Taking deliveries of concrete from truck mixers is a critical process in ensuring that the final concrete product meets the desired strength and workability. Improper handling during this phase can compromise both, leading to costly repairs and structural failures. This article provides a comprehensive guide to receiving concrete deliveries efficiently while maintaining the integrity of the mix.
1. Understanding Concrete Properties: Strength and Workability
Before discussing the process of taking deliveries, it is essential to grasp the fundamental properties of concrete—strength and workability. These characteristics are influenced by the mix design and handling.
- Strength: The compressive strength of concrete is its ability to withstand loads without cracking or failing. This is primarily determined by the water-cement ratio, the quality of the materials, and curing methods. Higher strength is achieved by lowering the water content and using well-graded aggregates.
- Workability: Workability refers to how easily the concrete can be mixed, placed, and finished. The right level of workability ensures proper placement without segregation or excessive bleeding. Admixtures like plasticizers can be used to improve workability without compromising strength.
Maintaining the balance between these two factors is key to a successful concrete project. A properly designed and handled concrete mix will not only be strong but also workable enough to be easily placed and finished.
2. Preparing for Concrete Deliveries: Site and Equipment Readiness
Before the concrete truck mixer arrives, the site must be ready to receive the delivery. Here’s how to prepare:
- Site Preparation: The delivery area should be clean, level, and free of obstructions to allow easy access for the truck mixer. Ensure the forms are properly installed and reinforced if necessary. The location where the concrete will be placed must be accessible, and if the concrete needs to be pumped, a clear path for the pump hose should be prepared.
- Equipment Check: All tools and equipment, including wheelbarrows, vibrators, trowels, and screeds, should be in place and functional. It’s also important to have backup tools on hand to avoid delays.
- Communication: Ensure the delivery crew is aware of the concrete specification, such as the mix type, strength requirements, and any special instructions like additives for retardation or plasticizing. Effective communication between the site crew and the driver is vital to minimize delays.
3. Steps for Taking Concrete Deliveries
Step 1: Receiving the Truck
When the truck mixer arrives, verify that the delivery matches the order specification. This includes:
- Checking the delivery ticket: The ticket should include important information such as the mix design, slump, water-cement ratio, and the time the truck was loaded. Ensure the details align with your order.
- Inspecting the concrete mix: Visually inspect the concrete for consistency, slump, and overall appearance. This step is essential in confirming that the mix hasn’t segregated during transit.
Step 2: Slump Test for Workability
A slump test helps determine the workability of the concrete. This simple test involves filling a cone-shaped mold with concrete, then lifting the cone and measuring how much the concrete “slumps” down.
- Procedure: Fill the cone in three layers, tamping each layer 25 times with a rod. Remove the cone, and measure the slump from the top of the mold to the highest point of the concrete.
- Results: If the slump is within the specified range, the concrete has the desired workability. If the slump is too high (too wet), the mix may have excessive water, leading to lower strength. If the slump is too low (too dry), the mix may be difficult to place and finish.
Step 3: Controlling Water Addition
Water-cement ratio is crucial to concrete strength. Adding extra water at the site can make the concrete more workable, but it will also reduce its strength. It is advisable not to exceed the allowable water-cement ratio specified in the mix design. If more workability is needed, consider using admixtures instead of water.
- Communicate with the driver: Ensure the truck operator does not add water without site approval. If additional water is necessary, document the amount added and note any changes in the slump.
Step 4: Placing the Concrete
Once the slump is confirmed, the next step is to place the concrete. Depending on the project size, this can be done directly from the truck chute, using wheelbarrows, or with the assistance of a concrete pump.
- Placement Techniques: Begin placing concrete at the farthest point from the truck and work towards it. Avoid pouring concrete from a height, as this can cause segregation, where the heavier aggregates settle at the bottom and the cement paste rises to the top.
- Compaction: Use vibrators to compact the concrete, ensuring that it fills the forms and eliminates air pockets. Proper compaction improves strength by reducing voids in the mix.
Step 5: Finishing the Surface
After the concrete has been placed and compacted, it’s time to finish the surface. Proper finishing ensures that the concrete will achieve the desired texture and durability.
- Screeding: This is the process of leveling the concrete surface using a straightedge, such as a screed board, to strike off the excess concrete. This creates a flat, even surface.
- Floating: Once the concrete has settled slightly, a float is used to smooth the surface. This step also embeds any coarse aggregate particles that may have risen to the top.
- Troweling: For a smoother finish, steel trowels can be used after floating. However, over-troweling can lead to a surface that’s prone to cracking or scaling.
Step 6: Curing the Concrete
Curing is crucial to the development of concrete strength. After the initial set, the concrete must be kept moist to allow proper hydration of the cement.
- Methods: Curing can be done by covering the surface with wet burlap, plastic sheets, or using a curing compound. For large pours, consider using a misting system to maintain moisture.
- Timing: Start curing immediately after finishing, and continue the process for at least 7 days. Proper curing improves the final strength and durability of the concrete.
4. Common Issues and Solutions
Issue: Segregation
- Solution: Ensure the concrete is placed as close to its final position as possible. Avoid pouring from heights or excessive vibration.
Issue: Delayed Delivery
- Solution: If the truck takes too long to arrive, concrete may begin to set in the mixer. In such cases, consider using a retarder to extend the setting time.
Issue: Cold or Hot Weather
- Solution: In hot weather, reduce evaporation by using windbreaks and sunshades, and consider a retarder. In cold weather, ensure the mix is warm, and consider using accelerators to speed up the setting time.
5. Final Thoughts on Strength and Workability
The goal of any concrete delivery is to strike a balance between strength and workability. Careful attention to the mix design, site preparation, and handling processes ensures that the concrete retains its desired properties. By following the steps outlined in this guide, you’ll be able to take deliveries of concrete from truck mixers in a way that maximizes the structural integrity and durability of the final product.
By maintaining strict control over the slump, minimizing water addition, and ensuring proper placement and curing, you safeguard the concrete’s strength while achieving the necessary workability for ease of use.