Concrete: 3500psi vs 5000psi

most cases like this will be OK with 3500 psi concrete, as long as it is thick enough and properly bedded wih the right seams and packing.

You need high strength cement for large heavy machines, like mills, motors etc and where you will have high rolling loads like fork lift trucks

Little Engineer,

You don't say what the concrete is to be used for.

Lots of pedestrians and occasional vehicles doesn't sound like a roadway.

They can both be made to cure quicker than normal, if necessary, so that's no reason to drive up the cost by using a more expensive, 5,000# mix.

If the vehicles are not very heavy, and the slab is designed properly, the 3,500# mix should be OK.

5,000# mix could produce a thinner slab. Use less concrete.

3,500# mix should be easier to finish, if that is a factor.

Whatever you do, cure it well.

Remember This: Concrete wants to be as dry as possible until it's set and as wet as possible until it's cured.

Do that and you'll get good concrete. Also, watch your cover on bars near the form.

Old Architect,

cgpedersen@aol.com

In terms of pros and cons:

• If stress is the defining factor, the 5000psi will make for less concrete but slightly higher reinforcment in kg/m3
• If displacement is the defining factor, the two are quite similar in amount of concrete and reinforcement
• The ground conditions and substrata to the slab will have a big impact on the relative costs of each solution
• Softer ground conditions will generally require a thicker slab and so the 3500psi will tend to be better
• 5000psi will allow for less cover to the bars (less concrete)
• 5000psi will allow for less lap to the bars (less reinforcement)

We are trying to make the most suitable most economic slab (both in construction and in maintenance costs). 3500psi is slightly more expensive than the 5000psi in $/m3 but the 5000psi can be less in volume which makes it worthwhile.

Thanks! This is helpful.

By the way, the area is meant to be a sidewalk, but its on a college campus so the plan is to be able to have the occasional service vehicle drive on it and also have the possibility of setting large tents for gatherings on special occasions.

As a sidewalk with pedestrian loads and light service vehicles, I don't see the need for 5,000# concrete.

The two most important considerations will likely be an adequate base and adequate expansion & control joints. The base should be at least as thick as the slab; thicker if you are in an area with wet soil. Obviously, all organic soil must be removed before the base is installed. Base needs to be a sharp crushed stone like CA-6, well compacted. Doweled, 3/4", expansion joints at 30' O.C. and control joints at 5' O.C. a minimum of 1" deep should insure a long lasting sidewalk. In consideration of the vehicle use, a 5" slab is warranted. Moderate size temperature mesh (6x6, #10 x #10, old terminology) should be OK. I'm sure you'll get lots of help in designing the slab. Don't forget air-entrainment.

Nothing that I've seen calls out for 5,000# concrete.

You don't say what part of the country the sidewalk will be constructed in. Weather can be a factor regarding the lifespan of concrete. Concrete will shrink and expand just like metal with heat and cold applications, and may carry a temporary heavy load greater in mild temperatures than either extreme hot or cold conditions. It is weaker near the edges and corners and often fails there first. curves offer unique problems, improperly designed curves will inevitably crack and walk if not properly reinforced and tied together at joints. I have personally seen construction joints that began as a 3/8 in. separation grow to exceed 7/8 in. in about one year. This was caused by the expansion and contraction process. As the concrete expands it will push against buildings curbs and other concrete placements. When it cools and contracts it will not pull these other materials with it unless attached to them. This will leave an open joint that will collect materials that don't compress well or at all. The next time the concrete warms up it will expand again moving further away from the other side of the joint, gradually walking the expansion joint wider. All concrete joints should be sealed for this purpose.

Concretes blends are as varied as are metal alloys. The materials used in the blends serve many different purposes. For instance a 3500# blend of basic portland cement, with sand and a 3/4 " granit agregate will be used for a different application than the similar mixture using 1 1/4" agregate. Mixes are specifically designed for it's purported use. Example; Blends used for airport runways are different than mixes used for side walks, and parking garages are designed different from sea walls or high rise bldgs. etc..

I suggest that you will save money in the long run by having the specific project designed by an engineer that specializes in the specific kind of project your undertaking as it will need to be designed for specific loads to be handled over a know period of time with the occasional heavier loads accounted for. TMF

The other posts have pretty much covered it pretty well, but I did want to point out that a higher cement factor gives added durability (resistance to weathering and freeze-thaw damage) as well as strength, and for that reason alone it may be worth using a richer mix in exposed locations Also, you want to make sure that you have enough entrained air, for the same reasons. 4 to 8% should work well.

I agree wholehardedly. would just like to add that I generaly use a 5 1/2 to 6 bag mix on all projects the slightly higher cost is worth it in that the higher cement mix gives a better finish, is easier to finish plus adds to the durability. I also have found great success in using fiber mesh as reinforcing instead of wire, the cost per square foot is about the same , but fiber mesh is a lot easier to use then wire ( i.e. no tripping over wire, or labors forgetting to pull wire up, ect.).

oilcan13