Civil Engineering

Dear Civil Engineer:-
I create this site to help you in your work even if you are a construction, design, quality control, quantity surveyor, etc..... So, I start now to introduce some informations to you which I believe that is a very important for all civil engineers.

Acceptance criteria for concrete compressive strength: the concrete strength is considerd to be satisfactory if both of the following requirements are met:

1. Every arithmetic average of any three consecutive strength tests equals or exceeds the specified strength.
2. No individual strength test (average of two cylinders) falls below the specified strength by more than 500 psi.

"According to ACI 318 item 5.6.3.3 & ASTM C 94 item 17.4"

Concrete Batch Plant: The project should has a central mix concrete manufacturing facility with a sustained production capacity of 80 cubic meters, it shall cosist of:-

1. Mixer with 1.5 m3 volume for one batch.
2. Control room provided with computer and printing facility to show the components of every one patch and the total actual, target, and difference weights. The batch recorder is an integral part of the batching facility and is not rely on manual entries.
3. Hot weather system with the capability to chill water such that the concrete temperature is not greater than 32 degree centigrade when discharged from the conveying vehicle.
4. Washing facility for coarse aggregate.
5. Main hopper with at least 4 sections.
6. Minimum number of 4 silos for cement storage.
7. Concrete truck mixers (No. of 4 minimum).

The batching facility should be calibrated according to NRMCA.

The above requirements is assigned for big projects like power plants, gas plant, petroleum or petrochemical plant,......etc.

Concrete discharge time limites:-

Discharge of the concrete shall be completed within 1 1/2- h time or 300-revolutions, whichever comes first, after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates.

According to ASTM C94 item11.7

Problem I faced and solve it:-

The above 3 informations are the most important informations should knowen by civil engineers, actually if I know this informations from along time may be I woulden't face the problem which I will tell you about in the next lines:-

When I do my regullar job as a quality control engiener for a main contractor I test the concrete compressive strength specimens and while I test it in the lab. I found the 28 days compressive strength not achieved, and all test resultes of 28 days compressive strength are failed and less than the required strength, in this time the problem starts because when the owner and consultant know this they orderd to stop all work and check all materials and batch plant facility and start to take core specimens from the two buildings which the resultes are about their concrete, the core test resultes showes that the resultes for one building is good and acceptable when the resultes of the seconde building are failed to meet the strength requirement mentioned in ACI 301 section1.6.7.3 , then owner and consultant require to demolish the seconde building and this was the start of my battle to proof that this building's concrete can be accepted and resist the loads (dead and live loads).

My project manager did a favour for me (because he believe in me) that he helped me to proof my idea and he take the risk and hire a special professor to start to study the situation and prepare a technical report about this problem, this professor start to study the building structural stability and serviceability that means to asses the acual compressive stress of concrete under the acting load or in other word to check the capability of concrete sections to resist the applied loads and the conclusions of this report was that when reanalysis of the building using both limit design theory and working design approach the structural elements were found safe regarding the low stress determined from core resultes.

The problem not finished because owner and consultant of the project reject this report and ask my company again to demolish the building and start a new one, at this time we made a challange to us this challenge that we will complete the building at it's present state with a good quality concrete and after that we will make a load test for a specific parte of the building, and after long negotiations they accept our offer but they ask to make this load test for the whole building, and we accept that.

The idea of loading the building is our professor idea and it's simple illustration is to complete all dead loads of the building (concrete+brick) and put a load on the building equal to the rest of dead loads (like tiles) and live loads the load intensity including dead load already in place shall be Pt=0.85(1.4D+1.7L), and after that he assign the critical sections of columns, beams, and slabs then deflection of beams and slabs measured and strain of columns, a group of dial gauges fixed for this purposes. The load applied in increments by using sand with height =0.7 m for the first floor and hight=0.35 m for the roof slab (the building is ground floor and first floor) and the dial gauges reading recorded after the increment take place and after 24 hour of increment applied and the same method when unloading, the test resultes analysis lead to the following conclusions:-

1. The building is structurally safe.
2. The measured deflections in all slabs and beams are within the allowable limits according to ACI 318.
3. Measured strains in columns showed that the equivalent concrete strength is about 87.86% of the required strength.
4. Visual inspection of all structural members showed no sign of failure.
5. Hair cracks were detected by visual inspection in some beams because the total applied moments are larger than the cracking moment.
6. No sign of cracking were observed in footings due to flexure or due to punching.

Finally this problem finished without demolish any structural element, after this problem I decide to follow a good concrete quality plan to avoid any problems in the future, my recommended concrete quality plan will be demonstrated in thge next lines.

Recommended concrete quality plan:-

A- Materials:-

* Cement:-

Any type of cement (according to project specification & requirements) shall conform to ASTM C-150, the following tests shall be done every week:-

1. Chemical analysis according to ASTM C-114.
2. Compressive strength of hydraulic cement mortar according to ASTM C-109.
3. Fitness of Portland cement according to ASTM C-204.
4. Time of setting of hydraulic cement by Vicat needle according to ASTM C-191.
5. Soundness, ESS 373-91.

Note:-

The above mentioned tests will have a frequency every week in case of new source (no quality history),

In case of a well known source (quality history already established), the rate of testing will be reduced to every month,

The quality history shall represent cement from the same source as the cement to be tested, and shall be based on available test results for not less than 40 test samples.

* For each shipment, mill certificate containing chemical and physical properties shall be submitted.

* Admixture:-

Any type of admixture shall conform to ASTM (according to project specification), name of the used material and dose required according to design mix prepared by a good qualified independent laboratory, QC certificate shall be submit for each shipment.

* Water:-

Water used for mixing concrete shall be free from impurities, oils, acid, salts, alkali, and organic matter, Chemical analysis for mixing water every month shall be done by a good qualified independent laboratory, Water of questionable quality shall be subject to the acceptance criteria of Table "1" ASTM C-94.

* Aggregates:-

The size of coarse aggregate will be determined according to project specification & requirements, the following tests will be carried out every week for coarse & fine aggregate:-

1. Unit weight, ASTM C-129.
2. Sieve analysis and fineness modulus, ASTM C-136.
3. Specific gravity and absorption ASTM C-128.
4. Material finer than sieve #200 by washing ASTM C-117.
5. Clay lumps and friable particles ASTM C-142.
6. Abrasion test (los angeles) ASTM C-131(for coarse aggregate) & Organic impurities ASTM C-40 (for fine aggregate).
7. Soundness ASTM C-88.
8. Sulfates & chlorides content BS-812-part 117 & 118.

** Potential reactivity test according to ASTM C-289 will be done every 60 days.

* Total salt content (chlorides & sulfates):-

Contractor shall insure that:

Total soluble chloride ion content & total sulphat content of any class of concrete mixture contain less than values mentioned in ACI 222 when tested according to ASTM C-1152.

* Fresh concrete tests:-

1- Slump test according to ASTM C 143 will be :

-Every 40 m3 deliverd or fraction of.

-When visual inspection indicates non-conformance.

-Not less than once per day.

-With each set of test cylinders.

2- Air content, unit weight, and temperature test according to ASTM C231 and ASTM C 138 will be;

-Every 40 m3 deliverd or fraction of.

-Not less than once per day.

-With each set of test sylinders.

If slump or temprature falls outside the specified limits, a check test will be made immediately on another portion of the same sample. In the event of a second failure, the concrete shall be considered to have failed the requirements of specification and is subject to rejection.

After I follow this quality plan for 18 month on more than 20,000 m3 of reinforced concrete a report prepared by independent laboratory to evaluate the strength test results of concrete along this period and the result was that the class of concrete control is "excellent" according to ACI 214, and I received an appreciation letter from owner and consultant.