Hollow Blockk testing 1

[gyroman10]

i am working for "Save the children" and have been recently deputed to Pakistan, for a construction project for the (2005) earthquake victims. This school construction project is scatered and is in very remote areas. the construction is hollow concrete block masonry with (non load bearing), nine schools at different locations have to be constructed this year. the concrete block testing lab is 12 hours drive from the construction sites, traveling also is not very safe here. I was thinking can i get a small mobile type Testing machine to test concrete hollow blocks ? which i can load in a pickup truck and move from site to site and test the strength of the blocks before they are placed. i am really worried about the strength of blocks as they will be manufactured at site manually using vibrators. Minimum design strength of block is 1200psi.
any suggestions.

 

[concretemasonry]

gyroman -

Check the Forney(sp?) web site. They manufacture many different testing machines.

The small one block machines are elctric powered and have a footprint (2' x 3').

How are you measuring the strength? Different specs and standards use different criteria - gross area? net area?. Current ASTM stardards use the net area, but older ASTM standards and some international standards use the gross area.

If you have reasonable aggregate and have vibration, 1200 psi can be acheived easily if there is adequate moisture for curing. Even rammed earth can meet that. It is not necessary to cap the units if you have access to gypsum board or plywood to use for a cap and apply a correction (15%-20%) that recognizes the lower indicated strength from the non-standard testing method. Testing without capping will be more variable and requires slower loading rates.

 

[civilperson]

Why test? Non load bearing walls have to carry only their own weight. If blocks weigh 35 pounds/square foot then any strength greater than 15 psi will suffice for self weight. Test only what is structurally significant.

 

[LCruiser]

Is this not in a substantial earthquake zone? I seem to recall a big one in Pakistan a couple of years ago.

 

[gyroman10]

civilperson

it is in a substantial quake zone. Zone 4, distance from source less than 5 Km. It needs strength for the lateral loading, 88000 people died here, mostly under lateral collapse of non load bearing stone masonry (most common construction technique here).

concretemasonry
i am unaware of any block testing standard, can you please put me in the right direction. i did visit the forney website, nothing small enough i could make mobile there.

 

[kelowna]

Cannot you make your own testing rig?

Small steel frame (say U sections on a square configuration welded or bolted, a few stiffeners where required) with a small jack on one side connected to a calibrated gauge and a hand pump. Steel plate to equaly distribute load between jack's ram and block to be tested.

It obviously will not be 'as per standard' but it will give you a very good idea of what the strength of the blocks is, which is, after all, what you need.
It will fit in the back of a pick up truck,it will be fairly cheap to fabricate, sturdy and easy to use, mark minimum strength required with a marker, it passes, good, it fails, bad.

 

[concretemasonry]

An appropriate standard for testing concrete masonry units is ASTM C140. ASTM stands for American Society for Testing and Materials. It covers all points like sampling, timing, dimensions, absorption and strength.

One of the problems with testing block is the size/area/geometry of the units in comparison to other materials like concrete cylinders. A lightweight tester would be hard pressed to give reasonably accurate results because of the deflection and distortion if they are ever challenged. One that occurs in testing is that the platen (plate on the top) distorts severely and redistributes the load causing severe excess load in the center and premature failure. Even a testing cannot use the same platen they use for concrete cylinders as for 8"x16" block(must be up to 4" thick steel). - If you think you can make your own test rig and justify the results, go for it since you only need to apply about 100,000 pounds!

Your required standard is so low compared to most standards, all you are probably doing is verifying the strength. You probably cannot switch block "suppliers". I don't think you want to prescribe the manufacturing methods, since making block is not at all like mixing concrete.

Dick

 

[civilperson]

gyroman,
Block unit compressive strength is not applicable for lateral seismic resistance. Rather, some minimum reinforcement is necessary for moment strength. That is why unreinforced masonry killed so many people.

 

[concretemasonry]

gyroman -

It sounds like you are in a position to verify the materials and the design is already done.

Unfortunately, most structural engineers (myself included) get lost in the details, load paths, etc., the subject of practical constructability and performance get lost in the philosophy. - This is based on seeing contruction in many developing countries, the available materials and the direction to carry out the necessary building.

If you can, please make some suggestions to make the common construction materials (fortunately now hollow concrete block are used) contribute more to the structure. Because you are now using concrete block instead of stone in your remote area, suggest vertical steel at openings such as doors and windows. Anchor the steel some how. Although joint reinforcement is thought of as a "non-structural, crack control" measure, it is very, very valuable in the maintenance of structural continuity and integrity. Somehow, the U.S. ACI 530 code does not address this important life safety issue, which is more of a design issue. - This is based on looking at/assesing the Northridge, CA seismic damage and the reusults of the two shakes in Turkey (and 15+ other developing/rebuilding countries).

Do what you can to enforce the material specifications and offer what constructability/long term performance measures you can.