Concrete should be placed within half an hour of mixing for optimal results in any environment. Ready-mix concrete for footings is the best choice and should be selected carefully. When combined with sand and aggregates, the mixture bonds to form a strong, rock-like material. Mixing quality concrete on the farm can be convenient and cost-effective, provided the right steps are followed. With a few workers and a decent cement mixer, a large amount of concrete can be mixed quickly.
Ready-mix concrete will have a strength rating that refers to the strength of cured concrete (see Table). Normal class ('N') concrete is available in grades N20, N25, N32, N40 and N50 (number refers to strength in megapascals).Once the screed concrete is in position, manual floats can be used to work the mortar on the surface, leveling the gaps between the coarse-grained aggregate particles. It is important to wait until the concrete has lost its shine before running a steel finishing trowel over the surface to avoid digging into the surface and possibly raising the water. Table 3 lists the appropriate surface finishes. Cement is the glue in a concrete mix.
Combined with water, it forms a paste that coats the aggregate and binds the mixture together. Aggregate makes up the bulk of the concrete structure, adding strength and reducing its cost. The individual stones in the coarse-grained aggregate intertwine and the sand fills the voids. Like most things in our lives, technology has affected bagged concrete. Frank Owens, Vice President of Marketing at Quikrete, says: “Today, bagged concrete mixes are designed for specific purposes due to the development of natural or manufactured chemical additives that improve certain properties of fresh or hardened concrete, such as workability or strength.
This allows mixes to be added correctly and mixtures intended for specific purposes. The strength of a concrete mix is determined by measuring the pressure required to crush a sample of test pieces cast from that mixture and allowed to harden for 28 days. All bag mixes have at least 4000 psi, and some specialty blends produce more than double that strength. The two most important conditions to consider when working with concrete are climate and weather. These factors become especially noticeable when pouring a slab, because the concrete has to partially set before it can be finished. When it's hot, the concrete sets faster, and you want to make sure there's time to finish laying the last mix before you have to finish the first few sections you laid.
Conversely, in cold climates, a slow setting mix can cause you to finish a slab with a headlight when you'd really like to be home having dinner. For slabs that are likely to see traffic in a short period of time, sidewalks and driveways, it is mainly worth looking for high-strength concrete early. Excessive watering is a common mistake when working with concrete. Theoretically, only enough water is needed to fully react with the amount of Portland cement in the concrete mix. Any water added beyond this results in a weaker concrete. That excess water expands the volume of the wet concrete, and some of it stays in place for a while after the concrete sets.
But eventually, excess water will evaporate, leaving the concrete less dense and not so strong. The key to achieving labeled psi force on a concrete bag is to keep the water-cement ratio as low as possible without sacrificing workability. Adding too much water can cause a number of problems. As wet concrete is compacted, water (the least dense component of the mix) moves upwards. If there is a lot of excess water, it can create vertical channels and get trapped below the course aggregate as it rises, creating voids. However, adding only the amount of water that is chemically necessary produces a mixture that is too stiff to mix and work with by hand.
More water is almost always needed to create a viable mix. That said, modern bagged concrete mixes often contain some type of plasticizer, a chemical that makes the mixture more manageable with less water. The instructions on the bag will tell you how much water to use - start there and only add more water if absolutely necessary. It's important to keep freshly finished concrete moist as it sets for 5-7 days after pouring. Common approaches are to cover it with plastic or burlap and keep it moist - this isn't done very often in residential jobs but makes all difference in its long-term strength. Except in water, bagged mixes come with just enough ingredients carefully dosed - but you can spoil it by adding too little or too much water.
Don't just pull out your garden hose and spray - use a graduated dosing container to distribute just enough water recommended by manufacturer in bag. Respirable silica has become construction safety concern of this decade - silica is crystalline mineral found in rocks and sand which when inhaled may scar lung tissue and make breathing difficult. Dry concrete mixes (as well as dust from demolition) contain respirable silica - The National Institute...
