Plasticizers are water-reducing admixtures that make fresh concrete easier to work with without adding extra water. They work by dispersing cement particles and reducing friction inside the paste, so the mixture flows more easily. This means you can achieve the desired workability with less water, or maintain the same workability with lower water content. Lowering the water–cement ratio while keeping workability improves the potential strength and durability of the hardened concrete. They don’t inherently change the setting time, and they don’t add cement.

The rebound hammer test is used to estimate surface hardness of concrete with a quick, nondestructive approach. A small mass inside the hammer is released to strike the concrete surface, and the device measures how far the mass rebounds. That rebound value, often called a rebound number, correlates with the hardness of the surface and, to some extent, with the local compressive strength. It’s fast, portable, and good for screening large areas or checking consistency between pours. However, readings come with caveats. The surface condition matters a lot—moisture, laitance, roughness, and carbonation can affect results, and the test only assesses the outermost layer, not the core. For accurate strength assessment, use rebound hammer readings alongside other tests and calibration with cylinder tests. Other non-destructive methods mentioned involve different principles: ultrasonic pulse velocity uses sound waves through the concrete to infer properties, not a hammer strike; chloride penetration tests measure chemical diffusion rather than hardness; and pull-out tests are destructive, pulling a rod or anchor to fail the concrete.

When the new slab bleeds, water rises to the surface and can create a laitance layer, leaving a weaker, more plastic surface. If this fresh slab is connected to an existing slab, movement is restrained as both pours cure, while they shrink at different rates. That restraint combined with the weaker surface promotes tensile stresses at the top, which express as cracks. So the surface is expected to crack, especially near the joint where the two slabs meet, rather than smoothing out or maintaining a uniform color.

Coarse aggregate is defined by particles that are large enough not to pass through a 4.75 mm sieve. In a sieve analysis, you separate the sample by passing it through progressively finer sieves; the portion that remains on the sieve with 4.75 mm openings is the coarse aggregate. The material that passes through that sieve is the fine aggregate. So the correct boundary is the 4.75 mm sieve, which is why the coarse portion is retained there.