The key to placing an order for ready-mixed concrete is to provide all the basic detailed information and to keep the requirements as simple as possible and relevant to the application. The ready-mixed concrete producer has mixture formulations for a wide variety of applications and can help decide the required mixture characteristics.
Some of the basic requirements to keep in mind when placing a concrete order are as follows:
Size of coarse aggregate. The important information is the nominal maximum size required, which should be smaller than the narrowest dimension through which concrete should flow, such as the thickness of the section and spacing of the reinforcing steel. For most applications, nominal maximum size of coarse aggregate is ¾ or 1 inch.
Slump. Concrete slump, a measure of its consistency, should be indicated. Typical slump range for most applications is 3 to 5 inches. For slip-form construction, a maximum slump of 2 inches is required, while higher slump to a maximum of 7 inches is typical for basement walls. The tolerance on the slump as delivered is ±1 to 1½ inches.
Entrained air. Air-entrained concrete should be used if concrete will be exposed to freezing temperatures in service, or even during construction. In many locations, air-entrained concrete is the default option. When non-air-entrained concrete is required, this should be clearly stated at the time of ordering. Typical air content depends on the size of the coarse aggregate, and the typical range is 4% to 6% of the concrete volume. The tolerance on air content as delivered is ±1.5%.
One method of ordering concrete is by specifying performance requirements, which is generally the concrete strength. A minimum strength of 3500 to 4000 psi may ensure that the concrete is durable enough to resist wear, abrasion, and freeze/thaw cycles. Other performance characteristics, such as permeability, shrinkage, or various durability requirements, may also be specified. The producer should be made aware of anticipated exposure and service conditions of the structure. The concrete producer will then proportion, mix, and furnish concrete for the desired performance.
Another option is to order concrete by specifying prescriptive requirements. The purchaser specifies limits on the quantities and types of ingredients in the mixture. The prescriptive limits may indicate minimum cement content, maximum water-cement ratio, and limits on the quantities of pozzolans, slag, or ad-mixtures. Frequently this approach is used when a particular prescriptive mixture formulation has worked well in the past.
Specifying both performance and prescriptive requirements is discouraged since the performance requirements may conflict with the prescriptive limits.
Quantity of concrete. Concrete is sold by volume, in cubic yards, in a freshly mixed, unhardened state as discharged from the truck mixer. The delivered volume, or yield, is calculated from the measured concrete density or unit weight. One cubic yard of concrete weighs about 4000 pounds. The typical capacity of a truck mixer is 8 to 12 cubic yards.
Order about 4% to 10% more concrete than is estimated from a volumetric calculation of the plan dimensions. This will account for waste or spillage, over-excavation, spreading of forms, loss of entrained air during placement, settlement of a wet mixture, truck mixer holdback, and change in volume—hardened concrete volume is 1% to 2% less than that of the fresh concrete. Reevaluate the needs during placement and communicate any changes to the concrete supplier.
This article is excerpted from CIP-31-Ordering Ready Mixed Concrete, part of the Concrete in Practice series from the National Ready Mixed Concrete Association. NRMCA has 38 CIPs on all aspects of concrete. They may be ordered online at www.nrmca.org or by calling 888-84NRMCA (888-846-7622).