properly cast, handled, stored, sealed, packaged for shipment, and shipped so as not to invalidate test results. For strict concrete control, test specimens should be cast in cast-iron or tin-can molds. Although widely used, cardboard or paper molds are not recommended for molding test cylinders for strict concrete control. If cardboard molds are used, they should conform with ASTM C470, Specifications for Molds for Forming Concrete Test Cylinders Vertically. Jobsite curing or the use of cardboard molds may contribute to low strength-test results. Grout strengths for special-type piles are determined by standard cube tests in accordance with ASTM C91, Specification for Masonry Cement. (See Auger-Grout Pile, Cast-in-Place Pile, and Minipiles under Special type Piles.)
Results of Tests. The pile inspector should be furnished with copies of the results of all concrete compression tests as called for in the specifications. It is advisable to obtain 3- and 7-day results at the beginning of the job in order to detect trends in concrete strengths. The results of 7-day tests are also valuable in monitoring concrete strength trends as the job progresses so that, if necessary, remedial measures can be taken before too much concrete is placed.
Strength Variations. Variations in concrete strength as determined by standard cylinder tests are normal. Several criteria are used to determine the acceptability of variations. For example, the concrete is considered satisfactory if the average of three consecutive tests is equal to or greater than the required 28-day strength and no test falls below the required 28-day strength by more than 500 psi (3447 kPa). Another acceptance criterion is that 80 percent of the tests show strengths greater than the design strength and that not more than 1 test in 10 is less than the required 28-day strength. A third is that the average strength from consecutive tests is greater than the required 28-day strength. ACI 214.3-88 provides recommendations on evaluating concrete strength-test results.
If the test results show that concrete strength is below that specified, the cause of low-strength concrete should be investigated. Low strength could be caused by unsatisfactory materials, by improper batching and mixing, or by the use of excess water in the mix. Low cylinder breaks could also result from improperly preparing, curing, handling, or testing cylinder specimens.
Verification of Concrete Strengths. If the results of standard cylinder tests are low, cores can be removed from piles for testing. Core tests are considered satisfactory if the average of three cores is equal to or greater than 85 percent of the required 28-day strength and if no core strength is less than 75 percent of the specified 28-day strength. The results of tests on cores are normally lower than those on standard cylinders owing to microfracturing of the concrete. It should be noted that pile concrete in a long steel shell embedded in the ground will cure at a rate slower than that for test cylinders or exposed concrete. Curing conditions are ideal, but the rate of strength gain is lower than normal. Concrete strength in completed piles can also be checked by various nondestructive methods such as penetration-resistance tests. See ASTM C803, Method of Tests for Penetration Resistance of Hardened Concrete.
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