11 concrete test procedure respecting a suspended slab

[Type text] Buildings Practice For ProfessionalsConstruction Method Statements

Chapter 11 Test Methods Respecting RCC Suspended SlabTest Methods Respecting RCC Suspended Slab

Scope1 Objective of this statement is to give an outline procedure wise effected from stage of

cube test failure to complete rescue of concrete, involving all technical & procedural steps in compliance to various codes pertinent to one 8 th level plant room suspended slab of a reinforced cement concrete eight storey building.

2 Slab was ordered in 1997 by Consulting Engineer to be demolished for reason of achieving less than permissible cube test characteristics compressive strength of concrete, but combined efforts by contractor & ready mix concrete supplier along with help by authorities, resulted into approval of slab past four months of issue of demolition order date, proving that sometimes, faulty testing machines can jeopardize whole project & erroneous test results may be disastrous.

3 Post twenty eight days curing after issue of demolition order was also, applied to add to replenish certain deficient chemical reaction in hardened concrete, which if possibly could be a cause, based on advice of ready mix supplier material research engineer.

4 That time, effect of supplementary cementing material was not known clearly, which accounted for such untoward testing results, while concrete achieved required strength.

Key Words 1 Cube test, Core Test, Loading, Characteristic Compressive strength, ACI (American

Concrete Institute), BS (British Standard), Engineer, Contractor, RMC Supplier

History1 Commercial Building 8th slab core test resulted in failure of its cube compression tests on

7th day/28th days. 2 Matter was reported to Engineer on 8th & on 28th day, who advised for a core test

according to ACI provisions, when requested by contractor, but core test failed, since test conducted by same laboratory that carried out cube test.

3 Consequently, Engineer ordered for removal of slab completely & reconstruct accordingly.

4 Ready mix concrete supplier & contractor expressed procedural doubts about laboratory testing equipment as well as, core capping material, but laboratory claimed that its machine was fully updated respecting calibration & core capping compound Sulpha being within its defined date of expiry.

5 Immediately, contractor conducted hammer test, which slab qualified, but Engineer did not approve slab.

6 Meanwhile, conducted another core test, which resulted better than previous test, but engineer did not pay any attention to this plea.

7 Another official core test was conducted successfully on request made to engineer by us & ready mix concrete supplier, but engineer still maintained demolition of slab & reconstruction.

8 Matter was referred to municipal engineer/buildings official, who approved/supported contractor’s proposal to carry out either a load test or one more core test.

9 Contractor conducted load test, which was too successful within provisions of deflection recovery, but even then, Engineer upheld demolition order, for it was not binding on Engineer to accept load test according to ACI provisions.

of 10 2012 Int. P Eng (India) Suraj Singh


Chapter 11 Test Methods Respecting RCC Suspended Slab10 Engineer contended that it was discretionary authority, whether or not, to accept load test

results. 11 Municipality was not in a position to approve slab contractually/legally, but requested

Engineer either to allow one more core test or department shall allow another Engineer take over management supervision job.

12 One more test was conducted too successfully & then Engineer approved slab consequently.

13 Contractor had also, cured slab post 28 days on advice by one material research engineer of concrete supplier, but this part of curing was meant only as preventive measure.

14 Reference tabulation- Cube & Cores Test result data 1, 2 & 3, Load Test result data 4

Slab Testing Procedures1 Cube Tests for soil testing & construction materials2 Tests were performed according to BS 1881 part 116; 1983 “method of determination of

compressive strength of concrete cubes”3 Cubes were tested in presence of consultant’s representative4 Tested cubes were observed normal in shape

Hammer Test1 Schmidt hammer test was conducted on June 28, 1997, which observed 34 N/mm^2

gauge reading indicating an equivalent of 40 N/mm^2 compressive strength2 July 12, 1997, Engineer rejected slab & required removal & reconstruction (Order Copy)

a “Studied core test results taken from top of roof slab. b Method of testing complied to BS 1881; part 120; 1983, applicable for

determination of compressive strength of concrete coresc From laboratory tests results of concrete cores, it was found that equivalent cube

strength of all cores is about 50% or more lower than specified design strength of concrete.

d Such poor results of concrete are unacceptable for structural safety. e Since, concrete core tests results have failed to achieve desired results as such

relevant top roof slab should be demolished immediately, following precautions as noted herewith.

f Any extra cost or time will not be allowed for demolishing & re concreting said slab according to drawing & specification.

g However, supplier for ready mix concrete is hereby suspended until further order & approval for same company is henceforth being withdrawn.

h Concrete cores, which were tested, should be analyzed further to determine ingredients of concrete with a view to justify mix design.

i Necessary arrangements should be made to send test specimen through our engineer to laboratory selected by us & cost of such results should be borne by you.

j Since, cores were taken from in situ concrete slab in presence of our representative & contractor as well as, testing of specimen was witnessed by us, as such, question of any further test for same does not arise.

k Special care should be taken to avoid damages to any structural element, while doing demolition.



Chapter 11 Test Methods Respecting RCC Suspended Slabl Should there be any damage to any part of existing structure, while carrying out

demolition then you will be held responsible to repair or replace same as instructed by us.

m Any extra cost & time will not be allowed for such consequences.

Precautions for demolition of roof slab1 Demolition should be done by concrete cutter & steel cutter with aid of mechanical

jackhammer of smaller size making cutting pieces not more than 30x30 cm2 Supporting columns should be protected from getting damaged while breaking slab3 Plywood shutter with supporting props, should be built 50 cm below top roof slab & 1 m

wide platform around slab level to hold concrete debris falling from cutting4 In any case, concrete pieces should not be allowed to fall on first roof slab to avoid any

damage from falling impact”

Chemical AnalysisChemical Analysis report by laboratory

1 Report # …..2 Concrete core sample cement concrete analysis ASTM C 1084-19923 Sample description4 4-inch dia. x 4-inch length cracked concrete core (core appeared to have already tested

for compression strength)5 Identification Nil6 Date received July 20, 19977 Date tested July 26, 1997

Results1 Concrete dry density (as oven dry basis) 2340 kg/m^32 Cement content percentage 18.1 %3 Cement content weight 424 kg/m^3

Appeal petition by contractor to municipality1 Seek a leave to admit this petition representing an appeal on you to approve 8 th slab on

above construction project contract XYZ supervised by Engineer. 2 Above engineer’s reference is en closed herewith covering details of representation filed

with engineer. 3 Representation is summarized as follows

a. Slab could not pass cube test compressive strength i.e. average being 37.68 N/mm^2

b. Above slab could not pass first official core test, which was an average 15.75 N/mm^2 (estimated cube strength) & in our representation, raised our doubts about testing procedures i.e. improper use of capping compound (substandard capping compound)

c. Slab passed 2nd official core test, which was an average 39.3 N/mm^2 (estimated cube strength) qualifying provision of ACI manual 1991 part 3 reference chapter 17 clause 17.3.2.2 & 17.3.2.3 i.e. Evaluation of acceptance of concrete strength



Chapter 11 Test Methods Respecting RCC Suspended Slabd. During chemical analysis, cement content of concrete has been certified 424

kg/m^3, which is satisfactorye. Engineer vide above reference instruction has confirmed previous instruction vide

dated 12.7.1997 to demolition & rebuild slabf. By admittance of this petition, prefer an appeal on final competent authority under

contractual & regulatory provision i.e. municipal authority to approve above slab & advise us further.

g. Should load test be required prior to approval, we propose following laboratories M/S ………………., Geo Consultants

Municipal Engineer response1 Your letter dated August 16, 19972 We have no objection to either your carrying out

Additional concrete tests by a third party laboratoryOr a load test on slab as per design, relevant code requirements & supervision of consultants.

3 Our recommendations will be based on results of either procedure4 Signed dated 19.8.1997

Load Test by laboratory (Result data 4)1 Between September 8 & 14, 1997, this test was performed in accordance with BS 8110

part 2, 1985 section 92 Tests were carried out on whole area of 8th slab as instructed by consultant with total 221

tonnes load approximately. 3 Applied using solid blocks evenly distributed covering 130 sq m in 5 equal increments as

advised by engineer4 Vertical movements of slab were measured using a set of 7 dial gauges having 0.01 mm

sensitivity installed at 7 locations at soffit. 5 After completion of loading & deflection measurement, load was left in position for one

hour 1st & 2nd cycle & 24th hours for 3rd cycle. 6 At end of one hour, load was removed in 5 equal decrements with deflection

measurements at each stage of unloading. 7 In case of 3rd cycle at end of 24 hours, load was removed altogether & deflection

measured.8 Visual examination of slab indicated no sign of any cracking on concrete during load test.

Core Test (Result data 2, 2a & 3) ………………………………………

1 Certificate of test on Chemical Compound July 28, 1997, 2 Delivery date July 22, 19973 Testing date July 26, 19974 Cube nominal size 50x50x50mm5 # of samples 3 cubes6 Description of materials Flake capping compound7 Tests results 1/2/3 ID are 48.8/75.6/76.1 N/mm^2 respectively



Chapter 11 Test Methods Respecting RCC Suspended Slab8 6 # of concrete cores were drilled out vertically from 8 th floor using rotary core cutting

machine with hollow diamond bit cooled with fresh water, ensuring no rebar being cut as well as, no portion of slab damaged.

9 Cores were then prepared, trimmed, capped with sulphur & soaked in water for 48 hours. 10 As received bulk density of specimen, were determined prior to capping.

Engineer’s approval letter 1 “Referring to your letter dated November 4, 1997 containing 3rd core test results of 8th

slab, hereby confirm that, due to satisfactory result of concrete cores, said slab is hereby acceptable.

2 However, you are requested to fill up all voids formed due to taking cores with non shrinking material having strength equivalent to design strength (40 N/mm^2 at 28 days) of concrete without extra cost & time “

Conclusion- 1 It is an extremely rare example of concrete cubes failure in field that came across, but

provide an impetus to personnel belonging to contract, supervision & control parties to have an opportunity of in depth study to practically, testing hardened concrete in all respects/full range in line/according to internationally, acceptable set procedures applying British & American codes.

2 Structural engineers supervising concrete working in construction field should be very careful, while dealing with tests & technicalities involved pertaining to dimensioning of core.

3 Sulpha flaky capping compound should be within its shelf life period & must demonstrate compressive strength more than proposed cube compression strength at least 1.5 times.

1 Result data 1 Cube & Core Tests

Cube size 150x150x150 mm, Specified strength 40 N/mm^2 June 5, 1997 Cube

IdentificationLocation Cast

dateTest date

Age(Days)

Density(gm/cm^3)

Compressive strength

(N/mm^2)- 8th slab May

29, 1997

June 5, 1997

7 2.494 27.5

- 8th slab May 29,

1997

June 5, 1997

7 2.485 26.1

- 8th slab May 29,

1997

June 5, 1997

7 2.469 27.3

- 8th slab May June 5, 7 2.485 25.6



Chapter 11 Test Methods Respecting RCC Suspended Slab29,

19971997

Cube size 150x150x150 mm, Specified strength 40 N/mm^2 June 26 , 1997- 8th slab May

29, 1997

June 26,

1997

28 2.449 35

- 8th slab May 29,

1997

June 26,

1997

28 2.459 39.2

- 8th slab May 29,

1997

June 26,

1997

28 2.482 38.8

- 8th slab May 29,

1997

June 26,

1997

28 2.478 37.7

1 Result data 1a Cube & Core Tests

Concrete Cores Crushing Sheets Laboratory

S # Core Measurements mm

Core 1 Core 2 Core 3

1 Location Top Slab Top Slab Top Slab2 Dia. mm 9.89 9.89 9.893 Max Length 165.4 172.7 1624 Min Length 140 152 1605 After Cutting 123 123.6 1156 After Capping 127.4 127.6 119.37 Steel Measurements8 Before cutting9 Dia. - - 1210 No of bars - - 111 Distance - - 33.012 After Cutting - - -13 No of bars - - -14 Distance - - -15 Core weight gm 2253 2239 229916 Failure load kN 108.2 99.7 118.417 Remarks 14.1 13.0 15.4

1 Result data 2 Core Tests



Chapter 11 Test Methods Respecting RCC Suspended SlabPhysical properties of coresCore

#Location Length cm Capped

length L cm

Dia. d cm Length diameter ratio

L/D

As received

bulk density g/cm^3

C1 8th slab 10.08 10.52 9.9 1.063 2.429C2 8th slab 10.12 10.69 9.9 1.08 2.482C3 8th slab 9.91 10.18 9.9 1.028 2.425

Mechanical properties of coresCore #

Location Core compressive strength N/mm^2

Dimension correction factor D

Reinforcement factor rd/cl

Estimated cube compressive strength N/mm^2

Remarks

C1 8th slab 41.8 0.942 - 39.4 Within Limits



Average 39.3 Within Limits

1 Result data 2a Core Tests Concrete Core Test Results

Client references Given above Given above Given aboveCore # C1 C2 C3Advised details-Core identification

8th floor slab 8th floor slab 8th floor slab

Advised direction of coring Vertical Vertical VerticalDate received July 22, 1997 July 22, 1997 July 22, 1997Date tested July 26, 1997 July 26, 1997 July 26, 1997Concrete age at test days 57 57 57Core length after preparationMinimum (mm) 100.8 101.2 99.1Maximum (mm) 100.8 101.2 99.1With sulphur capping (mm) 105.2 106.9 101.8Relation to as received length middle middle middleMean core diameter (mm) 99.0 99.0 99.0Length/ Diameter ratio 1.063 1.080 1.028Bulk density as received 2.429 2.482 2.425Saturation before test (days) 2 2 2Maximum load at failure (KN) 321.4 335.8 307.3



Chapter 11 Test Methods Respecting RCC Suspended SlabCompressive strengthMeasured core strength (N/mm^2)

41.8 43.6 39.9

Dimension correction factor 0.942 0.948 0.93Reinforcement factor - - -Estimated cube strength (N/mm^2)

39.4 41.3 37.1

Mode of failure Normal Normal Normal

1 Result data 3 Core Tests Brief visual examination of concrete BS 1881: Part 120: 1983

Client reference Given above Given above Given aboveCore # C1 C2 C3Advised details-Core identification

8th slab 8th slab 8th slab

Core dimension as receivedAverage dia (mm) 99.0 99.0 99.0Minimum/Maximum Length (mm)

121.0/130.0 124.0/138.0 149.0/161.0

Compaction & cracksSmall voids (0.5 to 3 mm)

None None None

Medium voids (3 to 6mm)

None None None

Large voids (>6mm)

None None None

Honey combing None None NoneCracks None None NoneAggregate Maximum size mm 20 20 20Petrological description (mm) Brownish dark grey crushed aggregate & screened natural

wadi sandDistribution of materials Graded Graded GradedReinforcementNumber of bars - - -Diameter of bars (mm) - - -Axis-end, as received (mm) - - -Axis-end, as prepared (mm) - - -Distance between bars (mm) - - -Other details failure plane passed through matrix with few crushed

aggregates

1 Result data 4 Load TestsLoad Test 8th Slab Report # 4897/A/17/9/1997 Laboratory



Chapter 11 Test Methods Respecting RCC Suspended SlabLoad Tonnes

Deflection mm



Chapter 11 Test Methods Respecting RCC Suspended SlabFirst cycle LocationLoading 1 2 3 4 5 6 70 0.0 0.0 0.0 0.0 0.0 0.0 0.068 0.03 0.01 0.00 0.36 0.00 0.00 0.19119 0.07 0.15 0.03 0.77 0.00 0.02 0.14170 0.15 0.12 0.12 1.25 0.10 0.10 0.83221 0 hour

0.25 0.01 0.16 1.71 0.22 0.14 1.22

221 1 hour

0.25 0.01 0.16 1.71 0.22 0.14 1.22

Unloading170 0.22 0.03 0.15 1.48 0.22 0.15 1.08119 0.13 0.03 0.15 1.47 0.22 0.12 1.0868 0.13 0.03 0.15 1.47 0.22 0.10 1.080 0.13 0.01 0.15 1.47 0.22 0.10 1.08Second cycle

Location

Loading 1 2 3 4 5 6 70 0.13 0.01 0.15 1.47 0.22 0.10 1.0868 0.24 0.10 0.23 1.47 0.34 0.29 1.09119 0.25 0.12 0.23 1.50 0.38 0.32 1.12170 0.29 0.14 0.24 1.92 0.41 0.32 1.15221 0 hour

0.30 0.17 0.25 2.35 0.42 0.33 1.45

221 1 hour

0.30 0.17 0.25 2.35 0.42 0.33 1.45

Unloading170 0.28 0.17 0.25 2.1 0.42 0.33 1.45119 0.28 0.17 0.24 2.07 0.42 0.33 1.4468 0.28 0.16 0.24 2.07 0.41 0.32 1.440 0.12 0.00 0.12 2.05 0.25 0.18 1.25Third cycle

Location

Loading 1 2 3 4 5 6 70 0.12 0.00 0.12 2.05 0.25 0.18 1.25221 0 hour

0.42 0.19 0.19 2.48 0.52 0.22 1.35

Unloading221 1 hour

0.48 0.22 0.20 2.55 0.54 0.24 1.38

0 0.17 0.08 0.14 2.06 0.38 0.21 1.09


11 concrete test procedure respecting a suspended slab

Engineering