Communicating about formwork

COMMUNICATING ABOUT FORMWORK

Dr J D Bapat

This post is a summary of an excellent article by: Mary Bordner Tanck, Concrete International, January 2009, pp 53-56.

The communication among the architect, structural engineer, general contractor, formwork designer and contractor is one of the most important issues on any concrete construction project. The main goal of the formwork drawings is to convey to the contractor how to build it. Good communication on a project can take many forms but drawings are the principal means of communication. It can be a huge help, especially on fast-track jobs, if the CAD files on formwork layout are shared with the contractor. This has the benefit of expediting the formwork layout and eliminating the need for extensive shop drawing revisions. It is important that items such as slab edges, beam faces, work points and anchorages are properly dimensioned.

The design live loads, live load reductions and superimposed dead loads should be noted in the contract documents. The reshoring designer needs these values to produce a safe design. When a pour strip is required (for example, on a post-tensioned slab), the engineer should indicate whether the cantilevers on each side of the pour strip can support their self-weight and any superimposed dead and live loads or if they are required to be backshored until the pour strip is closed.

There are many details that need to be conveyed by the formwork designer for the contractor to construct a safe formwork system. Of particular importance are bracing sizes and locations as well as the specifications for the materials and components to be used. The more detail a designer provides, the lower the risk of errors, both in the design and in its realization in the field. The shop drawings should give field superintendents all the information they need, including the locations for potential problems, so the risk of missing a key item is significantly reduced. The drawings can also help the formwork designer and field personnel work together to address potential problems. Depending on the formwork designer’s scope of work, the location each beam edge relative to a grid line and relative to adjacent beams should be shown. These dimensions can help the contractor lay out the job so that material can be moved to the appropriate location the first time, saving the contractor time, money, and crane usage. The formwork designers need to very clearly communicate the materials required for shoring, reshoring or backshoring. When timber is used, the grade and species of each timber member should be clearly called out on the plan.

When responsibility is passed onto the contractor for critical items such as cantilever spans, framing over an opening or slab framing along column lines, the risk of problems increases significantly, as forms may be cobbled together in the field with little regard to connections, safety factors or back-span conditions. The sequencing can be crucial to the success of a shoring and reshoring design.

The members of the project team should share a common goal: to get the structure built safely, on time and within the budget.

The readers are advised to go through the original article to know more details.

Using building code to improve construction quality

In the last few years , two major changes have taken place in the cement and construction industry in India. Firstly, the emphasis of construction industry has shifted from high – strength to high – performance concrete . The realisation has come on account of the fact that nearly 65 % of the total cement sales in the country presently go towards the repairs of old structures , most of which are built with OPC . Secondly, the mineral admixtures, namely the fly ash (FA), silica fume (SF) and the blast furnace slag (BFS), are being increasingly used to improve the long term strength and the durability characteristics of cement and concrete. The need to build durable structures is felt not only from the point of view of economy , but also for the conservation of resources , energy and environment . The present study reviews the provisions of the Building Code IS 456 – 2000 towards building durable structures

The durability of concrete incorporates , besides strength , its capacity to resist the effect of the internal and external deteriorating factors , such as sulphate attack , chloride attack manifested in the corrosion of the reinforcement , carbonation , alkali – aggregate reaction , freezing and thawing , so as to give a satisfactory performance during the economic life , for which it is designed . Some of the important provisions in the Building Code, related to quality and durability are as follows :

a/ Clause 5.2: The following mineral admixtures, conforming to relevant Indian Standards, are permitted in the concrete: fly ash, silica fume, rice husk ash, metakaoline and ground granulated blast furnace slag (BFS).

b/ Clause 5.5 and 10.3.3: These two Clauses give various provisions on chemical admixtures. The dosage of retarders, plasticisers and superplasticisers has been restricted to 0.5, 1.0 and 2.0 % respectively by weight of cementitious materials.

c/ Clause 7: In the revised Code, the workability of concrete has been expressed in terms of slump only, unlike old Code wherein it was expressed in terms of Vee-bee time/compacting factor.

The expression in terms of compacting factor is recommended only in case of ‘very low’ workability category, such as pavement quality concrete. The ‘very high’ workability category has been newly introduced in the revised Code, applicable to tremie concrete and the workability measurement by flow has been recommended there. It will also be applicable to self-compacting concrete. It is well known that it is always better to express workability in one particular unit i.e. slump, compacting factor or flow, as these units are not always compatible with each other.

The maximum water-cement ratio for reinforced concrete has been reduced (example, 0.55 instead of 0.6, for M-20 concrete) and the assumed standard deviation (Clause 9.2.4.2) has also been reduced for higher grades of concrete. It is observed, in line with the reduced water-cement ratio, that the values of slump allowed by the Code are also low for normal type of construction.

The Code is thus indirectly expecting a change towards mechanisation of placing and consolidation of concrete.

d/ Clause 8.1.1: The permeability of concrete to the ingress of deleterious agents has been identified as one of the major characteristics affecting the durability. The factors influencing the durability have been delineated as environment , cover to embedded steel , type and quality of construction materials , cement content and water-cement ratio of the concrete , workmanship to obtain full compaction and efficient curing and shape and size of the member

e/ Clause 8.2.2.1: The general environment , to which the concrete will be exposed during its working life , is classified into five levels of severity , namely mild , moderate , severe , very severe and extreme. In comparison to the old Code, two more intermediate weather conditions, with less cement content, have been added in the revised Code.

f/ Clause 8.2.2.3: The air – entraining admixtures have been recommended for use in the concrete where freezing and thawing actions under wet conditions exist

g/ Clause 8.2.2.4: Recommendations have been given for the type of cement , maximum free water-cement ratio and minimum cement content , to develop adequate resistance in the concrete exposed to different sulphate concentrations. It is stated that PSC conforming to IS 455, with slag content more than 50 %, exhibits better sulphate resisting properties. Under conditions where chloride is encountered along with sulphates, the Code recommends the use of OPC with C₃A in the range of 5-8 % instead of SRC, PSC with more than 50 % slag or a blend of OPC and slag

h/ Clause 8.2.4.1: Recommendations have been given on the minimum cement content, maximum free water – cement ratio and minimum grade of concrete , for different exposure conditions. The minimum grade of concrete for footings or elements under non-aggressive soil or ground water (moderate environment) has been specified as M- 25. Thus the design mix is obligatory even for small buildings/structures.

i/ Clause 8.2.4.2: The upper limit of the cement content , not including FA and GGBS , has been kept at 450 kg / m³ , considering the increased risk of cracking due to drying shrinkage in thin sections or early thermal cracking and the increased risk of damage due to alkali-aggregate reaction, at the higher cement contents. It is hoped that the provisions for maximum cement content made in other standards, like IS 1343 on prestressed concrete structures, will also be brought in line with that in IS 456 – 2000.

j/ Clause 8.2.5.2: The total amount of chloride content ( as Cl ) in the concrete , at the time of placing , has been specified . The maximum chloride content of 0.6 kg/m³ of concrete has been stipulated, for reinforced or plain concrete containing embedded metal. The maximum limit has been rationalised and revised upward, as that specified in the earlier Code was difficult to realise in practice.

k/ Clause 8.2.5.3: The maximum total water – soluble sulphate content of the concrete mix , expressed as SO₃ , has been specified as 4 % by mass of the cement in the mix

l/ Clause 8.2.5.4: As a precaution against alkali – aggregate reaction , recommendations have been given on the constituent materials, like use of non-reactive aggregates and low alkali Portland cement ( < style=""> Na₂O equiv.) , partial replacement of cement with the mineral admixtures , use of impermeable membranes to reduce the degree of saturation of concrete during service and limiting the cement content of concrete .

m/ Clause 8.2.8: Recommendations have been given on the concrete constructions in sea - water or directly exposed along the sea - coast , with respect to the Grade of the concrete , type of cement , mix design and the use of pre-cast members . The use of slag or pozzolana cement has been recommended under such conditions.

n/ Clause 9.1.2: The Code stipulates the following information to be included while specifying a particular grade of concrete: type of mix (design or nominal), concrete grade, cement type, maximum nominal size of aggregate, minimum cement content for design mix, maximum w/c ratio, workability, mix proportions for nominal mix, exposure conditions (as per the Code), maximum placing temperature, method of placing and the degree of supervision

o/ Clause 10.1: This Clause on Quality Assurance Measures has been incorporated to emphasise the good concreting practices.

p/ Clause 10.2: The use of ready-mixed concrete or concrete from on/off site

batching and mixing plant has been recommended for large and medium size projects.

q/ Clause 10.3: The Clause on concrete mixing makes an important new provision. It stipulates that the concrete mixers shall be provided with water measuring (metering) devices. The provision will go a long way controlling water-cement ratio, especially in site mixed concrete. The old provision of hand mixing the concrete with 10 % extra cement in case of breakdown of mixer, work in remote areas or when concrete quantity is very small, has been removed in the revised Code.

r/ Clause 12.3.2: The old provision of negative tolerance on the cover has been removed. Under the new provision, use of PVC cover blocks has been permitted.

s/ Clause 13.4: The provisions under this Clause on “Construction Joints and Cold Joints” have been improved

t/ Clause 13.5: The use of proper and adequate curing techniques has been stressed, to reduce the permeability of the concrete and enhance its durability by extending the hydration of cement. The difference has been maintained between recommended minimum period of curing for concrete containing ordinary Portland cement and that containing cement with mineral admixtures

u/ Clause 16: The acceptance criteria for strength requirement has been totally revised and divided into two parts, namely that for (I) compressive strength and (II) flexural strength of concrete

v/ Clause 21: The concept of fire resistance of concrete has been newly introduced and discussed.

w/ Clause 26.4: The minimum values for the nominal cover (a new term introduced in the Code) have been specified , to meet the durability requirements. The minimum cover for footings has been specified as 50 mm.

While going through these provisions, it becomes clear that besides improving the construction quality, concrete strength and durability, the revised Code also puts emphasis on conservation of building materials.

Reference: Bapat J. D., “ IS – 456 : 2000 and Further ” Indian Cement Industry Desk Book 2002, December 2002, pp 30-39