The key to achieving proper concrete strengths is a direct result of the correct mix design and the proper handling of the plastic (fluid) cement.
The project engineer will specify a minimum concrete strength to be utilized. There is nothing preventing the contractor or project owner from using stronger mix designs. The added cost is negligible and the resulting performance and benefits are significant.
The project contractor will discuss the project requirements with the ready-mix plant's "Mix Master" when they place an order for the concrete. They will discuss ultimate strength requirements, weather forecasts, traffic conditions, rate of placement (yards per hour), distance to pump and the pumping equipment.
The Mix Master will discuss the need for water reducers, plasticizers, air entrainment, set retarders or accelerators with the contractor. The aggregate size should be the largest allowed by the project engineer. The larger the aggregate the stronger the concrete will be.
Reduce the Water Content
Concrete is weakened by excess water. When water evaporates from concrete, it leaves behind a matrix of microscopic honeycombs. These voids weaken the concrete. They also allow ground or rain water to permeated into the concrete. This can result in corrosion of the reinforcement steel and eventual project failure.
Only that amount of water that is necessary to hydrate the cement particles is required. To increase the slump (liquidity) of the mix design, water reducers & plasticizers should be used. Think of these additives a being able to s-t-r-e-t-c-h the water. The resultant strength will be significant, though the initial concrete will be liquid, workable and pumpable.
A reputable ready-mix plant will even know the moisture content of their sand and aggregates. They will subtract from the water to be added to the mix, the water inherent in the aggregates. A load tag will accompany the delivery truck. The assay of the contents of the truck should be specified, along with the load time and starting count on the truck's rotating drum.
My ready-mix supplier even provides the assay of the aggregate in their mix. The load tag indicates the psi compressive strength of the rocks in the mix.
It makes absolutely NO SENSE to use concrete that contains rocks weaker than your objective. But, there are many ready-mix suppliers who use weak and inferior aggregates (e.g. quartz ). If they don't volunteer this information - ask. They are required by the building codes to tell you.
Achieving High Strength Concrete
Higher strength concretes are denser and more durable. This is achieved by "tightening" the matrix of the concrete. The increased cost of a higher "sack" mix is minimal (e.g. a 4 sack mix vs a 6 sack mix). A "sack" is equivalent to 94# of cement powder (about 1 cubic foot).
There are many means to increase concrete's strength. Many of the methods are used in conjunction with each other:
- Reduce the water content
- Increase the aggregate size from 3/8" to 3/4"
- Add pozzolans (Supplementary Cementitious Materials - SCM)
- Utilize crystalline or water proof admixes
- Corrosion inhibitors
- Air entrainment (freeze-thaw conditions)
- Metakaolis (help reduce efflorescence)
- Integral color pigments
Integrity of the Mix Design
It is the responsibility of the contractor to ensure that the mix design is not adultered by "well meaning" individuals. Concrete pump operators and ready-mix truck drivers are the worst culprits. When the truck arrives, the pump operator tells the driver to add water to the mix. Before the concrete is even out of the truck, the strength is being compromised.
After the truck arrives, if you hear the engine revving and the drum spinning rapidly they are adding and mixing additional water.
The concrete should not be allowed to sit in a truck, waiting for prior trucks to be unloaded. The concrete may achieve it's final set while in the truck, rendering it useless (unless set retarders were added).
The truck must achieve a minimum number of rotations prior to off loading. The dry ingredients are added to the truck at the ready-mix plant. It is the rotation of the drum and vanes that actually mix the concrete.
Concrete that is to be placed into tall forms, caisson holes, deep trenches or footings should be placed from the bottom up. It should not be pumped in from the top and allowed to fall to the bottom. This separates out the aggregate and compromises the strength. It also creates large voids and air pockets in the finished product.
Proper consolidation (vibration) must also be preformed. Excessive vibration will also cause the aggregate to settle to the bottom. The proper mix design can virtually eliminate the need for consolidation.
Concrete does not "dry," it cures. As the water hydrates the components of the concrete mix, crystalline structures are formed. These are what give the concrete it's density. Controlling the rate of water evaporation is critical. The goal to to have the water leave the concrete as slowly as possible. This is achieved through proper curing.
Coverings of burlap, carpet or plastic are the most rudimentary. This method cause surface discoloration due to trapped moisture under the covering. They are also susceptible to being moved or blown out of position.
Water curing is performed by maintaining the moisture content of the concrete by frequent sprinkling, spraying or flooding with water. This is not always practical as there may be water restrictions, drought conditions, lack of personnel 24/7, the project is too large or the site may become excessively muddy.
Chemical curing compounds are chemicals that are sprayed onto the finished concrete after it becomes stiff, but is still damp in appearance. Some chemically penetrate the concrete while others form a surface film. These are the method preferred by most State Highway Departments because they are the most infallible.
Care must be taken in selecting the proper curing solution. Some of them create a "bond breaker" that inhibits the bonding of subsequent surfaces. Subsequent decorative finishes of plaster, stucco, paint or stone work may not bond to the concrete.
Then why do they even make those types? Contractors who pour roadways, bridges, sidewalks and other high traffic surfaces are the largest consumers of these type of curing compounds. They are also the most inexpensive. Most contain some type of waxy substance. The curing film often dries white (or has a dye), to indicate areas of coverage and proper application. Over time, the film will eventually wear away from traffic, abrasion and weather.
Remember, they are concerned with lifetime performance of their structure (as should you!)
Paolo Benedetti - Aquatic Artist
"Creating water as art."™
Aquatic Technology Pool and Spa