Better concrete practices for residential construction: taking steps toward accountability and responsibility for the residential concrete market driveways, walkways, and patios
From findarticles.com
The winter of 2000-2001 was bad for concrete in the Midwest. It wasn't that winter temperatures were colder or that frost penetrated deeper than usual, rather it was the unexpectedly large number of freeze/thaw cycles. This caused an abnormal amount of scaling on concrete surfaces--mostly on residential driveways, patios, and walkways. Jerry Woods, marketing director for the Iowa Ready Mixed Concrete Association, Ankeny, Iowa, said that that winter was Iowa's worst in 40 years--in many subdivisions he was able to use a shovel to scoop off the top of a driveway slab. There were even rumors that one Iowa town considered prohibiting exterior concrete installations in residential settings.
As the least regulated and least sophisticated of all segments of the concrete industry, scaling and spalling problems in residential concrete should come as no surprise. Although both the American Concrete Institute (ACI) and the Building Officials and Code Administrators International (BOCA) have specifications and guidelines for residential concrete, they are seldom read or enforced. It's rare that cities and villages have anything more than a strength requirement for footings, foundations, and flatwork. (And often they don't even specify strength, instead indicating the number of bags of cement per cubic yard.) Typically there are no requirements for air entrainment, slump and water content, admixtures, the use or non-use of pozzolans, forming and subgrade preparation, control jointing, curing, or seasonal placement considerations. All this is left up to the individual contractor to decide.
John Albinger, president of Davidson Concrete, Frankfort, Ill., says that residential contractors rarely place orders for concrete where they indicate the ingredients and proportions. "Concrete is typically ordered as a cement mix or a performance mix," he says. "A cement mix consists of portland cement, aggregates, water, and air entrainment. Performance mixes include the addition of fly ash and a water-reducing admixture." Albinger notes that a "cement mix" for 3000-psi concrete for footings and foundation walls is typically batched with a 0.6 water/cement ratio and that's before it gets to the jobsite where the contractor will often add 1 to 4 gallons of water per cubic yard. Concrete mixes with a 4000-psi strength requirement, which is what ACI Committee 332 recommends for residential flatwork subjected to deicing salts, typically have a water-cement ratio around 0.5, while the ratio for a performance mix is about 0.47.
Curing is another issue. Curing materials are seldom used in residential work, and, when used, are often applied incorrectly. Woods thinks this is a major issue in Iowa. Another common problem is concrete placed late in the season: the moisture content in slabs is often too high when winter freeze/thaw cycling starts, making them more susceptible to scaling problems during the first winter.
What to do
To address these problems, the town of Joliet, Ill., asked two ready-mix producers to come up with a solution. These producers, in turn, asked the Illinois Ready Mixed Concrete Association (IRMCA) for help. What followed was a series of seven meetings involving developers, concrete contractors, ready-mix producers, testing companies (including Construction Technology Laboratories [CTL], Skokie, Ill.), and city engineers. The purpose of the meetings, according to Albinger, was to develop a guideline specification that all parties would regard as fair and achievable. That document, "Specifications for Residential Exterior Flatwork," is now available from IRMCA.
Defining responsibility and accountability
The guide specification defines the responsibilities as follows:
* The ultimate authority resides with the local department of engineering. Cities can adopt the guide specification for outdoor residential concrete in its entirety or elect to use parts.
* When concrete contractors take out a permit to install driveways, walkways, or patios in a locale that has adopted the guide specification, they are provided a copy and become responsible to install the work accordingly. Contractors are also responsible to provide the owner, if requested, with copies of ready-mix batch tickets. The tickets must display an accurate street address or lot number, the time the concrete arrived on the jobsite and the time that placement was completed, the amount of water added on the jobsite, and comments on any factors that might affect the durability of the concrete, such as weather conditions at the time of placement.
* City inspectors approve forming and grade preparation before placement begins.
* City inspectors are required to visit the jobsite while concrete is being placed. They may require slump and air entrainment testing. If the inspector notices violations to the specification, the contractor is instructed to make the appropriate changes. If there are more than three violations in one year, the city can deny that contractor any further work in its jurisdiction for some period of time.
* The contractor who places the concrete must warrant the work for 1 year. If problems develop, the owner may ask for a meeting between the contractor, the city, and other partners to the original contract. If the issues aren't resolved, the owner can have the concrete tested. The party at fault is responsible for paying for this testing.
Guidelines regarding mixes and admixtures
To make it easier for cities and towns to specify desired concrete mixes, the guide specification calls for mix designs that follow either BOCA or Illinois Department of Transportation (IDOT) recommendations, with a water-cement ratio that doesn't exceed 0.45. In freeze/thaw climates, air entrainment is required, ranging from 5% to 7% depending on the coarse aggregate size (5% for 1 1/2-inch aggregates, 7% for 1/2-inch aggregate mixes).
Contractors may not increase slump to more than 5 inches except through the use of midrange or high-range water reducers. All mixes must include an approved water-reducing admixture. If a load arrives at a jobsite with less than 5 inches of slump, the contractor can add water to bring the slump up to 5 inches, but the indiscriminate addition of water to increase slump is prohibited. And when water is added at the jobsite, the amount must be noted on the delivery ticket and signed by the person authorizing the addition.
To manage concrete placement during different weather conditions, contractors may elect to use retarding or accelerating admixtures, with introduction at the jobsite being permissible. This includes the use of calcium chloride in amounts not to exceed 2% of the weight of cementitious material. However, flake calcium chloride must be thoroughly dissolved in water before being added to the mix.
Placing, finishing, and jointing
Under normal circumstances, contractors have 90 minutes from the time the concrete was batched until the time placement is complete. The guide procedure is to place, bull float, edge, and broom. Contractors are discouraged from using jitterbugs, trowels, or fresnos.
Control joints may be installed in the plastic concrete with grooving tools or with concrete saws after initial set. Joint spacing should not exceed two to two-and-a-half times the slab thickness expressed in feet. For example, joint spacing for a 4-inch-thick slab will be 8 to 10 feet. The maximum joint width for any thickness should not be more than 12 feet for exterior applications.
Curing
Until the first of November, when temperatures begin to drop, curing compounds are mandatory. The curing requirements also include protection from either excessive hot or cold temperatures. Contractors are also encouraged to use evaporation retarders when weather conditions could cause excessive water evaporation from the slab.
Though the committee didn't agree on whether a sealer should be required, its use was encouraged, providing that it has good moisture-vapor transmission qualities. It's left as a homeowner option.
When things go wrong
Compliance to the specifications becomes the joint responsibility of all parties to the contract. This includes the owner, the builder, the concrete contractor, the concrete producer, and the city or village. In all cases, communication follows the lines of the contract between these parties.
The contractor who installs the concrete is required to warrant the work for 1 year against failures due to construction methods, placement, curing, or materials. This warranty includes such things as major cracking, scaling and spalling, and low compressive strength. If these issues arise, and if a solution can't be worked out between the owner and the contracting parties, the owner can authorize petrographic testing or the drilling of cores to check for compressive strength. The cost, as mentioned earlier, will be assumed by whomever the testing reveals to be responsible for the problem.
The importance of regulating residential concrete
Regulating the quality and durability of concrete for residential concrete is consistent with how other segments of the concrete industry work, such as with highway pavement or structural concrete for commercial buildings. Residential concrete is one of concrete's largest markets. Keeping and expanding this market depends on producing and installing durable concrete. The IRMCA guideline is a good first step. Another of its committees is currently working on similar guidelines for footings, foundation walls, and floor slabs.
To learn more about concrete mix designs and placement, refer to the Portland Cement Association's classic book, Design and Control of Concrete Mixtures by Steven Kosmatka, Beatrix Kerkhoff, and William Panarese, now in its 14th edition. (Portland Cement Association, Skokie, Ill., 847-966-6200, www.cement.org)
* For a copy of "Specifications for Residential Exterior Flatwork" contact Bruce Grohne, executive director, Illinois Ready Mixed Concrete Association, 309-862-2144