The 76 sparks at Woolwich Station have already staged two 24-hour stoppages in row over finishing bonuses.
Workers want a completion bonus of four weeks money – up to £5,000 in some cases.
Unite national officer for construction Jerry Swain said: “The blasé attitude of Balfour Beatty’s management is breathtaking.
“They appear to have absolutely no desire nor inclination to resolve this dispute.
“The morale of the workforce is at rock bottom and productivity levels have fallen off the cliff edge, since the strike action began.”
But Balfour said it keen to resolve the dispute.
A statement said: “We are continuing to engage with Unite and Balfour Beatty Kilpatrick’s workforce of electricians on Crossrail.
“Balfour Beatty engaged fully with Unite to negotiate and agree a productivity bonus for Balfour Beatty Kilpatrick’s workforce of electricians on Crossrail. Since then, Unite demanded a significant increase in the bonus level and a completion bonus.
“We are committed to resolving this dispute and to the continued progress of the project.”