Alternate for River Sand for Concrete Aggregate

Ordinary river sand is becoming extremely scarce due to the environmental impact of river bed dredging in the country. In some parts of the country, cost of sand is about Rs. 90 per cft whereas cost of cement may be about Rs. 180 per cft. Very ironically the inert inexpensive aggregate of concrete is now more costlier than cement in a nominal 1:1.5:3 concrete. Its cost share is much more high in cement mortar plasters, whose ratios may be nominal 1:5.

Replacement has been sought by manufacture of sand by crushing stones to sieve sizes of sand. Although appearing to be a less expensive solution, this has its own share of problems to the environment. There has also been apprehension about quality of grading and lack of quartzite composition of stone crushed fine aggregates.

One more alternative needs to be explored considering the gravitry of the problem, namely use of sea bed sand for fine aggregate in concrete. Over 15% of the sand and gravel currently used in England and Wales is won from the sea-bed and has been used in a wide range of reinforced concrete structures, according to a 1987 publication of Building Research Establishment UK [Gutt, W. and Collins, R. J.].According to the paper, the main difference between the majority of land-based aggregates and marine aggregate lies in the presence in the latter of sea salts, which usually have to be washed out to meet British Standard maximum limits for chloride and latest guidelines for limited alkali content to prevent alkali silica reaction (ASR).  Many island countries such as New Zealand are now exploring the possibilities.

Sea bed sand are not devoid of their own share of environmental problems. A summary of problems is presented at http://www.seafriends.org.nz/oceano/seasand.htm. The environmental and quality related apprehensions if addressed appropriately might lead to a significant solution to the construction industry in general.

Relative Economy of Pre-Engineered Buildings

Over past two decades, popularity of so-called Pre-Engineered Buildings have increased over conventional truss structures in industrial and warehouse construction. Technically, Pre-Engineered Buildings are portal frames with moment transferring connections and are not new. In fact, portal frames had been less popular alterrnative to truss buildings from economy considerations.

The current popularity of portal frames is due to the following reasons:

1. 
   
   Introduction of High Grade High Carbon Steel at prices comparable to that of Mild Steel. Use of Grade 350MPa Steel in place of conventional Grade 250MPa results in slender structures achieving greates economy.
2. 
   
   Advent of Submersion Arc Welding that imparts built-up sections homogenity and strengths comparable to that of Hot Rolled Standard Sections. However, SAW is not still feasible on all joints of built-up sections.
3. 
   
   Popularity of Cold Rolled Light Guage Sections such as Zed and Lipped Channnels for Purlins. Cost of these sections have now come down and their lower thickness is more friendly to self-tapping screws of Galvalume sheet roofing.
4. 
   
   Galvalume sheet roofing has itself served as a viable allternative replacing disdvantages of conventional forms of roofing.
5. 
   
   Flatter slopes are possible with Portal frames, (as less as 1 in 10) since slopes are not required from structural considerations. Flatter roofs reduces sheeting areas and make appearance more aesthetic.
6. 
   
   Aggressive marketing efforts of PEB manufacturing companies.

It is generally seen that use of high grade steel is more economical in PEB construction than using conventional MS sections. The cost saving in using 350MPa Grade Steel reduces weight of structture by around 20% whereas cost differences are about 10%. It is advisable to check supply of MS grade structures whereas designs are made for higher grades.

However, slender structures are vulnerable to higher defflections than conventional structures. In most cases, PEB structures cannot meet the requirements of deflection control as per IS:800. Bracing using rods also fails to satisfy the slenderness ratio criterion of IS Codes. In practice, performance of rod braces are subject to the extent of  prestressing achieved by tightening the same. These are specifications that have  to be decided between the client and the vendor.

It is generally seen that strength per unit weigt (in terms of moment carrying capacity) of built up sections are sensitive to section parameters in decreasing order as  follows: Width of flange, Thickness of Flange, Depth of Section, Thickness of Web. Stiffness per unit weight (in terms of MI on major axis) are sensitive respectively as Depth of section, Flange Dimensions, Thickness of Web.

Ready-Made-Steel in India

Recently ready made steel has been making strides in the construction industry, waiting to take it by storm as RMC did a few years back.

Ready made steel is reinforcement bars cut and bent to required size and shape so that it is only necessary to tie them in place before concrete.

The product is useful on the following grounds:
• Practically, however carefully you order steel from a vendor, the exact number or weight of steel bars do not match to your requirement. So conventional steel purchase always tend to be in excess of exact requirement. This leads to balance after usage.
• When steel is purchased and handed over to craftsmen, there is a tendency to cut the required lengths at random on an as required basis. This leads to un-optimized cutting schedules and results in wastage.
• There is always a lot of wastage or scrap of steel generated at site. Much of these get buried in construction debris and some of them are lost in theft/vandalism. And whatever scrap you recover after fabrication, is worth 25% of its original price.
• The space of steel yard is saved by using RMS and site looks clean of unwanted steel material.

Since RMS providers use top brands of steel and add the service charges for fabrication, RMS appears costlier than steel bent at site. The steel is transported to site with tags showing the bar marks. Note that in most cases, bending radius is specified on the outer dimensions. Sharp bends are not possible for stirrups.

Generally, RMS suppliers provide 8, 10 and 12mm bars cut to shape from coils. This ensures zero percent wastage on these bars. Coils are costlier than straight bars by about 5-10%. Bars more than 12mm in dia or bars cut from bent lengths generate scrap. The RMS suppliers generally use some optimizing routines to find the best cutting schedules. Despite this some scrap is generated – longer pieces are reusable elsewhere, shorter ones are not. Scrap that is not reusable by the company has to be purchased by the client.

Supply is generally of lengths less than 6m, although upto 12m can also be transported on trailers. Straight lengths less than 16mm can be bent to fit 6m length – this cannot be done on already bent bars.
By far the most important limitation of RMS is the necessity of a detailed bar bending schedule to be provided to the vendor. In most construction such schedules are not prepared as part of design. However, irrespective of using RMS or not, it is always advisable to prepare bar banding schedules (BBS) for all steel cutting works.

The position of lapping of steel reinforcement is the most difficult specification of a pre-determined BBS. The lapping requirement results in BBS such that total length of cut pieces is always greater than the total length required by the total length of number of lap splices. There is now an upcoming tendency to replace lap splices with butt splices using couplers.

Threaded couplers require ends of lapped bars too be threaded to accommodate the coupling sleeve. There are also couplers which can be crimped to bar profile at site, or inject filling of space between sleeve and rod. Bars can also be spliced with flanged butt connections. These types have not appeared in the Indian construction market yet. Threading has now appeared as a common splicing method in many major projects.

Even using coupler connections, it is necessary to foresee the staggering of laps at BBS preparation stages itself. Hopefully, RMS manufacturers may come up with automated solutions for preparing comprehensive BBS to tap the large market of middle level construction in the country.