How do I level (screed) the concrete over an area that is wider than my screed board.
You can tackle this several ways. The professionals will drive grade stakes inside the forms at convenient intervals (less than the width of the screed board). The grade stakes will have the proper level marked on the stake, usually a nail in a wooden stake. You can place some concrete in the forms and using the grade stakes as a guide, begin screeding the concrete. As you get the proper grade on the concrete, you can pull the grade stakes. As you can tell, this requires a little experience to get it just right, but beginners can do it with less accuracy.
Another method is to get a probe, such as a stick or steel stake, mark it with the depth you want the concrete to be, then place the concrete in the forms. Screed the concrete and probe to ensure the proper depth. This is not as efficient as the first method, but it will get the job done with a little more effort. You can set construction joints (1×4’s, metal keyway forms, etc) inside the forms at intervals of 25-30 times the depth of the slab. You can use these as screed guides.
Finally, you can set screed guides inside the forms using 2×4’s and stakes so that the bottom of the 2×4’s are at the top of where you want the concrete to be. Nail ears on the end and top of your screed board so that when you set the screed board on the screed guides, the bottom of your screed board is even with the bottom of the screed guide, thus level with the top of where your concrete will be. Once you get your concrete down and screeded, you can pull these screed guides and fill in the stake holes and re-level by eye.
What standard specifications should I know or understand regarding information received from a contractor in anticipation of signing a contract to have my concrete driveway replaced? What is better rebar, reinforcement mesh (both?) , fiberglass reinforced cement? Depth of the driveway poured, 2inches 4 or more? How far should expansion joints be put in to handle heat and cold.
Reinforcement steel in flatwork is strictly for crack control. The best crack control is affected by putting the steel in the upper half of the slab. If your contractor can guarantee he will keep the wire mesh in the upper half of the concrete, that will do as well as rebar. Make sure he gives at least 3/4″ cover to whatever reinforcement he uses. The standard driveway depth is 4″, but that doesn’t mean that depth is right for you. It really depends on the soil conditions and the load you will be putting on the concrete. Thickened edges (beams) are highly recommended for the outsides of the driveway. This will enable the edges to take that occasional load on the edge or coming onto or off of the edge. Normal
Portland cement concrete likes to crack every 10-12′, so jointing in all directions no more than 10′, with the joints being a minimum of 1/4 the slab depth, should help the expansion/contraction problem. There are two types of joints: contraction and construction. Contraction joints are usually installed with a concrete saw after the concrete gets hard or a jointing tool while the concrete is still fresh. Construction joints are preinstalled dividers such as redwood strips, aluminum keyways, or other physical barriers. Either type of joint will allow for expansion/contraction, so if you don’t want redwood every 10′, you don’t have to have it. Make your joints as square as you can. Try to avoid rectangular jointing sections. Make sure the contractor orders concrete that has 4-6% entrained air, since you live in conditions that probably require salting of roads. The air will enhance the concrete’s freeze/ thaw durability and make it less susceptible to salt damage. I could spend at least 8 hours going over things to look out for and things to avoid. The main thing is sitting down with the contractor before you sign a contract, and tell him how you want the thing to look when he gets finished. Discuss whether visible cracks are acceptable or not. Discuss a warranty. Tell them what you want in the end, put it in your contract, then get out of his way and let him give you what you want. If he doesn’t give you what he says he would in the contract, you have legal standing for a remedy. Also, let him buy the concrete so
he can’t blame you for buying an inferior product that he couldn’t work with.
Periodically, as a service for our readers, we’ll feature a select job posting with a top concrete industry company. This month we are proud to present a posting for a Ready Mix Dispatcher.
Candidates must have experience with Command Alcon software.
This is not an entry level position. Professional experience as a ready mix dispatcher is a prerequisite. The geographic location for this job is the Austin, Texas area.
How to apply: please email resume and requests to: hr@concrete.com
*No phone calls, please – email correspondence only - send resume in PDF or doc format
Concrete is truly a versatile building material. Concretes in use today are formulated with very specific performance characteristics in mind and include lightweight, heavyweight, porous, fiber-reinforced, mass, high-performance and cellular concretes to name just a few. Each provides specific characteristics or properties for their intended use. These properties are achieved by intentional formulation and control of such variables as cement content and type, pozzolan type and content, aggregate type, admixtures used, the addition time and rate of those admixtures, as well as other, often subtle, differences.
By George W. Seegebrecht and Steven H. Gebler
Contributing Editors
www.concrete.Com
One widely used specialty concrete is known as “shotcrete.” The major difference between shotcrete and its close cousin, concrete, is the placement method. Concrete is discharged from a ready-mix truck, placed on the ground or in forms and then must be vibrated for compaction. By contrast, the shotcrete process, whether using wet or dry material feed, does not require forming or compaction thereby enhancing design creativity and application flexibility, often resulting in a savings of time or money
Shotcrete, was originally called “Gunite” when Carl Akeley designed a doubled chambered cement gun in 1910. His apparatus pneumatically applied a sand-cement mixture at a high velocity to the intended surface. Other trademarks were soon developed known as Guncrete, Pneucrete, Blastcrete, Blocrete, Jetcrete etc. all referring to pneumatically applied concrete. Today Gunite equates to dry-mix process shotcrete while the term “shotcrete” usually describes the wet-mix shotcrete process. At point of application, both are typically referred to as shotcrete.
Dry-mix process shotcrete, introduces and mixes the required water at the application nozzle as the dry cementitious materials (fly ash, slag, silica fume etc.) and aggregates are delivered through the “gun” The nozzleman controls mix consistency, adjusting water addition to suit the changing conditions of the work area. The dry-mix process also is well suited for sporadic application operations since the majority of the water only comes into contact with the cementitious materials as it leaves the nozzle.The wet-mix process utilizes concrete delivered to the job that is thoroughly mixed excluding of any required accelerators. The ingredients are generally delivered in ready-mix trucks as with normal concrete. Accelerators or other admixtures may still be metered into the slurry at the nozzle along with air under pressure to increase the velocity of the material and improve control of the application or “shooting” process.
The impact velocity of properly applied shotcrete instantly compacts the material, yielding an “in-place” mix that is richer in cement and higher in strength than the same mixture prior to placement. Typically, a fine aggregate dry-mix shotcrete mix delivered in a 1:3 cement to aggregate proportion upon entering the application gun results in a 1:2 cement to aggregate ratio when in place. What appears to be a waste of materials and a dust nuisance known in the trade as “rebound” and overspray, actually results in dense, high-strength shotcrete as a portion of the aggregate ricochets off the receiving surface and away from the placement location. The loss through rebound will vary depending upon the dryness of the mix, the shooting distance from the surface, wind conditions, etc. The intended thickness is generally overshot, trimmed back to the design thickness and finished to the desired surface texture and appearance.
While the dry mix process sounds quick and economical, it requires precautions to ensure application quality. The nozzleman’s workmanship and experience are critical, since the nozzleman controls the critical water-to-mix ratio going into application equipment. With the wet-mix process, the nozzleman has no control over the consistency of the mix delivered to the job site, but can control the velocity of the materials and the addition of accelerators as the mix leaves the nozzle.
Just as in concrete mix designs, the water-to-cementitious materials ratio remains the single most important parameter influencing the compressive strength, shrinkage and overall durability of the final product. Application technique is also crucial and less forgiving than ordinary ready-mix. Good “shooting” technique can mean the difference between a dense high-strength material or one that looks good on the finished surface but actually has underlying sand pockets, voids and poorly encased reinforcing steel. Poor application technique increases the probability of cracking and its negative ramifications.The shotcrete process is more versatile than conventional concrete placement. If the shooting surface is sound, clean and accessible, shotcrete can be applied in very difficult or complex shapes or sections where conventional concrete formwork would prove difficult or impossible as well as cost prohibitive. Shotcrete is especially applicable for unique shapes desired in complex shapes, swimming pools and other unique features of aquatic parks. It can also be an excellent overlay and repair material for existing structures because of its potential to achieve good bond strength and low permeability.
The nuances and differences between concrete and shotcrete are too numerous to cover in a short article. Selecting a concrete placement method, whether it be conventional concrete, wet-mix or dry-mix process shotcrete, can be a challenging task, since there are positive aspects of each for almost every application. While it is true that one approach may be more applicable, adaptable or economical than another, the final concrete placement selection for the project should be based on project design, material performance criteria and overall budget.
So, you want to put in a patio, driveway or sidewalk, and you are going to use concrete. A very wise choice, we can all agree. One thing to know before you put in your concrete all concrete cracks. You say, “Wait a minute, I’ve seen concrete that doesn’t have any cracks. How can you say all concrete cracks?” Concrete typically consists of cement, rock, sand and water. In the fresh, or plastic stage, concrete is fluid.
By Kenneth Wayne Meyer
Contributing Editor
www.concrete.com
As it hardens, the cement and water begin to shrink, and the stresses created by this shrinking cannot be overcome by the small amount of strength developed by the young concrete. If you place the concrete on a windy day, the top may start to harden before the bottom, which will cause the concrete to shrink unevenly (plastic shrinkage cracks.) Also, if the ground underneath the concrete is not level, there will be an unequal dragging force while the concrete shrinks, also causing stresses the new concrete cannot withstand. So, how do you get concrete with no VISIBLE cracks in it? By following a few simple steps before and after you place the concrete, you will have a very nice looking structure that will require very little maintenance, and give you years of enjoyment.
Before you place the concrete, make sure your subgrade (ground beneath the concrete) is thoroughly compacted and level. The absolute best thing to do is get a garden tiller, till the soil to a depth of 6 inches, then rent a hand operated compactor and compact the soil vigorously. This will help ensure there are no soft spots. You can apply a layer of cushion sand if you want. This will help achieve a totally level surface and allow a consistent friction to the shrinking concrete. Four inches of washed sand ought to be plenty for the cushion. If you use a wire mesh for reinforcement, use panels and not rolls. The rolled wire mesh is extremely difficult to keep in the top half of the concrete, where it HAS to be in order to do its job. You can also use reinforcement bars (rebar) tied together with steel wire, but spacing and size requirements vary based on load and soil conditions, so it is hard to recommend a standard set up for that. If you do use rebar, it is essential that you keep it in the top half of the concrete. You can use stones, broken brick or you can buy plastic chairs that the steel will sit on to keep it in the proper position when you place the concrete. You can also have the ready mix concrete company supply fibers to the mix. These fibers are usually nylon or polypropylene. They help keep the cracking of the concrete on a micro level instead of a macro level (where you can see the cracks with your naked eye.) Steel reinforcement also helps keep cracking in check, but if cracking does occur, the steel, when properly placed in the concrete, will hold the concrete together, whereas fibers will not do that.
Okay, you’ve got your subgrade ready, you have placed a plastic vapor barrier on the subgrade for slabs that will support dwellings, your steel is sitting nicely on your plastic chairs in the proper position, and you now have 14 of your closest friends on their way over to help you place the concrete you have coming. When the concrete arrives, if you don’t have a vapor barrier, wet the subgrade without puddling the water so that the water in the concrete will not be absorbed by the dry subgrade, thus causing uneven drying and the dreaded plastic shrinkage cracks. Once the concrete is placed, make sure to protect it from high winds and direct sunlight so the concrete will dry evenly from top to bottom. You are now ready to perform the most important step in preventing noticeable cracking. Contraction joints are the secret to no cracking! By placing contraction joints that are at least 1/4th the depth of the concrete and on intervals of 25 to 30 times the depth of the concrete (usually easiest with a jointing trowel or tool while the concrete is still fresh), you will almost ensure there will be no visible cracking in your concrete. If your slab is 4 inches thick, the joints must be at least 1 inch deep and placed every 100 to 120 inches. If you cannot use a jointing tool to put the joints in, you can hire a concrete sawing contractor to do this for you. Make sure he cuts the joints a minimum of 1/4th the slab depth. This jointing method helps the concrete crack at the weakest point. This is why it is so important for the joints to be deep enough. Variations in subgrade levels could cause greater stress in the concrete in an area where the joint isn’t deep enough, and the concrete will crack outside the joint. Once your joints are in place, and the concrete has cured for about two weeks, you are ready to seal the joints. This will prevent water from migrating into the subgrade and expanding and contracting, or getting into the joints and freezing, causing the water to expand and breaking out the concrete around the joints. You now have a concrete structure that will serve you well.
