What Exactly Is Ready-Mixed Concrete?
The terms concrete and cement are often used interchangeably, but they are not the same product. Cement is a powdered ingredient that, when combined with water, acts as the binding agent that holds together the aggregates. A concrete mix itself is composed of cement, water, and both coarse and fine aggregates, such as crushed stone and sand.
Concrete mixes are specifically formulated to achieve desired characteristics, including workability (or slump) and strength, for a particular application. The right consistency, known as slump, is crucial for easy handling and placement. Additionally, concrete must possess sufficient strength and durability to withstand the intended loads once it has cured.
Ready-mixed concrete is delivered to the job site in its fresh, unhardened state. This concrete mix retains its plasticity for several hours, with the exact duration depending on the specific mixture type and the prevailing conditions during placement. Typically, concrete begins to set within two to twelve hours after mixing. It then continues to gain strength for months, or even years, provided it undergoes proper curing during its initial days.
Key Considerations When Ordering Ready-Mixed Concrete
When placing an order for ready-mixed concrete, it’s essential to provide all the fundamental details while keeping your requirements as straightforward and pertinent as possible. Concrete producers typically have a variety of mix designs available for different applications and can assist you in selecting the most suitable mixture.
Size of Coarse Aggregate
Understanding the nominal maximum size of coarse aggregate is important. This aggregate size should always be smaller than the narrowest dimension through which the concrete will flow. For instance, consider the thickness of the section being poured and the spacing of any reinforcing steel. For most common uses, a nominal maximum aggregate size of 3/4 or 1 inch is typical.
Slump
Indicate the desired slump of the concrete. A stiffer mixture will correspond to a lower slump value. For most applications, a typical slump range is 3 to 5 inches. For specialized uses, like slip-form construction, a maximum slump of 2 inches is often required, while basement walls might utilize a higher slump, up to 7 inches. The accepted tolerance for slump upon delivery is typically ±1 to 1-1/2 inches. Adding water at the jobsite to increase slump is permissible, provided it’s not excessive enough to cause segregation of the mix or reduce its ultimate strength and durability.
Entrained Air
For concrete that will be exposed to freezing temperatures, air-entrained concrete must be used. In many regions, air-entrained concrete is the standard default option. If you specifically require non-air-entrained concrete, you must explicitly state this to the concrete producer. The target air content depends on the size of the coarse aggregate, with a typical range of 4% to 6% of the concrete volume. The tolerance for air content upon delivery is ±1.5%. The concrete supplier is permitted to adjust the air content at the jobsite if, upon testing, it falls below the specified amount.
Required Quality Level
The most effective way to order concrete is by specifying its performance requirements. This primarily refers to the concrete’s required strength. When necessary, other performance characteristics like permeability, shrinkage, or durability can also be specified. Inform the producer about the anticipated exposure and service conditions of the structure. The concrete producer is best equipped to proportion, mix, and supply concrete that meets your desired performance criteria. A minimum strength of 3,500 to 4,000 psi is commonly specified.
Alternatively, concrete can be ordered by defining its prescriptive requirements. In this scenario, the purchaser specifies limits on the ingredients in the mixture. When this approach is used, the purchaser typically assumes responsibility for the concrete’s strength and overall performance. This method offers the concrete producer limited flexibility in the mixture, potentially hindering their ability to adapt to changes that might affect concrete performance.
Quantity of Concrete
Concrete is sold by volume, measured in cubic yards. The delivered volume is determined from the measured concrete density or unit weight. One cubic yard of concrete weighs approximately 4000 pounds. A typical truck mixer has a capacity of 8 to 12 cubic yards.
It’s advisable to order approximately 4% to 10% more concrete than what is estimated from a volumetric calculation based on your plan dimensions. This buffer accounts for potential waste or spillage, over-excavation, spreading of forms, loss of entrained air during placement, settlement of a wet mixture, truck mixer hold-back, and volume changes. Note that the volume of hardened concrete is 1% to 2% less than that of fresh concrete. Re-evaluate your needs during placement and communicate any changes to your concrete supplier immediately.
Disposing of returned concrete has significant environmental and economic implications for the ready-mixed concrete producer. Therefore, making an accurate estimate of the concrete required for the job before placing your order is crucial.
Additional Items
A variety of supplementary options are often available from the concrete producer. Chemical admixtures can be used to accelerate or retard the setting characteristics of concrete, which aids in placement and finishing during hot or cold weather. Water-reducing admixtures are utilized to increase slump without adding extra water to the concrete. Synthetic fibers can help reduce the potential for plastic shrinkage cracking. Additionally, color additives or special aggregates are frequently offered.
Scheduling and Responsibilities
Scheduling Delivery
Schedule the concrete delivery to align seamlessly with your overall construction timeline. Inform the producer of the correct address, exact location, nature of the pour, and your estimated delivery time. Contact the ready-mixed concrete producer well in advance of the required delivery date. Concrete is a perishable product, so your construction crew should be fully prepared for concrete placement when the truck arrives. Immediately notify the producer of any schedule changes or work stoppages.
Ensure that the truck mixer has proper access to the placement location. Remember, a concrete truck weighs over 60,000 lbs and may have difficulty maneuvering in certain jobsite conditions.
Responsibilities
The responsibilities of all parties involved in the construction process should be clearly defined during a pre-construction meeting. Key responsibilities typically include:
- The concrete producer is responsible for the concrete slump as specified for a period of 30 minutes after the requested delivery time or the truck’s arrival at the placement site, whichever is later.
- The concrete producer is obligated to deliver concrete at the requested slump and air content, within the accepted tolerances mentioned previously, as measured at the point of discharge from the transportation unit.
- When concrete placement procedures could potentially alter the characteristics of the fresh concrete (e.g., pumping concrete), it is the purchaser’s responsibility to inform the producer of any necessary changes to the mixture requirements to accommodate these effects.
- If a job involves more than one type of concrete mixture, it is the purchaser’s responsibility to verify the mixture delivered and direct it to the correct placement location.
- The purchaser should meticulously check and sign the delivery ticket and document any special occurrences directly on the ticket.
- The concrete producer cannot be held responsible for the quality of concrete if any modifications or additions are made to the mixture at the jobsite by the purchaser. This includes adding excessive water, admixtures, fibers, or special products, or if the truck is forced to wait for an extended period before discharging the concrete.
- When strength tests are used for concrete acceptance, samples should be obtained at the point of discharge from the transportation unit. The purchaser or their representative should ensure that appropriate facilities are available for onsite testing and that standard practices are rigorously followed. Certified personnel should conduct these tests. Test reports should be promptly forwarded to the producer to ensure any necessary adjustments can be addressed.