Concrete is what usually gets blamed for failures and it is concrete finishers who place, finish and cure the concrete. It is important to take a closer look at what happens with the concrete, how the problems might be avoided, and what finishers should look for, do and know. Not only should the concrete finishers be aware of these issues but the flooring trades and decorative concrete overlay installers also must be up to date on issues that may affect their materials and workmanship.
Concrete finishers should be able to successfully pour, finish and cure concrete slabs, as well as, have a basic understanding of concrete, its properties and field testing methods. A successful heart surgeon must not only be skilled with tools to perform the operation but must also understand how the heart works.
Concrete is the most widely used building product today yet how it works and “gets hard” is many times misunderstood. Concrete is a mixture of a fine aggregate (usually sand), coarse aggregate (usually crushed rock or natural stones), cement and water:
¨ 11% cement
¨ 16% water
¨ 6% air
¨ 26% sand
¨ 41% coarse aggregate
The cement is the binder for the concrete is made from a combination of materials. These materials include limestone, shale, clay and iron ore. They are ground and heated in a kiln at approximately 3000° F. In the heating process, these natural materials are made into calcium silicates and aluminates. A mixture of about 5% gypsum is then added and reground to form the powder like substance we call cement. When batched and mixed with water and the aggregates, the material then becomes the product we can concrete. The mix we call concrete does not get hard by drying but curing that takes place through a series of chemical reactions with the mixing water called hydration.
Today’s concrete mixes may also contain several supplementary cementitious materials (SCM’s). These materials can be either natural minerals or industrial byproducts. They may also contain admixtures for air-entraining, water reducing, and set control.
Concrete is poured and placed with several natural materials. Water occupies a certain volume of the concrete mix design ranging anywhere from 8-20%. This volume will leave the concrete as the hydration process begins quickly at first and then slower over time. This is where many problems can occur. People begin to think just because it is hard enough to walk on or even drive on, people can therefore do anything to it. The fact is that this just is not true.
Although hydration begins when the cement and water make contact, it takes weeks and even years for the concrete to fully reach its final strength. An old saying is “when it dries it dies” simply put, concrete keeps gaining strength until all the water is gone.
Over the years, the use of materials such as fly-ash and slag-cement slow early set and hold the hydration water in for longer periods of time. Simply put the finishing time might be about the same but days have been added to the so called 28-day curing time frame.
The use of better grading of coarse and fine aggregates in mixes today have been good for placing and finishing but the better grading also holds the hydration water for longer time. Some mixes contain smaller coarse aggregates which require higher water/cement ratios.
Knowing these things enables both finishers and overlay installers to finish and install materials and concrete with a greater understanding of the set times and components of concrete.
Putting the concrete into service too soon causes many issues as well. How many times have we seen a floor poured one day and the next day arrive on the job to find:
¨ Scissor lifts with workers on the floor.
¨ Job boxes, ladders or other equipment being stored on the slab.
¨ Masonry, pallets of brick or block, concrete mixers, etc. being stored on the slab.
¨ A form of acid cleaner used to wash finished walls dripping on fresh concrete slabs.
¨ The new slab being used as a cleaning area or a water source or the use of power-washers on the concrete before the concrete is 28 days old. This can raise water/cement ratios at the surface of the concrete and force water into the concrete during early strength development. This can also affect the aggregate-to-paste bond needed for a durable concrete slab.
Water-cement ratios are key to a modern concrete mix design. Lower water-cement ratios result in:
¨ A better product.
¨ Fewer cracks.
¨ Less curling.
¨ Lower permeability.
Many issues today can also be traced to new finishing methods and the fact that cement is ground finer resulting in tighter floors. The demand for fast turn around and early entry for buildings has also caused issues.
Power trowels are now “ride on types” and weigh much more than the walk behind machines of the 1970’s and 1980’s. As little as 15 years ago, most floors were finished with walk behind machines that weigh between 150-180 lb. New ride on machines approach weights of 2500 lb. The finish with these types of machines results in much “tighter” floor surface and helps hold more water for a longer time.
There are many other instances that cause moisture issues in concrete. Saw cuts allow for water entry into concrete floors either from rain or water sources during the construction process. Vapor barriers beneath the concrete force water to find a way out or evaporate.
VOC laws have changed over the last 10 years and have resulted in confusion with both adhesives and curing compounds.
Both curing and curing materials have changed greatly over the past decade. Understanding the materials, intended use of the concrete, coverage rates and application methods is very important.
In the concrete industry, when there is more water than necessary for the hydration process to take place it is called “water of convince”. The hydration process can begin with water -cement ratios of around 0.27 but the concrete cannot be placed efficiently or economically at that ratio. Concrete dries from the top during placement and then internally over time. The time frame for the concrete to properly complete the hydration process and for the construction timeline to continue on schedule often conflicts with one another.
During the past few years, installing a vapor barrier has become a part of concrete floor installation procedures. Many problems have arisen from this. Vapor barriers provide a service in that they do not allow for ground water transmission, the concrete floor to stay dry, protect coatings and stored materials and thus some of the “sweating” seen on older floors is eliminated.
When poured, concrete has an internal relative humidity of 100% top to bottom. As the slab dries it does so from top to the bottom. When a impermeable coating is applied the drying stops and humidity levels can once again reach 100% and result in failures. To avoid issues the internal moisture should be checked before coating are applied.
Following both ACI Hot and Cold weather placement methods can help avoid issues. Protection of fresh concrete in less than ideal conditions is vital for its success.
In hot weather, one might assume that hydration water is gone quickly while in many cases this may be true, more water might be added to the mix during placement, and many times during hot weather there is high humidity, and heavy rain storms which may affect the subgrade.
Temperature has one of the greatest effects on set and cure times of concrete. Cold weather results in a longer cure time. Provide a sub-grade that is within 25-30° F of the concrete temperature and never pour on frozen ground or snow/frost. In cold temperatures set time is delayed and finishing time slowed. It is sometimes a bad practice of finishers to begin finishing too soon and trap bleed water just below the paste.
Both hot and cold weather can affect the concrete surface and decorative overlay. Installers should ask when the concrete placed. With today’s use of the internet it is relatively easy to find weather conditions during placement.
¨ ACI 305
¨ ACI 306
¨ ACI 308 18.104.22.168 Liquid membrane-forming compounds
¨ ASTM C09
¨ ASTM C 156
¨ ASTM C 309 (Sodium silicate solutions are chemically reactive rather than membrane-forming and they do not meet the requirements of ASTM C 309 specification.)
¨ ASTM C 1315
See Moisture Issues in Concrete Floors Part 3 – How to Test For Moisture