Aci Concrete Time Limit
Julia Kozub
In April 2021, ASTM C94/C94M-21, Specification for Ready-Mixed Concrete, was updated with two significant changes that do away with the 90-minute default time limit for ready mixed concrete. Since its inception in 1933, the standard has required a time limit of 1.5 hours from the start of mixing to the completion of discharge.
The revisions were proposed because delivery time can vary on projects depending on distance or placement methods. Technological advances made in the production of concrete can extend the quality of freshly mixed concrete beyond 90 minutes.
According to the 2021 revisions, the time limit can be stated by the purchaser at the time concrete is ordered. If the limit is not stated by the purchaser, the producer should establish a time limit. Either way, the established time limit must be stated on the delivery ticket.
Establishing that time limit is important. There may be some mixtures in some conditions where 90-minute-old concrete is not workable or suitable without using special means or excessive addition of water. Whatever the time limit, the concrete quality must be acceptable to clients and meet their requirements.
Section 12.9: Discharge of the concrete shall be completed within time limit as established in 6.1.10. This limitation may be waived by the purchaser if the concrete is of such slump or slump flow after the specified time limit has been reached that it can be placed, without the addition of water to the batch.
Section 6.1.10: Purchaser shall state a time limit from the start of mixing defined in 12.3 or 12.5 to when the concrete discharge must be completed. If no time limit is stated by Purchaser, the Manufacturer shall establish and communicate the limit to the Purchaser prior to delivery. The time limit to complete discharge shall be stated on the delivery ticket.
A change in the ASTM C94-21 which removes time limit to discharge concrete would greatly benefit all involved with placing concrete. For the concrete pumping companies, it would allow them greater latitude to remedy problems they may arise: before the pour (traffic, flat tire), during prime out, equipment failure, etc. Oftentimes, the stress and hurriedness to save old concrete leads to shortcuts and unintended consequences which can result in accidents. It would also benefit the ready mixed companies from having to remove and dispose of returned concrete, which can be extremely costly and non-productive.
Time limit leniency would allow contractors to have greater control over their projects, eliminate waste and confrontations on job sites, and refrain from wasting money on concrete sent back that is perfectly within specifications if it were not for some antiquated standard created 90 years ago. For more information, visit _c0094m-21.html.
ASTM C94/C94M-21, Specification for Ready Mixed Concrete, includes a revision related to the time limit to discharge concrete; the time limit of 1 hours to discharge has been removed. This revision indicates that the time limit can be stated by the purchaser at the time concrete is ordered. If the limit is not stated by the purchaser, the producer should establish a time limit. The revision was proposed because delivery time can vary on projects depending on distance or placement methods. Additionally, there may be some mixtures in some conditions where the previous default time limit was not appropriate without using special means or could cause excessive addition of water. Another part of the revision requires the established time limit to be stated on the delivery ticket.
Because of our work with Counties, we have many remote sites. Such is the nature of the business, but since concrete begins to cure the moment it gets mixed we often run into problems with the prescribed time limits for concrete travel distance.
The only true concern with long concrete placement times is workability. The concrete is curing right from initial mixing, therefore a long wait time, and hence low slump during placement, will make it difficult to fill the forms adequately and avoid segregation.
The American Society of Concrete Contractors says that the 1 1/2 hr time limit is too conservative, and I tend to agree. One reason is that water reducing admixtures can easily extend the workability of concrete much longer than this. Superplasticizers are present in almost every concrete mix nowadays and they also give another boost.
That being said, contractors always have the urge to add water to the concrete to make it more workable. This should be avoided because it affects the structural properties of the concrete, but when the concrete truck has travelled a long distance this is likely to be a concern for the site inspector.
I am planning to pour 40 square feet of 4" slab to support light shed. I was thinking of using 60LB bags of regular premixed concrete (not fast set). I figured that getting it as ready mix in a truck is way too expensive and also the site is 150+' away from the street.
The answer depends on your level of physical ability and ambition, and is therefore mostly subjective. There are plenty of folks who could do it. I wouldn't try it myself at my age and level of fitness. Concrete is really freaking heavy. I would want some additional labor on hand.
I attempted to finish a sidewalk and garage apron on my own when my help had to leave, and there's little as nerve-wracking as finding oneself racing the clock with setting concrete. You never end up with a good finish in those cases, and if you try something foolish like watering the mix you'll end up with a weak surface prone to flaking and disintegration.
I'm imagining about 1 bag per square foot, so you want approximately 40 bags (that might be a conservative estimate for 60-lb bags).
You mention a concrete-mixing device which you roll back and forth several feet in order to mix. I'm not convinced, but for now I'll just assume that that product works exactly as described on amazon: "fill it, roll it 10 feet, and then back 10 feet, and you have perfectly mixed concrete".Now, the TIME LIMIT here is crucial: sakrete says you can wait AT MOST somewhere between 20 minutes and 4 hours between pouring and troweling. So this depends on your climate &etc, but if I were you I would want to pour the 40th bucket of concrete within an hour of pouring the first bucket.
When you figure that you want all this mixing and pouring to take place within the space of an hour or so, and would want to run 40 bags through that mixer, that's about 1 minute per mix. I don't think you could keep up that pace even on the FIRST bag, probably more like 5 to 10 minutes per bag for a single person, right? (include time to lift that completed mix and pour it into place). In order to achieve that pace, you would want two people on the mixer and one running water and smoothing as you go.
Concrete driveways are typically poured four inches thick, giving the concrete driveway enough strength to withstand loads of regular vehicles (up to approximately 8,000lbs). A fully loaded concrete truck can weigh over 70,000lbs, almost nine times heavier than your driveway was meant to withstand. Even if the concrete does not crack immediately, there will be issues down the road.
The concrete industry has been asking the Minnesota Department of Transportation (MnDOT) to lengthen the time allowed to deliver concrete. MnDOT is planning on constructing many small bridge projects that are difficult to reach within the existing 60-minute time limit for air-entrained concrete. This 60-minute time limit could unnecessarily increase the cost to construct these bridges. Although other state departments of transportation (DOTs) do allow longer transit times with the use of retarding admixtures, there are no known studies to verify whether the longer hauling time is detrimental to concrete performance. Also, there may be significant differences in the mix designs and materials that are used by other state DOTs, as well as the environments that the concrete is placed and expected to perform in. The goal of this project was to utilize the results of the testing programs and develop specification guidelines that allow the implementation of chemical admixtures to extend transport and delivery time from the current 60 minutes for air-entrained concrete up to 120 minutes.
A. Includes the requirements for the construction of full depth Portland Cement Concrete (PCC) pavement, pavement widening, and concrete curbs and gutters placed upon a prepared or corrected subgrade or previously constructed base or subbase. See Division 2 - Earthwork and Site Grading for subgrade and subbase construction specifications. This section shall also include final subgrade/subbase preparation for concrete paving.
C. Reference is made to the Iowa Department of Transportation Standard Specifications for Highway and Bridge Construction, Series 2001, and all current General Supplemental Specifications and Materials Instructional Memorandum by the term Iowa DOT Specifications and/or Iowa DOT I.M.
B. PCC Mix Design: Mixes are Class C, M, C-V47B, and M-V as defined in Iowa DOT 2301 and I.M. 529. Unless otherwise specified in the contract documents, maturity method for strength determination prior to opening to traffic will be allowed. Class A shall be used for concrete base or temporary pavement and recreational trails only. Class B shall be used for temporary pavement and recreational trails only. After October 31 all Class B concrete will be replaced with Class A concrete. In cases where early opening of pavement is desirable, the contract documents may require the use of Class M, or M-V concrete mixtures or may require using Type III cement with other concrete mixtures. Such sections of pavement may be opened to traffic in accordance with the limitations of Section 7010, 1.07.I.
1. Two weeks prior to commencing any Portland concrete placement, the Contractor shall submit a paving mix design for each different source of aggregate to be used for review and approval by the Jurisdictional Engineer. Mixes or mix designs approved by the Iowa Department of Transportation or an independent testing laboratory shall be submitted.
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