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Concrete cube failure, how does it portray on the design, designers and supervision agency?

posted May 23, 2011, 6:15 AM by jeffery jim   [ updated Jul 7, 2016, 10:16 AM ]

Introduction and case study

Have you ever gone through the misery when your concrete cubes failed to show indicative strength for the batches that was casted at site? Of course, many would argue that a single cube failure would not be good enough to indicates that the whole batches would be under strength.

It would not be a problem if those members are less critical structural members but what if this happened to the foundation itself. The sampling is similar to other structure members, yet it is the most important structure which distribute the load it carries to the ground via ground bearing distribution foundation, friction piles, end bearing piles or combination of these sort of transfers as the foundation.

In the case that sampling for foundation is in accordance to subclause 4.3 of Section D –Concrete Works from the Standard Specification of Building Works (hereafter known as JKR SSBW), the required strength for these cubes would be a minimum of 3N/mm2 as individual or 3N/mm2 as a group (based on Table 8A of the said section and subclause 4.10 for readymix concrete). As long as an individual cube failed and passed as a group, there would be easily justified as that particular cube would be categorized as under the required mean characteristic strength for the concrete however it is the subset of the allowable 5% of the failed population as described in BS EN 206-1:2000, Concrete, subclause 3.1.32.

Now, what happened when a batch of concrete are sampled as a group and it failed as a group? Traceability of the chit list and inspection form will indicates the locality of the said casted batch and action should be taken to render or to remedy the said problem. Will it resolved to hacking and recasting of the said member? Of course many will argue and say that statistical analysis will come in as it will surely save the day. It is agreeable that this statistical analysis would get you out of the problem when you have  casted so many batches, where the current margin value will surely not fall below the designed margin. Unfortunately, this is the foundation and it is the first few castings which did not have sufficient data to assist in statistical analysis as stipulated  in subclause 4.11. How do you maneuver to justify that this concrete is still within allowable design and durability envelop?

It is a PASS even though as a group it achieved only 31N/mm2 instead of characteristic strength of 35N/mm2 (Minimum pass is 32N/mm2).

 

Literature for Fresh concrete and derivation of it’s criteria

The fundamental for this failure is easily justified if ones could fully understand the criteria of each structural member and the relevant of it’s design to requirements such as strength, fire-rating and durability. Nowadays, most of the ordinary structures are assigned with C25/G30 (30N/mm2 characteristic strength for superstructures) while assigning a C30/G35 (35N/mm2 characteristic strength) for all foundations and substructures. This criteria can easily be recognized as a good step in providing a well compacted, low free water ratio; which will provide a denser concrete. Such grade of concrete will prolong from carbonation (XC Class) and sulfate (XD Class) attacks nevertheless, such design does not hold the ultimate key in minimizing such attacks. The thick concrete cover of 50mm and the combination with 50mm blinding shall work as an armor but to a state. In many cases, foundation concrete are specified based on normal acceptable code of practice which does not put into consideration of XA class attacks. Most of the designs only highlighted sulfate at the sea or XS class attacks as design consideration during marine structure design by assigning higher grade concrete.

XA class attacks or Soil Chemical attack has not been the point of discussion by many engineers and not so much of consideration has been taken to ensure the durability of the concrete itself. Specifying the concrete grade as a code practice without fundamental is futile. The foundation which are designed shall be less durable may likely to contribute to failure of the foundation itself and not due to workmanship.

Assigning the foundation concrete and reviewing the design mix should reaffirm with the consultant that the intended design are designed to intended serviceable period. The appropriate concrete design mix should  be compatible and to be reviewed earlier to the intended design. At most time, most of the designated concrete will follow according to the JKR SSBW, Section D which I reckoned that MS1195 to be in accordance to the practice of British Standard (parallel to EN standard) which adapts BS EN 206-1:2000 and BS 8500:2006  instead of BS5328-1:2007 series as mentioned in the JKR SSBW subclause 3.2. Table 6 in MS1195 is an adaptation of Table F.1 of BS EN 206-1:2000.

The differences between Table 14 of BS 5328-1:1997 and table BS EN 206-1:2000 is the actually derived from the same addendum between BS 8110-1:1985 and BS 8110-1:1997 where the significantly amplifying  on the durability.

 

The revolutions of codes and specifications

BS 8110-1:1985 clearly stated about durability at Chapter 6.2 Durability and 3.3.4 Exposure condition.

BS 8110-1:1997 than amplify the whole issue by introducing BS5328-1:1997 under subclause 2.2.4 Durability.

Eurocode 2 series than adapted the BS EN 206-1:2000 concept of classing concrete type to exposure for durability.

It is to be reminded that the whole shift of code and specifications emphasize on the concrete’s durability rather than strength-based concrete for structures. The adaptation of Eurocode 2 series and the introduction of BS EN 206-1:2000 and BS 8500-1:2006 have played a very minor role if not, is unknown to many structural designers especially on designing the foundation.

The foundation does not comes as C30/G35 as it is or spelled as a concrete which is only exposed to XC2 class only as most designer class their design as moderate under BS8110-1:1985. This clearly indicates that the designer have very low knowledge in regards to concrete design where, exposure are divided into 2 tables from BS EN 206-1:2000 which are Table 1 and Table 2 which underline exposure under XC, XS, XD and XA. The criteria for concrete design is summarized  in Table F.1 for characteristic strengths, minimum cement content and maximum water cement ratio.  But how are these determined by the designer; placed as instructions in drawings and during the review of the design mix?

One table does not  justify the selection and can be used as a point in determination the concrete criteria. In BS5328-1:1997, there are several issues needed for design which suppose to be taken in consideration which re-submerge in BS8500-1:2006 which earlier superseded BS8500-1:2002.

 

The actual scenario in accordance to in-place specification

The derivation of concrete say FND2 (foundation class) should not be as per Table F.1 (BS EN 206-1:2000) as ultimate guidance instead another source should be the read in the light for guidance in selecting the best design mix. Table 5 of BS 8500-2:2006 should be employed during the determining the best mix with provision of Table F.1.

Under moderate exposure say XC2 exposure for foundation only (an assumption by the designer), Table F.1 clearly indicates that the best mixed would have these provisions;-

  • The maximum Water/Cement ratio                         0.60
  • Minimum Strength Class                                      C25/G30
  • Minimum cement content                                    280kg/m3

These information is not sufficient in building up a design mix for the concrete, therefore it is reckon to cross refer table 5 of BS 8500-1:2006 which specified foundation Class FND2, with ordinary Portland Cement (Class CEM1), the mix should be as follows;-

  • The maximum Water/Cement ratio                         0.50
  • Minimum Strength Class                                      C25/G30
  • Minimum cement content                                    340kg/m3 with max aggregate of size 20mm

 

There are difference since it is clearly accepted that BS85001-2006 is the complimentary to BS EN 206-1:2000. Both should be read in details in reviewing the design. It is acceptable to fully utilized the Designated Mix of FND2 rather than just the general Exposure Classes, with the help of BRE Special Digest 1:2005 in case there is special chemical attack for aggressive condition as indicated in Table 2 BS EN 206-1:2000.

 

The ugly truth of over designing in practice.

The ugly truth had came unfolded because of the inability of designers to understand and embraced the latest code of practice, rather it has been a general practice blindly amongst many. The upgrade of concrete grade for the substructure came from BS5328-1:1997 from Table 13 where fully buried reinforced foundation should be of RC30 (or G35) with a nominal slump of 75mm. However based on the newest and on-placed specification which is the BS 8500-1:2006, Table A.13 clearly indicates C25/G30 is sufficient for reinforced buried foundation.

Therefore it is clearly acceptable for any cube of desired C30/G35 for foundation to be within the range of allowable strength for class C25/G30 to be a normal pass based on the on-place requirements. Designer should understand that, it is clearly highlighted in BS 8500-1:2006 that among the main changes would be the reduction of strength class for the FND designated concrete to C25/G30.

 

Summary

The failure of these cubes had been an eye opener where most of the design practices are not in accordance to in-place specifications but merely an assumption of what generally the other professionals had did repeatedly as a routine. In many cases, contractor is blamed and taking or switching places as scapegoats when the designers themselves initially failed to consider their part in making a basis for their designs and not based on speculation and norms.

Consider a designer maintain a G35 for footing design on pyrite infested soils. What would happened? Will it even reached the intended designed life span?

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