In brief
- A low aggregate grading bid often prices one graded stockpile but omits washing, re-screening, test records and rejected loads.
- Mix consistency falls when the supplier blends all-in gravel, so ask for a per-sieve grading record for each stockpile and compare loads against it.
- Close the gap by naming the grading required in the specification and asking for written unit rates for washing, screening and testing before award.
Contents
A low aggregate grading bid in Garowe usually means the estimate covers one screened stockpile and leaves out the work that keeps grading steady. Washing, re-screening, separated stockpiles, test records and rejected loads all carry cost. When a bid omits them, the buyer sees the saving now. The pour sees it later as price, workability and strength move between loads.
What does a low aggregate grading bid leave out?
It leaves out the operations that turn loose gravel into a controlled product. A single screened pile is cheap to sell but hard to hold to one grading curve. Washing removes silt and clay fines that coat the stones and stop cement paste gripping them. Re-screening after rain or after dusty delivery takes plant time and fuel. Separate bins for each size cost space, handling and a loader hour that nobody lists. Testing costs a laboratory visit and waiting days for results. Rejected loads cost the supplier freight both ways. A bidder who prices only delivery can look the cheapest while accepting all of that risk, and the buyer carries it instead. Ask for the bid to name each of those operations as a line. If a line is missing, the bid is not comparable with one that keeps it.
Why does poor mix consistency follow?
The grading of the aggregate sets how much cement paste and water the mix needs to stay workable. When fines, mid sizes and coarse stones arrive in different proportions from load to load, the same nominal mix produces different concrete. A load with too many fines needs more water for the same slump, so the water to cement ratio rises and strength drops. A load of mostly coarse stones segregates, and the surface finishes harsh. Batching by volume makes it worse, because a loose pile holds air and each shovel of gravel carries a different mass of stone. The crew then chases workability at the mixer, adding water until the mix feels right. Over a Garowe slab or column, that means one section strong and the next weak. The mechanism is the missing graded stockpile, not the mixer.
How do you check a supplier's grading claim?
Ask for a per-sieve grading record for the exact stockpile being sold, not a general product sheet. The record should list the sieve sizes used and the mass passing each one, with the date, the stockpile reference and the laboratory that ran it. Walk the yard or the site and look for separate piles for each size or a screened stockpile under cover. Wet the sample and rub it in your palm: a gritty, sticky residue means fines and clay. Ask the laboratory for its rate and turnaround, and confirm the acceptance limits against the project's specification or engineer. During supply, take a sample from each delivery or each day's stockpile and compare it with the record. Two supplies may grade differently, so ask both for written unit rates for washing, re-screening and testing before award. Ask the project engineer or whoever holds the records what material and grading records exist for the intended source.
What to do next
Before you award the aggregate supply, send every bidder a written schedule listing washing, screening, stockpile separation, grading tests and rejected-load handling as separate items, and ask for a unit rate against each. Then compare bids line by line, not on the total.
Frequently asked questions
Can a supplier simply wash the gravel at the site?
Washing at site needs water, a settlement area and somewhere for the silt to go. Ask the supplier to show the plant and the water source in the bid, and check what the contract allows for disposal.
How often should a grading record be renewed?
Renew it whenever the source or stockpile changes, and after heavy rain or a long dusty spell. Confirm the interval with the project engineer and write it into the supply schedule.
Does a low aggregate bid always mean poor quality?
No. A low bid can be genuine if the supplier holds a graded stockpile and prices testing inside the work. The check is whether the omitted operations appear as line items, not the total alone.
Local reporting: this article is written for building work in Garowe and Puntland. Ground conditions, prices and rules differ between places; confirm the details for your own site.