22.6.c. Placing and Finishing

• Surface must be clean, roughened, and saturated surface dry (SSD).
• Remove dust, grease, or laitance before placement.

Surface preparation is a critical step before placing Polymer-Modified Concrete (PMC). A clean surface ensures that the PMC can bond effectively with the substrate. This involves removing any dust, grease, or laitance, which is a layer of weak cement paste. The surface should also be roughened to provide more grip for the concrete and be in a saturated surface dry state, meaning it's moist but not surface wet, which helps prevent excessive water absorption during curing.

Think of surface preparation like cleaning a plate before serving food. If the plate has leftover grease or crumbs, the new food won't sit well or will mix with the remnants, affecting taste and appearance. Similarly, a clean and properly prepared surface ensures the new concrete adheres properly and performs well.

• Add polymer latex to mixing water.
• Mix aggregates and cement first, then add polymer water mix.
• Mixing should be done for at least 3–5 minutes for homogeneity.

Mixing the ingredients for PMC is a systematic process. First, the polymer latex should be added to the water to create a polymer solution. Then, the dry components like aggregates and cement should be mixed together before the polymer solution is included. This sequence ensures an even distribution of the polymer throughout the mix. A minimum of 3 to 5 minutes of mixing is necessary to achieve homogeneity, meaning all components are well-mixed and consistent in texture.

Imagine making a cake. You wouldn't just dump all ingredients in at once; you would first mix the dry ingredients (like flour, sugar, and baking powder) and then gradually add in the wet ingredients (like eggs and milk). Likewise, proper mixing of concrete ensures that the final product has a uniform consistency, resulting in better performance.

• Use conventional placing methods.
• Troweling and screeding should be done promptly.
• Finishing should be completed before polymer starts to set.

Once the mixing is done, the concrete can be placed using conventional methods, similar to traditional concrete placement. Troweling and screeding—the processes of smoothing and leveling the surface—must be done quickly because PMC has a limited working time before it starts to set due to the polymers. It's crucial to finish these tasks before the mix begins to harden to ensure a smooth and durable surface.

Placing and finishing PMC can be compared to icing a cake. After you've baked your cake (mixed the ingredients), you need to spread the icing on promptly while the cake is still warm and easy to work with. If you wait too long, the icing won't adhere well, and the result will be messy.

• Moist curing may not be necessary as polymer acts as a curing membrane.
• Air curing at ambient temperature is often recommended.
• For some polymers, protective coverings are advised for first 24–48 hours.

Curing is essential for concrete to reach its maximum strength, but with PMC, the presence of polymers can eliminate the need for constant moisture. Polymeric materials can act as a barrier, holding moisture in. Generally, air curing at ambient temperatures is sufficient. However, for some types of polymers, using protective coverings for the first 24 to 48 hours may be advisable to prevent premature drying and ensure optimal curing conditions.

Consider how a plant needs water to grow. When it rains, the soil is moist, allowing strong roots to develop. Similarly, the curing process ensures that the concrete remains adequately hydrated to gain strength while the polymers help retain this moisture. Protective coverings are like a greenhouse for plants, shielding them from the harsher conditions until they are well established.

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